Gene cloning, expression and serological evaluation of the 12-kDa antigen-B subunit from Echinococcus granulosus

A 12-kDa subunit of antigen B from Echinococcus granulosus has recently been cloned, expressed and used in diagnostic ELISA to test human sera for evidence of cystic echinococcosis. The performance of the ELISA based on the recombinant antigen (rAgB) was compared with that of similar assays based on native antigen B (nAgB) or hydatid-cyst fluid. For the preparation of the rAgB, total RNA was extracted from Ec. granulosus protoscoleces so that antigen-B complementary DNA could be synthesised, amplified by PCR, and then cloned into the pQE30 expression vector. The recombinant plasmid was transformed in Escherichia coli and induced using isopropyl-beta-D-thiogalactopyrano-side. Bacterial samples were collected, lysed and then analysed by SDS-PAGE and western blotting. The recombinant protein was purified by affinity chromatography. Although the performance of the ELISA based on cyst fluid appeared identical to that of the assay based on the recombinant antigen (with a sensitivity, specificity, positive predictive value and negative predictive value of 96.0%, 97.0%, 97.2% and 95.5%, respectively), the corresponding results for the ELISA based on nAgB (98.6%, 100%, 100% and 98.5%) were slightly better. Despite this difference (which was not statistically significant), the comparative ease with which large quantities of the recombinant antigen could be produced make the antigen a potentially useful tool in the diagnosis of cystic echinococcosis.