The Dendritic Cell System and Its Role in Immunogenicity1

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THE DENDRITIC CELL SYSTEM AND ITS ROLE IN IMMUNOGENICITY1

Ralph M. Steinman

Laboratory of Cellular Physiology and Immunologyand Irvington Institute, The Rockefeller University, NewYork, New York 10021

KEYWORDS: primary immuneresponse, antigen presenting cells, Langerhans cells, homing,antigen processing

Abstract Dendritic cells are a system of antigen presenting cells that function to initiate several immuneresponses such as the sensitization of MHC-restricted T cells, the rejection of organ transplants, and the formation of T- dependent antibodies. Dendritic cells are found in many nonlymphoid tissues but can migrate via the afferent lymphor the blood stream to the T-dependent areas of lymphoid organs. In skin, the immunostimulatory function of dendritic cells is enhanced by cytokines, especially GM-CSF. After foreign proteins are administered in situ, dendritic cells are a prin-

by Rockefeller University on 08/15/07. For personal use only. cipal reservoir of immunogen.In vitro studies indicate that dendritic cells only process proteins for a short period of time, whenthe rate of synthesis of MHCproducts and content of acidic endocytic vesicles are high. Antigen processing is selectively dampenedafter a day in culture, but the capacity Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org to stimulate responses to surface bound peptides and mitogens remains strong. Dendritic cells are motile, and efficiently cluster and activate T cells that are specific for stimuli on the cell surface. High levels of MHC class-I and -II products and several adhesins, such as ICAM-1and LFA- 3, likely contribute to these functions. Therefore dendritic cells are specialized to mediate several physiologic components of immunogenicity

~ Abbreviations: APC,antigen-presenting cell; LC, Langerhans cells; MLR,mixed leukocyte reaction; MHC,major histocompatibility complex. 271 0732-0582/91/0410-0271 $02.00 Annual Reviews www.annualreviews.org/aronline 272 STEINMAN such as the acquisition of antigens in tissues, the migration to lymphoid organs, and the identification and activation of antigen-specific T cells. The function of these presenting cells in immunologictolerance is just beginningto be studied.

INTRODUCTION Manytissues contain a trace populationof cells with an unusualdendritic shape, high levels of MHCclass-II products, and strong accessoryfunction for the stimulation of T lymphocytes.A clearer picture of the physiology of these cells is emergingand is reviewedhere. Dendritic cells comprisea system that occupies discrete portions of nonlymphoidand .lymphoid organs and is interconnected by defined pathwaysof movement(Table 1). In each site, dendritic cells share features of structure and function, the mostnotable being the ability to capture antigens and initiate T celt- mediatedimmunity. The mechanisms of dendritic cell action are important for understanding how T cells are primed and how one might begin to manipulate the immuneresponse at the early sensitization phase of immunity.Three areas of function are considered here, each exhibiting differences fromother antigen-presentingcells (APC):a sentinel role whichantigens are captured and presented, a migratoryfunction in which dendritic cells moveto the T-dependentareas of lymphoidorgans and bind antigen-specific T cells, and an adjuvant or activation role in which T-cell growthand effector function are induced. Recent progress on dendritic cells is reviewedhere. Moredetailed reports on manytopics are in Researchin ImmunologyVolume 140, International Reviewsin Immunology Volume6, Advancesin ImmunologyVolume 47, and EpidermalLangerhans Cells, ed. G. Schuler.

by Rockefeller University on 08/15/07. For personal use only. DENDRITIC CELLS: A WIDELY DISTRIBUTED AND CONNECTED SYSTEM OF POTENT ANTIGEN- PRESENTING CELLS Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org Thecharacterization of dendriticcells has requiredthe isolation of enriched populationsof whatis a trace cell type in all tissues. Mousespleen remains

Table 1 The dendritic cell system--distribution and nomenclature

Nonlymphoid organs Langerhanscells, interstitial dendritic cells Circulation Afferent lymphveiled cells, blood dendritic cells Lymphoid organs Lymphoiddendritic cells, interdigitating cells Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 273 the organ most readily analyzed, but manyother tissues have been utilized successfully. Typically, dendritic cells do not adhere to tissue culture sur- faces, especially after a day in culture, and they have low levels of Fc receptors (FcR) but high levels of MHCclass-II products. Macrophages in contrast are adherent, with abundant FcR but variable levels of class II. Wereview the isolation and function of dendritic cells in different sites and include newevidence that these antigen-presenting cells capture antigens in situ and movefrom nonlymphoid to lymphoid organs.

Nonlymphoid organs EPIDERMISEpidermal Langerhans cells are the best characterized non- lymphoiddendritic cells. It is feasible to prepare partially enriched (30- 60%)populations in mice, since keratinocytes can be depleted with ~-thy- 1 and complement plus adherence (1, 2). Partially enriched Langerhans ceils suffice in most studies of function and are a better starting material for purification by sorting and panning methods than are unfractionated cells (2, 3). For humanepidermis there is no mAblike c~-thy-1 which binds keratinocytes, but anti-CD 1 mAbselectively identify Langerhanscells (4). Mouseepidermal Langerhans cells, in spite of high levels of MHCclass- II products and detectable FoR are not active antigen-presenting cells for the mixed lymphocyte reaction (MLR)or ~-CD3response. Activity develops after a 1-3 day period of culture in which the cytokine GM-CSF plays a key role (2, 3, 5). GM-CSFdoes not act simply to maintain Langerhans cell viability, since another cytokine, TNF-cachectin, maintains viability without inducing accessory function (6). Structure and phenotype also change in culture. The Langerhans cells enlarge, express more MHCclass-II and adhesion molecules (below), and lose Fc receptors and Birbeck granules. Romani et al (7) and Teunissen et al (8) noted entirely analogous changes during the culture of humanLangerhans cells. by Rockefeller University on 08/15/07. For personal use only. As a result, cultured Langerhans cells fully resemble blood and lymphoid dendritic cells (1, 7, 9, 10). There is evidence that Langerhans cells can leave the skin and move

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org via the afferent lymph to draining lymphoid organs. Cells with Birbeck granules have been noted in lymph (11, 12) and lymph node (13). contact allergens are applied, allergen is found 8-24 hr later on lymphoid dendritic cells (14). Larsen (15) directly monitored the movementof gerhans cells out of the epidermis. Grafting itself leads to movementin either syngeneic or allogeneic hosts. Within a day, the Langerhans cells enlarge and express high levels of MHCclass II. Epidermal Langerhans cells numbers fall to about one third steady state, and the antigen-presenting cells appear in the dermis in what appear to be lymphatics. Similar events occur in organ cultures of skin fragments. Langerhans cells move Annual Reviews www.annualreviews.org/aronline 274 STE1NMAN into the dermis and then the culture medium(15). Kaplan et al suggest that dermal Langerhans cells also arise from the blood in delayed type hypersensitivity reactions (16). HEARa"Fabre, Hart, and McKenzieshowed that the interstitium of most organs, except brain, contain irregularly shaped cells that stain strongly with mAbto MHCclass-II products and the CD45leukocyte antigen (17- 19). Twokinds of resident CD45÷ leukocytes have been identified (20) with a new antimacrophage mAb: macrophages that react with the mAb and are radioresistant, and dendritic cells that do not react with the mAb but are strongly MHCclass II-positive and are radiosensitive. Larsen et al, studying mousecardiac allografts, found that the number of interstitial Ia÷ dendritic cells fell during the first four days after grafting (21). Using mAbspecific to polymorphisms of the heart donor, they saw donor-derived, MHCclass II-rich dendritic cells in the spleen, indicating migration from the heart via the blood stream. These results were con- firmed in a rat limb transplant model (22). L~WrtExcept for an initial report (23), there has been little study isolated dendritic cells. In situ these antigen-presenting cells are in the portal triads, in contrast to sinus-lining phagocytes(Kupffer cells) (18, 20). LtrNGIn rat (24, 25), mouse(26, 27), and human(28, 29), dendritic are selected on the basis of their low buoyant density, and lack of plastic adherence and FcR. The enriched ceils have abundant MHCclass-II and induce strong T cell-mediated immuneresponses. The parenchyma is the source of lung dendritic cells rather than the bronchoalveolar lavage fluid, a standard source of macrophages.Holt et al (30) localized ÷ dendritic cells to alveolar septae, as well as within and just beneath airway epithelium. In tangential sections, the networkof epithelial dendritic cells

by Rockefeller University on 08/15/07. For personal use only. is reminiscent of epidermal Langerhans cells. Macrophagesare juxtaposed to dendritic cells along the airway. After aerosolization of proteins, lung dendritic cells present antigen,

