Surface antigen SAG1 mediates fitness and host cell attachment in IFNγ-stimulated cells.

is an obligate intracellular protozoan parasite that can establish lifelong infections and cause severe disease in immunocompromised individuals. Interferon gamma (IFNγ) is a key host defense cytokine that induces a variety of toxoplasmacidal mechanisms. Recent CRISPR/Cas9 loss-of-function screens identified multiple genes important for fitness in IFNγ-stimulated cells. One consistent hit in several screens was the parasite surface antigen, SAG1. Here, we used CRISPR/Cas9 to generate a knockout strain and found that SAG1 is important for parasite fitness specifically in IFNγ-stimulated cells. Mechanistic studies revealed that host surface sialic acids are important for parasite attachment, especially in IFNγ-stimulated cells. SAG1-deficient parasites had reduced attachment efficiency, which was exacerbated in IFNγ-treated cells. These findings highlight the role of SAG1 in mediating robust parasite attachment, especially in the context of immune pressure.