Surface Of APCs Research Articles

MAIT Cell Recognition of MR1 on Bacterially Infected and Uninfected Cells

Mucosal-associated invariant T cells are a unique population of T cells that express a semi-invariant αβ TCR and are restricted by the MHC class I-related molecule MR1. MAIT cells recognize uncharacterized ligand(s) presented by MR1 through the cognate interaction between their TCR and MR1. To understand how the MAIT TCR recognizes MR1 at the surface of APCs cultured both with and without bacteria, we undertook extensive mutational analysis of both the MAIT TCR and MR1 molecule. We found differential contribution of particular amino acids to the MAIT TCR-MR1 interaction based upon the presence of bacteria, supporting the hypothesis that the structure of the MR1 molecules with the microbial-derived ligand(s) differs from the one with the endogenous ligand(s). Furthermore, we demonstrate that microbial-derived ligand(s) is resistant to proteinase K digestion and does not extract with common lipids, suggesting an unexpected class of antigen(s) might be recognized by this unique lymphocyte population.

The recruitment and activation of phosphatidylinositol 4-phosphate 5-kinases a critically regulate CD28-dependent signaling responses

The recruitment and activation of phosphatidylinositol 4-phosphate 5-kinases a critically regulate CD28-dependent signaling responses

Forced Expression of HLA-DM at the Surface of Dendritic Cells Increases Loading of Synthetic Peptides on MHC Class II Molecules and Modulates T Cell Responses

Adoptive transfer of autologous dendritic cells (DCs) loaded with tumor-associated CD4 and CD8 T cell epitopes represents a promising avenue for the immunotherapy of cancer. In an effort to increase the loading of therapeutic synthetic peptides on MHC II molecules, we used a mutant of HLA-DM (DMY) devoid of its lysosomal sorting motif and that accumulates at the cell surface. Transfection of DMY into HLA-DR(+) cells resulted in increased loading of the exogenously supplied HA(307-318) peptide, as well as increased stimulation of HA-specific T cells. Also, on transduction in mouse and human DCs, DMY increased loading of HEL(48-61) and of the tumor Ag-derived gp100(174-190) peptides, respectively. Interestingly, expression of DMY at the surface of APCs favored Th1 differentiation over Th2. Finally, we found that DMY(-) and DMY(+) mouse APCs differentially stimulated T cell hybridomas sensitive to the fine conformation of peptide-MHC II complexes. Taken together, our results suggest that the overexpression of HLA-DMY at the plasma membrane of DCs may improve quantitatively, but also qualitatively, the presentation of CD4 T cell epitopes in cellular vaccine therapies for cancer.

Enhanced Antigen Processing of Flagellin Fusion Proteins Promotes the Antigen-Specific CD8+ T Cell Response Independently of TLR5 and MyD88

Flagellin is a highly effective adjuvant for CD4(+) T cell and humoral immune responses. However, there is conflicting data in the literature regarding the ability of flagellin to promote a CD8(+) T cell response. In this article, we report that immunization of wild-type, TLR5(-/-), and MyD88(-/-) adoptive transfer recipient mice revealed the ability of flagellin fusion proteins to promote OVA-specific CD8(+) T cell proliferation independent of TLR5 or MyD88 expression by the recipient animal. Wild-type and TLR5(-/-) APCs were able to stimulate high levels of OVA-specific CD8(+) T cell proliferation in vitro in response to a flagellin fusion protein containing full-length OVA or the SIINFEKL epitope and 10 flanking amino acids (OVAe), but not to OVA and flagellin added as separate proteins. This effect was independent of the conserved regions of flagellin and occurred in response to OVAe alone. Comparison of IFN-γ production by CD8(+) effector cells revealed higher levels of SIINFEKL peptide-MHC I complexes on the surface of APCs that had been pulsed with OVAe-flagellin fusion proteins than on cells pulsed with OVA. Inhibition of the proteasome significantly reduced Ag-specific proliferation in response to OVAe fusion proteins. In summary, our data are consistent with the conclusion that flagellin-OVA fusion proteins induce an epitope-specific CD8(+) T cell response by facilitating Ag processing and not through stimulatory signaling via TLR5 and MyD88. Our findings raise the possibility that flagellin might be an efficient Ag carrier for Ags that are poorly processed in their native state.