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org since the isolated antigen-presenting cells directly stimulate antigen-specific T-cell lines (24). To detect antigen-presenting cells function, suppressive macrophages must be removed. 6tJa" Pavli et al succeeded in isolating dendritic cells from the lamina propria of mouse intestine (31). Again, depletion of suppressive macrophages helped uncover the presence and function of dendritic cells. The Circulation AFF~R~Na~ LVraPr~ Afferent but not efferent lymph contains leukocytes termed "veiled" cells in rabbit (32), pig (33), rat (34, 35), mouse Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 275 human(37), and sheep (38). Like dendritic cells, veiled cells have a buoyant density, low FcR and phagocytic activity, and high levels of MHC class-II and antigen-presenting cell activity. The origin of veiled cells is unclear. They may come from the interstitium and epithelium of many organs, or from a pool of cells that leaves the blood, moves through nonlymphoidtissues, and enters the lymph. If protein antigens are administered intradermally, afferent lymphdendritic cells carry the antigen in a form that will trigger antigen-specific T- cell lines (38). Therefore, dendritic cells in lymphcan carry antigens lymphoid tissues, as has been proposed for epidermal Langerhans cells. BLOODIn human blood, < 0.1% of the white cells are dendritic cells. Prior enrichment methods fell short of substantial purity, but >90% purity has nowbeen achieved (39). Enrichmententails successive depletion of T cells (sheep red cell rosetting), monocytes(adherence to plastic or coated plates), and B plus NKcells (pelleting in metrizamide). Together, and whenperformed in the above sequence, these steps yield an E rosette negative, nonadherent, low-density fraction with 30--60%dendritic cells. Further purity is obtained by panning or sorting with mAb, especially to CD45RA,that selectively react with the contaminants. Although this isolation approach is labor-intensive, one simultaneously secures populations of other cell types, each depleted of dendritic cells (39). The purity of the populations can be assessed by three approaches with comparable results. One approach uses mAband flow cytometry (39) phenotype the cell fractions. Markersthat dendritic cells lack are CD3(T cells), CD14(monocytes), CD19/20 (B cells), and CD56/57 (NK cells). Abundantproteins are classoI and -II MHCproducts plus a distinct array of receptors and adhesion molecules (see below). The second approach to observe cell fractions live by video microscopy (39). Dendritic cells

by Rockefeller University on 08/15/07. For personal use only. continually extend and retract large lamellipodia or "veils." Noother cell in blood shows this motility. The third approach is to test MLRstimulatory function. The dendritic cells are potent, e.g. in cultures of 105 T cells, 1-3 × 103 allogeneic dendritic cells induce an MLRcomparable to that Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org induced by the standard population of 105 bulk mononuclears. Monocytes and B cells have little or no stimulating activity (21). Dendritic cells in blood may be migrating from nonlymphoidtissues to spleen. Evidence for the latter was recently obtained in a rodent heart transplant model (39). Alternatively, dendritic cells maybe en route from the marrowto nonlymphoid tissues. Lymphoid Organs TONSILSubstantial purification of humantonsil dendritic cells also has been obtained (40). The cells are strong MLRstimulators, and their distinct Annual Reviews www.annualreviews.org/aronline

276 STEINMAN surface composition has been outlined with a panel of mAband immuno- peroxidase labeling. The results are included in the next section.

T-DEPENDENT REGIONS OF PERIPHERAL LYMPHOID ORGANS In sections of the T-dependent regions of lymphoid organs, there are "interdigitating cells" (41) which resemble dendritic cells in cytology and phenotype (42, 43). If dendritic cells from mousespleen (44) are administered into blood or foot pads, Austyn et al noted homing to the T-dependent areas. Fossuminjected radiolabeled afferent lymphdendritic cells into rats. By light and electron microscopy, the injected cells assumedthe location of interdigitating cells (45). Little is knownabout the efficiency of this homing. Possibly the only cells that are retained are those that find T cells specific to the presented antigens. The reason is that the flux of dendritic cells in lymphcan be 105/hr, yet these antigen-presenting cells do not accumulate in lymph node or efferent lymph. Given the substantial turnover of cells in lymph and in spleen (35, 46), most dendritic cells may not live long upon reaching the lymphoid organ. In spleen, dendritic cells are more numerousthan the interdigitating cells of the periarterial sheaths. Whendendritic cells are sorted from fresh spleen suspensions (see below), only a subpopulation label clearly with NLDC145(47), a mAbto interdigitating cells in the central periarterial sheaths (48). In section, nests of dendritic cells lie in the periphery of the T area interrupting the marginal zone of macrophages (49, 50). Both cells and antigens leave the arterial tree in the marginal zone, so in effect, dendritic cells are positioned like "doors" through whichT cells pass upon entry into the periarterial sheaths. Lymphoidorgans may contain two populations: peripheral [?migratory], short-lived dendritic cells, and central, long-lived interdigitating cells. by Rockefeller University on 08/15/07. For personal use only. DENDRITICCELLS FRESHLY ISOLATED FROM MOUSE SPLEEN AS evident above, dendritic cell isolation requires manysteps that can take a day or more. Since viability and function can be influenced by cytokines like GM- Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org CSF, IL-1, and TNF(2, 3, 6, 51, 52), it is important to characterize fresh isolates. This recently was done in mouse spleen using a new mAb,N418 to murine CD1lc (47, 49). Dendritic cells are the main cell expressing high levels of CD1lc in the steady state in spleen. Fresh suspensions contain about 1.5% N418+ cells, while peritoneal washings and blood have few. Two-color immunolabelingshows that splenic N418+ cells have the phenotype of dendritic cells (47). Sorted N418+ cells assume a typical veiled morphologyif cultured for a day. Fresh, sorted N418+ dendritic cells actively stimulate the MLRand present protein antigens to sensitized T cells (47). If proteins are given i.v. Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 277 or i.p., the N418+ fraction is the main source of immunogenwhen spleen fractions are applied to antigen-specific T cells (53).

"r~¥~ctJs Large, bone marrow-derived, class II-rich dendritic cells are found in the medulla (54, 55). These likely are the "interdigitating cells" noted by electron microscopy (56) or the "dendritic cells" in enzyme- digested thymus (57-59). Crowley et al succeeded in enriching mouse thymic dendritic cells to a high degree of purity, using the same approach as was used in spleen (10). Adherent cells with a low buoyant density were cultured overnight so that most dendritic cells dislodged. The dendritic cells were about twice the size as in spleen but similar in phenotype and function. If mgdoses of proteins are given i.v., thymic dendritic cells pick up antigen in a form stimulatory for T cells (58). Summary."a Dendritic Cell Lineage Nonlymphoidorgans, blood, afferent lymph, and lymphoid tissues (see Table 1) contain dendritic cells with different namesbut similar properties. To enrich them, one selects for low buoyant density, nonadherence to plastic especially after a day in culture, and absence of certain markers found on other cell types. These approaches simply deplete other cells but do not positively select for cells that are distinct in shape and have abundant MHCclass II plus strong antigen-presenting cell function. This indicates that the negatively selected dendritic cells are an independent cell type. Evidenceis growingthat dendritic cells in different tissues, as defined morphologically and by a distinct group of cell surface markers, are part of a system connected by movementand homing. Dendritic ceils in nonlymphoidorgans, such as epidermal Langerhanscells and heart interstitial cells, can give rise to "veiled cells" in the afferent lymphand blood which by Rockefeller University on 08/15/07. For personal use only. migrate to lymphoid tissues where they are isolated as "dendritic" or "interdigitating" cells. Coupled with these migratory abilities is the capacity to capture antigens in an immunogenicform in situ.

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org It seemsappropriate to classify the cells in Table I as a separate lineage given their distinct features and tissue distribution. There is no evidence that other white cells convert into dendritic cells or vice versa, even if challenged with a variety of cytokines. For example, macrophage and granulocyte factors, M-CSFand G-CSF, have no known effects on dendritic cells, in contrast to their profound effects on typical phagocytes. Nonetheless, the progenitor for the putative dendritic cell lineage has not been isolated. Dendritic cells in spleen and lymph originate from a proliferating pool of precursors and undergo rapid turnover (35, 46, 60), but the site for proliferation (3H-thymidine uptake) is not known. A bone Annual Reviews www.annualreviews.org/aronline 278 STEINMAN marrowprecursor exists (35, 46, 54, 61), but conditions have not been identified that direct its growthin culture.

THE DENDRITIC CELL SURFACE: SPECIALIZATIONS FOR ANTIGEN PRESENTATION

In mouseand human,the fluorescence activated cell sorter (FACS)now has provideddetailed descriptionsof the dendritic cell surface (10, 39, 47). The results are diagrammed(Figure 1). The surface is distinct among leukocytesin manyrespects and is consistent with their functional capaci- ties (see below), including antigen presentation (abundant surface products), active clustering with T cells (high levels of several adhesins), and weakphagocytosis (low amountsof Fc and complementreceptors). Mostantigens are expressed in a homogeneousfashion on dendritic cells fromdifferent sites, but deviationscan occur. Thesemay represent differences expressed as dendritic cells mature and migrate from peripheral tissues to the T areas. Products of the MHC Consistent with their strong antigen-presenting cell function, dendritic cells express all class-ll MHCgenes at high levels. Both I-A and I-E

MHC Class i q’-t- Class+++ II

CD19-- 29-- 21 --22--40 +Ig -- 1[ CD16--32 + or-- 64 --23 + by Rockefeller University on 08/15/07. For personal use only. Tcell ~1~ /~ ~2Integrins Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org

