Indirect Role for COPI in the Completion of Fcγ Receptor-mediated Phagocytosis

Recent evidence suggests that extension of pseudopods during phagocytosis requires localized insertion of endomembrane vesicles. The nature of these vesicles and the processes mediating their release and insertion are unknown. COPI plays an essential role in the budding and traffic of membrane vesicles in intracellular compartments. We therefore assessed whether COPI is also involved in phagosome formation. We used ldlF cells, a mutant line derived from Chinese hamster ovary cells that express a temperature-sensitive form of εCOP. To confer phagocytic ability to ldlF cells, they were stably transfected with Fc receptors type IIA (FcγRIIA). In the presence of functional COPI, FcγRIIA-transfected ldlF cells effectively internalized opsonized particles. In contrast, phagocytosis was virtually eliminated after incubation at the restrictive temperature. Similar results were obtained impairing COPI function in macrophages using brefeldin A. Notably, loss of COPI function preceded complete inhibition of phagocytosis, suggesting that COPI is indirectly required for phagocytosis. Despite their inability to internalize particles, COPI-deficient cells nevertheless expressed normal levels of FcγRIIA, and signal transduction appeared unimpeded. The opsonized particles adhered normally to COPI-deficient cells and were often found on actin-rich pedestals, but they were not internalized due to the inability of the cells to extend pseudopods. The failure to extend pseudopods was attributed to the inability of COPI-deficient cells to mobilize endomembrane vesicles, including a VAMP3-containing compartment, in response to the phagocytic stimulus.