Le click c'est chic: a plug-and-play virus-like particle vaccination platform enabled by non-canonical amino acid incorporation and click chemistry in the tobacco BY-2 cell-free protein synthesis system

Non-canonical amino acids (ncaas) can provide recombinant proteins with novel exciting functionalities beyond the limits of nature, such as orthogonal reaction groups. Notably, ncaa introduction can be used in vaccinology to enhance the adaptability and immunogenicity of putative vaccine candidates. Cell-free protein synthesis (CFPS) represents the most promising methodology to introduce ncaa into recombinant proteins of interest. However, traditionally used prokaryotic CFPS systems show limitations to produce complex proteins requiring post-translational modifications, whilst eukaryotic CFPS systems have historically been difficult to scale and show low protein yields. In this work, we establish the site-specific introduction of ncaas into complex proteins with the high-yielding and scalable eukaryotic tobacco BY-2 CFPS system (BYL), commercialised as ALiCE(R). The tyrosine transferase from Escherichia coli (eTyrT) was tested for amber suppression-mediated ncaa incorporation in BYL. eTyrT showed high incorporation yields of up to 2mg/ml recombinant protein for the azido-tyrosine and alkyne-tyrosine ncaas, with linear scalability up to 10ml without any losses in protein yield. We applied ncaa incorporation in BYL to enable click chemistry bioconjugation of the receptor binding domain (RBD) of influenza hemagglutinin to pre-assembled hepatitis B core (HBc) virus-like particles (VLPs). BYL efficiently produced the alkyne-modified RBD and azido-modified HBc VLPs, and their conjugation via copper-catalysed azide-alkyne cycloaddition (CuAAC) led to structurally intact, RBD-coated particles. VLP-RBD conjugates could efficiently hemagglutinate chicken erythrocytes where the individual proteins could not, proving both the sialic-acid binding activity of the RBD and its multivalent presentation by the HBc VLP. Finally, when used to vaccinate mice the conjugated RBD-VLPs showed a greater protection against live influenza challenge than free RBD. This research thus enables ncaa introduction for recombinant proteins produced in BYL, constructing a novel plug-and-play vaccine platform and further expanding the capabilities of BYL to produce vaccine candidates and other proteins of interest.