Theileria annulata:The Expression of Two Novel Macroschizont Antigens on the Surface of Infected Mononuclear Cells Differs duringin VitroAttenuation of a Virulent Cell Line

Preston, P. M., Jackson, L. A., Sutherland, I. A., Bell-Sakyi, L., Wilkie, G., Brown, D. J., Schofield, J., Melrose, T. R., Sanderson, A., and Brown, C. G. D. 1998.Theileria annulata:The expression of two novel macroschizont antigens on the surface of infected mononuclear cells differs duringin vitroattenuation of a virulent cell line.Experimental Parasitology89,228–240. The first part of this study of the biological mechanisms underlying attenuation of virulentTheileria annulatamacroschizont-infected cell lines screened four pairs ofT. annulata(Hisar)in vivo- andin vitro-derived macroschizont-infected cell lines (lines) and identified a singlein vivo-derived line, which induced lethal tropical theileriosis. The other seven lines were relatively avirulent. Analysis of the clinical, hematological, and parasitological responses of cattle immunized with different passages of the virulent line afterin vitroculture showed that it was partly attenuated by passage (p) 50 and avirulent by p130. Clones representing the three glucose phosphate isomerase (GPI) isotypes, which constituted the newly isolated virulent culture, were obtained from p3 by limiting dilution; p50 and p130 consisted of one isotype. The second part of the study raised monoclonal antibodies (MAbs) against macroschizont-infected cells, as reagents for detecting antigenic differences between virulent and avirulent parasites, and identified two MAbs that recognized the surface of infected cells as well as macroschizonts. MAb EU1 recognized an antigen expressed by all the lines tested, whetherin vitro- orin vivo-derived, whether uncloned or cloned, and irrespective of extent of subpassage in culture. MAb EU106 recognized an antigen whose expression by the virulent line and its clones disappeared on passage in culture. This antigen was not expressed at all by the avirulentin vitro-derived line prepared with cells from the same calf. Both antigens were expressed by lines infected with other stocks ofT. annulata,including two lines known to induce lethal disease. The different profiles of expression of the two novel antigens, recognized by MAbs EU1 and EU106, by the line undergoing attenuation suggest (1) that the two antigens interact differently with the bovine immune system; and (2) that there are two, very different, potential roles for these antibodies in the development of vaccines againstT. annulatainfections.