Arp2/3 Complex Is Required for Macrophage Integrin Functions but Is Dispensable for FcR Phagocytosis and In Vivo Motility

Abstract

SUMMARY The Arp2/3 complex nucleates branched actin, forming networks involved in lamellipodial protrusion, phagocytosis and cell adhesion. We derived primary bone marrow macrophages lacking Arp2/3 complex (Arpc2−/−) and directly tested its role in macrophage functions. Despite protrusion and actin assembly defects, Arpc2−/− macrophages competently phagocytose via FcR and chemotax towards CSF and CX3CL1. However, CR3 phagocytosis and fibronectin haptotaxis, both integrin-dependent processes, are disrupted. Integrin-responsive actin assembly and αM/β2 integrin localization are compromised in Arpc2−/− cells. Using an in vivo system to observe endogenous monocytes migrating toward full-thickness ear wounds we found that Arpc2−/− monocytes maintain cell speeds and directionality similar to control. Our work reveals that the Arp2/3 complex is not a general requirement for phagocytosis or chemotaxis, but is a critical driver of integrin-dependent processes. We demonstrate further that cells lacking Arp2/3 complex function in vivo remain capable of executing important physiological responses that require rapid directional motility.

Integrating Physical and Molecular Insights on Immune Cell Migration

“…Interestingly, similar observations have been made in neutrophils [23] as well as in T lymphocytes, in which mDia1 is required for migration [24,25] while Arp2/3 is not [26]. In agreement with these findings, Arp2/3 activity is mainly required for Integrin-dependent cellular functions of macrophages in vivo, this complex being dispensable for the locomotion of these cells in 3-dimensional environments [27]. It therefore appears that most leukocytes do not rely on branched actin nucleation for migration in confinement, consistent with their migration being independent of adhesion in such complex environment.…”

“…Formins might also play a role in the recently described "short-term directional memory of migrating HL-60 neutrophil-like cells" DCs can be fully attributed to impaired Arp2/3 activation has not been shown. Interestingly, recent results obtained on Arp2/3-deficient macrophages suggest that it is only required for these cells to follow immobilized gradient of chemokines, an integrin-dependent process referred to as haptotaxis [27].…”

Loss of Hem1 disrupts macrophage function and impacts on migration, phagocytosis and integrin-mediated adhesion

“…Phagocytosis requires protrusion of the plasma membrane to envelop the target particle, and is well accepted to involve both WASP and WAVE proteins (13,23,67,74). Earlier work already established that Arp2/3 complex -albeit not essential -is crucially contributing to phagocytosis (55,75), while the precise differential contributions of WASP and WRC remained elusive. Accordingly, phagocytosis was found here to be detectable even in the absence of Hem1, although the engulfment of different targets such as bacteria, yeast and opsonized red blood cells or beads was significantly delayed or defective to different degrees as compared to WT and WASP KO macrophages.…”

“…Indeed, Freeman and colleagues demonstrated in an elegant piece of work how integrin activation leads to the formation of a diffusion barrier around the opsonized target, which facilitates coordinated particle engulfment during Fc gamma receptor-initiated phagocytosis (87). Interestingly, another recent study using Arp2 knockout cells identified a critical role for Arp2/3 complex-mediated actin assembly in integrin activation, affecting all integrin-dependent processes in macrophages, including migration, adhesion and phagocytosis (75). However, the specific participation of single Arp2/3 complex activators could not yet be delineated, since all Arp2/3-mediated processes were simultaneously switched off.…”

Membrane-cytoskeleton mechanical feedback mediated by myosin-I controls phagocytic efficiency

“…It is generally accepted that the F-actin within the phagocytic cup is primarily nucleated by the Arp2/3 complex, although evidence of forminassisted nucleation also exists [50][51][52] . Indeed, staining the actin waves with Arp3 antibody showed discrete puncta in WT macrophages and significant Arp2/3 recruitment to the actin clumps of the dKO macrophages ( Fig.…”