Characterization of the immunological synapses formed by induced T regulatory cells (168.13)

Abstract T regulatory cells (Tregs) play a vital role in the maintenance of self-tolerance and in the suppression of active immune responses. FoxP3+ Tregs arise naturally in the thymus, but can also be induced both in vivo and in vitro. The immunological synapses (IS) formed by induced Tregs (iTregs) have not been studied. We show that iTregs form either immunological kinapses or stable IS depending on the level of CD80 on the surface of APCs. As has been shown for natural Tregs, iTregs are capable of downmodulating the costimulatory ligand CD80 on the surface of dendritic cells (DCs) in an antigen-specific, CTLA-4-dependent manner. On the same timescale as CD80 downmodulation, iTregs transition from stable to motile contacts with DCs. Experiments utilizing supported planar bilayers containing pMHC, ICAM-1 and CD80 show that iTregs form motile immunological kinapses with a highly polarized cell shape in the presence of low levels of CD80. However, iTregs arrest and form stable IS when interacting with bilayers containing high levels of CD80. In experiments with blocking antibodies, we demonstrate that CD28-CD80, but not CTLA-4-CD80, interactions modulate iTreg motility. Thus, iTregs are capable of downmodulating costimulatory molecules on the surface of DCs, which in turn alters their motility.

Alternative mechanism for glycoconjugate vaccine activation of the adaptive immune system (52.25)

Abstract Glycoconjugate vaccines, which are currently made by covalently coupling a bacterial capsular polysaccharide to a carrier protein, have been used to immunize many millions of people and have enormously reduced disease burden, particularly among children. However, the immunogenicity and efficacy of these vaccines have been relatively low and heterogeneous in some high-risk populations. The classic antigen presentation hypothesis for glycoconjugates proposes that helper CD4+ T cells recognize a peptide derived from the carrier protein. We investigated mechanisms of processing and presentation of a prototypic glycoconjugate vaccine in which group B streptococcal type III polysaccharide is coupled to a carrier. Upon uptake into the endosome of an antigen-presenting cell, the polysaccharide undergoes oxidative depolymerization, which, in conjunction with acidic protease-mediated digestion of the carrier protein, creates a carbohydrate T cell epitope conjugated to an MHCII binding peptide. These epitopes formed in the endosome bind to MHCII through the peptide and are subsequently presented on the APC surface, with the hydrophilic carbohydrate positioned for easy recognition by the T-cell receptor. Recognition of the carbohydrate in the presence of MHCII drives T-cell help for antibody-producing B cells. An enhanced understanding of immune mechanisms such as T-cell recognition of carbohydrates may lead to markedly improved vaccines against infectious diseases.

Expression of the SLAM family of receptors adapter EAT-2 as a novel strategy for enhancing beneficial immune responses to vaccine antigens.

Recent studies have shown that activation of the signaling lymphocytic activation molecule (SLAM) family of receptors plays an important role in several aspects of immune regulation. However, translation of this knowledge into a useful clinical application has not been undertaken. One important area where SLAM-mediated immune regulation may have keen importance is in the field of vaccinology. Because SLAM signaling plays such a critical role in the innate and adaptive immunity, we endeavored to develop a strategy to improve the efficacy of vaccines by incorporation of proteins known to be important in SLAM-mediated signaling. In this study, we hypothesized that coexpression of the SLAM adapter EWS-FLI1-activated transcript 2 (EAT-2) along with a pathogen-derived Ag would facilitate induction of beneficial innate immune responses, resulting in improved induction of Ag-specific adaptive immune responses. To test this hypothesis, we used rAd5 vector-based vaccines expressing murine EAT-2, or the HIV-1-derived Ag Gag. Compared with appropriate controls, rAd5 vectors expressing EAT-2 facilitated bystander activation of NK, NKT, B, and T cells early after their administration into animals. EAT-2 overexpression also augments the expression of APC (macrophages and dendritic cells) surface markers. Indeed, this multitiered activation of the innate immune system by vaccine-mediated EAT-2 expression enhanced the induction of Ag-specific cellular immune responses. Because both mice and humans express highly conserved EAT-2 adapters, our results suggest that human vaccination strategies that specifically facilitate SLAM signaling may improve vaccine potency when targeting HIV Ags specifically, as well as numerous other vaccine targets in general.

