High-affinity IgE recognition of a conformational epitope of the major respiratory allergen Phl p 2 as revealed by X-ray crystallography.

We report the three-dimensional structure of the complex between the major respiratory grass pollen allergen Phl p 2 and its specific human IgE-derived Fab. The Phl p 2-specific human IgE Fab has been isolated from a combinatorial library constructed from lymphocytes of a pollen allergic patient. When the variable domains of the IgE Fab were grafted onto human IgG1, the resulting Ab (huMab2) inhibited strongly the binding of allergic patients' IgE to Phl p 2 as well as allergen-induced basophil degranulation. Analysis of the binding of the allergen to the Ab by surface plasmon resonance yielded a very low dissociation constant (K(D) = 1.1 x 10(-10) M), which is similar to that between IgE and Fcepsilon;RI. The structure of the Phl p 2/IgE Fab complex was determined by x-ray crystallography to 1.9 A resolution revealing a conformational epitope (876 A(2)) comprised of the planar surface of the four-stranded anti-parallel beta-sheet of Phl p 2. The IgE-defined dominant epitope is discontinuous and formed by 21 residues located mostly within the beta strands. Of the 21 residues, 9 interact directly with 5 of the 6 CDRs (L1, L3, H1, H2, H3) of the IgE Fab predominantly by hydrogen bonding and van der Waals interactions. Our results indicate that IgE Abs recognize conformational epitopes with high affinity and provide a structural basis for the highly efficient effector cell activation by allergen/IgE immune complexes.

Recombinant allergen-based IgE testing to distinguish bee and wasp allergy.

BACKGROUND: The identification of the disease-causing insect in venom allergy is often difficult. OBJECTIVE: To establish recombinant allergen-based IgE tests to diagnose bee and yellow jacket wasp allergy. METHODS: Sera from patients with bee and/or wasp allergy (n = 43) and patients with pollen allergy with false-positive IgE serology to venom extracts were tested for IgE reactivity in allergen extract-based tests or with purified allergens, including nonglycosylated Escherichia coli-expressed recombinant (r) Api m 1, rApi m 2, rVes v 5, and insect cell-expressed, glycosylated rApi m 2 as well as 2 natural plant glycoproteins (Phl p 4, bromelain). RESULTS: The patients with venom allergy could be diagnosed with a combination of E coli-expressed rApi m 1, rApi m 2, and rVes v 5 whereas patients with pollen allergy remained negative. For a group of 29 patients for whom the sensitizing venom could not be identified with natural allergen extracts, testing with nonglycosylated allergens allowed identification of the sensitizing venom. Recombinant nonglycosylated allergens also allowed definition of the sensitizing venom for those 14 patients who had reacted either with bee or wasp venom extracts. By IgE inhibition studies, it is shown that glycosylated Api m 2 contains carbohydrate epitopes that cross-react with natural Api m 1, Ves v 2, natural Phl p 4, and bromelain, thus identifying cross-reactive structures responsible for serologic false-positive test results or double-positivity to bee and wasp extracts. CONCLUSION: Nonglycosylated recombinant bee and wasp venom allergens allow the identification of patients with bee and wasp allergy and should facilitate accurate prescription of venom immunotherapy.

Structure of the major carrot allergen Dau c 1.

Dau c 1 is a major allergen of carrot (Daucus carota) which displays IgE cross-reactivity with the homologous major birch-pollen allergen Bet v 1. The crystal structure of Dau c 1 has been determined to a resolution of 2.7 A, revealing tight dimers. The structure of Dau c 1 is similar to those of the major allergens from celery, Api g 1, and birch pollen, Bet v 1. Electron density has been observed in the hydrophobic cavity of each monomer and has been modelled with polyethylene glycol oligomers of varying length. Comparison of the surface topology and physicochemical properties of Dau c 1 and Bet v 1 revealed that they may have some, but not all, epitopes in common. This is in agreement with the observation that the majority of carrot-allergic patients have Bet v 1 cross-reactive IgE antibodies, whereas others have Dau c 1-specific IgE antibodies which do not recognize Bet v 1.

