Two Cellular Components of Bone Marrow Origin Contribute to Pulmonary Irradiation Fibrosis.

Following lung irradiation of C57BL/6NHsd mice, migration of marrow origin macrophages into the lungs is detectable at 100 days followed by migration of marrow origin fibroblasts into areas of developing organizing alveolitis/fibrosis. To demonstrate the importance of each cellular migration, mice were irradiated to 20 Gy to the pulmonary cavity. Beginning at day 80, the mice were injected (daily for 15 days I.P.) with antibodies to either mouse macrophages or to a pan-T-cell antigen (M1/70 or TIB-207, respectively). Groups of mice were injected at day 80 with DS-red labeled Smad3+/+ clonal bone marrow stromal cell line cells or a GFP+ Smad3−/− bone marrow stromal cell line. The mice were then followed for survival. Mice injected with the M1/70 antibody had increased survival compared to those receiving TIB-207 or control irradiated mice (percent survival at 180 days 90% and 60% compared to 40%, p=0.0225 and 0.8263). There was a decrease in the number of macrophages in the lungs of mice injected with M1/70 compared to TIB-207 injected or control irradiated mice. Mice injected with bone marrow stromal cells from the Smad3−/− line, which have abrogation of the TGF-β signaling pathway, showed decreased numbers of labeled cells in areas of organizing alveolitis/fibrosis compared to mice that were injected with Smad3+/+ cells at 120 days (5.4 ± 1.6% vs. 21.0 ± 3.3% cells per 100x high powered field in areas of organizing alveolitis/fibrosis, respectively, p = 0.0003). These results demonstrate a critical importance of both marrow hematopoietic (macrophage) and stromal fibroblasts to the development of irradiation-induced pulmonary fibrosis.