Antigenic diversity of a major merozoite surface molecule in Theileria annulata

The 30-kDa merozoite surface antigen of Theileria annulata (Ankara) has been characterised by using merozoites derived from differentiating macroschizont-infected cloned cell lines. This molecule was found to be 1 of the major polypeptides in extracts of both merozoites and piroplasms and was strongly detected by serum from an immune cow. Analysis of merozoite extracts derived from cloned cell lines infected with parasites of different genotypes detected 2 forms of this molecule, with respective molecular masses of 30 kDa and 32 kDa. These 2 molecules were shown to be related to each other by peptide mapping. In addition to the observed size differences, the molecules were found to be antigenically divergent. Periodate treatment of western blots demonstrated that at least 1 of the variable antigenic determinants was generated by secondary modification of the polypeptide and that a large amount of the antibody reactivity was directed against similar moieties. Parasite stocks isolated from different geographical regions were distinguished by analysis of the molecules within the 30-32-kDa molecular region. The relationship between T. annulata molecules and similar antigens described for other Theileria species is discussed.