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

BackgroundThe known cockroach allergens do not appear to account for the full repertoire of IgE responses.ObjectiveTo identify and investigate the importance of other Blattella germanica allergens contributing to cockroach allergy.MethodsA 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.ResultsThree 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 CaCl2 after calcium depletion increased IgE binding by approximately 2-fold, a finding not previously reported for calcium binding allergens.ConclusionBla g 6 is a troponin allergen with a calcium dependent IgE reactivity that may be involved in muscle contraction.Clinical implicationsBla g 6 homologous allergens may occur among other insects and cause cosensitization or allergenic cross-reactivity.