Temporal gradients limit the accumulation of neutrophils toward sources of chemoattractant

Neutrophil trafficking during inflammation is a highly orchestrated process, coordinating neutrophil recruitment, sterilization of the wound, and inflammation resolution. Although the chemotactic signals guiding neutrophil recruitment to sites of inflammation are relatively well understood, our knowledge of mechanisms controlling cessation of neutrophil recruitment and return to normal tissue physiology remains incomplete. To gain insights into these processes, we designed a microfluidic device with an array of chemoattractant reservoirs, which mimics the microenvironment in infected tissues, when multiple clusters of microbes are present. We monitored the temporal dynamics of neutrophil recruitment toward the chemoattractant reservoirs at single cell resolution, for 3 h. We observed robust neutrophil recruitment that reached a plateau after 1.5 h, despite the continuous presence of strong chemoattractant gradients around the reservoirs. The timing of the plateau was dependent on the geometry of the devices and was independent from the number of neutrophils. On the basis of these observations, we ruled out sub-population sensitivity, chemoattractant scavenging, and production of a self-limiting stop signal as potential mechanisms underpinning the plateau in neutrophil recruitment. We found a strong correlation between the temporal stabilization of concentration changes and the plateau in neutrophils recruitment. These results suggest that dynamic aspects of chemoattractant gradients are key for maximizing recruitment during the acute phase of infections and limiting the accumulation of neutrophils as soon as the infection is contained.