Abstract 259: S100A9 is a Master Regulator of Inflammation and Repair After Myocardial Infarction

Background: The innate immune response plays an important role in cardiac repair following an acute myocardial infarction. The pro-inflammatory alarmin S100A8/A9 is released in high amounts locally and systemically during acute coronary events. Here, we studied the effects of S100A9 blockade on the innate immune responses involved in post-ischemic myocardial inflammation and repair. Methods: We induced MI by permanent left coronary artery ligation in C57BL/6 mice, which were subsequently treated with the specific S100A8/A9 blocker ABR-238901 (30mg/kg) or with buffer for 21 days. Left ventricular function was assessed by echocardiography. Immune cell populations in the myocardium, blood, bone marrow and spleen were analysed by flow cytometry. Results: The treatment induced progressive deterioration of the left ventricular systolic function and increased left ventricular volumes in ABR-238901-treated mice compared with controls, suggesting defective repair and negative myocardial remodelling. After 21 days of treatment, infarction size was significantly higher in mice receiving S100A8/A9 blockade (16.1±5.4% vs. 9.9±3.4%, P=0.03). S100A9 blockade inhibited proliferation of hematopoietic stem cells in the bone marrow, and reduced neutrophil and monocyte trafficking from bone marrow and spleen to the circulation and myocardium. Neutrophil and monocyte counts increased in the bone marrow and spleen of mice receiving S100A8/A9 blockade, possibly due to impaired egression. The presence of reparatory CD11b + F4/80 + Ly6C low macrophages expressing the efferocytosis receptor MerTK was potently decreased in the myocardium by day 7 post-MI compared to controls (36 820 ± 2 538 vs. 72 371 ± 4 482 cells/heart, P=0.0001). The transcription factor Nur77 mediates phenotype switching from inflammatory Ly6C hi monocytes to reparatory Ly6C lo macrophages in MI. ABR-238901 lowered the expression of Nur77 in Ly6C hi/int monocytes in-vivo , total bone marrow derived monocytes in-vitro , and the activity of Nur77 in macrophages in-vitro . Conclusions: S100A8/A9 plays an important role in leukocyte trafficking after MI. Long-term S100A8/A9 blockade impairs efferocytosis and myocardial repair, leading to systolic dysfunction and detrimental remodelling.