Traffic Signals on Endothelium for Lymphocyte Recirculation And

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TRAFFIC SIGNALS ON ENDOTHELIUMFOR LYMPHOCYTE RECIRCULATION AND LEUKOCYTEEMIGRATION

Timothy A. Springer The Center for BloodResearch, HarvardMedical School, 200 LongwoodAvenue, Boston, Massachusetts02115

KEYWORDS: selectin, integrin, adhesion,homing, inflammation

INTRODUCTION

The circulatory and migratory properties of white blood cells have evolved to allow efficient surveillance of tissues for infectious pathogensand rapid accumulation at sites of injury and infection. Lymphocytescontinually patrol the body for foreign antigen by recirculating from blood, through tissue, into lymph, and back to blood. Lymphocytesacquire a predilection, based on the environment in which they first encounter foreign antigen, to hometo or recirculate through that same environment(39, 40). Granulocytes and monocytes cannot recirculate, but emigrate from the bloodstream in response to molecular changeson the surface of blood vessels that signal injury or infection. Lymphocytescan similarly accumulatein response to inflammatorystim- uli. The nature of the inflammatorystimulus determines whether lymphocytes, monocytes, neutrophils, or eosinophils predominate, and thus exercises specificity in the molecularsignals or "area codes" that are displayed on endothelium and control traffic of particular leukocyte classes. Recent findings showthat the "traffic signals" for lymphocyterecirculation and for neutrophil and monocytelocalization in inflammation are strikingly similar at the molecular level. These traffic signal or area code moleculesare 827 0066-4278/95/0315-0827505.00 Annual Reviews www.annualreviews.org/aronline

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Ig Family Selectin Chemoattractant Member

StepI Step2 Step3

FigureI Threesequential steps providethe traffic signalsthat regulate leukocytelocalization in the vasculature.Selectin moleculesthat bindcarbohydrate ligands, often displayedon mucin-like molecules,are responsiblefor the initial tetheringof a flowingleukocyte to the vesselwall and labile, rolling adhesions(Step 1). Tetheringbrings leukocytesinto proximitywith chemoattractantsthat are displayedon or releasedfrom the endotheliallining of the vessel wall. Chemoattractantsbind to receptors that span the membraneseven times on the surface of leukocytes. Thesecouple to G proteins,which transduce signals that activate integrinadhesiveness (Step 2). Theintegrins can then bindto immunoglobulinsuperfamily (IgSF) memberson the endothelium,increasing adhesiveness and resulting in arrest of the rolling leukocyte (Step 3). Followingdirectional cues from chemoattractantsand using integrins for traction, leukocytesthen cross the endotheliallining of the bloodvessel andenter the tissue.

displayed together on endothelium but act on leukocytes in a sequence that was first defined for neutrophils and appears to hold true with slight modification for lymphocyte homing as well (Figure 1). The selectin (Step 1) allows cells to tether and roll, the chemoattraetant (Step 2) tells cells to activate integrin adhesiveness and put on the brakes, and the Ig family member(Step 3) binds integrins and causes cells to come to a full stop. These three steps, Annual Reviews www.annualreviews.org/aronline

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with multiple molecularchoices at each step, provide great combinatorial diversityin signals. Accordingly,the selectiveresponses of different leukocyte classesto inflammatoryagents, as well as the preferentialreeirculation patterns of distinct lymphocytesubpopulations, can be explainedby their distinct re- ceptivity to combinationsof molecularsignals. Followingan overviewof leukocytesand endothelium,and of the moleculesimportant in their interactions, I reviewthe traffic signals that enableselective emigratorybehavior of monocytesand neutrophils, and then elaborate howa paradigmof three or four sequential signals can be extendedto lymphocyterecirculation. This review updates and extends about twofolda previous one (240). For recent reviewssee 12, 25, 26, 36, 42, 73, 90, 97, 143, 159, 160, 167, 172, 196, 206, 223, 280.

THE FUNCTION OF LEUKOCYTE CLASSES CORRELATES WITH CIRCULATORY BEHAVIOR Neutrophilic granulocytes are amongthe most abundantleukocytes in the bloodstreamand the first to appearat sites of bacterial infection or injury. Neutrophilsare producedat the prodigiousrate of 109cells/kg bodywt/day in the bonemarrow and have a half-life in the circulationof 7 h. Theirlifespan after extravasationis hoursor less (54). Their primaryfunction is to phago- cytose and eliminate foreign microorganismsand damagedtissue. Monocytesare far less numerousin the bloodthan neutrophils, wheretheir half-life is about24 h (113). Likeneutrophils, they are phagocyticand accumulatein responseto traumaticinjury or bacterial infection. However,monocytes differ fromneutrophils, in that they accumulateat sites whereT lymphocytes haverecognized antigen, as in delayedtype hypersensitivityreactions and graft rejection. Monocytesare importanteffector cells in antigen-specific T cell immunity,are activatedby T cell productssuch as "/-interferon, andcan organizearound parasites into protective structures called granulomas.After extravasation,monocytes may also differentiate into longer-livedtissue mac- rophagesor mononuclearphagocytes such as the Kupffercells of the liver, whichhave a half-life of weeksto months, In contrast to the neutrophil and monocyte,a lymphocytemay emigrate and recirculate manythousands of times duringits life history. Recirculationof lymphocytescorrelates with their role as antigenreceptor-bearing surveillance cells. Lymphocytesfunction as the reservoir of immunologicalmemory, and recirculate throughtissues to provide systemic memory.Few of the body’s lymphocytesare present at any one time in the bloodstream, wheretheir half-life is 2 h. Distinct subsets of lymphocytesextravasate through the microvasculaturein tissues such as skin and gut, and throughspecialized high endothelial venules(HEV) in lymphoidorgans (39, 159, 196). After migrating Annual Reviews www.annualreviews.org/aronline

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through tissue, lymphocytesfind their wayinto the lymphatics. Theypercolate through draining lymph nodes in the lymphatic system and finally enter the thoracic duet, through which they return to the bloodstream. This journey is completed roughly every 1 to 2 days.

ENDOTHELIUM

By displaying specific signals, the endothelium is the most active player in controlling leukocyte traffic. Vascular endotheliumis diversified at a number of levels. Large vessels differ from small vessels and capillaries, venular endotheliumdiffers from arterial endothelium, and endothelial phenotypevar- ies betweentissues. The preferential migration ofleukocytes from postcapillary venules maybe related to factors such as shear stress, which is lower there and hence more favorable for leukocyte attachment than in capillaries or arterioles, or to events that occur whenleukocytes pass through capillaries. However,when flow is controlled so that shear stress is equivalent in arterioles and venules (152), or when the direction of blood flow is reversed (182), attachment and emigration is far greater from venules, suggesting molecular differences in their endothelial surfaces. In agreementwith this, P-selectin is muchmore abundant on postcapillary venules than on large vessels, arterioles, or capillaries (168), and induction of E-selectin and vascular cell adhesion molecule- 1 (VCAM-l) expression in inflammation is most prominent on postcapillary venules (25). The mucin-like cluster of differentiation 34 (CD34) moleculeis well expressed on capillaries and is absent frommost large vessels (82), and CD36is expressed on microvascular but not large vessel endothelium (251). The extracellular matrix mayexert an influence on endothelial differentiation, as exemplified by modulation of adhesiveness (279). The high endothelium in lymphoid tissue, which expresses addressins for lymphocyte recirculation, is one of the most dramatic examplesof endothelial specialization (196). Inflammatorycytokines dramatically and selectively modulatethe transcrip- tion and expression of adhesion moleculesand chemoattractants in endothelial cells (203). Tumornecrosis factor (TNF)and interleukin-1 (IL-1) increase adhesiveness of endothelium for neutrophils and lymphocytes and induce intercellular adhesion molecule-1 (ICAM-1),E-selectin, and VCAM-1.IL-4, synergistically with other cytokines, increases adhesion of lymphocytesand induces VCAM-1(164, 259). It is likely that the precise mixture of chemoattractants and cytokines produced at inflammatorysites in vivo determines whichtypes of leukocytes emigrate. Thus injection into skin of IL-lt~ induces emigration of neutrophils and monocytes;as do lipopolysaccharide (LPS) and TNF-o~,but with more prolonged emigration of the monocytes. IFN-y induces emigration of monocytesbut not neutrophils (113). IFN-~/andTNF-a, but not Annual Reviews www.annualreviews.org/aronline

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IL-lot or LPS,recruit lymphocytes,and IL-4 is ineffective by itself but syn- ergizes with TNF(33, 60, 118). Actingmore quickly than cytokines, vasoactivesubstances such as hista- mineand thrombinmodulate endothelial function in secondsor minutes.They inducesecretion of the storage granulesof endothelial cells and platelets. Furthermore,they dilate arterioles and increaseplasma leakage, which raises the hematoeritwithin mierovessels, and thereby alters the rheologyof blood so as to increasethe collision of leukocyteswith the vessel wall (218). Fur- thermore,arteriolar dilation and the ensuingincreased blood flow in inflammatorysites are responsiblefor two of the cardinal signs of inflammation, rubor (redness)and calor (heat), and greatly enhancethe dischargeand, thus, accelerate the accumulationof leukocytes.

Area Code Molecules SELECTINSMultiple protein families, each with a distinct function, provide the traffic signals for leukocytes.The selectin familyof adhesionmolecules (Figure 2) has an N-terminaldomain homologous to Ca2÷-dependentlectins (25, 144, 167, 206, 238). Thename selectin capitalizes on the derivation lectin and select fromthe sameLatin root, meaningto separateby pickingout. Selectins are limited in expressionto cells of the vasculature(Figure 2). L-selectinis expressedon all circulatingleukocytes, except for a subpopulation of lymphocytes(85, 126, 151). P-selectin is stored preformedin the Weibel- Paladebodies of endothelialcells and the ~ granulesof platelets. In response to mediatorsof acute inflammationsuch as thrombinor histamine,P-selectin is rapidly mobilizedto the plasmamembrane to bind neutrophils and monocytes (86, 140, 168). E-selectin is inducedon vascular endothelial cells cytokinessuch as IL-1, LPS, or TNFand requires de novo mRNAand protein synthesis(27).

CARBOHYDRATESANDMUCIN-LIKE MOLECULES All seleetins appear to rec- ognizea sialylated carbohydratedeterminant on their counter-receptors(26, 143, 206). E-selectin andP-selectin recognizecarbohydrate structures that are distinct, but are both closelyrelated to the tetrasaccharidesialyl Lewisx and its isomersialyl Lewisa (Figure 2). Theactual ligand structures for E- and P-selectin are morecomplex than sialyl Lewisa or x, as shownby display of the ligandfor E-selectin,but not P-selectin,on fucosyltransferase-transfected cells that expresssialyl Lewisx (141). Theaffinity of E-selectin for soluble sialyl Lewisx or a is quite low, with Kd= 0.2-0.8 mM(183), whichsuggests that a higheraffinity ligand mayyet be identified. P-selectinis specific for carbohydratedisplayed on the P-selectin glycoproteinligand (PSGL-1),suggesting either that PSGL-1expresses a specific carbohydratestructure or that Annual Reviews www.annualreviews.org/aronline

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Endothellum Leukocytes GlyCAM-I--.. f Ne~ L-selectin / CD~ Lymphsubpop HEV, Monooyte Activated ~ Neutrophil endothelium -. Ls±a.~Le~sXso4 Fu~! j Eosinophil

Endothelialcell ~ P-selectin / CD62P/ PADGEM/ GMP-140 Neutrophi( Weibei-Paladegran Monocyte Platelet¢x granule Lymphsubpop )SGL-I~ NK

Neutrophil E-selectin/ CD62E/ ELAM-1 Monocyte Cytokine-activated Eosinophil endothelium Basophil ¯ LectJndomain Lymphsubpop ¯ EGFqikedomain NK O Shortconsensus repeat 10nm Figure2 Sclcctinsand their ligands.The sclcctins are shownto scale, basedon electron micrographsof P-selectin (260), the X-ray structure of E-sclectinlectin and EGF domains (91), estimatesof thesizes of theshort consensus repeats (SCR) (238). P-seleetin is shownpalmitylated ona transmembranecysteine (84). The carbohydrates arenot to scale.Sialyl Lewis a andx contain Gall~l-3(Fuctxl-4)GleNAeandGalI~I-4(Fuc(xl-3)GIcNAe linkages, respectively.

PSGL-1protein forms part of the ligand binding site (173). The affinity P-selectin for PSGL-1is high with a Kd= 70 nM(260).Structure-function studies suggest that the Ca2+-bindingsite and a cluster of basic residues on E-selectin coordinatewith the fucosyl andsialic acid carboxylatemoieties, respectively, of sialyl Lewisx (78, 91). The carbohydrateligands for L- and P-selectin are O-linked to specific mucin-likemolecules. Mucinsare serine- andthreonine-rich proteins that are heavily O-glycosylatedand have an extendedstructure. L-selectin recognizes at least two mucinsin HEV(Figure 3): glycosylation-dependentcell adhesion molecule-1 (GIyCAM-1),which is secreted (144), and CD34,which is on cell surface (17). Thecarbohydrate ligand for L-selectin is related to sialyl Lewis a andx (19, 83), contains sialic acid andsulfate, and is O-linked mucin-like structures of HEV(206). Structural studies on the carbohydrates of GlyCAM-Ishow that 6’ sulfated sialyl Lewis x (Figure 2) is a major oligosaccharidecapping group and is a candidatefor the ligand structure(101). Themucin-like P-selectin glycoproteinligand (PSGL-1)is a disulfide-linked dimerof 120-kDasubunits (173) that is sensitive to O-glycoprotease,which selectively cleaves mucin-like domains(186, 245). PSGL-1(Figure 3) isolated by screening for cDNAthat expressedligand activity (212). COScells mustbe transfected both with the PSGL-1eDNA and ix-3/4 fucosyl transferase eDNAto express P-selectin ligand activity. By contrast, COScells cotrans- fected with cDNAfor tx-3/4 fucosyl transferase andanother mucin-like molecule that is expressedby neutrophils,CD43, lack P-selectin ligand activity. Annual Reviews www.annualreviews.org/aronline

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10 n=

GlyCAM CD34 PSGL-1 Figure3Mucin-like carriers of selectin ligands. TheGIyCAM (144) and CD34(17,227) molecules synthesized by peripheral lymphnode HEVand MAdCAM-1molecule synthesized by mucosal HEV(see Figure5) bear O-linkedcarbohydrates that bindto L-selectin. CD34has a globulardomain that maybe Ig-like (13) and is resistant to O-glycoprotease(250). The PSGL-1molecule neutrophilsbears O-linkedcarbohydrates that bind to P-selectin (186, 212). A cysteine in the transmembraneregion is predictedto be palmitylated.O-linked sites and N-linkedsites are shown as bars and lollipops, respectively.The length of the mucin-likedomains, and the percentof serines and threoninesthat are O-glycosylated,are proportionedto measurementsfor CD43(45 nmper 224 aminoacids and 75-90%of O-glycosylation)(65).

FUNCTIONOF SELECTINSAND THEIR LIGANDSSelectins mediate functions unique to the vasculature, the tethering of flowing leukocytes to the vessel wall, and the formation of labile adhesions with the wall that permit leukocytes subsequently to roll in the direction of flow. One study demonstrated this with purified P-selectin incorporated into supported planar lipid bilayers on one wall of a flow chamber (147). At wall shear stresses within the range of those found in postcapillary venules, neutrophils formed labile attachments to the Annual Reviews www.annualreviews.org/aronline

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P-selectin in the bilayer and rolled in response to fluid drag forces. In other studies, intravascular infusion of a soluble L-selectinflgG chimera inhibited neutrophil rolling attachmentsin vivo (153), as did infusion of anti-L-selectin monoclonalantibodies (266). Morerecent studies have shownthat neutrophils roll on E-selectin in purified form(148) or on the endothelial cell surface both in vitro (1) and in vivo (188), that monoclonalantibody (mAb)to P-selectin decreases neutrophil rolling in vivo (28), and that neutrophil rolling in the microvasculature of mice genetically deficient in P-selectin is almost completely absent (165). P- and L-selectin may cooperate with one another because inhibition of either almost completely inhibits neutrophil rolling in vivo (153, 165, 266, 267). E- and L-selectin also appear to cooperate (135, 148, 200, 265). A class of ligand that is closely associated with L-selectin on the neutrophil surface is required for the initial tethering duringflow to E-selectin bilayers, after which another class of ligands that mediates rolling takes over (145). Selectins can mediatetethering of a flowingcell in the span of a millisecond. Other adhesion receptors require minutes to develop similar adhesive strength and do not mediate rolling (51, 147). It has been hypothesized that selectins differ from other adhesion moleculesnot in affinity (Keq), but in having much morerapid association (Kon)and dissociation (Koff) rate constants (147), recently been confirmed(Table 1). Rolling is intermittent and appears mediated by randomassociation and dissociation of selectin-ligand bonds, a small number of which tether a leukocyte to the vessel wall at any one time. A rapid association rate facilitates the initial tethering in flow. Arapid dissociation rate ensures that even with multiple selectin-ligand bonds, it will not take long before the bond that is most upstream randomlydissociates, allowing the cell

Table 1 Fast on and off rates of a selectin and affinity modulationof an integrin

Ko,~ Koch" Kd (M-’s-’) (s-’) (/zM)

P-selectin 1.4 × 107a 1b 0.07¢ LFA-1a low affinity 3 x 102 0.03 100 LFA-1e high affinity NDf ND 0.6

a Calculatedfrom Kon = Koft/Kd b At verylow P-selectin densities in lipid bilayers,neutrophils, attach transiently, i.e. they subsequentlydetach rather than roll. Measurementsof the cellular dissociationrate suggest that the ti/2 for dissociationof a single selectin-ligandbond is about 0.7 s (R Alon& Springer,unpublished data). ¢ For bindingof monomeric,truncated P-selectin to neutrophils(260). ’~ko,, koff, and Kdwere measuredby competitiveinhibition by monomeric,truncated ICAM-1of binding of Fab to LFA-Ion resting lymphocytes(157). c Sameas d but for phorbolester-stimulated lymphocytes. Approximately 20% of the cell surfaceLFA-1 was in the highaffinity state (157). fND= not determined. Annual Reviews www.annualreviews.org/aronline

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to roll forwarda smalldistance until it is held by the next mostupstream bond (96, 147). Theelongated molecular structure of selectins and mucinsand their seg- mentalflexibility (65, 260)are predictedto enhancetheir accessibility for bindingto counter-structureson closely opposedcells (147). P-selecdnand PSGL-1are currently the most elongated adhesionmolecules known (Figure 3 and 4) and can bridge together twocells with plasmamembranes about 0.1 gmapart. Expressionon cytoplasmicprotrusions further enhancesaccessibil- ity. L-selecdnis clusteredon microvilliof neutrophils(79, 200), whichproject about0.3 lamabove the surface of a cell with a diameterof 7 lam, andcontain 90%of the L-selectin(D Baintonet al, unpublisheddata). In keepingwith this topographicdistribution, roiling in vivorequires the integrity of the L-selectin cytoplasmicdomain and is inhibited by eytochalasin B (125). Lymphoeytes bind through microvilli to HEV(7, 261). Conversely,the mucin-like CD34 molecule(227) is concentratedon filopodia of nonspecializedendothelial cells foundin the microvasculatureof mosttissues (82). Thesefilopodia are con- eentrated near junctions betweenendothelial cells, and electron micrographs of granulocytesbinding to the microvasculaturein inflammatorysites suggest that the earliest bindingevent is to these filopodia(62).