NK cell ~~ O~her receptors +CD35-M’CSF- CD56--57-- ~"/~~.j CD25 Leukocyte common antigen CD"- 45+45R~ Foure 1. Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 279 are abundant in the mouse, and HLA-DP,DQ, DR likewise in human. Monocyteshave low levels of DPand DQ(39). Dendritic cells have surface invariant chain (CD74), as evidence by FACS(39) and precipitation ~25I-labeled cells with polyclonal antibody (62). Polymorphic class-I products also are abundant (39, 63-65) and can used to monitor the repopulation of dendritic cells in bone marrowchi- meras (66) or movementduring transplantation (22). Nonpolymorphic class I (such as Qa and TI), which maypresent antigens to the ~/6 subset (68), have not been evaluated on dendritic cells. CD1, whichis not encoded in the MHCbut is class I-like and mayact as a restriction element for y/6 T cells (69), is abundant on dendritic cells in skin (CDla) (4, 70) afferent lymph (CDlb) (38). Fc and C3 Receptors (FcR, C3R) Dendritic cells are not actively phagocytic. Consistent with this, there is little or no expression of FcyR (CD16, CD32, CD64) or C3R (CDllb, CD21, CD35), as assessed by staining with mAbor binding of opsonized particles. Yet some exceptions are apparent. In skin, CD32FcTR are noted (1, 71, 72), but their levels fall by 90%when the Langerhans cells are cultured (8, 73, 74). Only freshly isolated Langerhanscells actively process andpresent native proteins (see below); this fact leads to the idea that Fc~Ron fresh Langerhanscells are involved in antigen uptake (75). Sheep lymph dendritic cells have cytophilic Ig and specific antibody enhances antigen presentation (76). Fc~Rhave been studied on epidermal Langerhans cells. Binding of IgE is noted in situ in the skin of atopic patients (77), but the FceR-mediating bind.ing has not been identified. Low-affinity FceRII (CD23)are upregulated with IL-4 and IFN-7 (78). Langerhans cells are implicated in the transport of contact allergens as well (12, 79). by Rockefeller University on 08/15/07. For personal use only. C3bi (CD1lb) receptors typically are present at trace levels with the exception of mouseLangerhans cells where they are more readily detectable (1).

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org Other receptors used by phagocytes to scavenge particulates and micro- organisms have not been studied. Dendritic cells lack mannose-fucose receptors (35), but no studies have been published on other receptors, e.g. for lipoproteins and scavenging. The lack of phagocytic receptors does not meanthat particulate antigens are not processed. It is possible these antigen-presenting cells take up small numbersof particles at defined stages of their natural history, or that particles are processedat the surface. Afferent lymphdendritic cells do not internalize test particles, yet someof the cells have what appear to be phagocytic inclusions (35). The latter perhaps were acquired in the tissues prior to entry into the lymph. Annual Reviews www.annualreviews.org/aronline 280 STEINMAN Integrins and Adhesins These molecules contribute to cell binding and homing,which are both importantfeatures of dendritic cell function. Dendriticcells can havehigh levels of p150/90or CD1 l c but lowCD ~ 1 b (39, 49). CD11 a or LFA-1 found on dendritic cells in mousespleen and humanblood (39, 80) but not skin (8, 73). Dendriticcells also have high levels of other adhesins, ICAM-1(CD54), LFA-3 (CD58), and fl~ integrin (CD29)(7, Differentiation Antigens Manygroups are trying to prepare dendritic-cell specific mAb,but this has proven problematic. This is not for a lack of immunogenicity,since large numbers of mAbto shared markers are obtained following immu- nization with dendritic cells, e.g. mAbto MHCproducts and integrins (49). A groupof three mAbare useful in the mouse(50). 33D1reacts mostdendritic cells in spleen and Peyer’s patch, but not skin and thymic medulla(10). NLDC145reacts with dendritic cells in skin and in the dependentregions of several lymphoidorgans, but also binds to thymic epithelium(48). N418anti-CD1 lc reacts strongly with dendritic cells, but CD1lc can be upregulated on macrophages. Manymolecules are expressedin a cell-type restricted fashion and are useful for distinguishing dendritic cells. For example,dendritic cells in humanblood (39) lack CD3(T cells), CD19-22(B cells), CD13-15 the c-fms receptor for M-CSF(phagocytes), and CD56/57(NK cells). Dendritic cells can express the B cell-activation markerCD40 (7, 39, 40), and expression of CD4and CD8also occurs (10, 47). Someuseful markers that mousedendritic cells lack are CD45RA,thy-1, and certain phagocyte markersthat lack a CDdesignation (SER-4,RB-6) (50).

by Rockefeller University on 08/15/07. For personal use only. PRIMARY RESPONSES INDUCED BY DENDRITIC CELLS: DISTINCT ANTIGEN-PRESENTING CELL REQUIREMENTS FOR THE AFFERENT

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org AND EFFERENT LIMBS OF IMMUNITY Theterm "antigen-presentingcell" does not fully describe the function of dendritic cells. Their distinctive role is to initiate T-dependentresponses from quiescent lymphocytes.Once sensitized, the T lymphoblastsreadily interact with other antigen-presentingcells. In effect, efficient T-cell responses both in vitro (81-83) and in vivo (84) seem to occur in phases. In the afferent limb, dendritic cells are specializedto identify and activate trace clones of antigen-reactiveT cells whilein the efferent limb, the sensitized blasts find other antigen-presentingcells to induceeffector Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 281 functions such as macrophageactivation and IL-1 production, and B-cell antibodyresponses. In this section, several primaryresponses are reviewed, while in the next, aspects of mechanismare considered. The Primary Antibody Response Theneed for spleen adherent cells in the primaryantibody response by B and T lymphocytes(85) led to the discoveryof dendritic cells whichproved to be essential accessories (86). Whenforeign red blood cells are the antigen, dendritic cells stimulatethe productionof helper factors bypassing the needfor intact T cells (87). For haptencarrier conjugates,the needfor dendriticcells is to sensitize carrier-specificT cells whichdirectly interact with hapten-specificB cells as antigen-presentingcells (82). TheB cells becomeresponsive to antigen-nonspecificcytokines, and this all occurs in discrete aggregatesof dendritic, B, andT cells (88, 89). Someuncertainty exists concerningthe relative roles of dendritic cells and B cells as antigen-presentingcells for sensitizing helper cells in situ (90). The lymphnodes of mice that are treated with anti-# from birth cannot be primedwith proteins in adjuvant (91-94). The nodespresumably are depleted of B cells and not dendritic cells. In contrast, in studies of antigen-presentingcells functionfor antibodyresponses, there are in vitro data that mousespleen responses require dendritic cells (82, 88) and vivo findingsthat B cells in chickensare not the initial antigen-presenting cells for responsesto heterologousred cells (95). In a study whereantigen- pulsed antigen-presentingcells have beenused to induceantibody in situ, it has been foundthat dendritic cells inducethe formationof antiviral and antiidiotypic antibodies(96). Primary Responses to Protein Antigens In Situ If spleendendritic cells are exposedto proteinantigens in vitro andinjected by Rockefeller University on 08/15/07. For personal use only. into the foot pads of mice, CD4+ T cells in the draining lymphnode are sensitized (97). Responsivenesspeaks at day 5, wanesby day 9-12, and rapidly induced by rechallenge with antigen-pulseddendritic cells. When

