Roles of TLR4 in macrophage immunity and macrophage-pulmonary vascular/lymphatic endothelial cell interactions in sepsis.

In sepsis, acute lung injury (ALI) is a severe complication and a leading cause of death, involving complex mechanisms that include cellular and molecular interactions between immune and lung parenchymal cells. In recent decades, the role of Toll-like receptor 4 (TLR4) in mediating infection-induced inflammation has been extensively studied. However, how TLR4 facilitates interactions between innate immune cells and lung parenchymal cells in sepsis remains to be fully understood. This study aims to explore the role of TLR4 in regulating macrophage immunity and metabolism in greater depth. It also seeks to reveal how changes in these processes affect the interaction between macrophages and both pulmonary endothelial cells (ECs) and lymphatic endothelial cells (LECs). Using TLR4 knockout mice and the combined approaches of single-cell RNA sequencing and experimental validation, we demonstrate that in sepsis, TLR4-deficient macrophages upregulate Abca1, enhance cholesterol efflux, and reduce glycolysis, promoting M2 polarization and attenuating inflammation. These metabolic and phenotypic shifts significantly affect their interactions with pulmonary ECs and LECs. Mechanistically, we uncovered that TLR4 operates through multiple pathways in endothelial dysfunction: macrophage TLR4 mediates inflammatory damage to ECs/LECs, while endothelial TLR4 both directly sensitizes cells to lipopolysaccharide-induced injury and determines their susceptibility to macrophage-derived inflammatory signals. These findings reveal the complex role of TLR4 in orchestrating both immune-mediated and direct endothelial responses during sepsis-induced ALI, supporting that targeting TLR4 on multiple cell populations may present an effective therapeutic strategy.