Where and why do neutrophils stall in retinal microcirculation: Data from low-grade systemic inflammation model in the mouse

Excessive capillary flow heterogeneity results in a microcirculatory dysfunction and hinders optimal oxygen delivery into central nervous system (CNS). Irregularities in cellular flow, like dynamic stalls in capillaries is a potential origin of this heterogeneity. Because under systemic inflammatory conditions neutrophils can become activated and stiffen, they may potentially contribute to capillary stalling of higher frequency and duration. In this study, low dose intraperitoneal lipopolysaccharide (LPS) was administered to transgenic mice with fluorescent capillary pericytes, to induce a low grade systematic inflammatory state, triggering neutrophil activation without hemodynamic instability. LPS resulted in increased submembrane actin density in neutrophils as early as 90 minutes, and this accumulation was prevented by in vivo application of an actin polymerization inhibitor, latrunculin. By repeated in vivo retinal imaging of microcirculation and fluorescently labeled leukocytes before and after LPS, we evaluated leukocyte kinetics, stall parameters and categorized the stall locations, with respect to their spatial association with pericyte somas and bifurcation points. Under baseline conditions, majority of neutrophil stalls occurred at bifurcations with significant association with pericyte somas. Low systemic inflammation resulted in an increased ratio of stalling leukocytes, in capillaries, complying with their submembrane actin condensation. Initial evaluation suggests that latrunculin can have a modulatory effect over neutrophil stiffness in vivo, improving stall parameters and microcirculation quality. Our data will disclose the detailed characteristics of leukocyte flow irregularities in CNS microcirculation under inflammatory conditions and will help understand whether targeting of neutrophil actin skeleton can be a therapeutic option for improving microcirculation dynamics.