A 330-residue carboxy-terminal antigenic fragment of the Toxoplasma gondii 54-kDa rhoptry protein (ROP2) was expressed in Escherichia coli as a fusion polypeptide containing a 48-amino-acid sequence derived from phage lambda protein Cro and E. coi protein LacI followed by six consecutive histidyl residues. Metal chelate affinity chromatography provided an easy way to isolate the recombinant product in a highly purified form (>95%). When this material was used to develop an immunoglobulin G (IgG) enzyme-linked immunosorbent assay for diagnosis of toxoplasmosis, the test reached a sensitivity of 89%o. The sensitivity of the assay was similar whether the sera contained T. gondii-specific IgM or were devoid of such IgM. It was also found that ROP2 is a conserved antigen since antibodies against the recombinant antigen could be detected in mice experimentally infected with 11 independent T. gondii isolates originating from infected human tissues tested. Thus, the 54-kDa rhoptry antigen could advantageously complement other previously described T. gondii antigens for the development of more-sensitive and more-informative recombinant antigen-based tests for toxoplasmosis diagnosis.

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Prenatal diagnosis of congenital toxoplasmosis was attempted in 50 pregnant women at risk for giving birth to an affected child. Fifteen of these patients seroconverted during pregnancy and 35 had a high initial antibody level in their first serum sample. Prenatal diagnosis consisted of a combination of ultrasound screening, amniocentesis, and funipuncture at about 20 weeks' gestation. Diagnosis of congenital toxoplasmosis was based on a positive toxoplasma culture of amniotic fluid or fetal blood and on the presence of specific immunoglobulin M antibodies in fetal blood. In addition, alterations in fetal hematology, cellular immunology, and fetal liver tests were indicative of infection. Fetal infection was detected in six fetuses; two died in utero as a consequence of the infection and four were born after 37 weeks' gestation. Despite antibiotic treatment with pyrimethamine and sulfadiazine, one child has internal hydrocephalus and chorioretinitis and another has unilateral chorioretinitis. In the two other children, the disease is still subclinical. Of the 44 children born after a negative prenatal diagnosis, 35 have reached the age of 1 year; toxoplasma antibodies have disappeared in all of them. Investigation of the remaining nine children showed a decrease in toxoplasma antibodies, suggesting that none of them are affected. Prenatal diagnosis was never associated with fetal loss, and premature delivery occurred in only two cases. We conclude that prenatal diagnosis of congenital toxoplasmosis is safe and reliable.

In order to characterize Toxoplasma gondii antigens, we have produced a panel of monoclonal antibodies specific for the parasite. A total of 22 hybridomas were derived from the spleen cells of mice immunized either with a 100,000 g supernatant of a sonicate from the RH strain (called F3), or chronically infected with the Wiktor or the 76K strain. Except for one hybridoma producing an IgM, all the hybridomas derived from mice immunized with F3 produced IgG1 antibodies while those obtained from chronically infected mice produced antibodies belonging to the IgG2b, IgG2a and IgM subclasses. Western-blot analysis showed that the panel of monoclonal antibodies defines at least 7 distinct antigens or antigen families. An antigen of apparent Mw 25 kD present exclusively in the 100,000 g supernatant of the T. gondii sonicate was recognized by the majority of monoclonal antibodies derived from mice immunized with the F3 fraction. Two other antigens of apparent Mw 27 kD and 29 kD present in the soluble and insoluble fractions of the sonicate were also identified. Monoclonal antibodies against the previously described 21 kD and 31 kD surface antigens and belonging to the IgG2a but also to the IgG1 subclasses were able to mediate lysis of the parasite in the presence of human non immune serum. The 22 monoclonal antibodies did not identify antigenic differences between the two independently isolated RH and Wiktor strains.

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