ALBERT

Description: We have isolated a novel cell surface molecule, the human homolog of osteoclast-associated receptor (OSCAR). Unlike mouse OSCAR, hOSCAR is widely transcribed in cells of the myeloid lineage. Notably, hOSCAR is expressed on circulating blood monocytes and CD11c+ dendritic cells but not on T and B cells. hOSCAR is continually expressed during differentiation of CD14+ monocytes into dendritic cells and maintained after maturation. hOSCAR associates with the FcRγ as shown by translocation of FcRγ to the cell surface in presence of hOSCAR and coimmunoprecipitation from transfected cell lines and ex vivo cells. Engagement of hOSCAR with specific mAb leads to Ca2+ mobilization and cytokine release, indicators of cellular activation. Endocytosis of the receptor in dendritic cells was observed, followed by passage of the internalized material into Lamp-1+ and HLA-DR+ compartments, suggesting a role in antigen uptake and presentation. Dendritic cells were able to stimulate a T-cell clone specific for an epitope of mouse IgG1 after uptake and processing of the hOSCAR-specific antibody, demonstrating the capacity of this receptor to mediate antigen presentation. hOSCAR thus represents a novel class of molecule expressed by dendritic cells involved in the initiation of the immune response.

Description: CD4+CD25+ regulatory T cells (Treg) are essential negative regulators of immune responses. However, the mechanisms of immune suppression and the spectrum of cells they target remain incompletely defined. In particular, although Treg directly suppress conventional T cells in vitro, they might also affect antigen presenting cells (APC). Here, we studied the maturation of human myeloid (mDC) and plasmacytoid (pDC) dendritic cells activated with Toll-like receptor (TLR) ligands in the presence of CD4+ CD25high regulatory T cells in vitro. T cells and DC subsets were purified from normal human peripheral blood. LPS, CpG ODN 2216 and R-848 were used to trigger the maturation of mDC, pDC or both through TLR4, TLR9 and TLR7/8 respectively. Preactivated CD4+ CD25high Treg had no effect on the maturation of pDC. Conversely, they strongly suppressed TLR-triggered mDC costimulatory molecules up-regulation, pro-inflammatory cytokines secretion and their antigen presentation capacity, as opposed to conventionnal T cells (Tconv). At a ratio of 3 Treg for 1 DC, the percentage of mDC acquiring CD80 was reduced 5 fold (from 75% to 16%) while the Mean Fluorescence Intensity was decreased by approximately 65% for CD80 and 35% for CD86 after LPS stimulation and by 50% and 20% after R-848 stimulation. Furthermore, Treg dramatically decreased the secretion of IL-12p40, TNF-α, and IL-6 by mDC (95%, 93% and 50% average inhibition respectively) after LPS activation and to a lesser but still significant extent (38%, 35%, and 38% average inhibition respectively) after R848 stimulation. Finally, we found that Treg-conditionned mDC had a reduced ability to trigger naïve T cell proliferation in a mixed leukocyte reaction. Suppression of mDC activation by Treg appeared to require cell-cell contact. Moreover, the inhibition of pro-inflammatory cytokines secretion, but not of phenotypic maturation, was almost completely restored using an anti-IL10 receptor monoclonal antibody, but not anti-TGFβ nor anti-CTLA-4 blocking antibodies. Those data suggest that Treg prevent the co-stimulatory molecules up-regulation on mDC through contact dependent mechanisms, while the modulation of cytokines secretion appears to be largely mediated by IL-10. Overall, our results provide the first evidence of a direct inhibition of human mDC but not pDC maturation by CD4+ CD25high Treg. Therefore, by restraining the maturation of mDC, human Treg may enlist those cells for the initiation and the amplification of tolerance in vivo.