Pseudomonas aeruginosa infection is associated with alveolar macrophage M2 polarization via MERTK-mediated NLRC4 inflammasome activation

Introduction. As a leading cause of severe pulmonary infections, such as hospital-acquired pneumonia, Pseudomonas aeruginosa (PA) poses a significant threat to public health. Macrophage polarization plays a central role in the control of PA infection; however, its precise regulatory mechanisms remain to be fully elucidated. Hypothesis. It remains unclear whether MERTK participates in the regulation of macrophage polarization induced by PA infection, as well as its potential downstream molecular mechanisms, especially the association with the NLRC4 inflammasome. Aim. This study was designed to investigate the specific role and underlying molecular mechanisms of MERTK-mediated macrophage phenotypic switching during PA infection, with the goal of defining the MERTK–NLRC4 macrophage polarization regulatory axis. Methodology. The expression dynamics of MERTK in alveolar macrophages from PA-infected mice were detected via Reverse transcription quantitative real-time PCR (RT‑qPCR) and Western blotting (WB). Flow cytometry was employed to determine the proportions of M1 (CD86+F4/80+) and M2 (CD206+F4/80+) macrophages. ELISA was utilized to quantify the levels of M1-associated (TNF- α , IL-6) and M2-associated IL-10, transforming growth factor (TGF)- β inflammatory cytokines, while the phagocytic activity of macrophages against PA was detected. WB was further applied to detect the expression of cleaved caspase-1 and N-GSDMD in the NLRC4 inflammasome pathway. The secretion of IL-1 β and IL-18 and the release of lactate dehydrogenase were assessed. Results. PA infection induced the upregulation of MERTK in alveolar macrophages. MERTK knockdown facilitated macrophage polarization toward the M1 phenotype while suppressing M2 polarization. Mechanistically, MERTK knockdown impaired NLRC4 inflammasome activation. Functional rescue experiments validated that MERTK overexpression activated M2 polarization by activating NLRC4 inflammasomes, which was reversed by NLRC4 knockdown. Conclusion. By upregulating MERTK to activate NLRC4 inflammasomes, PA facilitated M2 polarization of alveolar macrophages. This discovery furnished a critical theoretical foundation for the development of novel therapeutic strategies for PA infection targeting the MERTK–NLRC4 inflammasome–macrophage polarization axis.