Two forms of the low-affinity Fc receptor for IgE differentially mediate endocytosis and phagocytosis: identification of the critical cytoplasmic domains.

Abstract

We have previously identified two species of the low-affinity human Fc receptor for IgE, FceRIIa and FceRIIb, which differ only in a short stretch of amino acids at the N-terminal cytoplasmic end. (1)(2)(3). FceRII has been known as a B-cell differentiation antigen, CD23 (4-7), and has been proposed to be involved in several B-cell functions such as the growthpromoting effect (8, 9), cell adhesion (10), and IgE-mediated antigen presentation.(11). On the other hand, FceRII has also been reported to be expressed on macrophages, eosinophils, and platelets and to be an effector molecule of IgE-mediated immunity, including immediate-type allergy and cytotoxicity against certain parasites (2). IgE can mediate release of varieties of chemical mediators from eosinophils, monocytes, and platelets (12)(13)(14).We have previously identified two species of human FceRII, FceRIIa and FceRIIb (15, 16). They are generated by utilizing different transcriptional initiation sites and 5' exons of the single genomic gene, and they differed only in a short amino acid stretch in the N-terminal cytoplasmic end. FceRIIa is constitutively, but cell-type-specifically, expressed on B cells. On the other hand, monocytes and eosinophils express only FceRIlb. FceRIIb is expressed on normal peripheral blood B cells and monocytes only after stimulation with interleukin 4, which is known to be the cytokine responsible for the isotype switching of B cells to 5030The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement"in accordance with 18 U.S.C. §1734 solely to indicate this fact.

A Novel B Cell-Mediated Transport of IgE-Immune Complexes to the Follicle of the Spleen

“…Especially when using EBV-transformed B cell lines, which should be free of contaminating cell types, contribution of DCs to the T cell stimulation/Ag presentation is excluded (26 -28). This, taken along with the fact that human B cells have been shown to internalize IgE/Ag via CD23 (57,58), strongly suggest that CD23 ϩ B cells can indeed present IgE-Ag complexes to specific T cells in vitro. Whether this also takes place in vivo is more difficult to ascertain.…”

The high-affinity IgE receptor (FcεRI): a critical regulator of airway smooth muscle cells?

“…CD23 is widely expressed in hematopoietic cells including B cells, eosinophils, platelets, and monocytes (11). Two variants of CD23 have been described, CD23a and CD23b, resulting from alternative splicing and translational initiation at different sites (1,114,115). CD23a is constitutively expressed on B cells and subpopulations of eosinophils, whereas CD23b is inducible by IL-4 on various immune cell types, including B cells, monocytes, macrophages, neutrophils, and others (11).…”

Intracellular Trafficking of CD23: Differential Regulation in Humans and Mice by Both Extracellular and Intracellular Exons

“…The two CD23 species mainly differ by their intracellular trafficking properties; although CD23a undergoes efficient endocytosis, CD23b is not efficiently internalized, but is able to mediate phagocytosis of IgE-opsonized particles when expressed in macrophages (15).…”