Mechanism(s) of TH17-mediated pathologic lesions. TH17 cells were generated as described in “Methods.” (A) TNF-α expression in in vitro–differentiated cells (left), TH17 cells isolated from the lung 8 days after transplantation (WT TH17 cells; middle and IFN-γ^−/− TH17 cells; right). (B) Serum TNF-α concentrations on day 12 after transplantation from recipients of TH17 cells, total T cells, and BM only. TH17 cells (5 × 10⁶) from WT mice were transferred with T cell–depleted bone marrow cells into lethally irradiated B6D2 recipients. Animals were treated with 500 μg of anti–TNF-α (n = 6), 200 μg of anti–IL-17A (n = 6), or 500 μg rat IgG (n = 7) biweekly for 4 weeks. Animals were monitored for survival (C), signs of GVHD (*P < .01 [anti–TNF-α vs rat IgG] and remained significant from day 7 on, D), and weight loss *P < .01 [anti–TNF-α vs rat IgG] (**P values remained significant from day 18 on) (E). (F) Comparison of pathologic skin lesions in recipients of 5 × 10⁶ TH17 cells receiving anti–IL-17A (left animal), anti–TNF-α (middle animal), or rat IgG (right animal). (G) TH17 cells were generated as described in “Methods.” Cells were then stimulated with PMA and ionomycin for 4 hours followed by RNA extraction. IL-21 and IL-22 expression were determined by quantitative PCR. Data are presented as fold change compared with naive PMA and ionomycin stimulated CD4⁺/CD25⁻ cells.