Activation of TLR4 on endothelium alone initiates neutrophil adhesion within the liver microcirulation during endotoxemia (102.22)

Abstract During sepsis and endotoxemia, neutrophils are recruited to the liver where they cause tissue damage and vascular dysfunction. We have previously reported that neutrophils adhere within sinusoids of the endotoxemic liver via interactions between neutrophil CD44 and endothelial hyaluronan. In this study, we aimed to characterize the cellular sentinels that detect circulating LPS via TLR4 and the pathways leading to initiation of neutrophil adhesion via CD44-hyaluronan interactions. Intravital microscopy of bone marrow chimeric mice revealed that TLR4 expression by non-bone marrow derived cells was required for neutrophil recruitment into the liver during endotoxemia (1 mg/kg LPS, i.v.). Furthermore, LPS-induced neutrophil adhesion in sinusoids was equivalent between wild-type mice and transgenic mice that express TLR4 only on endothelium (tlr4-/-Tie2tlr4, named endoTLR4), but surprisingly, sinusoidal occlusion (vascular damage) was attenuated in endoTLR4 mice. Intravital immunofluorescence imaging demonstrated that stimulation of endothelial TLR4 induced the deposition of serum-derived hyaluronan associated protein (SHAP) within liver sinsusoids, which was required for activation of endothelial hyaluronan to bind neutrophil CD44. These data reveal that endothelial cells are the key sentinels that recognize circulating LPS and initiate neutrophil adhesion in the liver during endotoxemia, but the development of liver pathology requires TLR4 activation on additional cell types.