Inhibition of Macrophage Complement Receptor CRIg by TRIM72 Polarizes Innate Immunity of the Lung.
[Rationale] The complement system plays a critical role in immune responses against pathogens. However, its identity and regulation in the lung are not fully understood. [Objectives] This study aims to explore the role of tripartite motif protein 72 (TRIM72) in regulating complement receptor (CR) of the immunoglobulin superfamily (CRIg) in alveolar macrophage (AM) and innate immunity of the lung. [Methods] Imaging, absorbance quantification and flow cytometry were used to evaluate in vitro and in vivo AM phagocytosis with normal, or altered, TRIM72 expression. Pull down, co-immunoprecipitation and gradient binding assays were applied to examine TRIM72 and CRIg interaction. A pneumonia model was established by intratracheal injection of Pseudomonas aeruginosa. Mortality, lung bacterial burden and cytokine levels in BALF and lung tissues were examined. [Results] Our data showed that TRIM72 inhibited CR-mediated phagocytosis, and release of TRIM72 inhibition led to increased AM phagocytosis. Biochemical assays identified CRIg as a binding partner of TRIM72, and TRIM72 inhibited formation of the CRIg-phagosome. Genetic ablation of TRIM72 led to improved pathogen clearance, reduced cytokine storm and improved survival in murine models of severe pneumonia, specificity of which was confirmed by adoptive transfer of WT or TRIM72KO AMs to AM-depleted TRIM72KO mice. TRIM72 overexpression promoted bacteria-induced NF-κB activation in MH-S cells. [Conclusions] Our data revealed a quiescent non-inflammatory bacterial clearance mechanism in the lung via AM CRIg, which is suppressed by TRIM72. In vivo data suggest that targeted suppression of TRIM72 in AM may be an effective measure to treat fetal pulmonary bacterial infections.