Macrophages in the infarcted heart acquire a fibrogenic phenotype, expressing matricellular proteins, but do not undergo fibroblast conversion.

Although some studies have suggested that macrophages may secrete structural collagens, and convert to fibroblast-like cells, macrophage to fibroblast transdifferentiation in infarcted and remodeling hearts remains controversial. Our study uses linage tracing approaches and single cell transcriptomics to examine whether macrophages undergo fibroblast conversion, and to characterize the extracellular matrix expression profile of myeloid cells in myocardial infarction. To examine whether infarct macrophages undergo fibroblast conversion, we identified macrophage-derived progeny using the inducible CX3CR1CreER mice crossed with the PDGFRαEGFP reporter line for reliable fibroblast identification. The abundant fibroblasts that infiltrated the infarcted myocardium after 7 and 28 days of coronary occlusion were not derived from CX3CR1+ macrophages. Infarct macrophages retained myeloid cell characteristics and did not undergo conversion to myofibroblasts, endothelial or vascular mural cells. Single cell RNA-seq of CSF1R+ myeloid cells harvested from control and infarcted hearts showed no significant expression of fibroblast identity genes by myeloid cell clusters. Moreover, infarct macrophages did not express significant levels of genes encoding structural collagens. However, infarct macrophage and monocyte clusters were the predominant source of the fibrogenic growth factors Tgfb1 and Pdgfb, and of the matricellular proteins Spp1 /Osteopontin, Thbs1 /Thrombospondin-1, Emilin2 , and Fn1 /fibronectin, while expressing significant amounts of several other matrix genes, including Vcan /versican, Ecm1 and Sparc. ScRNA-seq data suggested similar patterns of matrix gene expression in human myocardial infarction. In conclusion, infarct macrophages do not undergo fibroblast or myofibroblast conversion and do not exhibit upregulation of structural collagens but may contribute to fibrotic remodeling by producing several fibrogenic matricellular proteins. Infarct fibroblasts and myofibroblasts are not derived from macrophages. Infarct macrophages do not produce significant amounts of structural matrix proteins but contribute to fibroblast activation through secretion of matricellular proteins, such as EMILIN2, Thrombospondin-1, Osteopontin, but also fibronectin, versican, ECM1 and SPARC. Created with Biorender.com. [Display omitted] • Infarct macrophages do not express fibroblast identity genes. • Lineage tracing shows no significant macrophage to fibroblast conversion. • Infarct macrophages do not express structural collagens. • Human and mouse infarct macrophages express Spp1 , Emilin2, and Thbs1. • Infarct macrophages are a major source of Mmp8, Mmp9, Mmp12, Mmp13, Tgfb1 , and Pdgfb. [ABSTRACT FROM AUTHOR]