Heat Shock Protein 90 Mediates Efficient Antigen Cross Presentation through the Scavenger Receptor Expressed by Endothelial Cells-I

Ag cross presentation is an important mechanism for CD8(+) T cell activation by APCs. We have investigated mechanisms involved in heat shock protein 90 (Hsp90) chaperone-mediated cross presentation of OVA-derived Ags. Hsp90-OVA peptide complexes bound to scavenger receptor expressed by endothelial cells (SREC-I) on the surface of APCs. SREC-I then mediated internalization of Hsp90-OVA polypeptide complexes through a Cdc42-regulated, dynamin-independent endocytic pathway known as the GPI-anchored protein-enriched early endosomal compartment to recycling endosomes. Peptides that did not require processing could then be loaded directly onto MHC class I in endosomes, whereas longer peptides underwent endosomal and cytosomal processing by aminopeptidases and proteases. Cross presentation of Hsp90-chaperoned peptides through this pathway to CD8(+) T cells was highly efficient compared with processing of free polypeptides. In addition, Hsp90 also activated c-Src kinase associated with SREC-I, an activity that we determined to be required for effective cross presentation. Extracellular Hsp90 can thus convey antigenic peptides through an efficient endocytosis pathway in APCs and facilitate cross presentation in a highly regulated manner.

Small Modular ImmunoPharmaceutical (SMIP™) Molecules Directed at the TCR Complex (CD3) Block T Cell Activation and Cause Minimal Cytokine Release In Vitro (96.25)

Abstract SMIP molecules are single chain binding proteins based on a scFv-CH2-CH3 format. We have generated SMIP proteins using binding domains from well characterized mitogenic and non-mitogenic anti-CD3 antibodies, and have found dramatic differences in the biologic consequences on human T cells exposed to the compounds. In particular, compared to antibodies, SMIP proteins cause a unique Ca++ flux, are better able to down modulate the TCR complex from the cell surface, and can inhibit naïve and activated T cells in various in vitro assays. Importantly, SMIP proteins cause a much reduced level of cytokine release, notably, when compared to second generation anti-CD3 antibodies. Most known anti-CD3 antibodies cause activation of T cells, especially when cross linked via FcR on the surface of APCs. Without cross linking, these antibodies have minimal activation of T cells in vitro. In the clinic, second generation anti-CD3 antibodies are still plagued with cytokine release syndrome, despite showing little detectable cytokine release from primary T cells in vitro. We have developed an activated T cell blast assay sensitive enough to detect in vitro cytokine release caused by anti-CD3 antibodies in second generation formats. Results from these assays indicate that features in the SMIP format could generate novel CD3-directed therapeutics with superior efficacy and reduced potential for the serious side effects of Cytokine Storm.

Endogenous MR1 is transiently expressed on the cell surface and can be stabilized with a unique mAb for activation of mucosal-associated invariant T cells (130.19)

Abstract MHC-related protein 1 (MR1) is highly conserved among mammals and restricts the development of mucosal-associated invariant T (MAIT) cells. MAIT cells bear an invariant TCRα chain preferentially paired with limited β chains. MAIT cells reside predominantly in the mucosal tissues but are absent in mice lacking commensal bacteria or B cells. Upon activation, MAIT cells rapidly secret cytokines including IFN-γ, IL-4, and IL-10. Surprisingly, endogenous MR1 has not been detected at the cell surface; but, surface MR1 is detected on cells overexpressing MR1 by transfection or transduction. To better characterize endogenous MR1 expression, we generated mAbs to mouse MR1. Using these mAbs, we show that select B cell lines express low levels of endogenous MR1 at the cell surface. Interestingly, a unique mAb to MR1 was found to enhance MAIT cell activation, whereas other mAbs to MR1 blocked MAIT cell activation. The enhancing and the blocking mAbs recognize different MR1 epitopes mapped by a panel of MR1 mutants and by mAb competition assays. Mechanistically, the enhancing mAb stabilizes MR1 at the cell surface suggesting it either promotes assembly of the MR1 complex or allosterically induces a more favorable conformation. These findings demonstrate that endogenous MR1 transiently visits the cell surface of APCs, and that induced levels of MR1/ligand complexes are required for MAIT cell activation.

Reassessing the role of HLA‐DRB3 T‐cell responses: Evidence for significant expression and complementary antigen presentation

