This study investigates the role of macrophages in the transition from acute myocardial ischemia reperfusion (MIR) injury to chronic cardiac fibrosis. Key findings include: 1. **Macrophage Infiltration and Phenotypic Switching**: Following MIR, macrophages infiltrate the heart tissue, with an initial M1 pro-inflammatory phenotype that transitions to a reparative M2 phenotype by day 7 post-MIR. This transition is crucial for fibrosis repair. 2. **GMCSF/CCL2/CCR2 Signaling**: Granulocyte-macrophage colony-stimulating factor (GMCSF) released by mouse cardiac microvascular endothelial cells (MCMecs) recruits macrophages, and CCL2 released by GMCSF-induced macrophages further enhances their migration. CCR2+ macrophages are early responders to inflammation and play a significant role in the inflammatory response. 3. **NLRP3/Caspase-1/IL-1β Signaling**: GMCSF/CCL2/CCR2 signaling activates the NLRP3 inflammasome, leading to the production of inflammatory mediators such as IL-1β, which amplifies the inflammatory response. 4. **TGF-β Release and Fibrosis Progression**: CCL2-mediated CCR2+ macrophages release transforming growth factor-β (TGF-β), promoting the transformation of cardiac fibroblasts into myofibroblasts and advancing fibrosis. 5. **CCR2 Inhibition**: Knocking out the CCR2 gene or using a CCR2 inhibitor effectively reduces infarct size, down-regulates inflammatory mediators, and protects cardiac function, suggesting that CCR2 is a therapeutic target for reducing MIR injury and fibrosis. 6. **Mass Cytometry**: Mass cytometry reveals changes in the cardiac microenvironment, showing increased CD45+ cells, particularly macrophages and dendritic cells (DCs), on day 7 post-MIR. CCR2−/− mice exhibit reduced infiltration of macrophages and DCs, confirming the role of CCR2 in macrophage recruitment. In conclusion, this study highlights the critical role of macrophages in the transition from MIR injury to fibrosis, with GMCSF/CCL2/CCR2 signaling and TGF-β release playing key roles in this process. CCR2 inhibition offers a potential therapeutic strategy to mitigate MIR injury and fibrosis.This study investigates the role of macrophages in the transition from acute myocardial ischemia reperfusion (MIR) injury to chronic cardiac fibrosis. Key findings include: 1. **Macrophage Infiltration and Phenotypic Switching**: Following MIR, macrophages infiltrate the heart tissue, with an initial M1 pro-inflammatory phenotype that transitions to a reparative M2 phenotype by day 7 post-MIR. This transition is crucial for fibrosis repair. 2. **GMCSF/CCL2/CCR2 Signaling**: Granulocyte-macrophage colony-stimulating factor (GMCSF) released by mouse cardiac microvascular endothelial cells (MCMecs) recruits macrophages, and CCL2 released by GMCSF-induced macrophages further enhances their migration. CCR2+ macrophages are early responders to inflammation and play a significant role in the inflammatory response. 3. **NLRP3/Caspase-1/IL-1β Signaling**: GMCSF/CCL2/CCR2 signaling activates the NLRP3 inflammasome, leading to the production of inflammatory mediators such as IL-1β, which amplifies the inflammatory response. 4. **TGF-β Release and Fibrosis Progression**: CCL2-mediated CCR2+ macrophages release transforming growth factor-β (TGF-β), promoting the transformation of cardiac fibroblasts into myofibroblasts and advancing fibrosis. 5. **CCR2 Inhibition**: Knocking out the CCR2 gene or using a CCR2 inhibitor effectively reduces infarct size, down-regulates inflammatory mediators, and protects cardiac function, suggesting that CCR2 is a therapeutic target for reducing MIR injury and fibrosis. 6. **Mass Cytometry**: Mass cytometry reveals changes in the cardiac microenvironment, showing increased CD45+ cells, particularly macrophages and dendritic cells (DCs), on day 7 post-MIR. CCR2−/− mice exhibit reduced infiltration of macrophages and DCs, confirming the role of CCR2 in macrophage recruitment. In conclusion, this study highlights the critical role of macrophages in the transition from MIR injury to fibrosis, with GMCSF/CCL2/CCR2 signaling and TGF-β release playing key roles in this process. CCR2 inhibition offers a potential therapeutic strategy to mitigate MIR injury and fibrosis.