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Next Article 
Blood, Vol. 90 No. 9 (November 1), 1997:
pp. 3245-3287
REVIEW ARTICLE
Dendritic Cells: Unique Leukocyte Populations Which Control the Primary Immune Response
By
Derek N.J. Hart
From the Haematology/Immunology/Transfusion Medicine Research Group, Christchurch School of Medicine/Christchurch Hospital, Christchurch, New Zealand.
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INTRODUCTION |
The description of skin dendritic cells (DC) by Langerhans in 1868 was followed by prolonged speculation as to their function. Steinman and Cohn identified mouse spleen DC in 19731 and initiated a series of experiments that established lymphoid tissue-derived DC as potent stimulators of primary immune responses.2-5 The observation that similar cells were present in the nonlymphoid tissues of both rodents6-8 and humans,9,10 combined with early evidence that they played an important role in heart and kidney transplant rejection,11-14 generated further interest in DC. However, a paucity of markers for DC, the difficulty distinguishing DC from monocytes/macrophages (Mo/MØ), and the problems involved in purifying DC made for slow progress. Nonetheless, several laboratories persisted with their investigations, leading to the current acceptance that DC represent discrete leukocyte population(s) of specialist or "professional" antigen presenting cells (APC), with an extraordinary capacity for initiating primary (and secondary) T-lymphocyte responses.15,16
It is now possible to grow DC-like cells in culture17-21 and the first monoclonal antibodies (MoAbs)22,23 that react with human DC are available. DC migration24 is probably directed by a range of mediators and recent data suggest that reciprocal T-lymphocyte feedback provides antigen (Ag)-specific control of DC Ag presentation.16,25,26 This new understanding of DC function may provide opportunities for therapeutic intervention in bone marrow transplantation (BMT), solid organ transplantation, and autoimmune disease. Protocols for clinical immunotherapy programs, targeted on malignant cell Ag or infectious agents, are being designed to exploit DC as "nature's adjuvant" for optimal therapeutic vaccination.
A practical issue facing hematologists is the lack of a clear definition of a DC. This, combined with uncertainty as to the ontogeny of DC, limited data as to the interrelationship of different DC populations, and the fact that activation/differentiation events may dramatically change the morphology, molecular expression (phenotype), and function of DC, has created difficulties. This review will concentrate mainly on the myeloid lineage-derived DC, which associate with T lymphocytes within lymphoid tissue, to provide critical APC activity for initiating specific T-lymphocyte activation and proliferation.15,27,28 Follicular dendritic cells (FDC), which are present within the B-lymphoid follicle, do not originate from the BM, have a different phenotype, and retain Ag for prolonged periods, thereby restimulating B lymphocytes and perhaps T lymphocytes, will not be reviewed. Nor will dendritic epithelial cells, a specialized subset of T lymphocytes with extensive membrane processes, which are found in epithelial surfaces in mice but are rare in humans. Myeloid DC should be further distinguished from thymic DC, which appear to derive from a lymphoid stem cell.29 Thymic DC (and perhaps other lymphoid lineage-derived DC) have an entirely different role30,31; they delete maturing T lymphocytes and the prediction that they may have different properties from the myeloid-derived DC, although controversial, has received experimental support.32
The cardinal properties of the myeloid lineage-derived DC (hereafter DC) include (1) the ability to take up, process, and present Ag; (2) the ability to migrate selectively through tissues; and (3) the ability to interact with, stimulate, and direct T-lymphocyte responses. DC may be the only cell capable of stimulating a naive T lymphocyte but other "nonprofessional" APC (and DC) can stimulate experienced (activated or memory) T lymphocytes. As such, DC have unique cell interaction capabilities, some of which relate to their extensive cell membrane processes that are acquired late during DC differentiation/activation (Fig 1). DC have been defined, as of necessity, by these specialized functional characteristics15,27,33 to distinguish them from Mo and MØ.
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| Fig 1.
The morphology of human (see text for description) DC. (A) Fresh "lineage negative" blood DC, isolated without a period of tissue culture, using immunoselection.57 (May-Grünwald-Giemsa [MGG], original magnification [OM] × 1,433). (B) CMRF-44 sorted, Nycodenz gradient purified cultured blood DC77 MGG, OM × 1,433). (C) Tonsil low-density cultured DC stained with anti-HLA-DR. The veils and dendritic processes are more obvious in these preparations (OM × 1,433). (D) Mo-derived DC preparation stained with CMRF-44 using an immunoenzyme (brown, Peroxidase-DAB) technique (in preparation) (OM × 1,433). (E and F ) Fresh "lineage negative" blood DC clustered with CD4+ purified autologous T lymphocytes in the presence of staphylococcal entertoxin A (SEA) and MGG stained. The DC is stained in another cluster (F ) for the costimulator molecule CD86 using an immunoenzyme (Alkaline Phosphatase-Fast Blue) technique (OM × 1,197). (G) EM appearances of a CMRF-44-positive cultured blood DC. Note the mitochondria endosomes and lysosomal vacuoles (OM × 17,000). (H) DC in the interstitial tissues of rat heart identified by anti-MHC class II staining (immunofluorescence) (OM × 479). (I) Dermal CMRF-44+ DC (red, Peroxidase-AEC) and T lymphocytes (blue, Alkaline Phosphatase-Fast Blue) within a section of normal skin adjacent to a hair follicle (OM × 479). (J) Lymph node interfollicular (T lymphocyte) area containing CMRF-44+ IDC (brown, Peroxidase-DAB) compared with CD14 Mo and CD20 B lymphocytes (blue, Alkaline phosphatase-Fast Blue) (OM × 143). (K) Lymph node interfollicular region with CMRF-44+ IDC (blue, Alkaline phosphatase-Fast Blue) showing nuclear labeling for the transcription factor Rel B (brown, Peroxidase-DAB) (OM × 1,197). Bar = 10 µm.
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TOWARD A DEFINITION OF DC |
The Existence of a Lineage?
