MMP12 and pulmonary emphysema: IL-17RA−/− mice showed lower levels of monocyte chemotactic protein-1 (MCP-1) in BAL fluid and lower expression of the matrix metalloproteinase (MMP)-9 and MMP-12 in the lung, compared to wild-type mice, suggesting an important role for IL-17RA in macrophage recruitment to the inflamed lung and for development of emphysema.