CHEMOATI’RACTANTSChemoattractants are important in activation of integrin adhesivenessand in directingthe migrationof leukocytes.In chemotaxis, cells movein the direction of increasingconcentration of a chemoattractant, whichtypically is a soluble moleculethat can diffuse awayfrom the site of its production,where its concentrationis highest(70, 274). Leukocytes,which can sensea concentrationdifference of 1%across their diameter,move steadily in the direction of the ehemoattraetant. There is muchinterplay between adhesionmolecules and chemoattractantsbecause adhesionto a surface is required to providethe traction necessaryfor migrationdirected by chemoattractants, and chemoattractantscan activate adhesiveness. Thealternative mechanismto chemotaxisis haptotaxis. In haptotaxis, cells migrateto the regionof highest adhesiveness(45). Thuson a gradient of adhesiveligand affixed to the surface of other ceils or to the extracellular matrix, and in the absenceof a chemotacticgradient, motilecells will tend to accumulatein the region of highest ligand density. Both chemotaxisand haptotaxiscan contribute to cell localization, but haptotaxis has yet to be demonstratedin vivo. Classical leukocytechemoattraetants act broadly on neutrophils, eosinophils, basophils, and monocytes(Table 2). A recently describedfamily chemoattractivecytokines, termedchemokines, are 70-80residue polypeptides and have specificity for leukocyte subsets (12, 172). Twosubfamilies ehemokineshave been defined by sequence homologyand by the sequence Annual Reviews www.annualreviews.org/aronline

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Table 2 Leukocytechemoattractants

Chemoattractant Origin Respondingcells Classicala chemoattractants N-formylpeptides Bacterialprotein processing Monocyte,neutrophil, eosinophil, basophil C5a Complementactivation Monocyte,neutrophil, eosinophil, basophil LeukotrieneB4 Arachidonatemetabolism Monocyte,neutrophil Platelet-activatingfactor Phosphatidylcholinemetabolism Monocyte,neutrophil, eosinophil (PAF) CXCb chemokines IL-8/NAP-1 T lymphocyte,monocyte, endothe- Neutrophil,basophil lial cell, fibroblast,keratinocyte, chondrocyte,mesothelial cell CTAP-III//3- SuccessiveN-terminal cleavage of Neutrophil,basophil, fibroblast thromboglobulin/ platelet basicprotein released NAP-2 from a-granules gro/MGSA Fibroblast, melanoma,endothelial Neutrophil,melanomas, fibroblast cell, monocyte ENA-78 Epithelium Neutrophil CCc chemokines MCP-I T lymphocyte,monocyte, fibrob- Monocyte,T lymphocytesub- last, endothelialcell, smooth population,basophil muscle MIP-lot Monocyte,B and T lymphocyte Monocyte,T lymphocytesub- population,basophil, eosinophil RANTES T lymphocyte,platelet Monocyte,T lymphocytesub- population,eosinophil 1-309 T lymphocyte,mast cell Monocyte References: a(70, 232),1~(12, 29, 43, 137,172), c(2, 12, 43, 124,172, 208, 214-216, 253,

around two cysteine residues (Table 2). The CXCor a chemokines tend to act on neutrophils and nonhematopoietic cells involved in wound healing, whereas the CC or I~ chemokines tend to act on monocytes and in some cases on eosinophils and lymphocyte subpopulations. It has long been debated whether chemoattractants can act in the circulation, where they would be rapidly diluted and swept downstream by blood flow. Tethering and rolling of leukocytes through selectins would enhance exposure to chemoattractants by prolonging contact with the vessel wall. However, retention of chemoattractants at their site of production by noncovalent interactions with molecules on the vessel wall and within the inflammatory site may also be important. Heparin-binding sites on chemokines provide a mech- Annual Reviews www.annualreviews.org/aronline

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anism for retention in the extracellular matrix (109) to enhanceconcentration gradients and perhaps to present chemokineson the endotheliumto circulating leukocytes (207, 253).

CI-IEMOA’I"rRACTANTRECEPTORS Leukocyte chemoattractant receptors have multiple functions. Theynot only direct migration, but also activate integrin adhesiveness, and stimulate degranulation, shape change, actin polymerization, and the respiratory burst (232). Chemoattractantreceptors are G protein-coupled receptors that span the membraneseven times. Ligand binding to the seven membrane-spanner is coupled to exchange of GTPfor GDPbound to the associated G protein heterotrimer and results in activation by the G protein t~ and ~y subunits of signaling effectors such as phospholipase C-[~2 (277). This results in release of diacylglycerol and inositol phosphates, and mobili- zation of Ca2÷. Neutrophils and lymphocytesexpress G~i2and Go~i3subunits (18, 232). TheG~x subunits of the ~i class are ADP-ribosylatedand irreversibly inactivated by pertussis toxin. All the biological effects of leukocyte chemoattraetants are inhibited by pertussis toxin. Couplingthrough G~i subunits has been confirmedby reconstitution in transfected cells (277). The lipid mediators LTB4and PAFare as active as formylated bacterial peptides C5a and IL-8 in stimulating chemotaxis,but less active in stimulating the respiratory burst and other functions of neutrophils (232); this correlates with their ability to couple to distinct Go~subunits in transfected cells (6). Cloning of the receptors for formylated bacterial peptides C5aand PAFhas shownthat they are expressed on both neutrophils and monocytes, whereas the receptor for IL-8 is expressed only on neutrophils (181). The receptor for MCP-1is expressed on monocyticcells but not on neutrophils (52). Thus the specificity of chemoattractantsis regulated by the cellular distribution of their receptors.

INTEGRINSIntegrins are perhaps the most versatile of the adhesion molecules. Integrin adhesivenesscan be rapidly regulated by the cells on which they are expressed. Each integrin contains a noncovalentlyassociated ot and ~ subunit, with characteristic structural motifs (Figure 4). Five integrins are important the interaction of leukocyteswith endothelial cells. Their cellular distribution, ligand specificity, and structure are summarizedin Table 3 and Figure 4.

ACTIVATIONOF INTEGRINSThe adhesiveness of LFA-1 and VLA-4 on T lymphocytesis activated by cross-linking of the antigen receptor and other surface molecules (73, 223, 238). Increased adhesiveness occurs within a few minutes, is not accompaniedby any change in quantity of surface expression, and appears to result from both conformationalchanges that increase affinity for ligand, and altered interaction with the cytoskeleton (73, 81, 88). However, Annual Reviews www.annualreviews.org/aronline

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A. SubunltPrimary Structure B. ScaleModel I domain EFhand Membrane

(xL((xM, X)

EF hsnd

Conserved Cyste|ne- region richrepeats

~’ N- Gly©osylatlonSite ¯ Cystelne20O amino 8cldl

Figure4 Integdns that bind endothelial ligands. (A) Schematicsof representative integrin ¢t and subunits. The structures of ~tL (142) and ~2 (134) integrin subunits are shownas representative ¢tMand ctx or Ill and [~7, respectively; cysteines are identical, and glycosylationsites vary but arc sparse in the I domainand EF hand repeats. The EFhand repeats are divalent metal-binding motifs that may bind Ca2+ or Mg2+ (labeled Me). A binding site for 2+ and Mn2 but not Ca2+ has be en identified in the I domain(17 I). The4 integrin subunit has a posttranslational proteolytic cleavage site (252). A putative divalent cation binding site has been defined in the conserveddomain of the integrin l~3 subunit and is shownfor [~2 056). (B) Scale model of an integrin based on electron micrographsof the intggrins gpIIbIIIa (44) and VLA-5(ctS[~l) (184),

it is unlikely that recognitionby T cell receptors of antigen on endothelial cells (204) is a step in lymphocytetrafficking because traffic of lymphocytesthat can and cannot recognize specific antigen is increased in antigen-induced inflammation. Although evidence has been presented that binding of neutrophils to selectins can activate adhesivenessof integrins (155), other evidence has failed to confirm this (148, 158; T Diacovo & T Springer, unpublished data). Thus far the best candidates for activation of integrin adhesivenesswithin the vasculature are chemoattractants. Adhesiveness of Mac-1and LFA-1on neutrophils and monocytesis activated by N-formylatedpeptide and IL-8 (38, 74, 154, 154, 229, 276). In contrast to LFA-1on lymphocytesand neutrophils, Mac-1on neutrophils is increased about tenfold on the surface by chemoattractant-stimulated fusion of secretory granules with the plasma membrane(221); however, this is neither sufficient nor necessary for increased adhesiveness (195,262). The transient nature of the activation of integrin adhesiveness(76, 154) provides a mechanismfor de-adhesionand, perhaps, for retraction of the trailing edge of a leukocyte from the substrate during cell migration. Conformational changes in LFA-1and Mac-1that are associated with increased adhesiveness are suggested by the reaction of mAband antigen-binding fragments (Fab), i.e. they react only with these molecules after cellular Annual Reviews www.annualreviews.org/aronline

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activation (72, 127, 138, 201). After chemoattractantactivation of neutrophils, saturation binding shows that 10%of the surface Mac-1molecules express an activation epitope, yet mAbto this epitope completely blocks binding to ligands such as ICAM-1and fibrinogen, which suggests that ligand binding is mediated by a subpopulation of activated Mac-1 molecules (72). The I domain of leukocyte integrins is important in ligand binding (71,171) and expresses activation epitopes (72, 138). Recent measurementsof the affinity of cell surface LFA-1for soluble, monomericICAM-1 (Table 1) have directly demonstrated that cellular activation increases the affinity of a subpopulationof LFA-1molecules by approximately 200-fold (157). Surprisingly, the integrin VLA-4,by contrast to LFA-1and Mac-l, appears to be capable of supporting rolling. Lymphocytescan tether in flow and subsequently roll on VCAM-1.If activated while rolling by phorbol ester or TS2/16 mAbto the [~l subunit, the lymphocytes arrest and develop finn adhesion. Activated lymphocytestether as efficiently as resting lymphocytes but do not roll. Fibronectin can support developmentof finn adhesionin static conditions but not in tethering or rolling in flow. VCAM-1is less efficient than selectins in mediating tethering and rolling (R Alon et al, submitted).

IgSF MEMBERSON ENDOTHELIUM AS INTEGRIN LIGANDS In a paradigm first established with ICAM-1binding to LFA-l, several immunoglobulinsuperfamily (IgSF) members,expressed on endothelium, bind to integdns expressed on leukocytes (Figure 5). ICAM-I, ICAM-2, and ICAM-3are products of distinct and homologousgenes and wereall initially identified by their ability to interact with LFA-1(68, 210, 243). ICAM-1has also been found to bind to Mac-1through a distinct site in its third Ig domain(74, 75, 230) (Figure Induction of ICAM-1on endothelium and other cells by inflammatory cytokines mayincrease cell-cell interactions and leukocyte extravasation at inflammatory sites, whereas constitutive expression of ICAM-2may be important for leukocyte trafficking in uninflamedtissues, as in lymphocyterecirculation. ICAM-3is restricted to leukocytes. All three ICAMscontribute to antigen- specific interactions, thus inhibition with mAbto all three is required to completely block LFA-1-dependentantigen-specific T cell responses (67). VCAM-1is inducible by cytokines on endothelial cells, and on a more restricted subset of nonvascular cells than ICAM-1(25). A single VCAM-1 gene gives rise through alternative splicing to a seven domainisoform and to a second isoform that contains either six domains or three domains and a glycosyl phosphatidylinositol membraneanchor (130, 176, 259) (Figure

VCAM-1is a ligand for the integrin O~41~1 (VLA-4)and binds weakly7to (g41~ (50, 77, 211). In contrast to the shorter isofonns, the seven domainisoform VCAM-Ihas two binding sites for VLA-4in highly homologousdomains 1 and 4 (191,192, 268, 269). Annual Reviews www.annualreviews.org/aronline

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Mac-1 LFA-1 VLA-4 LPAM-1 ~a ~2 ~L ~2 .. ~1~7

10 nm

hu,mo hu mo ICAM-1 ICAM-2 I 7D 6D 3D ] MAdCAM-1 CD54 CD102 VCAM-1 CD106 Figure5 Ig supeffamilyadhesion receptors on endothelium,and their integrin-bindingsites. Membersof the Ig supeffamilysham the immunoglobulindomain, composed of 90 to 100 amino acidsarranged in a sandwichof twosheets of anti-parallel ~-strands,which is stabilizedby oneor (in the N-terminaldomain of the moleculesshown) two disulfide bonds. Theimmunoglobulins and T cell receptors are the onlyknown members of this familythat undergosomatic diversification. Thefunction of the IgSFin adhesionevolutionadly predates specialization for antigenrecognition. Theshape and size of the ICAM-1 molecule, with its unpairedIg domainsand bend,was determined by electron microscopy(131,242), as was that of VCAM-1(192). Immunoglobuliadomains ellipsoidswith a lengthof 4 nmparallel to the II-strands and2.5 nmin the other dimensions.The mucin-like region of MAdCAM-1is modeled as described in the legend to Figure 3. N-linked glycosylationsites in the Ig domainsof this andthe other moleculesare not shown.References for structures(in parentheses)and for localizationof the domainsto whichintegrins bind(in brackets) follow: ICAM-1(226, 244) [75, 242]; ICAM-2(243); VCAM-1(176, 190, 205) [191,192, 269]; MAdCAM-I(34).

An addressin for lymphocyte recirculation to mucosa is expressed on Peyer’s patch HEVand on other venules (248). Now termed mucosal addressin cell adhesion molecule (MAdCAM-1),it contains three Ig-like domains and mucin-like region interposed between domains 2 and 3 (34) (Figure MAdCAM-1binds the integrin ~41~7 but not t~4~l (24, 108). Furthermore, carbohydrates attached to the mucin-like domain of MAdCAM-1bind L- selectin and mediate lymphocyte rolling (20). Thus MAdCAM-1has a dual function as an integrin and selectin ligand.