The original description of mouse spleen DC drew attention to their irregular shape with numerous cell membrane processes, including spiny dendrites, bulbous pseudopods, and lamellipodiae or veils.1 Distinctive electron microscopic features with a paucity of intracellular organelles and prominent mitochondria were also described. The presence of endosomes and lysosomes essential for Ag processing have been emphasized more recently.34,35 Mouse spleen DC adhere to plastic initially but become nonadherent after overnight tissue culture1 and this critical property, plus their low density, allowed them to be separated from MØ. Spleen DC were further distinguished from MØ on the basis of cytochemical reactions, a lack of phagocytic activity, and an apparent lack of Fc and complement (C) receptors, features which may be present in less mature DC. Similar cells were identified in lymph node (LN) preparations.2 Subsequent studies (reviewed by Steinman15 ) showed that DC express major histocompatibility complex (MHC) molecules, particularly class II loci products, in high density (mouse MØ have low-density MHC class II) and are exceptionally effective stimulators of primary T-lymphocyte responses, both in allogeneic mixed leukocyte reactions (allo-MLR)3 and Ag-specific systems.36
Leukocytes with a high density of MHC class II molecules and irregular cell membrane processes were noted in the interstitium of most rat tissues, except the immuno-privileged sites of brain and testes.6-8,37 These interstitial DC were shown to be BM-derived, relatively poorly phagocytic, and to have similar characteristics to the lymphoid tissue-derived DC.7 Interstitial DC stimulate strong allograft responses13,14 and isolated interstitial DC stimulate T lymphocytes.24 Human interstitial DC were described in kidney initially9 and subsequently in other tissues.10
Initial emphasis on their Fc and C receptors as MØ-like features delayed appreciation that Langerhans cells (LC) represent a stage of DC differentiation.33 Electron microscopy distinguished the Birbeck granule as a distinctive membrane inclusion within LC33 but this was difficult to follow technically and is rarely present in other DC types. When techniques were evolved to isolate LC from both mouse38 and human39 skin, cultured LC were shown to develop the morphological and phenotypic features of lymphoid tissue-derived DC. Functional studies confirmed that LC could stimulate a strong primary allo-MLR, but in some studies only after a significant period of culture.40-42 Similar allostimulatory properties were attributed to veiled cells and soon it was appreciated that these migrating DC gave rise to the interdigitating dendritic cell (IDC) within draining nodes.
Thus, the concept grew that DC have a growth and differentiation pathway (see A DC Hematopoietic Lineage and Migration Pathway), which interrelates with their migration24 and functional activation.15,16 BM precursors2,7 give rise to maturing DC, which can be distinguished from other myeloid cell types within human BM.44 Small numbers of DC precursors (and mature or recirculating forms) circulate in the blood and exit from the bloodstream into the tissues. These interstitial DC (LC form a subset in the skin33 and other epithelia) have extended cell processes and act as sentinel cells. After exposure to Ag or inflammatory stimuli, interstitial DC migrate out of the tissues into afferent lymph, where they are identified as veiled cells.45 Upon entering draining LN the DC are found as IDC in the T-lymphoid areas of LN. IDC are also found in tonsil and the white pulp of spleen.15,16,46 The spleen contains a second population of (red pulp, ie, blood-derived) marginal-zone DC.47
Functional Activity of DC
Although DC lack substantial phagocytic activity, this relative deficiency has been over-emphasized.16 The lack of phagocytic activity is a feature of more differentiated or mature DC and less differentiated or immature DC have selective phagocytic activity48-50 (see Antigenic Material). In vitro studies show DC to have marked cell motility and the ability to extend/retract their cell-membrane processes.51
The allo-MLR has been used widely to test DC Ag presenting activity.3,52 Although certain features (notably the high-responder T-lymphocyte precursor frequency) are not typical of a true naive primary immune response,53,54 the ability of DC to stimulate a potent allo-MLR distinguished them from other leukocytes. Activated B lymphocytes also have allostimulatory activity in vitro55,56 but are poor allostimulators compared with DC.57,58
DC stimulate an autologous MLR,59-61 which may equate to a primary response to exogenous Ags (see Autologous Ag).
The extraordinary ability of DC to take up and present Ag for primary specific T-lymphocyte responses, in vitro62 and in vivo,63-65 is their main distinguishing feature.
The unique properties of DC become most evident when pure populations of DC, Mo, and B lymphocytes are compared.15,27 B lymphocytes probably contribute little to primary immune responses in vivo.66,67 DC are also active APC for secondary responses as are MØ, endothelial cells, and activated B lymphocytes.
Isolation of DC
A comprehensive review of methodology27 is available. DC purifications generally use one of two main approaches.
The first exploits the fact that DC lack certain lineage-specific markers expressed on other defined leukocyte populations (Fig 1A), eg, T-lymphocyte (CD2 [at least in high density], CD3), B lymphocyte (CD19, CD20, CD24), natural killer (NK) cell (CD16, CD56, CD57), Mo (high-density CD14), and granulocyte (CD15) markers. Various MoAb cocktails have been used with a variety of immunoselection systems to isolate "lineage negative" populations of DC from cell suspensions of hematopoietic tissue or after enzymatic digestion of other tissues such as skin.
The second approach has been to capitalize on the low density of DC, which results after a period of in vitro culture.68 Culture of peripheral blood mononuclear cells (PBMC) (often sheep red blood cell [SRBC] rosette negative), particularly in fetal calf serum at 37°C, activates DC and changes their size as well as their density, aiding purification over density gradients.17,69,70 Various gradient media have been used and the original bovine serum albumin (BSA) gradient71 has been replaced by metrizamide,51,72,73 Nycodenz (Nycomed Pharma, Oslo, Norway),68 or Percoll (Amrad, Pharmacia Biotech, Aukland, New Zealand).17,69,70 The Nycodenz gradient has proved very effective (Fig 1B).68 The gradient media all perform a similar function but these compounds and their osmotic effects may induce phenotypic or functional changes.25,74 For example, Kabel et al75 have suggested that metrizamide may alter Mo cells so that they resemble DC.
The major selective procedure in mouse DC preparation, transient adherence of DC to plastic,1,15 is ineffective in humans.16 Human DC, Mo, and other cell types appear to segregate variably after adherence to both plastic and teflon surfaces. Adherence enriches human Mo but this does not lead to reliable increases in stimulatory cells.16 In addition, adherence to different surfaces may influence the phenotype of Mo76 and perhaps DC.
Elutriation has been used27,75 but its application is limited by the availability of equipment. Methods that used other physical characteristics which may discriminate DC from other cells, eg, phagocytosis and clustering,27 have fallen into abeyance.
