Allergy-associated FcRbeta is a molecular amplifier of IgE- and IgG-mediated in vivo responses

Markedly different pathogenicity of four immunoglobulin G isotype-switch variants of an antierythrocyte autoantibody is based on their capacity to interact in vivo with the low-affinity Fcgamma receptor III

Using three different Fcgamma receptor (FcgammaR)-deficient mouse strains, we examined the induction of autoimmune hemolytic anemia by each of the four immunoglobulin (Ig)G isotype-switch variants of a 4C8 IgM antierythrocyte autoantibody and its relation to the contributions of the two FcgammaR, FcgammaRI, and FcgammaRIII, operative in the phagocytosis of opsonized particles. We found that the four IgG isotypes of this antibody displayed striking differences in pathogenicity, which were related to their respective capacity to interact in vivo with the two phagocytic FcgammaRs, defined as follows: IgG2a > IgG2b > IgG3/IgG1 for FcgammaRI, and IgG2a > IgG1 > IgG2b > IgG3 for FcgammaRIII. Accordingly, the IgG2a autoantibody exhibited the highest pathogenicity, approximately 20-100-fold more potent than its IgG1 and IgG2b variants, respectively, while the IgG3 variant, which displays little interaction with these FcgammaRs, was not pathogenic at all. An unexpected critical role of the low-affinity FcgammaRIII was revealed by the use of two different IgG2a anti-red blood cell autoantibodies, which displayed a striking preferential utilization of FcgammaRIII, compared with the high-affinity FcgammaRI. This demonstration of the respective roles in vivo of four different IgG isotypes, and of two phagocytic FcgammaRs, in autoimmune hemolytic anemia highlights the major importance of the regulation of IgG isotype responses in autoantibody-mediated pathology and humoral immunity.

Regulation of B lymphocyte differentiation

LEARNING OBJECTIVES

Type I hypersensitivity reactions uniquely involve the IgE class of immunoglobulins (Ig). IgE differs from other classes of Ig in that the majority of the antibodies are bound to high affinity IgE Fc(epsilon)Rs that are expressed on a variety of cell types. Some of these cell types, most notably, mast cells and basophils, are triggered to undergo rapid activation, degranulation, and release of bioactive mediators following binding of antigen to Fc(epsilon)RI-bound IgE. Because of the central role that IgE antibodies and these mediators play in the tissue injury typical of type I hypersensitivity, this article will review the various stages of B lymphocyte development, activation, and differentiation and comment, where appropriate on potential sites of deregulation in allergic disease.

DATA SOURCES

A literature search of the stages of B lymphocyte differentiation with emphasis on events that concern IgE expression was performed.

RESULTS

B lymphocyte differentiation into IgE expressing cells is dependent upon three types of signals. The first signal is delivered through the B cell antigen receptor and is pivotal in determining the antigenic specificity of the response. The second signal is provided primarily by cytokines derived from T helper 2 (TH2) cells, ie, interleukin (IL)-4 and IL-13. These cytokines are under tight regulation and their role appears to be the stimulation of transcription through the Ig constant region genes. Finally, the third signal is provided via the interaction between the constitutively expressed CD40 molecule on B lymphocytes and CD154 (CD40 ligand), a molecule expressed on T lymphocytes following activation. Elevated levels of IgE in atopic individuals may result from the preferential activation of TH2 cells.

CONCLUSIONS

A greater understanding of the regulation of IgE expression may be central to the development of more effective immunotherapy strategies designed to attenuate IgE synthesis.

A common FCER1B gene promoter polymorphism influences total serum IgE levels in a Japanese population

Genetic factors are important in defining total serum IgE levels. Linkage analyses have localized a gene or genes that influence atopic phenotype at chromosome 11q13. Variants of the FCER1B gene have been identified, which are associated with an increased risk of developing atopy and bronchial asthma. Given uncertain functional consequences and low frequencies of these coding variants of FCER1B, we screened for new mutations using 24 subjects with atopic asthma. A common -109C/T polymorphism at the promoter region of FCER1B was identified, although no variant was found in the entire coding region. We genotyped this promoter polymorphism in 226 healthy control subjects and 226 asthmatic subjects using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis. Allele frequencies were 0.697 for -109T and 0.303 for -109C in 226 healthy control subjects. No significant difference in the distribution of -109C/T polymorphism was found between asthmatic subjects and healthy control subjects. A homozygosity for the -109T allele, however, was associated with increased total serum IgE levels in 226 subjects with asthma (p = 0.0015). The strongest evidence for an association between total serum IgE levels and -109C/T polymorphism (p = 0.0004) was obtained when age at onset of asthma was incorporated into the analysis. Our findings may represent genetic heterogeneity and complex interactions between genetic and environmental components involved in the regulation of total IgE levels, providing evidence that the -109C/T polymorphism of the FCER1B promoter region is one of the genetic factors identified thus far, which affects total serum IgE levels in a Japanese population.

