Rationale: Cardiac macrophages (cMP) are increasingly recognized as important regulators of myocardial homeostasis and disease, yet the role of noncoding RNA in these cells is largely unknown. Small RNA sequencing of the entire miRnomes of the major cardiac cell fractions revealed microRNA-21 (miR-21) as the single highest expressed microRNA in cMPs, both in health and disease (25 and 43% of all microRNA reads respectively). MiR-21 has been previously reported as a key microRNA driving tissue fibrosis.

Objective: To determine the function of macrophage miR-21 on myocardial homeostasis and disease-associated remodeling.

Methods and results: Mice with macrophage-specific (Cx3cr1-Cre-mediated) genetic deletion of miR-21 were protected from interstitial fibrosis and cardiac dysfunction when subjected to pressure overload of the left ventricle. Single cell sequencing of pressure-overloaded hearts from these mice revealed that miR-21 in macrophages is essential for their polarization towards a M1-like phenotype. Systematic quantification of intercellular communication mediated by ligand-receptor interactions across all cell types revealed that miR-21 primarily determined macrophage-fibroblast communication, promoting the transition from quiescent fibroblasts to myofibroblasts. Differentiation of isolated macrophages in vitro towards a pro-inflammatory (M1) phenotype activated myofibroblast transdifferentiation of cardiac fibroblasts in a paracrine manner and was dependent on the rapid induction of miR-21 in cMPs.

Conclusions: Our data indicate a critical role of cMPs in pressure overload-induced cardiac fibrosis and dysfunction and reveal macrophage miR-21 as a key molecule for the pro-fibrotic role of cMPs.