Sophisticated flow cytometric and immunomagnetic bead selection have now become the norm and a new era of positive selection with the CD83 and CMRF-44 reagents has made high-purity preparations of low-density DC feasible.77,78 An effective method to prepare DC for immunostimulation involves gradient separation over a standard density gradient to obtain PBMC, T-lymphocyte depletion (optional), overnight incubation for 16 hours in autologous serum/medium (±cytokines), and Nycodenz gradient separation. A significant lineage-negative, CMRF-44+ cell population (Figs 1B and 2) can then be isolated by positive selection.77 A similar approach using CD83 to select DC from a metrizamide gradient is also effective.78
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| Fig 2.
The costimulatory capacity of blood derived CMRF-44 purified DC compared with CD14+ Mo and CD19+ B lymphocytes. (a) The flow cytometry labeling of Nycodenz low-density cells is shown before sorting CMRF-44+ CD14- DC and CMRF-44- CD14+ Mo. (b) Allogeneic T-lymphocyte proliferative responses measured with 3H thymidine, are shown using each FACS purified cell populations as stimulators.77 The allostimulatory capacity of sorted DC, Mo, and B lymphocytes (CD19+ sorted) are compared at a ratio of 1 APC to 50 allogeneic T lymphocytes. (c) DC and Mo are compared in their ability to process and present the soluble protein antigens to KLH and tetanus toxoid to autologous T lymphocytes at an APC:T lymphocyte ratio of 1:20.
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Cytochemical Characteristics
DC lack many of the enzymes associated with classic Mo/MØ. Cytochemical reactions show that DC lack myeloperoxidase1,7,44,57,79 and have low levels of 5' nucleotidase, dipeptidyl peptidase (DPP1), and cathepsin B activity.33,73,80 Other intracellular enzymes (eg, nonspecific esterase [NSE], acid phosphatase) or lysosomal Ags (eg, CD68)46 may be present in DC but their intensity is less or their intracellular distribution differs from Mo and MØ. NSE staining decreases with DC activation.81
Problems Defining the DC Lineage
There are notable species differences. In mice and rats, BM and blood DC precursors express few MHC Ags and are poor stimulators in the allo-MLR.20,82,83 However, in humans, BM is allostimulatory44,84 and at least one or more fresh DC populations in blood are potent allostimulatory cells.52,57,85 Other surface molecules on DC have different patterns of expression between the species, eg, CD11c86 and CD187 (see Cell Surface and Other DC-Associated Molecules).
Inbred mouse and rat strains7,14 have different DC counts and their number may decrease with age.88,89 Informal reports (routine blood DC counts are just available) suggest considerable individual variations in DC numbers circulating in humans.
The different methods of purifying DC have considerable and variable effects on DC activation, which may modify DC phenotype and function. It remains a problem as to how to select for "resting" DC in human blood and thereby remove the contaminating small lymphoid population, stem cells, basophils, endothelial precursors, etc. Other cell populations, notably adherent cells, may be contaminated by numbers of highly activated DC.16 Thus, the original adherent preparations of Mo and MØ for APC studies were performed, not surprisingly, without regard for the existence of DC. Only recently, when CD14+ Mo were selected to exclude DC, did it become clear that Mo were poor APC compared with DC.57,80
A Working Definition of DC
The putative DC lineage encompasses DC populations with very different properties, characteristic of their differentiation/activation state. Although DC were named because of their distinctive morphology,1,7 this feature is insufficient to define a DC. A definition of a DC must emphasize its ability to migrate and to stimulate a primary T-lymphocyte response. A consensus working definition might be:
- The ability to stimulate a primary T-lymphocyte response (this may require differentiation/activation of the DC population).
- Marked cell motility and the ability to extend and retract cell membrane processes freely at 37°C in vitro. The ability to migrate through tissues and track to the T-lymphocyte-dependent areas of lymph node.
- Relatively specialized phagocytic activity (in vitro)
 DC uptake of extracellular material is probably greater than hitherto realized in vivo (see Antigen Uptake, Processing, and Presentation by DC). Active fluid-phase endocytosis is a feature of DC.
- Spontaneous initial and rapid clustering with T lymphocytes at 37°C in vitro.
- A cell-surface Ag phenotype distinguishing it from other leukocytes, notably Mo/MØ and B lymphocytes (see Cell Surface and Other DC-Associated Molecules).
- Expression of certain DC-associated Ags according to their differentiation/activation state (see Cell Surface and Other DC-Associated Molecules).
- Cytochemical reactions, which differ from Mo/MØ (see earlier).
Two additional features, "dendritic" morphology and high-density membrane MHC class II Ag, are confirmatory only (particularly in humans), as other cells, notably B lymphocytes46,90 and fibroblasts, can adopt a very similar appearance. DC morphology is also influenced by temperature.21 Note that FDC, dendritic epithelial cells, and lymphoid DC (for the moment) are excluded by these criteria.
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CELL SURFACE AND OTHER DC-ASSOCIATED MOLECULES |
DC express a repertoire of molecules common to other leukocytes, eg, MHC molecules, CD45 (leukocyte common) isoforms, adhesion molecules etc. These have dominated attempts to produce DC lineage specific MoAbs but persistence has resulted in some useful new reagents (Table 1).
Relatively DC-Specific Ags
The difficulties in isolating pure DC for immunization and screening of MoAb undoubtedly limited endeavors and hence the reagents available. It seems unlikely that DC lack unique membrane features and cell-surface molecules. Indeed, when sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) protein analysis of pure human tonsil DC proteins was compared with other leukocytes,91 DC appeared to have a very different protein composition, even if this level of resolution did not identify DC-specific molecules.
Mouse DC
The 33D1 rat MoAb92 identifies a low-density Ag on mouse (marginal zone) spleen DC. The antibody does not stain DC in cryostat sections and does not react with LC. No biochemical data on the Ag are available. Nonetheless, this antibody has proved extremely useful for C lysis of mouse spleen DC.
The hamster MoAb N41886 reacts with an epitope of the  2 integrin family heterodimer p150/95 (CD11c), which is expressed in high density on mouse DC and other leukocytes including MØ.
The rat NLDC-145 MoAb was raised by immunizing with mouse lymphoid tissue stroma and screening for reactivity on DC in tissue sections.93 It stains DC in T lymphocyte areas, including spleen white pulp but not spleen marginal-zone DC or LC. The Ag is also expressed on activated MØ and thymic cortical epithelium.93 Subsequent isolation of the Ag recognized by NLDC-145 and protein sequencing enabled a full-length mouse cDNA to be isolated.94 Now renamed DEC-205 (as a result of a revised molecular weight), the molecule, which has some similarities with mannose receptors,95 is postulated to act as a receptor for Ag uptake.94
Other Animal Species
Another potential member of the integrin family that is preferentially expressed on rat DC is defined by the MoAb MRC OX62.96 The Ag is found on other cells but has considerable potential for defining DC.49
Although DC have been studied in pigs, sheep, and monkeys, no reagents specific to DC in these species have been produced. Sheep and rabbit, but not mouse, DC express CD1.
