Ferroptotic MSCs protect mice against sepsis via promoting macrophage efferocytosis

The therapeutic effect of mesenchymal stem cells (MSCs) on sepsis has been well-known. However, a comprehensive understanding of the relationship between MSCs and macrophages remains elusive. Superparamagnetic iron oxide (SPIO) is one of the most commonly used tracers for MSCs. Our previous study has shown that SPIO enhanced the therapeutic effect of MSCs in a macrophage-dependent manner. However, the fate of SPIO-labeled MSCs (MSC(SPIO)) after infusion remains unknown and the direct interaction between MSC(SPIO) and macrophages remains unclear. Mice were injected intravenously with MSC(SPIO) at 2 h after Escherichia coli infection and sacrificed at different times to investigate their distribution and therapeutic effect. We found that MSC(SPIO) homed to lungs rapidly after infusion and then trapped in livers for more than 10 days. Only a few MSC(SPIO) homed to the spleen and there was no MSC(SPIO) detectable in the brain, heart, kidney, colon, and uterus. MSC(SPIO) tended to stay longer in injured organs compared with healthy organs and played a long-term protective role in sepsis. The mRNA expression profiles between MSCs and MSC(SPIO) were rather different, genes related to lipid metabolism, inflammation, and oxidative stress were changed. The levels of ROS and lipid peroxide were elevated in MSC(SPIO), which confirmed that SPIO-induced ferroptosis in MSC(SPIO). Ferroptosis of MSC(SPIO) induced by SPIO enhanced the efferocytosis of macrophages and thus enhanced the protective effect on septic mice, while the benefits were impaired after MSC(SPIO) were treated with Ferrostatin-1 (Fer-1) or Liproxtatin-1 (Lip-1), the inhibitors of ferroptosis. SPIO-induced ferroptosis in MSCs contributes to better therapeutic effects in sepsis by enhancing the efferocytosis of macrophages. Our data showed the efficacy and advantage of MSC(SPIO) as a therapeutic tool and the cell states exert different curative effects on sepsis.