A neonatal S100A8+ macrophage subset orchestrates inflammation resolution and cardiac regeneration after myocardial infarction

Background Myocardial infarction (MI) remains a leading cause of heart failure due to the adult mammalian heart's limited regenerative capacity, which precludes the restoration of lost myocardium. In contrast, neonatal mice retain a transient ability to regenerate after injury within the first seven days of life (P7). During this regenerative process, macrophages have been shown to play a pivotal role in cardiac regeneration. The underlying cellular and molecular mechanisms, however, remain poorly understood. Methods & Results Using single-cell RNA sequencing of cardiac macrophages isolated from neonatal and adult mouse hearts ten days post-MI, we identified a neonatal-enriched population with granulocyte-like transcriptional features and high S100A8 expression (S100A8⁺ macrophages). Flow cytometry confirmed a marked expansion of these cells during the subacute phase in neonatal hearts, coinciding with accelerated neutrophil clearance, whereas adult hearts exhibited persistent neutrophil accumulation and inflammation. Genetic ablation of S100A8⁺ macrophages in MRP8-Cre-iDTR mice impaired post-MI cardiac regeneration, as evidenced by reduced cardiomyocyte proliferation, exacerbated inflammation and fibrosis, confirming their essential role in neonatal cardiac repair and regeneration. Ligand-receptor analyses demonstrated that S100A8⁺ macrophages promote cardiomyocyte proliferation and angiogenesis through NRG1–ErbB4 and VEGFA-associated signaling, respectively. Furthermore, in vitro and in vivo assays confirmed that S100A8⁺ macrophages drive neutrophil apoptosis, a process likely mediated by THBS1–CD47 and ANXA1–FPR2 interactions, as specific enrichment of Thbs1 and Anxa1 in this subset was confirmed by independent RNA-Seq. Importantly, re-analysis of historic human cardiac datasets confirmed the infiltration of S100A8⁺ macrophages in the human failing heart, suggesting evolutionary conservation of this subset and highlighting its potential role in myocardial repair. Conclusion Our findings identify S100A8⁺ macrophages as a functionally specialized subset that orchestrates inflammation resolution, cardiomyocyte proliferation, and angiogenesis during neonatal cardiac regeneration, likely through paracrine signaling. Targeting S100A8⁺ macrophage subset and the signaling pathways it engages, therefore, represents a promising therapeutic strategy to enhance myocardial repair and functional recovery following MI.