Identification of a B-cell epitope of hyaluronidase, a major bee venom allergen, from its crystal structure in complex with a specific Fab.

The major allergens of honeybee venom, hyaluronidase (Hyal) and phospholipase A2, can induce life-threatening IgE-mediated allergic reactions in humans. Although conventional immunotherapy is effective, up to 40% of patients develop allergic side effects including anaphylaxis and thus, there is a need for an improved immunotherapy. A murine monoclonal anti-Hyal IgG1 antibody (mAb 21E11), that competed for Hyal binding with IgEs from sera of bee venom allergic patients, was raised. The fragment of these IgG antibodies which bind to antigen (Fab) was produced and complexed (1:1) with Hyal. The crystal structure determination of Hyal/Fab 21E11 complex (2.6 A) enabled the identification of the Hyal-IgG interface which provides indirect information on the Hyal-IgE interaction (B-cell epitope). The epitope is composed of a linear array of nine residues (Arg138, His141-Arg148) located at the tip of a helix-turn-helix motive which protrudes away from the globular core and fits tightly into the deep surface pocket formed by the residues from the six complementarity determining regions (CDRs) of the Fab. The epitope is continuous and yet its conformation appears to be essential for Ab recognition, since the synthetic 15-mer peptide comprising the entire epitope (Arg138-Glu152) is neither recognized by mAb 21E11 nor by human IgEs. The structure of the complex provides the basis for the rational design of Hyal derivatives with reduced allergenic activity, which could be used in the development of safer allergen-specific immunotherapy.

Crystal structure of the major celery allergen Api g 1: molecular analysis of cross-reactivity.

Many patients who have been sensitised to pollen, display allergic symptoms after ingestion of certain plant food such as fresh fruit, vegetables and nuts. The cause is the cross-reactivity between structurally very similar major plant allergens. In particular, allergy to celery is very frequently associated with birch and mugwort pollen sensitization, known as to the birch-mugwort-celery syndrome. The crystal structure of the major celery allergen Api g 1, a homologue of the major birch pollen allergen Bet v 1, has been determined to a resolution of 2.9 A. The structure of Api g 1 is very similar to that of Bet v 1 with major differences occurring in the segment comprised of residues 23-45, preceding the well conserved glycine-rich P-loop, as well as in loops beta3-beta4 and beta5-beta6. In particular, Api g 1 lacks E45, which has been shown to be a crucial residue for antibody recognition in the crystal complex of Bet v 1 with the Fab fragment of a murine monoclonal IgG (BV16) antibody. The absence of E45 and the structural differences in the preceding segment suggest that this region of the Api g 1 surface is probably not responsible for the observed cross-reactivity with Bet v 1. A detailed analysis of the molecular surface in combination with sequence alignment revealed three conserved surface patches which may account for cross-reactivity with Bet v 1. Several residues of Bet v 1 which have been shown by mutagenesis studies to be involved in IgE recognition belong to these conserved surface regions. The structure of Api g 1 and the related epitope analysis provides a molecular basis for a better understanding of allergen cross-reactivity and may lead to the development of hypoallergens which would allow a safer immunotherapy.

Crystal structure of a hypoallergenic isoform of the major birch pollen allergen Bet v 1 and its likely biological function as a plant steroid carrier.

Bet v 1l is a naturally occurring hypoallergenic isoform of the major birch pollen allergen Bet v 1. The Bet v 1 protein belongs to the ubiquitous family of pathogenesis-related plant proteins (PR-10), which are produced in defense-response to various pathogens. Although the allergenic properties of PR-10 proteins have been extensively studied, their biological function in plants is not known. The crystal structure of Bet v 1l in complex with deoxycholate has been determined to a resolution of 1.9A using the method of molecular replacement. The structure reveals a large hydrophobic Y-shaped cavity that spans the protein and is partly occupied by two deoxycholate molecules which are bound in tandem and only partially exposed to solvent. This finding indicates that the hydrophobic cavity may have a role in facilitating the transfer of apolar ligands. The structural similarity of deoxycholate and brassinosteroids (BRs) ubiquitous plant steroid hormones, prompted the mass spectrometry (MS) study in order to examine whether BRs can bind to Bet v 1l. The MS analysis of a mixture of Bet v 1l and BRs revealed a specific non-covalent interaction of Bet v 1l with brassinolide and 24-epicastasterone. Together, our findings are consistent with a general plant-steroid carrier function for Bet v 1 and related PR-10 proteins. The role of BRs transport in PR-10 proteins may be of crucial importance in the plant defense response to pathological situations as well as in growth and development.

