In silico design of Phl p 6 variants with altered folding stability significantly impacts antigen processing, immunogenicity and immune polarization

Abstract

AbstactIntroductionProtein fold stability has been proposed to represent an intrinsic feature contributing to immunogenicity and immune polarization by influencing the amount of peptide-MHC II complexes (pMHCII). Using in silico prediction, we introduced point mutations in proteins that either increase or decrease their fold-stability without altering immunodominant epitopes or changing the overall structure of the protein. Here, we investigated how modulation of the fold-stability of the grass pollen allergen Phl p 6 affects its ability to stimulate immune responses and T cell polarization.MethodsUsing the MAESTRO software tool, stabilizing or destabilizing mutations were selected and verified by molecular dynamics simulations. The mutants were expressed in E. coli, purified tag-free, and analyzed for thermal stability and resistance to endolysosomal proteases. The resulting peptides were analysed by degradome assay and mass spectrometry. The structure of the most stable mutant protein was obtained by X-ray crystallography. We evaluated the capacity of the mutants to stimulate T cell proliferation in vitro, as well as antibody responses and T cell polarization in vivo in an adjuvant-free BALB/c mouse model.ResultsFour stabilizing and two destabilizing mutations were identified by MAESTRO. Experimentally determined changes in thermal stability compared to the wild type protein ranged from -5 to +14 °C. Destabilization led to faster proteolytic processing in vitro, whereas highly stabilized mutants were degraded very slowly. However, the overall pattern of identified peptides remained very similar. This was confirmed in bone marrow derived dendritic cells that processed and presented the immune dominant epitope from a destabilized mutant more efficiently. In vivo, stabilization resulted in a shift in immune polarization as indicated by higher levels of IgG2a and increased secretion of TH1/TH17 cytokines.ConclusionMAESTRO was very efficient in detecting single point mutations that increase or reduce fold-stability. Thermal stability correlated well with susceptibility to protease resistance and presentation of pMHCII on the surface of dendritic cells in vitro. This change in processing kinetics significantly influenced the polarization of T cell responses in vivo. Modulating the fold-stability of proteins thus has the potential to optimize and polarize immune responses, which opens the door to more efficient design of molecular vaccines.