Genetic restrictions in olive pollen allergy

BACKGROUND

The major antigen of olive tree pollen, Ole e 1, produces an IgE response restricted by DQ2.

OBJECTIVE

Our purpose was to further analyze the genetic restrictions associated with IgE and IgG antibodies against Ole e 1 and IgE against the recently described antigen Ole e 3.

METHODS

Twenty-two nuclear olive pollen-allergic families (n = 88) were selected. DRB1 and DQB1, TCR-Valpha 8.1, the high-affinity receptor of IgE (FcepsilonRI-beta) Rsa I exon 7 and intron 2 and TNF-beta (LTalpha-Nco I) polymorphisms were determined by PCR and analyzed for association with allergic traits by the multiallelic transmission disequilibrium test.

RESULTS

Significant associations were found among HLA-DQB1*0201 (n = 29) and high levels of IgG (P =.023) and IgE (P =.0136) antibodies to Ole e 1 and with IgE specific to Ole e 3 (P =.0368). DRB1*0701 was associated with high levels of total serum IgE (P =.04) and IgG against Ole e 1 (P =.025). The FcepsilonRI-beta Rsa I exon 7, allele 1 (n = 39), was associated with high levels of total serum IgE (P =. 01), IgE antibodies against Olea europaea extract (P =.004), and specific antibodies to Ole e 1, IgG (P =.04), and IgE (P =.006). The FcepsilonRI-beta Rsa I intron 2, allele 2 (n = 33), was associated with IgE antibodies to O europaea extract (P =.003) and specific antibodies to Ole e 1, IgG (P =.025), and IgE (P =.05).

CONCLUSIONS

We found a new association between IgE antibody response to Ole e 3 and DQB1*0201 and verified the previously reported association between Ole e 1-specific response and DQB1*0201. Also, the association between FcepsilonRI-beta and IgE antibodies against Ole e 1 was demonstrated.

Adenosine and inosine increase cutaneous vasopermeability by activating A(3) receptors on mast cells

Adenosine has potent effects on both the cardiovascular and immune systems. Exposure of tissues to adenosine results in increased vascular permeability and extravasation of serum proteins. The mechanism by which adenosine brings about these physiological changes is poorly defined. Using mice deficient in the A(3) adenosine receptor (A(3)AR), we show that increases in cutaneous vascular permeability observed after treatment with adenosine or its principal metabolite inosine are mediated through the A(3)AR. Adenosine fails to increase vascular permeability in mast cell-deficient mice, suggesting that this tissue response to adenosine is mast cell-dependent. Furthermore, this response is independent of activation of the high-affinity IgE receptor (FcepsilonR1) by antigen, as adenosine is equally effective in mediating these changes in FcepsilonR1 beta-chain-deficient mice. Together these results support a model in which adenosine and inosine induce changes in vascular permeability indirectly by activating mast cells, which in turn release vasoactive substances. The demonstration in vivo that adenosine, acting through a specific receptor, can provoke degranulation of this important tissue-based effector cell, independent of antigen activation of the high-affinity IgE receptor, supports an important role for this nucleoside in modifying the inflammatory response.

Signalling through the high-affinity IgE receptor Fc epsilonRI

The Fc epsilonRI complex forms a high-affinity cell-surface receptor for the Fc region of antigen-specific immunoglobulin E (IgE) molecules. Fc epsilonRI is multimeric and is a member of a family of related antigen/Fc receptors which have conserved structural features and similar roles in initiating intracellular signalling cascades. In humans, Fc epsilonRI controls the activation of mast cells and basophils, and participates in IgE-mediated antigen presentation. Multivalent antigens bind and crosslink IgE molecules held at the cell surface by Fc epsilonRI. Receptor aggregation induces multiple signalling pathways that control diverse effector responses. These include the secretion of allergic mediators and induction of cytokine gene transcription, resulting in secretion of molecules such as interleukin-4, interleukin-6, tumour-necrosis factor-alpha and granulocyte-macrophage colony-stimulating factor. Fc epsilonRI is therefore central to the induction and maintenance of an allergic response and may confer physiological protection in parasitic infections.

