How Mitochondria Steer Macrophages Between Inflammation and Immune Paralysis in Sepsis

Why this matters: Sepsis is a dysregulated host response to infection that causes organ dysfunction. The review cites roughly 48.9 million cases and more than 11 million deaths a year, nearly 20% of global mortality. Outcomes remain poor despite better antibiotics and ICU care, especially in the elderly and immunocompromised. Sepsis typically begins with a hyperinflammatory phase and often moves into prolonged immunosuppression that invites secondary infections. Macrophages orchestrate both phases, so understanding what controls their behavior is a major therapeutic goal.

What the review covers: This is a narrative review, not a systematic analysis or a new experiment. It synthesizes work on how mitochondrial metabolism shapes macrophage polarization. The text available for analysis was truncated partway through the mtROS section, so the points below rest on the abstract, introduction, polarization and early metabolism sections. The authors explain that the classic M1/M2 split is too simple. Single-cell RNA sequencing shows that macrophages in septic tissue often carry mixed or hybrid inflammatory and suppressive features, along with mixed glycolytic and oxidative metabolism.

Key mechanisms: M1 macrophages shift to aerobic glycolysis (a Warburg-like state) and break the TCA cycle at isocitrate dehydrogenase and succinate dehydrogenase. This causes citrate and succinate to accumulate. Succinate stabilizes HIF-1α and boosts IL-1β in a positive feedback loop. Mitochondrial ROS, oxidized mtDNA and potassium efflux activate the NLRP3 inflammasome. Leaked mtDNA also activates cGAS-STING. M2-like cells depend on oxidative phosphorylation and fatty acid oxidation, supported by PPARγ, PGC-1α, SIRT3 and TREM2, with lower mTORC1 and higher AMPK activity. Itaconate dampens inflammation by inhibiting SDH and NLRP3. Lactate can promote M2 genes through histone lactylation and can restrain cGAS via lactylation. Fragmented mitochondria and impaired mitophagy (DRP1, MFN1/2, PINK1–Parkin) undermine the metabolic flexibility needed to move from an inflammatory to a reparative state.

Clinical and therapeutic implications: Prolonged M1-skewed profiles are linked to acute lung injury, acute kidney injury and cardiovascular collapse. Later, immunoparalysis features lower MHC class II, endotoxin tolerance and raised PD-L1, TREM2 and Arg1. High TREM2 on circulating monocytes has been correlated with poor prognosis. Preclinical approaches include the mitochondria-targeted antioxidant MitoTEMPO, which reduces mtROS and NLRP3 activation, and AMPK activators such as metformin, which favor M2 polarization. Others are HIF-1α inhibition, PD-L1 or TREM2 blockade, stem cell-derived extracellular vesicles, and nanocarrier-delivered RNA targeting PDK1 or SIRT3. The abstract also notes that metabolic reprogramming differs by organ (kidney, lung, heart), which supports a precision-medicine framing.

Caveats: Most supporting evidence comes from cell and animal models. The therapeutic ideas are not yet validated in sepsis patients. A reductive M1-versus-M2 framework risks oversimplifying a dynamic, time-dependent disease. Timing is likely critical, because suppressing inflammation during the immunosuppressive phase could be harmful.