Can promotion of neutrophil apoptosis enhance repair in the infarcted myocardium and resolution of sterile peritonitis?
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Efferocytosis, the clearance of apoptotic cells including apoptotic neutrophils by macrophage phagocytosis, is a key cellular mechanism for resolution of inflammation and tissue repair. Cyclin-dependent kinases (CDKs) 7 and 9 phosphorylate RNA polymerase II that is vital for neutrophil transcriptional capacity. CDK inhibitors such as R-roscovitine, and the more selective inhibitor AT7519, induce neutrophil apoptosis and promote resolution of several mouse models of inflammation including acute lung inflammation. The hypothesis investigated here was that AT7519 would promote neutrophil apoptosis (i) in the infarcted heart, leading to macrophage polarisation, angiogenesis, reduced infarct expansion and retention of cardiac function and (ii) in the peritoneum, enhancing resolution of sterile peritonitis. AT7519 (1μM) induced apoptosis of mouse unstimulated-bone marrow derived neutrophils and thioglycollate-stimulated neutrophils in vitro in a time- and caspase-dependent manner, but did not alter activation assessed by calcium flux in response to the synthetic formyl peptide (fMLF) or platelet-activating factor (PAF). Only high concentrations of AT7519 (10 μM) induced monocyte/macrophage apoptosis and this was likely due to saturated phagocytosis of apoptotic neutrophils induced by high concentration of AT7519. Myocardial infarction (MI) was induced by coronary artery ligation in adult male mice and infarct volume was assessed 7 or 21 days later by in vitro optical projection tomography (OPT). The novel use of OPT for this purpose was validated by demonstrating correlation with infarct volume obtained by late-gadolinium enhanced magnetic resonance imaging in vivo and with infarct area assessed by histological staining (Masson’s Trichrome) in tissue sections. AT7519 (30 mg/kg i.p.) increased the number of apoptotic neutrophils (cleaved caspase-3 and Ly6G +ve) in the heart when administered after MI, but this was not associated with any subsequent alteration in macrophage polarisation, vessel density, infarct expansion or structural and functional remodelling of the left ventricle. In contrast, induction of neutrophil apoptosis by AT7519 (30mg/kg i.p.) successfully promoted macrophage polarisation and the resolution of inflammation associated with peritonitis elicited by either 10% thioglycollate or by 1mg zymosan. AT7519 treatment also reduced the number of CD19+ B cells, Foxp3+CD4+ T cells and eosinophils in peritoneal lavage, and prolonged the phase of monocyte recruitment in zymosan-induced peritonitis. In conclusion, AT7519 successfully induced mouse neutrophil apoptosis in vitro, as well as in vivo in experimental MI and peritonitis. Subsequent promotion of inflammation resolution in peritonitis was not matched by improved outcome following MI. Unexpected effects of CDK inhibition on monocytes, T cells and eosinophils that are necessary for myocardial infarct repair may have compromised any beneficial effects resulting from promotion of in situ neutrophil apoptosis. CDK inhibition may therefore have therapeutic potential for the treatment of peritonitis, but not for prevention of infarct expansion and detrimental ventricular remodelling after MI.