Abstract

The rapid and selective accumulation of neutrophils into the lungs is thought to underlie the pulmonary failure that leads to sepsis-related death. In this study we investigated whether neutrophil TLR4 is important in LPS-induced pulmonary neutrophil recruitment by creating chimeric mice (transferring bone marrow between TLR4(+/+) and TLR4(-/-) mice). In TLR4(+/+) mice receiving TLR4(-/-) bone marrow, 6 weeks after transplant TLR4 was absent in all circulating leukocytes as well as in resident macrophages (these mice were termed LeukocyteTLR4(-/-)), and these cells were completely nonresponsive to LPS. In TLR4(-/-) mice receiving TLR4(+/+) bone marrow, endothelial cells but not leukocytes were deficient in TLR4 (EndotheliumTLR4(-/-)). Surprisingly, systemic LPS (0.5 mg/kg) induced a dramatic increase in neutrophil sequestration into the lungs of LeukocyteTLR4(-/-) mice over the first 4 hours. Concomitantly, numbers of circulating leukocytes decreased by 90%. By contrast, EndotheliumTLR4(-/-) mice showed very little increase in neutrophil sequestration in the lungs, suggesting that endothelium rather than leukocyte TLR4 was important. Intravital microscopy of peripheral microcirculation in the cremaster muscle revealed about 30-fold more leukocyte-endothelial cell interactions in LPS-treated EndotheliumTLR4(-/-) mice than in LPS-treated LeukocyteTLR4(-/-) mice. This is consistent with less sequestration of leukocytes into the lungs of EndotheliumTLR4(-/-) mice. In conclusion, our data challenge the view that LPS directly activates neutrophils to trap in lungs and suggest a far more important role than previously appreciated for the endothelial cells.