Crystal structure of Sol I 2: a major allergen from fire ant venom.

Sol i 2 is a potent allergen from the venom of red imported fire ant, which contains allergens Sol i 1, Sol i 2, Sol i 3, and Sol i 4 that are known to be powerful triggers of anaphylaxis. Sol i 2 causes IgE antibody production in about one-third of individuals stung by fire ants. Baculovirus recombinant dimeric Sol i 2 was crystallized as a native and selenomethionyl-derivatized protein, and its structure has been determined by single-wavelength anomalous dispersion at 2.6 Å resolution. The overall fold of each subunit consists of five helices that enclose a central hydrophobic cavity. The structure is stabilized by three intramolecular disulfide bridges and one intermolecular disulfide bridge. The nearest structural homologue is the sequence-unrelated odorant binding protein and pheromone binding protein LUSH of the fruit fly Drosophila, which may suggest a similar biological function. To test this hypothesis, we measured the reversible binding of various pheromones, plant odorants, and other ligands to Sol i 2 by the changes in N-phenyl-1-naphthylamine fluorescence emission upon binding of ligands that compete with N-phenyl-1-naphthylamine. The highest binding affinity was observed for hydrophobic ligands such as aphid alarm pheromone (E)-ß-farnesene, analogs of ant alarm pheromones, and plant volatiles decane, undecane, and ß-caryophyllene. Conceivably, Sol i 2 may play a role in capturing and/or transporting small hydrophobic ligands such as pheromones, odors, fatty acids, or short-living hydrophobic primers. Molecular surface analysis, in combination with sequence alignment, can explain the serological cross-reactivity observed between some ant species.

Crystal structure of the major allergen from fire ant venom, Sol i 3.

Fire ant venom is an extremely potent allergy-inducing agent containing four major allergens, Sol i 1 to Sol i 4, which are the most frequent cause of hypersensitivity reactions to hymenoptera in the southern USA. The crystal structure of recombinant (Baculovirus) major fire ant allergen Sol i 3 has been determined to a resolution of 3.1 A by the method of molecular replacement. The secondary-structure elements of Sol i 3 are arranged in an alpha-beta-alpha sandwich fold consisting of a central antiparallel beta-sheet surrounded on both sides by alpha helices. The overall structure is very similar to that of the homologous wasp venom allergen Ves v 5 with major differences occurring in the solvent-exposed loop regions that contain amino acid insertions. Consequently, the limited conservation of surface chemical properties and topology between Sol i 3 and Ves v 5 may explain the observed lack of relevant cross-reactivity. It is concluded that Sol i 3 recognizes immunoglobulin E antibodies with a distinct set of its own epitopes, which are different from those of Ves v 5. Indeed, the molecular area in Sol i 3 covered by non-conserved residues is large enough to accommodate four unique Sol i 3 epitopes.

Characterization of the N-glycans of recombinant bee venom hyaluronidase (Api m 2) expressed in insect cells.

