Bovine γ/δ TcR+ T Lymphocytes are Stimulated to Proliferate by Autologous Theileria annulata‐Infected Cells in the Presence of Interleukin‐2

An in vitro model system has been developed in which freshly isolated resting WC1+γ/δTcR+ T cells proliferate in response to cells transformed by the protozoan parasite Theileria annulata, providing a strategy in which the basis of activation of naive γ/δ T cells can be investigated. Irradiated parasite‐transformed cells stimulate the proliferation, but not cytolytic activity, of autologous peripheral blood mononuclear cells (PBMC) from non‐immune animals. The proliferating cells are mainly WC1+γ/δ T cells. The majority of WC1+γ/δ T cells in freshly isolated PBMC express CD25 at a low level that increases when stimulated with T. annulata‐infected cells. Purified WC1+γ/δ T cells fail to proliferate when cultured with irradiated T. annulata‐infected cells and produce a small proliferative response to IL‐2, but proliferate strongly to irradiated or lightly fixed Theileria‐infected cells in combination with IL–2. The Theileria‐infected cells express cytokine transcripts encoding IL‐1α, IL‐1β, IL‐6 and IL‐10, but not IFNγ, IL‐2, IL‐4 and IL–7. Purified WC1+γ/δ T cells stimulated with T. annulata‐infected cells with or without IL‐2 fail to produce IL‐2 transcripts, but do produce those for TNFα. These experiments show that WC1+γ/δ T cells recognize a surface determinant on T. annulata‐infected cells, that together with a second signal, which can be provided by exogenous IL‐2, stimulates their proliferation.