Guanylate-binding Protein 1 (Gbp1) Contributes to Cell-autonomous Immunity against Toxoplasma gondii

Abstract

IFN-γ activates cells to restrict intracellular pathogens by upregulating cellular effectors including the p65 family of guanylate-binding proteins (GBPs). Here we test the role of Gbp1 in the IFN-γ-dependent control of T. gondii in the mouse model. Virulent strains of T. gondii avoided recruitment of Gbp1 to the parasitophorous vacuole in a strain-dependent manner that was mediated by the parasite virulence factors ROP18, an active serine/threonine kinase, and the pseudokinase ROP5. Increased recruitment of Gbp1 to Δrop18 or Δrop5 parasites was associated with clearance in IFN-γ-activated macrophages in vitro, a process dependent on the autophagy protein Atg5. The increased susceptibility of Δrop18 mutants in IFN-γ-activated macrophages was reverted in Gbp1−/− cells, and decreased virulence of this mutant was compensated in Gbp1−/− mice, which were also more susceptible to challenge with type II strain parasites of intermediate virulence. These findings demonstrate that Gbp1 plays an important role in the IFN-γ-dependent, cell-autonomous control of toxoplasmosis and predict a broader role for this protein in host defense.

Ubiquitin systems mark pathogen-containing vacuoles as targets for host defense by guanylate binding proteins

“…We confirmed previous observations that hypervirulent strains of T. gondii block GBP accumulation on PV membranes (18,25,58). In light of our discovery of the causal link between the deposition of GKS proteins, PV ubiquitination, and p62-dependent GBP delivery, we were able to explain how hypervirulent strains of T. gondii can escape GBP-mediated immunity through the inhibition of GKS proteins.…”

“…Interference with ATG8 lipidation diminishes GBP and GKS protein recruitment to PVs, demonstrating the importance of these molecules in immunotargeting of PVs (24,25,42,48,(55)(56)(57). Whether and how the ATG8 pathway intersects with the newly discovered IRG-driven ubiquitination pathway will need to be explored further in the future.…”

“…In IRGMdeficient cells, however, GKS proteins enter the active state prematurely, form protein aggregates, mislocalize, and thus fail to bind to PVs (17,21). Although these previous observations help explain how IRGM proteins promote the delivery of GKS proteins to PVs, IRGM proteins also control the subcellular localization of GBPs through an uncharacterized mechanism (6,17,(24)(25)(26).…”

Toxoplasma GRA7 effector increases turnover of immunity-related GTPases and contributes to acute virulence in the mouse

“…The activity of ROP18 is regulated by a related pseudokinase ROP5 (17), which also binds to monomeric Irga6 and blocks its assembly in vitro (18,19). Recent evidence implicates ROP5 and ROP18 in avoidance of guanylate binding proteins, which also contribute to restriction of T. gondii in IFN-γ-activated macrophages (20).Kinases often interact with pseudokinases and scaffolding proteins for regulation or substrate recruitment (21, 22). Consequently, we were interested in whether ROP18 forms a complex with other parasite proteins that might influence its activity or function.…”

“…The activity of ROP18 is regulated by a related pseudokinase ROP5 (17), which also binds to monomeric Irga6 and blocks its assembly in vitro (18,19). Recent evidence implicates ROP5 and ROP18 in avoidance of guanylate binding proteins, which also contribute to restriction of T. gondii in IFN-γ-activated macrophages (20).…”