Structure, function and clinical relevance of the low affinity receptor for IgE

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

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

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.

Effect of immunotherapy on sCD23 levels in patients allergic to Hymenoptera venom

BACKGROUND

sCD23 is the designation given to the low affinity IgE receptor. The soluble fragment of this receptor (sCD23) participates in the regulation of IgE synthesis.

OBJECTIVE

The purpose of this study is to examine the effect of a venom immunotherapy regimen on sCD23 levels.

METHODS

We measured sCD23 levels by ELISA in Hymenoptera venom-allergic patients (positive skin tests and a history of systemic reactions to Hymenoptera sting) in serial sera collected over a course of venom immunotherapy with a mean duration of 54 months. Mean pre-sCD23 and post-sCD23 levels were compared using a Student's two-tailed t test.

RESULTS

sCD23 levels were found to be unchanged over the course of venom immunotherapy.

CONCLUSIONS

This is the first longitudinal study that has been done. It suggests that while both immunotherapy and sCD23 are known to be involved in the regulations of IgE synthesis in the atopic patient, the immunomodulation seen in venom immunotherapy is not mediated through sCD23 in any simple regulatory manner.

Serum levels of interleukin 4 and soluble CD23 in children with allergic disorders

In order to clarify the clinical significance of serum interleukin 4 (IL-4) levels, we measured serum IL-4 concentrations in allergic and non-allergic children using a highly sensitive sandwich ELISA. The limit of detection of the assay was 0.15 pg/ml in serum samples. Serum IL-4 was detected in 96.3% (53/55) of non-allergic controls, in 92.9% (183/197) of allergic children, in 70% (7/10) of cord blood samples and in 86.7% (26/30) of neonates. The IL-4 levels in sera from non-allergic controls were relatively constant during the ages examined and all samples were under 1.5 pg/ml. In allergic children, the serum levels of IL-4 were significantly elevated, particularly at age 13-24 months. The serum levels of IL-4 did not differ in children with different clinical manifestations of allergy, such as bronchial asthma, and atopic dermatitis. The serum level of soluble CD23 (sCD23) showed an age-dependent change in allergic and non-allergic children and was significantly higher in allergic than in non-allergic infants aged 7 to 12 months, but not in other age groups. There was no significant correlation among serum levels of IL-4, sCD23 and IgE.

CONCLUSION

It is suggested that the measurement of serum IL-4 and sCD23 is helpful in the examination of allergic patients in infancy and early childhood, but neither the serum level of IL-4 nor sCD23 directly reflects in vivo IgE production.

Comparison of peptides bound to spleen and thymus class II

In the past we and others have suggested that positive selection of developing thymocytes may depend upon interaction between the alpha beta receptors on these cells and major histocompatibility complex (MHC) proteins bound to peptides found uniquely in the selecting tissue, thymus cortical epithelium. To test this hypothesis, peptides were isolated from MHC class II proteins of spleen, thymus cortical plus medullary epithelium, or thymus cortical epithelium alone. The results showed that the major peptides bound to class II on thymus cortical epithelium were also associated with spleen class II. Some peptides could only be detected in isolates from spleen, probably because of differences in the distribution or uptake of the donor proteins between spleen and thymus. Thus, although we found some tissue-specific distribution of self-peptides, our data suggest that there are no fundamental differences among these tissues in the occupancy of class II MHC by self-peptides. These results limit hypotheses which depend on a specialized mechanism of peptide generation and/or MHC class II loading to account for the positive selection of T cells on thymic cortical epithelium.

Atopic dermatitis

AD is a complex, multifactorial, cutaneous manifestation of the atopic diathesis. Observations from bone marrow transplantation cases have shown transmission of the disease from atopic donors [30] and indicate that the basic defect is carried in immune and inflammatory cells which infiltrate skin lesions. Mast cells appear to be important in the initiation of inflammatory events and eosinophils may have an important role in perpetuating the response. New evidence suggests that IL-4 may be a crucial factor controlling mast cells as well as IgE production in allergic disease. The significance of the Fc&RII/CD23 in regulating IgE synthesis and its role in Langerhans' cell/antigen interactions in atopic dermatitis represents an intriguing area in need of further study.

Interleukin 4 and interferons alpha and gamma regulate Fc epsilon R2/CD23 mRNA expression on normal human B cells

Human interleukin 4 (IL-4) upregulates Fc epsilon R2/CD23 expression on the surface of B lymphocytes. Here it is shown that IL-4 induces expression of CD23 mRNA in normal human B lymphocytes whereas recombinant IL-1, IL-2, IL-5, IFN gamma, IFN alpha 2b and semi-purified low molecular weight B cell growth factor were unable to do so. CD23 mRNA expression could be observed in B cells after 6 hr incubation with IL-4 and was maximal for 24-72 hr. Costimulation of the B cells with anti-IgM antibody enhanced the IL-4 induced CD23 mRNA expression. In contrast, IFN gamma and IFN alpha 2b inhibited IL-4 induced CD23 mRNA expression in normal B lymphocytes. Thus the regulatory effects of IL-4 and interferons on the CD23 membrane expression are linked to an increase and a decrease of CD23 transcripts respectively.

Molecular structure and expression of the murine lymphocyte low-affinity receptor for IgE (Fc epsilon RII)

The cDNA encoding the murine low-affinity receptor for IgE (Fc epsilon RII) has been isolated from a cDNA library prepared from B cells activated with lipopolysaccharide and interleukin 4. It encodes a 37-kDa protein of 331 amino acids with two potential N-linked glycosylation sites. Analogous to its human counterpart, there is no signal sequence and the putative transmembrane region is close to the amino terminus, indicating an inverse membrane orientation with the carboxyl terminus at the cell exterior. The predicted murine Fc epsilon RII amino acid sequence demonstrates a 57% identity with its human counterpart. The murine sequence has an additional internal repeat motif of 21 amino acids giving four repeats as compared to three in the human sequence. Furthermore, the murine Fc epsilon RII is truncated at the carboxyl terminus and the Arg-Gly-Asp sequence, a common recognition site of integrin receptors, which is found in the reverse configuration in human Fc epsilon RII, is missing. B cells activated with interleukin 4 and lipopolysaccharide have an increased amount of Fc epsilon RII mRNA as compared with resting or lipopolysaccharide-stimulated B cells. Con A-activated normal T cells, the TH-2 cell line D10, as well as the macrophage cell line J774 have no detectable Fc epsilon RII mRNA. Expression analysis using transiently transfected COS cells revealed that recombinant murine Fc epsilon RII binds anti-Fc epsilon RII as well as mouse and rat IgE but does not bind human IgE or mouse IgG. Fc epsilon RII expressed in COS cells has a molecular mass of 45 kDa whereas the Fc epsilon RII from B-cell lines is a 49-kDa protein.