Macrophage TMEM175-Mediated Transcriptional Regulation of Cathepsins Drives Bacterial Killing in Sepsis.

Sepsis is a life-threatening condition characterized by a dysregulated host response to infection, in which lysosomal acidification and hydrolytic activity are crucial for effective pathogen clearance by macrophages. Transmembrane Protein 175 (TMEM175) is a key regulator of lysosomal pH, and its deficiency is reported to cause lysosomal over-acidification and impaired cathepsin D (CTSD) activity. However, the role of TMEM175 in sepsis remains uncertain. We hypothesize that TMEM175 deficiency exacerbates sepsis by impairing CTSD activity. TMEM175 mRNA expression levels were measured in peripheral blood mononuclear cells (PBMCs) from septic patients and healthy controls (HCs). Using a cecal ligation and puncture (CLP) mouse model, we evaluated the effects of the TMEM175 inhibitor 2-phenylcyclopentylamine (2-PPA) on mortality and bacterial load in target organs. In vitro, TMEM175-knockdown in macrophages was performed to assess its impact on bacterial clearance, phagocytosis, lysosomal acidification, and cathepsin maturation. Results showed that TMEM175 expression was significantly downregulated in PBMCs from septic patients. Pharmacological inhibition of TMEM175 in CLP mice exacerbated bacterial burden and increased mortality. Mechanistically, TMEM175 deficiency impaired macrophage-mediated bacterial clearance, despite having no effect on the initial phagocytic uptake. This dysfunction was attributed to TMEM175, which directly regulates cathepsin transcription. Additionally, TMEM175 deficiency suppressed PI3K-Akt signaling, reducing inflammatory cytokine production.