Enhanced frequency of M2 polarized macrophages in peri-infarct region and attenuation of scar formation after transplantation of mesenchymal stem cells

Abstract

Introduction: Ventricular remodelling following myocardial infarction is strongly associated with inflammation, mainly mediated by macrophages. In contrast to the M1 polarized type of macrophages, the M2 type macrophage has been described to exhibit potent regenerative and anti-inflammatory characeristics. Since mesenchymal stem cells (MSC) can influence the M1/M2 polarization of macrophages, we tested MSC mediated M2 macrophage specific effects in a small animal model of myocardial infarction. Methods: Murine bone marrow derived MSC were specifically expanded in vitro and characterized for cell type defining characteristics. MSC were transplanted intramyocardially in a murine model of myocardial infarction. After 1 and 2 weeks hearts from MSC transplanted and sham operated animals were excised and analyzed histologically for expansion of fibrotic scar (Masson's Trichrome staining) and distribution of M2 polarized macrophages (Relm-alpha and F4/80 expression). Tissue from the peri-infarct region was analyzed for expression of M2 macrophage and inflammation specific gene expression by quantitative real-time PCR (qPCR). Results: MSC were validated by flow cytometry (CD44: 64.1 ± 4.1%; Sca-1: 98.1 ± 0.9%) and by successful in vitro adipo-, chondro- and osteogenic differentiation. Hearts from MSC transplanted and sham operated animals showed no significant difference in scar size after one week. However, after two weeks the scar size was significantly smaller in MSC treated animals (12.4 ± 2.3 mm2 vs. 17.8 ± 1.8 mm2; p < 0.05). Furthermore, an increase in F4/80 and Relm-alpha positive M2 macrophages was observed histologically in the peri-infarct area after 2 weeks in MSC transplanted hearts. Compared to sham treated animals qPCR analyzes revealed an increased expression of M2 macrophage associated genes in the peri-infarct region of MSC treated animals after two weeks (CD206: +51.4 ± 8.8%; TSG6: +11.3 ± 1.2%; both p < 0.05), an increase of pan-macrophage marker CD11b expression (+87.2 ± 8.7%; p < 0.05) and a strong increase in expression of anti-inflammatory cytokine IL-10 (+172.3 ± 18.2%; p < 0.01). Conclusion: Intramyocardial transplantation of MSC in the acute phase of myocardial infarction can attenuate the remodeling process and is linked to increased M2 polarization of resident macrophages. The underlying cellular, presumably paracrine mechanisms need to be resolved in detail in order to guarantee safe and efficient potential clinical application.