Indirect ELISAs based on recombinant and affinity-purified glycoprotein E of Aujeszky's disease virus to differentiate between vaccinated and infected animals

Two indirect ELISAs for the detection of antibodies against glycoprotein E (gE) of Aujeszky's disease virus (ADV) in sera have been developed. The rec -gE-ELISA is based on the E. coli-expressed recombinant protein containing the N-terminal sequences of gE (aa 1-125) fused with the glutathione S-tran sferase from Schistosoma japonicum. The affi-gE-ELISA is based on native gE , which was purified from virions by affinity chromatography. The tests wer e optimised and compared with each other, as well as with the recently deve loped blocking gE-ELISA (Morenkov et al., 1997b), with respect to specifici ty and sensitivity. The rec-gE-ELISA was less sensitive in detecting ADV-in fected animals than the affi-gE-ELISA (sensitivity 80% and 97%, respectivel y), which is probably due to the lack of conformation-dependent immunodomin ant epitopes on the recombinant protein expressed in E. coli. The specifici ty of the rec-gE-ELISA and affi-gE-ELISA was rather moderate (90% and 94%, respectively) because it was necessary to set such cut-off values in the te sts that provided a maximum level of sensitivity, which obviously increased the incidence of false positive reactions. Though the indirect ELISAs dete ct antibodies against many epitopes of gE, the blocking gE-ELISA, which det ects antibodies against only one immunodominant epitope of gE, showed a bet ter test performance (specificity 99% and sensitivity 98%). This is most pr obably due to rather high dilutions of the sera used in the indirect gE-ELI SAs (1:30) as compared to the serum dilution in the blocking gE-ELISA (1:2) . We conclude that the indirect gE-ELISAs are sufficiently specific and sen sitive to distinguish ADV-infected swine from those vaccinated with gE-nega tive vaccine and can be useful, in particularly affi-gE-ELISA, as additiona l tests for the detection of antibodies to gE.