IgA Fc Receptors

Abstract

The IgA receptor family comprises a number of surface receptors including the polymeric Ig receptor involved in epithelial transport of IgA/IgM, the myeloid specific IgA Fc receptor (FcαRI or CD89), the Fcα/μR, and at least two alternative IgA receptors. These are the asialoglycoprotein receptor and the transferrin receptor, which have been implicated in IgA catabolism, and tissue IgA deposition. In this reviewwe focus on the biology of FcαRI (CD89). FcαRI is expressed on neutrophils, eosinophils, monocytes/macrophages, dendritic cells, and Kupffer cells. This receptor represents a heterogeneously glycosylated transmembrane protein that binds both IgA subclasses with low affinity. A single gene encoding FcαRI has been isolated, which is located within the leukocyte receptor cluster on chromosome 19. The FcαRI α chain lacks canonical signal transduction domains but can associate with the FcR γ-chain that bears an activation motif (ITAM) in the cytoplasmic domain, allowing activatory functions. FcαRI expressed alone mediates endocytosis and recyling of IgA. No FcαRI homologue has been defined in the mouse, and progress in defining the in vivo role of FcαRI has been made using human FcαRI transgenic (Tg) mice. FcαRI-Tg mice demonstrated FcαRI expression on Kupffer cells and so defined a key role for the receptor in mucosal defense. The receptor functions as a second line of antibacterial defense involving serum IgA rather than secretory IgA. Studies in FcαRI-Tg mice, furthermore, defined an essential role for soluble FcαRI in the development of IgA nephropathy by formation of circulating IgA-FcαRI complexes. Finally, recent work points out a role for human IgA in treatment of infectious and neoplastic diseases.