Lipopolysaccharide-Induced Leukocyte-Endothelial Cell Interactions: A Role for CD14 Versus Toll-Like Receptor 4 Within Microvessels

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Abstract
The objective of this study was to systematically assess leukocyte-endothelial cell interactions in vivo in response to LPS in CD14-deficient (CD14−/−) and Toll-like receptor 4-deficient (TLR4d; C3H/HeJ) mice. Local injection of LPS (0.05 μg/kg) into muscle at a concentration that did not cause systemic effects produced a significant reduction in the speed with which leukocytes roll and a substantial increase in leukocyte adhesion and emigration 4 h postinjection. There was no response to LPS in the muscle microvasculature of CD14−/− mice or TLR4d animals. Systemic LPS induced leukopenia and significant sequestration of neutrophils in lungs in wild-type mice but not in CD14−/− or TLR4d mice. P-selectin expression was examined in numerous mouse organs using a dual radiolabeling mAb technique. The results revealed a 20- to 50-fold increase in P-selectin expression in response to LPS in all wild-type tissues examined but no response in any TLR4d tissues. Surprisingly, there was consistently a partial, significant increase in P-selectin expression in numerous microvasculatures including skin and pancreas, but no increase in P-selectin was detected in lung, muscle, and other organs in CD14−/− mice in response to LPS. Next, the skin and muscle microcirculation were visualized using intravital microscopy after systemic LPS treatment, and the results confirmed a CD14-independent mechanism of leukocyte sequestration in skin but not muscle. In summary, our results suggest that the LPS-induced leukocyte sequestration to some tissues is entirely dependent on both CD14 and TLR4 but there are CD14-independent, TLR4-dependent endothelial cell responses in some microvascular beds.