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CD+ cells from CsA-treated DA rats with long-surviving PVG heart allografts specifically suppress the capacity of naive CD4+ cells to restore allograft rejection in irradiated DA rats, but have normal donor-specific alloreactivity in MLC. CD4+ suppressor cells from CsA-treated DA rats cultured for 3 days against either PVG or DA spleen cells lost the capacity to transfer suppression into irradiated DA rats grafted with PVG hearts and regained the ability to mediate rejection. However, these cells retained suppressor function when stimulated with donor-specific alloantigen in media supplemented with 20% Con A supernatant. CD4+ cells from CsA-treated rats cultured against either third-party stimulator cells or syngeneic cells expressing anti-PVG idiotype in media supplemented with Con A supernatant failed to maintain suppressor cell function. CD4+ cells from CsA-treated rats cultured in media supplemented with Con A supernatant alone also failed to maintain suppressor function. Suppressor cell function in culture was not maintained by rIL-2. mAb to the IL-2 receptor a chain (CD25) prevented the maintenance of suppressor cell function in media supplemented with Con A supernatant. Con A supernatant is rich in IFN-γ, but addition of an anti-IFN-γ mAb to the culture did not affect the maintenance of suppressor cells. These studies demonstrate that the CD4+ suppressor cell from CsA-treated rats with long-surviving grafts is short-lived; its survival is dependent upon contact with specific alloanti-gens and cytokines, one of which is IL-2. In the absence of cytokines and/or specific alloantigen, the CD4+ cells regain the capacity to initiate graft rejection in irradiated rats, suggesting that within the CD4+ subpopulation there is a fragile balance between cells with the capacity to suppress and effect rejection.