In humans, several HLA-DRB loci (DRB1/3/4/5) encode diverse beta-chains that pair with alpha-chains to form DR molecules on the surface of APC. While DRB1 and DRB5 have been extensively studied, the role of DRB3/4 products of DR52/DR53 haplotypes has been largely neglected. To clarify the relative expression of DRB3, we quantified DRB3 mRNA levels in comparison with DRB1 mRNA from the same haplotype in both B cells and monocytes, observing quantitatively significant DRB3 synthesis. In CD19+ cells, DRB1*03/11/13 was 3.5-fold more abundant than DRB3, but in CD14+ this difference was only two-fold. Monocytes also had lower overall levels of DR mRNA compared with B cells, which was confirmed by cell surface staining of DRB1 and DRB3. To evaluate the functional role of DRB3, tetramer-guided epitope mapping was used to detect T cells against tetanus toxin and several influenza antigens presented by DRB3*0101/0202 or DRB1*03/11/13. None of the epitopes discovered were shared among any of the DR molecules. Quantitative assessment of DRB3-tetanus toxin specific T cells revealed that they are present at similar frequencies as those observed for DRB1. These results suggest that DRB3 plays a significant role in antigen presentation with different epitopic preferences to DRB1. Therefore, DRB3, like DRB5, serves to extend and complement the peptide repertoire of DRB1 in antigen presentation.

Hypochlorous Acid: A Natural Adjuvant That Facilitates Antigen Processing, Cross-Priming, and the Induction of Adaptive Immunity

The production of hypochlorous acid (HOCl) is a characteristic of granulocyte activation, a hallmark of the early phase of innate immune responses. In this study, we show that, in addition to its well-established role as a microbicide, HOCl can act as a natural adjuvant of adaptive immunity. HOCl enhances the T cell responses to the model Ag OVA, facilitating the processing and presentation of this protein via the class II MHC pathway. HOCl modification also enhances cross-presentation of the tumor Ag tyrosinase-related protein 2 via class I MHC. The adjuvant effects of HOCl are independent of TLR signaling. The enhanced presentation of HOCl-modified OVA is mediated via modification of the N-linked carbohydrate side chain rather than formation of protein aldehydes or chloramines. HOCl-modified OVA is taken up more efficiently by APCs and is degraded more efficiently by proteinases. Atomic force microscopy demonstrated that enhanced uptake is mediated via specific receptor binding, one candidate for which is the scavenger receptor lectin-like oxidized low-density lipoprotein receptor, which shows enhanced binding to chlorinated OVA. A function of HOCl is therefore to target glycoprotein Ags to scavenger receptors on the APC surface. This additional mechanism linking innate and adaptive immunity suggests novel strategies to enhance immunity to vaccines.

Antigen processing of glycoconjugate vaccines; the polysaccharide portion of the pneumococcal CRM 197 conjugate vaccine co-localizes with MHC II on the antigen processing cell surface

Pneumococcal (Pn) polysaccharides (PS) are T-independent (TI) antigens and do not induce immunological memory or antibodies in infants. Conjugation of PnPS to the carrier protein CRM 197 induces PS-specific antibody in infants, and memory similar to T-dependent (Td) antigens. Conjugates have improved immunogenicity via antigen processing and presentation of carrier protein with MHC II and recruitment of T cell help, but the fate of the PS attached to the carrier is unknown. To determine the location of the PS component of PnPS-CRM 197 in the APC, we separately labeled PS and protein and tracked their location. The PS of types 14-CRM 197 and 19F-CRM 197 was specifically labeled by Alexa Fluor ® 594 hydrazide (red). The CRM 197 was separately labeled red in a reaction that did not label PS. Labeled antigens were incubated with APC which were fixed, permeabilized and incubated with anti-MHC II antibody labeled green by Alexa Fluor ® 488, followed by confocal microscopy. Labeled CRM 197 was presented on APC surface and co-localized with MHC II (yellow). Labeled unconjugated 14 or 19F PS did not go to the APC surface, but PS labeled 14-CRM 197 and 19F-CRM 197 was internalized and co-localized with MHC II. Monoclonal antibody to type 14 PS bound to intracellular type 14 PS and PS-CRM 197. Brefeldin A and chloroquine blocked both CRM 197 and PS labeled 14-CRM 197 and 19F-CRM 197 from co-localizing with MHC II. These data suggest that the PS component of the CRM 197 glycoconjugate enters the endosome, travels with CRM 197 peptides to the APC surface and co-localizes with MHC II.

