Chemotaxing neutrophils enter alternate branches at capillary bifurcations.

Upon tissue injury or microbial invasion, a large number of neutrophils converge from blood to the sites of injury or infection in a short time. The migration through a limited number of paths through tissues and capillary networks seems efficient and ‘traffic jams’ are generally avoided. However, the mechanisms that guide efficient trafficking of large numbers of neutrophils through capillary networks are not well understood. Here we show that pairs of neutrophils arriving closely one after another at capillary bifurcations migrate to alternating branches in vivo and in vitro. Perturbation of chemoattractant gradients and the increased hydraulic resistance induced by the first neutrophil in one branch biases the migration of the following neutrophil towards the other branch. These mechanisms guide neutrophils to efficiently navigate through capillary networks and outline the effect of inter-neutrophil interactions during migration on overall lymphocyte trafficking patterns in confined environments. Neutrophils mobilize rapidly and travel through the vasculature and microcirculation en masse in response to inflammatory stimuli. Here the authors use a microfluidic device and intravital microscopy to show neutrophils move through tissues in a highly ordered pattern, taking turns to file into alternate branches at bifurcations.