Proliferation and Lymphocyte Stimulatory Capacity of Theileria-infected Lymphoblastoid Cells Before and After the Elimination of Intracellular Parasites

A prominent pathogenic facet of Theileria infections is that the parasite infects lymphocytes and 'transforms' them into parasitized lymphoblastoid cells which are highly proliferative and can be cultured indefinitely in vitro. To analyse the relationship between the intracellular parasite and lymphocyte transformation we have studied the effects of eliminating parasites from these lymphoblastoid cells using the naphthoquinone derivative 993.C. Treatment of Theileria-infected lymphoblastoid cells with 993.C gradually eliminates intracellular parasites but cell proliferation is not inhibited until several days after parasite elimination. The proliferating cells no longer contain schizont particles and are medium-sized lymphocytes and small blast cells. The surface phenotype of these cells, as defined by lectins and monoclonal antibodies, remains unchanged after parasite elimination. The division of non-parasitized cells, under the culture conditions examined, was not indefinite. The compound 993.C itself does not appear to be mitogenic and possible mechanisms for this continued division are discussed. It has been previously reported that co-cultivation of irradiated Theileria-infected lymphoblastoid cells with autologous lymphocytes induces marked DNA synthesis in the latter. We examined further the relevance of this observation for immunity to Theileria by using lymphoblastoid cells treated with 993.C. Elimination of intracellular parasites by this compound does not impair the ability of these cells to stimulate DNA synthesis in autologous lymphocytes. Furthermore, lymphocytes from Theileria-immune or non-immune cattle react similarly. The reaction differs from a classical mixed lymphocyte reaction induced by antigens encoded in the main histocompatibility complex since the stimulator cells are exclusively T lymphocyte-derived cells, the magnitude of response is greater and stimulated lymphocytes are able to act as 'stimulator cells' to fresh autologous lymphocytes. Thus we question the immunological relevance of the observed lymphocyte division. The possibility that Theileria-infected lymphoblastoid cells carry viral genomes or infectious virus particles is discussed. Preliminary electron microscopic studies have not revealed any virus particles.