C[D4.sup.+]C[D25.sup.+] regulatory T cells are not required for mesenchymal stem cell function in fully MHC- mismatched mouse cardiac transplantation

Although the immunomodulative properties of mesenchymal stem cells (MSCs) open up attractive possibilities in solid-organ transplantation, information concerning the optimal dose, route, timing of administration, major histocompatibility complex (MHC)-restriction and relevant mechanisms is currently lacking. Therefore, better characterization of MSC immunoregulatory activity and elucidation of its mechanisms are crucial. In this study, we confirmed that MSCs did not elicit proliferation by allogeneic C[D4.sup.+] T cells, suggesting that MSCs were not immunogenic. By using C57BL/6 mouse MSCs as donor-derived or recipient-derived or as third-party MSCs, we discovered that MSCs suppressed C[D4.sup.+] T cell proliferation and prolonged mouse cardiac allograft survival in a dose-dependent and non-MHC-restricted manner. We also found that intraperitoneal administration favored survival prolongation, although this prolongation was weaker than that via the intravenous route. Only infusion at earlier time points favored survival prolongation. Depletion of C[D4.sup.+]C[D25.sup.+] T cells did not affect the immunosuppression of MSCs on C[D4.sup.+] T cells. Moreover, MSCs did not induce regulatory T cells. The in vivo data revealed that MSCs did not increase the percentage of C[D4.sup.+]C[D25.sup.+] T cells and FoxP3 expression. More importantly, we demonstrated for the first time that depletion of C[D4.sup.+]C[D25.sup.+] T cells did not hinder MSC-induced survival prolongation, indicating that C[D4.sup.+]C[D25.sup.+] regulatory T cells were not essential for the prolongation of MSC-mediated allograft survival. Keywords Mesenchymal stem cells * Immunosuppression * C[D4.sup.+]C[D25.sup.+] regulatory T cells * MHC-mismatch * Mouse heart transplantation