We have recently shown that a short course of high-dose interleukin-2 (IL-2) can markedly inhibit the graft-versus-host disease (GVHD)- promoting activity of donor CD4+ T cells. The difficulty in dissociating GVHD-promoting from graft-versus-leukemia (GVL) effects of alloreactive donor T cells currently prevents clinical bone marrow transplantation (BMT) from fulfilling its full potential. To test the capacity of IL-2 treatment to promote such a dissociation, we have developed a new murine transplantable acute myelogenous leukemia model using a class II major histocompatibility complex-positive BALB/c Moloney murine leukemia virus-induced promonocytic leukemia, 2B-4–2. BALB/c mice receiving 2.5 x 10(5) 2B-4–2 cells intravenously 1 week before irradiation and syngeneic BMT died from leukemia within 2 to 4 weeks after BMT. Administration of syngeneic spleen cells and/or a 2.5- day course of IL-2 treatment alone did not inhibit leukemic mortality. In contrast, administration of non-T-cell-depleted fully allogeneic B10 (H-2b) spleen cells and T-cell-depleted B10 marrow led to a significant delay in leukemic mortality in IL-2-treated mice. In these animals GVHD was inhibited by IL-2 treatment. GVL effects were mediated entirely by donor CD4+ and CD8+ T cells. Remarkably, IL-2 administration did not diminish the magnitude of the GVL effect of either T-cell subset. This was surprising, because CD4-mediated GVHD was inhibited in the same animals in which CD4-mediated GVL effects were not reduced by IL-2 treatment. These results suggest a novel mechanism by which GVHD and GVL effects of a single unprimed alloreactive T-cell subset can be dissociated; different CD4 activities promote GVHD and GVL effects, and the former, but not the latter activities are inhibited by treatment with IL-2.

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