The aim of this work was to identify the
immunodominant regions of the
Em18 antigen to improve the specificity in diagnosis of
alveolar echinococcosis (AE). Two recombinant
antigens ReEm18-1 and ReEm18-2, which have the same sequence except that nine
amino acid residues are absent in ReEm18-2, were tested by ELISA and Western Blot (WB) for their diagnostic efficiency. Serological evaluation of the two
antigens demonstrated that the sensitivity of both
antigens was 95.5% in ELISA and WB, and the specificity was 93.6% and 95.7% in ELISA, and 81.4% and 82.9% in WB, respectively. Five more expression clones (EmS1-EmS5), which contain different regions of the Em18 sequence, were constructed for defining the
immunodominant regions of the
antigen. Fourteen
monoclonal antibodies (mAbs) against ReEm18-2
antigen and the sera from different groups of patients were used to identify the
epitope regions of the five
antigen fragments. Results showed that the
epitopes recognized by the mAbs are located in the N-terminal third of the sequence, but the immunodominant area recognized by native serum
antibodies may be located further downstream (C-terminal) in the sequence. The nonspecific cross-reactivity is due to
epitopes present in the C-terminal third of the sequence. The
antigen fragments that contain the first two-thirds of the sequence have the same sensitivity to AE sera as those of the ReEm18-1 and ReEm18-2
antigens, but removal of the C-terminal third of the sequence improved the specificity of the assay from 93.6% to 99.3% (ELISA) and 81.4% to 90.7% (WB). We conclude that the necessary part of the ReEm18
antigen sequence for AE diagnosis is the N-terminal half to two-thirds of the entire sequence.