Evasion of interferon-gamma responses by Toxoplasma gondii in murine and human fibroblasts

The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. ABSTRACT Co-evolution of pathogen and host helps drive biological diversity. Unlike viral-host interactions, little is known about the co-evolution of eukaryotic pathogens with their hosts. The intracellular parasite Toxoplasma gondii is an excellent model organism for co-evolution studies because its hosts include all warm-blooded animals, and genetically diverse Toxoplasma strains may be adapted to specific hosts. Toxoplasma must evade host immunity without killing the host to establish a chronic infection and ensure transmission. Interferon-gamma (IFNγ) activation of non-immune cells is crucial for host defense against Toxoplasma. In murine cells, interferon-inducible immune-related GTPases (IRGs) are essential to the IFNγ response because they disrupt the parasitophorous vacuole (PV). However, Toxoplasma secretes the contents of apical secretory organelles into the host cell during invasion, and some of these proteins strain-specifically promote mouse virulence by inactivating the IRGs. Here, we show that two secreted Toxoplasma factors, the protein kinase ROP18 and the pseudokinase ROP5, determine IRG evasion. We demonstrate that ROP5 binds to and inhibits the oligomerization of Irga6, allowing ROP18 to phosphorylate the IRGs to inhibit PV accumulation. However, humans lack interferon-inducible IRGs, and ROP5 and ROP18 do not affect Toxoplasma growth inhibition in human cells, suggesting these factors specifically evolved to battle the IRG system. Both ROP5 and the IRGs exhibit diversifying selection, and these proteins may provide a model for study of eukaryotic pathogen-host co-evolution. We also uncover a novel mechanism of IFNγ-mediated Toxoplasma growth inhibition in human fibroblasts that correlates with host cell death that cannot be abrogated by inhibiting cell death pathways. Furthermore, we observed parasite egress from IFNγ-stimulated cells before replication, but inhibition of egress did not prevent cell death. Thus, the inhospitable intracellular environment of dying IFNγ-stimulated human fibroblasts triggers parasite egress. This disrupts the intracellular niche, prevents replication and could promote immune clearance or depletion of parasite secretory factors. This work highlights the need for a parasite to balance immune evasion for increased parasite propagation with limiting parasite burden for host and parasite survival. Thus, host immune factors and parasite immune evasion factors have co-evolved, and strain differences may be due to adaptation to different hosts.