IVIg decrease antigen-dependent T cell activation by reducing the FcγR-dependent antigen presentation ability of APC (78.36)

Abstract Intravenous immunoglobulins (IVIg) have been shown to have anti-inflammatory effects in a variety of autoimmune diseases, but their mechanisms of action remain unclear. In autoimmune diseases, activated self-reactive T cells invade specific tissues and activate local APC which in turn present autoantigens on their surface, leading to T cell restimulation in a positive feedback loop. Although it was shown that IVIg could reduce T cell activation and modify their cytokine secretion pattern, it is still not clear whether this effect occurs following a direct interaction of IVIg with T cells or through an interference of IVIg with the ability of APC to present autoantigens. To address this question, we used an in vitro antigen presentation system using ovalbumin as model antigen, to study the effects of IVIg on antigen-mediated T cell activation. The results obtained showed that IVIg inhibited T cell activation but that this effect was the indirect consequence of a reduction in the antigen presentation ability of APC. We further showed that this inhibitory effect was not due to modulation of expression of MHC II or CD80/86 costimulatory molecules on the surface of APC and was independent of inhibitory FcγRIIb. Finally, we showed that F(ab')2 fragments of IVIg had no effect on antigen-mediated T cell activation, suggesting that IVIg must interact with activating FcγRs to mediate their inhibitory effect.

Cross-Linking of CD80 on CD4+ T Cells Activates a Calcium-Dependent Signaling Pathway

CD80 expressed on the surface of APCs provides a positive costimulatory signal to naive CD4+ T cells via CD28 during activation. However, CD80 is also expressed on the surface of activated CD4+ T cells, and cross-linking CD80 on the surface of CD4+ T cells activated in the presence of Th1-promoting cytokines induces a direct up-regulation of T-bet, IFN-gamma, and Bcl(XL) expression in primary CD4+ T cells. The present data show that naive CD4+ T cells activated in Th1-promoting conditions in the presence of anti-CD80 mAb increase the level of IFN-gamma produced by increasing the rate of IFN-gamma mRNA transcription, which is supported by an increase in the level of T-bet phosphorylation and T-bet binding to the third intronic enhancer in the IFN-gamma locus. Furthermore, anti-CD80 mAb-induced increase in IFN-gamma expression and T-bet phosphorylation is dependent upon the activation of a Ca2+-dependent pathway as shown by anti-CD80 mAb-induced intracellular Ca2+ flux following CD80 cross-linking. These findings indicate a novel regulatory role for CD80-mediated intracellular signals in CD4+ T cells and have important implications for disease therapies using anti-costimulatory mAbs as use of an intact CD80 mAb may lead to CD80 cross-linking on activated T cells and enhanced proinflammatory cytokine production.

Antigen-independent adhesion and cell spreading by inducible costimulator engagement inhibits T cell migration in a PI-3K-dependent manner

Engagement of the costimulatory protein ICOS activates effector/memory T cells in tissue by enhancing TCR-mediated proliferation and cytokine production. We now report that in an antigen-independent manner, ICOS also induces adhesion and spreading in human effector/memory T cells, consequently inhibiting cell migration. T cell spreading and elongation after ICOS ligation are accompanied by the formation of two types of actin-rich membrane protrusions: thin, finger-like structures similar to filopodia and short, discrete microspikes. Although filopodia/microspike formation occurs independently of the PI-3K signaling cascade, ICOS-mediated T cell elongation depends on PI-3K activity, which inhibits the accumulation of GTP-bound RhoA. Further inhibition of RhoA activation exacerbates the ICOS-mediated, elongated phenotype. We propose that in inflamed tissue, ICOS engagement by ICOS ligand on a professional or nonprofessional APC prevents the forward motility of the T cell by inhibiting RhoA-dependent uropod retraction. The resulting ICOS-induced T cell spreading and filopodia/microspike formation may promote antigen recognition by enhancing a T cell's scanning potential of an adherent APC surface.

Redundancy renders the glycoprotein 96 receptor scavenger receptor A dispensable for cross priming in vivo

CD8(+) T cells (T(CD8+)) differentiate into effector cells following recognition of specific peptide-major histocompatibility complex (MHC) class I complexes (pMHC-I) on the surface of professional APCs (pAPCs), such as dendritic cells. Antigenic pMHC-I can be generated from two spatially distinct sources. The direct presentation pathway involves generation of peptide from protein substrate synthesized within the cell that is presenting the pMHC-I. Alternatively, the cross presentation pathway involves presentation of antigen that is not synthesized within the presenting cell, but is derived from exogenous proteins synthesized within other donor cells. The mechanisms by which cross presentation of exogenous antigens occur in vivo remain controversial. The C-type lectin scavenger receptor A (SR-A) has been implicated in a number of potential cross presentation pathways, including the presentation of peptide bound to heat shock proteins, such as glycoprotein 96 (gp96), and the transfer of pMHC-I from a donor cell to the pAPC. We demonstrate here that initiation of T(CD8+) responses is normal in mice lacking SR-A, and that the redundancy of ligand binding exhibited by the SR family is likely to be an important mechanism that ensures cross presentation in vivo. These observations emphasize the requirement to target multiple receptors and antigen-processing pathways during the rational design of vaccines aimed at eliciting protective T(CD8+).

