Induction of Autophagy by Carbon Monoxide Conditioning Enhances Human Mesenchymal Stromal Cell Function During Sepsis

Rationale: Cell therapy using mesenchymal stromal cells (MSCs) is presently under investigation for the treatment of sepsis. We previously demonstrated that conditioning of MSCs with the cytoprotective gas carbon monoxide (CO) improves MSCs function. CO is known to be protective during experimental sepsis, via inhalation of the gas or using CO-releasing molecules, and CO induces autophagy. Our hypothesis is that CO-induced autophagy is critical for improved hMSCs function during sepsis, through their interaction with innate immune cells.

Methods: Human bone marrow-derived MSCs (hMSCs) were conditioned with CO (250 ppm x 4 hours), in the presence or absence of the autophagy inhibitor Spautin1. Cells or vehicle were injected via the tail vein in mice 6 hours after the onset of polymicrobial sepsis induced by cecal ligation and puncture (CLP). To assess the paracrine actions of MSCs, conditioned medium from the cells was also used to assess neutrophil phagocytosis and macrophage efferocytosis.

Results: Injection of hMSCs conditioned with CO after the onset of CLP-induced sepsis led to an increase in mouse survival, compared with mice receiving vehicle or hMSCs+CO in the presence of Spautin1. hMSCs+CO were also able to alleviate organ injury, improve bacterial clearance, and promote the resolution of inflammation. However, after inhibition of autophagy by Spautin1, this CO effect was lost. Exposure of hMSCs to CO, and their conditioned media, were able to increase neutrophil phagocytosis of bacteria and macrophage efferocytosis of apoptotic cells, however these effects were significantly blunted in hMSCs+CO+Spautin1.

Conclusions: Taken together, these data reveal that autophagy is a key mediator in regulating the ability of CO (ex vivo) to enhance hMSCs function in experimental models of sepsis, in part through their impact on innate immune cells, both by cell-to-cell and paracrine actions of hMSCs.