INTRODUCTION: Mitochondrial transplantation (MTx) is an emerging strategy for restoring cellular bioenergetics and mitigating ischemia-reperfusion (IR) injury. We previously demonstrated that MTx improved neurological outcomes and survival in a rat model of cardiac arrest (CA). However, the mechanisms underlying these benefits, particularly regarding immune modulation and transcriptional regulation, remain unclear. METHODS: Adult C57BL/6 mice and Sprague-Dawley rats underwent CA and resuscitation protocols, followed by intravenous MTx with species-matched donor mitochondria. Survival and neurological outcomes were assessed up to 72 h. Biodistribution and cellular uptake of fluorescent dye-labeled mitochondria were analyzed via in vivo imaging and flow cytometry. Gene expression related to mitochondrial dynamics, inflammation, and immune regulation was evaluated using qPCR. RESULTS: MTx improved 72-hour survival (33.3% vs. 0%, P = 0.006) and neurological scores compared to vehicle treatment. Reduced brain edema was observed in MTx-treated animals. Mitochondrial uptake was significantly enhanced in the brain and spleen post-CA, with key infiltrating and resident immune cell populations-including monocytes, macrophages, microglia, astrocytes, and endothelial cells-preferentially internalizing transplanted mitochondria. Circulating myeloid cells rapidly internalized functional mitochondria, with 53.9% uptake in MTx-treated CA animals versus 10.6% in controls (P <
0.001). MTx also modulated immune profiles, reducing pro-inflammatory macrophages and enhancing cytotoxic T cell numbers. Gene expression analysis showed that MTx downregulated Fission 1, preserved Mitofusin 2, and upregulated protective genes, including Hmox1, Sirt1, and Entpd1. CONCLUSIONS: MTx improves outcomes after CA, accompanied by mitochondrial uptake by immune cells and redistribution to injured tissues. This process likely modulates immune responses, enhances mitochondrial fusion, and activates cytoprotective gene expression.