THE P-29 AND P35 IMMUNODOMINANT ANTIGENS OF NEOSPORA-CANINUM TACHYZOITES ARE HOMOLOGOUS TO THE FAMILY OF SURFACE-ANTIGENS OF TOXOPLASMA-GONDII

Neospora caninum is an apicomplexan parasite that is closely related t o Toxoplasma gondii and has been found to be associated with neurologi cal disorders in dogs and congenital infections and abortions in cattl e. We have identified two surface proteins of 29 and 35 kDa (designate d Ncp29 and Ncp35, respectively) from N. caninum tachyzoites that are the predominant antigens recognized by antisera from Neospora-infected animals. Monoclonal antibodies against Ncp29 and Ncp35 were used to a nalyze several independent and diverse N. caninum isolates; both antig ens were recognized in all isolates, suggesting that they are well con served. Localization studies and surface labeling with biotin demonstr ated that Ncp29 and Ncp35 are membrane associated and displayed on the surface of the parasite. After treatment with phosphatidylinositol-sp ecific phospholipase C, parasite lysates were analyzed with antibodies against the cross-reacting determinant of glycosylphosphatidylinosito l anchors. Approximately six glycolipid-anchored surface proteins were identified, with the two most prominent corresponding to Ncp29 and Nc p35. Sequence comparisons of Ncp29 and Ncp35 with GenBank indicated th at they are most similar to the T. gondii surface antigen 1 (SAG1) and surface antigen 1-related sequence 2 (SRS2), respectively. Consequent ly, Ncp29 has been designated NcSAG1 and Ncp35 has been designated NcS RS2. Both NcSAG1 and NcSRS2 contain a tandemly duplicated motif and 12 absolutely conserved cysteines which are also found in all of the SAG and SRS proteins of T. gondii. Maintenance of these motifs and the 12 cysteine residues suggests that these surface antigens share a simila r secondary and tertiary structure that is presumably important for a conserved function that these antigens serve during infection.