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.

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Fig. 1: Substantial structural divergence between VSGs.
Fig. 2: Structural identification of O-linked carbohydrate on the VSG3 surface.
Fig. 3: Identification of heterogeneous O-linked glycans in surface loops of multiple VSGs.
Fig. 4: The presence of O-linked glycan impairs immune function.

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Acknowledgements

We thank G. Cross (Rockefeller University) and H. Wardemann (DKFZ) for critical reading of the manuscript and for general advice, M. Sanches-Vaz and L. Figueiredo (IMM, Lisbon) for help with mouse experiments and M. Chandra (DKFZ) for providing us with purified VSG615. We also thank the staff at Argonne National Laboratories (NE-CAT) for beamline support, and D. Oren at the Structural Biology and the High-Throughput Sequencing and Spectroscopy Resource Centers at Rockefeller University. NE-CAT is funded by an NIH/NIGMS grant (P41 GM103403) and the Pilatus 6M detector on 24-ID-C beam line is funded by an NIH-ORIP HEI grant (S10 RR029205). The Advanced Photon Source, within which NE-CAT is located, is a US Department of Energy (DOE) User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. This work was also supported by funds to C.E.S. and F.N.P. from the German Cancer Research Center (DKFZ, Heidelberg) and Rockefeller University, by NIH/NIAID (AI085973) to F.N.P. and by a Wellcome Trust Senior Investigator Award (101842) to M.A.J.F. The University of Dundee MS facility is supported by Wellcome Trust grant 097045.

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J.P., D.N., C.E.S., L.A., M.A.J.F. and F.N.P. conceived and designed the experiments. J.P., D.N., M.L., F.N.P. and F.A.B. carried out the protein purification. C.E.S. and F.A.B. performed the structural prediction analyses. D.N. and C.E.S carried out the crystallography analyses. L.A. and M.A.J.F. performed the MS analyses. J.P., S.C., F.A.B., F.N.P., J.V. and J.R. carried out the trypanosome genetics and growth analyses, antibody assays and mouse infection studies. D.N., L.A., J.P., M.L., S.C., F.A.B., H.-S.K., F.N.P. and C.E.S. contributed reagents, materials and analysis tools. J.P., C.E.S., F.N.P., D.N., L.A. and M.A.J.F. wrote the paper.

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Correspondence to Michael A. J. Ferguson or F. Nina Papavasiliou or C. Erec Stebbins.

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Pinger, J., Nešić, D., Ali, L. et al. African trypanosomes evade immune clearance by O-glycosylation of the VSG surface coat. Nat Microbiol 3, 932–938 (2018). https://doi.org/10.1038/s41564-018-0187-6

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