*Purpose: Immune rejection of transplanted stem/progenitor cells is a major stumbling block in designing effective therapy for myocardial infarction (MI). Human neonatal mesenchymal stem cells (nMSCs) showed superior cardiac functional recovery compared to adult MSCs in immune competent rat MI model. However, molecular mechanisms underlying immune evasion by transplanted nMSCs in the infarcted myocardium was unexplored. Here, we demonstrate for the first-time the expression and function of CD47 in human nMSCs and its novel mechanism of immune evasion increases its regenerative potential in rat MI model.

*Methods: Cardiac functional outcome following nMSCs/aMSCs in rat MI model was measured by echocardiography. Transplanted cell retention, in vivo phagocytosis and CD68⁺ cells was assessed by immunohistochemistry. In vitro phagocytosis analysis for aMSCs/nMSCs were performed with THP-1 derived macrophages. THP-1 differentiated macrophages were validated for the expression of CD68 and absence of CD31. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used for proteomic analysis of nMSCs and aMSCs. CD47 expression in nMSCs was blocked by anti-CD47/ siRNA transections/lenti-virus expressing CD47 shRNAs approaches. CD47 knock down efficiency by siRNA and lenti-virus were validated by immunoblot. Isotype antibody, scrambled siRNA and lentivirus expressing empty vector served as control.

*Results: Transplanted nMSCs showed significant increased cell retention, reduced phagocytosis and CD68⁺ cells compared to aMSCs in vivo rat MI model. Comparative proteomic analysis between nMSCs and aMSCs showed that CD47, a transmembrane protein highly upregulated in nMSCs. Increased CD47 expression in nMSCs inhibited phagocytosis compared to aMSCs in vitro and in vivo (p<0.05). Further, CD47 blockade in nMSCs using anti-CD47, siRNA and shRNA lentiviral based approaches increased in vitro, in vivo phagocytosis, CD68⁺ cells and reduced cell retention and MI recovery in vivo.

*Conclusions: In conclusion, increased CD47 expression in nMSCs evade phagocytosis by CD47/SIRPα immune regulatory axis to demonstrates its immune evasion potential and its therapeutic applications in regenerative medicine.

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