The many faces of FcgRI: implications for therapeutic antibody function
Fcg receptor I (FcgRI or CD64) is the sole human Fc receptor with high affinity for monovalent IgG. While it contains an immunoreceptor tyrosine based activation motif in its cytoplasmic domain, binding of FcgRI can result in a complex array of activating and inhibitory outcomes. For instance, binding of monomeric IgG provides a low intensity tonic signal through FcgRI that is necessary for full interferon g receptor signaling in the same cell. Interaction of FcgRI with larger high avidity complexes can result in phagocytosis, the generation of reactive oxygen species, as well as the synthesis and release of inflammatory cytokines. However, numerous reports also document potent anti inflammatory effects brought about by FcgRI engagement with immune complexes such as the inhibition of IFNg and TLR4 signaling, and secretion of interleukin 10. This has led to conflicting hypotheses regarding the function of FcgRI, especially with regard to its role in the efficacy of several therapeutic monoclonal antibodies. While many of these issues are still unclear, continued characterization of the regulation and context dependence of FcgRI function, as well as the molecular mechanisms responsible for these various outcomes, will improve our understanding of FcgRI biology as well as the therapeutic strategies designed to harness or constrain its actions.
Immunological reviews