OTHERMOLECULES CD31 is an IgSF member expressed on leukocytes, platelets, and at cell-cell junctions on endothelium (3, 4, 177, 185, 228, 24"/, 254). Annual Reviews www.annualreviews.org/aronline

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CD31can bind homophilicallyto itself and also heterophilically to an un- characterizedcounter-receptor, mAb cross-linking of CD31,similarly to many but not all other lymphocytesurface molecules,can trigger integdn adhesiveness (254). Interaction betweenCD31 on endothelial junctions and CD31 leukocytesappears to be required for transmigrationbut not for integdn-mediated binding of leukocytesto endothelium(178). CD31-CD31interaction mayrepresent a fourth step in transendothelial migrationthat overlaps the integrin-mediatedstep and maycontribute to the maintenanceof the permeability barrier functionof endotheliaduring transmigration. CD44is a widelydistributed moleculein the bodythat is homologouswith cartilage-linkprotein, is extensivelyalternatively spliced, andcan bear heparin sulfate or chondroitinsulfate side chains(98). Thebest understoodfunction of CD44is as a majorsurface receptorfor hyaluronate(I i, 64). Alternatively spliced formsof CD44are importantin tumormetastasis (92) and in localization of antibody-secretingcells (9). CD44(H-CAM, Hermes) was at one mistakenlythought to be the humanequivalent of murinemel-14 (L-selectin). It participates in vitro in lymphocyteinteraction with HEVand activated endothelium(119, 189). However,lack of cell surface CD44has no effect lymphocyterecirculation in vivo (41). Toward a Multi-Step Model of Neutrophil Emigration in Inflammation INTEGRINSAND SELECTINS Patients whoare genetically deficient in the leukocyteintegrins, owingto mutationsin the common132 integrin CD18 subunit, providedearly evidencethat adhesionmolecules are required for leukocyte extravasationin vivo (8, 133). Leukocyteadhesion deficiency-I (LAD-I) tients havelife-threatening bacterial infections, and neutrophilsin these patients fail to cross the endotheliumand accumulateat inflammatorysites, despite higher than normallevels of neutrophilsin the circulation. In vitro, LAD-Ineutrophils or normalneutrophils treated with mAbto the leukocyte integrins are deficient in bindingto and migratingacross resting or activated endothelialmonolayers (35, 231). Eventhough capable of bindingto activated endotheliumthrough selectins, LAD-Ineutrophils fail to transmigrate(231). mAb~t to the leukocyteintegrin 132subunit, and in somecases the integrin ~t subunit, have been found to have profoundeffects in vivo (97). ThesemAb prevent the neutrophil-mediatedinjury that occurs whenischemic tissue is reperfused, and thus can prevent death from shock after blood loss, limb necrosisafter frostbite or after amputationand replantation, and tissue necrosis frommyocardial ischemia and reperfusion, mAbto leukocyteintegrins and to ICAM-1can also inhibit lymphocyteand monocyte-mediatedantigen-specific responsesin vivo, includingdelayed-type hypersensitivity, granulomaforma- tion, andallograft rejection(97). Annual Reviews www.annualreviews.org/aronline

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WhereasmAb to the leukocyte integrin ~2 subunit blocked accumulation of leukocytes in tissue in response to chemoattractants, and prevented stable adhesionof leukocytes in the local vasculature, it had no effect on the number of rolling leukocytes on the vessel wall (10). Furthermore,leukocyte integrins were found to mediate binding of neutrophils to endothelial monolayersin a parallel wall flow chamberat subphysiologic, but not at physiologic, shear stresses foundin postcapillary venules (146, 231). Parallel studies showedthat selectins were required for leukocyte accumulation in vivo and acted at an early step. Antagonists of L-selectin and E- selectin inhibit neutrophil and monocyteinflux into skin, peritoneal cavity, and lung in response to inflammatoryagents (122, 123, 151, 180, 272). mAb to L-selectin was shownto inhibit neutrophil accumulationon cytokine-stimulated endotheliumat physiologic shear stress (229). Stimulation of neutrophils with chemoattractants results in shedding into the mediumwithin minutes of L-selectin, with kinetics similar to upregulation of surface expression of the integrin Mac-1. Based on this, and the evidence reviewed above, it was hypothesized that selectins mightact at a step prior to integrins (132). Further studies showedthat selectins mediate rolling, and function prior to developmentof firm adhesion through integrins. At sites of inflammation, leukocytes first attach to the vessel wall in a rolling interaction, then become arrested or firmly adherent at a single location on the vessel wall before diapedesis (56). This process was fully reconstituted with purified components of the endothelial surface (147). At physiologic shear stresses, neutrophils attach to and form labile rolling adhesions on phospholipidbilayers containing purified P-selectin, but not on bilayers containing ICAM-1.Chemoattractants stimulate strong, integrin-mediated adhesion to bilayers containing ICAM-1 under static conditions but not in shear flow. At physiologic shear stresses, if both P-selectin and ICAM-1are present in the phospholipid bilayer, resting neutrophils attach and roll identically as on bilayers containing P-selectin alone. However,when chemoattractant is added to the buffer flowing through the chamber,the rolling neutrophils arrest, spread, and firmly adhere through the integrin-ICAM-1interaction. Chemoattractantdoes not enhance interaction of neutrophils with bilayers containingP-selectin alone, but rather inhibits it. These findings show that purified adhesion molecules and chemoattractants representing the endothelial signals can reproduce the key events in leukocyte localization in vivo, and prove that the selectin-mediatedstep is a prerequisite for the chemoattractant and integrin-mediated steps (147). Complementary vivo studies showedthat mAbto L-selectin, or L-selectin/IgG chimeras, decreased the numberof rolling leukocytes (153, 266) and the numberof leukocytes that subsequently became firmly adherent, whereas mAbto the 1~2 integrin subunit only decreased firm adherence of leukocytes. This suggests that L-selectin acts at a step prior to leukocyteintegrins (266). In static assays, Annual Reviews www.annualreviews.org/aronline

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a factor derived from cytokine-stimulated endothelium induced shedding of L-selectin, and if transmigration was blocked with CD18mAb, induced release of neutrophils from inverted endothelial monolayers,also suggesting that L- selectin acted prior to leukocyte integrin-mediatedemigration (229). In elegant confirmation of a three-step model in a static assay of neutrophil adhesion to histamine-stimulated endothelium, juxtacrine cooperation betweenP-selectin and platelet-activating factor (PAF)was found (158). P-selectin tethered trophils to endothelium and thereby augmentedstimulation by PAFof CD18- dependent neutrophil adhesion. Stimulation of adhesiveness was by PAFand not by P-selectin, as shownwith PAFreceptor antagonists. The requirement for the carbohydrate ligands of selectins for leukocyte emigration in vivo has received strong support from studies of two patients with a genetic defect in biosynthesis of fucose and whotherefore lack the ligands for E-selectin and P-selectin (80, 264). The defect, designated LAD-II, has manyclinical similarities to LAD-Iincluding strikingly depressed neutrophil emigration into inflammatorysites.

CHEMOATTRACTANTSChemoattractants are required for transendothelial migration in vitro and in vivo and can induce all steps required for transmigration in vivo. Injection of chemoattractants into skin or muscle leads to robust emigration of neutrophils from the vasculature and accumulationat the injection site (58). Injection of lipopolysaccharide or cytokines that induce IL-8 synthesis also elicits neutrophil emigration. Moreover,mAb to IL-8 markedly inhibits neutrophil emigration into lung and skin in several models of inflammation (179, 220). The effects of pertussis toxin provide further evidence for the importance of Gt~ protein-coupled receptors in leukocyte emigration in vivo. Pretreatment of neutrophils with pertussis toxin inhibits emigration into inflammatorysites (187,234). Chemoattractants impart directionality to leukocyte migration. By contrast to intradermal injection, intravascular injection of IL-8 does not lead to emigration (99). Cytokine-stimulated endothelial monolayers grown on filters secrete IL-8 into the underlyingcollagen layer. Neutrophilsadded to the apical compartmentemigrate into the basilar compartment, but not when the IL-8 gradient is disrupted by addition of IL-8 to the apical compartment(109). AlthoughIL-8 acts as an adhesion inhibitor in someassays (87), the results are partially attributable to disruption of a gradient of IL-8 on activated endothelial monolayers when exogenous IL-8 is added on the same side as the neutrophils. Chemoattractantsact on the local tissue, as well as on ieukocytes. Neutrophil chemoattractants injected into the same skin site hours apart will stimulate neutrophil accumulation the first but not the second time, whereas a second Annual Reviews www.annualreviews.org/aronline

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injection into a distant site will stimulate accumulation.Desensitization occurs for homologouschemoattractants only (57, 59). Thus chemoattractants must act on and homologouslydesensitize a cell type that is localized in tissue. In somecases this localized cell maybe the mast cell. Somechemoattractants stimulate the mast cell (which localizes in tissue adjacent to the vasculature), or its better studied relative the basophil, to release histamine (29, 137) and TNF(270). Histamine induces P-selectin and TNFinduces E-selectin on endothelium. Thus chemoattractants mayindirectly increase selectin expression on endothelium, as well as directly activate integrin adhesiveness on leukocytes. A Three-Step Area Code for Signaling Neutrophil and MonocyteTraffic The above evidence indicates that emigration from the vasculature of neutrophils and monocytesis regulated by at least three distinct molecular signals (Figure 1, Figure 6A). A key feature is that selectin-carbohydrate, chemoattractant-receptor, and integrin-Ig family interactions act in sequence,not in parallel. This concept has been confirmedby the observation that inhibition of any one of these steps gives essentially complete,rather than partial, inhibition of neutrophil and monocyteemigration, Animportant consequence of a sequenceof steps, at any one of whichthere are choices of multiple receptors or ligands that have distinct distributions on leukocyte subpopulations or endothelium,is that it provides great combinatorialdiversity for regulating the selectivity of leukocyte localization in vivo, as has been emphasizedin several reviews (36, 143, 223, 240, 280). The term area code for modelsfor cell localization in the body (106, 239) is particularly apt becauseit is nowknown that at least three sequential steps are involved. The concepts of area codes and traffic signals can be combined by thinking of howtelephone traffic is routed by digital signals. Each type of leukocyte respondsto a particular set of area code signals. Inflammationalters the expression and location of the signals on vascular endothelium.It is as if leukocytes carry "cellular phones." An example of howthis model works is shownfor the two cell types for whichthe signals are best understood, neutrophils and monocytes(Figure 6B). Chemoattractants provide the greatest numberof molecularchoices (or "digits") and the greatest cellular specificity. Refinementsto the three-step modelare in order. First, selectins actually mediatetwo steps, initial tethering to the vessel wall and rolling (Figure 6A), which can be distinguished for E-selectin by dependenceon different classes of neutrophil ligands (145). Thus selectins can cooperate, and someselectin- ligand combinationsmay be moreimportant in tethering and others in rolling. Second,the steps are overlapping, rather than strictly sequential (Figure 6A). AlthoughL-selectin is shed from neutrophils soon after activation (132), the Annual Reviews www.annualreviews.org/aronline

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Figure 6 The three-step area code model. (A) Selectins, chemoattractants, and integrins act in sequence with some overlap. The sequence in which these signals act on neutrophils and lymphocytes may differ. (B) Combinatorial use of different molecules at each step can generate a large number of different area codes, and specificity for distinct leukocyte subpopulations. All of the known selectin and integrin interactions are shown in the hundreds and ones place, respectively: however, only a subset of the chemoattractants is shown in the tens place (see Table 1) due to space limitations. The area codes symbolize how specificity for monocytes, neutrophils, or both can be generated at inflammatory sites.

kinetics of shedding by neutrophils in whole blood (min) are much slower than the transition from rolling to integrin-mediated attachment (ms-s) (266). L- selectin is shed more slowly from lymphocytes than from neutrophils (121, 235). Furthermore, ligands for P-selectin (173) and E-selectin (145) remain on Annual Reviews www.annualreviews.org/aronline

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MCP-1IL-8 C5e PAF B ~lal’l Rsc Rec Rec Rec PSGL-1L-selectln L~wl: x "~ ~’~"~ ~ I~! /. Mac-1 LFA-1 VLA-4

100 200 300 I0 20 30 40 2 3 4

o O MCP-1 IL-8 CSa PAF

P-s~lectinCD34 E-~ele~’tin ICAM-1 ICAM-2 VCAM-1

MonocyteArea Codes111,211,311,112, 212, 312, 113, 213, 313, 114, 214, 314, 134,234, 334, 144, 244, 344 Neutrophil AreaCodes 121,221,321,122, 222, 322, 123, 223, 323 Monocyteand Neutrophil 131,231,331,132, 232, 332, 133, 233, 333, AresCodes 141,241,341,142,242, 342, 143, 243, 343 Null AreaCodes 124,224, 324

the neutrophilsurface after activation. Thusinteractions with selectins will continueafter activationof integrins, probablypersisting until transendothelial migrationis completed.Chemoattractants are requirednot only for activation of integrin adhesiveness,but also for directional cues duringthe subsequent step of transendothelialmigration. Finally, I]1 integrinsthat bindto extracellular matrix componentsare undoubtedlyrequired during migrationthrough the subendothelial basementmembrane. LymphocyteRecirculation: Distinct Traffic Patterns for Naive and Memory Lymphocytes Patrolling the bodyin searchof foreign antigen, lymphocytesfollow circuits throughnonlymphoid and lymphoidtisues (Figure 7). Theperipheral lymph nodesdraining skin and muscle,and the gut-associatedlymphoid tissues such Annual Reviews www.annualreviews.org/aronline

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Pos~-~ capillary venule

Figure7 Lymphocyterecirculation routes.

as Peyer’s patch, differ in the types of antigens to which lymphocytesare exposed. Whencollected from lymph draining gut or skin, lymphocytesfrom adult animals, but not newborns, show a twofold or higher preference to recirculate to the type of organ from whichthey cameand to reappear in the draining lymph (39, 40, 117, 159). This suggests that priming by specific antigen in a particular environmentmay induce expression of surface receptors that enable preferential recirculation to the type of secondary organ where specific antigen wasfirst encountered. Evidenceexists for separate streams of lymphocytesthat recirculate throughthe skin, gut, and lung and that drain into their associated lymphoidtissues (159, 196). Our understanding of the mechanismsof this selectivity has been advanced by the discovery that "naive" and "memory"lymphocytes prefer different Annual Reviews www.annualreviews.org/aronline

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recirculation pathways (162). Whennaive lymphocytes encounter antigen, those lymphocyteswith receptors specific for the antigen are stimulated to elonally expand and are converted to memorylymphocytes that have altered expression of adhesion receptors and circulatory patterns. Lymphocytesthat emigrate in the hind leg of a sheep through flat endotheliumin the skin and drain through the afferent lymphatics to the popliteal lymphnode are all of the memoryphenotype. By contrast, lymphocytesin the efferent lymph from the popliteal lymphnode, derived mostly from traffic through HEV,are pre- dominantlyof the naive phenotype. Thus, at least for peripheral tissues and lymph nodes, memorylymphocytes emigrate preferentially through tissue endothelium, whereas naive lymphocytes enter the lymph node through HEV (Figure 7). Memorylymphocytes are more sensitive to specific antigen than naive lymphocytesand thus better able to respond to antigen in peripheral tissues, whichhave fewer antigen-presenting cells than do lymphnodes (160). Traffic through HEV The high or cuboidal-shaped endothelial cells found in HEVare specialized for emigration of lymphocytesinto peripheral lymphnodes that drain skin and the lymphoidtissues of the mucosa:Peyer’s patches, tonsils, and appendix. Emigration into the spleen, by contrast, involves sinusoidal endothelia and molecular mechanismsthat are distinct and not yet characterized. About 25% of lymphocytesthat circulate through an HEVwill bind and emigrate, a much higher percent than through nonspecialized flat venules (30, 275). HEVphe- notype is developmentallyregulated. The carbohydrate ligands for L-selectin are absent from peripheral lymphnode HEVat birth but are displayed at adult levels by 6 weeks (196). If peripheral lymphnodes are deprived of afferent lymph, the HEVconvert from a high to a fiat-walled endothelial morphology, lose expression of L-selectin ligands, and lose the ability to support lymphocyte traffic (169, 170). Introduction of antigen into the node leads to a full resto- ration of HEVphenotype and function, Furthermore, intense antigenic stimulation can induce formation of HEVin diverse non-lymphoidtissues (161, 196). Molecular Mechanisms Defined by the HEV-Binding Assay Whenlymphocyte suspensions are overlayed on thin sections cut from frozen lymphnodes, the lymphocytesspecifically bind to the morphologicallydistinct HEV(241). Specific differences have been demonstrated between binding peripheral lymph node and Peyer’s patch HEV(37, 275). T lymphocytesbind one and one-half-fold better than B lymphocytes to peripheral lymph node HEVin vitro, and showa similar preference to recirculate to this site in vivo. B lymphocytesbind two- to threefold better to Peyer’s patch than to peripheral lymphnode HEVand showsimilar preference in recirculation in vivo. These Annual Reviews www.annualreviews.org/aronline

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preferences are reflected in the preponderanceof T cells in peripheral lymph nodes, and the preponderance of B lymphocytesin Peyer’ s patch, where they are important in secretion of IgA and IgMinto the mucosa(246). Certain lymphomacells possess marked preference for binding to Peyer’s patch or peripheral lymphnode HEVin vitro (37) and for metastasis in vivo to mucosal or peripheral lymphoid tissue, respectively (16). Assay of lymphomacell binding to HEVin the Stamper-Woodruffassay has led to the identification of two important adhesion pathways.

MOLECULESINVOLVED IN BINDINGTO PERIPHERALNODE HEV The L-selectin moleculewas initially defined in the mousewith the Mel-14mAb as a molecule on lymphocytesrequired for binding to peripheral lymphnode, but not Peyer’s patch, HEV(85). Conversely, the MECA-79carbohydrate antigen was defined with mAbthat bound specifically to peripheral lymph node HEVand blocked lymphocyte binding. The isolated MECA-79antigen, termed the peripheral node addressin (249), binds to L-selectin on lymphocytes(22). AnL-selectin/ IgG chimera was also found to specifically bind to HEVin peripheral lymph node and to block lymphocytebinding (273). The L-selectirdlgG chimera was used to isolate two distinct mucin-like ligands, GIyCAM-1,which is secreted by HEV(144), and CD34, a surface molecule on HEV(17). MECA-79 recognizes a carbohydrate determinant that is expressed on multiple protein species in HEV,including GiyCAM-Iand CD34and, comparedto L-selectin, recognizesan overlappingbut distinct set ofglycoproteins(22, 144). Sialylation and sulfation of the O-linked side chains of the GlyCAM-1and CD34molecules are required for activity in binding to L-selectin (22, 111,206). HEVdiffer from other tissues in carbohydrate processing; GlyCAM-1and CD34expressed in transfectants, and CD34expressed in other vascular endothelia, do not bind L-selectin chimera under conditions in which binding to HEVis detectable (144). However,an L-selectin ligand with a presumably lower affinity certainly present on most endothelia, as shownby L-selectin-dependentrolling in vivo and binding in vitro (125, 153, 229, 236, 237, 265, 266).

