IRG- and GBP-specific virulence mechanisms of Toxoplasma gondii

• Applicant: Privatdozent Dr. Tobias Steinfeldt
• Subject Area: Parasitology and Biology of Tropical Infectious Disease Pathogens
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 277942395

Immunity-Related GTPases (IRG proteins) and Guanylate Binding Proteins (GBP proteins) drive an essential mechanism of early cell-autonomous resistance against intracellular pathogens in mice. Some strains of the protozoan parasite Toxoplasma gondii are virulent in laboratory mice because they secrete kinases and pseudokinases that cooperate to specifically inactivate IRG proteins. We could identify a T. gondii kinase, ROP39, interacting with ROP5B, the predominant ROP5 isoform for virulence in laboratory mice. We will investigate the impact of this obvious novel kinase complex on parasite virulence, identify its cellular targets and thereby demonstrate its molecular function. Another part of this proposal comprises the analysis of a potential interplay of IRG and GBP proteins in terms of T. gondii control. We could demonstrate direct interaction of Irgb10 with mGBP6, the first IRG:GBP pair identified so far. We will continue to identify and describe these molecular interactions and in the end reveal a novel aspect of the IRG- and GBP-mediated control of murine T. gondii infections. In some wild-derived Eurasian mice, polymorphic IRG proteins inhibit the replication of virulent T. gondii strains. We could demonstrate that resistance is due to direct binding of the tandem IRG protein Irgb2-b1 to the T. gondii virulence effector ROP5B. The Irgb2-b1 interface of this interaction is highly polymorphic and under positive selection. Arypical South American T. gondii strains are virulent even in wild-derived Eurasian mice. We were able to demonstrate that this difference in virulence is due to polymorphic ROP5 isoforms that are not targeted by Irgb2-b1. We will characterise the ROP5 interface of that interaction and keep investigating molecular virulence mechanisms of South American T. gondii strains associated with IRG and GBP proteins.In human cells, resistance against T. gondii infections is, among other things, mediated by GBP proteins but the underlying molecular mechanism has not been revealed yet. Virulent but not avirulent T. gondii strains can inhibit the activation of the inflammasome and we were able to demonstrate a significant impact of effector proteins ROP5/ROP18. Interaction of ROP5 and ROP18 with GBP5 and GBP3 and experiments in respective GBP ko cells indicate a significant impact of ROP5/ROP18 as well as both GTPases. We seek to reveal the underlying molecular mechanism for ROP18 and ROP5 interaction with GBP5 and GBP3. These results will contribute to decipher the function of GBP proteins against T. gondii infections in human cells.

• DFG Programme: Research Grants