Humans
The CD1 gene family includes at least three relatively well-characterized gene products: CD1a, CD1b, and CD1c, which have similarity with MHC class I molecules and are expressed by cortical thymocytes. LC express CD1a and variable amounts of CD1c87,97 while CD1b has been reported on dermal and migrating LC.98
IDC in LN draining the skin may express CD1a, CD1b, and CD1c.99 Blood57,70 and tonsil46 DC do not express CD1a, although the former have been reported to both express CD1c78 and to lack CD1c.51 CD1 expression may be influenced by exogenous 2 -microglobulin levels in media.87 The CD1 Ags are inducible on Mo100 and probably on other DC populations in certain circumstances.18,101,102
CD11c (p150/95) reagents have been investigated in humans as potential DC markers. This integrin has been described on a wide range of human leukocytes including Mo and MØ.103 CD11c reagents probably differ in the epitopes recognized,104 and have not proved particularly useful for identifying human DC. Nonetheless, CD11c expression on a subset of activated freshly isolated blood DC has been reported,78,85,105 although it is absent from tonsil DC.46
Other reagents, including an MoAb, X-11, which detects a neo-epitope of C9106 and the "HLA-DQ-associated Ag" defined by RFD-1,107 also found on MØ, have received less attention. These Ags and the molecule present within LC identified by the Lag MoAb108 have yet to be characterized biochemically. An antibody, IRAC, which stains IDC in LN sections,109 has a wider tissue distribution by flow cytometry. Factor XIIIa, which identifies a dermal cell with dendritic morphology,110 has been suggested to be a marker for a tissue MØ population.104
Two new reagents, HB15 (CD83) and CMRF-44, which react with DC membrane Ags are contributing toward better identification of DC in humans.22,23 These MoAbs and a third reagent, CMRF-56 (submitted), react with separate differentiation/activation Ags, which appear in higher density on cultured blood DC, as follows:
- The CD83 antibody HB15 has limited reactivity with activated B lymphocytes but stains cultured human blood DC,78 LC74, and some interdigitating cells in LN.23,74 The Ag is encoded by a cDNA that defines a member of the Ig gene superfamily.23 The mouse homologue maps to the MHC region.111 As yet it has no assigned function, although preliminary data suggest soluble CD83 may influence T-lymphocyte proliferation.
- The MoAb CMRF-44 reacts strongly with cultured human blood DC22 and identifies a small population of freshly isolated DC in human blood.77 The fact that these DC subsequently express CD83 upon activation77 suggests that these are partially differentiated/activated cells rather than recirculating cells. The CMRF-44 Ag is expressed in moderate density on isolated LC, consistent with the postulated DC differentiation/activation pathway.74 Although inducible on Mo/MØ by high doses of interferon-
 (IFN-),22 the CMRF-44 Ag expression on Mo/MØ is more limited in physiologic circumstances. Thus, the substantially higher density of CMRF-44 Ag expressed on DC allows positive selection of DC.77 The Ag is also weakly expressed on B lymphocytes. Biochemical studies have yet to fully characterize the Ag.
- The third reagent CMRF-56 identifies a DC differentiation/activation Ag of unknown function, which is not present on Mo/MØ (submitted).
Finally, the sequence for human DEC-205 has been obtained (submitted) and reagents to this molecule are eagerly anticipated.
Non-Lineage-Restricted Membrane Ags on DC
Broadly Expressed Molecules
DC express the CD45 Ag60 including the CD45RA, CD45RO, and CD45RC isoforms.51,78,112,113 The CD45RO is probably induced by activation.105 Signaling via the phosphatase active CD45 cytoplasmic portion may contribute to DC function.60
The CD43 Ag (leukosialin) is less evident on human tonsil DC than on most Mo/MØ.114 Human blood DC preparations57,78 and perhaps mouse DC115 also show variable low-level expression.
DC also express CD48, CD148 (a molecule with phosphatase activity), and several other non-lineage-restricted Ags.78,113
Myeloid Ags
In humans, most blood DC express CD33, an early marker of myeloid differentiation.44,80 LC and dermal DC express some CD33.116,117 Expression of CD33 appears to be reduced on tonsil DC,46,118 suggesting that it downregulates as DC differentiate. CD33 has been postulated to be a sialoadhesin119 but its function on DC is unknown.
CD13 is expressed on blood DC precursors but not on more differentiated DC.46,85,120 In contrast, tissue MØ are CD13+.103
The CD14 molecule is expressed in high density on Mo and acts as a lipopolysaccharide (LPS) receptor.121,122 A soluble form is released by direct secretion and by cleavage of the phosphatidylinositol (PI)-linked membrane form which may bind to other cells. Low and variable levels of CD14 have been reported on blood DC80,85 while other flow cytometry studies suggest DC are CD14-.57,77,78,123 The CD14 gene structure makes reverse transcriptase polymerase chain reaction (RT-PCR) analysis of CD14 mRNA difficult124 but the presence or absence of CD14 transcripts in DC needs clarification. Given that CD14 binds LPS,121 which triggers cellular responses, including DC migration,24 this is a pertinent question. The expression of CD14 can be modified by cytokines, notably IL-4125 and IFN-.126 CD14 bright cells have little allostimulatory activity57,58 but there may be a CD14+ stage of DC differentiation.84,127
Human blood DC are CD36+ (thrombospondin receptor)78 and CD36 identifies a perivascular subset of probable DC in skin.104 CD36 stains subsets of Mo128 and MØ104 as well.
Human blood DC are weakly CD15s+78 but, like tonsil DC,46 CD15-. DC do not stain with certain MØ-specific reagents, eg, CD115 (M-CSFR) and AM-3K.113,129
Lymphoid Ags
Freshly isolated human blood DC probably express low levels of CD278,130 as do rat DC. However, cultured human blood DC,70,130 LC,117 and tonsil DC131 lack CD2 Ag. The CD5 Ag is found on blood DC.78,85,120 The CD4 Ag was identified on tonsil DC,46 LC,97 and recently on blood DC.105,120,132 Lymphoid-derived mouse DC express CD8.133
Adhesion Molecules
Clearly, DC are involved in a number of adhesive interactions during their migration and subsequent interaction with T lymphocytes. The initial tethering and rolling of DC precursors on endothelium may prove to be mediated by selectins. Firmer DC adhesion and migration through vessel walls is likely to involve integrins and intercellular adhesion molecules (ICAMs) or other Ig superfamily members. Integrins, the CD44 Ag or the syndecans may mediate interstitial tissue interactions. The more stable DC interactions with epithelia probably involve the cadherins.
