Single-cell RNA sequencing reveals S100a9hi macrophages promote the transition from acute inflammation to fibrotic remodeling after myocardial ischemia‒reperfusion

Rationale:The transition from acute inflammation to fibrosis following myocardial ischemia-reperfusion (MIR) significantly affects prognosis.Macrophages play a pivotal role in inflammatory damage and repair after MIR.However, the heterogeneity and transformation mechanisms of macrophages during this transition are not well understood.Methods: In this study, we used single-cell RNA sequencing (scRNA-seq) and mass cytometry to examine murine monocyte-derived macrophages after MIR to investigate macrophage subtypes and their roles in the MIR process.S100a9 -/-mice were used to establish MIR model to clarify the mechanism of alleviating inflammation and fibrosis after MIR.Reinfusion of bone marrow-derived macrophages (BMDMs) after macrophage depletion (MD) in mice subjected to MIR were performed to further examine the role of S100a9 hi macrophages in MIR.Results: We identified a unique subtype of S100a9 hi macrophages that originate from monocytes and are involved in acute inflammation and fibrosis.These S100a9 hi macrophages infiltrate the heart as early as 2 h post-reperfusion and activate the Myd88/NFκB/NLRP3 signaling pathway, amplifying inflammatory responses.As the tissue environment shifts from proinflammatory to reparative, S100a9 activates transforming growth factor-β (Tgf-β)/p-smad3 signaling.This activation not only induces the transformation of myocardial fibroblasts to myofibroblasts but also promotes fibrosis via the macrophage-to-myofibroblast transition (MMT).Targeting S100a9 with a specific inhibitor could effectively mitigate acute inflammatory damage and halt the progression of fibrosis, including MMT.Conclusion: S100a9 hi macrophages are a promising therapeutic target for managing the transition from inflammation to fibrosis after MIR.