Effect of cytokines on growth of Toxoplasma gondii in murine astrocytes

Abstract

Cytokines play a significant role in the regulation of Toxoplasma gondii in the central nervous system. Cytokine-activated microglia are important host defense cells in central nervous system infections. Recent evidence indicates that astrocytes can also be activated by cytokines to inhibit intracellular pathogens. In this study, we examined the effect of gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), interleukin-6 (IL-6), and IL-1 on the growth of T. gondii in a primary murine astrocyte culture. Pretreatment of astrocytes with IFN-gamma resulted in 65% inhibition of T. gondii growth. Neither TNF-alpha, IL-1, nor IL-6 alone had any effect on T. gondii growth. IFN-gamma in combination with either TNF-alpha, IL-1, or IL-6 caused a 75 to 80% inhibition of growth. While nitric oxide was produced by astrocytes treated with these cytokines, inhibition of T. gondii growth was not reversed by the addition of the nitric oxide synthase inhibitor NG-monomethyl-L-arginine. Furthermore, IFN-gamma in combination with IL-1, IL-6, or TNF-alpha also induced inhibition in astrocytes derived from syngeneic mice deficient in the enzyme inducible nitric oxide synthase. This finding suggests that the mechanism of cytokine inhibition is not nitric oxide mediated. Similarly, the addition of tryptophan had no effect on inhibition, indicating that the mechanism was not mediated via induction of the enzyme indoleamine 2, 3-dioxygenase. The mechanism of inhibition remains to be elucidated. Results from this study demonstrate that cytokine-activated astrocytes are capable of significantly inhibiting the growth of T. gondii. These data indicate that astrocytes may be important host defense cells in controlling toxoplasmosis in the brain.

FIG. 1

Effect of cytokines on growth of T. gondii in mouse astrocytes. Cells were incubated with the cytokines IFN-γ (100 U/ml), IL1 (1 ng/ml), IL-6 (100 U/ml), and TNF-α (100 U/ml) for 72 h before infection. Control cultures were incubated in medium alone. [³H]uracil (2.5 μCi/ml) was added 2 h after infection, and cells were harvested 48 h later. Results are averages of three separate experiments. Bars indicate ± standard error of the mean. ∗, significance at the P < 0.05 level versus control as calculated by Student’s t test. (Insert) Percentages of cells (mean ± standard deviation) infected with T. gondii ME49 as determined by microscopy (multiplicity of infection, 5:1). The difference between each cytokine treatment and the control was significant at P < 0.05; there was no significant difference between any of the various cytokine treatments.