Alloreactive ICOS^-/- T cells display Th2 immune deviation. Lethally irradiated (1300 cGy split) C3FeB6F1 recipients received transplants with B6 TCD allo-HSCs (5 × 10⁶) and splenic T cells (1 × 10⁶) from WT or ICOS^-/- B6 donors. Mice from 6 independent transplantations (n = 5 to 10) were killed at different days after transplantation, and cytokine production was analyzed. (A) Th2 cytokine profile allo-HSCT recipients of ICOS^-/- T cells. IFN-γ, IL-10, and IL-4 levels were determined by ELISA in serum of allo-HSCT recipients of WT or ICOS^-/- T cells. ⋆Statistical analyses for IFN-γ: day 3, P = .01; day 8, P = .05; day 10, P = .04. Statistical analyses for IL-10: P = .004. Statistical analyses for IL-4: P = .006. These results are representative of 2 independent experiments at each time point. (B-C) Impaired IFN-γ production in ICOS^-/- T cells. Donor T cells were isolated from mice with GVHD at day 10 after allo-HSCT and stimulated with syngeneic (B6) or allogeneic (C3FeB6F1) splenocytes for 5 days. IFN-γ levels were determined by ELISA(B) or by intracellular cytokine staining of T cells from individual mice (C) (ELISA n = 5, intracellular stain n = 2). ⋆Statistical analysis: P < .02; nd indicates not detected.