T cell activation in Theileria annulata infection : implications for immunity & pathogenesis

This thesis sets out to understand interactions between T cells and T. annulata infected cells, both in vitro and in vivo and the consequences for the generation of immunity. In vitro stimulation of peripheral blood T cells from naive animals by IC caused the cells to proliferate, peaking 5 days post stimulation. Phenotypic analysis showed that CD25 and MHC class II were expressed upon the surface of all T cells (CD4, CD8 and γδT cells) within 24hrs of stimulation, reaching a peak at 48hrs and remaining stably expressed for up to 7 days post activation. The parasite infected cells could activate both "memory" and "naive" CD4 T cells, with little change in the CD45RB isoforms during activation. Activation of T cells was contact dependent. T annulata infected cells can therefore cause the activation of the majority of T cells from naive animals irrespective of memory status and , presumably, antigen specificity. The cytokines produced by IC stimulated T cells 1-7 days post stimulation were assessed by reverse transcription polymerase chain reaction (RT-PCR) using primers for IL2, IL2 receptor (IL2R), IL4 and interferon gamma (IFNγ). None of these cytokines were found to be expressed by IC. T cells within PBM expressed mRNA for IL2, IL2R, IL4 and IFNγ 24-48 hours post IC stimulation. IL2 and its receptor were still expressed at day 5 (peak proliferation), and waned by day 7. IFNγ was expressed by all tested animals' cells at all timepoints, while IL4 was intermittently found at day 5 and was always absent at day 7. IL4 was only expressed by CD4 T cells, while IL2/IL2R/IFNγ was expressed by all T cell types. The presence of CD4 cells was required for IL2 and IL2R expression by non CD4 T cells. In summary, this thesis has shown that T. annulata infected cells possess an innate ability to activate naive T cells. Although all T cell types are activated in PBM, this is dependent upon cytokine release by CD4 cells, subsequently leading to a type 1 response. In vivo, a similar mechanism leads to activation of DLN T cells primarily by IC. Such interactions do not lead to the induction of an antigen specific immune response, but to the loss of GC and suppression of further T cell activation.