The Ig superfamily members ICAM-1(CD54), ICAM-2 (CD50), and ICAM-3 (CD102), which are all ligands for LFA-1(CD11a), are expressed on DC.134-136
ICAM-1 is present in low density initially on blood DC but its expression is readily upregulated.135 ICAM-2 is present and its density seemingly changes little with activation.136 In contrast, ICAM-3 is present in highest apparent density as judged by flow cytometry staining and appears to be the most important LFA-1 ligand for initial DC-T lymphocyte adhesion.135 ICAM-1, which upregulates,137 and ICAM-3, which is not upregulated by culture in vitro, are also expressed by LC.138-140
LFA-3 is a PI-linked protein that is expressed on DC and LC.134 This molecule probably has an adhesive function as well as signaling via T lymphocyte CD2.
Another family member, V-CAM (CD106), although not present on resting DC, is inducible at least on gut DC,141 possibly by IL-4 exposure.
PECAM-1 (CD31) is found on blood78 and tonsil DC.131
Several integrin family members are expressed on DC.
Certain 1 integrins (VLA subfamily  1-9, V ) are expressed selectively on LC.142 VLA-5 (fibronectin receptor) and VLA-6 (laminin receptor) may be involved in DC interstitial reactions. The 2 integrin, LFA-1 (CD11a), is expressed on mouse and human DC in high density.134,143 The expression of CD11b is limited to, at most, a subset of DC with low-density expression.57,85 CD11c appears to be present in higher density (N418 staining) on mouse lymphoid tissue-derived DC but it is also on a subset of human blood DC.85,105 CD11b and CD11c are also reported on human LC.104,144 The 3 subfamily (11b) and the vitronectin receptor (v) are probably absent from DC. Data on the 4-8 integrins are awaited.
The cadherins are transmembrane glycoproteins that localize to adhering cell junctions.
E-cadherin is present on both mouse145 and human146 LC. This molecule is expressed on blood DC and downregulates as LC migrate, suggesting an isologous adhesion interaction with skin epithelial cells.
The selectins are likely to be expressed on DC and may account for some of their trafficking.
It is reported that human blood DC lack L selectin (CD62L),78 which is widely expressed on other leukocytes.147 However, it is shed after cell activation and thus perhaps during DC preparation. LC express the E-selectin ligand, cutaneous lymphocyte-associated Ag (CLA).148 The carbohydrate (CHO) moiety, sialyl Lewis X (CD15s), on DC is related to the ligands for E and P selectins.
The CD44 molecule is present in very high density on DC134 and the V3, V6, V9 isoforms are preferentially expressed on Mo-derived DC (Mo-DC).149
A comprehensive analysis of CD44 isoforms in DC is justified, as CD44 acts as a ligand for hyaluronate, a connective tissue component likely to stabilize DC interactions with the tissue interstitium. CD44 may also have costimulatory functions.
The syndecans (CD138) bind growth factors and matrix molecules (fibronectin, collagen, laminin, and vitronectin) extracellularly and associate with the intracellular cytoskeleton.
Thus, their presence on DC is of great interest.
Low levels of surface CD68 on DC may act as a ligand for E selectin.150 DC stain strongly for endoglin (CD105),78,113 a molecule found on endothelium, which may also have adhesion properties and act as part of the TGF receptor complex.151 They also express neurothelin (CD147), another endothelial expressed molecule with a suggested adhesion function.113 The fact that DC express the urokinase plasminogen activation receptor (CD87) implies this may be involved in tissue extravasation.113 Finally, CD36 may encourage DC binding to wound surfaces.152
Potential Ag Uptake (C, Fc, pattern recognition) Receptors
Intact pathogens or antigenic components must bind to DC via opsonization (C and antibody) or other nonspecific recognition receptors.
Trace amounts of C receptors are detected on DC, particularly mouse24,40 and human153 LC.
The CR1 (CD35) receptor is absent, as is CR2 (CD21),46 but trace levels of CD11b (CR3, see above) are documented. CD11c on DC may have potential as another C receptor. C5a receptors (CD88) are present on dermal DC and a subpopulation of LC.154 Blood DC are CD55+ and CD59+, providing some protection against C-mediated damage.
Early in their differentiation, DC probably express some CD32 (FcRII),80,97,155 and CD64 (FcRI)155 but no CD16 (FcRIII).80
These may be downregulated with activation. A recent publication emphasized that the preparative method may affect FcR expression by DC and suggested CD64 and CD32 were both present in high density and functional (to a lesser degree than Mo).155 Human LC express CD32 and the high-affinity receptor for IgE (Fc RI)156 as well as the low-affinity FcRII (CD23).157 The FcRI on DC is a multimeric structure containing FcRI and chains but lacks the chain present in basophils.158 A molecule with considerable homology to the poly Ig receptor, the CMRF-35 antigen, has been described159 and similar molecules are expressed on human blood DC, LC, and tonsil DC (submitted).
Nonspecific "pattern recognition" receptors exhibit binding specificities for structural patterns typically displayed by cell surface molecules of many microorganisms (eg, LPS and glycans) but not normally found on the surface of host cells.150
Membrane-bound proteins, including the mannose receptor,95,160 DEC-205,94 or other lectin-like molecules,161 may act as non-self-recognition receptors on DC. Functional assays have recently identified the MØ mannose receptor on Mo-DC,162,163 although previous studies on rat DC were negative.164 The DEC-205 molecule on DC is predicted to have glycan binding activity associated with its CHO recognition domains.94 CD11c165 may be more relevant than CD14 as a DC LPS receptor. The scavenger receptors include the two class A MØ receptors SR-AI and SR-AII,150 belonging to a cysteine-rich (SRCR) family of proteins, which may be expressed on some DC.166 A third class A scavenger receptor described only on MØ to date, MARCO,167 remains to be studied on DC. The class B scavenger receptors CD36 and SR-BI are primarily lipid binding proteins150; the former is present on DC.78 The collectins are soluble serum proteins with lectin like specificity168 that may also interact with DC.
