Identification of Tropomyosin as the Major Cross-Reacting Crustacean Allergen

Abstract

Seafood including crustaceans, on ingestion, are known to provoke gastrointestinal as well as systemic allergic reactions. Crustaceans are aquatic arthropods with a chitinous exoskeleton and include shrimp, lobster, prawn and crab. Earlier studies in our laboratory have led to the identification and characterization of three allergens from shrimp, designated as Sa-I, Sa-I1 and Sa-III. The former two were shown to be heat stable proteins with a mol. wt. of 8.4 and 34 kDa respectively, while Sa-III was identified as tRNA Arg and TRNATyr ). Sa-II was found to be the major allergen contributing to more than 50% of the allergenic activity. There are several reports on the existence of cross-reactivity among atopic allergens, in particular food allergens. It is well known that individuals with shrimp allergy often complain of adverse reactions following the ingestion of other re1ated crustaceans. Recognition of crustacea as a group causing adverse reactions in sensitive individuals has a basis in the close phylogenetic relationship of shrimp, lobster, crab and prawn. Thus, one could expect appreciable similarity in the IgE binding epitopes of the offending allergens from related crustaceans. The present study was, therefore, aimed towards the identification of the major cross-reacting crustacean allergen and localization of its IgE binding epitopes. Cross-reactivity among a1lergens from shrimp, prawn, crab and lobster was evaluated by immunochemical methods. Antigenic cross-reactivity was established by immunodiffusion using shrimp-specific rabbit IgG. Competitive ELlSA inhibition experiments using sera of shrimp sensitive patients revealed a high degree of allergenic cross-reactivity between different crustaceans. SDSPAGE and immunoblot analysis using the sera of shrimp sensitive patients have identified a 34 kDa protein as the cross-reacting crustacean allergen. Using shrimp as a model system and Sa-II as a representative crustacean allergen, further studies were carried out to get an insight into the structural and molecular basis of allergenic cross-reactivity. The strategies adopted were, (1) to raise allergen specific anti-idiotypic antibodies and explore the possibility of using these anti-idiotypic antibodies as surrogate allergens for diagnosis of crustacea allergy and (2) to identify the IgE binding epitopes on the major shrimp allergen Sa-II, which may be shared by the 34 kDa allergen from the related crustaceans. In order to explore idiotypic, anti-idiotypic and anti-anti-idiotypic responses to Sa-II, Balb/c mice were immunized with affinity purified human idiotypic antibodies directed against the purified allergen. This resulted in the production of anti-idiotypic antibodies which were quantitated using rabbit idiotypic antibodies raised against the same allergen. The mouse anti-idiotypic antibodies recognized shrimp-specific human idiotypic antibodies of the IgE isotype from 18 of 20 individuals, and IgG antibodies from 14 of 20 shrimp sensitive patients. Immunization of Balb/ c mice with affinity purified, allergen-specific anti-idiotypic antibodies induced anti-allergen IgE and IgG responses in the absence of the allergen. The induction of anti-anti-idiotypic antibodies functionally identical to allergen-specific idiotypic antibodies confirmed that the anti-idiotypic antibodies generated, are indeed a mirror image of the allergen. The present study thus provides evidence that anti-idiotypic antibodies raised against allergen-specific idiotypic antibodies may substitute for the original allergen in the induction of allergen-specific idiotypic antibodies. The demonstration of shared idiotopes on IgG and IgE antibodies in the sera of shrimp sensitive patients supports the use of allergen-specific anti-idiotypic antibodies as surrogate allergens. These anti-anti-idiotypic antibodies not only recognized Sa-II, but also the 34 kDa allergen from prawn, lobster and crab. Cross-reactivity studies using polyclonal sera of shrimp sensitive patients and Sa-II anti-anti-idiotypic antibodies have attributed the allergenic cross-reactivity observed among the related crustaceans to the presence of highly conserved IgE binding epitopes on the 34 kDa crossreacting allergen from shrimp, crab, lobster and prawn. In order to identify the igE binding epitopes on Sa-11, it was subjected to limited tryptic digestion and the peptides were separated by reverse phase HPLC. Amino acid sequence analysis of these peptides and several other peptides generated by Asp N and Lys C treatment revealed an 861 homology with the muscle protein tropomyosin from the fruit fly Drosophila melanogaster, suggesting that the major shrimp allergen is tropomyosin. To establish that Sa-II is indeed tropomyosin, the latter was isolated from shrimp and its physicochemical and immunochemical properties were compared with those of Sa-II. Both tropomyosin and Sa-II had the same molecular mass and focused in the isoelectric pH range of 4.8-5.4. In the presence of 6 M urea, the mobility of both Sa-I1 and shrimp tropomyosin shifted to give an apparent molecular mass of 50 kDa, which is a characteristic property of tropomyosins. Shrimp tropomyosin bound to specific IgE antibodies in the sera of shrimp sensitive patients as assessed by competitive ELISA inhibition and immunoblot analysis. Tropomyosin, similar to Sa-I1 was subjected to limited tryptic digestion and the tryptic maps of both Sa-II and tropomyosin as obtained by reverse phase HPLC were found to be super imposable. Dot blot immunoassay and competitive ELISA inhibition assay using the sera of shrimp sensitive patients identified two peptides, 6 and 9 that exhibited allergenic activity. Both the peptides were purified to homogeneity and sequenced. Peptide 6 is a nonapeptide corresponding to the amino adds 153-161 and peptide 9 has 17 amino acids corresponding to the aminoacid residues 50-66. The peptides individually blocked upto 50% the binding of allergen-specific IgE to hropomyosin. Sa-II specific mouse anti-anti-idiotypic antibodies recognized not only tropomyosin, but also the two allergenic peptides, thus confirming that these peptides represent the major IgE binding epitopes. The IgG binding activity was found to be associated with peptides 6 and 9 as assessed by dot blot immunoassay using the sera of shrimp sensitive patients. Thus, it was found that both IgG and IgE binding epitopes on shrimp tropomyosin are identical. Tropomyosins from both phylogenetically related and unrelated species were assessed for allergenic activity using the sera of shrimp sensitive patients. It was found that allergenic activity was associated with tropomyosins from related crustaceans and from Drosophila melanogaster which shares 86% homology with shrimp tropornyosin. However, tropomyosins from totally unrelated species like yeast, chicken, bovine, rat, rabbit and human did not exhibit allergenic activity. A comparison of the amino acid sequence of shrimp tropomyosin in the region of IgE binding epitopes with the corresponding regions of bopomyosins from different species confirmed lack of allergenic cross-reactivity. The allergenic peptides 6 and 9 were able to inhibit the binding of tropomyosins from related crustaceans to shrimp tropomyosin-specific IgE antibodies to the same extent, confirming the presence of highly conserved IgE binding epitopes. It has been established for the first time that the major crustacean allergen is the heat stable muscle protein, tropomyosin, and extensive cross-reactivity between different members of crustacea is due to the presence of highly conserved IgE binding epitopes on tropomyosins from these sources. Thus, from the present study, information with respect to the amino acid sequence of tropomyosin and localization of its 1gE binding epitopes, could be used to design synthetic peptides corresponding to the B cell and T cell epitopes which would find application in the diagnosis and desensitization of individuals allergic to crustacea.