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

Abstract

Theileria annulata is a protozoan parasite of cattle, causing the lymphoproliferative disease tropica] theileriosis. Ir susceptible animals, disease progresses rapidly, with the host apparently unable to mount an effective immune response. The parasite infects MHC class II+ monocytes and macrophages, during its pathogenic macroschizont stage. This leads to an augmentation of their antigen presenting capability in vitro - infected cells (IC) can induce proliferation of autologous resting T cells from naive animals. Preliminary studies showed that such altered APC function was a barrier to isolating Theileria specific T cells from immune animals. Infection of APC leading to a change in their function may provide T.annulata with a mechanism to evade the host immune response. 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 76T 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 dependant. 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.vation 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 (IFNy). None of these cytokines were found to be expressed by 1C. T cells within PBM expressed mRNA for IL2, IL2R, IL4 and IFNy 24-48 hours post IC stimulation. IL2 and its receptor were still expressed at day 5 (peak proliferation), and waned by day 7. IFNy 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 lL2/IL2R/IFNy was expressed by all T cell types. The presence of CD4 cells was required for 1L2 and IL2R expression by non CD4 T cells.In vivo examination was of the draining lymph node (DLN) which is the principal site of parasite development, ideally placing the parasite to interfere with the induction of immunity. DLN exhibited radically altered distributions of T cells expressing activation markers from accepted steps in immune response development. Infected cells were first manifested as proliferating non T non B cells which disseminated throughout the node and which expressed monocyte surface markers. Medullary T cells (predominantly CD4) clustered around 1C and expressed CD25 within 2 days of infection. This pattern persisted 4 days post infection, but numbers of CD25 T cells subsequently declined and none were found in the node by 10 days post infection. Altered function of DLN T cells was most obvious in the destruction of DLN germinal centres (GC), particularly the T cell dependent light zones. Although T cell activation marker expression waned, CD3 cells were not depleted from GC light zones or from the node in general. This may suggest that soluble factors play a role in interfering with immunity. Cytokine analysis of DLN cells early in infection demonstrated that production of IL4 within the entire node ceased within 4 days of infection, while in contrast IFNy production remained and has been shown in other experiments to become elevated. The control of B cell proliferation and differentiation depends partially upon the balance between stimulatory IL4 and inhibitory IFNy. The dominant production of IFNy may be leading to the disruption observed in GC by both changing the IL4 production capabilities of GC T cells and directly inhibiting B cell differentiation.In summary, this thesis has shown that T.annulata infected cells posses an innate ability to activate naive T cells. Although all T cell types are activated in pbm, this is dependant upon cytokine release by CD4 cells, subsequently leading to a type I 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.