MOLECULESINVOLVED IN BINDINGTO PEYER’S PATCH HEV Elegant screens for mAbwith specificity for Peyer’s patch HEV,and ability to block lymphocyte binding to HEV,yielded mAbMECA-367 to the mucosal addressin now termed MAdCAM-1(248). MAdCAM-1is expressed on endothelia in mucosal tissues, not only on HEVin Peyer’s patch, but also on venules in intestinal lamina propria and in the lactating mammarygland (213, 248). MAdCAM-1 has both IgSF domains and a mucin-like domain(34) (Figure Similar elegant screens for mAbswith specificity for lymphomacells that bound to Peyer’s patch HEVand with the ability to block binding to HEVin the Stamper-Woodruffassay yielded mAbsto the ct4 subunit of the Peyer’s Annual Reviews www.annualreviews.org/aronline

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patch homingreceptor (104). The 4 subunit was fo und tobe associated with a novel 13 subunit, 13p (105), whichis identical to 137 (108). Theintegdn ~x4~7, but not 0:413~binds to Peyer’s patch HEV(108), and o:"137 binds directly MAdCAM-1(24),

An Area Code Model for Lymphocyte Migration Through HEV

PERIPHERALLYMPH NODE HEV Although the L-selectin:mucin and 0t4137: MAdCAM-1interactions were identified in parallel assays, recent studies suggest that multiple steps are involved in lymphocyteinteraction with HEV and raise the possibility that these interactions function in distinct, rather than parallel, steps in this process. Soonafter its discovery as a peripheral lymph node homingreceptor, L-selectin also was found to be present on neutrophils and eosinophilsand to be importantin emigrationof, at least, neutrophils (151). As expected from their strong expression of L-selectin, neutrophils and other leukocytes can bind avidly to HEVin the Stamper-Woodruffassay, yet do not normally hometo peripheral lymph nodes in vivo. Injection of Escherichia coil supernatant induces acute emigration of neutrophils through HEVof the draining lymph node. Thus signals other than those mediated by L-selectin can regulate the class of leukocyte that homeinto a lymphnode (151). Al- though peripheral node HEVis far richer than any other site in the body in expressionof the carbohydratereceptor for L-selectin (112), this is insufficient to explain the specificity of lymphocytehoming to this organ. The findings suggest that L-selectin is required for lymphocyteemigration through peripheral lymph node HEVand mayhelp regulate recirculation of the L-selectin- positive subset of lymphocytes;however, L-selectin is insufficient to determine the specificity of the cell types that emigrate, and other currently undefined moleculesare required to achieve specificity. In vivo studies strongly suggest that lymphocyteemigration through HEV is a multi-step process that utilizes area code modelssimilar to those of other leukocytes, mAbto L-selectin almost completely blocks emigration of lymphocytes from blood into peripheral lymph nodes (85, 94). However,mAb the integrin LFA-1also markedly reduces or almost completely abolishes lymphocytemigration into peripheral lymphnodes (41, 95). Thus molecules of step 1 and 3 (see Figure 1) are required for homingto peripheral lymph nodes in vivo. LFA-1on blood lymphocytesrequires activation for binding to its counter-structures ICAM-1and ICAM-2(238), which are expressed HEV(69, 76). Binding of L-selectin does not trigger activation of LFA-1 because lymphocytesattach and roll in flow on purified peripheral node addressin identically whetheror not purified ICAM-1is present on the substrate; an additional stimulus is required before lymphocyteswill arrest and strengthen adhesion through LFA-1(M Lawrence et al, in preparation). Annual Reviews www.annualreviews.org/aronline

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Tethering Activation Firm Adheelon

Peripheral Node HEV

Sldn

Novel P~thway

Figure 8 The three- or four-step area code paradigmfor lymphocytes, For skin and gut, pathwaysshown may mediate ~circulation and increased accumulationin inflammation. The novel pathway shown at the bottom may b~ important when VCAM-Iexpression on endoth~lium is induced by cytokines and maycooperale with the other illustrated pathways.For each organ, the interacting moleculesare shownon the lop for lymphocylcsand on the bottom for endothelia. text for suppo~for the molecularassignments at each step. based primaf|y on in vivo dala, Annual Reviews www.annualreviews.org/aronline

ENDOTHELIALSIGNALING 853

G protein-coupled receptors are required for lymphocyterecirculation and likely provide the signals required to activate the adhesiveness of LFA-1. Pertussis toxin causes lymphocytosis and profoundly depresses lymphocyte recirculation (271). Murinelymphocytes treated with pertussis toxin in vitro and reinfused fail to emigrate into either peripheral lymph nodes or Peyer’s patches (175). This suggests that G protein-coupled receptors of the oq class are required for lymphocyteemigration through HEV.Results with mice with a transgene for the ADP-ribosylating subunit of, pertussis toxin selectively expressed in the T lineage suggest that Goqproteins are not only required for emigration from the bloodstream, but also for emigration from the thymus (48, 49). Despite lack of emigration, pertussis toxin-treated lymphocytes bind normally to lymph node HEVin vitro. These findings provided the basis for an early proposal for a two-step model, in which G protein-coupled receptors function subsequent to binding of lymphocytesto HEV(233). Thus emigration of lymphocytes through peripheral node HEVrequires three sequential area code signals that are analogous to those involved in neutrophil emigration from the bloodstream (Figure 8). Identification of putative lymphocytechemoattractant secreted by peripheral lymphnode HEV, and a chemoattractantreceptor that is predicted to be selectively expressedon the naive subset of lymphocytesthat recirculate through peripheral node HEV, will be a subject of intense research interest in comingyears.

PEYER’S PATCHmAb to L-selectin block 50%of lymphocyte emigration from blood to Peyer’s patch and to the remainderof the intestine (93, 94). This consistent with the lower level of L-selectin ligand in Peyer’s patch HEVthan in peripheral lymph node HEV05, 163, 273). mAbto certain epitopes on the integrin od and ~7 subunits inhibit recirculation by approximately 50%of lymphocytesto Peyer’s patch and intestine, but have no effect on recirculation to peripheral lymphnode; furthermore, mAbspecific for the ct4~7 complexare equally as effective as mAbto a4 (93). Moreover,recirculation is inhibited mAbto MAdCAM-1(248), implicating ~x4~7 binding to MAdCAM-1in recirculation to mucosaltissue, mAbto LFA-1block recirculation to Peyer’s patch by 50 to 80%, but have no effect on recirculation to the remainder of the intestine (41, 95). Thus both LFA-1and ~4[~7 contribute to emigration into mucosal lymphoid tissue. Gprotein-coupled receptors act subsequentto a rolling interaction in Peyer’s patch HEV.In contrast with peripheral lymphnodes, Peyer’s patches maybe visualized by intrayital microscopy(30). Normally, lymphocytesroll along Peyer’s patch HEVfor only a few seconds, then arrest and emigrate. However, prior treatment of lymphocyteswith pertussis toxin completelyblocks arrest and emigration, and prolongs the rolling indefinitely, so that the lymphocytespass Annual Reviews www.annualreviews.org/aronline

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out of the Peyer’s patch rather than emigrate (14). It remains to be established, but seems likely, that a chemoattractant presented or secreted by Peyer’s patch binds to a G0q-coupled receptor on lymphocytes and activates LFA-1 and tx4~7 to mediate arrest and emigration (Figure 8). Lymphomacells or lymph node lymphocytes can bind to Peyer’s patch HEVor purified MAdCAM-1without any apparent need for activation; however, activation increases the strength of binding to MAdCAM-1(24, 108). The pertussis toxin studies suggest that activation of blood lymphocytes is required for the last step of arrest and emigration (14, 233). Truncation of the cytoplasmic domain of ~7 greatly decreases binding to HEV. Thus interactions with the cytoplasmic domain can regulate the avidity of ct4~ for MAdCAM-1(63), similar to regulation of the avidity of LFA-1 for ICAM-1 ( 102, 103 ) by the [~2 integrin subunit cytoplasmic domain.

Recirculation of MemoryLymphocytes

DISTINCT PATHWAYSTHROUGH SKIN ANDGUT Memory lymphocytes are im- printed so that they are more likely to return to the type of tissue, such as skin or mucosa, where they first encountered antigen (39, 40, 159). The surface phenotypes of gut and skin-homing memorycells are distinct (163). Further- more, staining of lymphocytes in sections of skin and gut with mAbshows distinctive expression of adhesion molecules that may contribute to selective extravasation in these tissues, or to subsequent localization within these tissues in specific anatomic compartments (Table 4).

Tablea 4 Naive and memorylymphocyte subsets

Molecule Naive lymphocytes Memorylymphocytes CD45RO Negative Positive CD45RA High Low CD2 Low High LFA-3 Negative Positive L-selectin Positive Positiveand negativesubsets a4 Low High Memorylymphocytes subsets Gut associated Skin associated CLA Negative Positive a~/37 (HML-1) Positive Negative O/4j~b 7 High Low a4/~ Low High a6 Low High aReferences:(107, 163,219), bbut see (129). Annual Reviews www.annualreviews.org/aronline

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SKINHOMING LYMPHOCYTES Lymphocytes that extravasate in the skin and appear in afferent lymph have a distinct pattern of expression of adhesion molecules (163) (Table 4). Furthermore, as shownby staining of tissue tions, T lymphocyteslocalized in the skin, but not in the gut, express a carbohydrate termed cutaneous lymphocyte-associated antigen (CLA)(198). CLAis closely related to sialyl Lewisa and x (21) and is a ligand for E-selectin (23). Binding of a subpopulation of memorylymphocytes that bears CLA E-selectin maycontribute to the tropism of this subset to the skin (89, 197, 224). E-selectin is induced on dermal endothelial cells in delayed type hypersensitivity (61) and in chronically inflamed skin (197). ClonedT cells derived from challenged skin express high levels of CLAand bind to E-selectin, whereas T cell clones derived from blood lymphocytesdo not (5). Both types of clones bind to P-selectin.

GUTHOMING LYMPHOCYTES The most organized lymphoid structures in the wall of the gut are the Peyer’s patches, They underlie follicle-associated epithelia that contain Mcells, which are specialized for uptake of antigen from the gut lumen. Other lymphocyteslocalize more diffusely in the lamina propria underlyingthe digestive epithelium and in the epithelial layer. Studies on gut afferent lymphreveal the presence of memoryand naive lymphocytes (163); whether there is differential migration of naive and memorylymphocytes through Peyer’s patch HEVand lamina propda postcapillary venules, both of which contribute to gut afferent lymph(Figure 7), remains unclear. Gut-homingmemory lymphocytes display a surface phenotype distinct from skin-homing lymphocytes (Table 4). Wheninjected into the bloodstream, memorylymphocytes from gut afferent lymphdisplay a strong preference to return to gut afferent lymph, whereas naive lymphocytesredistribute ran- domly (163). Gut afferent memorylymphocytes display an 4 high, I~ l integrin low phenotype, suggesting they are t~4~÷ (163) in commonwith subpopulation of memorylymphocytes in blood (219). Expression of MAd- CAM-1on Peyer’s patch HEVand postcapillary venules in lamina propria (248), and 50%inhibition by mAbto ~4 and 1~7 of migration into Peyer’s patches and intestine (93), suggest a role for u41~7 interaction with MAdCAM- 1 in both sites. A subpopulation of gut lymphocytesdistinct from those in lamina propria localize within the epithelium on the external surface of the basementmembrane and express the humanmucosal lymphocyte(HML-1) integrin a~l~7 (47, 128, 193). Theo~ ~ integrin subunit contains an I domainand a novel proteolytic cleavage site precededby a stretch of acidic residues, just N-terminalto the I domain(222). Bindingof intraepithelial lymphocytes(IEL) to epithelial monolayersin vitro is inhibited by mAbto t~E, which suggests that o~ may help mediate localization of IEL in epithelia in vivo (46). Intraepithelial Annual Reviews www.annualreviews.org/aronline

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lymphocytesmay undergo thymus-independent differentiation in situ, and their recirculation pattern is undefined. HML-1is expressed on a subpopulation (2-6%) of blood T cells, which are in the memorysubset and are CLA-and L-selectin- (199). Transforminggrowth factor 13 (TGF-13)together with mitogen induces expression of HML-1 on peripheral T cells and increases expression on IEL (128, 193). TGF-13also induces switching of B lymphocytes production of the IgA class of immunoglobulin(55), the predominant class secreted in the mucosa.These dual effects on differentiation of mucosa]lymphocytes suggest the possibility that TGF-I~may be an environment-specific cytokine that imprints lymphocytes,when first exposedto antigen, to recirculate selectively to the gut. Alteration of Lymphocyte Trafficking in Inflammation Antigen injected into the tissue of sensitized individuals induces localized accumulation of lymphocytes. These lymphocytes, and those accumulating in tissues in autoimmunedisease, are almost all memorycells (120, 202). The phenotypeof these cells is quite similar to that of lymphocytestrafficking through these sites under basal conditions. This suggests that the signals for lymphocytestrafficking maybe qualitatively the same in the basal and inflammatory states and that they are upregulated in inflammation. Accumulationof lymphocytesinduced by specific antigen, or by injection of IFN-~/or TNF-~x, is4 significantly inhibited by mAbto either the LFA-10tor the integrin ot subunit (53, 114, 115, 217, 278). A combination of mAbto LFA-1and 4 gives almost complete inhibition of lymphocyteemigration and the resulting induration and plasma leakage (116). mAbto E-selectin and VCAM-1also inhibit lymphocyteaccumulation in delayed type hypersensitivity in skin (225). Multiple signals are thus required for augmentedtrafficking of lymphoeytes into skin in inflammation(Figure 8). Both antigen responsive and nonresponsive lymphocytestraffic into sites of antigenic stimulation (166). Antigen-specific lymphocytesmay accumulate in the site because stimulation through their antigen receptors increases adhesiveness of integrins and causes them to be retained, whereas nonresponsive lymphocytesmore rapidly enter the lymphatics and leave the site. The interaction between VCAM-Iand VLA-4mediates rolling and finn adhesion(R Alonet al, J. Cell, Biol., in press), thus it doesnot fit neatly into the three-step paradigm established for neutrophils, mAbto LFA-1or VLA-4 alone do not completely inhibit lymphocyte accumulation in inflammation, and patients with LAD-Ishow delayed-type hypersensitivity reactions. This suggests that the functions of VLA-4and LFA-1are partially overlapping in the step of firm adhesion, but they mayalso act in series, as in VLA-4-mediated rolling followed by LFA-l-mediated firm adhesion. VLA-4may act together Annual Reviews www.annualreviews.org/aronline

ENDOTHELIALSIGNALING 857

with selectins to augmentT lymphocytetethering and rolling in the vasculature. All or most memoryT lymphocyteslack L-selectin (32, 126, 163, 257). The CLA+ subset can bind E-selectin, and T lymphocytescan also bind P-selectin (66, 174). Peripheral blood T lymphocytesare substantially less efficient than neutrophils in tethering in hydrodynamicflow to E-selectin and P-selectin (T Diacovo et al, unpublished data); therefore, cooperation of VCAM-Iwith E-selectin or P-selectin, or amongall three molecules, maybe important in enhancing lymphocyte accumulation in inflammation. Inflammationalso affects traffic through HEV.Antigen injected into tissue drains to the regional lymphnode and greatly increases blood flow to the node and traffic of naive lymphocytes through HEV(161). Furthermore, memory lymphocytesnow appear to enter the node directly; this is associated with induction of VCAM-Ion non-HEVvascular endothelia within the node (161). Entry is inhibited by mAbto a4, which suggests a role for interaction of VCAM-Iwith ~4~1 (114, 161). Lymphocytechemoattractants are interesting candidates for the step 2 signal (see Figure 1) for lymphocyteaccumulation at inflammatorysites. Pertussis toxin treatment inhibits lymphocyteemigration in response to antigen in delayed-type hypersensitivity (234). Identification of lymphocytechemoattractants has been hamperedby the low motility of lymphocytes compared to monocytesor neutrophils (194), and by the low signal-to-backgroundratio, typically less than 2, in most chemotaxisassays. Recent interest has focused on chemokines(Table 2). A numberof chemokines,all of which were isolated based on chemoattractive activity for neutrophils or monocytes,or by cloning genes of unknownfunction, have subsequently been tested and found to be chemoattractive for lymphocytesubpopulations (12, 172). These include IL-8 (139) (but see 136, 150), RANTES(216), MIP-1B(253), MIP-la and 255), and IP-10 (256). Thereare differences amongreports in the subsets found to be chemoattracted, and somereports use lymphocytespreactivated by T cell receptor cross-linking, which maybe relevant to migration within inflammatory sites, but not to emigration from blood. Of interest, MIP-11~can induce binding of the naive, CD8÷ subset to VCAM-1,either in solution or when immobilized on a substrate, mimickingpresentation by an endothelial cell surface (253; S Shaw,personal communication);the specific effect is modest, equal to backgroundbinding. The RANTEScytokine, by contrast to MIP-1I~, selectively attracts the memoryT lymphocytesubset (216). Vascular endothelium maypresent chemoattractant to lymphocytes in a functionally relevant way, as well as provide a permeability barrier that stabilizes the chemoattractant gradient. A transendothelial chemotaxisassay more accurately simulates lymphocyteemigration from the bloodstream than filter chemotaxisassays and yields signals >10 times background(43). Since Annual Reviews www.annualreviews.org/aronline

