Identification of the Schistosoma japonicum 22.6–kDa Antigen as a Major Target of the Human IgE Response: Similarity of IgE–Binding Epitopes to Allergen Peptides

Abstract

Human resistance to reinfection with Schistosoma mansoni and Schistosoma haematobium correlates with elevated IgE titers against worm antigens (soluble worm antigen preparation, SWAP). In S. mansoni infection, low levels of reinfection following chemotherapy are associated with the recognition of a cloned tegumental protein Sm22.6. Because of potential species–specific differences in resistance to schistosomes, we attempted to identify Schistosoma japonicum antigens recognized by human IgE. Following a survey of 176 infected individuals in Leyte, Philippines, we show that IgE antibodies from the majority of older, high–IgE/SWAP responders recognize antigens in the 22 (Sj22)–, 45–, 78– and 97–kDa range in SWAP. Limited IgE cross–reactivity between Sj22 and Sm22 was observed following a comparison of Filipino IgE responses to these antigens. The antigen was cloned from an adult S. japonicum λ–ZAP cDNA library (Mindoro strain) by immunoscreening with pooled high–titer IgE antisera and a rabbit anti–Sj22 polyclonal antibody. The deduced amino acid sequence of the identified cDNA clone, MJ–1, showed significant homology to Sm22.6 (74%) and Sj22.6 (99%). Although the molecular sequence of Sj22.6 has already been reported, this is the first demonstration of its recognition by human IgE, thereby strengthening its potential as a vaccine candidate. Using an overlapping peptide approach, four IgE–binding epitopes were identified in Sj22.6, two of which exhibited similarities to known IgE–binding epitopes from codfish (Gad c 1) and β–lactoglobulin–related allergens. These findings suggest that allergy and protective immunity to helminth infection may be linked by the structural similarities of epitopes recognized by human IgE.