Expression and biological effects of high levels of serum IgE in epsilon heavy chain transgenic mice

Genetic variations of the FCER2 gene and asthma susceptibility in north Indian children: a case-control study

CONTEXT AND OBJECTIVE

The findings showed that the low-affinity IgE receptor plays a pivotal role in allergic immune response and it is a pharmacogenetic predictor in asthma disease. This study aims to investigate the association of genetic variations: rs28364072 and rs7249320 with asthma and its severity in north Indian children.

METHODS

Case-control-based genetic association study was performed among 550 children.

RESULTS

Statistical analysis demonstrated significant association between asthma and genotypes frequency of both the SNPs.

CONCLUSIONS

Our data indicate that the studied variations are strongly associated with asthma susceptibility and might be risk factor among north Indian asthmatic children.

Polyclonal hyper-IgE mouse model reveals mechanistic insights into antibody class switch recombination

Preceding antibody constant regions are switch (S) regions varying in length and repeat density that are targets of activation-induced cytidine deaminase. We asked how participating S regions influence each other to orchestrate rearrangements at the IgH locus by engineering mice in which the weakest S region, Sε, is replaced with prominent recombination hotspot Sμ. These mice produce copious polyclonal IgE upon challenge, providing a platform to study IgE biology and therapeutic interventions. The insertion enhances ε germ-line transcript levels, shows a preference for direct vs. sequential switching, and reduces intraswitch recombination events at native Sμ. These results suggest that the sufficiency of Sμ to mediate IgH rearrangements may be influenced by context-dependent cues.

IgE knock-in mice suggest a role for high levels of IgE in basophil-mediated active systemic anaphylaxis

Immunoglobulin E (IgE) production is tightly regulated at the cellular and genetic levels and is believed to be central to allergy development. At least two cellular pathways exist that lead to systemic anaphylaxis reactions in vivo: IgE-sensitized mast cells and IgG1-sensitized basophils. Passive anaphylaxis, by application of allergen and allergen-specific antibodies in mice, indicates a differential contribution of immunoglobulin isotypes to anaphylaxis. However, analysis of a dynamic immunization-mediated antibody response in anaphylaxis is difficult. Here, we generated IgE knock-in mice (IgE(ki) ), which express the IgE heavy chain instead of IgG1, in order to analyze the contribution of IgG1 and IgE to active anaphylaxis in vivo. IgE(ki) mice display increased IgE production both in vitro and in vivo. The sensitization of IgE(ki) mice by immunization followed by antigen challenge leads to increased anaphylaxis. Homozygous IgE(ki) mice, which lack IgG1 due to the knock-in strategy, are most susceptible to active systemic anaphylaxis. The depletion of basophils demonstrates their importance in IgE-mediated anaphylaxis. Therefore, we propose that an enhanced, antigen-specific, polyclonal IgE response, as is the case in allergic patients, is probably the most efficient way to sensitize basophils to contribute to systemic anaphylaxis in vivo.

Inefficient processing of mRNA for the membraneform of IgE is a genetic mechanism to limit recruitment of IgE-secreting cells

Immunoglobulin E (IgE) is the key effector element in allergic diseases ranging from innocuous hay fever to life-threatening anaphylactic shock. Compared to other Ig classes, IgE serum levels are very low. In its membrane-bound form (mIgE), IgE behaves as a classical antigen receptor on B lymphocytes. Expression of mIgE is essential for subsequent recruitment of IgE-secreting cells. We show that in activated, mIgE-bearing B cells, mRNA for the membrane forms of both murine and human epsilon (epsilon) heavy chains (HC) are poorly expressed compared to mRNA for the secreted forms. In contrast, in mIgG-bearing B cells, mRNA for the membrane forms of murine gamma-1 (gamma1) and the corresponding human gamma4 HC are expressed at a much higher level than mRNA for the respective secreted forms. We show that these findings correlate with the presence of deviant polyadenylation signal hexamers in the 3' untranslated region (UTR) of both murine and human epsilon genes, causing inefficient processing of primary transcripts and thus poor expression of the proteins and poor recruitment of IgE-producing cells in the immune response. Thus, we have identified a genetic steering mechanism in the regulation of IgE synthesis that represents a further means to restrain potentially dangerous, high serum IgE levels.