Stimulatory function of paired immunoglobulin-like receptor-A in mast cell line by associating with subunits common to Fc receptors

Paired Ig-like receptors (PIR) are polymorphic type I transmembrane proteins belonging to an Ig superfamily encoded by multiple isotypic genes. They are expressed on immune cells such as mast cells, macrophages, and B lymphocytes. Two subtypes of PIR have been classified according to the difference in the primary structure of the PIR transmembrane and cytoplasmic regions. These subtypes are designated as PIR-A and PIR-B. In this study, the transmembrane and cytoplasmic regions of the PIR-A subtype were shown to mediate activation signal events such as cytoplasmic calcium mobilization, protein tyrosine phosphorylations, and degranulation in rat mast cell line RBL-2H3. The association of the Fc receptor gamma and beta subunits with PIR-A was shown to be responsible for PIR-A function but not required for membrane expression of PIR-A on COS-7 cells. We further revealed the role of two charged amino acid residues in the transmembrane region, namely arginine and glutamic acid, in PIR-A function and its association with the above subunits. In contrast to the inhibitory nature of the PIR-B subtype, present findings reveal that PIR-A potentially acts as a stimulatory receptor in mast cells, suggesting a mechanism for regulation of mast cell functions by the PIR family.

Requirement for a negative charge at threonine 60 of the FcRgamma for complete activation of Syk

Aggregation of FcepsilonRI on mast cells results in the phosphorylation of the FcepsilonRIgamma chain on tyrosine and threonine residues within the immunoreceptor tyrosine-based activation motif. In the present study we sought to identify the site of threonine phosphorylation in FcepsilonRIgamma and investigate its functional importance. We found that threonine 60 was phosphorylated in vitro and in vivo. Expression of a mutated FcepsilonRIgamma (T60A), in either FcepsilonRIgamma-deficient or gamma-null mast cells, resulted in a delay of FcepsilonRI endocytosis, inhibition of TNF-alpha mRNA production, and inhibition of degranulation but did not affect FcepsilonRI-induced cell adhesion. Tyrosine phosphorylation of the T60A mutant gamma chain was normal, but Syk phosphorylation was dramatically reduced in these transfectants. This correlated with reduced co-immunoprecipitation of FcepsilonRIgamma with Syk. Substitution of an aspartic residue for threonine 60 of the FcepsilonRIgamma reconstituted complete activation of Syk and co-immunoprecipitation of FcepsilonRIgamma with Syk. We conclude that the negative charge provided by phosphorylation of threonine 60 of the FcepsilonRIgamma is required for the appropriate interaction and activation of Syk. This is a likely requirement for immunoreceptor tyrosine-based activation motifs involved in Syk activation.

The Transcription Factors Elf-1 and GATA-1 Bind to Cell-Specific Enhancer Elements of Human High-Affinity IgE Receptor α-Chain Gene

Key regulatory regions necessary for the expression of the gene encoding FcεRI α-chain, a component of the high-affinity IgE receptor primarily responsible for IgE-dependent allergic response, were investigated. Two regions, −74/−69 and −55/−47, which contained binding motifs for proteins belonging to the Ets family and the GATA family, respectively, were shown to be necessary for the activation of the α-chain promoter. Both the regulatory elements enhanced the promoter activity only in α-chain-producing cells PT18 and RBL-2H3 (mast cell lines), indicating that the elements required specific trans-acting proteins present in the α-chain-producing cells. EMSA using nuclear extracts and in vitro-translated proteins revealed that Elf-1 and GATA-1 bound to the enhancer elements. This is the first report describing the regulation in the expression of the FcεRI α-chain.

A Novel Lyn-Binding Peptide Inhibitor Blocks Eosinophil Differentiation, Survival, and Airway Eosinophilic Inflammation1, 2

Receptor antagonists block all receptor-coupled signaling pathways indiscriminately. We introduce a novel class of peptide inhibitors that is designed to block a specific signal from a receptor while keeping other signals intact. This concept was tested in the model of IL-5 signaling via Lyn kinase. We have previously mapped the Lyn-binding site of the IL-5/GM-CSF receptor common β (βc) subunit. In the present study, we designed a peptide inhibitor using the Lyn-binding sequence. The peptide was N-stearated to enable cellular internalization. The stearated peptide blocked the binding of Lyn to the βc receptor and the activation of Lyn. The lipopeptide did not affect the activation of Janus kinase 2 or its association with βc. The inhibitor blocked the Lyn-dependent functions of IL-5 in vitro (e.g., eosinophil differentiation from stem cells and eosinophil survival). It did not affect eosinophil degranulation. When applied in vivo, the Lyn-binding peptide significantly inhibited airway eosinophil influx in a mouse model of asthma. The lipopeptide had no effect on basophil histamine release or on the proliferation of B cells and T cells. To our knowledge, this is the first report on an inhibitor of IL-5 that blocks eosinophil differentiation, survival, and airway eosinophilic inflammation. This novel strategy to develop peptide inhibitors can be applied to other receptors.

