FIG. 1.

Inhibition of N. caninum growth in human cells and abrogation of inhibition by addition of tryptophan. A total of 3 × 10⁴ HFFs (A), astrocytoma cells (86HG39 cells) (B), or human lung cells (A549 cells) (C) were stimulated with IFN-γ (0 to 1,000 U/ml) in the presence or absence of l-tryptophan (final concentration, 0.6 mM). After 3 days of incubation, N. caninum tachyzoites (2 × 10⁴ tachyzoites per well) were added with or without l-tryptophan. Parasite growth was determined as described in Materials and Methods. The data are means and standard errors of the means of four independent experiments, each performed in triplicate. Significant inhibition of N. caninum growth by IFN-γ-activated cells is indicated by one asterisk, and significant abrogation of inhibition by l-tryptophan is indicated by two asterisks (P < 0.05).