IFNγ prevents the development of idiopathic pneumonia syndrome. Tissue was collected at day 9 after transplantation from recipients of G-CSF–mobilized allogeneic or TCD grafts. (A) Semiquantitative histopathology was scored as described in “Materials and methods.” GI tract, *P < .03 (n = 4-5 per group); liver (n = 4-5 per group); skin **P < .01 (n = 11 in T-cell-replete and n = 4 in TCD groups); lung, **P < .01 (n = 6 per group). (B) Representative example of histological pulmonary tissue from wt and IFNγR^−/− recipients of T-cell-replete and TCD grafts 9 days after transplantation (100 ×). (C) Donor (H-2D^d+) cellular infiltrate in pulmonary tissue was determined by flow cytometry. Significant increases in donor CD4⁺ T cells (*P < .02), CD8⁺ T cells, and F480⁺ macrophages (**P < .01) were seen in the lungs of IFNγR^−/− recipients of T-cell-replete allogeneic grafts compared with wt recipients. Ly6G⁺ neutrophils were significantly decreased in IFNγR^−/− recipients (**P < .01). Data represents mean plus or minus SE cells per right upper lobe of lung.