Characterisation of Alternaria alternata manganese-dependent superoxide dismutase, a cross-reactive allergen homologue to Asp f 6

It is well known that Alternaria alternata presents a significant level of allergenic cross-reactivity with several other phylogenetically related and non-related allergenic moulds. To improve the molecular diagnosis, the identification and characterisation of all clinically relevant allergens, including both species-specific and cross-reacting proteins, is required. In this study we report the molecular and immunological characterisation of the A. alternata manganese-dependent superoxide dismutase (Alt a MnSOD) and its cross-reactivity with Asp f 6, a diagnostic marker allergen in allergic bronchopulmonary aspergillosis (ABPA). The cDNA coding for Alt a MnSOD sequence was isolated by RACE and PCR. Alt a MnSOD is a protein of 191 amino acids that presented significant homology and potential cross-reactive epitopes with Asp f 6. The recombinant protein was produced in Escherichia coli and the immunoreactivity was evaluated in patient sera. Immunoblotting analyses showed that seven of sixty-one A. alternata-sensitised patient sera and two ABPA patient sera reacted with the recombinant Alt a MnSOD. The native counterpart contained in both A. alternata and Aspergillus fumigatus extracts inhibited IgE binding to the recombinant molecule. The allergen was named Alt a 14 by the official Allergen nomenclature subcommittee. Thus, Alt a 14 is a relevant allergen in A. alternata sensitisation that may be used to improve diagnostic procedures. Evidence of cross-reactivity between Asp f 6 and Alt a 14-recognition by ABPA patient sera suggest the existence of an Alt a 14-mediated mechanism that, similar to Asp f 6, may be related to the pathogenesis of ABPA.