Human Fc-Conjugated Receptor Binding Domain-Based Recombinant Subunit Vaccines with Short Linker Induce Potent Neutralizing Antibodies against Multiple SARS-CoV-2 Variants

Abstract

The coronavirus disease-19 (COVID-19) pandemic has been ongoing since December 2019, with more than 6.3 million deaths reported globally as of August 2022. Despite the success of several SARS-CoV-2 vaccines, the rise in variants, some of which are resistant to the effects of vaccination, highlights the need for a so-called pan-coronavirus (universal) vaccine. Here, we performed an immunogenicity comparison of prototype vaccines containing spike protein receptor-binding domain (RBD) residues 319–541, or spike protein regions S1, S2 and S fused to a histidine-tagged or human IgG1 Fc (hFC) fragment with either a longer (six residues) or shorter (three residues) linker. While all recombinant protein vaccines developed were effective in eliciting humoral immunity, the RBD-hFc vaccine was able to generate a potent neutralizing antibody response as well as a cellular immune response. We then compared the effects of recombinant protein length and linker size on immunogenicity in vivo. We found that a longer recombinant RBD protein (residues 319–583; RBD-Plus-hFc) containing a small alanine linker (AAA) was able to trigger long-lasting, high-titer neutralizing antibodies in mice. Finally, we evaluated cross-neutralization of wild-type and mutant RBD-Plus-hFc vaccines against wild-type, Alpha, Beta, Delta and Omicron SARS-CoV-2 variants. Significantly, at the same antigen dose, wild-type RBD-Plus-hFc immune sera induced broadly neutralizing antibodies against wild-type, Alpha, Beta, Delta and Omicron variants. Taken together, our findings provide valuable information for the continued development of recombinant protein-based SARS-CoV-2 vaccines and a basic foundation for booster vaccinations to avoid reinfection with SARS-CoV-2 variants.

Preclinical assessment of a recombinant RBD-Fc fusion protein as SARS-CoV-2 candidate vaccine

“…Therefore, developing new vaccines with more efficient immunogenicity highlights the necessity of further research in this area. Besides inducing neutralizing antibody, recombinant subunit vaccines have several advantages including high safety and yield as well as easy storage and transportation [11]. Studies have shown that RBD comprises of several immunodominant epitopes and therefore is a major target for neutralizing antibodies, which blocks the interaction of RBD-ACE2 and inhibits virus entrance to the host cells [24,[27][28][29], however, the immunogenicity of RBD is not strong enough.…”

“…Studies have shown that RBD comprises of several immunodominant epitopes and therefore is a major target for neutralizing antibodies, which blocks the interaction of RBD-ACE2 and inhibits virus entrance to the host cells [24,[27][28][29], however, the immunogenicity of RBD is not strong enough. Using Fc domain of human IgG as a fusion partner has several advantages, including interaction with FcRn, increasing the half-life and stability of the fusion protein, binding to Fc receptor on immune cells and enhancing their immunological functions, enhancing immunogenicity by increasing uptake via FcγR by antigen presenting cells, and also efficient purification of the fusion protein by protein A-or G-affinity chromatography [11,[30][31][32].…”

“…The vaccine contains the receptor-binding domain (RBD) of the viral spike protein, which is responsible for binding to the angiotensin-converting enzyme 2 (ACE2), the virus receptor on host cells [9,10]. Fusing the IgG Fc domain to recombinant RBD improves the stability of RBD and also induces an efficient immune response by increasing antigen uptake through Fc-receptors on immune cells [11].…”

Intranasal Boosting with Spike Fc-RBD of Wild-Type SARS-CoV-2 Induces Neutralizing Antibodies against Omicron Subvariants and Reduces Viral Load in the Nasal Turbinate of Mice

“…In this study, we used a Fc-linked trimeric spike RBD to improve the uptake and presentation of the trimeric spike RBD, and we used intranasal administration to achieve a more robust immunity in the upper airway. Twelve studies [ 29 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 43 , 44 , 45 ] have tried to use Fc-RBD subcutaneously or intramuscularly with mostly alum, one Freund’s, one MF59, manganese, or Montanide TM ISA720 adjuvants in mice, and one non-human primate model; but only one study has used intranasal vaccination in Balb/c mice, and it did not try to use it for boosting after priming with intramuscular inactivated vaccine.…”