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 disease 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 ineffectual response, efferent lymph cells' ability to proliferate to IL‐2 drops, coinciding with loss of MoAb binding to CD2 by CD8+ cells. When animals were treated with the anti‐parasite drug ‘Butalex’, IL‐2 responses, anti‐CD2 antibody binding by CD8+ cells and strong CTL activity were restored within 24 h. The initial activation of CD4+ T cells by parasite‐infected cells altering the IL‐2 production in the draining lymph node is the likely cause of the failure of CTL responses.