Antigen interaction and heat inactivation expose new epitopes on human IgE

Natural anti-FcεRIα autoantibodies may interfere with diagnostic tests for autoimmune urticaria

IgG autoantibodies against the alpha-chain of the high affinity IgE receptor are claimed to play a pathogenetic role in autoimmune urticaria. The best methods for detection of functional autoantibodies are currently the autologous serum skin test and the basophil histamine release assay. A simplified and feasible screening test would facilitate the diagnosis of autoimmune urticaria. Here we offer an explanation for the difficulties in establishing a screening test for autoantibodies directed against the alpha-chain of the high affinity IgE receptor in autoimmune urticaria. Identical autoantibodies in chronic urticaria patients and healthy donors belonging to the natural autoantibody repertoire were found by sequence analysis of anti-alpha-chain autoantibodies isolated by repertoire cloning from antibody libraries. These natural autoantibodies bound to the receptor and triggered histamine release but only if IgE was previously removed from the receptor. Diagnostic assays used for detection of antibodies directed against the IgE receptor may require signal comparison with and without the artificial removal of IgE, immune complexes, and complement in order to avoid false positive or negative results. After IgE removal diagnostic tests will detect natural autoantibodies against the high affinity IgE receptor regardless of whether they are pathogenic or not.

Current progress in the understanding of IgE-FcepsilonRI interaction

The last decade has seen a wealth of studies aimed at the characterization of the binding between IgE and its high-affinity receptor, FcepsilonRI. IgE-FcepsilonRI complex formation is a major molecular event in atopic allergy. IgE-FcepsilonRI binding connects allergen recognition to cellular triggering, ultimately leading to disease manifestations. Consequently, pharmacological intervention at this site is of universal relevance for atopic allergy. Until recent years, the complexity of IgE-FcepsilonRI binding, together with the difficulty in obtaining fully functional recombinant IgE and FcepsilonRI derivatives, often led to confusion and difficulty in data interpretation. Major advances in the understanding of this intricate protein-protein interaction have now been accomplished. Most of the current knowledge on the IgE-FcepsilonRI recognition mode derives from long-lasting efforts in the field of structural biology. Protein engineering, high-throughput screening, immunological and biochemical studies also made relevant contributions in this domain. The data accumulated to date predict that IgE and FcepsilonRI use their modular architecture to approach each other in an asymmetric stepwise manner determining a 1:1 stoichiometry. This recognition appears to be enhanced by conformational changes occurring upon binding, leading to the well-known high-affinity. In conclusion, the vast amount of high-quality data available broadened our knowledge on the IgE-FcepsilonRI system; however, the fine structural details of the recognition process are still largely hypothetical. More studies are necessary to provide the experimental comprehensive picture required to carefully design efficient drugs acting at the IgE-FcepsilonRI interface.

Molecular aspects of allergy

Atopic diseases such as asthma, rhinitis, eczema and food allergies have increased in most industrialised countries of the world during the last 20 years. The reasons for this increase are not known and different hypotheses have been assessed including increased exposure to sensitising allergens or decreased stimulation of the immune system during critical periods of development. In allergic diseases there is a polarisation of the Th2 response and an increase in the production of type 2 cytokines which are involved in the production of immunoglobulin E and the development of mast cells, basophils and eosinophils leading to inflammation and disease. The effector phase of atopy is initiated by interaction with Fc epsilon RI expressed on effector cells such as mast cells and basophils but also found on an ever increasing list of cells. Binding of a polyvalent allergen to the variable part of IgE leads to a cross-link of the receptor that triggers the cell to release histamine and pharmacological mediators of the symptomatic allergic response. Cross-linking of Fc epsilon RI by autoantibodies against the alpha-chain of the Fc epsilon RI, causing subsequent histamine release is thought to be involved in the pathogenesis of other diseases such as chronic idiopathic urticaria (CIU). To date, most therapeutic strategies are aimed at inhibiting and controlling components of the inflammatory response. Recently, new treatment strategies have emerged that focus on the development of preventive and even curative treatments. The most promising therapeutic approaches are aimed at inhibiting the IgE-Fc epsilon RI interaction with the use of non-anaphylactogenic anti-IgE or anti-Fc epsilon RIalpha autoantibodies. Clinical trials in humans using an humanised anti-IgE antibody showed that this antibody was well tolerated and reduced both symptoms and use of medication in asthma and allergic rhinitis. Thus interruption of the atopic cascade at the level of the IgE-Fc epsilon RI interaction with the use of non-anaphylactogenic antibodies is effective and represents an attractive therapy for the treatment of atopic disease.

