Pseudomonas aeruginosa is an important opportunistic human pathogen that lives in biofilm-like cell aggregates at sites of chronic infection, such as those that occur in the lungs of patients with cystic fibrosis and nonhealing ulcers. During growth in a biofilm, P. aeruginosa dramatically increases the production of filamentous Pf bacteriophage (Pf phage). Previous work indicated that when in vivo Pf phage production was inhibited, P. aeruginosa was less virulent. However, it is not clear how the production of abundant quantities of Pf phage similar to those produced by biofilms under in vitro conditions affects pathogenesis. Here, using a murine pneumonia model, we show that the production of biofilm-relevant amounts of Pf phage prevents the dissemination of P. aeruginosa from the lung. Furthermore, filamentous phage promoted bacterial adhesion to mucin and inhibited bacterial invasion of airway epithelial cultures, suggesting that Pf phage traps P. aeruginosa within the lung. The in vivo production of Pf phage was also associated with reduced lung injury, reduced neutrophil recruitment, and lower cytokine levels. Additionally, when producing Pf phage, P. aeruginosa was less prone to phagocytosis by macrophages than bacteria not producing Pf phage. Collectively, these data suggest that filamentous Pf phage alters the progression of the inflammatory response and promotes phenotypes typically associated with chronic infection.

Several members of the matrix metalloproteinase (MMP) family function in various processes of innate immunity, particularly in controlling leukocyte influx. Epilysin (MMP-28) is expressed in numerous tissues and, in adult mice, it has the highest expression in lung, where it is detected in bronchial epithelial cells (Clara cells). Epilysin is also expressed by bone marrow-derived macrophages, but not by alveolar macrophages, suggesting that its expression by macrophages is dependent on localization and differentiation. To assess the role of this MMP, we generated epilysin-null (Mmp28−/−) mice. Although epilysin is constitutively expressed in normal tissues, Mmp28−/− mice have no overt phenotype. However, using a murine model of Pseudomonas aeruginosa pneumonia, we found that Mmp28−/− mice had an early increase in macrophage recruitment into the lungs, as well as enhanced bacterial clearance and reduced pulmonary neutrophilia, which we predicted were due to accelerated macrophage influx. Macrophage depletion in WT and Mmp28−/− mice confirmed a role for macrophages in clearing P. aeruginosa and regulating neutrophil recruitment. Furthermore, we observed that macrophages derived from Mmp28−/− mice migrated faster than did wild-type cells to bronchoalveolar lavage fluid from P. aeruginosa-treated mice of either genotype. These observations indicate that epilysin functions as an intrinsic negative regulator of macrophage recruitment by retarding the chemotaxis of these cells.

Members of the MMP family function in various processes of innate immunity, particularly in controlling important steps in leukocyte trafficking and activation. MMP28 (epilysin) is a member of this family of proteinases, and we have found that MMP28 is expressed by macrophages and regulates their recruitment to the lung. We hypothesized that MMP28 regulates other key macrophage responses, such as macrophage polarization. Furthermore, we hypothesized that these MMP28-dependent changes in macrophage polarization would alter fibrotic responses in the lung. We examined the gene expression changes in WT and Mmp28-/- BMDMs, stimulated with LPS or IL-4/IL-13 to promote M1 and M2 cells, respectively. We also collected macrophages from the lungs of Pseudomonas aeruginosa-exposed WT and Mmp28-/- mice to evaluate changes in macrophage polarization. Lastly, we evaluated the macrophage polarization phenotypes during bleomycin-induced pulmonary fibrosis in WT and Mmp28-/- mice and assessed mice for differences in weight loss and total collagen levels. We found that MMP28 dampens proinflammatory macrophage function and promots M2 programming. In both in vivo models, we found deficits in M2 polarization in Mmp28-/- mice. In bleomycin-induced lung injury, these changes were associated with reduced fibrosis. MMP28 is an important regulator of macrophage polarization, promoting M2 function. Loss of MMP28 results in reduced M2 polarization and protection from bleomycin-induced fibrosis. These findings highlight a novel role for MMP28 in macrophage biology and pulmonary disease.

MMP-10 is expressed by macrophages and epithelium in response to injury, but its functions in wound repair are unknown. We observed increased collagen deposition and skin stiffness in Mmp10−/− wounds with no difference in collagen expression or re-epithelialization. Increased collagen deposition in Mmp10−/− wounds was accompanied by less collagenolytic activity and reduced expression of specific metallocollagenases, particularly MMP-8 and MMP-13, where MMP-13 was the key collagenase. Ablation and adoptive transfer approaches and cell-based models demonstrated that the MMP-10-dependent collagenolytic activity was a product of alternatively activated (M2) resident macrophages. These data demonstrate a critical role for macrophage MMP-10 in controlling the tissue remodeling activity of macrophages and moderating scar formation during wound repair.

Inflammatory bowel diseases such as ulcerative colitis represent serious health burdens, both because of the tissue-damaging disease itself, and because of an elevated risk of colon cancer. The increased expression of many members of the matrix metalloproteinase (MMP) family of enzymes that occurs in colitis, has long been associated with the destructive nature of the disease. Recent findings in cancer and other MMP-associated diseases, however, led us to question whether MMPs are indeed detrimental in the setting of colitis. Here, we focus on a single MMP family member, MMP10, and assess its role in a murine model of colonic tissue damage induced by dextran sulphate sodium (DSS) treatment. Using mice genetically deficient for MMP10, we find that absence of this enzyme leads to significantly worse disease scores and failure to resolve inflammation even after extended recovery periods. We show that MMP10 is produced predominantly by infiltrating myeloid cells in both murine and human colitis. Through bone marrow transplant experiments, we confirm that bone marrow-derived MMP10 contributes to colitis severity. Mice lacking MMP10 have a significantly higher propensity for development of dysplastic lesions in the colon after two rounds of DSS exposure. Thus, we conclude that MMP10 is required for resolution of DSS-induced colonic damage, and in its absence, chronic inflammation and ultimately dysplasia occurs.