Delayed donor Th2 cell infusion permits a graft-versus-tumor (GVT) effect to occur with subsequent amelioration of established graft-versus-host disease (GVHD). Relative to GVHD controls (B6-into-BALB/c model), recipients of delayed Th2 cells (day 14 post-BMT) had increased survival (3/3 experiments [exp]; each exp P < .0001) and reduced GVHD by histology analysis 5 days post-Th2 infusion without increased tumor burden (3 of 3 exp; each exp P < or = .02). Th2 cell-mediated amelioration of GVHD was associated with greatly reduced allospecific IFN-gamma secretion, in vivo augmentation of allospecific IL-4 and IL-10 secretion, and reduction in donor CD8(+) T cell number post-BMT (3 of 3 exp; each comparison, P < or = .003). To better understand the molecular mechanism of this GVHD therapy, Th2 cells were generated from wild-type (WT), IL-4 deficient (KO), or IL-10 KO donors: remarkably, recipients of IL-4 or IL-10 KO Th2 cells had no survival advantage, no improvement in GVHD by histology, no reduction in CD8(+) T cell expansion post-BMT, and no in vivo shift toward type II cytokines. We reasoned that IL-2 and alloantigen availability may be limiting factors for Th2 cell therapy, and as such, evaluated whether coadministration of IL-2 or coinfusion of host-type antigen-presenting cells (APC) might intensify the anti-GVHD effect. However, contrary to these hypotheses, concomitant IL-2 therapy or APC administration fully abrogated the Th2 cell-mediated survival advantage and histology-defined GVHD reduction, reduced Th2 cell expansion in vivo while promoting CD8(+) T cell expansion from cells originating from the initial allograft, and impaired type II polarization in vivo. In conclusion, Th2 cell therapy can rapidly ameliorate severe GVHD via IL-4 and IL-10 mediated mechanisms, and potentially, via IL-2 consumption and APC modulation mechanisms.