A gene family expressing a host-protective antigen of Echinococcus granulosus.

Echinococcus granulosus causes cystic hydatidosis in humans. A recombinant antigen vaccine has been developed, for use in the parasite's natural animal intermediate hosts, that may provide a new tool for control of hydatid disease transmission. The antigen, designated EG95, is encoded by a cDNA the features of which indicate it to be an incomplete copy of the associated mRNA. Characterisation of the gene(s) encoding the antigen was undertaken in order to enable subsequent study of genetic variability in the gene and associated protein in different parasite isolates. Southern hybridisation studies of E. granulosus genomic DNA probed with the eg95 cDNA revealed that the gene belonged to a gene family. DNA sequence analysis of cloned genomic fragments indicated that the gene family consists of at least seven members, one of which is a pseudogene. The gene having identity with the eg95 cDNA was cloned and sequenced, and the full length mRNA characterised. Genomic sequence and structure of the eg95 gene family members are highly conserved with respect to the gene encoding EG95. Four eg95-related genes are predicted to express an identical EG95 protein and all four were shown to be expressed in the oncosphere life-cycle stage. The full length EG95 protein has a predicted molecular mass of 16.9 kDa, secretory signal sequence, carboxy-terminal glycosylphosphatidylinositol hydrophobic anchor motif and a fibronectin type III domain. PCR amplification conditions were established which allow gene-specific characterisation of the eg95 gene in E. granulosus isolates from different host species and geographical locations.