Cardiac macrophages adopt profibrotic/M2 phenotype in infarcted hearts: Role of urokinase plasminogen activator

Background: Macrophages (mac) that over-express urokinase plasminogen activator (uPA) adopt a profibrotic M2 phenotype in the heart in association with cardiac fibrosis. We tested the hypothesis that cardiac macs are M2 polarized in infarcted mouse and human hearts and that polarization is dependent on mac-derived uPA. Methods: Studies were performed using uninjured (UI) or infarcted (MI) hearts of uPA overexpressing (SR-uPA), uPA null, or nontransgenic littermate (Ntg) mice. At 7 days post-infarction, cardiac mac were isolated, RNA extracted and M2 markers Argl, YM1, and Fizz1 measured with qrtPCR. Histologic analysis for cardiac fibrosis, mac and myofibroblasts was performed at the same time-point. Cardiac macs were also isolated from Ntg hearts and RNA collected after primary isolation or culture with vehicle, IL-4 or plasmin and M2 marker expression measured. Cardiac tissue and blood was collected from humans with ischemic heart disease. Expression of M2 marker CD206 and Ml marker TNFalpha was measured. Results: Macs from WT mice had increased expression of Argl and Yml following MI (41.3 +/- 6.5 and 70.3 +/- 36, fold change vs UI, n = 8, P < 0.007). There was significant up-regulation of cardiac mac Argl and YM1 with MI in both WT and uPA null mice (n = 4-9 per genotype and condition). Treatment with plasmin increased expression of Arg1 and YIVIl in cultured cardiac macs. Histologic analysis revealed increased density of activated fibroblasts and M2 macs in SR-uPA hearts post-infarction with associated increases in fibrosis. Cardiac macs isolated from human hearts with ischemic heart disease expressed increased levels of the M2 marker CD206 in comparison to blood-derived macs (4.9 +/- 13). Conclusions: Cardiac macs in mouse and human hearts adopt a M2 phenotype in association with fibrosis. Plasmin can induce an M2 phenotype in cardiac macs. However, M2 activation can occur in the heart in vivo in the absence of uPA indicating that alternative pathways to activate plasmin are present in the heart. Excess uPA promotes increased fibroblast density potentially via potentiating fibroblast migration or proliferation. Altering macrophage phenotype in the heart is a potential target to modify cardiac fibrosis. 2016 Elsevier Ltd. All rights reserved.