Costimulatory/Signaling Molecules (see DC Functional Properties section for functional data)
The CD40 molecule was first identified on human tonsil DC46 and then LC39 after activation in vitro. Blood DC express low levels of CD40 and upregulate its expression in vitro and after cytokine exposure.26
CD80/86 Ags are not found on resting blood DC,25,169 may be on LC,170 and are upregulated rapidly upon DC activation (see DC Functional Properties section).
The heat stable antigen (HSA) identified in mice as a heavily glycosylated molecule with a small PI-linked protein core has been described on a subset of mouse spleen DC, LC,171,172 liver173 and thymic DC.174 The human homologue, CD24,175 was described initially as an Ag recognized by several MoAbs, many reacting with CHO epitopes. CD24a (IgM) antibodies react with a CD24 CHO epitope (contained within mucin) and CD24b antibodies (predominantly IgG) react with the CD24 protein core.176 Blood DC were found to be negative for CD24 using both CD24b MoAb and RT-PCR but specific staining with CD24a MoAb recognized CHO epitopes on a potentially novel molecule(s) (DC24) on DC.177 This reactivity was associated with a more immature DC phenotype.
Activation Markers
Several inducible (activation) surface Ags are found on DC. CD25 (IL-2R39,81 ) is induced on DC (cytokine receptors are discussed in Molecular Events Involved in DC Clustering and Signaling to T Lymphocytes section). Other markers, eg, CD98 (4F2)113 and CMRF-37178 are found on DC. The Reed Sternberg/Hodgkin cell (RSC/HC) associated CD30 Ag179 has not proved inducible on DC in limited studies to date. CD38 is expressed on tonsil DC (in preparation).
Inhibitory Molecules
Migrating LC undergo apoptosis in vitro.180 DC have also been shown to undergo steroid and UV light-induced apoptosis.181 They are predicted to express the Fas Ag (CD95)16,182 but confirmation of a functional effect on DC is awaited. Human DC express Fas-ligand. A subset of mouse CD8+ spleen DC express Fas-ligand.183
Cytoplasmic Molecules
The hamster MoAb M342 stains an intracellular granule-associated Ag within mouse IDC and some B lymphocytes.47 Absent from freshly isolated spleen DC, the Ag is induced by culturing. Another MoAb, MIDC-8, stains similarly, but this Ag is not present in B lymphocytes.47
The lysosomal-associated CD68 Ag (FAII in mice,184 ED1 in rats49 ) has been a moderately useful marker.185 CD68 staining has a limited (dot) perinuclear distribution in DC compared with a widespread cytoplasmic distribution in MØ.185-188 The S100 MoAb detects a series of intracellular isomers that were first considered specific to nerve tissues. Technical factors limit its use as an intracellular marker of activated DC for histological studies on formalin fixed section.189,190
A 55-kD actin bundling protein that is induced in B lymphocytes by Epstein-Barr virus (EBV) infection has been shown to be expressed in the cytoplasm of the majority of human blood DC but not other leukocytes.191 Restin is a novel 160-kD cytoskeletal protein found in the RSC/HC and is associated with the intermediate filament cytoskeletal network.192 RT-PCR detected its expression by human DC (unpublished).
Novel DC-associated proteases that might be involved in processing Ag or other DC-secreted products (apart from chemokines or cytokines), which might contribute to DC migration via the extracellular matrix, may be anticipated.193
Transcription Factors
It is logical that certain transcription factors (or combinations) will be associated selectively with the DC lineage. The early data reinforce the concept that DC form a unique lineage, with distinct transcriptional control of several genes.
Rel-B is expressed in murine thymus, spleen, and LN DC.194 Immunoblotting techniques have shown expression of c-rel, rel-B, NF- Bp65, and NF-Bp50 in DC grown from peripheral blood with granulocyte-macrophage colony-stimulating factor (GM-CSF ).195 Interestingly, these DC lacked the widely expressed transcription factor SP-1. The lack of CD14 on DC may relate to the fact that SP-1 is critical for the expression of CD14 on monocytes (Mo).196 Rel-B has also been identified in the Mo-DC197 and human tonsil DC (Fig 1K).198
Fascinating studies with v-rel transformed chicken BM precursors suggested that when v-rel was induced, B-lymphoid cells resulted: suppression of v-rel caused the transformed cells to generate DC-like characteristics.199 The absence of rel B in gene-targeted mice is associated with a lack of functional mature DC (discussed later).200,201
Novel DC Molecules
The search for novel molecules that play a role in the special functions of DC as an APC continues apace using both MoAbs and the new differential display technology. Several novel molecules expressed relatively selectively in DC have been identified, and further information on these will soon be available.
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A DC HEMATOPOIETIC LINEAGE AND MIGRATION PATHWAY |
Immunophenotypic and functional analyses have defined the different DC populations (Fig 3). The interrelationship of nonlymphoid and lymphoid tissue DC has been inferred from the behavior of murine LC both in vitro and in vivo, the homing of DC injected into recipients and Ag tracking studies. The ability of DC to migrate into the tissues and from there to the LN is critical to their overall function as APC.
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| Fig 3.
A putative hematopoietic differentiation pathway for myeloid and lymphoid DC. Lymphoid DC have different properties (?tolerizing) from the myeloid DC, which is immunostimulatory in most circumstances. The surveillance tissue-based DC:LC in the skin, DC in the respiratory tract, gut, or other nonlymphoid tissues, migrate after exposure to infection, tissue damage/inflammation, or antigen (ie, danger signals) via the afferent lymphatics to the T-lymphocyte-dependent areas of the draining LN. It is possible that epithelial based CD1+ DC have an independent derivation from the stem cell. The ability of Mo and Mø to convert to DC in vivo has yet to be established.
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Bone Marrow
BM isolated from mice20 and rats83 does not have constitutive allostimulatory activity. Mouse BM cultured in low21 or high concentrations82 of GM-CSF, with further mechanical (decanting) selection against granulocytic development, enables a DC-like population to emerge. Curiously, these cell preparations lacked significant expression of the mouse DC markers, NLDC-145 and N418,20 but were allostimulatory and homed to LN. Subsequent reports by others describe the generation of DC from mouse BM using GM-CSF and interleukin-4 (IL-4).202,203 OP/OP M-CSF-deficient mice204 have deficiencies in MØ populations but have been reported to have normal LC and DC networks,205 although LC were said to be reduced by 40% in number and abnormal in morphology.206
In rats, conditioned medium, GM-CSF,83 and a novel cytokine combination including M-CSF with IL-3, linoleic acid, tocopherol, and cholecalciferol,207 generated DC-like cells from BM.