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org spleen cells, which are 60%B lymphocytes,are pulsed with antigen and then comparedto dendritic cells, the spleencells are muchless efficient as antigen-presenting cells in vivo (97). × 106 spleen cells (t he maximum that can be given readily in one doseinto a foot pad) induceweak responses whereas2 × 105 dendritic cells are muchmore active. As such, spleen B cells lack direct sensitizing function at least for bulk exogenousproteins. Peritoneal macrophagesare totally inactive as antigen-presentingcells in this in vivo assay system. Theinjected dendritic cells do not simplycarry antigens to be presented by host antigen-presenting cells. This can be shownby injecting pulsed, Annual Reviews www.annualreviews.org/aronline 282 Sa’Er~MAr~ parentalstrain dendriticcells into F 1 recipients. TheT cells that are primed respondto antigen in the context of MHCclass-II products of the injected dendritic cells (97). This is the first instancein whichantigen-pulsed antigen-presentingcells have primedT cells in an MHC-restrictedfashion in vivo. Viral Antigens The standard assay for studying the generation of antiviral CTLis to prime a mouseby exposure to virus and induce CTLby rechallenge in vitro. Dendritic cells from mice that have been infected with Moloney leukemiavirus are active antigen-presentingcells for inducingvirus-specific CTL(98). Dendriticcells also present Sendaito virus-primedspleen (98). eachcase, dendritic cells are 30-100times moreactive than unfractionated spleen. Macatoniaet al found that dendritic cells serve as antigen-presenting cells for primary CTLresponses in the influenza system (99). A special culture systemwas needed. 105 cells werecultured in 20 #1 drops that hung fromwells of inverted Terasaki plates, a systemwhich may improve gaseous exchange.A puzzling feature is that high levels of CTLactivity developin a primaryfive-day culture. This implies that the frequencyof precursors for flu-specific CTLin naive mice is high. Hengelet al also noted that dendritic cells inducethe formationof CTLto herpes simplexin a primary system(100). Limiting dilution assays indicated that herpes-specific CTL precursors represent 1 in 400 of nylonwool passed, spleen T cells. The Primary Mixed Leukocyte Reaction Thestrong stimulating activity of dendritic cells in the primary mixed leukocytereaction (MLR)is established (101). Small B cells and monocytes trigger little or no responsefrom resting CD4+ T cells (39) but do stimulate by Rockefeller University on 08/15/07. For personal use only. activated T-cell blasts and T-cell clones (102, 103). Certain B blasts and manyB-cell lines are in contrast capableof stimulatingthe mixedleukocyte reaction(80). Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org Several lymphokinesform whendendritic cells stimulate the mixed leukocyte reaction. IL-2 is noted on day 1 (81, 83, 104, 105) and other factors soon thereafter: IL-4, IFN-y, and T cell replacing factors for antibody responses (81, 104, 106). Detailed studies have not yet been publishedthat comparedendritic cells with other antigen-presentingcells for defined TH1and Tn2subsets. Dendritic cells also stimulate an MLRfrom CD8+ T cells, but higher antigen-presenting cell doses are needed(83, 105). Allospecific CTLare detected at day 4-5 using dendritic to T-cell ratios of 1 : 10 or 1 : 30. The Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 283 needfor relatively high dosesof dendritic cells couldmean that the antigen- presenting+ cells are killed during the course of the response. TheCD8 mixedleukocyte reaction is sizeable, with 30-40%of the culture at day 5 being T blasts that express CD25and MHCclass-II activation antigens. Monocytesdo not stimulate resting CD8+ cells but serve as targets in the efferent limbof the response,i.e. for the CTLthat are inducedby dendritic cells. As few as 1 allogeneic dendritic cell per 100 CD4-responders also induces strong NKactivity (K562targets) (105). Theprecursors to these NKcells can be removedselectively prior to the MLRby panning the CD4-population with a-CD1lb mAb(105). That NKand allospecific CD8+ cells are inducedin the absence of CD4+ helpers indicates that IL- 2 and perhaps other cytokines can be generated in substantial amountsin the cultures. COMPONENTSOF DENDRITIC CELL FUNCTION --MECHANISMS OF IMMUNOGENICITY Becausedendritic cells induceprimary responses in vitro and in vivo, they are important for studying immunogenicityin a physiologic context. The limitation is that only small numbersof dendritic cells are available from any tissue, and no cell lines exist. Yet dendritic cell numbersare large relative to their specializedtask, i.e. to identify and activatethe verysmall numbersof antigen-specific T cells that are present at the onset of an immuneresponse. To describe these specializations, I do not use the "signal 1/signal 2" terminology,since the notion of two signals oversimplifiesthe manyfeatures that contribute to dendritic cell function and does not considerthe distinct physiologicproperties of different antigen-presenting cells. Dendriticcell functioncan be consideredin three parts. by Rockefeller University on 08/15/07. For personal use only. Sentinel Function--Processin#and Presentation of Antigen SUMMARYOF RECENT ANTIGEN-PULSING EXPERIMENTS If pulsed with rela-

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org tively low doses of soluble protein (100 #g/ml), dendritic cells stimulate antigen-specific, primedT cells (62, 97). Chloroquineblocks pulsing (62) as it doesin other types of antigen-presentingcells (107, 108), implying need for an acidic intracellular compartment.Several features of antigen- pulsed dendritic cells are apparent: 1. Thenumber of pulsed dendritic cells neededto stimulate T cells is low, about 30-100times less than for mousespleen (62, 75, 97), a standard antigen-presenting cell population which contains about 1%dendritic cells. Annual Reviews www.annualreviews.org/aronline 284 STEINMAN 2. Antigen-pulsed dendritic cells, cultured for 1-2 days without further exposureto antigen, retain activity (62, 97). This could meanthat the levels of MHC-peptidecomplexes are high, the turnover of complexes is slow, or that fewer complexesare needed for presentation by dendritic cells. In contrast, if macrophages are pulsed, the turnover is rapid; the t~/2 of presenting activity is a few hours (109). 3. Antigen-pulseddendritic cells, but not other antigen-presenting cells, can be administered in situ to prime T cells without additional adjuvants (97). Primingis restricted to the draining lymphoidtissue and is direct. F 1 T cells are primedwith parental dendritic cells, sensitization is restricted to the MHCof the parental antigen-presenting cells. 4. The processing activity of dendritic cells is regulated. Only fresh isolates from epidermis (62, 75) and spleen (97) present native proteins. After a day in culture, the dendritic cells handle proteins poorly but are the most potent antigen-presenting cells for stimuli that do not require processing, e.g. peptides, alloMHC,and mitogens. 5. It is not yet possible to detect regurgitation of peptides processed by other antigen-presenting cells onto dendritic cells. Such experiments are possible with cultured dendritic cells which can present pcptides but not proteins. If MHC-mismatchedspleen or peritoneal macrophages (97) Ia- epidermal cells (62) are mixedwith protein plus cultured dendritic cells, antigen-presenting cell function does not occur. If other cells regurgitate immunogenicpeptides, one would expect the dendritic cells to have stimulated the T cells.

MECHANISMS OF ANTIGEN PROCESSING IN DENDRITIC CELLS Several vari- ables are being analyzed to explain the differences between fresh and cultured dendritic cells (see above). Both have high levels of surface MHC class-II products, with cultured Langerhanscells expressing at least five by Rockefeller University on 08/15/07. For personal use only. times more class II than fresh Langerhans cells (9, 110). Whenrhodamine- ovalbumin is used to monitor endocytosis, fresh and cultured Langerhans cells internalize protein into intracellular granules (62). However,it is not

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org yet possible to rigorously quantitate endocytic activity in fresh and cultured Langerhanscells, since these antigen-presenting cells are so weak. Stossel et al recently madethe fascinating observation that freshly isolated Lan- gerhans cells have more numerousacidic vacuoles, presumably endosomes, whereas cultured mouse and humanepidermal Langerhans cells have some acidic lysosomesbut few acidic endocytic vesicles (111). Pure’ et al have identified two more striking differences between fresh and cultured Langerhans cells (62). Fresh Langerhans cells actively synthesize class-II products, while cultured Langerhanscells synthesize other proteins but not class II. By blocking synthesis with cycloheximide, the Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 285 efficiency with whichLangerhans cells can be pulsed with protein antigen is reduced. In addition, fresh but not cultured Langerhanscells express high levels of cytoplasmicinvariant chain. Thelatter enhancespresentation by cell lines transfected with MHCclass-II genes(112). Thedata suggest that the efficient processingof native proteins by fresh Langerhanscells is linked to the synthesis of MHCclass-II products. Perhaps newly syn- thesized molecules,associated with invariant chains, are shuttled to an acidic compartmentwhere there is ready access to peptide antigens. Theability to downregulatethe presentation pathway,including a down- regulation of class-II biosynthesis and a loss of invariant chains, is of uncertain consequenceat this time. Antigen-pulseddendritic cells retain antigen for long periods in an immunogenicform (see above). Perhaps acquiredpeptides wouldbe displacedif antigen processingwere to continue in an unregulated fashion. Continuedantigen processing, whendendritic cells are migrating from nonlymphoidtissues to lymphoidT areas, also wouldallow self-peptides to predominateover exogenousones, and might lead to autoimmunity(see Discussion). Althoughdendritic cells can capture protein antigens, it is striking that the absolute amountof protein that accumulatesintracellularly is small (97). This suggests that while endocytic activity generates MHC-peptide complexes,the relevant levels of processing and presentation are small especially whencompared to the size and activity of the vacuolar system that is used to scavengeand degrade antigens in macrophages.One must distinguish betweenendocytosis that is primarily directed towardantigen scavenging, the hallmark of the macrophage,and the muchlower amounts that are associated with antigen-presentingcells function in dendritic and B cells. It has been knownfor sometime that macrophagesinternalize and completelydegrade at least 10,000protein moleculesper hour at the doses of protein that are used traditionally to pulse antigen-presentingcells. by Rockefeller University on 08/15/07. For personal use only. This scavengingfunction makesendocytosis, but not necessarily antigen processing, easy to visualize in macrophages(113).

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org CAPTUREOF ANTIGENSIN SITUWhen antigens are administered in situ, the dendritic cells can be isolated and shownto be carrying the antigen in an immunogenicform. This is tested by adding in vivo pulsed dendritic cells to specific T cells. T-cell stimulationoccurs with lungdendritic cells after aerosol administration of proteins (24), with afferent lymphsheep dendritic cells after intradermaladministration of protein (38), with draining lymphnode dendritic cells after application of a contact allergen (14), and with thymicdendritic cells after an i.v. bolus of protein (58). If mice are givenforeign proteins i.v. or i.p., dendriticcells seemto be the principal source of immunogenrelative to other cell types (53). Thelatter experi- Annual Reviews www.annualreviews.org/aronline 286 STEINMAN merits used selective depletion of in vivo pulsed antigen-presenting cells with 33D1anti-dendritic cell mAband complement, as well as positive selection with N418 anti-CD1 lc mAband the FACS(47).

