Diminished IL-2 responses and alteration of CD2 expression on CD8(+) T cells are associated with a lack of cytotoxic T cell responses during Theileria annulata infection

Theileria annulata is a tick-borne protozoan parasite which causes the dise ase bovine tropical theileriosis. In immunized or drug-treated animals, the pathogenic macroschizont stage of the parasite is destroyed by MHC class I -restricted cytotoxic T lymphocytes (CTL). Here we show that although CD8() T cells increase greatly in number and display activation markers during an acute infection, they exhibit no killing of infected cells. During the i neffectual response, efferent lymph cells' ability to proliferate to IL-2 d rops, coinciding with loss of MoAb binding to CD2 by CD8(+) cells. When ani mals were treated with the anti-parasite drug 'Butalex', IL-2 responses, an ti-CD2 antibody binding by CD8(+) cells and strong CTL activity were restor ed within 24 h. The initial activation of CD4(+) T cells by parasite-infect ed cells altering the IL-2 production in the draining lymph node is the lik ely cause of the failure of CTL responses.