Macrophage TRIM21 knockout inhibits septic acute lung injury by downregulating autophagy regulator protein ubiquitination

Acute lung injury (ALI) caused by sepsis is a fatal disease characterized by an systemic inflammatory response to invading pathogens. Inducing macrophage macroautophagy/autophagy is a critical strategy to combat the inflammatory response in septic ALI. The E3 ubiquitin ligase TRIM21 plays important roles in autophagy. However, the mechanism connecting macrophage TRIM21-associated autophagy to ALI development remains unclear. Therefore, this study was aimed to investigate the role of macrophage TRIM21 in septic ALI in human and mice. TRIM21 levels were significantly increased in the macrophages of septic mice and in the peripheral blood mononuclear cells and bronchoalveolar lavage fluid of septic ALI patients relative to the controls. Intriguingly, Trim21-specific agonist administration exacerbated ALI and inflammatory responses in septic mice. To elucidate the role of macrophage TRIM21 in the development of septic ALI, we developed a clinically relevant macrophage trim21-specific knockout mouse sepsis model (trim21^M-KO). trim21 deficiency significantly reduced mortality in septic ALI model mice by inhibiting sepsis-induced pulmonary edema and inflammatory infiltration, thereby improving the mechanical barrier properties of the alveolar mucosal epithelium and permeability of the alveolar membrane. Mechanistically, TRIM21 inhibits macrophage autophagy by enhancing the K11-linked ubiquitination of the autophagy-regulating proteins ULK1, SQSTM1/p62, BECN1/beclin1, and MAP1LC3B/LC3B and accelerating their ubiquitination-dependent proteasome degradation. This further promotes pro-inflammatory M1 macrophage polarization, aggravating the inflammation of septic lung tissue and exacerbating ALI. Collectively, our data demonstrate a novel role for macrophage TRIM21 in mediating autophagy to accelerate septic ALI. These new findings may provide a framework for potential interventions against septic ALI.Abbreviations: AL: autolysosome; ALI: acute lung injury; ARG1: arginase, liver; ATG12: autophagy related 12; Baf A1: bafilomycin A₁; BALF: bronchoalveolar lavage fluid; Bbox: B-box; BMDM: bone marrow-derived macrophages; CC: coiled-coil; CCL2/MCP1: C-C motif chemokine ligand 2; CHX: cycloheximide; CLP: cecum ligation puncture; co-IP: co-immunoprecipitation; CQ: chloroquine; DEG: differentially expressed genes; ELISA: enzyme-linked immunosorbent assay; GO: gene ontology; HE: hematoxylin and eosin; IL1B/IL-1β: interleukin 1 beta; KEGG: Kyoto Encyclopedia of Genes and Genomes; LPS: lipopolysaccharide; 3-MA: 3-methyladenine; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MRC1/CD206: mannose receptor, C type 1; NLRP3: NLR family, pyrin domain containing 3; NOS2/iNOS: nitric oxide synthase 2, inducible; PYCARD/ASC: PYD and CARD domain containing; RNA-seq: RNA-sequencing; RT-qPCR: reverse transcription quantitative PCR; TNF/TNF-α: tumor necrosis factor; ULK1: unc-51 like kinase 1.