Memory CD4⁺ T cells are suppressed by CD8⁺ regulatory T cells in vitro and in vivo

Background: Acute graft rejection mediated by alloreactive memory CD4⁺ T cells is a major obstacle to transplantation tolerance. It has been reported that CD8⁺ T regulatory cells (Tregs) have the ability to induce graft tolerance by restraining the function of activated CD4⁺ T cells, but not including memory T cells. The aim of this study is to elucidate the effect of CD8⁺ Tregs on alloreactive memory CD4⁺ T cells. Methods: We detected Qa-1 expression and performed proliferative assay on memory CD4⁺ T cells. All memory CD4⁺ T cells were purified from mice receiving skin allografts. We performed inhibitory and cytotoxic assays on CD8⁺ Tregs, which were isolated from a T cell vaccination mouse model, and IL-2, IL-4, IL-10 and IFN-γ levels were measured in co-culture supernatants by ELISA. To confirm CD8⁺ Tregs inhibition of memory CD4⁺ T cells in-vivo, we utilized a murine model of cardiac allograft transplantation. Results: Memory CD4⁺ T cells mediated acute allograft rejection, and CD8⁺ Tregs suppressed the proliferation of memory CD4⁺ T cells. In vitro, memory CD4⁺ T cells were inhibited and lysed by CD8⁺ Tregs. There was a positive correlation between IFN-γ levels, and cell lysis rate induced by CD8⁺ Tregs. In-vivo studies demonstrated CD8⁺ Tregs prolonged graft survival times, by inhibiting CD4⁺ memory T cells, through a Qa-1-peptide-TCR pathway. Conclusions: CD8⁺ Tregs inhibit CD4⁺ memory T cell-mediated acute murine cardiac allograft rejection, and further prolong graft survival times. These results provide new insights into immune regulation of organ rejection.