In the last decade, the interest in ferritin-based vaccines has been increasing due to their safety and immunogenicity. Candidates against a wide range of pathogens are now on Phase I clinical trials namely for influenza, Epstein-Barr, and SARS-CoV-2 viruses. Manufacturing challenges related to particle heterogeneity, improper folding of fused antigens, and antigen interference with intersubunit interactions still need to be overcome. In addition, protocols need to be standardized so that the production bioprocess becomes reproducible, allowing ferritin-based therapeutics to become readily available. In this review, the building blocks that enable the formulation of ferritin-based vaccines at an experimental stage, including design, production, and purification are presented. Novel bioengineering strategies of functionalizing ferritin nanoparticles based on modular assembly, allowing the challenges associated with genetic fusion to be circumvented, are discussed. Distinct up/down-stream approaches to produce ferritin-based vaccines and their impact on production yield and vaccine efficacy are compared. Finally, ferritin nanoparticles currently used in vaccine development and clinical trials are summarized.

Ferritin (Ft) nanoparticles have become versatile platforms for displaying antigens, being a promising technology for vaccine development. While genetic fusion has traditionally been the preferred method for antigen display, concerns about improper folding and steric hindrance that may compromise vaccine efficacy or stability have prompted alternative approaches. Bioconjugation offers the advantage of preserving native protein structure and function, with recent advancements improving efficiency and specificity. In this study, we used tyrosinase (TYR) to bioconjugate the receptor binding domain of the SARS-CoV-2 spike protein, tagged with a tyrosine (RBD-Y), to native cysteines on Ft, resulting in RBD-Y-Ft nanoparticles. We quantified available cysteines on ferritin using Ellman's assay and monitored their reduction during the reactions. Denaturing analytics (via SDS-PAGE, Western blot, and LC-TOF-MS) confirmed the formation of RBD-Y-Ft monomers with an expected molecular weight of 46 kDa. Mass photometry and HPLC estimated a molecular weight of RBD-Y-Ft nanoparticles of 680 kDa, which was higher than that of nonfunctionalized ferritin (480 kDa), indicating successful binding of up to eight RBD-Y antigens per 24-mer Ft nanoparticle. This work enhances our understanding of how Ft nanoparticles can be engineered to present antigens, leveraging them as a robust scaffold for producing tailored-made candidate vaccines in a timely manner.

Rift Valley fever (RVF) is a WHO-prioritized zoonotic, vector-borne disease with no licensed prophylaxis available for humans, highlighting the need for effective vaccine strategies. Nanoparticle-based platforms for antigen presentation offer a promising approach for vaccine development. In this work, we engineered ferritin (Ft) nanoparticles to display the immunogenic Gn domain of RVF virus (GnFt) and systematically assessed the production, purification, and physico-chemical properties of the purified nanoparticles. Baculovirus-based expression systems were evaluated in insect (Sf9, High-Five™, Tnao38, and Tnms42) and mammalian cells (HEK293 and CHO), revealing Sf9 cells as the most efficient host for producing GnFt nanoparticles. In addition, affinity-based chromatography was explored, yielding GnFt nanoparticles of > 95% purity (as assessed by SDS-PAGE) and an overall production yield of 0.2 mg/L culture. Biophysical characterization (e.g., high-performance liquid chromatography, dynamic light scattering, electron microscopy, and mass photometry) confirmed proper 24-mer nanoparticle assembly (1,344 kDa and 20 nm) and structural integrity. Binding affinity to Gn-targeting monoclonal antibodies was demonstrated by biolayer interferometry, with dissociation constants in the nM range, indicating retained antigenic functionality. These findings demonstrate the successful development of a platform for producing structurally stable, pure, and functional Gn-presenting ferritin nanoparticles, supporting their potential use for RVF vaccine development.

Ferritin (Ft) nanoparticles are promising scaffolds for antigen display in vaccine design due to their stability, defined architecture, and biocompatibility. Enzymatic methods, such as tyrosinase catalysis, enable covalent antigen conjugation by oxidizing tyrosine residues into o-quinones that react with accessible cysteine thiols. Here, we engineered Pyrococcus furiosus ferritin (PfFt) by introducing single cysteines at defined positions (K8C, D33C, and E92C) to enable site-specific bioconjugation. All PfFt variants retained their quaternary nanoparticle structure, as confirmed by mass spectrometry, dynamic light scattering, HPLC, and mass photometry. Thiol accessibility was verified by Ellman’s assay. Using tyrosinase-mediated catalysis, we conjugated two tyrosine-tagged antigens, Rift Valley fever virus Gn and SARS-CoV-2 receptor-binding domain, to the engineered cysteines. Up to 13 antigens were displayed per 24-mer nanoparticle. Conjugation was highly specific to the engineered cysteines, and the resulting antigen-PfFt conjugates bound neutralizing antibodies with nanomolar affinities (2–7 nM), comparable to their soluble antigen counterparts. This work establishes a robust and modular strategy for precise antigen display on ferritin nanocages using tyrosinase-mediated cysteine conjugation. The platform shows strong potential for next-generation protein-based vaccines and other bioconjugate therapeutics.