Bleomycin-induced pulmonary fibrosis is independent of eosinophils

Eosinophils have been shown to increase in tissues during many fibrotic con ditions and consequently have been suggested to contribute to the developme nt of fibrosis, This study tested the hypothesis that eosinophils are essen tial in the development of lung fibrosis in mice in response to bleomycin ( BLM), Anti-IL-5 antibody was administered intraperitoneally into mice 2 h p rior to endotracheal BLM inoculation and thereafter, every other day. Lung eosinophilia was evaluated by measurement of eosinophil peroxidase activity and confirmed by eosinophil counts in histologic sections. Lung fibrosis w as evaluated by hydroxyproline content and confirmed by collagen staining i n histological sections. Results demonstrated that BLM induced pronounced l ung eosinophilia, which was maximal 7 days after BLM treatment and remained elevated through day 14, in C57Bl/6 SCID mice and CBA/J mice. In contrast, eosinophilia was a minor component in the lungs of wildtype C57Bl/6 mice a fter BLM treatment, although lung fibrosis developed similarly in all three strains of mice, Treatment with anti-IL-5 completely abrogated eosinophili a but failed to block. pulmonary fibrosis induced by BLM in all mouse strai ns, including C57Bl/6 SCID, wildtype C57Bl/6 mice, and CBA/J mice. Analysis of cytokine mRNA by RNase-protection assay in C57Bl/6 SCID mice indicated that BLM treatment caused enhanced expression of the cytokines, TNF-alpha, and IL-6 at days 3, 7, and 14 post-BLM inoculation, regardless of whether e osinophils were depleted by anti-IL-5, Finally, the importance of eosinophi ls in lang fibrosis was examined in IL-5 gene kockout mice (IL-(tm1Kopf)). BLM treatment induced significant lung fibrosis in IL-5 knockout mice in th e absence of eosinophilia. These findings indicate that eosinophils are not an absolute requirement for BLM-induced pulmonary fibrosis in the mouse.