PATHOGENICITY OF THEILERIA PAWN IS INFLUENCED BY THE HOST-CELL TYPE INFECTED BY THE PARASITE

Theileria parva has been shown to infect and transform B cells and T c ells at similar frequencies in vitro. However, the majority of parasit ized cells in the tissues of infected cattle are alpha/beta T cells, T he aim of this study was to determine whether the cell type infected w ith T, pawn influenced the pathogenicity of the parasite. The initial approach, which involved inoculation of cattle with autologous cloned cell lines of different phenotypes, failed to resolve the issue, becau se the prolonged period of culture required to clone and characterize the cell lines resulted in attenuation of the cells, As an alternative approach, cattle were inoculated with purified populations of autolog ous cells that had been incubated in vitro with T, parva sporozoites f or 48 h. As few as 3 x 10(4) peripheral blood mononuclear cells (PBMC) treated in this way were found to produce severe clinical reactions w ith high levels of parasitosis. Infections of similar severity were pr oduced with purified populations of CD2(+), CD4(+), and CD8(+) T cells , By contrast, infected B cells gave rise to mild self-limiting infect ions even when administered at a 10-fold-higher dose, In animals that received infected CD4(+) or CD8(+) T cells, the parasitized cells in t he lymph nodes on day 11 of infection were all within the CD4(+) and C D8(+) populations, respectively, indicating that there had been minima l transfer of the parasite between cell types, Phenotypic analyses of cultures of PBMC infected in vitro with saturating concentrations of s porozoites revealed that parasitized B cells were abundant in the cult ures after 1 week but were subsequently overgrown by T cells, The resu lts of these experiments indicate that the cell type infected by T. pa rva influences the pathogenicity of the parasite.