Augmented early aged neutrophil infiltration contributes to late remodeling post myocardial infarction

Neutrophils oscillate in number and phenotype after being released from bone marrow. Myocardial infarction (MI) outcome is associated with the time-of-day of ischemia onset. However, the underlying contributive factors of neutrophils to cardiac remodeling post MI remain unknown. We examined neutrophil infiltration into the heart and cardiac function and remodeling in C57BL/6J MI model created by permanent coronary ligation at different zeitgeber times (ZT). We found that cell surface markers (CD62L, CXCR2, CXCR4) of neutrophils in peripheral blood lost diurnal oscillation 24 h post MI. Meanwhile, circadian gene Bmal1, Nr1d1, and Clock mRNA expression displayed disrupted diurnal patterns. Flow cytometry showed augmented aged neutrophil (CD11b⁺Ly6G⁺CD62L^low) infiltration into the heart along with increased circulating aged neutrophils in MI groups with more infiltration at ZT5 (p < 0.05), but no difference for aged neutrophil infiltration at different ZT points in late stage. Infiltrated neutrophils had significantly higher CXCL2 and CXCR2 but lower CXCR4 gene expression (p < 0.05). Mice that underwent ligation at ZT5 had high mortality rate and large infarct size. Echocardiography showed that those mice had significantly larger end diastolic and systolic volume and lower ejection fraction (p < 0.05). Immunohistology revealed that those mice displayed more fibrosis, cardiomyocyte hypertrophy, and less angiogenesis compared to ZT13 or ZT21 group (p < 0.05). However, treatment with anti-CXCL2 antibody significantly reduced LV dilatation, fibrosis, hypertrophy and improved cardiac function. These results indicate greater aged neutrophil infiltration into the heart contributes to cardiac hypertrophy, fibrosis, and dysfunction which suggests that blocking neutrophil aging may be a therapeutic alternative following acute myocardial infarction.