Bleomycin-induced pulmonary fibrosis is independent of eosinophils

Abstract: Eosinophils have been shown to in-crease in tissues during many fibrotic conditions and consequently have been suggested to contrib-ute to the development of fibrosis. This study tested the hypothesis that eosinophils are essential in the development of lung fibrosis in mice in response to bleomycin (BLM). Anti-IL-5 antibody was adminis-tered intraperitoneally into mice 2 h prior to en-dotracheal BLM inoculation and thereafter, every other day. Lung eosinophilia was evaluated by mea-surement of eosinophil peroxidase activity and confirmed by eosinophil counts in histologic sec-tions. Lung fibrosis was evaluated by hydroxypro-line content and confirmed by collagen staining in histological sections. Results demonstrated that BLM induced pronounced lung eosinophilia, which was maximal 7 days after BLM treatment and re-mained 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 after BLM treatment, although lung fibrosis developed similarly in all three strains of mice. Treatment with anti-IL-5 completely abro-gated eosinophilia but failed to block pulmonary fibrosis induced by BLM in all mouse strains, in-cluding C57Bl/6 SCID, wildtype C57Bl/6 mice, and CBA/J mice. Analysis of cytokine mRNA by RNase-protection assay in C57Bl/6 SCID mice in-dicated that BLM treatment caused enhanced ex-pression of the cytokines, TNF-a, and IL-6 at days 3, 7, and 14 post-BLM inoculation, regardless of whether eosinophils were depleted by anti-IL-5. Finally, the importance of eosinophils in lung fibro-sis was examined in IL-5 gene knockout mice (IL-5tm1Kopf). BLM treatment induced significant lung fibrosis in IL-5 knockout mice in the absence of eosinophilia. These findings indicate that eosino-phils are not an absolute requirement for BLM-induced pulmonary fibrosis in the mouse. J. Leu-koc. Biol. 68: 515–521; 2000. Key Words: lung eosinophilia z cytokines z knockout mic