C‐C chemokine receptor 2 (CCR2) deficiency improves bleomycin‐induced pulmonary fibrosis by attenuation of both macrophage infiltration and production of macrophage‐derived matrix metalloproteinases

Macrophage infiltration is implicated in various types of pulmonary fibrosis. One important pathogenetic process associated with pulmonary fibrosis is injury to basement membranes by matrix metalloproteinases (MMPs) that are produced mainly by macrophages. In this study, C-C chemokine receptor 2-deficient (CCR2-/-) mice were used to explore the relationship between macrophage infiltration and MMP activity in the pathogenesis of pulmonary fibrosis, using the bleomycin-induced model of this disease process. CCR2 is the main (if not only) receptor for monocyte chemoattractant protein-1/C-C chemokine ligand 2 (MCP-1/CCL2), which is a critical mediator of macrophage trafficking, and CCR2 -/- mice demonstrate defective macrophage migration. Pulmonary fibrosis was induced in CCR2-/- and wild-type (CCR2+/+) mice by intratracheal instillation of bleomycin. No significant differences in the total protein concentration in bronchoalveolar lavage (BAL) fluid, or in the degree of histological lung inflammation, were observed in the two groups until day 7. Between days 3 and 21, however, BAL fluid from CCR2-/- mice contained fewer macrophages than BAL fluid from CCR2+/+ mice. Gelatin zymography of BAL fluid and in situ zymography revealed reduced gelatinolytic activity in CCR2-/- mice. Immunocytochemical staining showed weaker expression of MMP-2 and MMP-9 in macrophages in BAL fluid from CCR2-/- mice at day 3. Gelatin zymography of protein extracted from alveolar macrophages showed reduced gelatinolytic activity of MMP-2 and MMP-9 in CCR2-/- mice. At days 14 and 21, lung remodelling and the hydroxyproline content of lung tissues were significantly reduced in CCR2-/- mice. These results suggest that the CCL2/CCR2 functional pathway is involved in the pathogenesis of bleomycin-induced pulmonary fibrosis and that CCR2 deficiency may improve the outcome of this disease by regulating macrophage infiltration and macrophage-derived MMP-2 and MMP-9 production.