Fifty years later: Emerging functions of IgE antibodies in host defense, immune regulation, and allergic diseases

Abstract

Fifty years ago, after a long search, IgE emerged as the circulating factor responsible for triggering allergic reactions. Its extremely low concentration in plasma created significant hurdles for scientists working to reveal its identity. We now know that IgE levels are invariably increased in patients affected by atopic conditions and that IgE provides the critical link between the antigen recognition role of the adaptive immune system and the effector functions of mast cells and basophils at mucosal and cutaneous sites of environmental exposure. This review discusses the established mechanisms of action of IgE in pathologic immediate hypersensitivity, as well as its multifaceted roles in protective immunity, control of mast cell homeostasis, and its more recently revealed immunomodulatory functions.

Keywords: IgE; anaphylaxis; mast cells.

FIG 1

IgE CSR. Before switching, the IgH locus in a B cell is in its germline configuration, with exons encoding the heavy chain constant region domains distributed over 150 kb of genomic DNA. Stimulation with IL-4 initiates ε-germline transcription through the Sε region. Clustering of Gs results in a very tight interaction between the transcribed RNA and DNA template, leaving a single nontemplate DNA strand. Secondary structures arising in the single strand cause stalling of RNAse polymerase II (Pol II), which results in recruitment of AID. AID catalyzes dC→dU conversions. The resultant dU nucleotides are deaminated by uracil N-glycosylase (UNG), generating a basic sites, which are substrates for apurinic/apyrimidinic endonuclease 1 (APE1). Single-stranded DNA breaks introduced at high density by this enzyme ultimately lead to DSBs. Breaks in Sµ and Sε are then annealed by means of classical nonhomologous end-joining (C-NHEJ), a DNA repair process involving the enzymes Ku70, Ku80, and DNA-dependent protein kinase, catalytic subunit. The products of this reaction are an episomal switch excision circle along with a functional IgE gene in which the VDJ cassette encoding the heavy chain variable regions is juxtaposed to the Cε exons encoding the constant domains.

FIG 2

Structures of the mature ε-heavy chain gene and IgE protein. The ε-heavy chain gene generated in the IgH locus through deletional CSR (Fig 1) contains a VDJ cassette encoding the heavy chain variable regions juxtaposed to the Cε exons encoding the constant domains. A hybrid Sµ/Sε sequence remains between the V_H and Cε exons, a by product of the isotype-switching process. The secreted IgE protein has 4 constant regions, 1 more of these domains than IgG. Intrachain disulfide bonds are contained within each of the immunoglobulin domains. IgE molecules are heavily glycosylated (7 N-linked sites as indicated by ovals). The N394 oligosaccharide is essential for IgE-FcεRI binding. The transmembrane form of IgE, which contains M1 and M2 exons encoded by ε-mRNA splice isoforms, is expressed at the surface of IgE⁺ B cells.

FIG 3

IgE receptors. The high-affinity IgE receptor FcεRI is expressed in its tetrameric form (αβγ₂) on mast cells and basophils. In human subjects a trimeric form (αγ₂) is found on a number of lineages, including various types of professional APCs. CD23, the low-affinity IgE receptor, is broadly distributed and is a type II transmembrane protein (N-terminus intracellular) assembled as a multimer with α-helical coiled-coil stalks terminating in IgE-binding C-type lectin heads. Protease-sensitive sites in the stalks can be cleaved by endogenous proteases (including ADAM 10) or exogenous proteases (including the Der p 1 protease of dust mites).

FIG 4

Proposed adjuvant and immunoregulatory functions of IgE and FcεRI. Mast cells and basophils residing in mucosal and skin sites produce IL-4 in response to antigen-induced IgE-FcεRI signaling. IL-4 promotes the induction of T_H2 cells and sustains their local survival. These provide the IL-4 and cognate T-B interactions critical for driving IgE class-switching in mucosal B cells. Mast cells suppress Treg cell expansion and function, possibly through cytokines, including IL-4 and IL-6. Trimeric FcεRI present on APCs facilitates antigen uptake for presentation to local T cells.