Association study of the chromosomal region containing the FCER2 gene suggests it has a regulatory role in atopic disorders

On the basis of studies with animal models, the gene for the low-affinity receptor for immunoglobulin E (IgE) (FCER2, CD23) has been implicated as a candidate for IgE-mediated allergic diseases and bronchial hyperreactivity, or related traits. Given evidence for genetic complexity in atopic disorders, we sought to study two European subpopulations, Finnish and Catalonian. We studied three phenotypic markers: (1) total serum IgE level; (2) asthma; and (3) specific IgE level for a mixture of the most common aeroallergens in Finland. Altogether, eight polymorphic markers spanning a region of 10 cM around the FCER2 gene on chromosome 19p13 were analyzed in 124 families. The physical order of the markers and the location of the FCER2 gene were confirmed by using radiation hybrids. The allele and haplotype association study showed a suggestive haplotype association (significance of p </= 0.03 based on a permutation test) for a high serum IgE response. In a subset of chromosomes segregating with asthma in families with two or more affected members, a single haplotype was found to be highly enriched (p = 8.3 x 10(-6)). However, sequence polymorphisms, which would verify structural differences in the FCER2 gene, were not detected in the coding region of the receptor. Our results suggest that chromosome 19p13 might harbor a genetic determinant of IgE-related traits. Studies in other population samples are needed to verify this finding.

Establishment of antigen-specific IgE transgenic mice to study pathological and immunobiological roles of IgE in vivo

We have established transgenic mice that carry the genes coding for heavy and light chains of TNP-specific IgE. They produced high titers of TNP-specific IgE (20-40 microg/ml in serum) and their mast cells were heavily loaded with IgE. The level of FcepsilonRI expression on their mast cells was 6-8 times higher than that in non-transgenic littermates. The expression of low-affinity IgE receptor FcepsilonRII (CD23) on splenic B cells was also 6-8 times higher in the transgenic mice. Consistent with this, substantial amounts of IgE were detected on B cells in the transgenic mice. When challenged with i.v. administration of the corresponding antigen, the transgenic mice exhibited systemic anaphylactic symptoms such as a drastic drop of body temperature and extravasation of administered dye. Biphasic (immediate and delayed) ear swelling response was also elicited in a TNP-specific manner by epicutaneous antigen challenge without any prior sensitization. Thus, IgE produced in the transgenic mice was found to be biologically active to induce both local and systemic allergic reactions in vivo upon the challenge of the corresponding antigen. Taken together, the antigen-specific IgE transgenic mice established for the first time in this study appear to provide an attractive model system to study the pathological roles of IgE in acute and chronic phases of allergic inflammation as well as their immunobiological roles in vivo. They may also be useful to develop novel therapeutic strategies for atopic disorders.

Negative feedback regulation of IgE synthesis by murine CD23

Immunoglobulin E is found in nanogram amounts in normal human and mouse serum. It is increased during parasitic infestations and mediates allergy. CD23, the low-affinity receptor for IgE (Fc epsilon RII), has been proposed as an important regulator of IgE synthesis. The type-II transmembrane lectin CD23 is expressed in the mouse on B cells and follicular dendritic cells. In humans there are two forms of CD23 which differ in their intracellular amino-terminal 6/7 amino acids; expression of the A-form corresponds to that of murine CD23, whereas the B-form is also found on T and other haematopoietic cells. CD23 has been implicated in cellular adhesion, antigen presentation, as a growth and differentiation factor for human B, T and plasma cells, and as a signal transduction molecule (reviewed in refs 3, 8). Here we disrupt the gene coding for murine CD23 (ref. 9) to clarify the role of CD23 in vivo and find that B- and T-cell development is normal in these CD23-deficient mice. Immune responses to the helminth Nippostrongylus brasiliensis are unaffected. In contrast, immunization with thymus-dependent antigens leads to increased and sustained specific IgE antibody titres compared with controls. Formation of germinal centres is normal. These results suggest that murine CD23 acts as a negative feedback component of IgE regulation.

High-level expression of a human immunoglobulin gamma 1 transgene depends on switch region sequences

We describe that chimeric mouse-human immunoglobulin heavy chain (IgH) genes lacking a switch region and controlled by an IgH promoter and the intronic enhancer are only weakly expressed in transgenic mice. Insertion of part of the human C gamma 1 or murine Cmu switch region into the major intron of the chimeric IgH gene results in a 10(2)-to 10(3)-fold increase in transgene expression. Analysis of B cell hybridoma clones from transgenic mice suggests that switch sequences influence IgH transgene expression at the cellular level. However, the effect of switch region sequences on IgH gene expression observed in vivo is not apparent in transfected B cell lines. These results indicate that switch region sequences which are located proximal to the constant part of the gene and which are normally present in a rearranged IgH gene after class switching represent a novel type of regulatory element that plays a critical role in IgH gene expression in vivo.