Rapamycin or tacrolimus alone fails to resist cardiac allograft accelerated rejection mediated by alloreactive CD4+ memory T cells in mice

Donor-reactive memory T (Tm) cells undermine transplanted organs more readily than naive T cells. Rapamycin (RAPA) and tacrolimus (FK-506) are current mainstay immunosuppressants used for preventing acute allograft rejection. Although their efficacy in suppressing naive T cell is established, their suppressing effect on memory T cells is undefined. This study was conducted to investigate the inhibiting capability of RAPA or FK-506 against transferred alloreactive CD4+ Tm cells in a mouse cardiac transplant model. We found that these drugs alone prolonged the median survival time (MST) of allograft from 5 days to 9 days in recipient mice with CD4+ Tm infusion (P < 0.01), which however was not significantly longer than that (8 days) in untreated recipient mice without CD4+ Tm infusion (naive control). Mean histologic rank of rejection activity in section of cardiac allograft on day 5 postgrafting was Grade 4 in the Tm control recipients versus Grade 3A in both of the immunosuppressant treatment recipients with CD4+ Tm infusion. RAPA or FK-506 alone failed to completely suppress proliferation and differentiation of the alloreactive CD4+ Tm, which was confirmed by in vitro mixed lymphocyte reaction (MLR) and by flow cytometry (FCM) of the splenocytes for detecting CD44highCD62L− effector/memory as well as CD69+/CD25+ activation phenotype cells from the respective recipients. Furthermore, the agent alone didn't completely inhibit the activation of CD4+ Tm, for serum level of IFN-γ and its gene expression at the cardiac allograft from the immunosuppressant-treated recipients were as still high as the untreated naive control. Thus, RAPA or FK-506 alone couldn't completely suppress the proliferation and activation of the alloantigen-primed CD4+ Tm cells responding to the alloantigen, indicating that alloreactive CD4+ Tm was insensitive to these immunosuppressants. The characteristics of alloreactive CD4+ Tm to resist immunosuppressants and its potency to initiate quick and vigorous rejection despite treatment with the immunosuppressant make it to be a critical barrier to prolongation of allograft survival and induction of transplant tolerance.