Abstract 5054: Bone Marrow MyD88 Contributes to Ischemic Myocardial Infarction and Modulates Neutrophil Function and Chemokine Receptor Expression

Abstract

INTRODUCTION: MyD88 is an adaptor protein critical for innate immune signaling and plays a pivotal role in host defense against infection, but its role in non-infectious myocardial inflammation and injury after ischemia-reperfusion (I/R) is unclear. Our previous studies in MyD88 −/− (KO) mice have demonstrated that MyD88 is essential for I/R-induced myocardial injury. The current study was to delineate the specific contribution of cardiac and bone marrow (BM)-derived MyD88 in mediating ischemic cardiac injury and to explore the underlying mechanisms. METHODS: Chimeric models were generated by lethal irradiation and BM reconstruction. For I/R model, the coronary artery was ligated for 60 min and followed by reperfusion. At the end of reperfusion, the hearts were examined for ischemic and infarct sizes. To assess neutrophil mobility function, thioglycollate was injected intra-peritoneally. Six hours later, peritoneal cells were counted. Neutrophils were labeled with anti-Ly-6C and counted by flow cytometry. CXCR2 expression, which is critical for neutrophil mobility function, was measured by flow cytometry. RESULTS: There was a significant decrease in infarct sizes in KO compared to WT (22±1% vs. 37±2%, p <0.001). Compared to WT→WT control (Donor→Recipient), KO→WT mice, which lacked MyD88 in BM cells but maintained intact MyD88 signaling in the heart, had smaller MI sizes ( p < 0.05). Surprisingly, infarct sizes of WT→WT mice were also significantly reduced compared to WT ( p < 0.05). To determine if MyD88 deficiency inhibits neutrophil function that contributes the observed cardiac benefit, we examined the neutrophil mobility and CXCR2 expression. Indeed, neutrophil mobility was dramatically decreased in KO mice compared to WT mice ( p < 0.01). Neutrophil mobilization also decreased in WT→WT and KO→WT chimeric mice ( p < 0.01). Moreover, there was a significant decrease in CXCR2 expression on the peritoneal neutrophils of KO and KO→WT mice ( p < 0.01), but not in WT→WT mice. CONCLUSIONS: Taken together, these findings suggest that MyD88 signaling, most likely that of BM-derived neutrophils plays a critical role in mediating myocardial I/R injury. MyD88 may mediate I/R injury partly by modulating CXCR2 expression and thus neutrophil function. This research has received full or partial funding support from the American Heart Association, Founders Affiliate (Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Vermont).