Detection of nucleoside triphosphate hydrolase as a circulating antigen in sera of mice infected with Toxoplasma gondii

The nucleoside triphosphate hydrolase, which is present in the tachyzoite form of Toxoplasma gondii, was detected as a circulating antigen in sera of mice infected with a virulent (RH) or an avirulent (Beverly) strain of T. gondii. The enzyme was detected with a monoclonal antibody incorporated into an enzyme-linked immunosorbent assay. The lower limit of sensitivity of the assay was about 0.3 ng/ml, and standard assays provided a linear plot of nucleoside triphosphate hydrolase concentration over a range of 0.3 to 12 ng/ml. In mice inoculated intraperitoneally with tachyzoites of the RH strain, nucleoside triphosphate hydrolase emerged in the serum 1 day after injection, and then the concentration increased and reached a value of 30 μg/ml on day 5. In mice inoculated intraperitoneally with cysts of the Beverly strain, nucleoside triphosphate hydrolase was detected at day 3 after injection, and a peak concentration of 89 ng/ml was seen on day 10. The concentration of enzyme decreased thereafter, and the enzyme disappeared from the circulation on day 56.