Recognition and characterization of stage-specific oocyst/sporozoite antigens of Toxoplasma gondii by human antisera.

Human infection with Toxoplasma gondii is presumed due to the ingestion of either tissue cysts containing bradyzoites or oocyst/sporozoites that are excreted in the feces of infected cats. The incidence of human infection in the general population by either of these routes is unknown. We have previously described unique stage-specific oocyst/sporozoite antigens identified by murine hybridoma monoclonal antibodies. We obtained acute and convalescent antitoxoplasma antisera from patients in an epidemiologically well-documented outbreak of oocyst-transmitted infection associated with the ingestion of contaminated water. An enzyme-linked immunosorbent assay comparing equal numbers of tachyzoites (invasive stage) and oocyst/sporozoite (excreted stage) indicated that these antisera recognized antigens from both life forms. Absorption of pooled antisera with purified oocyst/sporozoites reduced both the antioocyst immunoglobulin G (IgG) and immunoglobulin M (IgM) titer but had only minimal effect on the antitachyzoite titer. Absorption of the antisera with tachyzoites reduced the IgG and IgM antioocyst and antitachyzoite titer. A sodium dodecyl sulfate-polyacrylamide gel analysis of radioiodinated oocyst/sporozoites shows that the principal stage-specific surface proteins of the oocyst/sporozoite have approximate Mr of 67,000 and 25,000. Periodic acid and silver stain of purified oocyst/sporozoite identified bands of similar molecular weight not present in the tachyzoite preparation. Western blot analysis of purified parasites assayed with human antioocyst antisera identified specific oocyst/sporozoite antigens not present on the tachyzoites. At least two major stage-specific oocyst/sporozoite antigens of approximate Mr of 67,000 and 190,000 were identified by the infected patients' antisera and not by the normal controls. Reaction to these oocyst/sporozoite antigens was seen primarily in the IgM fraction of the acute phase and the IgG fraction of convalescent phase antisera. Neither absorption of the antisera with tachyzoites nor periodate treatment of the oocyst/sporozoites reduced the antibody recognition of these stage-specific antigens. These data suggest that individuals infected by a presumed oocyst-transmitted route develop antibodies against unique stage-specific oocyst/sporozoite antigens.