Background Neutrophils play a major role in inflammation after myocardial ischemia-reperfusion (I/R) injury. The effects of mesenchymal stem cells (MSCs) on neutrophils in I/R are complex and not fully understood. This study was designed to investigate the effects and mechanism of MSCs on alleviating myocardial I/R injury in rats. Methods and Results MSCs induced M2 macrophages polarization in vitro and enhanced macrophage efferocytosis of apoptotic neutrophils, measured by fluorescence-activated cell sorting analysis and immunofluorescence staining. Rats myocardial I/R were induced by transient ligation of left anterior descending coronary. Adipose-derived MSCs or vehicle were infused at initiation (immediate after reperfusion) or peak of inflammation (24 hours after I/R). Hematoxylin and eosin, 2,3,5-triphenyltetrazolium chloride/Evans Blue staining and immunofluorescence staining were applied within 72 hours after cell infusion. Cardiac function was assessed by echocardiography and left cardiac catheterization analysis at 28 days post-operation. MSCs infused immediately and 24 hours later both markedly ameliorated myocardial I/R injury, and immediate infusion had more significant outcome. These improvements were associated with neutrophils infiltration, measured by fluorescence-activated cell sorting analysis and immunofluorescence staining. When infused immediately, MSCs did not significantly change neutrophil number at 24 hours but CD11b expression was significantly higher. When infused at 24 hours, MSCs markedly decreased neutrophil number by enhanced M2 macrophage infiltration and macrophage efferocytosis of neutrophils within 72 hours. Conclusions Efferocytosis is pivotal to relieve neutrophil-mediated I/R injury and initial the immune response for healing. MSCs infusion improves cardiac function in rats after myocardial I/R via the possible mechanism of enhancing M2 macrophages-induced efferocytosis of apoptotic neutrophils.
Keywords: efferocytosis; inflammation; mesenchymal stem cells; myocardial ischemia; reperfusion injury.