Human BM contains allostimulatory DC-like cells.44 Further fractionation of the morphologically heterogenous allostimulatory "lineage negative" population of BM mononuclear cells established that some of the CD34+ progenitor cells were also allostimulatory.84 Putative precursors of LC-type DC have been identified in human BM18,102,208,209 using CD1a labeling. CD1a is inducible on Mo by GM-CSF100 and is not present on the uncultured BM allostimulatory population44 or fresh-blood DC.57 More recently the CLA Ag was shown to define a subpopulation of blood CD34+ cells, which appear to give rise to a CD1a+ Lag+ LC.210
Analysis of human BM precursor growth in semisolid media has enabled DC-like cells to be defined morphologically,18,127,211 which when isolated had functional properties very akin to DC. The cells isolated by Young et al212 have virtually all the characteristics of DC and it is noteworthy that a significant proportion are CD1a-.
Human BM cultured in liquid suspension84,213 with a range of cytokines has produced DC-like populations (see In Vitro Cultivation of DC and DC Lines). A CD14 stage of DC differentiation may emerge in vitro,84,127,213 although no CD14+ precursor was evident in directly sorted BM.44,84 The purity of the resulting DC-like populations has been hard to assess but these cell populations certainly include potent allostimulatory cells. Curiously, few CMRF-44+ or CD83+ cells have been identified in these first culture studies and prolonged culture in specific conditions may be required.84,127
It is possible that a second DC differentiation pathway commits independently from a CD34+ precursor to provide Lag+ CD1+ epithelial-associated LC.210
Galy et al214 suggested a human CD10+ lymphoid precursor gave rise to T, B, NK cells, and DC. In addition, a CD34+ CD38 (dim) human thymic progenitor has been shown to give rise to T lymphocytes, NK cells, and DC.215 These intriguing results will encourage more functional studies on putative lymphoid lineage DC.
Not surprisingly, SCF (c-kit ligand) improves yields of DC generated in vitro from human BM.211,212,216 It also increases DC (and MØ) colony yields in semisolid media.211 The alternative growth factor flt-3 ligand has been suggested to drive both myeloid and lymphoid DC growth in mice217 and facilitates human DC growth twofold.218 GM-CSF, either exogenous or endogenous, seems important in vitro for DC growth but lower doses than generally used may be sufficient.211 Curiously, both types of DC are found in mice in the absence of GM-CSF,219 suggesting other cytokines control DC production in vivo. IL-6216 and IL-4 almost certainly contribute to DC hematopoiesis. The latter downregulates CD14125 and suppresses MØ generation.220 Interestingly, IL-13 may reduplicate this IL-4 effect,221 as may IL-7. Tumor necrosis factor (TNF ) may contribute more to late DC differentiation/activation.
The high cytokine concentrations in vitro may drive aberrant costimulatory function and skew differentiation of committed Mo/MØ lineage cells. It will be reassuring to see culture conditions evolved that allow significant upregulation of "DC lineage" markers. Studies using CD34+ cord blood generated DC are discussed in the In Vitro Cultivation of DC and DC Lines section.
Blood DC
Mouse peripheral blood mononuclear cells generate DC-like cells when cultured in GM-CSF alone,19 suggesting a circulating DC precursor. These are destined to provide interstitial DC in nonlymphoid tissues, including the skin and liver.49
Human peripheral blood DC were first isolated using methods similar to those used for the isolation of murine splenic DC.71 DC circulate in low numbers in blood and were difficult to isolate in quantity or high purity at this time.61,71 Most workers estimate DC (and their precursors) to represent only 0.1% of PBMC (a routine 300 to 400 mL blood donation provides 105 to 106 DC).
Highly purified human blood Mo, including a CD16 subpopulation,222 are poor stimulators in the allo-MLR compared with "lineage-negative" DC-enriched fractions.57,58 These "freshly isolated" blood DC57,58,105,155 or "immature" DC are smaller and lack dendritic morphology but have the nuclear features of DC (Fig 1A). A small population of larger cells are CMRF-44+ may be activated. These immature DC develop classic DC-like features after culture. It is noteworthy that these "lineage-negative" fresh DC preparations are not homogeneous and may include five or more cellular subsets.77
Gradient separation of cultured T-lymphocyte-depleted PBMC removes contaminating small "lymphoid cells" (perhaps even some DC precursors) and with further cell sorting, relatively homogeneous DC preparations result, perhaps by coordinating the differentiation/activation of the DC precursor and CMRF-44+ DC populations. These have the following (MGG stain) cytological features: an indented nucleus with open chromatin pattern and rare nucleoli, substantial by a large pale blue cytoplasm with a prominent light Golgi (perinuclear) zone, minimal granules but some minor vacuolation, and more extensive membrane protrusions, veils, or dendrites (Fig 1B).
In the presence of human serum (ie, perhaps normal physiological conditions) human Mo differentiate into MØ,223,224 apparently distinct from DC. These results and the separation of circulating DC precursors (±an activated circulating DC population) from Mo suggest two separate lineages evolve from a common precursor (Fig 3). Recent data127 again support this concept but an independent CD1a+ intermediate precursor of LC remains a definite but elusive possibility.
As illustrated in Fig 3, a third differentiation pathway may also occur at least in vitro whereby, in certain conditions,223,225 nonproliferating Mo differentiate into Mo-DC (Fig 1D). This involves high concentrations (? nonphysiological) of cytokines in combinations, which may not occur in vivo. The critical differentiation event in this process appears to be cytokine downregulation of the M-CSF receptor, which allows a DC phenotype to emerge197 (see In Vitro Cultivation of DC and DC Lines).
Nonlymphoid Tissue-Derived Interstitial DC
There is an extensive network of interstitial DC encompassing virtually all organs except the brain, parts of the eye, and the testes.7,10,37,226 These cells develop from precursors in the blood7 and provide a sentinel system of APC. LPS,115 GM-CSF,227 IL-6, and probably other stimuli recruit precursors to the tissues. Mo-DC migrate in response to classical chemoattractants (formyl peptides and C5a) and some chemokines (MCP-3, MIP-1, and RANTES) but not IL-8.228 A basal rate of tissue entry is boosted (5- to 10-fold) by tissue damage/inflammation, ie, "danger" signals.229 The kinetics of the DC response in the lung are rapid, similar to the neutrophil response. Freshly recruited mouse LC and other interstitial DC are N418-, NLDC-145- but become NLDC-145 + in time and may express low levels of the macrophage-associated F4/80 Ag.
