Neutrophil swarm formation on large targets.

Neutrophils are smaller in size than most fungal hyphae. Yet, neutrophils can engage in collective actions known as swarming, during which they cover the hyphae and block their growth. To understand how neutrophil swarms cover large targets efficiently, we employed particle image velocimetry (PIV). We found that the process is initiated at a small number of sites on the surface of a large target. Radiating from these initial sites, waves of intracellular Ca2+ increase in surrounding neutrophils, and neutrophils start converging toward these sites. Multiple swarms form which later merge and ultimately fully encapsulate the target. Neutrophils from immunocompromised patients after liver transplant or hematopoietic stem cell transplant produce swarms that grow slowly and end up smaller compared with those from healthy donors. In the future, the methods and metrics of neutrophil swarming toward large targets may help diagnose and monitor innate immune impairment in transplant patients.