MHC Class II-Peptide Complexes Displayed on Activated T Cells Guide Treg Suppression

Abstract

Background: Regulatory T cell (Treg) therapy remains a promising strategy for minimizing immunosuppression and extending allograft survival. Treg suppression requires initial activation through Treg-target cell contacts and T cell receptor (TCR) engagement, however, the precise nature of these interactions in the context of allogeneic settings remains to be elucidated. Studies indicate that inflammation facilitates transfer of peptide-MHC-II complexes(pMHCII) from antigen presenting cells (APCs) to activated T cells. Thus, we hypothesize that during inflammation associated with organ transplantation, pMHC-II complexes relocate to alloreactive T effector cells and serve as Treg activation signals. Methods: Transfers of pMHC-II complexes onto activated T effector cell surface were studied in co-cultures of C57BL/6 (B6) CD4+ Teff cells and B6 B lymphocytes isolated from I-Ab-GFP transgenic mice. Suppression assays in vitro were done in co-cultures of APCs (CD90 depleted splenocytes), CD4+CD25- effector T cells (Teff), and CD4+CD25+ Tregs from various MHC backgrounds. Experimental read-out was Teff cell proliferation following stimulation by allogeneic APCs. Similar combinations of Teff and Treg cells were tested in vivo in B6 Rag 1-/- mice reconstituted with Treg/Teff (1:1 or 2:1 ratios) injected IV one day prior to grafting of allogeneic (C3H or BALB/c) tail skin grafts. Results and Conclusions: Data from more than 150 Teff-Treg-APC co-culture experiments unequivocally show that in vitro Treg suppression occurs when Treg and Teff cells have the same MHC-II background, but independently of Treg matching with APC MHC-II. Suppression experiments to determine the origin of pMHC-II complexes were extended in vivo using Teff, Treg, and skin allografts with different MHC-II matching or mismatching. Results recapitulate the in vitro data confirming that Treg activation/suppression proceeds through recognition of “suppress me” pMHC-II tags exposed on Teff cells, leading to graft survival. Collectively, these data imply that Treg regulation is the result of semidirect recognition of donor and/or recipient peptides exposed on activated effector cells. They also suggest that recognition of a limited set of Treg activator signals on Teff cells improves local Treg function by directing suppression only toward activated cell targets.