Honeybee venom hyaluronidase (Api m 2) is a major glycoprotein allergen. Previous studies have indicated that recombinant Api m 2 expressed in insect cells has enzyme activity and IgE binding comparable with that of native Api m 2. In contrast, Api m 2 expressed in Escherichia coli does not. In this study, we characterized the carbohydrate side chains of Api m 2 expressed in insect cells, and compared our data with the established carbohydrate structure of native Api m 2. We assessed both the monosaccharide and the oligosaccharide content of recombinant Api m 2 using fluorophore-assisted carbohydrate electrophoresis and HPLC. To identify the amino acid residues at which glycosylation occurs, we digested recombinant Api m 2 with endoproteinase Glu-C and identified the fragments that contained carbohydrate by specific staining. Recombinant Api m 2 expressed in insect cells contains N-acetylglucosamine, mannose, and fucose, as well as trace amounts of glucose and galactose, and the oligosaccharide analysis is consistent with heterogeneous oligosaccharide chains consisting of two to seven monosaccharides. No sialic acid or N-acetylgalactosamine were detected. These results are similar to published data for native Api m 2, although some monosaccharide components appear to be absent in the recombinant protein. Analysis of proteolytic digests indicates that of the four candidate N-glycosylation sites, carbohydrate chains are attached at asparagines 115 and 263. Recombinant Api m 2 expressed in insect cells has enzymic activity and IgE binding comparable with the native protein, and its carbohydrate composition is very similar.

Mutational analysis of amino acid positions crucial for IgE-binding epitopes of the major apple (Malus domestica) allergen, Mal d 1.

BACKGROUND: Individual amino acid residues of the major birch pollen allergen, Bet v 1, have been identified to be crucial for IgE recognition. The objective of the present study was to evaluate whether this concept was applicable for the Bet v 1-homologous apple allergen, Mal d 1. METHODS: A Mal d 1 five-point mutant was produced by PCR techniques, cloned into pMW 172 and expressed in Escherichia coli BL21(DE3) cells. To evaluate the allergenic properties of the engineered protein compared to Mal d 1 wild-type IgE immunoblotting, ELISA, peripheral blood monocytes proliferation assays, and skin prick tests were performed. RESULTS: The Mal d 1 mutant showed reduced capacity to bind specific IgE as compared to wild-ype Mal d 1 in in vitro assays in the majority of the sera tested. In ELISA, 10 out of 14 serum samples displayed an 88-30% decrease in IgE binding to Mal d 1 mutant compared to wild-type Mal d 1. Skin prick tests in apple-allergic patients (n = 2) confirmed the markedly decreased ability of the Mal d 1 mutant to induce allergic reactions in vivo. However, the relevant T cell epitopes were present in the mutated molecule according to peripheral blood mononuclear cell proliferation assays. CONCLUSIONS: Our findings suggest that it is possible to modulate the IgE-binding properties of allergens by single amino acid substitutions at crucial positions which might be useful for future immunotherapy of birch-pollen-associated food allergies which are not ameliorated by birch pollen immunotherapy.

A major allergen gene-fusion protein for potential usage in allergen-specific immunotherapy.

BACKGROUND: Specific immunotherapy is a common treatment of allergic diseases and could potentially be applied to other immunologic disorders. Despite its use in clinical practice, more defined and safer allergy vaccine preparations are required. Differences between epitopes of IgE that recognize the 3-dimensional structure of allergens and T cells that recognize linear amino acid sequences provide a suitable tool for novel vaccine development for specific immunotherapy. OBJECTIVE: The aim of the study was to delete B-cell epitopes and prevent IgE crosslinking, but to preserve T-cell epitopes by fusion of 2 major allergens of bee venom because of a change in the conformation. METHODS: By genetic engineering, we produced a fusion protein composed of the 2 major bee venom allergens: phospholipase A 2 (Api m 1) and hyaluronidase (Api m 2). RESULTS: The Api m [1/2] fusion protein induced T-cell proliferation and both T H 1-type and T H 2-type cytokine responses. In contrast, IgE reactivity was abolished, and profoundly reduced basophil degranulation and type 1 skin test reactivity was observed. Pretreatment of mice with Api m [1/2] fusion protein significantly suppressed the development of specific IgE as well as other antibody isotypes after immunization with the native allergen. CONCLUSION: The novel fusion protein of 2 major allergens bypasses IgE binding and mast cell/basophil IgE FcepsilonRI crosslinking and protects from IgE development.