Rapamycin Generates Graft-Homing Murine Suppressor CD8+ T Cells That Confer Donor-Specific Graft Protection

It has been reported that Rapamycin (RPM) can induce de novo conversion of the conventional CD4+Foxp3- T cells into CD4+Foxp3+ regulatory T cells (iTregs) in transplantation setting. It is not clear whether RPM can similarly generate suppressor CD8+ T cells to facilitate graft acceptance. In this study, we investigated the ability of short-term RPM treatment in promoting long-term acceptance (LTA) of MHC-mismatched skin allografts by generating a CD8+ suppressor T cell population. We found that CD4 knockout (KO) mice (in C57BL/6 background, H-2b) can promptly reject DBA/2 (H-2d) skin allografts with mean survival time (MST) being 13 days (P<0.01). However, a short course RPM treatment in these animals induced LTA with graft MST longer than 100 days. Adoptive transfer of CD8+ T cells from LTA group into recombination-activating gene 1 (Rag-1)-deficient mice provided donor-specific protection of DBA/2 skin grafts against co-transferred conventional CD8+ T cells. Functionally active immunoregulatory CD8+ T cells also resided in donor skin allografts. Eighteen percent of CD8+ suppressor T cells expressed CD28 as measured by flow cytometry, and produced reduced levels of IFN-γ, IL-2, and IL-10 in comparison to CD8+ effector T cells as measured by ELISA. It is unlikely that CD8+ suppressor T cells mediated graft protection via IL-10, as IL-10/Fc fusion protein impaired RPM-induced LTA in CD4 KO mice. Our data supported the notion that RPM-induced suppressor CD8+ T cells home to the allograft and exert donor-specific graft protection.