Donor-derived Th17 are highly proinflammatory and plastic in their cytokine profile. (A-C) CD4⁺YFP⁺ development was assessed in lethally irradiated allogeneic mice transplanted with G-CSF mobilized grafts (B6.IL-17^CreRosa26^eYFP → B6D2F1). (A) Representative FACS analysis showing IL-17A, IFNγ, IL-22, IL-13, TNF, GM-CSF, and IL-10 cytokine expression posttransplant by splenic CD4⁺YFP⁻ or CD4⁺YFP⁺ populations after short-term in vitro restimulation (d7). (B) Frequencies of cytokine-expressing CD4⁺ T cells isolated from spleen and assessed at d7 or d21 post-SCT (mean ± SEM, n = ≥5 mice per group, *P < .05, **P < .01, ***P < .001). (C) GM-CSF and TNF coexpression by IL-17A⁺IFNγ^– and IL-17A⁺IFNγ⁺ CD4⁺ T cells at d7 after allo-SCT (n = 6 mice from 1 representative experiment, *P < .0001). DN, double negative (TNF⁻GM-CSF⁻).