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lymphocytes, responding to specific antigen in tissue, signal emigration of further lymphocytesinto the site, a chemoattractant was sought in material secreted by mitogen-stimulated mononuclearcells. Purification to homoge- neity guided by the transendothelial lymphocyteehemotaxis assay revealed that MCP-1,previously thought to be solely a monocytechemoattractant, is a major lymphocyte chemoattractant (43). Subsequent studies using the transendothelial chemotaxis assay have confirmed that lymphocytesrespond to RANTESand MIP-10~ (CC chemokines), but do not respond to IL-8 IP-10 (CXC chemokines) (209). MCP-1, RANTES,and MIP-lct selectively attract the memoryT lymphocyte subset and the CD4and CD8subsets. All also attract monocytes, but not neutrophils, with MCP-1being more potent than RANTESor MIP-lct as a monocyte chemoattractant. The physiologi- cally relevant transendothelial assay suggests that CCchemokines tend to attract both monocytesand lymphocytes, in agreement with the long-standing clinical observation that lymphocyteemigration into inflammatory sites is always accompaniedby emigration of monocytes. The converse is not true. Monocytessometimes emigrate in the absence of lymphocytes, correlating with activity of ehemoattractants such as C5a and PAFon monocytes, but not on lymphocytes. Teleologically, it is important that monocytesaccom- pany lymphocytesinto inflammatory sites in order to present antigen and to carry out effector functions in which monocytesare activated by T lymphocytes. MCP-1is abundantly expressed at sites of antigen challenge and autoimmune disease (149, 172, 263), and together with MIP-1 t~ and RANTES,is an excellent candidate to provide the step 2 signal required to activate integrin adhesiveness and emigration of both monocytesand lymphocytes in vivo (Figure 8). The finding that resting T lymphocytesthat tether and roll on VCAM-Ican spontaneously arrest and develop firm adhesion on VCAM-1(R Alon et al, J. Cell Biol., in press) has provocative implications for the multi-step model.It suggests that the VLA-4:VCAM-Iinteraction not only can mediate the steps of rolling and firm adhesion, but mayalso short-circuit the step of stimulation by chemoattractants of firm adhesion through integrins. This is intriguing; although a twofold stimulation of adhesiveness of VLA-4to VCAM-1has been demonstrated by MIP-1I~in one system (253), with the chemoattractant that is mosteffective in eliciting transendothelial chemotaxisofT lymphocytes, MCP-1,it is difficult to detect stimulation of integrin adhesiveness on lymphocytes (M Carr & T Springer, unpublished data). Therefore, an alternative pathway mayexist in which VCAM-Ican mediate both tethering and arrest of lymphocytes,perhaps in cooperation with other endothelial molecules, prior to stimulation by chemoattractants. After arrest, chemoattractants wouldguide transendothelial migration, and perhaps stimulate further increases in the ad- Annual Reviews www.annualreviews.org/aronline

ENDOTHELIALSIGNALING 859

hesiveness of the integrins VLA-4and LFA-1,which are important in migration across the endothelium and basement membrane.

CONCLUDING REMARKS

A three-step or area code modelof leukocyte emigration from the bloodstream, established and validated in vitro and in vivo with neutrophils (Figure 1 and 6B), appears extendible with only slight modification to all subclasses of leukocytes including lymphocytes(Figure 8). Multiple adhesion and chemoattractant receptors are used combinatorially in a series of steps that enable leukocytes to progress from initial tethering in flow to firm adhesion and emigration. The distinct distribution of receptors on leukocyte subsets for signals that are displayed on endotheliumregulates selection of the subclasses of leukocytes that emigrate at inflammatorysites and the distinctive recirculation behavior of lymphocytesubsets. Manyimportant developmentsawait. Strong evidence suggests that G protein-coupled receptors are required for lymphocyterecirculation, but manyof the putative lymphocytechemoattractants specific to HEV,mucosa, and skin, and the receptors for these chemoattractants on lymphocytes, remain to be identified. Specific mucin-like moleculesthat present carbohydrateligands to selectins have recently been identified. Are there similar mucin-likemolecules on lymphocytesthat present carbohydrates to P-selectin or E-selectin, and do these differ from the PSGL-Imolecule on neutrophils? It is likely that endothelial cells express molecules that retain chemoattraetants on the luminal surface, preventing them from being washed awayby blood flow, as already suggested for MIP-1~and IL-8. Are these molecules specifically regulated? The mucin-like ligands of selectins have manyfeatures such as extended structure, sulfation, and negative charge in commonwith proteoglycans, and thus might have a second function of binding chemokinesthrough their heparin-binding sites and presenting them to leukocytes. Presenting molecules might be required not only to prevent chemoattractants from being washed awayby blood flow, but also to generate maximalchemoattractant activity, analogous to proteoglycans that must bind fibroblast growth factor to enable signaling through a second receptor molecule. It will be interesting to determine whether chemoattractant receptors on leukocytes couple to distinct G proteins and signaling effeetors, allowing for selectivity in whichintegrins are upregulated in avidity. For example, do chemoattractants differ in ability to upregulate adhesiveness of two integrins such as LFA-1and VLA-4expressed on the samecell? Finally, after the area code is dialed and cells emigrate across the endothelium, muchremains to be learned about the "7 digit code" that Annual Reviews www.annualreviews.org/aronline

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regulates leukocytemigration and localization withinspecific anatomiccom- partments.

ACKNOWLEDGMENTS I thankthe NationalInstitutes of Healthfor supportingmost of the cited work and Uli yonAndrian for commentson the manuscript.

AnyAnnual Review chapter, as well as any article cited in an AnnualReview chapter, maybe purchased from the Annual ReviewsPreprints and Reprints service. 1-800-347-8007; 415-259-5017; emaih [email protected]

LiteratureCited

1. Abbassi O, Kishimoto TK, Mclntire LV, tastasis-inducing splice variant of CD44. Anderson DC, Smith CW. 1993. E- Science 257:682-85 seleetin supporls neutrophil rolling in 10. Arfors KE, Lundberg C, Lindbom L, vitro under conditions of flow. J. Clin. Lundberg K, Beatty PG, Hadan JM. Invest. 92:2719--30 1987. A monoclonal antibody to the 2. AlamR, Forsythe PA, Stafford S, Lett- membrane glycoprotein complex CD18 Brown MA, Grant JA. 1992. Macro- inhibits polymorphonuclear leukocyte phage inflammatory protein-let accumulation and plasma leakage in activates basophils and mast ceils. J. vivo. Blood 69:338--40 Exp. Med 176:781-86 11, Aruffo A, Stamenkovic I, Melnick M, 3. Albelda SM, Muller WA, Buck CA, Underhill CB, Seed B. 1990. CD44is NewmanPJ. 1991. Molecular and cel- the principal cell surface receptor for lular properties of PECAM-I(endo- hyaluronate. Cell 61:1303-13 CAM/CD31):a novel vascular cell-cell 12. Baggiolini M, Dewald B, Moser B. adhesion molecule. J. Cell Biol. 114: 1994. Interleukin-8 and related chemo- 1059-68 tactic cytokines-CXC and CC ¢hemo- 4. Albelda SM, Oliver PD, Romer LH, kines. Adv. Immunol. 55:97-179 Buck CA. 1990. EndoCAM: a novel 13. Barclay AN, Birkeland ML, Brown MH, endothelial cell-cell adhesion molecule. Beyers AD, Davis SJ, et al. 1993. The J. Cell Biol. 110:1227-37 Leucocyte Antigen Facts Book. London: 5. Alon R, Rossiter H, Springer TA, Academic Kupper TS. 1994. Distinct cell surface 14. Bargatze RF, Butcher EC. 1993. Rapid ligands mediate T lymphocyte attach- G protein-regulated activation event in- ment and rolling on P- and E-selectin volved in lymphocyte binding to high under physiological flow. J. Cell Biol. endothelial venules. J. Exp. Med. 178: In press 367-72 6. Amatruda TT, Gerard NP, Gerard C, 15. Bargatze RF, Streeter PR, Butcher EC. Simon MI. 1993. Specific interactions 1990. Expression of low levels of pe- of chemoattractantfactor receptors with ripheral lymphnode-associated vascular G-proteins. J. Biol. Chem. 268:10139- addressin in mucosai lymphoid tissues: possible relevance to the dissemination 7. Anderson AO, Anderson ND. 1976. of passaged akr iymphomas. J. Cell. Lymphocyte emigration from high en- Biochem. 42:219-27 dothelial venules in rat lymph nodes. 16. Bargatze RF, Wu NW, Weissman IL, Immunology 31:731--48 Butcher EC. 1987. High endothelial ven- 8. Anderson DC, Springer TA. 1987. Leu- ule binding as a predictor of the dissem- kocyte adhesion deficiency: An inher- ination of passaged murine lymphomas. ited defect in the Mac-l, LFA-1, and J. Exp. Med. 166:1125-31 p150,95 glycoproteins. Annu. Rev. Med. 17. Baumhueter S, Singer MS, Henzel W, 38:175-94 Hemmerich S, Renz M, et al, 1993, 9. Arch R, Wirth K, Hofmann M, Ponta Binding of L-selectin to the vascular H, MatzkuS, et al. 1992. Participation sialomucin, CD34.Science 262:436-38 in normal immuneresponses of a me- 18. Beals CR, Wilson CB, Pedmutter RaM. Annual Reviews www.annualreviews.org/aronline

ENDOTHELIAL SIGNALING 861

1987. A small multigene family encodes thelial interactions in vivo. Science 231: Gi signal-transduction proteins. Proc. 402-5 Natl. Acad. Sci. USA 84:7886-90 31. Bochner BS, Luseinskas FW, Gimbrone 19. Berg EL, Magnani J, WarnockRA, Rob- MAJr, NewmanW, Sterbinsky SA, et inson MK, Butcher EC. 1992. Compar- al. 1991. Adhesion of humanbasophils, ison of L-selectin and E-selectin ligand eosinophils, and neutrophils to interleu- specificities: TheL-selectin can bind the kin 1-activated humanvascular endothe- E-selectin ligands sialyl Lex and sialyl lial cells: contributions of endothelial Lea. Biochem. Biophys. Res. Commun. cell adhesion molecules. J. Exp. bled. 184:1048-55 173:1553-56 20. Berg EL, McEvoy LM, Berlin C, 32. Bradley LM, Atkins GG, Swain SL. Bargatze RF, Butcher EC. 1993. L- 1992. Long-term CD4+memory T cells selectin-mediated lymphocyterolling on from the spleen lack MEL-14,the lymph MAdCAM-1.Nature 366:695-98 node homingreceptor. J. lmmunol. 148: 21. Berg EL, Robinson MK, Mansson O, 324-31 Butcher EC, MagnaniJL. 1991. A car- 33. Briscoc DM,Cotran RS, Pober JS. 1992. bohydrate domain commonto both sialyl Effects of minor necrosis factor, lipo- Lea and sialyl Lex is recognized by the polysaccharide, and IL-4 on the expres- endothelial cell leukocyte adhesion sion of vascular cell adhesion mole- modecule ELAM-I../. Biol. Chem. 266: cule-1 in vivo. J. lmmunol. 149:2954-60 14869-72 34. Briskin MJ, McEvoyLM, Butcher EC. 22. Berg EL, Robinson MK, Warnock RA, 1993. MAdCAM-1has homology to im- Butcher EC. 1991. The human periph- munoglobulin and mucin-like adhesion eral lymphnode vascular addressin is a receptors and to lgA1. Nature 363:461- ligand for LECAM-I,the peripheral lymph node homing receptor, ,L Cell 35. Buchanan MR, Crowley CA, Rosin RE, Biol. 114:343-49 Gimbrone MA,Babior BM. 1982. Stud- 23. Berg EL, Yoshino T, Rott LS, Robinson ies on the interaction between GP-180- MK, Warnock RA, et al. 1991. The deficient neutrophils and vascular endo- cutaneous lymphocyteantigen is a skin thelium. Blood 60:160-65 lymphocytehoming receptor for the vas- 36. Butcher EC. 1991. Leukocyte-endothe- cular lectin endothelial cell-leukocyte lial cell recognition: Three (or more) adhesion molecule 1. J. Exp. Med. 174: steps to specificity and diversity. Cell 1461-66 67:1033-36 24. Berlin C, Berg EL, Briskin MJ, Andrew 37. Butcher EC, Scollay RG, WeissmanIL. DP, Kilshaw PJ0 ctal. 1993. ~41~7 ino 1980. Organ specificity of lymphocyte tegrin mediates lymphocyte binding to migration: Mediationby highly selective the mucosal vascular addressin MAd- lymphocyteinteraction with organ-spe- CAM-1.Cell 74:185-95 cific determinants on high endothelial 25. Bevilacqua MP. 1993. Endothelial-leu- venules. Eur. J. lmmunol. 10:556-61 kocyte adhesion molecules. Annu. Rev. 38. Buyon JP, Abramson SB, Philips MR, lmmunol. 11:767-804 Slade SG, Ross GD,et al. 1988. Disso- 26. Bevilacqua MP, Nelson RM. 1993. ciation between increased surface ex- Selectins. J. Clin. Invest. 91:379-87 pression of Gp165/95 and homotypie 27. Bevilacqua MP, Pober JS, Mendrick neutrophil aggregation. J. Immunol.140: DL, Cotran RS, Gimbronc MA. 1987. 3156-60 Identification of an inducible endo- 39. Cahill RNP,Poskitt DC, Frost H, Trnka thelial-leukocyte adhesion molecule, Z. 1977. Twodistinct pools of reeireu- ELAM-1.Proc. Natl. Acad. Sci. USA lating T lymphocytes: Migratory char- 84:9238--42 acteristies of nodal and intestinal T 28. Bienvenu K, Granger DN. 1993. Mo- lymphocytes. J. Exp. Med. 145:420--28 lecular determinants of shear rate-de- 40. Cahill RNP,Poskitt DC, Hay JB, Heron pendent leukocyte adhesion in post- I, Trnka Z. 1979. The migration of lym- capillary venules. Am. J. Physiol. 26,1: phocytes in the fetal lamb. Eur. J. lm- H1504-8 munol. 9:251-53 29. BisehoffSC, Krieger M, Brunner T, Rot 41. CampRL, Scheynius A, Johansson C, A, Tschamer VV, et al. 1993. RANTES Pur6 E. 1993. CD44is necessary for and related ehemokinesactivate human optimal contact allergic responsesbut is basophil granulocytes through different not required for normal leukocyte ex- munol.G protein-coupled 23:761-67 receptors. Eur. Z lm- travasation. J. Exp. Med. 178:497-507 42. Carlos TM, Harlan JM. 1994. Leu- 30. Bjerknes M, Cheng H, Ottaway CA. kocyte-endothelial adhesion molecules. 1986. Dynamics of lymphocyte-endo- Blood. 84:2068-2101 Annual Reviews www.annualreviews.org/aronline

