Zinc-Assisted Microscale Granules Made of the SARS-CoV-2 Spike Protein Trigger Neutralizing, Antivirus Antibody Responses

The development of new and more efficient vaccination approaches is a constant need, due to the pressure of historical and emerging infectious diseases and the limited efficacy and universality of the current vaccination technologies. Peptides and recombinant proteins have been explored for decades as subunit vaccines for bacterial and viral infections, presented either as soluble protein species or as virus-like assemblies. Recently, synthetic secretory protein-only microscale granules have been developed as dynamic depots for sustained protein release. They are based on the reversible coordination between ionic Zn and histidine residues, which promotes a fast formation of granular particles in vitro out of soluble protein and a slow release of such building block protein in vivo through the physiological chelation of the metal. Such an endocrine-like platform represents a new drug delivery system fully validated in oncology by which soluble and functional protein drugs are progressively released from the granules and made available for antitumor activities upon subcutaneous administration. By exploring such an approach for immunization here, microparticles made of a recombinant form of the receptor binding domain (RBD) of SARS-CoV-2 were tested as an antigen delivery system for induction of antibody responses against the virus upon administration of the material in the absence of added adjuvants. Also, the comparison between protein materials produced in bacterial, mammalian, or insect cell factories has demonstrated a moderate impact of protein glycosylation on the final immunological performance of the system. Therefore, we propose the consideration of synthetic protein secretory granules as a new sustainable immunization platform based on fully manageable, self-organized, and self-formulated immunogens, aimed at reducing the dosage, costs, and complexity of vaccination regimens.