IL-17A production from donor T cells is required for the Mn-induced lung GVHD exacerbation. Lethally irradiated B6D2F1 mice were transplanted with 4 × 10⁶ TCD-BM plus 4 × 10⁶ T cells from WT or IL-17A^−/− B6 mice. Survival (A) and clinical GVHD scores (B, means ± standard error) in WT controls (closed circles), IL-17A^−/− controls (open circles), Mn-treated mice with WT donors (closed diamonds), and Mn-treated mice with IL-17A^−/− donors (open diamonds) from 3 independent experiments were combined and shown (n = 12-15 per group). Representative lung sections were stained with hematoxylin and eosin (C) and the pathology scores of the lungs were analyzed 3 weeks after BMT from: (D) Allo controls with WT donors (gray bar), Allo controls with IL-17A^−/− donors (vertical-striped bar), Mn-treated Allo with WT donors (black bar), and Mn-treated Allo with IL-17A^−/− donors (horizontal-striped bar). Data from 3 independent experiments were combined and shown as means ± standard error. Original magnification ×200. Scale bars represent 100 μm (D). *P < .05; **P < .01.