Tregs suppress antigen-specific CD8+ T cells in vivo by depleting pMHC-I complexes from Dendritic Cells

Abstract

Abstract T Regulatory cells (Tregs) regulate antigen-specific immune responses by using a variety of mechanisms. Recent studies have shown that antigen-specific Treg can suppress CD4+ effector T cells in vitro and in vivo by depleting peptide-MHC-II complexes from the DC surface. It remains unclear as to how Treg suppress CD8+ T effectors, particularly in vivo. To explore Treg-mediated suppression of CD8+ T cells, we generated CD4+ iTregs from OT-II mice specific for Ova323–339 in association with I-Ab and determined their capacity to suppress CD8+ T cells from OT-I mice specific for Ova257–264 (SIINFEKL) in association with H-2Kb. CD4+ OT-II iTreg suppressed the in vitro proliferation of OT-I T cells in the presence of DCs pulsed with both Ova323–339 and Ova257–264 or with DCs singly pulsed with each peptide. Interestingly, the expansion of OT-I CD8+ T cells in vivo was not suppressed by OT-II iTreg when the two peptides were presented on separately pulsed DCs, but was suppressed when both peptides were presented on the same DCs. OT-II Tregs depleted the Kb-SIINFEKL complexes from the DC surface in vitro by a process of trogocytosis only in the presence of their cognate antigen. The uptake of Kb-SIINFEKL complexes by OT-II Treg was not secondary to leakage of free SIINFEKL from the DC surface as OT-II Treg from MHC-I deficient mice were as efficient in uptake of pMHC-I as Treg from wild type mice. Multiphoton imaging microscopy in vivo reveals a close interaction between Tregs and CD8+ T cells suggesting that suppression of CD8+ T cells in vivo requires close proximity between Treg and responder CD8+ T cell may involve an artificial synapse between Treg and CD8+ T cell.