The high-affinity IgE receptor (Fc epsilon RI): from physiology to pathology

The high affinity receptor for immunoglobulin E (designated Fc epsilon RI) is the member of the antigen (Ag) receptor superfamily responsible for linking pathogen-or allergen-specific IgEs with cellular immunologic effector functions. This review provides background information on Fc epsilon RI function combined with more detailed summaries of recent progress in understanding specific aspects of Fc epsilon RI biology and biochemistry. Topics covered include the coordination and function of the large multiprotein signaling complexes that are assembled when Fc epsilon RI and other Ag receptors are engaged, new information on human receptor structures and tissue distribution, and the role of the FcR beta chain in signaling and its potential contribution to atopic phenotypes.

Chromosome 11q13 and atopic asthma

Asthma is a complex syndrome in which bronchial inflammation and smooth muscle hyperactivity lead to labile airflow obstruction. The commonest form of asthma is that due to atopy, which is an immune disorder where production of IgE to inhaled antigens leads to bronchial mucosal inflammation. The ultimate origins of asthma are interactive environmental and genetic factors. The genetics is acknowledged to be heterogeneous, and one chromosomal region of interest and controversy has been 11q13. To clarify the nature of the chromosome 11q13 effect in atopy and asthma, we conducted a genetic association study in subjects with marked atopic asthma and matched controls, which incorporated the study of 13 genetic variants over a distance of 10-12 cM and which took account of detailed immune and clinical phenotyping. Association with high IgE levels was limited to the interval flanked by D11S1335 and CD20 in a 0.8-Mb interval and was greatest for variants of Fc epsilonRIbeta and HTm4; these variants also associated with asthma (recurrent wheeze with labile airflow obstruction and need for regular inhaler treatment). At the more telomeric marker, D11S480, variants associated with asthma, but not with high IgE levels. The data might support the possibility of multiple loci relevant to atopic asthma on chromosome 11q13.

Inhibition of IgE-mediated triggering of mast cells by complement-derived peptides interacting with the Fc epsilon RI

Mucosal type mast cells, in contrast to the serosal type ones, do not respond to cationic agents, or to the complement-derived peptides C3a and C5a. Earlier we have found that while C3a does not activate the rat mucosal type mast cells (line RBL-2H3), it strongly inhibits the IgE-mediated triggering of these cells, by interfering with the Fc epsilon RI-initiated signaling pathway. In the present study we further investigated the mechanism of this process. It is shown, that C3a interacts with the beta-chain of the Fc epsilon RI complex. Binding of the complement peptide to the cells apparently causes a decrease in the proximity of the IgE-binding Fc epsilon RI. Investigating certain sequences of C3a we found that the inhibition is caused by the C-terminal sequences of the complement-peptide, ranging from positions 56 to 77 and also by a shorter sequence, ranging from positions 56 to 64. The inhibitory effect of these peptides was observed both in the case of RBL-2H3 cells and mouse bone marrow derived mast cells.

Fc receptor beta subunit is required for full activation of mast cells through Fc receptor engagement

The high-affinity IgE receptor (Fc epsilonRI) and the low-affinity IgG receptor (Fc gammaRIII) on mast cells are the key molecules involved in triggering the allergic reaction. These receptors share the common beta subunit (FcRbeta) which contains an immunoreceptor tyrosine-based activation motif and transduces the signals of these receptors' aggregation. In rodents, FcRbeta is essential for the cell surface expression of the Fc epsilonRI. In humans, the FcRbeta gene was reported to be one of the candidate genes causing atopic diseases. However, the role of FcRbeta in vivo still remains ambiguous. To elucidate the functions of FcRbeta, we developed the mice lacking FcRbeta [FcRbeta(-/-)]. The FcRbeta(-/-) mice lacked the expression of the Fc epsilonRI on mast cells and IgE-mediated passive cutaneous anaphylaxis (PCA) was not induced in FcRbeta(-/-) mice as was expected. In these mice, the expression of IgG receptors on mast cells was augmented but the IgG-mediated PCA reaction was attenuated. Although with bone marrow-derived cultured mast cells from FcRbeta(-/-), adhesion to fibronectin and Ca2+ flux upon aggregation of IgG receptors were enhanced, mast cells co-cultured with 3T3 fibroblasts exhibited impaired degranulation on receptor aggregation. These observations indicate that FcRbeta accelerates the degranulation of mature mast cells via the IgG receptor in connective tissues.