Yeast-produced RBD-based recombinant protein vaccines elicit broadly neutralizing antibodies and durable protective immunity against SARS-CoV-2 infection

Novel and efficient yeast-based strategies for subunit vaccine delivery against COVID-19

Yeast shows promise as a delivery system for drugs and vaccines due to its specific targeting and immunogenic properties. The objective of this research is to create novel and effective yeast-based methods for delivering subunit vaccines. Through the modification of yeast expression plasmids and optimization of expression techniques, a new dual-expression system has been developed. We have successfully generated a S. cerevisiae vaccine strain exhibiting stable dual expression of RBD, as well as an inducible S. cerevisiae vaccine strain with dual expression of RBD. The vaccine efficacy assay in mice indicated that the dual-RBD S. cerevisiae vaccine elicited a significantly more robust humoral and mucosal immune response in comparison to the conventional S. cerevisiae vaccine expressing RBD solely on Aga2p. This study demonstrated a cost-effective dual-expression S. cerevisiae system that not only exhibits potential in combating COVID-19, but also harbors the capacity to foster vaccine development against other infectious diseases.

Protein Expression Platforms and the Challenges of Viral Antigen Production

Several protein expression platforms exist for a wide variety of biopharmaceutical needs. A substantial proportion of research and development into protein expression platforms and their optimization since the mid-1900s is a result of the production of viral antigens for use in subunit vaccine research. This review discusses the seven most popular forms of expression systems used in the past decade-bacterial, insect, mammalian, yeast, algal, plant and cell-free systems-in terms of advantages, uses and limitations for viral antigen production in the context of subunit vaccine research. Post-translational modifications, immunogenicity, efficacy, complexity, scalability and the cost of production are major points discussed. Examples of licenced and experimental vaccines are included along with images which summarize the processes involved.

Improvement of RBD-FC Immunogenicity by Using Alum-Sodium Alginate Adjuvant Against SARS-COV-2

BACKGROUND

Adjuvants use several mechanisms to boost immunogenicity and to modulate immune response. The strength of adsorption of antigen by adjuvants can be a determinant factor for significant improvement of immunopotentiation.

METHODS

We expressed recombinant RBD-FC in PichiaPink Strain 4 and examined the vaccination of mice by vaccine formulation with different adjuvants (sodium alginate and aluminum hydroxide, alone and together).

RESULTS

Sodium alginate significantly increased the immunogenicity and stability of RBD-FC antigen, so RBD-FC formulated with combined alginate and alum (AlSa) and sodium alginate alone showed higher antibody titer and stability. Immunogenicity of RBD-FC:AlSa was determined by serological assays including direct enzyme-linked immunosorbent assay (ELISA) and surrogate virus neutralization test (sVNT). High levels of IgGs and neutralizing antibodies were measured in serum of mice immunized with the RBD-FC:AlSa formulation. On the other hand, cytokines IL-10 and INF-γ were severely accumulated in response to RBD-FC:AlSa, and after 10 days, their accumulation was significantly declined, whereas IL-4 showed the highest and the lowest accumulation in response to alum and alginate, respectively.

CONCLUSIONS

Our data may suggest that combination of alum and sodium alginate has a better compatibility with RBD-FC in vaccine formulation.

Diverse approaches to express recombinant spike protein: A comprehensive review

The spike protein of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is responsible for infecting host cells. It has two segments, S1 and S2. The S1 segment has a receptor-binding domain (RBD) that attaches to the host receptor angiotensin-converting enzyme 2 (ACE2). The S2 segment helps in the fusion of the viral cell membrane by creating a six-helical bundle through the two-heptad repeat domain. To develop effective vaccines and therapeutics against COVID-19, it is critical to express and purify the SARS-CoV-2 Spike protein. Extensive studies have been conducted on expression of a complete recombinant spike protein or its fragments. This review provides an in-depth analysis of the different expression systems employed for spike protein expression, along with their advantages and disadvantages.

The impact of N-glycans on the immune response of plant-produced SARS-CoV-2 RBD-Fc proteins

Plant-based manufacturing has the advantage of post-translational modifications. While plant-specific N-glycans have been associated with allergic reactions, their effect on the specific immune response upon vaccination is not yet understood. In this study, we produced an RBD-Fc subunit vaccine in both wildtype (WT) and glycoengineered (∆XF) Nicotiana benthamiana plants. The N-glycan analysis: RBD-Fc carrying the ER retention peptide mainly displayed high mannose. When produced in WT RBD-Fc displayed complex-type (GnGnXF) N-glycans. In contrast, ∆XF plants produced RBD-Fc with humanized complex N-glycans that lack potentially immunogenic xylose and core fucose residues (GnGn). The three recombinant RBD-Fc glycovariants were tested. Immunization with any of the RBD-Fc proteins resulted in a similar titer of anti-RBD antibodies in mice. Likewise, antisera from subunit RBD-Fc vaccines also demonstrated comparable neutralization against SARS-CoV-2. Thus, we conclude that N-glycan modifications of the RBD-Fc protein have no impact on their capacity to activate immune responses and induce neutralizing antibody production.

