Inducible Nitric Oxide Synthase Is a Key Host Factor forToxoplasmaGRA15-Dependent Disruption of the Gamma Interferon-Induced Antiparasitic Human Response

Toxoplasma,an important intracellular parasite of humans and animals, causes life-threatening toxoplasmosis in immunocompromised individuals. Gamma interferon (IFN-γ) is produced in the host to inhibit the proliferation of this parasite and eventually cause its death. Unlike mouse disease models, which involve well-characterized virulence strategies that are used byToxoplasmato suppress IFN-γ-dependent immunity, the strategies used byToxoplasmain humans remain unclear. Here, we show that GRA15, aToxoplasmaeffector protein, suppresses the IFN-γ-induced indole-2,3-dioxygenase 1-dependent antiparasite immune response in human cells. Because NLRP3-dependent production of IL-1β and nitric oxide (NO) inToxoplasma-infected human cells is involved in the GRA15-dependent virulence mechanism, blocking NO or IL-1β production in the host could represent a novel therapeutic approach for treating human toxoplasmosis.