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The kinetics of induction of T cell responses were examined in efferent lymph from a node draining the site of a primary inoculation of Toxoplasma gondii. The numbers of T cells increased after infection, due initially to an expansion of the CD4+ T cell population followed by an increase in the number of CD8+ T cells which coincided with the peak lymphoblast response. Proliferative responses of CD4+ T cells to T. gondii antigen were detectable from day six after infection and immune efferent lymph cells inhibited the intracellular multiplication of T. gondii in vitro. Optimum inhibition was achieved using CD8+ T cells restimulated in vitro, and the effector function appeared to be directed preferentially against the autologous rather than the allogeneic infected target cell. The results provide unique information on the induction of immune responses to T. gondii in vivo and provide evidence that both CD4+ and CD8+ T cells are necessary for the development of protective immunity induced by the S48 strain of T. gondii which is used as a live vaccine in sheep.

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A model for the in vitro infection of ovine cells with Toxoplasma gondii tachyzoites has been developed and used to investigate the effect of treatment with ovine recombinant interferon-gamma (ov.rIFN gamma) on parasite replication. Treatment of both alveolar macrophages and fibroblast cells either 24 h pre-infection or 2 h post-infection with ov.rIFN gamma inhibited replication of T. gondii and was quantified by suppression of 3H uracil uptake by the parasite. Replication of T. gondii in the fibroblast cells was significantly inhibited by treatment with 200-300 U/ml ov.rIFN gamma, whereas concentrations as low as 1 U/ml suppressed parasite replication in the alveolar macrophages.