Flt3-dependent mitochondrial transfer from macrophages alleviates heart failure after myocardial infarction through SEC22B de-ISGylation

Fms-related receptor tyrosine kinase 3 (Flt3) is upregulated in ischemic hearts, yet its macrophage-specific role in post-ischemic remodeling remains unclear. Here, we identify macrophages as the predominant Flt3-expressing cells in the infarcted heart and demonstrate that activation of Flt3 by its ligand (FL) initiates a critical cardioprotective program. FL stimulation drives macrophages from pro-inflammatory M1-like to reparative M2-like phenotypes and promotes the release of extracellular vesicles containing structurally preserved and bioenergetically competent mitochondria, accompanied by metabolic reprogramming and suppression of interferon signaling. Functionally, systemic FL administration or intramyocardial injection of FL-primed macrophage-derived mitochondrial vesicles markedly reduced infarct size, attenuated fibrosis, and preserved cardiomyocyte mitochondrial integrity. Conversely, macrophage-targeted Flt3 knockdown abrogated these benefits, and mitochondrial uncoupling with CCCP abolished the cardioprotective effects of transferred vesicles, establishing that functional mitochondrial transfer is indispensable for cardiac repair. Mechanistically, Flt3 activation promotes STAT3 Y705 phosphorylation, which transcriptionally upregulates TRIM16. TRIM16-mediated K48-linked ubiquitination and degradation of ISG15 prevents ISGylation of the vesicle trafficking protein SEC22B, which, together with USP18-mediated deISGylation, facilitates SEC22B-dependent mitochondrial vesicle transfer to injured cardiomyocytes. The role of this cascade was confirmed by adoptive transfer of TRIM16- or SEC22B-deficient macrophages in vitro and in vivo, which abolished FL-enhanced mitochondrial transfer and cardiac repair. Collectively, our findings uncover a STAT3-TRIM16-ISG15-SEC22B regulatory axis governing macrophage-to-cardiomyocyte mitochondrial crosstalk and identify macrophage Flt3 as a promising therapeutic target for post-infarction heart failure.