Differential recognition of Toxoplasma gondii recombinant nucleoside triphosphate hydrolase isoforms by naturally infected human sera.

Toxoplasma gondii possesses a highly active nucleoside triphosphate hydrolase, which has been shown to be an immunodominant antigen in mice and humans. Two isoforms (I and II) which exhibit different activities with respect to hydrolysis of ATP exist. Past studies suggest that all strains of T. gondii contain the less active nucleoside triphosphate hydrolase II, whilst only virulent strains contain the nucleoside triphosphate hydrolase I isoform. In order to further investigate the correlation between nucleoside triphosphate hydrolase isoform and biological significance, we cloned and expressed as glutathione S-transferase fusion proteins the full-length nucleoside triphosphate hydrolase I and II isoforms and two truncations of the nucleoside triphosphate hydrolase I isoform in Escherichia coli. We then used ELISAs with the full-length recombinant nucleoside triphosphate hydrolases as antigens to examine 188 naturally infected T. gondii-positive sera and 83 T. gondii-negative sera for antibody reactivity. All positive sera reacted to T. gondii whole tachyzoite lysate antigen, 31 sera reacted to both nucleoside triphosphate hydrolase isoforms, three sera reacted specifically to nucleoside triphosphate hydrolase I and two sera reacted to only nucleoside triphosphate hydrolase II. Immunoblot analysis of the five sera reacting to either nucleoside triphosphate hydrolase I or II revealed both quantitative and qualitative differences in reactivity to the two isoforms. Comparative immunoblot analysis using the truncations of the nucleoside triphosphate hydrolase I isoform, and one of these positive sera identified a presumptive differential epitope between the nucleoside triphosphate hydrolase I and II isoforms within an 81 amino acid region (amino acids 445-526) at the C-terminus of the nucleoside triphosphate hydrolase I isoform. This differential reactivity was further localised to the 12-residue region of greatest variability between the two isoforms (residues 488-499) using synthetic peptides. This is the first report where naturally infected human sera have been used to identify a differential epitope. Because this region is essential for substrate binding, an antibody response to this region may play some role in inhibition of this highly active enzyme.