THE LEVELS OF ANTIGEN/MHC ON THE DENDRITIC CELL SURFACE The high levels ofclass-I and -II MHCmolecules on dendritic cells could function to carry manydifferent epitopes, as in the presentation of complexinfectious agents or foreign cells, and/or to accumulate large numbersof individual kinds of MHC-peptidecomplexes to better identify and stimulate resting T cells. It is not yet possible to enumeratespecific MHC-peptidecomplexes on dendritic cells. However,Romani et al (73) studied the numberofT cells that bind and respond to presentation of anti-CD3 on defined numbers of Langerhans cells. As few as 250 FcR per Langerhans cell are needed to drive a T cell into cell cycle, and each Langerhanscell on average handles 10-20 T cells. Migratory Functions-- the Binding of Antigen-Specific T Cells

MIGRATIONTO T-DEPENDENTAREAS AS detailed above, dendritic cells can migrate via blood or afferent lymph to the T areas of spleen and lymph node respectively. This places the antigen-charged dendritic cell in the path of recirculating T cells, enhancing the chance that relevant T-cell clones can be selected. The factors that stimulate and direct the movementof dendritic cells in vivo are not clear. Simply grafting skin to isogeneic hosts or placing skin in organ culture induces egress of at least two thirds of the epidermal Langerhanscells (l 5). This suggests that movementis T cell independent. However,homing may be T dependent since dendritic cells are not retained in the spleens of nude mice (44). Dendritic cells express molecules that are

by Rockefeller University on 08/15/07. For personal use only. implicated in leukocyte homing, e.g. f12 integrins and CD44. The migratory properties of dendritic cells do not preclude a role in stimulating immunity locally. This may occur in secondary responses, where dendritic cells are found in association with T cells at delayed type Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org hypersensitivity sites (16). However,in certain primary responses, like sensitization to skin transplants (114) and contact allergens (115), afferent lymphatics need to be intact--a point consistent with the idea that a flux of dendritic cells is critical to immunogenicity.

CLUSTERING OF ANTIGEN-SPECIFIC T CELLS A feature that helps explain dendritic cell function is the capacity to form stable clusters with antigen- specific T cells during primary responses. Other antigen-presenting cells do not aggregate resting T cells but do bind sensitized T blasts (81, 82, 103, 116). Annual Reviews www.annualreviews.org/aronline DENDRITIC CELLS 287 In the living state, thin sheets or lamellipodia extend from the dendritic cell body in manydirections (33, 39, 117). The processes extend, retract, and bend as if the antigen-presenting cells are probing the environment around them. This motility, which is distinct amongwhite cells, likely enables the dendritic cell to survey T cells and select antigen-specific ones. Twoexamples indicate that the initial T-cell sampling occurs by an antigen-independent mechanism.First, dendritic cells bind to T blasts that are directed to specificities different from that present on the antigen- presenting cells (116). Second, whencultured in close proximity, dendritic cells bind resting T cells in large numbers, too manyfor all to be antigen- specific (118). In both instances, bound T cells do not produce IL-2 unless another stimulus like a mitogen is added. Antigen-independent binding could provide a temporary state of antigen-presenting cell T-cell contact, which may be essential for the appropriate TCR-antigen-MHCinteraction to occur. The molecules that mediate antigen-independent clustering have yet to be identified. As mentioned, adhesins like ICAM-1and LFA-3are abundant on dendritic cells (39, 119). To date, mAbto//2 integrins have not blocked heterotypic clustering with T cells in the mixedleukocyte reaction (49, 80, 120), although ~-CD18mAb does block when the stimulus sodium periodate (121). Other adhesins are under study, including CD2 and CD28. Adjuvant Function-- T-Cell Activation IL-I ANDDENDRITIC CELL FUNCTION For years the main hypothesis for the mechanism of T-cell activation has been that macrophage-derivcd IL-1 induces IL-2 release or responsiveness. Dendritic cells and B cells are active antigen-presenting cells but are not knownto makeIL-1 (122-125), except for a report of IL-1 in freshly isolated epidermal Langerhanscells (126).

by Rockefeller University on 08/15/07. For personal use only. It remains to be shown that a response of primary T cells, rather than chronically stimulated cell lines, can be blocked with antibodies to IL-1 or IL-1 receptors. For example, T lymphoblasts can engage macrophages

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org and induce IL-1 production (102, 103), but the T-cell proliferation that occurs in these cultures is insensitive to neutralizing anti-IL-1 (103). Attempts have been madeto identify other activating factors that might be produced under dendritic cell control. These have yet to be found. Inaba et al placed dendritic-T-cell clusters on one side of a millipore filter, and T cells exposed to u-CD3on the other side. IL-2 was produced by the clusters and crossed the filter, but no activation occurred even though the TCRcomplex was occupied by ~-CD3(118). This suggests that the signals provided by dendritic cells for T-cell activation are either active over short distances or remain membranebound. Annual Reviews www.annualreviews.org/aronline 288 STEINMAN

CYTOKINES AMPLIFY THE FUNCTION OF DENDRITIC CELLS While cytokine production by dendritic cells is not yet well documented, cytokines can enhance T cell-mediated immunity by amplifying the viability and function of these antigen-presenting cells. Interestingly, each active cytokine has distinct effects. Cachectin-TNFmaintains Langerhans cell viability but does not enhance sensitizing function (6). IL-l enhances the function skin, spleen, and thymic dendritic cells but does not increase viability (3, 51,127). GM-CSFenhances both viability and function (2, 3, 52, 128). As yet no cytokine has been identified that upregulates the level of dendritic cell MHCproducts. Lymphokines like IFN-~ and IL-4 induce class-II products on macrophages and B cells, respectively, but do not similarly increase expression on dendritic cells. Specific cytokine receptors have not been identified on dendritic cells except for the low affinity, p55, IL-2 receptor chain (CD25),first detected on mouseepidermal Langerhans cells (129). CD25has since been found dendritic cells from manysites--mouse spleen (10), rat afferent lymph (52), humanblood (39)--but the function on these antigen-presenting cells is not known. By influencing dendritic cell function, cytokines mayplay a pivotal role in immunogenicity, e.g. in skin, GM-CSFrelease by keratinocytes (2) may mobilize stimulatory dendritic cells. The phenotype of dendritic cells in blood, lymph, and lymphoidorgans is that of a cell that has been activated by cytokines, since leukocytes commonly upregulate MHCproducts, adhesins, and IL-2 receptors upon activation. All of these molecules are expressed at high levcls on dendritic cells (see above).

SURFACE MOLECULES CONTRIBUTING TO DENDRITIC CELL FUNCTION Many ligand receptor systems, such as LFA-1/ICAMand LFA-3/CD2may contribute both to cell-cell adhesion and to signalling (130). Studies are under- by Rockefeller University on 08/15/07. For personal use only. wayto test the effects ofmAbto these molecules on dendritic cell function, particularly in clusters of antigen-presenting cells and T cells that are the microenvironment for manyresponses in vitro. Because of the difficulty

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org encountered in identifying secreted lymphocyteactivating factors (above), it is important to search for specific dendritic cell surface molecules that contribute to T-cell signalling. However,it is possible that dendritic cells use surface signalling molecules that can be expressed by manycell types, such as LFA-3 and ICAM-1, but that the key feature of these antigen- presenting cells is the capacity to upregulate manyadhesion/signalling systems in a T-independentfashion at the site of antigen deposition. Recent studies of humanLangerhans cells indicate that dendritic cells rapidly upregulate adhesion molecules in vitro, in the apparent absence of T cells (7, 8). These maturation events, coupled with the other specializations (see Annual Reviews www.annualreviews.org/aronline

DENDRITIC CELLS 289 above) in antigen handling and migration, help explain the immunogenicity of antigen-pulsed dendritic cells.

CLONALEXPANSION OF T CELLSFor many mitogens, such as lectins and anti-CD3 mAb, the accessory function of macrophages compares with that of dendritic cells. Mitogens maymediate macrophage-Tcell clustering, which is not observed in primary antigen-specific responses. However, the T-cell function being measured is the onset of DNAsynthesis, not long-term clonal expansion. Whenantigen-presenting cell requirements for the latter were addressed (131), dendritic cells proved to be remarkably more efficient than other antigen-presenting cells. A thousand dendritic cells mediate the optimal cloning efficiency of single T cells in the presence of lectin and exogenousIL-2. Someclones develop with just one dendritic cell, while few clones develop with up to 104 monocytes.In effect, dendritic cells are able to induce long-term T-cell responsiveness to growth factors like IL-2. Using dendritic cells as immunoadsorbents,Pancholi et al cloned T cells that had been bound in clusters (132). Macrophages had to be removed to enhance cluster formation. Antigen was needed to select but not expand the clones. These findings, plus those showing antigen capture in vivo, suggest a use for dendritic cells to select clones that are specific for physio- logically relevant antigens on antigen-presenting cells in vivo.

DISCUSSION

This review has emphasized dendritic cells and immunogenicity. These antigen-presenting cells are in essence nature’s adjuvant and allow one to study responses to specific antigens both in tissue culture and in whole by Rockefeller University on 08/15/07. For personal use only. animal models. The data discussed above on distinct antigen handling functions, the lack of IL-1 production, the ability to migrate and form stable contacts with T cells, the afferent and efferent limbs of antigen- Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org presenting cells function, all have becomeapparent via direct analyses of this trace but specialized subset of antigen-presenting cells. Given their role in presentation of antigens on self and allogeneic MHC molecules,it will be important to test the contribution of dendritic cells to self-tolerance. Metzinger & Guerder found that dendritic cells tolerized cells in organ cultures of fetal mousethymus (133). Inaba et al obtained data in an Mls system that neonatal tolerance can be induced with dendritic cells, but it is the result of clonal anergy not deletion (134). Fairchild Austyn worked out methods for placing dendritic cells within the medulla of thymic organ cultures (135); these methods should be useful for future Annual Reviews www.annualreviews.org/aronline

290 STEINMAN studies. The capacity of dendritic cells to induce suppressor cells against self-reactivity has not been tested. Given the likelihood that manyself-reactive T cells are not tolerized (136), one must consider that the avoidance of self can also reflect the extent to which a given protein is captured and presented by dendritic cells, and the extent to which dendritic cells are activated and mobilized by cytokines or other factors. In other words, tolerance in the adult operates not only at the level of the T cell but also at the level of the dendritic cell. For manyantigens, the distinction is not between foreign and self, but in the extent to which a given protein, foreign or self, is captured and presented by stimulatory dendritic cells.

