Matrix metalloproteinase-12 produced by Ly6Clow macrophages prolongs the survival after myocardial infarction by preventing neutrophil influx

Background Various immune cells are involved in different phases of cardiac repair after myocardial infarction (MI). Especially, Ly6Clow M2-like macrophages (Ly6Clo macrophages) are vital for cardiac repair after MI. However, the molecular mechanisms how Ly6Clo macrophages promote wound healing after MI are still largely unknown. Methods and results Transcriptome analysis of Ly6Clo macrophages and Ly6Chigh M1-like macrophages (Ly6Chi macrophages) harvested from the infarcted heart revealed that Ly6Clo macrophages highly expressed matrix metalloproteinase (MMP)-12 mRNA compared to Ly6Chi macrophages. MMP-12 expression was enhanced in the infarcted heart and preferentially observed in Ly6Clo macrophages. Importantly, the survival rate and cardiac function after MI were significantly impaired in MMP-12-deficient (mmp12−/−) mice compared with those in wild-type mice. In addition, the extent of myocardial fibrosis and the number of myofibroblasts in the infarct area were decreased in mmp12−/− mice. MMP-9 expression and neutrophils, which are the major cellular source of MMP-9, in the infarcted heart were increased in mmp12−/− mice. Moreover, mRNA expression of neutrophil-attracting chemokines including CXCL1, CXCL2, and CXCL5 was significantly higher in mmp12−/− mice. Consistently, treatment with anti-CXCR2 antibody significantly decreased neutrophil numbers and MMP-9 expression in the infarcted heart in mmp12−/− mice. Finally, the administration of recombinant MMP-12 into the infarcted heart decreased neutrophil numbers in the infarcted heart and promoted wound healing in both wild-type mice and mmp12−/− mice. Conclusion MMP-12 produced by Ly6Clo macrophages improves the survival after MI possibly through the promotion of wound healing by reducing neutrophil infiltration.