Differential Recruitment of Activating and Inhibitory FcγRII during Phagocytosis

Abstract

Human myeloid cells express both activating and inhibitory receptors of the FcγRII family. FcγRIIA mediates processes associated with cell activation, including phagocytosis of IgG-opsonized particles, whereas coengagement of the inhibitory FcγRIIB downregulates such signaling. We analyzed the relative recruitment of these two receptors during phagocytosis of IgG-coated particles by ts20 Chinese hamster fibroblast cells cotransfected with both receptors carrying distinguishable fluorescent protein tags. We found that FcγRIIA is substantially enriched at sites of particle binding relative to its inhibitory counterpart, with a greater than 2-fold increase in the local ratio of activating to inhibitory receptor compared with that for the plasma membrane as a whole. Experiments with chimeric receptors revealed that the preferential enrichment of FcγRIIA results from differences between the extracellular domains of the receptors, and indicated that the lesser recruitment of FcγRIIB limits its ability to effectively inhibit FcγRIIA-mediated phagocytosis. Mutagenesis studies indicated that FcγRIIA residues leucine 132 and phenylalanine 160, which lie in IgG-binding regions of FcγRIIA and which differ in FcγRIIB, both contribute to the local relative enrichment of FcγRIIA by increasing its affinity for IgG1 relative to that of FcγRIIB. In human monocytes, engagement of approximately equal amounts of FcγRIIB was required to substantially inhibit FcγRIIA-mediated phagocytosis. These results demonstrate that differences in affinity for IgG between activating and inhibitory FcγR can result in substantial local changes in their relative concentrations during phagocytosis, with important functional consequences.

Trastuzumab mediates antibody-dependent cell-mediated cytotoxicity and phagocytosis to the same extent in both adjuvant and metastatic HER2/neu breast cancer patients

“…However, reduced expression levels did not necessarily correlate with reduced phagocytic activity as monocytes of the t-naive group had an almost normal CD32 expression level but reduced phagocytic activity. We did not find CD32 expression on NK cells, which supports the notion that CD32 is primarily expressed on monocytes and is enriched during phagocytosis [51]. Recently, a novel polymorphism affecting the locus encoding human Fc receptors has been reported that leads to inhibitory CD32 expression on NK cells and has been shown to negatively regulate IgG-induced NK cell activation [52].…”

Polymorphisms and Interspecies Differences of the Activating and Inhibitory FcγRII of Macaca nemestrina Influence the Binding of Human IgG Subclasses

“…Varying the sequence at position 160 of the human receptor by replacement of Phe160 with Ala substantially increased binding of both human IgG1 and IgG2 (50, 52). Conversely, the replacement of Phe160 with Tyr reduced binding of IgG (53); note that Tyr occupies this position in mnFcγRIIa (Fig. 1).…”

Viremic HIV Controllers Exhibit High Plasmacytoid Dendritic Cell–Reactive Opsonophagocytic IgG Antibody Responses against HIV-1 p24 Associated with Greater Antibody Isotype Diversification

“…Thus, IgG2 antibodies can exist as multiple structural isoforms consisting of different heavy chain/light chain/hinge covalent complexes that result from differences in disulfide bonding between the hinge and Fab regions of the molecule (23–25) and also form covalent dimers at the hinge region (26, 27). In addition, while IgG2 antibodies bind to FcRs with lower affinity than other IgG subclasses, binding is predominantly to the 131H genotype of FcγRIIa (28), which is carried by 75% of individuals from all racial groups and is the major FcγR mediating phagocytosis (29). Furthermore, IgG2 and IgG1 antibodies are more effective than IgG3 antibodies in activating intracellular pathways of dendritic cells following phagocytosis via FcγRIIa (30).…”