ACKNOWLEDGMENTS This work has been supported by grants AI13013, AI24540, and AI24775 from the National Institutes of Health.

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DENDRITIC CELLS 291 J., Baer, R. L., Rosenthal, S. A., Bere- macrophages isolated from various rat zowsky, V. 1976. Antigen-bearing Lan- tissues- J. Exp. Med. 156; 1-19 gerhans cells in skin, dermal lymphatics 24. Holt, P. G., Schon-I-Iegrad, M. A., and in lymph nodes. Cell. ImmunoL25: Ofiver, J. 1987. MHCclass II antigen- 137-51 bearing dendritic cells in pulmonarytis- 13. Hoefsmit, E. C. M., Duijvestijn, A. M., sues of the rat. Regulation of antigen Kamperdijk, W. A. 1982. Relation presentation activity by endogenous between Langerhanscells, veiled cells, macrophage populations. J. Exp. Med. and interdigitating cells. Immuno- 167:26~74 biology 161:255 65 25. Rochester, C. L., Goodell, E. M., Stop 14. Macatonia, S. E., Knight, S. C., tent~org, J. K., Bowers, W. E. 1988. Edwards, A. J., Griffiths, S., Fryer, P. Dendritic cells from rat lung are potent 1987. Localization of antigen on lymph accessory cells. Am. Rev. Respir. 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Med. 165:763 Weissler, J. C., Lyons, C. R., Alberton, 76 J., Toews, G. B. 1987. Mononuclear 17. Hart, D.N.J.,Fabre, J.W. 1981.Dem- cells in humanlung parenchyma: char- onstration and characterization of la- acterization of a potent accessory cell positive dendritic cells in the interstitial not obtained by bronchoalveolar connective tissues of rat heart and other lavage. Am. Rev. Respir. Dis. 136: 818- tissues, but not brain. J. Exp. Med.154: 23 347-61 29. Nicod, L. P., Lipscomb, M. F., 18. Prickett, T. C. R., McKenzie, J, L., Weissler, J. C., Toews, G. B. 1989. Hart, D. N. J. 1988. Characterization Mononuclear cells from human lung of interstitial dendritic cells in human parenchy~na support antigen-induced liver. Transplantation A,6:754-61 T lymphocyte proliferation. J. Leuk. 19. Hart, D. N. J., McKenzie, J. L. 1990. Biol. 45: 336~A Interstitial dendritic cells. Int. Rev. 30, Holt, P. G., Schon-Hegrad, M. A,, Immunol. 6:128-49 MeMenarnin, P. G. 1990. Dendritic 20. Spencer, S. C., Fabre, J. W. 1990. Char- cells in the respiratory tract. Int. Rev. acterization of the tissue macrophage Immunol. 6:139-49 by Rockefeller University on 08/15/07. For personal use only. and the interstitial dendritic cell as dis- 31. Pavli, P., Woodhams,C. E., Doe, W. tinct leukocytes normally resident in F., Hume, D. A. 1990, Isolation and the connective tissue of rat heart. J. characterization of antigen-presenting Exp. Med. 171:1841-51 dendritic cells from the mouse intes- 21. Larsen, C. P., Morris, P. J., Austyn, J. tinal lamina propria. Immunology 70: Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org M. 1990. Migration of dendritic leu- 4O-47 kocytes from cardiac allografts into 32. Knight, S. C., Balfour, B. M., O’Brien, host spleens: a novel pathway for J., Buttifant, L., Sumerska,T., Clark, J. initiation of rejection. J. Exp. Med. 1982. Role of veiled cells in lymphocyte 171:307-14 activation. Eur, J. Immunol. 12: 105% 22. Codner, M. A., Shuster, B. A., Stein- 6O man, R. M., Harper, A. D., LaTrenta, 33. Drexhage, H. A., Mullink, H., de G. S., Hoffman, L. A. 1990. Migration Grout, J., Clarke, J., Balfour, B, M. of donor leukocytes from limb allo- 1979. A study of cells present in pe- grafts i~ato host lymphoidtissues. Ann. ripheral lymphof pigs with special ref- Plastic Surg. 25:353-59 erence to a type of cell resembling the 23. Klinkert, W. E. F., Labadie, J. H., Langerhans cells. Cell Tiss. Res. 202: Bowers, W. E. 1982. Accessory and stim- 4O7-30 ulating properties of dendritic cells and 34. Mason, D. W., Pugh, C. W., Webb, M. Annual Reviews www.annualreviews.org/aronline 292 STEINMAN 1981. The rat mixed lymphocyte reac- Z, A. 1974. Identification of a novel cell tion: roles of a dendritic cell in intes- type in peripheral lymphoid organs of tinal lymphand T cell subsets defined mice. III. Functional properties in vivo. by monoclonal antibodies. Immu- J. Exp. Med. 139:143145 nology 44:75-87 47. Crowley, M. T., Inaba, K., Witmer- 35. Pugh, C. W., MacPherson, G. G, Pack, M. D., Gezelter, S., Steinman, Steer, H. W. 1983. Characterization of R. M. 1990. Use of the fluorescence nonlymphoid cells derived from rat activated cell sorter to enrich dendritic peripheral lymph. J. Exp. Med. 157: cells from mouse spleen. J. Immunol. 1758 79 Methods. In press 36. Rhodes, J. M., Agger, R. 1987. Com- 48. Kraal, G,, Breel, M., Janse, M., Bruin, parison of membrane antigens of G. 1986. Langerhanscells, veiled cells, mouse dendritic cell types. Immunol. and interdigitating cells in the mouse Lett. 16:107-12 recognized by a monoelonal antibody. 37. Spry, C. J. F., Pflug, A. J., Janossy, G., J. Exp. Med. 163:981-97 Humphrey, J. H. 1980. Large mono- 49. Metlay, J. P., Witmer-Pack, M. D., nuclear (veiled) cells with "Ia-like" Agger, R., Crowley, M. T., Lawless, membrane antigens in human afferent D., Steinman, R. M. 1990. The distinct lymph. Clin. Exp. Immunol. 39:750 leukocyte integrins of mouse spleen 38. Bujdoso, R., Hopkins, J., Dutia, B. M., dendritic cells as identified with new Young, P., McConnell, I. 1989. Char- hamster monoclonal antibodies. J. acterization of sheep afferent lymph Exp. Med. 171:1753-72 dendritic cells and their role in antigen 50. Agger, R., Crowley, M. T., Witmer- carriage. J. Exp. Med. 170:1285-1302 Pack, M. D. 1990. The surface of den- 39. Freudenthal, P., Steinman, R. M. 1990. dritic cells in the mouseas studied with Surface antigens and motility of human monoclonal antibodies. Int. Rev. dendritic cells enriched from blood by Immtmol. 6:89-101 an improved technique. Proc. Natl. 51. Koide, S. L., Inaba, K., Steinman, R. Acad. Sci. USA. In press M. 1987. Interleukin-1 enhances T- 40. Hart, D. N., McKenzie,J. L. 1988. Iso- dependent immuneresponses by ampli- lation and characterization of human fying the function of dendritic cells. J. tonsil dendritic cells. J. Exp. Med.168: Exp. Med. 165:515 30 157 70 52. MacPherson, G. G., Fossum, S., Har- 41. Veldman, J. E., Kaiserling, E. 1980. rison, B. 1989. Properties of lymph- Interdigitating cells. In The Reticulo- borne (veiled) dendritic cells in culture endothelial System, Morphology, edited II. Expressionof the IL-2 receptor: role by I. Carr and W. T. Daems, p. 381- of GM-CSF. Immunology 68:108 13 416. New York: Plenum 53. Crowley, M., Inaba, K., Steinman, R. 42. Witmer, M. D., Steinman, R. M. 1984. M. 1990. Dendritic cells are the prin- The anatomy of peripheral lymphoid cipal cell in mouse spleen bearing organs with emphasis on accessory immunogenicfragments of foreign pro- cells: light microscopic, immuno- teins. J. Exp. Med. 172:383-86 cytochemical studies of mouse spleen, 54. Barclay, A. N., Mayrhofer, G. 1981. lymph node and Peyer’s patch. Am. Bone marroworigin of Ia-positive cells by Rockefeller University on 08/15/07. For personal use only. J. Anat. 170:465-81 in the medulla of rat thymus. J. Exp. 43. Dijkstra, C. D. 1982. Characteriza- Med. 153:1666-71 tion of nonlymphoidcells in rat spleen, 55. Guillemot, F. P., Oliver, P. D., Peault, with special reference to strongly Ia- B. M., LeDourain, N. M. 1984. Cells positive branchedcells in T-cell areas. expressing Ia antigen in the avian Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org J. Reticuloendothelial Soc. 32:167-78 thymus. J. Exp. Med. 160:1803-19 44. Austyn, J. M., Kupiec-Weglinski, J. 56. Kaiserling, E., Stein, H., Mueller-Her- W., Hankins, D. F., Morris, P. J. 1988. melink, H. K. 1974. Interdigitating Migration patterns of dendritic cells in reticulum cells in the human thymus. the mouse. Homingto T cell-dependent Cell Tiss. Res. 155:4~55 areas of spleen, and binding within 57. Duijvestijn, A. M., Schutte, R., Kohler, marginal zone. Z Exp. Med. 167: 646- Y. G., Korn, C., Hoefsmit, E. C. M. 51 1983. Characterization of the popu- 45. Fossum, S. 1989. 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DENDRITIC CELLS 293 entation of circulating non-MHCanti- CD4-CD8-cytolytic T lymphocytes. gens by medullary dendritic cells. An ~Vature 341: 447 antigen-dependent microenvironment 70. Ray, A., Schmitt, D., Dezutter-Dam- for T cell differentiation. J. Exp. Med. buyant, C., Fargier, M.-C., Thivolet, J. 163:231~,6 1989. Reappearance of CDla antigenic 59. 0liver, P. D., Le Dourain, N. M. 1984. sites after endocytosis on humanLan- Avian thymic accessory cells. J. Immu- gerhans cells evidenced by immunogold nol. 132:1748-55 relabeling. J. Invest. Dermatol. 92:217 60. Fossum, S. 1989. The life history of 24 dendritic leukocytes [DL]. In Current 71. Tamaki, K., Stingl, G., Gullino, M., Topics in Pathology, O. H. Ivessen, pp. Sachs, D. H., Katz, S. I. 1979. Ia anti- 101-24. Berlin: Springer-Verlag gens in mouse skin are predominantly 61. Katz, S. I., Tamaki, K., Sachs, D. H. expressed on Langerhans cells. J. 1979. Epidermal Langerhans cells are Immunol. 123:784-87 derived from cells originating in bone 72. Haines, K. A., Flotte, T. J., Springer, marrow. Nature 282:324-26 T. A., Gigli, I., Thorbecke,13. J. 1983. 62. Pure’, E., Inaba, K., Crowley, M. T., Staining of Langerhans cells with Tardelli, L., Witmer-Pack, M. D., monoclonal antibodies to macrophages Ruberti, G., Fathman, G., Steinman, and lymphoid cells. Proc. Natl. Acad. R. M. 1990. Antigen processing by epi- Sci. USA 80:3448-51 dermal Langerhanscells correlates with 73. Romani, N., Inaba, K., Witmer-Pack, the level of biosynthesis of MHCclass M., Crowley, M., Pure’, E., Steinman, II molecules and expression of R. M. 1989. A small number of anti- invariant chain. J. Exp. Med. 172:1459 CD3molecules on dendritic cells stimu- 63. Boog, C. J. P., Boes, J., Melief, C. J. late DNAsynthesis in mouse T lym- M. 1988. Role of dendritic cells in the phocytes. J. Exp. Med. 169:1153-68 regulation of class I restricted cytotoxic 74. Schmitt, D. A., Hanau, D., Bieber, T., T lymphocyte responses. J. Immunol. Dezutter-Dambuyant, C., Schmitt, D., 140:3331-37 Fabre, M., Pauly, G., Cazenave, J.-P. 64. Nussenzweig, M. C., Steinman, R. M., 1990. Human epidermal Langerhans Unkeless, J. C., Witmer, M. D., Gut- cells express only the 40-kilodalton Fc chinov, B., Cohn, Z. A. 1981. Studies gammareceptor [FcRII]. J. Imrnunol. of the cell surface of mouse dendritic 144:4284-90 cells and other leukocytes. J. Exp. Med. 75. Romani, N., Koide, S., Crowley, M., 154:168-87 Witmer-Pack, M., Livingstone, A. M., 65. Steinman, R. M., Kaplan, G., Witmer, Fathman, C. G., Inaba, K., Steinman, M. D., Cohn, Z. A. 1979. Identification R. M. 1989. Presentation of exogenous of a novel cell-type in peripheral lymph- protein antigens by dendritic cells to T oid organs of mice. V. Purification of cell clones: intact protein is presented spleen dendritic cells, newsurface mar- best by immature, epidermal Lan- kers, and maintenancein vitro. J. Exp. gerhans cells. J. Exp. Med. 169:1169- Med. 149:1-16 78 66. Lenz, A., Heufler, C., Rammensee,H.- 76. Bujdoso, R., Harkiss, G., Hopkins, J.,

