TNF-α production by donor cells is critical to the development of IPS.Lethally irradiated B6D2F1 mice received BMT from either syngeneic (B6D2F1, □), allogeneic wild-type (B6129SF2/J, ▪), or allogeneic TNF-α^-/- (B6.129S6-Tnf^tmGk1, ▦) donors as described in “Materials and methods.” Animals were analyzed at week 5 for (A-B) lung histopathology (hematoxylin and eosin; magnification × 200), (C) BAL fluid cellularity, and (D) BAL fluid TNF-α levels. Data are presented as mean ± SEM and are from 1 of 2 comparable experiments; n = 5 to 9 per group; ^*P < .05. In a second set of experiments, bm1 or bm12 mice received BMT from either syngeneic (bm1 or bm12, □), allogeneic wild-type (B6129SF2/J, ▪), or allogeneic TNF-α^-/- (B6.129S6-Tnf^tmGk1, ▦) donors as described in “Materials and methods.” (E) Lung histopathology and (F) BAL fluid cellularity were decreased after TNF-α^-/- BMT in both systems. Data are presented as mean ± SEM and are from 1 of 2 comparable experiments; n = 4 to 7 per group; ^*P < .05, ▦ versus ▪.