Allogeneic CD8+ T cells mediate both a graft-vs-leukemia (GVL) effect and graft-vs-host disease (GVHD). To evaluate whether CD8 cells of defined cytokine phenotype differentially mediate these processes, alloreactive donor CD8+ T cells preferentially secreting type I or type II cytokines were generated by alloantigenic priming in vitro in the presence of IL-12 or IL-4, respectively. Both cytokine-secreting subsets lysed allogeneic tumor targets in vitro ("Tc1" and "Tc2" subsets). A transplantation model was established (B6 into B6C3F1, 1050 cGy host irradiation) using the 32Dp210 myeloid line (bcr/abl transfected, H-2k; 1 x 10(4) tumor cells/recipient). Compared with leukemia controls (death at 12.9 days post-bone marrow transplantation), both Tc1 and Tc2 recipients were conferred a survival advantage. At cell doses of 2 to 2.5 x 10(7), the Tc1-mediated GVL effect (mean survival of 34.2 days) was more potent than the Tc2-mediated GVL effect (mean survival of 20.5 days; Tc1 > Tc2, p = 0.009). On day 15, histologic examination showed that Tc1 recipients had undetectable tumor burdens, whereas Tc2 recipients had extensive leukemic infiltrates. However, Tc2 recipients had essentially no histologic evidence of GVHD, whereas Tc1 recipients had mild to moderate GVHD (average GVHD scores of 1/40 and 9.3/40, respectively). In contrast, recipients of uncultured CD8+ donor T cells developed severe GVHD (average GVHD score of 26.7/40). Because in vitro-generated, alloreactive Tc1 and Tc2 populations mediated GVL with reduced GVHD, we conclude that both subsets may improve the therapeutic outcome of allogeneic T cell transfers in patients with leukemia.