The Lung is a Host Defense Niche for Immediate Neutrophil-Mediated Vascular Protection

Bloodstream infection is a hallmark of sepsis, a medically emergent condition requiring rapid treatment. However, up-regulation of host defense proteins through Toll-like receptors (TLRs) and nuclear factor κB requires hours after endotoxin detection. Using confocal pulmonary intravital microscopy, we identified that the lung provides a TLR4–Myd88 (myeloid differentiation primary response gene 88)–dependent and abl tyrosine kinase–dependent niche for immediate CD11b-dependent neutrophil responses to endotoxin and Gram-negative bloodstream pathogens. In an in vivo model of bacteremia, neutrophils crawled to and rapidly phagocytosed Escherichia coli sequestered to the lung endothelium. Therefore, the lung capillaries provide a vascular defensive niche whereby endothelium and neutrophils cooperate for immediate detection and capture of disseminating pathogens.