Combined therapy of CD4+CD25+ regulatory T cells with low-dose sirolimus,but not calcineurin inhibitors,preserves suppressive function of regulatory T cells and prolongs allograft survival in mice

Our previous study proved that sirolimus is a potent immunosuppressant which induces long-term allograft survival depends on persistence of alloantigens. CD4⁺CD25⁺ regulatory T (Treg) cells are potent suppressors in transplantation. Our objectives focus on whether combined-therapy of Tregs with immunosuppressants could prolong allograft survival in mice.The study showed that inhibition of Tregs was maintained by co-cultured with sirolimus (1 nM) in vitro, but not tacrolimus (1 nM) or CsA (1 nM). When the concentration was increased > 100 nM, suppression was fallen. Based on the ability of sirolimus to target effector T cells, but retaining the inhibition of Tregs, an adoptive infusion of donor alloantigen specific Tregs combined with 30-day sirolimus (1 mg/kg) and 3-day ATG (20 mg/kg) was found to prolong heart allograft survival in mice. Even though the cell numbers of CD4⁺ T cells were found to decrease in sirolimus-treated mice, sirolimus selectively enhanced the numbers of CD4⁺CD25⁺ cells and increased the expression of Foxp3 in spleens and lymph nodes, respectively, in recipients. However, combined therapy with low-dose CsA (5 mg/kg) or tacrolimus (1 mg/kg) reduced significantly the expression of Foxp3 and failed to prolong the allograft survival.In summary, expanded Tregs exposed to sirolimus can survive, proliferate, and preserve inhibition in vitro. Tregs are more resistant to sirolimus than other T cells. Combined with Tregs, sirolimus rather than calcineurin inhibitors, prolongs the allograft survival. Sirolimus may be the best copartner for Tregs therapy. It also suggests calcineurin-dependent signals may be required in the development of Tregs.