862 SPRIHGER

43. Carr MW,Roth SJ, Luther E, RoseSS, 55. CoffmanRL, LehmanDA, Shrader IR. Springer TA. 1994. Monocytechemo- 1999.Transforming growth factor [~ spe- attractant protein-1 is a majorT lym- cifically enhances IgA production by Acad.phocyte Sci.chemoattractant. USA91:3652-56 Proc. Natl. lipopolysaccharide-stimula~odmurine B lymphocytes.J. Exp. Med.170:1039-44 44. Carrell NA,Fitzgerald LA,Steiner B, 56. CohnbeimJ. 1889. Lectures on General EricksonHE Phillips DR.1985. Struc- Pathology:A Handbookfor Practition- ture of humanplatelet membranegly- ers and Students. London: The New coproteinslib and Ilia as determinedby SydenhamSoc. electronmicroscopy, J. Biol. Chem,260: 57. Colditz IG. 1991.Dcsensitisation mech- 1743-49 anismsregulating plasmaleakage and 45. Carter SB. 1967. Haptotaxis and ~he neutrophll emigration.In VascularEn- mechanismof ceil motility. Nature213: dothellum:Interactions with Circulating 256-6O Cells, ed. JL Gordon,pp. 175-87. New 46. Cepek KL, Parker CM,Madara JL, York:Elsevier Brenner MB.1993. Integrin ~Ei87me- 58. ColditzIG. 1992.Sites of antigenicstim- diates adhesion of T lymphocytesto ulation: Roleof cytokines and chemo- epithelial cells. J. lmmunol.150:3459- tactic agonistsin acute inflammation.In 70 AnimalHealth and Production in the 47. Ccrf-BensussanN, Jarry A, Brousse 21st Century, ed. KJ Beh. Melbourne: Lisowska-GrospierreB, Guy-GrandD, CSIRO Griscelli C. 1987.A monoclonalanti- 59. Colditz IG, MovatHZ. 1984. Desensi- body (HML-I)defining a novel mem- tization of acute inflammatorylesions brane molecule present on humanin- to ehemotaxinsand endotoxin. J. Im- testinal lymphocytes.Eur. J. immunol. munol. 133:2163-68 17:1279-85 60. Colditz IG, WatsonDL. 1992. The effect 48. Chaffin KE, Beals CR, Wilkie TM, of cytokines and chemotacticagonists ForbushKA, Simon MI, Perlmutter RM. on the migrationof T lymphocytesinto 1990.Dissection of thymocytesignaling skin. Immunology76:272-78 pathwaysby in vivo expressionof per- 61. Cotran RS, GimbroneMA Jr, Bevil- tussis toxin ADP-ribosyltransferase. aequa MP, Mendrick DL, Pober JS. EMBOJ. 9:3821-29 1986.Induction and detection of a hu- 49. ChainKE, Perlmutter RM.1991. A per- manendothelial activation antigen in tussis toxin-sensitiveprocess controls vivo. J. Exp. Med.164:661--66 thymocyteemigration. £ur. J. lmmunol. 62. Cross AH,Raine CS.1992. Central ner- 21:2565-73 vous systemendothelial ceil-polymor- 50. Chart BMC,Elices MJ, Murphy E, phonuclear cell interactions during HemlerME. 1992. Adhesionto vascular autoimmune demyelination. Am. J. cell adhesion molecule 1 and fibro- Pathol. 139:1401-9 nectin: comparisonof (~4~1 (VLA-4) 63. CroweDT, Chiu H, Fong S, Weissman and o~4~7on the humanB c¢11line IL. 1994.Regulation of the avidity of J. Biol. Chem.267:8366-70 integrin a41~7by the 1~7 cytoplasmic 51. Chart P-Y, LawrenceMB, Dustin ML, domain.J. Cell Biol. 269:14411-18 FergusonLM, Golan DE,Springer TA. 64. Culty M, Miyake K, Kineade PW, 1991.Influence of receptor lateral mo- Sikorski E, Butcher EC, Underhill C. bility on adhesion strengthening be- 1990. The hyaluronate receptor is a tween membranescontaining LFA-3and memberof the CD44(H-CAM) family CD2.J. Cell Biol. 115:245-55 of cell surface glycoproteins.J. Cell 52. CharoIF, MyersS J, HermanA, Franci Biol. 111:2765-74 C, ConnollyAJ, Coughlin SR. 1994. 65. Cyster JG, Shotton DM,Williams AF. Molecularcloning and functional 1991. The dimensionsof the T lympho- press/on of twomonocyte chemoattract- cyte glycoproteinleukosialin and iden- ant protein 1 receptors reveals alter- tification of linear proteinepitopes that native splicing of the carboxyl-terminal can be modified by glyeosylation. tails. Proc. Natl. Acad. Sci. USA91: EMBOJ. 10:893-902 2752-56 66. DamleNK, KlussmanK, Dietsch MT, 53. ChisholmPL, Williams CA, Lobb RR. MohagheghpourN, Aruffo A. 1992. 1993.Monoclonal antibodies to the in- GMP-140(P-selectin/CD62) binds tegrin o~--4subunit inhibit the murine chronically stimulated but not resting contact hypersensitivityresponse. Eur. CD4+ T lymphocytesand regulates their J. immunol.23:682-88 production of proinflammatorycyto- 54. Cline MJ. 1975. The White Cell. Cam- kines. Eur. J. lmmunol.22:1789-93 bridge: HarvardUniv. Press 67. deFougerollesAR, Qin X, Springer TA. Annual Reviews www.annualreviews.org/aronline

ENDOTHELIAL SIGNALING 863

1994. Characterization of the function (LECAM-1) adherence receptors of ICAM-3and comparison to ICAM-1 human neutrophils by high-resolutlon and ICAM-2in immuneresponses. J. field emission SEM.£ Histoche~ Cyto- Exp. Med, 179:619-29 chem. 41:327-33 68. deFougerolles AR, Springer TA. 1992. 80. Etzioni A, FrydmanM, Pollack S, Avi- Intercellular adhesionmolecule 3, a third dot I, Phillips ML,et al. 1992. Recurrent adhesion counter-receptor for lympho- severe infections caused by a novel leu- cyte function-associated molecule i on kocyte adhesion deficiency. N. Engl. Z resting lymphocytes. J. Exp. Med. 175: Med. 327:1789-92 185-90 81. Faull P,J, Kovach NL, Harlan HM, 69. deFougerolles AR, Stacker SA, Ginsberg MH.1994. Stimulation of in- Schwarfing R, Springer TA. 1991. Char- tegrin-mediated adhesion of T lympho- acterization of ICAM-2and evidence cytes and monocytes: Twomechanisms for a third counter-receptor for LFA-I. with divergent biological consequences. J. Exp. Med. 174:253-67 d. Exp. Med. 179:1307-16 70. Devreotes PN, ZigmondSH. 1988. Che- 82. Fina L, Molgaard HV, Robertson D, motaxis in eukaryotic cells: A focus on Bradley NJ, MonaghanP, et al. 1990. leukocytes and Dictyostelium. Anna. Expression of the CD34gene in vascular Rev. Cell Biol. 4:649-86 endothelial cells. Blood 75:2417-26 71. Diamond MS, Garcia-Aguilar J, 83. Foxall C, Watson SR, Dowbenko D, Bickford JK, Corbi AL, Springer TA. Fennie C, Lasky LA, et al. 1992. The 1993. The I domain is a major recogni- three membersof the selectin receptor tion site on the leukocyteintegrln Mac-1 family recognize a commoncarbohy- (CD11b/CD18)for four distinct adhe- drate epitope, the sialyl Lewisx oligo- sion ligands. J. Celt Biol. 120:1031-43 saccharide. J, Cell Biol. 117:g95-902 72. Diamond MS, Springer TA. 1993. A 84. FujimotoT, Stroud E, WhatleyRE, Pres- subpopulation of Mac-1 (CD11 b/CD18) cott SM, Muszbek L, et al. 1993. P- molecules mediates neutrophil adhesion selectin is acylated with palmitie acid to ICAM-1and fibrinogen. J. Cell Biol. and steadc acid at eysteine 766 through 120:545-56 a thioester linkage, d. Biol. Chem.268: 73. Diamond MS, Springer TA. 1994. The 11394-400 dynamicregulation of integdn adhesive- 85. Gallatin WM,Weissman IL, Butcher ness. Curt. Biol. 4:506-17 EC. 1983. A cell-surface molecule in- 74. Diamond MS, Staunton DE, de- volved in organ-specific homingof lym- Fougerolles AR, Stacker SA, Garcia- phocytes. Nature 31M:30-34 Aguilar J, et al. 1990. ICAM-I(CD54): 86. Geng J-G, Bevilacqua MP, Moore KL, A counter-receptor for Mac-1 (CDI lb/ Mclntyre TM, Prescott SM, et al. 1990. CD18). J. Cell Biol. 111:3129-39 Rapid neutrophil adhesion to activated 75. Diamond MS, Staunton DE, Marlin SD, endothelium mediated by GMP-140. Springer TA. 1991. Binding of the in- Nature 343:757-60 tegrin Mac-1(CDI Ib/CD18) to the third 87. Gimbrone MA, Obin MS, Brock AF, Ig-like domain of ICAM-1(CD54) and Luis EA, Hass PE, et al. 1989. Endo- its regulation by glycosylation. Cell 65: thelial interleukin-8: A novel inhibitor 961-71 of leukocyte-endothelial interactions. 76. Dusfin ML, Springer TA. 1989. T cell Science 246:1601-3 receptor cross-linking transiently stimu- 88. Ginsberg MH, Du X, Plow EF. 1992. lates adhesiveness through LFA-1. Na- Inside-out integrin signalling. Curr. ture 341:619-24 Opin. Cell Biol. 4:766-71 77. Elices MJ, OsbomL, Takada Y, Crouse 89. Graber N, Gopal TV, Wilson D, Beall C, Luhowskyj S, et al. 1990. VCAM-1 LD, Polte T, NewmanW. 1990. T cells on activated endotheliuminteracts with bind to cytokine-activated endothelial the leukocyte integrin VLA-4at a site cells via a novel, inducible sialoglyco- distinct from the VLA-4/fibronectin protein and endothelial leukocyte adhe- binding site. Cell 60:577-84 sion molecule-1. Z lmmunol. 145: 819- 78. Erbe DV, Wolitzky BA, Presta LG, 30 Norton CR, Ramos RJ, et al. 1992. 90. Granger DN, Kubes P. 1994. The micro- Identification of an E-selectin region circulation and inflammation: Modula- critical for carbohydrate recognition tion of leukocyte-endothelial cell adhe- and cell adhesion. J. Cell. Biol. 119: sion. J. Leukocyte Biol. 55:662-75 215-27 91. Graves BJ, Crowther RL, Chandran C, 79. Eflandsen SL, Hasslen SR, Nelson RD. RumbergerJM, Li S, et al. 1994. Insight 1993. Detectionand spatial distribution into E-selectirVligand interaction from of the ~2 integrin (Mac-l) and L-selectin the crystal structure and mutagenesisof Annual Reviews www.annualreviews.org/aronline

864 SPRINGER

the Iec/EGFdomains. Nature 367:532- LFA-1integrin beta subunit. Science 38 251:1611-13 92. Giinthert U. 1993. CD44:a multitude 104. HoizmannB, Mclntyre BW,Weissman ofisoformswith diverse functions,Curr. IL, 1989, Identification of a routine Top. Microbiol. lmmunol.184:47-63 Peyer’s patch-specific lymphocyte 93. HamannA, Andrew DP, Jablonski- homingreceptor as an integrin molecule Westrich D, HolzmannB, Butcher EC. with an alpha chain homologousto 1993.The role of ~t4 integdnsin lym- humanVLA-4 alpha. Cell 56:37-46 phocytehoming to mucosaltissues in 105. Holzmann B, Weissman IL. 1989. vivo. J. lmmunol.152:3282-93 Peyer’s patch-specific lymphocyte 94, HamannA, Jablonski-Westdch D, homingreceptors consist of a VLA-4- Jonas P, Thiele H-G. 1991. Homing like o~ chain associated with either of receptors reexamined: mouseLECAM- twointegrin 1~ chains, one of whichis 1 (MEL-14antigen) is involvedin lym- novel, EMBOJ. 8:1735-41 phocytemigration into gut-associated 106. HoodL, HuangHV, Dreyer WJ. 1987. lymphoidtissue. Eur. J. Immunol.21: The area-code hypothesis: The immune 2925-29 systemprovides clues to understanding 95. HamannA, WestrichDJ, Duijevstijn A, the genetic and molecularbasis of cell ButcherEC, BaischH, et al. 1988. Ev- recognition during development. J. idence for an accessoryrole of LFA-I Supramol.Struc. 7:531-59 in lymphocyte-highendothdium inter- 107. Horgan KJ, Luce GEG, Tanaka Y, action duringhoming. J. immunol.140: SchweighofferT, ShimizuY, et al. 1992. 693-99 Differential expressionof VLA-ct4and 96. HammerDA, Apte SM. 1992. Simula- VLA-I~Idiscriminates multiplesubsets tionof cell rolling and adhesion on of CD4+CD45R0+ "memory"T cells. Z surfaces in shear flow: general results ImmunoL149:4082-87 and analysis of selectin-mediatedneu- 108. Hu MC-T, Crowe DT, WeissmanIL, trophil adhesion.Biophys. J. 63:35-57 HolzmannB. 1992. Cloning and ex- 97. Harlan JM, Winn RK, Vedder NB, pression of mouseintegdn q~p(~): DocrschukCM, Rice CL, 1992, In vivo functionalrole in Peyer’spatch-specific modelsof leukocyteadherence to endo- lymphocytehoming. Prec. Natl. Acad. thelium.In Adhesion:Its Rolein Inflam- Sci. USA89:8254-58 matoryDisease, ed. JR Hadan,D Liu, 109. Huber AR, Kunkel SL, Todd RF IIl, pp. 117-50. NewYork: Freeman Weiss SJ. 1991. Regulation of trans- 98, HaynesBF, Liao H-X,Patton KL.1991. endothelial neutrophil migrationby en- The transmembranehyaluronate recep- dogenousintedeukin-8. Science 254: tor (CD44):Multiple functions, multiple 99-102 forms. CancerCells 3:347-50 110. HynesRe. 1992.Integrins: Versatility, HechtmanDH, Cybuisky MI, Fuchs modulation,and signaling in cell adhe- Baker JB, GimbroneMA Jr. 1991. In- sion. Cell 69:11-25 travascular IL-8: Inhibitor of polymor- 111. Imai Y, Lasky LA, Rosen SD. 1993. phonuclearleukocyte accumulationat Sulphation requirementfor GIyCAM-1, sites of acute inflammation.J. lmraunol. an endothelialligand for L-selectin.Na- 147:883-92 ture 361:555-57 HemlerME. 1990. VLAproteins in the 112. Imai Y, Singer MS,Feunie C, Lasky integrin family:Structures, functions, LA,Rosen SD. 1991. Identification of and their role on leukocytes.Annu. Rev. a carbohydratebased endothelial ligand lmmunol, g:365-400 for a lymphocytehoming receptor. J. 101. HemmedehS, RosenSD. 1994. 6’-Sul- Cell Biol. 113:1213-21 fated sialyl Lewisx is a majorcapping 113. Issekutz AC,Issekutz TB. 1993. Quan- group of GIyCAM-1.Biochemistry 33: titation and kinetics of bloodmonocyte 4830-35 migrationto acute inflammatoryreac- 102. HibbsML, Jakes S, Stacker SA,Wallace tions, and IL-lct, TNF-~,and IF’N-’/.J. RW,Springer TA.1991. The cytoplas- Immunol.151:2105-15 mic domainof the integdn lymphocyte 114. IssekutzTB. 1991. Inhibition of in rive function-associatedantigen 1 subunit: lymphocytemigration to inflammation sites requiredfor bindingto intercellular and homingto lymphoidtissues by the adhesion molecule I and the phorboi TA-2monoclonal antibody: A likely ester-stimulatedphosphorylation site. J. role for VLA-4in vivo. J. lmmunol. Exp. Med. 174:1227-38 147:4178-84 103. Hibbs ML,Xu H, Stacker SA, Springer 115. Issekutz TB. 1992. Inhibition of lym- TA. 1991. Regulation of adhesion to phocyteendothelial adhesion and in vivo ICAM-1by the cytoplasmic domainof lymphocytemigration to cutaneousin- Annual Reviews www.annualreviews.org/aronline

ENDOTHELIAL SIGNALING $65

flammation by TA-3, a new monocional CG. 1988, A monoclonalantibody antibody to rat LFA-1.J. lmmunoL149: (NKI-LI6)directed against a unique 3394-402 epitopeon the alpha-chainof human 116. Isscku~z "lB. 1993. Dual inhibition of leukocytefunction-associated antigen I VLA-4and LFA-1 maximally inhibils induceshomotypic cell-cell interactions. cutaneous delayed type hypersensitivity- J. Immunol.140:1393-400 induced inflammation. Am. J. Pathol. 128.Kilshaw PJ, MutantSJ. 1990.A new 143:1286-93 surface antigen on intraepithelial lym- 117. Issekntz TB, Chin W, Hay JB. 1982. phocytes in the intestine. Eur. J. lm. The characterization of iymphocytesmi- munol. 20:2201-7 grating through chronically inflamedtis- 129. Kilshaw PL Mutant SJ. 1991. Expres- sues. Immunology46:59--66 sion and regulation of ~([~p) integrins 118. Issekutz TB, Stoltz JM, Meide PVD. on mouse lymphocytes: Relevance to 1988. Lymphocyterecruitment in de- the mucosai immunesystem. Eur. J. layed-type hypersensitivity: The role of Immunol. 21:2591-97 IFN-¥. J. lmmunol. 140:2989-93 130. Kinashi T, St. Pierre Y, Huang C-H, 119. Jalkanen $, Bargatze RF, de los Toyos Springer TA. 1993. Expression of gly- J, Butcher EC. 1987. Lymphocyterec- cophosphatidylinositol (GPI)-anchored ognition of high endothelium:antibodies and non GPI-anchored isoforms of vas- to distinct epitopes of an 85-95 kD cular cell adhesion molecule I (VCAM- glycoprotein antigen differentially in- I) in stromal and endothelial cells. J. hibit lymphocytebinding to lymphnode, Leukocyte Biol. In press mucosaland synovial endothelial cells. 131. Kirchhausen T, Staunton DE, Springer J. Cell Biol. 105:983-93 TA. 1993. Location of the domains of 120. Janossy G, Bofili M, RoweD, Muir J, ICAM-1by immunolabeling and single- Beverley PC. 1989. The tissue distribu- molecule electron microscopy. J. Leu- tion of T lymphocytes expressing dif- kocyte Biol. 53:342-46 ferem CD45polypeptides. Immunology 132. Kishimoto TK, Jutila MA, Berg EL, 66:517-25 Butcher EC, 1989, Neutrophii Mac-1 121. Jung TM, Gailatin WM,Weissman IL, and MEL-14 adhesion proteins inversely Dailey MO.1988. Down-regulation of regulated by chemotactic factors. Sci- homingreceptors after T cell activation. ence 245:1238-41 J. lmmunol. 141:4110-17 133. Kishimoto TK, Larson RS, Corbi AL, 122. Jutila MA, Lewinsohn D, Berg EL, Dustin ML, Staunton DE, Springer TA. Butcher E. 1988. Homingreceptors in 1989. The leukocyte integrins: LFA-1, lymphocyte, nentrophil, and monocyte Mac-1, and p 150,95. Adv. lmmunol. interaction with endothelial cells. In 46:149--82 Leukocyte Adhesion Molecules, ed. TA 134. Kishimoto TK, O’Connor K, Lee A, Springer, DCAnderson, AS Rosenthal, Roberts TM, Springer TA. 1987. Clon- R Rothlein, pp. 227-35. NewYork: ing of the beta subunit of the leukocyte Springer-Veriag adhesion proteins: Homologyto an ex- 123. Jutila MA,Rott L, Berg EL, Butcher tracellular matrix receptor defines a EC. 1989. Function and regulation of novel supergene family. Cell 48:681-90 the neutrophil MEL-14antigen in vivo: 135. Kishimoto TK, WamockRA, Jutila MA, Comparison with LFA-1 and MAC-I. Butcher EC, Lane C, et al. 1991. Anti- J. lmmunol. 143:3318-24 bodies against human neutrophil 124. KameyoshiY, Dtrschner A, Mallet AI, LECAM-1 (LAM- 1/Leu-8/DREG-56 Christophers E, Schr’tder JM. 1992. antigen) and endothelial cell ELAM-1 Cytokin¢ RANTESreleased by thrum- inhibit a commonCD18-independent bin-stimulated platelets is a potent at- adhesion pathway in vitro. Blood 78: tractant for humaneosinophils. J. Exp. 805-11 Med. 176:587-92 136. Kudo C, Araki A, MatsushimaK, Sendo 125. Kansas GS, Ley K, Munro JM, Tedder F. 1991. Inhibition of IL-8-indueed TF. 1993. Regulation of leukocyte roll- W3/25÷ (CIM÷) T lymphocyte recruit- ing and adhesion to high endothelial ment into subcutaneous tissues of rats venules through the cytoplasmic domain by selective depletion of in vivo neu- of L-selectin. J. Exp. Med. 177:833-38 lmmunol.trophils with174:2196-201 a monoclonalantibody. J. 126. Kansas GS, Wood GS, Fishwild DM, Engleman EG. 1985. Functional char- 137. Kuna P, Reddigari SR, Schall TJ, acterization of human T lymphocyte Rucinski D, Sadick M, Kaplan AP. subsets distinguished by monoclonal 1993. Characterization of the humanba- anti-leu-8. J. lmmunol. 134:2995-3002 sophil response to cytokines, growthfac- 127. Keizer GD, Visser W, Vliem M, Figdor tors, and histamine releasing factors of Annual Reviews www.annualreviews.org/aronline

