The inability of donor accessory cells to produce TNF-α results in reduced IPS severity. 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 (hatched bar). 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, gray and hatched bars versus black bar; #P < .05, ▥ versus □.