Multinucleated giant cell formation exhibits features of phagocytosis with participation of the endoplasmic reticulum

Macrophage fusion leading to formation of multinucleated giant cells during chronic inflammation is poorly understood in mechanism and physiological significance. To address this, we developed a system of human macrophage fusion that utilizes IL-4, IL-13, or α-tocopherol to generate large foreign body-type giant cells (FBGC). Extending our previously demonstrated requirements for F-actin and mannose receptor (MR) activity, we find that macrophage fusion exhibits further features of a phagocytic process. Pharmacological inhibition of IL-4-induced FBGC formation indicates critical roles for vacuolar-type ATPase, microtubules, the endoplasmic reticulum (ER), and calcium-independent phospholipase A2 (iPLA2), but not calcium-dependent PLA2 (cPLA2), secretory PLA2 (sPLA2), cyclooxygenase, or lipoxygenase. Immunocytochemistry confirms iPLA2 expression and absence of cPLA2 or sPLA2 expression in macrophages/FBGC. As markers of ER-mediated phagocytosis, calnexin and calregulin are detectable on non-permeabilized fusing macrophages and also concentrated at fusion interfaces where they co-localize with actin in permeabilized macrophages/FBGC. Furthermore, ER markers co-localize with concanavalin A reactivity on non-permeabilized fusing macrophages, suggesting that the ER may present MR ligand during fusion events. These data demonstrate for the first time that the mechanism of macrophage fusion leading to formation of multinucleated giant cells exhibits multiple features of phagocytosis with potential participation of the ER.