Diverse mechanisms employed by Toxoplasma gondii to inhibit IFN-γ-induced major histocompatibility complex class II gene expression

The intracellular parasite Toxoplasma gondii is able to establish persistent infections in immunocompetent hosts and this may be facilitated by different immune evasion mechanisms. In the present study, we describe that infection of murine monocyte/macrophage RAW 264.7 cells with T. gondii blocks the IFN-γ-induced upregulation of major histocompatibility complex (MHC) class II mRNAs and proteins. Heat inactivation of the parasites prior to host cell invasion, but not inhibition of the intracellular replication of T. gondii abolished the inhibition of MHC class II upregulation. Interestingly, a T. gondii lysate (TL) mimicked the inhibitory effect of viable parasites on MHC class II expression. Nuclear translocation of the signal transducer and activator of transcription in response to IFN-γ were normal both in cells incubated with TL or infected with viable parasites. Transcript levels of the class II transactivator and consequently H2-Ab were nevertheless diminished by both viable parasites and TL. In contrast, interferon regulatory factor-1 mRNA was only decreased in response to viable T. gondii. Luciferase reporter assays confirmed differential effects of viable parasites and TL on minimal or complex IFN-γ-responsive promoters. Furthermore, only TL, and not viable parasites, strongly induced the secretion of IL-10 by murine macrophages. Whereas TL also inhibited MHC class II expression in macrophages from IL-10-deficient mice, increased IL-10 secretion by wild type macrophages did not mediate the block in MHC class II upregulation. In conclusion, T. gondii employs different mechanisms to inhibit MHC class II expression, suggesting a complex regulation of this immune evasion strategy.