Matrix metalloproteinases (MMPs) comprise a family of extracellular proteinases that function in various processes of innate immunity, such as leukocyte trafficking. MMP28 (epilysin) is one of the newest members of the MMP family, and unlike most MMPs, it functions to dampen inflammatory responses. We have found MMP28 is expressed by lung epithelium, Ly6chigh monocytes, and recruited CD11bhigh pulmonary macrophages. MMP28-null (Mmp28-/-) mice have increased early macrophage recruitment to the lung and other tissues, with increased pro-inflammatory markers in the lung and serum, suggesting that broadly, MMP28 serves to inhibit inflammation. In addition, we have found that Mmp28-/- macrophages migrate faster to chemotactic stimuli and secrete more pro-inflammatory cytokines in response to LPS or bacteria, suggesting that MMP28 participates in inhibiting early macrophage recruitment and pro-inflammatory function in a cell-autonomous fashion. We hypothesize that MMP28 may have evolved as a mechanism by which macrophages, skewed towards an anti- inflammatory phenotype, are recruited during later stages of inflammation. We also hypothesize that these effects are mediated via modulation of a receptor, such as a chemokine receptor or other adhesion protein, localized to the monocyte/macrophage surface. Unpublished studies suggest that reduced MMP28 expression in human monocytes is associated with development of emphysema in smokers, and we show that Mmp28-/- have increased susceptibility to cigarette-smoke induced emphysema.
In aim 1, we will test our hypothesis that absence of MMP28 promotes development of emphysema in smoke-exposed mice via enhanced macrophage recruitment and greater M1 polarization. We will expose wildtype and Mmp28-/- mice to chronic cigarette smoke, and will characterize monocyte and macrophage numbers and phenotypes during several time- points, including regulation of elastinolytic proteinases by MMP28.
In aim 2, we will test our hypothesis that macrophage-derived MMP28 is the critical source that confers resistance to emphysema. To evaluate the relative contribution of MMP28-expression by epithelial cells vs. myeloid cells in macrophage recruitment and lung destruction, we plan to generate bone marrow chimeras and expose them to chronic cigarette smoke along with confirmatory rescue experiments. In our third aim, we plan to identify and validate novel MMP28 substrates involved in macrophage recruitment and polarization using deduction and unbiased approaches with membrane proteomics. We present data on several candidate substrates to be included in validation studies, and results of these studies will lead to novel mechanisms to limit emphysema.
Macrophages are key orchestrators of inflammatory responses in the lung, and they are important contributors to emphysema. MMP28 is a protein that dampens lung inflammation and injury by both restraining macrophage migration and suppressing their pro-inflammatory functions. MMP28 is protective in emphysema, and this proposal will uncover a novel pathway by which inflammatory diseases in the lung may be modified.