"Danger" signals, ie, tissue damage/inflammation, also mobilize DC from the tissues. After sensing inflammatory change230 or antigenic exposure,229 tissue DC migrate into afferent lymph.231 The properties of fresh LC and cultured LC28 (and fresh and cultured interstitial DC from other sites24,173 ) define two separate DC immune functions: (1) Ag acquisition and processing, and (2) Ag presentation to naive T lymphocytes in association with costimulatory signals.
LC
Epidermal LC form an extensive suprabasal network with branched cytoplasmic processes extended in physical contact with multiple adjacent LC.33 CD1a+ LC containing Birbeck granules are found in the dermis97 and may represent cells moving into the afferent lymphatic system. The number of LC ( 1,000/mm2 ) vary according to site.232 LC in the skin are subject to neuroendocrine control and are intimately associated with nerve endings. A low level of in vivo LC proliferation (? recently entered blood precursors) has been described.233 A population of CD1a- dermal DC (immunohistology97 ) or CD1adim (immunofluorescence/isolated cells) is also present.74 Factor XIIIa positive dermal DC may be another potential subset110 among a substantial dermal MØ population.104
The isolation of LC from skin is achieved by separating epidermal sheets from the dermis by short (<1 hour) incubations in trypsin39,42,138,234,235 or dispase.74 Epidermal cell suspensions are then produced by mechanical disruption and keratinocytes depleted. LC may be isolated at this stage, or after a period of culture, by density gradient centrifugation. Positive selection using CD1 or MHC class II MoAbs allows high purity LC preparations.235,236 Dermal DC are isolated by collecting the cells that migrate from dermal fragments.98,137
As with blood DC, there are clear phenotypic and functional differences between fresh236,237 and cultured LC.39,42,234 Periods of culture, particularly in the presence of keratinocytes which produce copious cytokines, notably GM-CSF and IL-1, stimulate upregulation of MHC class II Ags, CD40, and allostimulatory activity. Low-level expression of functional Fc and C receptors on LC40 are lost on activation.231 Culture causes LC veiled processes to increase in number and length, whereas acidic organelles and Birbeck granules decrease or disappear. The characteristic CD1a marker also disappears. Normal skin contains only a small proportion of activated CD1a+ CMRF-44+ CD83+ DC (Fig 1I) mainly in association with T lymphocytes.74 CD1a+ DC have recently been identified in other epithelial surfaces such as the urothelium238 and gut epithelium.232
Interstitial DC
First noted as strongly MHC class II positive cells with irregular membrane processes in rat tissue sections,6-8,37 these DC are distinguished from MØ by phenotype and radiosensitivity.239 Similar DC have been observed in mouse heart and kidney,43 although the level of MHC class II expression in the mouse appears reduced, hindering detection. In humans, DC have been documented in most organs, including liver,185 kidney,9 heart, and other connective tissue and tend to be associated with vascular structures.10
Liver DC are preferentially associated with portal triads whereas Kupffer cells are located in the sinusoids. Their histological location suggests that DC enter lymphatics in portal triads or capsular lymphatics.185 DC in liver were poorly phagocytic cells compared with Kupffer cells,7 but recently divided BM-derived DC precursors enter the liver with phagocytic capacity for colloidal carbon.49 Rat liver DC have now been isolated and shown to have a relatively immature DC phenotype and functional capability,173 differentiating when exposed to type I collagen.
Interstitial DC in the heart (Fig 1H) show close association with small capillaries.7,239 They are not found in heart valves. Isolated mouse heart DC (N418- and NLDC-145-) have limited phagocytic capacity and upregulate allostimulatory activity upon in vitro culture characteristics reminiscent of the LC.43
Interstitial DC in kidney are prominent in the cortex and are noted between renal tubules close to renal capillaries.6,9 They also occur in the medulla. Again they show limited phagocytic capability and upregulate allostimulatory activity in vitro.43 DC are found throughout the rest of the urogenital system, particularly associated with vascular structures below the urothelium. The presence of DC in the sclera and limbus of the eye, with a decreasing number of DC more centrally in the cornea, has been documented.226,240 Recently, DC have been isolated from the rat iris a technical tour de force, and these too have similar properties to LC.241
Interstitial DC are found in endocrine organs, notably thyroid, adrenal, and islet cells.8 These cells have no unique characteristics as yet but it is curious to note that thyroxine, vitamin D, and other medications influence DC function. DC were not obvious in nervous tissue7 but DC-like cells are reported in rat pineal.242
Skeletal muscle and other supporting tissues all contain interstitial DC.10 Indeed, DC are found in major blood vessels, below the endothelium, and have been suggested to contribute to the atheromatous reaction.150
DC are associated with the supporting structures of normal and abnormal joints. Arthritic synovial fluid is a source of human tissue-derived DC (see Autoimmune Disease).
Interstitial DC, like the epithelial associated LC, are believed to form a sentinel network of Ag-receptive cells, which subsequently move Ag centrally to provide activated APC and generate T-lymphocyte responses. Although both subpopulations are considered to parallel each other in terms of putative (? independent) DC differentiation pathways (Fig 3), the differential CD1a and CLA expression is notable and probably functionally important.
Mucosal Surface-Associated DC
An interdigitating sentinel epithelial network of DC has been described in the mucosa of the oral cavity, intestinal tract, and the respiratory tract, of mice, rat, and humans.243-246 These mucosal DC mature after weaning and their phenotype is responsive to external environmental influences.247 Inflammatory stimuli also recruit DC to the mucous membranes.248 Mucosal DC are also likely to have properties peculiar to the epithelial (external environment) surface that they serve.
Isolated oral mucosal DC express CD1a and contain Birbeck granules.243 CD1a+ cells are found in the oral and rectal mucosa.232 MHC class II positive DC have been identified in the epithelium of stomach, small and large bowel, lamina propria,244,249 and in the vagina and cervix.232 The draining mesenteric lymphatic DC population contains strongly MHC class II positive allostimulatory cells, with inclusion bodies indicative of a postphagocytic history. |
Introduction
References