Kinetics and regulation of NK activity by interleukin-2 and interferon in acute toxoplasmosis

Natural killer (NK) activity against Toxoplasma gondii tachyzoites and tumour cells during acute toxoplasmosis was investigated using a single-cell NK assay. During the course of infection the percentage of lymphocytes binding tachyzoites and tumour cells did not vary significantly and NK activity was enhanced due to an increase in the specific cytolytic capacity per cell. To determine whether regulatory mechanisms mediated by cytokines might explain the increased NK activity, the kinetics of interleukin-2 (IL-2) and interferon (IFN) production and the correlation between their concentrations and the level of NK activity were analysed at the same time. As the infection progressed NK activity increased in spite of the fact that IL-2 production decreased (except for a small increase during the first day of infection). However, IFN production increased gradually in close temporal and quantitative association with the overall increase in NK activity. These results suggest that T. gondii, via its ability to produce interferon, enhances NK activity against itself and other cells.