Virulent and Avirulent Strains of Toxoplasma gondii Which Differ in Their Glycosylphosphatidylinositol Content Induce Similar Biological Functions in Macrophages

Laura J Knoll,Marco A Campos,Terry K Smith,Ralph T Schwarz,Jean-François Dubremetz,Nahid Azzouz,Françoise Debierre-Grockiego,Ricardo T Gazzinelli,Sebastian Niehus

doi:10.1371/journal.pone.0085386

Abstract

Glycosylphosphatidylinositols (GPIs) from several protozoan parasites are thought to elicit a detrimental stimulation of the host innate immune system aside their main function to anchor surface proteins. Here we analyzed the GPI biosynthesis of an avirulent Toxoplasma gondii type 2 strain (PTG) by metabolic radioactive labeling. We determined the biological function of individual GPI species in the PTG strain in comparison with previously characterized GPI-anchors of a virulent strain (RH). The GPI intermediates of both strains were structurally similar, however the abundance of two of six GPI intermediates was significantly reduced in the PTG strain. The side-by-side comparison of GPI-anchor content revealed that the PTG strain had only ∼34% of the protein-free GPIs as well as ∼70% of the GPI-anchored proteins with significantly lower rates of protein N-glycosylation compared to the RH strain. All mature GPIs from both strains induced comparable secretion levels of TNF-α and IL-12p40, and initiated TLR4/MyD88-dependent NF-κBp65 activation in macrophages. Taken together, these results demonstrate that PTG and RH strains differ in their GPI biosynthesis and possess significantly different GPI-anchor content, while individual GPI species of both strains induce similar biological functions in macrophages.

Highlights

The obligate intracellular protozoan Toxoplasma gondii ranks among the most common parasites of animals and humans, with an estimated worldwide incidence of 30% of chronically infected individuals [1]
We demonstrate that the GPIs of the virulent T. gondii RH and the avirulent PTG strains have similar structures and induce comparable biological effects in macrophages but differ in terms of total GPI-anchor content and degree of GPI-anchored protein N-glycosylation, both were significantly increased in the virulent RH strain
These findings are in line with several earlier studies that have shown that purified GPIs from protozoan parasites are able to activate immune cells of both myeloid and lymphoid origin [51]

Summary

Introduction

The obligate intracellular protozoan Toxoplasma gondii ranks among the most common parasites of animals and humans, with an estimated worldwide incidence of 30% of chronically infected individuals [1]. The high virulence of type 1 strains is largely due to the rhoptry protein (ROP) kinase, the closely related ROP5 pseudokinase, and the ROP16 kinase [6,11,12,13,14] The former kinases target host immunity by disrupting the function of immunity-related GTPase (IRG)-mediated parasite clearance in macrophages, while this process is not affected by type 2 and 3 strains, where IRGs cause disruption of the PVM and subsequent parasite destruction [15,16,17]. This detrimental overstimulation of host immunity is likely triggered by parasite components, such as profilin, cyclophilin-18, and glycosylphosphatidylinositol (GPI) anchors [22,23,24]

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Conclusion