Donor T cell-derived TNF-α critically contributes to the development of IPS. Lethally irradiated B6D2F1 mice underwent transplantation as described in Figure 2 (syngeneic, ▥; allogeneic wild-type, ▪; allogeneic TNF-α^-/-, ▦). A third allogeneic group received allogeneic TNF-α^+/+ bone marrow cells mixed with allogeneic TNF-α^-/- T cells (▥). Lung injury was assessed 35 days after transplantation by lung histopathology (A), BAL cellularity (B), and BAL fluid TNF-α levels (C). Data are presented as mean ± SEM and are from 1 of 2 comparable experiments; n = 4 to 8 per group; ^*P < .05, ▦ and ▥ versus ▪.