In a recent study posted to the bioRxiv* preprint server, researchers demonstrated that decorating the capsid surface of adenovirus (Ad) vaccine vectors with a newly discovered protein superglue, DogTag/DogCatcher, generates potent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific humoral and cellular immunity.
In response to the SARS-CoV-2 pandemic, potent Ad vector-based vaccines against coronavirus disease 2019 (COVID-19) have been developed and distributed worldwide. Thanks to their rapid development, expandability, cost-effectiveness, and longer shelf-life, Ad vaccines gained particular importance during the SARS-CoV-2 pandemic. Currently, four Ad-based SARS-CoV-2 vaccines named: AZD1222 vaccine, Johnson & Johnson vaccine, Sputnik V vaccine, and Convidecia vaccine are clinically approved for prophylactic use in COVID-19, using a wide breadth of serotypes.
Although the Ad-based COVID-19 vaccines produce robust T cell responses, the enhancement of pathogen-specific antibody responses during homologous boosting is moderate relative to other approaches such as virus-like-particle (VLP) based recombinant protein technologies and nanoparticles. Further, humans exhibit substantial pre-existing neutralizing antibodies (NAbs) against frequently seen human Ad serotypes, and the most clinically progressive serotypes have already been used to vaccinate people during the current COVID-19 pandemic. Hence, the efficacy of reusing these platforms for boosting immunity against SARS-CoV-2 is questionable.
About the study
In the present study, the researchers demonstrated a technique that allows the modular decoration of the Ad capsid surfaces with protein antigens as big as the receptor-binding domain (RBD) of SARS-CoV-2 and the generation of a robust humoral response against these antigens.
The ligand was attached to the vector through a covalent isopeptide bond created by a quick and spontaneous reaction requiring simple co-incubation of vector and ligand components. For the ligation process, the team used a recently discovered protein superglue called DogTag/DogCatcher, which was comparable to the commonly used SpyTag/SpyCatcher ligation system but operates better in loop structures. The DogCatcher-fused ligands were attached to the antigens by inserting DogTag into surface-exposed loops in the Ad hexon protein.
The results indicate that covalent ligands as big as the RBD of the SARS-CoV-2 spike (S) linked to hexon in three insertion sites – hexon hypervariable region 5 (HVR5), HVR2, and HVR1 – obtained broad coverage of the Ad capsid against SARS-CoV-2 antigens. Further, each case demonstrated the retention of vector infectivity. These inferences suggest that the reactivity was not based on the insertion site, which shows the generalizability of this approach to other Ad serotypes.
Capsid decoration protected the particles from the neutralizing antibodies targeted against the Ad. The capsid shielding was more evident with the DogCatcher-RBD than the DogCatcher-NANP18 against the monoclonal antibody (mAb) 9C12, probably because of the high coverage of the capsid surface associated with the larger RBD ligand.
The cryogenic electron microscopy (cryo-EM) data indicated that the interaction of Ad fiber protein with the coxsackie and Ad receptor (CAR) were unlikely to be interfered with by the hexon conjugation because of the large-sized ligands tested, indicating efficient transduction of the cells by the decorated Ads in vivo and in vitro. These inferences suggest that the cellular and humoral transgene-specific immune responses were not impaired by the capsid decoration.
Proof of principle suggested that COVID-19 Ad vaccines decorated with the SARS-CoV-2 RBD produced over 10-times higher SARS-CoV-2 neutralizing antibody titers than an undecorated Ad vector expressing SARS-CoV-2 S in prime-boost regimens.
The decorated Ad vectors preserved a high level of T cell immunogenicity to the encoded SARS-CoV-2 antigens, which was a fundamental feature of Ad vector vaccines.
According to the authors, this is the first study demonstrating Ad vector's capsid decoration through protein superglue-facilitated covalent ligation. The study provides a novel approach to enhance the efficacy of Ad-based therapeutics and vaccines. The Ad-based platform suggested in the study boosted cellular and humoral immunity since the capsid surface displayed antigenic targets facilitating antibody stimulation, and the vector genome encoded the T cell epitopes. In addition, it enhanced the efficacy of multi-shot regimens.
Apart from prophylactic vaccination, the capsid decoration approach proposed in the study has applications in therapeutic vaccines against cancer and chronic viral pathogens and devising personalized therapies. This technique can be generalized to develop pan-CoV and pan-influenza vaccines via combining exchangeable capsid ligands producing significant neutralizing humoral immunity with broad and conserved T cell immunity from encoded antigens.
Nonetheless, further investigations are required to determine the restrictions of size and structure of ligands that can be displayed on capsids.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.