Recognition by the human immune system of candidate vaccine epitopes of Toxoplasma gondii measured by a competitive ELISA.

A competitive ELISA for the detection of human antibody to specific candidate vaccine epitopes of Toxoplasma gondii is described. The method is based on competition between antibody in human serum and murine monoclonal antibody to bind to a crude sonicated antigen of T. gondii. A total of 301 normal human sera were tested against a panel of four anti-T. gondii murine monoclonal antibodies. The results obtained with two of the monoclonal antibodies (FMC 22 and FMC 23) gave good agreement with those of an indirect ELISA for human IgG to T. gondii, a measure of previous exposure to the parasite. The epitopes against which these two monoclonal antibodies are directed were recognized by the immune system of all patients previously exposed to the parasite, while greater than 92 and greater than 96% respectively of these patients recognized the epitopes which the other two monoclonal antibodies (FMC 18 and FMC 19) are directed against. An ELISA double antibody binding system was used to ascertain relationships in the epitope binding sites of the four monoclonal antibodies. The data obtained demonstrated that FMC 19 and FMC 22 bound to related epitopes. FMC 18, FMC 22 and FMC 23 also bound to related epitopes, but the binding of FMC 19 was found to be independent of the binding of FMC 18 and FMC 23 to their epitope binding sites.