IL-17A is generated predominantly from donor CD8 T cells after transplantation. (A) Survival by Kaplan-Meier analysis of lethally irradiated B6 recipients (n = 20 per group) transplanted with bone marrow and purified splenic T cells from nonmobilized BALB/c.WT or BALB/c.IL-17A^−/− donors. A non-GVHD control group received TCD bone marrow only (n = 7). Data combined from 2 replicate experiments. Final clinical scores from week 7 after BMT are also shown. (B) Semiquantitative histopathology of skin 7 weeks after BMT (n = 9 per GVHD group and n = 2 in TCD group). **P < .01, P < .05, WT vs IL-17A^−/−. (C) Representative histopathology at week 7 after BMT (original magnification ×200). (D) Lethally irradiated B6 recipients received nonmobilized WT splenocytes or G-CSF–mobilized splenocytes from WT or IL-17A^−/− BALB/c donors (n = 5-14 per group). Seven days after SCT, splenic T cells were stimulated for 12 hours with CD3 and cytokine levels determined in culture supernatants. **P < .01, *P < .05, G-CSF–mobilized WT and IL-17A^−/− vs nonmobilized. ND indicates not detected. (E) Ratio of effector (CD4⁺FoxP3^neg) to regulatory (CD4⁺FoxP3⁺) T cells in spleen of respective B6 recipients of BALB/c grafts. (F) Representative plots of IL-17A and IFN-γ secretion in splenic (SP) and inguinal lymph node (LN) cells 7 days after SCT. (Right) The number of respective cytokine-producing CD4 and CD8 T cells per spleen (n = 4 per group, one of 3 experiments). *P < .05, CD8 vs CD4. (G) Relative expression of RORγt in sort-purified donor CD4 and CD8 T cells 7 days after SCT (n = 5 per group). (H) Representative plots of IL-17A and IL-21 secretion in splenic CD4 and CD8 T cells 7 days after SCT.