Mesenchymal stromal cells (MSC) anti-microbial effect in acute respiratory distress syndrome (ARDS) is mediated in part by enhanced alveolar macrophage phagocytosis through cell contact-dependent mitochondrial transfer leading to improved macrophage bioenergetics

Background: Previous studies have shown that mitochondrial transfer from MSC to lung endothelial and epithelial cells is an important mechanism of MSC protective effects in pre-clinical models of inflammatory lung diseases. Aims: This study was carried out to investigate the importance of MSC mitochondrial transfer to pulmonary macrophages in the context of ARDS. Methods: In vitro : human bone marrow-derived MSC were co-cultured with primary human monocyte-derived macrophages (MDM). MSC mitochondria were labelled with MitoTrackerRED (Life Sciences), transfer was assessed by flow cytometry and confocal microscopy. In vivo: a mouse model of E.coli pneumonia was used, MSC were given 4 h post infection. Results: Mitochondrial transfer from MSC to macrophages was evident both in vitro and in vivo . Confocal imaging revealed MSC formation of mitochondria positive tunnelling nanotubes (TNT) and microvesicles. MDMs co-cultured with MSC demonstrated enhanced phagocytosis and increased ATP production. Inhibition of TNT formation in MSC by pre-incubation with Cytochalasin B significantly decreased transfer (by 50%) to MDMs and abrogated MSC effects on MDM phagocytosis and ATP production as well as the anti-microbial effect of MSC in vivo . Conclusions: MSC transfer mitochondria to macrophages at least partially through TNT. This donation results in enhancement of macrophage phagocytosis and bioenergetics representing a novel mechanism of the anti-microbial effect of MSC in bacterial-induced ARDS. Funded by MRC UK MR/L017229/1.