Toxoplasma IWS1 determines fitness in interferon-?-activated host cells and mice by regulating ROP18 mRNA expression

Toxoplasma gondii secretes various virulence effector molecules into host cells to disrupt host interferon-? (IFN-?)-dependent immunity. Among the effectors, ROP18 directly phosphorylates and inactivates IFN-inducible GTPases, such as immunity-related GTPases (IRGs) and guanylate-binding proteins (GBPs), leading to subversion of IFN-inducible GTPase-induced cell-autonomous immunity. The modes of action of ROP18 have been studied extensively; however, little is known about the molecular mechanism of how ROP18 is produced in the parasite itself. Here, we report a role of T. gondii transcription factor IWS1 in ROP18 mRNA expression in the parasite. Compared with wild-type virulent type I T. gondii, IWS1-deficient parasites showed dramatically increased loading of IRGs and GBPs onto the parasitophorous vacuole membrane (PVM). Moreover, IWS1-deficient parasites displayed decreased virulence in wild-type mice but retained normal virulence in mice lacking the IFN-? receptor. Furthermore, IWS1-deficient parasites showed severely decreased ROP18 mRNA expression. Ectopic expression of ROP18 in IWS1-deficient parasites restored the decreased loading of effectors onto the PVM and in vivo virulence in wild-type mice. Taken together, these data demonstrate that T. gondii IWS1 regulates ROP18 mRNA expression to determine fitness in IFN-?-activated host cells and mice. Overall design: We compared global gene expression profiles in freshly isolated tachyzoites of wild-type and IWS1-deficient type I T. gondii as triplicate.