Detection of IgG and IgE serum antibodies to Culicoides salivary gland antigens in horses with insect dermal hypersensitivity (sweet itch)

We postulated that all horses exposed to the bites of Culcoides (midges) would have an antibody response to the antigen secreted in Culcoides saliva, but that IgE antibody would be restricted to allergic individuals. Using immunohistology on sections of fixed Culicoides, we have demonstrated the presence of antibodies in horse serum which recognise Culicoides salivary glands. Antibodies were detected in the serum of horses with insect dermal hypersensitivity and in the serum of normal horses exposed to Culicoides bites. In contrast, no antibodies were detected in serum from native Icelandic ponies which had not been exposed to Culicoides. Anti-salivary gland IgG antibodies were detected in serum from both allergic and healthy horses exposed to Culicoides. IgE antibodies were only detected in horses with signs of insect dermal hypersensitivity, they were not found in serum of healthy controls nor in the serum of horses with a history of hypersensitivity but in remission at the time of sampling. Using western blotting we confirmed the presence of antibodies to Culicoides antigens and demonstrated that individual horses react to different numbers of antigens. This paper demonstrates the ability of serum from allergic horses to detect Culcoides antigens and will enable further studies to isolate and characterise the allergens.

Conditional autoantibodies in urticaria patients: a unifying hypothesis

Chronic urticaria may be characterized by conditional autoantibodies against the alpha-chain of the high-affinity receptor for IgE (FcepsilonRI). These autoantibodies are termed conditional as they only recognize unoccupied FcepsilonRI. The same conditional reactivity pattern has also been found in sera of atopic and normal healthy donors. Any condition resulting in accessibility of FcepsilonRI will render these autoantibodies anaphylactogenic. This finding offers a unifying hypothesis for the manifestation of different forms of urticaria. Non-immunologic triggers may thereby influence directly or indirectly the number of accessible FcepsilonRI allowing the conditional autoantibodies to induce urticaria symptoms.

Molecular basis for nonanaphylactogenicity of a monoclonal anti-IgE antibody

IgE Abs mediate allergic responses by binding to specific high affinity receptors (FcepsilonRI) on mast cells and basophils. Therefore, the IgE/FcepsilonRI interaction is a target for clinical intervention in allergic disease. An anti-IgE mAb, termed BSW17, is nonanaphylactogenic, although recognizing IgE bound to FcepsilonRI, and interferes with binding of IgE to FcepsilonRI. Thus, BSW17 represents a candidate Ab for treatment of IgE-mediated disorders. By panning BSW17 against random peptide libraries displayed on phages, we defined mimotopes that mimic the conformational epitope recognized on human IgE. Two types of mimotopes, one within the Cepsilon3 and one within the Cepsilon4 domain, were identified, indicating that this mAb may recognize either a large conformational epitope or eventually two distinct epitopes on IgE. On the basis of alignments of the two mimotopes with the human IgE sequence, we postulate that binding of BSW17 to the Cepsilon3 region predominantly blocks binding of IgE to FcepsilonRI, leading to neutralization of IgE. Moreover, binding of BSW17 to the Cepsilon4 region may explain how BSW17 recognizes FcepsilonRI-bound IgE, and binding to this region may also interfere with degranulation of IgE sensitized cells (basophils and mast cells). As a practical application of these findings, mimotope peptides coupled to a carrier protein may be used for the development of a peptide-based anti-allergy vaccine by induction of anti-IgE Abs similar to the current approach of using humanized nonanaphylactogenic anti-IgE Abs as a passive vaccine.

Interaction of human IgE with Fc epsilon RI alpha exposes hidden epitopes on IgE

BACKGROUND

Binding of human IgE via the heavy-chain constant region domain 3 (Cepsilon3) to the alpha-chain of its high affinity receptor (FcepsilonRIalpha) is a key event in mediating allergic reactions. We wanted to identify epitopes within Cepsilon3 that are stable to denaturation and to evaluate whether such structures are involved in receptor binding. The existence of stable epitopes would facilitate the generation of compounds that inhibit the IgE-FcepsilonRIalpha interaction.

METHODS

Monoclonal anti-human IgE-antibodies against recombinant bacterially synthesized Cepsilon3, which is known to be partly misfolded, were raised in mice. These antibodies were probed for binding to native, immobilized and receptor-bound IgE, respectively, providing tools for the identification of the indicated stable epitopes.

RESULTS

Two of the generated antibodies (8E7, 3G9) discriminate between IgE in solution and IgE attached to FcepsilonRIalpha, pointing towards a steric rearrangement within Cepsilon3 induced upon receptor binding. The described antibodies represent tools for studying the mechanism of the Fcepsilon-FcepsilonRIalpha interaction and may be of diagnostic value since serum IgE from various human donors was differently recognized by 8E7, which is indicative for naturally occurring IgE molecules with different steric conformation.

CONCLUSION

The presented data support the hypothesis of a conformational change within IgE Cepsilon3 upon receptor binding by showing that monoclonal antibodies raised against recombinant Cepsilon3 differently recognize soluble and receptor-bound IgE. The presence of an IgE portion in sera of human donors that is recognized by 8E7 indicates the existence of IgE molecules in different steric conformations in human blood, which may be related to pathologic parameters.