Notably, we uncovered a monocyte-derived reparative Arg1+ macrophage subtype that exhibited higher enrichment in P1 hearts after injury, while a monocyte-derived proinflammatory monocyte subpopulation characterized by S100a9 and Ly6c expression was uniquely abundant in P10 hearts 3 days after MI. Here, S100A9 is linked to myocardial infarction.