by Rockefeller University on 08/15/07. For personal use only. G., Glassl, H., Koch, F., Romani, N., McConnell, I. 1990. Afferent lymph Schuler, G. 1989. Murine epidermal dendritic cells: A model for antigen Langerhans cells express significant capture and presentation in vivo. Int. amounts of class I major histo- Rev. Immunol. 6:177-86 compatibility complex antigens. Proc. 77. Bieber, T., Dannenberg, B., Prinz, J.

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org Natl. Acad. Sci. USA86:7527-31 C., Rieber, E. P., Stolz, W., Braun- 67. Suthanthiran, M. 1990. A novel model Falco, O., Ring, J. 1989. Occurrence of for antigen-dependent activation of IgE bearing epidermal Langerhans cells normal human T cells. J. Exp. Med. in atopic eczema: A study of the time 171:1965-80 course of the lesions and with regard to 68. Bonneville, M., Ishida, I., Itohara, S., the IgE serumlevel. J. Invest. Dermatol. ¯ Verbeek, S., Berns, A., Kanagawa, O., 93:215-19 Haas, W., Tonegawa,S. 1990. Self-tol- 78. Bieber, T., Rieger, A., Neuchrist, C., erance to transgenic gammadelta T Prinz, J. C., Rieber, E. P., Boltz-Nitu- cells by intrathymic inactivation. lescu, G., Scheiner, O., Kraft, D., Ring, Nature 344:163~55 J., Stingl, G. 1989. Induction of 69. Porcelli, S., Brenner, M.B., Greenstein, FcR/CD23 on human epidermal Lan- J. L., Balk, S. P., Terhorst, C., Bleicher, gerhans cells by human recombinant P. A. 1989. Recognition of cluster of interleukin-4 and interferon-gamma. J. differentiation I antigens by human Exp. Med. 170:309-14 Annual Reviews www.annualreviews.org/aronline 294 STEINMAN 79. Shelley, W.B., Juhlin, L. 1976. Lan- induction of antigen-reactive pro- gerhanscells forma reticuloepithelial liferating T cells in B cell-deprived trap for external contact allergens. mice. Eur. J. Immunol.11:964-68 Nature261 : 46-47 92. Janeway,C. A. Jr., Ron, J., Katz, M. 80. Metlay,J. P., Pure’, E., Steinman,R. E. 1987.The B cell is the initiating anti- M.1989. Distinct features of dendritic gen-presentingcell in peripheral lymph cells and anti-immunoglobulinacti- nodes. J. Immunol.138:1051-55 vated B cells as stimulatorsof the pri- 93. Kurt-Jones, E. A., Llano, D., Hay marymixed leukocyte reaction. J. Exp. Glass, K. A., Benacerraf, B., Sy, Med. 169:239-54 M.-S., Abbas,A. K. 1988. The role of 81. Inaba, K., Steinman,R. M. 1984.Rest- an.tigen-presen.ting Bcells in T cell ing and sensitized T lymphocytesexhi- priming in vlvo. Studies of B cell- bit distinct stimulatory (antigen-pre- deficient mice. J. Immunol.140: 3773- senting cell) requirementsfor growth 76 and lymphokinerelease. J. Exp. Med. 94. Ron,Y., Sprent, J. 1989.T cell priming 160:1717-35 in vivo: a majorrole for B cells in pre- 82. Inaba, K., Steinman,R. M. 1985. Pro- senting antigen to T cells in lymph tein-specific helper T lymphocytefor- nodes. J. Imrnunol.138:2848-56 mationinitiated by dendritic cells. Sci- 95. Lassila, O., Vainio, O., Matzinger,P. ence 229:475-79 1988.Can B cells turn on virgin T cells? 83. Inaba, K., Young,J. W., Steinman,R. Nature 334:253-55 M. 1987. Direct activation of CD8+ 96. Francotte, M., Urbain, J. 1985. cytotoxic T lymphocytesby dendritic Enhancementof antibody responses by cells. J. Exp. Med.166:182-94 mouse dendritic cells pulsed with 84. Inaba, K., Metlay, J. P., Crowley,M. tobacco mosaic virus or with rabbit T., Witmer-Pack,M., Steinman,R. M. antiidiotypic antibodies raised against 1990. Dendritic cells as antigen pre- a private rabbit idiotype. Proc. Natl. sentingcells in vivo. Int. Rev. lmmunol. Acad. Sci. USA82:8149-52 6:197-206 97. Inaba, K., Metlay, J. P., Crowley,M. 85. Mosier, D. E. 1967. A requirementfor T., Steinman, R. M. 1990. Dendritic two cell types for antibodyformation cells pulsed with protein antigens in in vitro. Science158:1573-75 vitro can primeantigen-specific, MHCo 86. Inaba, K., Steinman, R. M., Van restricted T cells in situ. J. Exp.Med. Voorhis, W. C., Muramatsu,S. 1983. 172:631-40 Dendritic cells are critical accessory 98. Kast, W. M., Boog, C. J. P., Roep, B. cells for thymus-dependentantibody O., Voordouw,A. C., Melief, C. J. M. responses in mouse and man. Proc. 1988. Failure or success in the res- Natl. Acad. Sci. USA80:6041-45 toration of virus-specific cytotoxic T 87. Inaba, K., Granelli-Piperno,A., Stein- lymphocyteresponse defects of den- man,R. M.1983. Dendriticcells induce dritic cells. J. Immunol.140:3186-93 T lymphocytesto release B cell stimu- 99. Macatonia, S. E., Taylor, P. M., lating factors by an interleukin 2 Knight, S. D., Askonas,B. A. 1989. dependent mechanism. J. Exp. Med. Primarystimulation by dendritic cells

by Rockefeller University on 08/15/07. For personal use only. 158:2040-57 induces anti-viral proliferative and 88. Inaba, K., Witmer, M. D., Steinman, cytotoxic T cell responsesin vitro. J. R. M. 1984. Clustering of dendritic Exp. Med. 169:1255~54 cells, helper T lymphocytes,and histo- 100. Hengel, H., Lindner, M., Wagner,H., compatible B cells, during primary Heeg, K. 1987. Frequency of herpes

