Activation of toll‐like receptor 4/nuclear factor‐kappa B signaling by triggering a receptor expressed on myeloid cells 1 promotes alveolar macrophage M1 polarization and exacerbates septic acute lung injury

Abstract Background Septic acute lung injury (ALI) is a life‐threatening condition commonly occurring in the intensive care unit. Inflammation is considered as the basic pathological response of septic ALI. Triggering receptor expressed on myeloid cells 1 (TREM1) is a member of the immunoglobulin superfamily receptors that regulates the inflammatory response. However, the role of TREM1 in septic ALI has not yet been reported. Methods Cell viability was tested using the MTT assay. TdT‐mediated dUTP nick end labeling assay and flow cytometry were used for apoptosis. The level of protein was detected using western blot analysis. The levels of tumor necrosis factor‐ α and interleukin‐1 β were assessed using enzyme‐linked immunosorbent assay. The lactate dehydrogenase content was assessed using the assay kit. Myeloperoxidase activity was determined using an assay. Histology of lung tissue was further analyzed through hematoxylin–eosin staining. Results We found that TREM1 knockdown by transfection with si‐TREM1 inhibited lipopolysaccharide (LPS)‐induced cell apoptosis of alveolar macrophage cell line MH‐S. The LPS stimulation caused M1 polarization of MH‐S cells, which could be reversed by TREM1 knockdown. In vivo assays proved that si‐TREM1 injection improved lung injury and inflammation of cecal ligation and puncture‐induced ALI in mice. In addition, TREM1 knockdown suppressed the activation of toll‐like receptor 4/nuclear factor‐kappa B signaling, implying the involvement of TLR4 in the effects of TREM1 in response to LPS stimulation. Conclusions This study examined the proinflammatory role of TREM1 in septic ALI and its regulatory effect on alveolar macrophage polarization. These results suggest that TREM1 could potentially serve as a therapeutic target in the prevention and treatment of ALI.