In a recent study published on the medRxiv* preprint server, researchers described a novel protein subunit vaccine comprising the receptor-binding domain (RBD) of the ancestral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein, dimerized with an immunoglobulin (Ig)G1-fraction crystallizable (Fc) domain.
Study: Broad immunity to SARS-CoV-2 worrisome variants mediated by SARS-CoV-2 receptor-binding domain protein vaccine. Image credit: PIC SNIPE/Shutterstock
Background
Due to the highly infectious and immune-evasive nature of SARS-CoV-2 and the continuous emergence of its variants, the world will require continued efforts to develop novel coronavirus disease 2019 (COVID-19) vaccines tailored to the SARS-CoV-2 variants of concern (MOOSE). Perhaps the biggest obstacle to redirecting immune responses to VOCs is the phenomenon of immunological imprinting, also called antigenic original sin. This was the first recognized problem of influenza infection and vaccination.
Immunoprinting limits the boosting effect of S-based booster vaccines incorporating VOC-specific S protein sequences. Because the immune system preferentially targets shared epitopes between the mutant S vaccine and the parent strain, this reduces the effect of all currently approved COVID-19 vaccines targeting it.
Although an RBD-based protein subunit vaccine cannot completely overcome the imprinting problem, it can limit the distraction of the immune system to S epitopes located outside the RBD. Furthermore, all RBD epitopes, whether VOC-specific or shared with the ancestral strain, were more likely to induce neutralizing antibodies (nAbs). Over 90% of anti-SARS-CoV-2 nAbs target the SARS-CoV-2 RBD, the small region in its S protein that facilitates binding to the host cell’s angiotensin-converting enzyme-2 (ACE-2) receptor.
About the research
In the current study, researchers generated a vaccine candidate using the SARS-CoV-2 RBD and Fc fusion protein to facilitate multimeric presentation of the immune system. Furthermore, this protein subunit vaccine engages Fc receptor (FcR)+ antigen-presenting cells (APCs) for improved immunological priming. They tested its multiple formulations with different adjuvants, namely Toll-receptor 2 (TLR2) agonist R4-di-palmitoyl-S-glycerol cysteine (Pam2Cys), T (NKT) cell agonist natural killer glycolipid alpha-galactosylceramide and MF59 ® oil-in-water emulsion adjuvant. Notably, the team constructed this vaccine by fusing the N334-P527 region of the RBD to the central hinge region of mouse IgG1 via a short serine/glycine linker. They confirmed the activity of the engineered vaccine candidate by demonstrating that it specifically binds ACE2-transduced HEK-293T cell lines, but not HEK-293T cells transduced with an irrelevant protein (control).
The researchers vaccinated groups of BALB/c mice with this vaccine by subcutaneous or intranasal routes; they then measured elicited nAb titers using an in vitro SARS-CoV-2 microneutralization (MN) assay and a surrogate virus neutralization test (sVNT). Additionally, they used a microbead-based assay to determine whether sera from immunized mice exhibited neutralizing activity against a wide range of SARS-CoV-2 RBD variants. Additionally, the researchers challenged immunized mice with VIC2089 and collected their lungs and turbinates three days post-infection (dpi).
They also developed and clinically tested a “beta variant” version of their RBD human IgG1-Fc vaccine combined with adjuvant MF59®. The team also tested the ability of this modified version of the vaccine as a heterologous booster in mice previously vaccinated with two doses of the SARS-CoV-2 S protein vaccine. Thus, they evaluated its efficacy in real community settings where most people are vaccinated with S-based vaccines.
Research results
In a murine model of COVID-19, each formulation of the RBD-Fc vaccine elicited strong nAb responses and provided durable and highly protective immunity against lower and upper respiratory tract infection, regardless of route of administration, but only when used in the presence of adjuvant. R4-Pam2Cys-adjuvanted or a-GalCer-adjuvanted vaccine is highly effective, especially after intranasal administration. It also provided complete protection to the lungs of the mice. Moreover, the immune protection was durable as the mice were infected 75 days after the booster dose.
Of note, the Beta RBD-Fc vaccine, when used as a booster after priming with a precursor (WT) strain S vaccine, resulted in higher mean antibody levels, including nAb, compared to a WT booster dose or beta variant, S vaccine. It also elicited cross-reactive nAbs against alpha, gamma, delta, delta+, lambda, Mu, and Omicron BA.1 and BA.2 sublines. It was most effective at a low dose of one to 10 micrograms (µg). If such a dose remains optimal in humans, this vaccine candidate could be mass-produced. In addition, stability studies have shown that this vaccine is stable for up to nine months at 2-8 °C and two weeks at 37 °C. This means it will be very amenable to transport and storage in countries that lack cold chain infrastructure.
Conclusions
Overall, the n RBD-Fc protein subunit vaccine candidate described in the present study provides complete and sustained protection against lower and upper respiratory tract infection in mice. Its beta-variant version promotes potent nAb responses that target beta and several other volatile organic compounds of SARS-CoV-2 in in vitro MN assays and mouse models. It is also emerging as a suitable candidate for commercialization. It is in a phase I clinical trial as a fourth booster dose for individuals primed and boosted with licensed SARS-CoV-2 vaccines.
*Important message
medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guiding clinical practice/health-related behavior or treated as established information.
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