Abstract
Ferritin-based, self-assembling protein nanoparticle vaccines are being developed against a range of viral pathogens, including SARS-CoV-2, influenza, HIV-1, and Epstein-Barr virus. However, purification of these nanoparticles is often laborious and requires customization for each potential nanoparticle vaccine. We propose that the simple insertion of a polyhistidine tag into exposed flexible loops on the ferritin surface (His-Fer) can mitigate the need for complex purifications and enable facile metal-chelate-based purification, thereby allowing for optimization of early stage vaccine candidates. Using sequence homology and computational modeling, we identify four sites that can accommodate insertion of a polyhistidine tag and demonstrate purification of both hemagglutinin-modified and SARS-CoV-2 spike-modified ferritins, highlighting the generality of the approach. A site at the 4-fold axis of symmetry enables optimal purification of both protein nanoparticles. We demonstrate improved purification through modulating the polyhistidine length and optimizing both the metal cation and the resin type. Finally, we show that purified His-Fer proteins remain multimeric and elicit robust immune responses similar to those of their wild-type counterparts. Collectively, this work provides a simplified purification scheme for ferritin-based vaccines.