866 SPRINGER

the intcrcfine/chemokinefamily. J. lm- 151. LcwinsohnDM, Bargatz¢ RF, Butcher munoL150:1932-43 EC. 1987. Leukocyte-endothelial¢¢11 138. Landis RC,Bennett RI, HogsN. 1993. recognition: Evidenceof a commonmo- A novel LFA-1activation epitope maps lecular mechanismshared by neutro- to the I domain.J. Cell Biol. 120:1519- phils, lymphocytes, and other 27 leukocytes.J. Immunol.138:4313-21 139. Larsen CG, AndersonAO, Appella E, 152. LeyK, GaehtgensP. 1991.Endothelial, OppenheimJJ, MatsushimaK. 1989. not hemodynamic,differences are re- Theneutrophil-ectivating protein (NAP- sponsiblefor preferentialleukocyte roll- 1) is also chemotacticfor T lympho- ing in rat mesentedcvenules. Circ. Res. cytes. Science 241:1464-66 69:1034-41 140. LarsenE, Celi A, Gilbert GE,Furie BC, 153. Ley K, GaehtgensP, Fennie C, Singer Ethan JK, et al. 1989. PADGEMpro. MS,Lasky LA, Rosen SD. 1991. Lectin- tein: A receptorthat mediatesthe inter- like cell adhesionmolecule I mediates action of activated platelets with leukocyterolling in mesenteficvenules neutrophils and monocytes.Cell 59: in vivo. Blood77:2553-55 305-12 154. Lo SK, DetmersPA, Levin SM,Wright 141. Larsen GR, Sako D, AhemTJ, Shaffer SD.1989. Transient adhesion of neutro- M,Ethan J, et al. 1992.P-selectin and phils to cndothclium.J. Exp. Med.169: E-selectin:Distinct but overlappingleu- 1779-93 kocyte iigand specificities. J. Biol. 155. Lo SK, Lee S, RamosRA, Lobb R, Chem.267:11104-10 RosaM, et al. 1991.Endothdial-leuko- 142. Larson RS, Corbi AL, Berman L, cyte adhesionmolecule 1 stimulates the SpringerTA. 1989. Primary structure of adhesiveactivity of leukocyteintegrin the LFA-1alpha subunit: Anintegrin CR3 (CDllb/CD18, Mac-l,t~2) with an embeddeddomain defining a humanneutrophils. J. Exp. Med.173: protein superfamily.J. Cell Biol. 108: 1493-500 703-12 156. Lotus JC, O’TooleTE, PlowEF, Glass 143. LaskyLA, 1992. Selcctins: Interpreters A, Frelinger ALIII, GinsbergMH. 1990. of cell-specific carbohydrateinforma- A63 integrin mutationabolishes ligand tion during inflammation.Science 258: bindingand alters divalent cation-de- 964-69 pendent conformation. Science 249: 144. Lasky LA, Singer MS, DowbenkoD, 915-18 Imai Y, Henzel WJ, et al. 1992. An 157. Lollo BA, Chan KWH,Hanson EM, endothelial ligand for L-selectin is a MoyVT, Brian AA.1993. Direct evi- novel mucin-like molecule. Cell 69: dence for two affinity states for lym- 927-38 phocytefunction-associated antigen 1 145. LawrenceMB, Balnton DF, Springer on activated T cells. £ Biol. Chem. TA.1994. Neutrophiltethering to and 268:1-8 rolling on E-selectin are separable by 158. Lorant DE, Patel KD, McIntyre TM, requirementfor L-selectin. Immunity1: McEverRP, Prescott SM, Zimmerman 137-45 GA. 1991. Coexpression of GMP-140 146. Lawrence MB,Smith CW,Eskin SG, and PAFby endotheliumstimulated by Mclntire LV. 1990. Effect of venous histamineor thrombin:A juxtacrine sys- shear stress on CD18-mediatedneutro- temfor adhesionand activation of neu- phil adhesionto cultured endothelium. trophils. Z Cell Biol. 115:223-34 Blood 75:227-37 159. MackayCR. 1992. Migration pathways 147. LawrenceMB, SpdngerTA. 1991. Leu- and immunologic memory among T kocytesroll on a selectin at physiologic lymphocytes.Semin. lmmunol.4:51-58 flow rates: distinction fromand prereq- 160. Mackay CR. 1993. Immunological uisite for adhesionthrough integrins. memory.Adv. immunol. 53:217-65 Cell 65:859-73 161. Mackay CR, Marston W, Dudler L. 148. LawrenceMB, SpringerTA. 1993. Neu- 1992.Altered patterns of T cell migra- trophils roll on E-selectin.J. lmmunol. tion throughlymph nodes and skin fol- 151:6338-46 lowingantigen challenge. Eur. J. lm- 149. LeonardEJ, YoshimuraT. 1990. Human munol. 22:2205-10 monocytechemoattractant protein-1 162. Mackay CR, Marston WL,Dudler L. (MCP-I). immunol. Today11:97-101 1990. Naive and memoryT cells show 150. Leonard F_J, YoshimuraT, TanakaS, distinct pathwaysof lymphocyterecir- Raffeld M.1991. Neutrophil recruitment eulation. J. Exp. Med.171:801-17 by intradermallyinjected neutrophilat- 163. MackayCR, Marston WL,Dudler L, tractant/activationprotein-1. J. Invest. Spenini O, TedderTF, HeinWR. 1992. Dermatol. 96:690-94 Tissue-specific migrationpathways by Annual Reviews www.annualreviews.org/aronline

ENDOTHELIAL SIGNALING 867

phenotypicallydistinct subpopulations distribution of transfused lymphocytes of memoryT cells. Eur. J. Immunol, in pertussis-treated and normalmic~, £ 22:887-95 Exp. Med. 132:663-72 164. MasinovskyB, Urdal D, Gallatin WM. 176. MoyP, Lobb R, Tizard R, Olson D, 1990.IL-4 acts synergisticallywith IL-1 HessionC. 1993.Cloning of an inflam- I~ to promotelymphocyte adhesion to mation-specificphosphatidylinositol- microvascularendothelium by induction linked formof murin©vascular e~ll ad- of vascularcell adhesionmolecule-1. J. hesion molecule-l. J. Biol. Chent268: lmmunol. 145:2886-95 8835-41 165. MayadasTN, Johnson RC, RaybumH, 177. Muller WA,Ratti CM,McDonnell SL, Hynes RO, WagnerDD. 1993. Leuko- CohnZA. 1989. A humanendothelial cyte rolling and extravasationare se- cell-restricted, externallydisposed plas- verely compromisedin P-selectin- malemmalprotein enriched in intercel- deficient mice. Cell 74:541-:54 lular junctions. J. Exp. Med. 170: 166. McCluskey RT, Benacerraf B, Mc- 399-414 CluskyJW. 1963. Studies on the spec- 178. Muller WA,Weigl SA, DengX, Phillips ificity of the cellularinfiltrate in delayed DM.1993. PECAM-1is required for type hypersensitivityreactions, d. lm- transendothelial migrationof leuko- munol. 90:466 cytes. J. Exp. Med.178:449-60 167. McEverRP. 1991.Selectins: Novelre- 179. Mulligan MS, Jones ML, Bolanowski ceptors that mediateleukocyte adhesion MA,Baganoff MP, Deppeler CL, et al. during inflammation.Thromb. Haemost. 1993. Inhibition of lung inflammatory 65:223-28 reactions in rats by an anti-humanIL-8 168. McEverRP, Beckstead JH, MooreKL, antibody. J. lmmunol.150:5585-95 Marshall-CarlsonL, BaintonDF. 1989. i 80. MulliganMS, Varani J, DameMK, Lane GMP-140,a platelet alpha-granule CL, Smith CW,et al. 1991. Role of membraneprotein, is also synthesized endothelial-leukocyte adhesion mole- by vascularendothelial cells and is lo- cule I (ELAM-1) in neutrophil-mediated calizedin WeibeI-Paladebodies. £ Clin. lunginjury in rats. J. Clin. Invest. 88: Invest. 84:92-99 1396-406 169. MeblusRE, DowbenkoD, Williams A, 181. MurphyPM. 1994. The molecular biol- Fennie C, LaskyLA, Watson SR. 1993. ogy of leukocytechemoattractant recep- Expression of GIyCAM-I,an endothe- tors. Annu.Rev. lmmunol.12:593-633 lial iigandfor L-selectin,is affectedby 182. NazziolaE, HouseSD. 1992. Effects of afferent lymphaticflow. J. immunol. hydrodynamicsand leukocyte-endothe- 151:6769-76 lium specificity on leukocyte-endothe- 170. Mebius RE, Streeter PR, Breve J, lium interactions. Microvasc.Res. 44: Duijvestijn AM,Kraal G. 1991. The 127-42 influence of afferent lymphaticvessel 183. Nelson RM, Dolieh S, Aruffo A, interruption on vascular addressin ex- Cecconi O, Bevilaequa MP. 1993. pression, d. Cell Biol. 115:85-95 Higher-affinityoligosacchafide ligands 171. Michishita M, VidemV, Arnaout MA. for E-selectin.J. Clin. Invest. 91:1157- 1993. Anovel divalent cation-binding 66 site in the Adomain of the 1~2integrin 184. Nermut MV, Green NM, Eason P, CR3(CDllb/CDI8) is essential for YamadaSS. YamadaKM. 1988. Elec- ligand binding. Cell 72:857-67 tron microscopyand structural modelof 172. Miller MD,Krangel MS. 1992. Biology humanfibronectin receptor. EMBOJ. and biochemistryof the chemokines:A 7:4093-99 familyof chemotacticand inflammatory 185. NewmanPJ, Berndt Me, Gorski J, cytokines. Crit. Rev. immunol.12:17-46 White GC, LymanS, et al. 1990. 173. MooreKL, Stults NL, Diaz S, Smith PECAM-I(CD31) cloning and relation DF,Cummings RD, et al. 1992.Identi- to adhesion molecules of the immu- ficationof a specific glycoproteinligand noglobulingene superfamily. Science for P-selectin (CD62)on myeloidcells. 247:1219-22 J. Cell Biol. 118:445-56 186. NorgardKE, Moore KL, Diaz S, Stults 174. Moore KL, ThompsonLF, 1992, P- NL,Ushiyama S, et al. 1993. Charac- selectin (CD62)binds to subpopulations terization of a specific ligand for P- of humanmemory T lymphocytes and selectin on myeloidcells: A minorgly- natural killer cells. Biochem.Biophys. coproteinwith sialylate.d 0-1inkedoligo- Res. Commun.186:173-81 saccharides. J. Biol. Chem.268:12764- 175. MorseSI, Barron BA.1970. Studies on 74 the leukocytosisand lymphocytosisin- 187. NoursharghS, WilliamsTJ. 1990. Evi- ducedby Bordetellaper~ussi. III. The dencethat a receptor-operatedevent on Annual Reviews www.annualreviews.org/aronline

868 SPRINGER

the neutrophil mediatesneutrophil ac- 199. Picker LJ, TerstappenLWMM, Rott IS, cumulationin vivo. Z Immunol.145: Streeter PR, Stein H, ButcherEC. 1990. 2633-38 Differential expressionof homing-asso- 188. OlofssonAM, Affors K-E, RamezaniL, ciated adhesion moleculesby T cell WolitzkyBA, Butcher EC, yon Andfian subsets in man. J, lmmunol.145:3247- UH.1994. E-seleefin mediates leukocyte 55 rolling in interleukin-1treated rabbit 200. Picker LJ, Warnoek RA, BumsAR, mesenteryvenules. Blood,In press DocrschukCM, Berg EL, Butcher F.,C. 189. Oppenheimer-MarksN, Davis LS, Lip- 1991. The neutrophil selectin LECAM-1 sky PE. 1990. HumanT lymphocyte presentscarbohydrate ligands to the vas- adhesionto endothelialcalls and trans- cular selectins ELAM-Iand GMP-140. endothelialmigration: alteration of re- Cell 66:921-33 ceptoruse relates to the activationstatus 201. Pitcher H, GroscurthP, BaumhutterS, of both the T cell and the endothelial Aguet M, Zinkernagel RM,Hengartner cell. J. Irrununol.145:140-48 H. 1986.A monoclonalantibody against 190. OsbornL, HessionC, TizardR, Vassallo altered LFA-1induces proliferation and C, LuhowskyjS, et al. 1989. Direct lymphokinerelease of cloned T cells. cloning of vascularcell adhesionmole- Eur, J. hnmunol.16:172-81 cule 1 (VCAM-I),a eytokine-induced 202. Pitzalis C, Kingsley G, Haskard D, endothelial protein that binds to lym- PanayiG. 1988.The preferential accu- phocytes. Cell 59:1203-1211 mulation of helper-inducer T lymph- 191. OsbomL, Vassailo C, BenjaminCD. ocytesin inflammatorylesions: evidence 1992.Activated endothelium binds lym- for regulationby selective endothelial phocytesthrough a novel binding site and homotypicadhesion. Eur. J. Im- in the alternately spliceddomain of vas- munol. 18:1397--404 cular cell adhesionmolecule-1. J. Exp. 203. Pober JS, Cotran RS. 1990. Cytokines Med. 176:99-107 and endothelial cell biology. Physiol. 192. Osborn L, Vassallo C, BrowningBG, Rev. 70:427-52 Tizard R, Haskard DO,et al. 1994. 204. Pober JS, DoukasJ, HughesCCW, Sav- Arrangementof domains, and amino age COS,Munro JM, Cotran RS. 1990. acid residues required for binding of Thepotential roles of vascularendothe- vascularcell adhesionmoleeule-I to its iium in immunereactions. Hum.lm- counter-receptorVLA-4(ct4131). J. Cell munol. 28:258-62 Biol. 124(4):601-8 205. Polte T, NewmanW, Gopal TV. 1990. 193. Parker CM,Cepek K, Russell GJ, Shaw Full length vascular cell adhesionmol- SK, PosnettD, et al. 1992.A familyof ecule 1 (VCAM-1).Nucleic Acids ICes. 1~7 integrins on humanmueosal lympho- 18:5901 cytes. Proc. Natl. Acad. Sci. USA89: 206. RosenSD. 1993.Cell surface lectins in 1924-28 the immunesystem. Semin. Immunol. 194. Parrott DMV,Wilkinson PC. 1981. 5:237-47 Lymphocytelocomotion and migration. 207. Rot A. 1992. Endothelial cell binding Prog. Allergy 28:193-284 of NAP-1/IL-8:role in neutrophil emi- 195. Philips MR,Buyon JP, WinchesterR, gration. Immunol.Today 13:291-94 WeissmanG, AbramsonSB. 1988. Up- 208. Rot A, Krieger M, BrunnerT, Bischoff regulation of the iC3breceptor (CR3) SC, Schall TJ, Dahinden CA. 1992. is neither necessarynor sufficient to RANTESand macrophage inflamma- promoteneutrophil aggregation. J. Clln. tory protein la induce the migration Invest. 82:495-501 and activation of normalhuman eosin- 196. Picker L J, ButcherEC. 1992. Physio- ophil granulocytes. J. Exp. Med.176: logical and molecular mechanismsof 1489-95 lymphocytehoming. Annu. Rev. lmmu- 209. Roth SJ, Carr MW,Rose SS, Springer nol. 10:561-91 TA. 1994. Characterization of trans- 197. Picker LJ, KishimotoTK, Smith CW, endothelial ehemotaxisof T lympho- Warnock RA, Butcher EC. 1991. cytes. Am.J. Pathol. Submitted ELAM-Iis an adhesion molecule for 210. Rothlein R, Dustin ML,Marlin SD, skin-homingT cells. Nature349:796-98 Springer TA.1986. A humanintercel- 198. Picker LJ, MichieSA, Rott LS, Butcher lular adhesionmolecule (ICAM-1) dis- EC. 1990. A uniquephenotype of skin- tinct from LFA-1. J. Immunol. 137: associatedlymphocytes in humans:pref- 1270-74 erential expression of the HECA-452 211. Rtiegg C, Postigo AA,Sikorski EE, epitope by benignand malignantT cells ButcherEC, Pytela R, Erie DJ. 1992. at cutaneoussites. Am.J. Pathol. 136: Roleof integdn ot4l~7Rz4l~Pin lympho- 1053-68 cyte adherence to fibronectin and Annual Reviews www.annualreviews.org/aronline

