The bovine autologous Theileria mixed leucocyte reaction: influence of monocytes and phenotype of the parasitized stimulator cell on proliferation and parasite specificity.

In the autologous Theileria mixed leucocyte reaction (MLR), irradiated Theileria parva-infected cells induce proliferative responses in autologous peripheral blood mononuclear leucocytes (PBM), irrespective of the immune status of the donor animal. In this paper we have analysed the cellular basis of this response in naive and immune cattle to determine the Theileria specificity of the response. The magnitude of proliferation is dependent on two parameters, namely the presence or absence of monocytes in the responder population, and the phenotype of the parasitized stimulator cells, both of which appeared to be independent of the immune status of the donor animal. Monocyte-depleted responders invariably gave stronger proliferative responses but generated cytotoxicity from immune cattle that tended to be less genetically restricted. Marked differences were observed in the stimulatory capacity of cloned parasitized T-cell and non-T cell lines. At least part of this variation was associated with differences in the capacity of the parasitized cells to secrete soluble suppressive factors and possibly also stimulatory factors. Two observations indicated that, in immune cattle, part of the proliferative response in the autologous Theileria MLR is parasite-specific. First, stimulator cells fixed with glutaraldehyde stimulated proliferative responses in monocyte depleted PBM from immune animals but not naive animals. Second, in autologous Theileria MLRs with intact PBM, genetically restricted cytotoxic cells were generated from immune but not naive animals. While monocytes seem not to be required for induction of the parasite-specific component of the response, their absence from the assay when viable stimulator cells are utilized appears to enhance the non-specific component of the proliferative and cytotoxic responses.