African trypanosomes evade immune clearance by O-glycosylation of the VSG surface coat

Abstract

The African trypanosome Trypanosoma brucei spp. is a paradigm for antigenic variation, the orchestrated alteration of cell surface molecules to evade host immunity. The parasite elicits robust antibody-mediated immune responses to its variant surface glycoprotein (VSG) coat, but evades immune clearance by repeatedly accessing a large genetic VSG repertoire and 'switching' to antigenically distinct VSGs. This persistent immune evasion has been ascribed exclusively to amino-acid variance on the VSG surface presented by a conserved underlying protein architecture. We establish here that this model does not account for the scope of VSG structural and biochemical diversity. The 1.4-Å-resolution crystal structure of the variant VSG3 manifests divergence in the tertiary fold and oligomeric state. The structure also reveals an O-linked carbohydrate on the top surface of VSG3. Mass spectrometric analysis indicates that this O-glycosylation site is heterogeneously occupied in VSG3 by zero to three hexose residues and is also present in other VSGs. We demonstrate that this O-glycosylation increases parasite virulence by impairing the generation of protective immunity. These data alter the paradigm of antigenic variation by the African trypanosome, expanding VSG variability beyond amino-acid sequence to include surface post-translational modifications with immunomodulatory impact.

A Parasite Coat Protein Binds Suramin to Confer Drug Resistance

“…2 and Table 1). VSGsur differs markedly from previous VSG structures determined (VSG1, VSG2, VSG3, and ILTat1.24) [24][25][26][27] . While the core 3-helix bundle scaffolding of the VSG family is present, VSGsur possesses several divergent features.…”

“…Despite the wide genetic variance in the VSGs, only a handful of the membrane distal, large NTD domains have been characterized at the atomic level [24][25][26][27] . The recent crystal structure of VSG3 revealed topological divergence from the folds of other VSG NTDs, demonstrating that there is significantly more variation in architecture of this protein family than previously considered 25 . This structure also uncovered an unexpected post-translational modification (O-15 linked sugar at the top of VSG3) that strongly modulates the host immune response.…”

POFUT2-mediated O-glycosylation of MIC2 is dispensable for Toxoplasma gondii tachyzoites

“…The recent recognition of the importance of glycosylation at the host-pathogen interface in parasites such as P. falciparum (19) and Trypanosoma brucei (29) has demonstrated the need to better understand glycosylation within phylogenetically distinct eukaryotic parasites.…”

Evolution of Antigenic Variation in African Trypanosomes: Variant Surface Glycoprotein Expression, Structure, and Function

“…The second development is the crystal structure of the NTD of a class B subtype, VSG3 (aka MITat1.3, VSG224), which yielded two surprises . First is the previously unknown presence of a short variable O ‐linked glycan on the outer face of the VSG.…”