Background: Regulatory T cell (Tregs) therapy remains a promising strategy for minimizing immunosuppression and extending allograft survival. Although Treg suppression requires initial activation through cell contacts and T cell receptor (TCR) engagement, the precise nature of these interactions in the context of allogeneic settings remains to be elucidated. Several studies indicate that inflammation facilitates transfer of peptide-MHC-II (pMHCII) complexes 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 IA^b-GFP transgenic mice. Suppression assays in vitro were done in co-cultures of APCs (CD90 depleted splenocytes), CD4⁺CD25^neg 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. To confirm that captured pMHC-II originated from host, not graft APCs, and that they provide appropriate “suppress me” signals on activated Teff cells, we extended the suppression experiments 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 indirect 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.

CITATION INFORMATION: Fryer M, Junius S, Germana S, Turka L, LeGuern C. MHC Class II-Peptide Complexes Displayed on Activated T Cells Guide Treg Suppression. Am J Transplant. 2017;17 (suppl 3).

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