Annual Reviews www.annualreviews.org/aronline Annu.Rev. Physiol. 1995. 57:827-72 Copyright©1995 by Annual Reviews Inc. All rightsreserved 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 accu- mulation 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 mono- cytes cannot recirculate, but emigrate from the bloodstream in response to molecular changeson the surface of blood vessels that signal injury or infec- tion. Lymphocytescan similarly accumulatein response to inflammatorystim- uli. The nature of the inflammatorystimulus determines whether lymphocytes, monocytes, neutrophils, or eosinophils predominate, and thus exercises spec- ificity in the molecularsignals or "area codes" that are displayed on endothe- lium 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 828 SPRINGER Ig Family Selectin Chemoattractant Member StepI Step2 Step3 FigureI Threesequential steps providethe traffic signals that regulate leukocytelocalization in the vasculature.Selectin moleculesthat bind carbohydrateligands, 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 bind to 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 modifi- cation 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 ENDOTHELIALSIGNALING 829 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 interac- tions, 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 accu- mulatein responseto traumaticinjury or bacterial infection. However,mono- cytes differ fromneutrophils, in that they accumulateat sites whereT lymph- ocytes 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 mi- crovasculaturein 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 830 SPRINGER 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 post- capillary 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 differ- entiation, as exemplified by modulation of adhesiveness (279). The high en- dothelium in lymphoid tissue, which expresses addressins for lymphocyte recirculation, is one of the most dramatic examplesof endothelial specializa- tion (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 chemo- attractants 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 ENDOTHELIALSIGNALING 831 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