ENDOTHELIAL SIGNALING 869

VCAM-1and in homotypic cell cluster- 224. Shimizu Y, Shaw S, Graber N, Gopal ing. Z Cell Biol. 117:179--89 TV, HorganKJ, et al. 1991. Activation- 212. Sake D, Chang X-J, Barone KM, independent binding of human memory Vachino G, While HM, et al. 1993. T cells to adhesion molecule ELAM-1. Expression cloning of a functional gly- Nature 349:799-802 coprolein ligand for P-selectin. Cell 75: 225. Silber A, NewmanW, Sasseville VG, 1179-86 Pauley D, Beail D, et al. 1994. Re- 213. San GabrieI-Masson C. 1992. Adhesion cruitment of lymphocytes during cuta- of lymphocytes to the lactating mam- neous delayed hypersensitivity in non- mary gland in the mouse. PhDthesis. human primates is dependent on E- Penn. State Univ. 105 pp. selectin and VCAM-1.J. Clin. Invest. 214. Schall TJ. 1991. Biology of the 93:1554-63 RANTES/SIScytokine family. Cytokine 226. Simmons D, Makgoba MW, Seed B. 3:165-83 1988. ICAM, an adhesion ligand of 215. Schail T3, Bacon K, Camp RDR, LFA-1,is homologousto the neural cell Kaspari JW, Goeddel DV. 1993. Human adhesion molecule NCAM.Nature 331: macmphage inflammatory protein 624-27 t~(MIP-10~)and MIP-l[I chemokinesat- 227. SimmonsDL, Satterthwaite AB, Tenen tract distinct populations of lympho- I)(3, Seed B. 1992. Molecular cloning cytes. J. Exp. Med. 177:1821-25 of a cDNAencoding CD34,a sialomucin 216. Schail TJ, Bacon K, Toy KJ, Gocddel of humanhematopoietie stem cells. 3. DV.1990. Selective attraction of mono- lmmunol. 148:267-71 tyros and T lymphocytes of the memory 228. Simmons DL, Walker C, Power C, phenotyp¢ by cytokine RANTES.Na- Pigott R. 1990. Molecular cloning of ture 347:669-71 CD31,a putative intercellular adhesion 217. Scheynius A, CampRL, Par6 E. 1993. molecule closely related to carcino- Reduced contact sensitivity reactions embryonic antigen. J. Exp. Med. 171: in mice treated with monoclonal an- 2147-52 tibodies to leukocyte function-associ- 229. Smith CW, Kishimoto TK, Abbass O, ated molecule-I and intercellular ad- HughesB, Rothlein R, et al. 1991. Che- hesion molecule-1. J. lmmunol. 150: motaetic factors regulate lectin adhesion 655--63 molecule 1 (LECAM-1)-dependent neu- 218. Schmid-SchOnbein GW, Usami S, trophil adhesion to cytokine-stimulated Skalak R, ChienS. 1980. The interaction endothelialcells in vitro. Z Clin. Invest. of leukocytes and erythrocyles in capil- 87:609-18 lary and postcapillary vessels. Micro- 230. Smith CW, Marlin SD, Rothlein R, vase. Res. 19:45-70 Toman C, Anderson DC. 1989. Coop- 219. Schweighoffer T, Tanaka Y, Tidswell erative interactions of LFA-1and Mac-1 M, Eric DJ, Horgan KJ, et al. 1993. with intercellular adhesion molecule-1 Selective expression of integrin tz4[~7 in facilitating adherence and trans- on a subset of human CD4+ memoryT endothelial migration of humanneutro- cells with hallmarks of gut-trophism. J. phils in vitro. J. Clin. Invest. 83:2008-17 lmmanol. 151:717-29 231. Smith CW, Rothlein R, Hughes B J, 220. Sekido N, Mukaida N, Harada A, Mariscalco MM,Schmalstieg FC, An- Nakanishi I, Watanabe Y, Matsushima derson DC. 1988. Recognition of an K. 1993. Prevention of lung repeffusion endothelial determinant for CD18-de- injury in rabbits by a monoclonalanti- pendent neutrophil adherence and trans- body against interleukin-8. Nature 365: endothelial migration. J. Clin. Invest. 654-57 82:1746-56 221. Sengelov H, Kjeldsen L, Diamond MS, 232. Snyderman R, Uhing RJ. 1992. Chemo- Springer TA, Borregaard N. 1993. Sub- attractant stimulus-responsecoupling. In cellular localization and dynamics of Inflammation: Basic Principles and Mac-1(~Xm~2) in humanneutrophils. Clinical Correlates, ed. JI Gallin, IM Clin. Invest. 92:1467-76 Goldstein, R Snyderman, pp. 421-39. 222. Shaw SK, Cepek KL, Murphy EA, Rus- New York: Raven sell GJ, Brenner MB,Parker CM.1994. 233. Spangrude GJ, Braaten BA, Daynes RA. Molecular cloning of the humanmucosal 1984. Molecular mechanisms of lym- lymphocyteintegrin otE subunit. J. Biol. phocyte extravasation. 1. Studies of two Chem. 269:6016--25 selective inhibitors of lymphocyterecir- 223. Shimizu Y, Newman W, Tanaka Y, culation. Z Immunol. 132:354-62 ShawS. 1992. Lymphocyteinteractions 234. SpangmdeGJ, Sacchi F, Hill HR, Van with endothelial cells, lmmunol. Today Epps DE, Daynes RA. 1985. Inhibition 13:106-12 of lymphocyte and neutrophil chemo- Annual Reviews www.annualreviews.org/aronline

$70 SPRINGER

taxis by pertussis toxin. J. immunol. 246. Stevens SK, WeissmanIL, Butcher EC. 135:4135-43 1982. Differences in the migration of B 235. Spertini O, Kansas GS, MunroJM, Grif- and T lymphocytes: Organ-selective lo- fin JD, Tedder TF. 1991. Regulation of calization in vivo and the role of lym- leukocyte migration by activation of the phocyte-endothelial cell recognition. J. leukocyte adhesion molecule (LAM-1) lmmunol. 2:844-51 selectin. Nature 349:691-94 247. Stockinger H, Gadd SJ, Eher R, Majdie 236. Spertini O, Luscinskas FW, Gimbrone O, Schreiber W, et al, 1990. Molecular MAJr, Tedder "IF. 1992. Monocyte characterization and functional analysis attachment to activated humanvascular of the leukocyte surface protein CD31. endotheliumin vitro is mediatedby leu- J. Immunol. 145:3889-97 kocyte adhesion molecule- 1 (L-selectin) 248. SWeeter PR, Lakey-Berg E, Rouse BTN, under nonstatic conditions. J. Exp. Med. Bargatze RF, Butcher EC. 1988. A tis- 175:1789-92 sue-specific endothelial cell molecule 237. Spcrtini O, Luscinskas FW, Kansas GS, involved in lymphocyte homing. Nature Munro JM, Griffin JD, et al. 1991. 331:41-46 Leukocyte adhesion molecule-1 (LAM- 249. Streeter PR, Rouse BTN, Butcher EC. 1, L-selectin) interacts with an induc- 1988. Immunohistologic and functional ible endothelial cell ligand to support characterization of a vascular addressin leukocyte adhesion. J. Immunol. 147: involved in lymphocytehoming into pe- 2565-73 ripheral lymphnodes. J. Cell Biol. 107: 238. Springer TA. 1990. Adhesion receptors 1853-62 of the immunesystem. Nature 346:425- 250. Sutherland DR, Marsh JCW, Davidson 33 J, Baker MA, Keating A, Mellors A. 239. SpringerTA. 1990. Areacode molecules 1992. Differential sensitivity of CD34 of lymphocytes.In Cell to Cell Interac- epitopes to cleavage by Pasteurella tion: a Karger Symposium, ed. MM haemolytica glycoprotease: Implica- Burger, B Sordat, RMZinkemagel, pp. tions for purification of CD34-positive 16-39, Basel: Karger progenitor cells, Exp. Hematol, 20:590- 240. Springer TA. 1994. Traffic signals for 99 lymphocyterecirculation and leukocyte 251. Swediek RA, Lee KH, Wick TM, emigration: The multi-step paradigm. Lawley TJ. 1992. Humandermal micro- Cell 76:301-14 vascular endothelial but not humanum- 241. Stamper HB Jr, Woodruff JJ. 1976. bilical vein endothelial cells express Lymphocytehoming into lymph nodes: CD36in vivo and in vitro. J. lmmunol. In vitro demonstration of the selective 148:78-83 affinity of reeireulating lymphoeytesfor 252. Takada Y, Eliees MJ, Crouse C, Hemler high-endothelial venules. J. Exp. Med. ME. 1989. The primary structure of ~-4 144:828 subunit of VLA-4: Homologyto other 242. Staunton DE, Dustin ML, Erickson HP, integrins and possible cell-cell adhesion Springer TA. 1990. The arrangement of function. EMBOJ. 8:1361-68 the immunoglobulin-like domains of 253. Tanaka Y, Adams DH, Hubseher S, ICAM-1and the binding sites for LFA-1 Hirano H, Siebenlist U. ShawS. 1993. and rhinovirus. Cell 61:243-54 T-cell adhesion induced by proteo- 243. Staunton DE, Dustin ML, Springer TA. glycan-immobilized cytokine MIP-1[~. 1989. Functional cloning of ICAM-2, Nature 361:79-82 a cell adhesion ligand for LFA-1 ho- 254. Tanaka Y, Albelda SM, Horgan KJ, Van mologous to ICAM-1. Nature 339:61- Seventer GA, Shimizu Y, et al. 1992. CD31expressed on distinctive T cell 244. Staunton DE, Marlin SD, Stratowa C, subsets is a preferential amplifier of 1~I Dustin ML, Springer TA. 1988. Primary integrin-mediated adhesion. J. Exp. structure of intercellular adhesionmol- Med. 176:245-53 ecule 1 (ICAM-I)demonstrates interac- 255. Taub DD, Conlon K, Lloyd AR, Op- tion between members of the immu- penheim JJ, Kelvin DL 1993. Pref- noglobulin and integrin supergene fam- erential migration of activated CD4+ ilies. Cell 52:925-33 and CDS+ T cells in response to 245. Steininger CN, Eddy CA, Leimgruber MIP-lot and MEP-I~. Science 260: RM, Mellors A, Welply JK. 1992. The 355-58 glycoprotease of Pasteurella haemol- 256. Taub DD, Lloyd AR, Conlon K, Wang ytica A1 eliminates binding of myeloid JM, Ortaldo JR, et al. 1993. Recombi- cells to P-selectin but not to E-selectin. nant humaninterferon-inducible protein Biochem. Biophys. Res. Commun.188: 10 is a ehemoattraetant for humanmono- 760-66 cytes and T lymphocytes and promotes Annual Reviews www.annualreviews.org/aronline

ENDOTHELIAL SIGNALING 871

T cell adhesionto endothelial cells. J. LECAM-1and the leukocyte [~2 in- Exp. Med. 177:1809-14 tegrins in vivo. Proc. Natl. Acad. Sci. 257. Tedder TF, MatsuyamaT, Rothstein D, USA 88:7538-42 Schlossman SF, Morimoto C, 1990, 267, yon Andrian UH, Hansell P, Chambers Humanantigen-specific memoryT cells JD, Berger EM, Filho IT, et al. 1992. express the homing receptor (LAM-I) L-selectin function is required for ~2- necessary for lymphocyterecirculation. integrin.mediated neutrophil adhesion at Eur. J, lmmunol. 20:1351-55 physiological shear rates in vivo. Art 258. Terry RW, Kwee L, Levine JF, Labow J. Physiol. 263:H1034--44 MA. 1993. Cytokine induction of an 268. Vonderheide RH, Springer TA. 1992, alternatively spliced murine vascular Lymphocyte adhesion through VLA-4: cell adhesion molecule (VCAM)mRNA Evidencefor a novel binding site in the encoding a glycosylphosphatidylino- alternatively spliced domain of VCAM- sitol-anchored VCAMprotein. Proc. 1 and an additional ct4 integdn counter- Natl. Acnd Sci. USA 90:5919-23 receptor on stimulated endothelium. J. 259. Thornhill MH, Wellicome SM, Ma- Exp. Med. 175:1433-42 hiouz DL, Lanchbury JSS, Kyan-Aung 269. Vonderheide RH, Tedder TF, Springer U, Haskard DO. 1991. Tumor necrosis TA, Staunton DE. 1994. Residues within factor combines with IL-4 or IFN- a conserved amino acid motif of do- gammato selectively enhance endothe- mains I and 4 of VCAM-1are required lial cell adhesiveness for T cells: The for binding to VLA-4.J. Cell Biol. contribution of vascular cell adhesion 125:215-22 molecule- 1 -dependent and -independent 270. Walsh L J, Lavker RM, Murphy binding mechanisms. J. Immunol. 146: 1990. Biology of disease. Determinants 592-98 of immunecell trafficking in the skin. 260. Ushiyama S, Laue TM, Moore KL, Er- Lab. Invest. 63:592--600 ickson HP, McEverRP. 1993. Structural 271. Wardlaw AC, Patton R. 1983. 8orde- and functional characterization of mo- tella pertussis toxins. Pharmacol.Ther. homeric soluble P-selectin and compar- 19:1-53 ison with membraneP-selectin. J. Biol. 272. Watson SR, Fennie C, Lasky LA. 1991. Chem. 268:15229-37 Neutrophil influx into an inflammatory 261. Van Ewijk W, Brons NHC, Rozing site inhibited by a soluble homingre- J. 1975. Scanning electron microscopy ceptor-lgG chimaera. Nature 349:164- of homing and recirculating lympho- 67 cyte populations. Cell. lmmunol. 19: 273. Watson SR, Imai Y, Fennie C, Geoffrey 245-61 JS, Rosen SD, Lasky LA. 1990. A hom- 262. Vedder ND, Harlan JM. 1988. Increased ing receptor-lgG chimera as a probe for surface expression of CBllb/CDI8 is adhesive ligands of lymph node high not required for stimulated neutrophil endothelial venules. J. Cell Biol. 110: adherence to cultured endothelium. J. 2221-29 Clin. Invest. 81:676-82 274. Wilkinson PC. 1982. Chemotaxis and 263. Villiger PM, Terkeltaub R, Lotz M. Inflammation. London: Churchill Liv- 1992. Production of monocyte chemo- ingstone. 249 pp. attractant protein-1 by inflamedsynovial 275. Woodruff J J, Clarke LM, Chin YH. tissue and cultured synoviocytes. J. lm- 1987. Specific cell-adhesion mecha- munol. 149:722-27 nisms determining migration pathways 264. yon Andrian UH, Berger EM, Ramezani of recirculating lymphocytes.Annu. Rev. L, ChambersJD, Ochs HD, et al. 1993. lmmunol. 5:201-22 In vivo behavior of neutrophils from 276. Wright SD, Meyer BC. 1986. Phorbol two patients with distinct inherited leu- esters cause sequential activation and kocyte adhesion deficiency syndromes. deactivation of complementreceptors on J. Clin. Invest. 91:2893-97 polymorphonuclear leukocytes. J. lm- 265. yon Andrian UH, Chambers JD, Berg munol. 136:1759-64 EL, Michie SA, BrownDA, et al. 1993. 277. WuD, LaRosa GJ, Simon MI. 1993. G L-selectin mediates neutrophil rolling in protein-coupled signal transduction inflamedx- venules through sialyl Lewis pathwaysfor interleukin-8. Science 261: dependent and -independent recognition 101-3 pathways. Blood 82:182-91 278. YednockTA, Cannon C, Fritz LC, San- 266. yon Andrian UH, Chambers JD, Mc- chez-Madrid F, Steinman L, Karin N. Evoy LM, Bargatze RF, Affors KE, 1992. Prevention of experimental auto- Butcher EC. 1991. Two-step model of immuneencephalomyelitis by antibod- leukocyte-endothelial cell interaction ies against ct4[ll integrin. Nature356: in inflammation: Distinct roles for 63-66 Annual Reviews www.annualreviews.org/aronline

872 SPRINGER

Zhu D, Cheng C-F, Paali BU. 1991. 280. ZimmermanGA, Prescott SM,Mclntyre Mediationof lungmetastasis of routine TM.1992. Endothelial cell interactions melanomasby a lung-specific endothe- withgranuiocytes: tethering and signal- lial cell adhesionmolecule. Proc. Natl. ing molecules, lmmunol.Today 13:93- Acad. Sci. USA88:9568-72 100