MHC-derived allopeptide activates TCR-biased [CD8.sup.+] Tregs and suppresses organ rejection

In a rat heart allograft model, preventing T cell costimulation with CD40Ig leads to indefinite allograft survival, which is mediated by the induction of [CD8.sup.+][CD45RC.sup.lo] regulatory T cells ([CD8.sup.+]CD40Ig Tregs) interacting with plasmacytoid dendritic cells (pDCs). The role of TCR-MHC-peptide interaction in regulating Treg activity remains a topic of debate. Here, we identified a donor MHC class Il-derived peptide (Du51) that is recognized by TCR-biased [CD8.sup.+]CD40Ig Tregs and activating [CD8.sup.+]CD40Ig Tregs in both its phenotype and suppression of antidonor alloreactive T cell responses. We generated a labeled tetramer (MHC-I [RT1.A.sup.a]/ Du51) to localize and quantify Du51-specific T cells within rat cardiac allografts and spleen. [RT1.A.sup.a]/ Du51-specific [CD8.sup.+]CD40Ig Tregs were the most suppressive subset of the total Treg population, were essential for in vivo tolerance induction, and expressed a biased, restricted Vβ11-TCR repertoire in the spleen and the graft. Finally, we demonstrated that treatment of transplant recipients with the Du51 peptide resulted in indefinite prolongation of allograft survival. These results show that [CD8.sup.+]CD40Ig Tregs recognize a dominant donor antigen, resulting in TCR repertoire alterations in the graft and periphery. Furthermore, this allopeptide has strong therapeutic activity and highlights the importance of TCR-peptide-MHC interaction for Treg generation and function.