Glutamine alleviates immunosuppression in polymicrobial sepsis by augmenting bacterial phagocytosis through sustaining the GFAT-DRP1 dependent mitochondrial calcium dynamics.

Sepsis triggers impaired macrophage bacterial phagocytosis, rendering the host more vulnerable to secondary infections, a manifestation termed sepsis-associated immunosuppression. Glutamine (Gln) is a vital nutrient in critical illness that not only supports energy production and biomass synthesis but also potentially exerts immunomodulatory effects. The aim of the present study was to investigate whether supplementation of Gln modulates macrophage phagocytosis and mitigates sepsis-induced immunosuppression. Using a murine model of polymicrobial sepsis, we evaluated the effects of Gln supplementation on bacterial load, cytokine production, and survival. In multiple in vitro assays, we employed molecular and pharmacological approaches to dissect Gln-dependent signaling pathways in recovering the immunosuppressive macrophages. We found that Gln deficiency impaired macrophage phagocytosis and exacerbated sepsis-induced immunosuppression. In contrast, exogenous Gln supplementation restored macrophage function and improved survival in septic mice-effects that were abolished upon macrophage depletion. Mechanistically, Gln promoted glutamine-fructose-6-phosphate transaminase (GFAT)-dependent protein O-GlcNAcylation, leading to dynamin-related protein 1 (DRP1) oligomerization. Concurrently, Gln activated a GFAT-mediated, cyclin-dependent kinase 1-dependent pathway that induced DRP1 phosphorylation at Ser-616 irrelevant of O-GlcNAcylation. These effects enhanced DRP1-mediated mitochondrial fission, increased mitochondrial calcium efflux, and sustained cytosolic calcium levels essential for phagocytosis. In conclusion, our study demonstrates that Gln strengthens macrophage phagocytosis and alleviates immunosuppression in sepsis through a dual GFAT-DRP1 mechanism co-ordinating mitochondrial dynamics and calcium signaling, highlighting the GFAT-DRP1-calcium axis as a potential therapeutic target for treating sepsis-induced immunosuppression.