In a recent study published on medRxiv* preprint server, researchers have described a novel protein subunit vaccine comprising the receptor-binding domain (RBD) of the ancestral respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein severe acute, dimerized with immunoglobulin (Ig) Fraction G1, crystallizable domain (Fc).
Due to the highly infectious and immuno-evasive nature of SARS-CoV-2 and the continued emergence of its variants, the world will require continued efforts to develop suitable novel coronavirus disease 2019 (COVID-19) vaccines. SARS-CoV-2 variants of concern (COV). Perhaps the greatest obstacle to redirecting immune responses to VOCs is the phenomenon of immunological imprinting, also known as original antigenic sin. This was the first problem recognized for influenza infection and vaccination.
Immunofingerprinting limits the boost effect of S-based booster vaccines incorporating VOC-specific S protein sequences. Since the immune system preferentially targets shared epitopes between the S-mutant vaccine and the ancestral strain, this decimates the effect of all currently used approved COVID-19 vaccines aligned to it.
While an RBD-based protein subunit vaccine cannot completely overcome the imprinting problem, it could limit the distraction of the immune system to S epitopes hosted outside the RBD. Additionally, all RBD epitopes, whether specific to VOCs or shared with the ancestral strain, are more likely to induce neutralizing antibodies (nAbs). Over 90% of anti-SARS-CoV-2 nAbs target the SARS-CoV-2 RBD, the small region of its S protein that facilitates binding to angiotensin-converting enzyme-2 (ACE-2) of the host cell receptor.
About the study
In the current study, researchers generated a candidate vaccine using the SARS-CoV-2 RBD and an Fc fusion protein to facilitate multimeric presentation to the immune system. Additionally, this protein subunit vaccine engaged Fc receptor (FcR) + antigen-presenting cells (APC) for enhanced immunological priming. They tested its multiple formulations having different adjuvants, namely, the toll receptor 2 (TLR2) agonist R4-di-palmitoyl-S-glycerol cysteine (Pam2Cys), a natural killer T (NKT) cell agonist glycolipid alpha- galactosylceramide and MF59 ® oil-in-water emulsion adjuvant. Notably, the team designed 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 modified vaccine candidate by demonstrating that it specifically binds HEK-293T cell lines transduced with ACE2, but not HEK-293T cells transduced with an irrelevant (control) protein.
The researchers vaccinated groups of BALB/c mice with this vaccine either subcutaneously or intranasally; then they measured the obtained nAb titers using a in vitro SARS-CoV-2 micro-neutralization (MN) test and by 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 mice immunized with VIC2089 and harvested their lungs and nasal turbinates three days post-infection (dpi).
They have also developed and clinically tested a “beta variant” version of their human IgG1-Fc vaccine RBD, associated with the adjuvant MF59®. Additionally, the team tested the ability of this modified version of the vaccine as a heterologous booster in mice previously vaccinated with two doses of a SARS-CoV-2 S protein vaccine. In this way, they assessed its efficacy in real-life community settings, where most individuals are vaccinated with S.
In the mouse model of COVID-19, each formulation of the RBD-Fc vaccine elicited strong nAb responses and conferred 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 an adjuvant. Vaccine adjuvanted with R4-Pam2Cys or a-GalCer adjuvant was highly effective, especially after intranasal administration. It also conferred complete protection on the mice’s lungs. Additionally, the immune protection was durable, as the mice were challenged 75 days after the booster dose.
Notably, the Beta RBD-Fc vaccine, when used as a post-priming booster with an ancestral strain (WT)S vaccine, resulted in higher mean levels of antibodies, including nAb, compared to a booster dose of WT, or a 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 1 to 10 micrograms (µg). If a similar dosage remains optimal for humans, this vaccine candidate could be manufactured on a large scale. In addition, stability studies have shown this vaccine to be stable for up to nine months at 2-8°C and two weeks at 37°C. This implies that it will lend itself very well to transport and storage in countries without cold chain infrastructure.
Overall, the n RBD-Fc protein subunit vaccine candidate described in the present study conferred complete and persistent protection against lower and upper respiratory tract infection in mice. Its beta-variant promoted potent nAb responses that targeted beta and several other SARS-CoV-2 COVs in in vitro MN assays and mouse models. It also emerged as a suitable candidate for commercialization. It is in a phase I clinical trial as a fourth booster dose for people primed and boosted with licensed SARS-CoV-2 vaccines.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.