The release of host-derived antibodies bound to the variant surface glycoprotein (VSG) of Trypanosoma brucei cannot be explained by pH-dependent conformational changes of the VSG dimer

Open Res Eur. 2024 Apr 24:4:87. doi: 10.12688/openreseurope.16783.1. eCollection 2024.

Abstract

Background: Trypanosoma brucei is a protozoan parasite that evades the mammalian host's adaptive immune response by antigenic variation of the highly immunogenic variant surface glycoprotein (VSG). VSGs form a dense surface coat that is constantly recycled through the endosomal system. Bound antibodies are separated in the endosome from the VSG and destroyed in the lysosome. For VSGs it has been hypothesized that pH-dependent structural changes of the VSG could occur in the more acidic environment of the endosome and hence, facilitate the separation of the antibody from the VSG.

Methods: We used size exclusion chromatography, where molecules are separated according to their hydrodynamic radius to see if the VSG is present as a homodimer at both pH values. To gain information about the structural integrity of the protein we used circular dichroism spectroscopy by exposing the VSG in solution to a mixture of right- and left-circularly polarized light and analysing the absorbed UV spectra. Evaluation of protein stability and molecular dynamics simulations at different pH values was performed using different computational methods.

Results: We show, for an A2-type VSG, that the dimer size is only slightly larger at pH 5.2 than at pH 7.4. Moreover, the dimer was marginally more stable at lower pH due to the higher affinity (ΔG = 353.37 kcal/mol) between the monomers. Due to the larger size, the predicted epitopes were more exposed to the solvent at low pH. Moderate conformational changes (ΔRMSD = 0.35 nm) in VSG were detected between the dimers at pH 5.2 and pH 7.4 in molecular dynamics simulations, and no significant differences in the protein secondary structure were observed by circular dichroism spectroscopy.

Conclusions: Thus, the dissociation of anti-VSG-antibodies in endosomes cannot be explained by changes in pH.

Keywords: Trypanosoma brucei; conformational shift; epitopes; molecular dynamics; variant surface glycoprotein.

Associated data

  • figshare/10.6084/m9.figshare.24716316

Grants and funding

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 955910. This study was also supported by the Russian Science Foundation (22-65-00022). This work was supported by the Ministry of Science and Higher Education of the Russian Federation, goszadanie no. 2019-1075. We thank the ITMO Fellowship and Professorship Program and Priority 2030 for infrastructural support.&Thomas Dandekar acknowledges funding by DFG (German Research Society) grant 270563345-GRK2157-TP11.