Bla g 6: A troponin C allergen from Blattella germanica with IgE binding calcium dependence

Abstract

Background: The known cockroach allergens do not appear to account for the full repertoire of IgE responses. Objective: To identify and investigate the importance of other Blattella germanica allergens contributing to cockroach allergy. Methods: A B germanica cDNA library was screened with pooled sera from patients with cockroach allergy. Three isoallergens of troponin C (Bla g 6) were cloned and expressed in Pichia pastoris. Homology modeling was performed by using Swiss-Model. IgE responses to purified allergens were simultaneously measured in 104 sera by using a fluorescent multiplex array system. The effect of calcium on IgE binding was investigated by ELISA. Results: Three isoallergens, Bla g 6.0101, Bla g 6.0201, and Bla g 6.0301, were identified which share homology with insect troponin Cs and vertebrate calmodulins (61% to 78% and 42% to 44% amino acid identity, respectively) and have 2 EF-hand calcium binding domains. Molecular models of Bla g 6 showed 2 structurally homologous lobes connected by a linker that confers flexibility to the allergen. The prevalence of IgE binding to recombinant Bla g 6 was 14%. Calcium depletion by 10 mmol/L ethyleneglycol-bis-(β-aminoethylether)-N,N,N′,N′-tetraacetic acid did not significantly affect IgE binding in most cases. Interestingly, addition of 10 mmol/L CaCl 2 after calcium depletion increased IgE binding by approximately 2-fold, a finding not previously reported for calcium binding allergens. Conclusion: Bla g 6 is a troponin allergen with a calcium dependent IgE reactivity that may be involved in muscle contraction. Clinical implications: Bla g 6 homologous allergens may occur among other insects and cause cosensitization or allergenic cross-reactivity. © 2006 American Academy of Allergy, Asthma and Immunology.