Interference by the intracellular parasite Theileria parva with T-cell signal transduction pathways induces transformation and protection against apoptosis.

The intracellular parasite Theileria parva transforms bovine T-lymphocytes, inducing uncontrolled proliferation. Upon infection, cells cease to require antigenic stimulation and exogenous growth factors to proliferate. Earlier studies have shown that pathways triggered via stimulation of the T-cell receptor are silent in transformed cells. This is reflected by a lack of phosphorylation of key signalling molecules and the fact that proliferation is not inhibited by immunosuppressants such as cyclosporin and ascomycin that target calcineurin. This suggests that the parasite bypasses the normal T-cells activation pathways to induce proliferation. Among the MAP-kinase pathways, ERK and p38 are silent, and only Jun N-terminal kinase is activated. This appears to suffice to induce constitutive activation of the transcription factor AP-1. More recently, it could be shown that the presence of the parasite in the host cell cytoplasm also induces constitutive activation of NF-kappa B, a transcription factor involved in proliferation and protection against apoptosis. Activation is effectuated by parasite-induced degradation of I kappa Bs, the cytoplasmic inhibitors which sequester NF-kappa B in the cytoplasm. NF-kappa B activation is resistant to the antioxidant N-acetyl cysteine and a range of other reagents, suggesting that activation might occur in an unorthodox manner. Studies using inhibitors and dominant negative mutants demonstrate that the parasite activates a NF-kappa B-dependent anti-apoptotic mechanism that protects the transformed cell form spontaneous apoptosis and is essential for maintaining the transformed state of the parasitised cell. (C) 1999 Elsevier Science B.V. All rights reserved.