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DENDRITIC CELLS 295 103. Bhardwaj, N., Lau, L. U, Friedman, Med. 128:197-221 S. M., Crow,M. K., Steinman, R. M. 115. Frey, J. R., Wenk,P. 1957. Exper- 1989. Interleukin-1 productionduring imental studies on the pathogenesisof accessory cell-dependent mitogenesis contact eczemain the guinea pig. Int. of T lymphocytes. J. Exp. Med.169: Arch. Allergy Appl. lmmunol.11: 81- 1121-36 100 104. Freudenthal, P., Bhardwaj,N. 1990. 116. Inaba, K., Steinman, R. M. 1986. Dendritic cells in humanblood and Accessory cell-T lymphocyteinter- synovial exudates. Int. Rev. Immunol. actions: antigen dependentand inde- 6:103-116 pendentclustering. J. Exp. Med.163: 105. Young,J. W., Steinman, R. M. 1990. 247-61 Dendritic cells stimulate primary 117. Steinman, R. M., Cohn, Z. A. 1973. humancytolytic lymphocyteresponses Identification of a novel cell type in in the absenceof CD4+helper T cells. peripheral lymphoidorgans of mice. I. J. Exp. Med. 171:1315-30 Morphology,quantitation, tissue dis- 106. Pure’, E., Inaba, K., Metlay,J. 1988. tribution. J. Exp. Med.137:1142-62 Lymphokineproduction by murine T 118. Inaba, K., Romani,N., Steinman, R. cells in the mixedleukocyte reaction. J. M. 1989. An antigen-independentcon- Exp. Med. 168:795-800 tact mechanismas an early step in T- 107. Ziegler, H. K., Unanue,E. R. 1982. cell proliferativeresponses to dendritic Decreasein macrophageantigen catab- cell. J. Exp. Med.170:527-42 olism caused by ammoniaand chloro- 119. Landry, D., Doyon,L., Poudrier, J., quine is associated with inhibition of Lafontaine, M, Pelletier, M, Mont- antigenpresentation T cells. Proc.Natl. plaisir, S. 1990. Accessoryfunction Acad. Sei. USA79:175-78 of humanthymic dendritic cells in a 108. Lanzavecchia, A. 1985. Antigen-spe- con A-inducedproliferation of auto- cific interaction betweenT and B cells. logous thymocytesubsets. J. Immunol. Nature 314:537-39 144:836-44 109. Harding, C. V., Roof, R. W., Unanue, 120. Inaba, K., Steinman,R. M.1987. Mono- E. R. 1990. Turnover of Ia-peptide clonal antibodies to LFA-1and to complexesis facilitated in viable anti- CD4inhibit the mixedleukocyte reac- gen-presentingcells: Biosyntheticturn- tion after the antigen-dependentclus- over of Ia vs. peptide exchange,Proc. tering of dendritic cells and T lym- Natl. Acad. Sei. USA86:4230-34 phocytes. J. Exp. Med.165:1403-17 110. Shimada, S., Caughman,S. W., Shar- 121. Austyn,J. M., Morris,P. J. 1988.T cell row, S. O., Stephany,D., Katz, S. I. activation by dendritic cells: CD18- 1987. Enhancedantigen-presenting dependentclustering is not sufficient for capacity of cultured Langerhanscells mitogenesis. Immunology63:537-43 is associated with markedlyincreased 122. Koide, S. L., Steinman, R. M. 1987. expression of Ia antigen. J. Immunol. Induction of murine interleukin-l: 139:2551-55 stimuli and responsive primarycells. 111. Sttssel, H., Koch, F., K~impgen,E., Proe. Nail. Aead. Sei. USA84: 3802- Sttger, P., Lenz, A., Heufler, C., 16

by Rockefeller University on 08/15/07. For personal use only. Romani,N., Schuler, G. 1990. Dis- 123. Bhardwaj,N., Lau, L., Rivelis, M., appearanceof certain acidic organelles Steinman, R. M. 1988. Interleukin-1 [endosomesand Langerhanscell gran- production by mononuclearcells from ules! accompaniesloss of antigen pro- rheumatoidsynovial effusions. Cell. cessing capacity uponculture of epi- Immunol.114; 405-23

Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org dermal Langerhanscells. J. Exp. Med. 124. McKenzie,J. L., Prickett, T. C. R., 172:1471-82 Hart, D. N. J. 1989. Humandendritic 112. Stoekinger,B., Pessara,U., Lin, R. H., ceils stimulateallogeneic T cells in the Habicht, J., Grez, M., Koch,N. 1989. absence of IL-1. Immunology67: 290- A role of In-associatedinvariant chains 97 in antigenprocessing and presentation. 125. Vakkila, J., Sihvola, M., Hurme,M. Cell 56:683-89 1990. Humanperipheral blood-derived 113. Steinman, R. M., Cohn, Z. A. 1972. dendritic cells do not produce inter- The uptake, distribution and fate of lcukin-1 alpha, interleukin-1 beta, or soluble horseradish peroxidase in interleukin-6. Seand. J. Immunol.31: mouseperitoneal macrophagesin vitro. 345-52 J. Cell Biol. 55:186-204 126. Sauder, D. N., Dinarello, C. A., 114. Barker, C. F., Billingham,R. E. 1968. Morhenn,V. B. 1984. Langerhanscell The role of afferent lymphaticsin the productionof interleukin-1. J. Invest. rejection of skin homografts.J. Exp. DermatoL82:605-7 Annual Reviews www.annualreviews.org/aronline

296 STEINMAN 127. lnaba, K., Witmer-Pack,M. D., Inaba, Clonal expansion of human T lym- M., Muramatsu,S., Steinman, R. M. phocytesinitiated by dendritic cells. J. 1988. The function of Ia÷ dendritic Exp. Med. 169:315-20 cells, and Ia- dendritic cell precursors, 132. Pancholi, P., Steinman,R. M., Bhard- in thymocytemitogenesis to lectin and waj, N. 1990.Dendritic cell-T cell clus- lectin plus IL-1. J. Exp.Med. 167: 149- ters are an enrichedsource for selecting 62 antigen specific CD4+ T cell clones. 128. Naito, K., Inaba, K., Hirayama,Y., Submitted Inaba-Miyama,M., Sudo, T., Mura- 133. Matzinger,P., Guerder, S. 1989. Does matsu, S. 1989. Macrophagefactors T-cell tolerance require a dedicated which enhance the mixed leukocyte antigen-presenting cell? Nature 338: reactioninitiated by dendritic cells: J. 74-76 Immunol. 142:1834-39 134. Inaba, M., Inaba, K., Hosono, M., 129. Steiner, G., Tschachler, E., Tani, M., Kumamoto, T., Ishida, T., Mur- Malek,T. R., Shevach,E. M., Holter, amatsu, S., Masuda,T., Ikehara, S. W., Knapp,W., Wolff, K., Stingl, G. 1990. Distinct mechanismsof neo- 1986. Interleukin 2 receptors on cul- natal tolerance induced by dendritic tured murine epidermal Langerhans cells and thymicB cells. Submitted cells. J. Immunol.137:155-59 135. Fairchild, P. J., Austyn,J. M. 1990, 130. Makgoba, M. W., Sanders, M. E., Thymicdendritic cells: Phenotypeand Shaw, S. 1989. The CD2-LFA-3and function. Int. Rev. Immunol.6:187-96 LFA-I-ICAMpathways; relevance to 136. Schild, H., Rotzschke,O., Kalbacher, T-cell recognition. Immunol.Today 10: H., Rammensee,H.-G. 1990. Limit of 417-22 T cell toleranceto self proteins by pep- 131. Langhoff, E., Steinman, R. M. 1989. tide presentation. Science 247:1587-89 by Rockefeller University on 08/15/07. For personal use only. Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org by Rockefeller University on 08/15/07. For personal use only. Annu. Rev. Immunol. 1991.9:271-296. Downloaded from arjournals.annualreviews.org by Rockefeller University on 08/15/07. For personal use only.