The FGL2‐FcγRIIB pathway: A novel mechanism leading to immunosuppression

Fibrinogen-like protein 2 (FGL2) is a multifunctional protein, which has been implicated in the pathogenesis of allograft and xenograft rejection. Previously, FGL2 was shown to inhibit maturation of BM-derived DC and T-cell proliferation. The mechanism of the immunosuppressive activity of FGL2 remains poorly elucidated. Here, we focus on identification of FGL2-specific receptor(s) and their ability to modulate APC activity and allograft survival. Using flow cytometry and surface plasmon resonance analysis, we show that FGL2 binds specifically to Fc gamma receptor (FcgammaR)IIB and FcgammaRIII receptors, which are expressed on the surface of APC, including B lymphocytes, macrophages and DC. Antibody to FcgammaRIIB and FcgammaRIII, or deficiency of these receptors, abrogated FGL2 binding. FGL2 inhibited the maturation of BMDC from FcgammaRIIB+/+ mice but not from FcgammaRIIB(-/-) mice and induced apoptosis in the FcgammaRIIB+ mouse B-cell line (A20) but not the A20IIA1.6 cell line that does not express FcgammaRIIB. Recombinant FGL2 infused into FcgammaRIIB+/+ (C57BL/6J, H-2b) mice but not FcgammaRIIB(-/-) mice inhibited rejection of fully mismatched BALB/cJ (H-2d) skin allografts. The identification of specific receptor binding has important implications for the pathogenesis of immune-mediated disease and suggests a potential for targeted FGL2 therapy.

Conferring indirect allospecificity on CD4+CD25+ Tregs by TCR gene transfer favors transplantation tolerance in mice

T cell responses to MHC-mismatched transplants can be mediated via direct recognition of allogeneic MHC molecules on the cells of the transplant or via recognition of allogeneic peptides presented on the surface of recipient APCs in recipient MHC molecules - a process known as indirect recognition. As CD4(+)CD25(+) Tregs play an important role in regulating alloresponses, we investigated whether mouse Tregs specific for allogeneic MHC molecules could be generated in vitro and could promote transplantation tolerance in immunocompetent recipient mice. Tregs able to directly recognize allogeneic MHC class II molecules (dTregs) were obtained by stimulating CD4(+)CD25(+) cells from C57BL/6 mice (H-2(b)) with allogeneic DCs from BALB/c mice (H-2(d)). To generate Tregs that indirectly recognized allogeneic MHC class II molecules, dTregs were retrovirally transduced with TCR genes conferring specificity for H-2K(d) presented by H-2A(b) MHC class II molecules. The dual direct and indirect allospecificity of the TCR-transduced Tregs was confirmed in vitro. In mice, TCR-transduced Tregs, but not dTregs, induced long-term survival of partially MHC-mismatched heart grafts when combined with short-term adjunctive immunosuppression. Further, although dTregs were only slightly less effective than TCR-transduced Tregs at inducing long-term survival of fully MHC-mismatched heart grafts, histologic analysis of long-surviving hearts demonstrated marked superiority of the TCR-transduced Tregs. Thus, Tregs specific for allogeneic MHC class II molecules are effective in promoting transplantation tolerance in mice, which suggests that such cells have clinical potential.

CD83 on murine APC does not function as a costimulatory receptor for T cells

The transmembrane glycoprotein CD83 is rapidly upregulated on murine and human DC upon maturation and therefore a costimulatory function for T cell activation has been suggested. Studies employing human APC indeed showed that CD83 expression was positively correlated to the stimulatory capacity of the APC. Murine APC that were CD83 deficient however, did not display a reduced capacity to activate T cells. To elucidate this contradiction, we thoroughly compared the stimulatory capacity of CD83-overexpressing and CD83-deficient APC. Here we show that CD83 expression levels on APC did not affect the capacity of the APC to activate CD8 + T cells. CD83 expression levels did not significantly affect CD4 + T cell activation in vivo, but a weak positive correlation of CD83 expression with CD4 + T cell activation was observed in vitro under suboptimal stimulation conditions. As CD83 expression also positively correlated with MHC-II but not with MHC-I expression, this differential stimulation specifically of CD4 + T cells could be explained by a higher density of MHC-II peptide complexes on the APC surface. Taken together, our results strongly suggest that CD83 does not deliver crucial costimulatory signals to murine T cells.