Reactive oxygen species suppress phagocyte surveillance by oxidizing cytoskeletal regulators
Despite their superficial similarities, the phagocytosis of pathogens differs from that of apoptotic cells in their recognition mechanisms and downstream signaling pathways. While the initial stages of these processes have been studied, the cytoskeletal reorganization that follows particle uptake is not well understood. By comparing the uptake of phosphatidylserine (PS)- coated targets versus IgG-opsonized targets of identical size, shape, and rigidity, we noted remarkable differences in the accompanying changes in cell morphology, adhesion and migration that persisted long after phagocytosis. While myeloid cells continued to survey their microenvironment after engulfing PS-coated targets, the uptake of IgG-opsonized targets caused phagocytes to round up, decreased their membrane ruffling, and led to the complete disassembly of podosomes. These changes were associated with increased activation of Rho and a concomitant decrease of Rac activity that collectively resulted in the thickening and compaction of the cortical F-actin cytoskeleton. Rho/formin-induced actin polymers were fastened to the membrane by their preferential interaction with Ezrin-Radixin-Moesin (ERM) proteins, which were necessary for cell compaction and podosome disassembly following ingestion of IgG-coated particles. The source of the distinct responses to PS- versus IgG-targets was the differential activation of the respiratory burst mediated by the NADPH oxidase: reactive oxygen species (ROS), emanating from phagosomes containing IgG-opsonized targets - but not those containing PS-coated ones - directly led to the activation of Rho. Similar findings were made with phagocytes that encountered pathogens or microbial-associated molecular patterns (MAMPS) that instigate the activation of the NADPH oxidase. These results implicate a connection between sensing of harmful particulates, the oxidation of cytoskeletal regulators, and the immune surveillance by myeloid cells that have potentially important consequences for the containment of pathogens.