Protective RBD-dimer vaccines against SARS-CoV-2 and its variants produced in glycoengineered Pichia pastoris

Protective vaccines are crucial for preventing and controlling coronavirus disease 2019 (COVID-19). Updated vaccines are needed to confront the continuously evolving and circulating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. These vaccines should be safe, effective, amenable to easily scalable production, and affordable. Previously, we developed receptor binding domain (RBD) dimer-based protein subunit vaccines (ZF2001 and updated vaccines) in mammalian cells. In this study, we explored a strategy for producing RBD-dimer immunogens in Pichia pastoris. We found that wild-type P. pastoris produced hyperglycosylated RBD-dimer protein containing four N-glycosylation sites in P. pastoris. Therefore, we engineered the wild type P. pastoris (GS strain) into GSΔOCH1pAO by deleting the OCH1 gene (encoding α-1,6-mannosyltransferase enzyme) to decrease glycosylation, as well as by overexpressing the HIS4 gene (encoding histidine dehydrogenase) to increase histidine synthesis for better growth. In addition, RBD-dimer protein was truncated to remove the R328/F329 cleavage sites in P. pastoris. Several homogeneous RBD-dimer proteins were produced in the GSΔOCH1pAO strain, demonstrating the feasibility of using the P. pastoris expression system. We further resolved the cryo-EM structure of prototype-Beta RBD-dimer complexed with the neutralizing antibody CB6 to reveal the completely exposed immune epitopes of the RBDs. In a murine model, we demonstrated that the yeast-produced RBD-dimer induces robust and protective antibody responses, which is suitable for boosting immunization. This study developed the yeast system for producing SARS-CoV-2 RBD-dimer immunogens, providing a promising platform and pipeline for the future continuous updating and production of SARS-CoV-2 vaccines.

A superior heterologous prime-boost vaccination strategy against COVID-19: A bivalent vaccine based on yeast-derived RBD proteins followed by a heterologous vaccine

Various vaccines have been challenged by SARS-CoV-2 variants. Here, we reported a yeast-derived recombinant bivalent vaccine (Bivalent wild-type [Wt]+De) based on the wt and Delta receptor-binding domain (RBD). Yeast derived RBD proteins based on the wt and Delta mutant were used as the prime vaccine. It was found that, in the presence of aluminium hydroxide (Alum) and unmethylated CpG-oligodeoxynucleotides (CpG) adjuvants, more cross-protective immunity against SARS-CoV-2 prototype and variants were elicited by bivalent vaccine than monovalent wtRBD or Delta RBD. Furthermore, a heterologous boosting strategy consisting of two doses of bivalent vaccines followed by one dose adenovirus vectored vaccine exhibited cross-neutralization capacity and specific T cell responses against Delta and Omicron (BA.1 and BA.4/5) variants in mice, superior to a homologous vaccination strategy. This study suggested that heterologous prime-boost vaccination with yeast-derived bivalent protein vaccine could be a potential approach to address the challenge of emerging variants.

Impact of cell wall polysaccharide modifications on the performance of Pichia pastoris: novel mutants with enhanced fitness and functionality for bioproduction applications

BACKGROUND

Pichia pastoris is a widely utilized host for heterologous protein expression and biotransformation. Despite the numerous strategies developed to optimize the chassis host GS115, the potential impact of changes in cell wall polysaccharides on the fitness and performance of P. pastoris remains largely unexplored. This study aims to investigate how alterations in cell wall polysaccharides affect the fitness and function of P. pastoris, contributing to a better understanding of its overall capabilities.

RESULTS

Two novel mutants of GS115 chassis, H001 and H002, were established by inactivating the PAS_chr1-3_0225 and PAS_chr1-3_0661 genes involved in β-glucan biosynthesis. In comparison to GS115, both modified hosts exhibited a looser cell surface and larger cell size, accompanied by faster growth rates and higher carbon-to-biomass conversion ratios. When utilizing glucose, glycerol, and methanol as exclusive carbon sources, the carbon-to-biomass conversion rates of H001 surpassed GS115 by 10.00%, 9.23%, and 33.33%, respectively. Similarly, H002 exhibited even higher increases of 32.50%, 12.31%, and 53.33% in carbon-to-biomass conversion compared to GS115 under the same carbon sources. Both chassis displayed elevated expression levels of green fluorescent protein (GFP) and human epidermal growth factor (hegf). Compared to GS115/pGAPZ A-gfp, H002/pGAPZ A-gfp showed a 57.64% higher GFP expression, while H002/pPICZα A-hegf produced 66.76% more hegf. Additionally, both mutant hosts exhibited enhanced biosynthesis efficiencies of S-adenosyl-L-methionine and ergothioneine. H001/pGAPZ A-sam2 synthesized 21.28% more SAM at 1.14 g/L compared to GS115/pGAPZ A-sam2, and H001/pGAPZ A-egt1E obtained 45.41% more ERG at 75.85 mg/L. The improved performance of H001 and H002 was likely attributed to increased supplies of NADPH and ATP. Specifically, H001 and H002 exhibited 5.00-fold and 1.55-fold higher ATP levels under glycerol, and 6.64- and 1.47-times higher ATP levels under methanol, respectively, compared to GS115. Comparative lipidomic analysis also indicated that the mutations generated richer unsaturated lipids on cell wall, leading to resilience to oxidative damage.

CONCLUSIONS

Two novel P. pastoris chassis hosts with impaired β-1,3-D-glucan biosynthesis were developed, showcasing enhanced performances in terms of growth rate, protein expression, and catalytic capabilities. These hosts exhibit the potential to serve as attractive alternatives to P. pastoris GS115 for various bioproduction applications.

Expression of codon-optimized PDCoV-RBD protein in baculovirus expression system and immunogenicity evaluation in mice

Porcine deltacoronavirus (PDCoV) is a global epidemic enteropathogenic coronavirus that mainly infects piglets, and causes huge losses to the pig industry. However, there are still no commercial vaccines available for PDCoV prevention and controlment. Receptor-binding domain (RBD) is located at the S1 subunit of PDCoV and is the major target for developing viral inhibitor and vaccine. In this study, the characteristics of the RBD were analyzed by bioinformatic tools, and codon optimization was performed to efficiently express the PDCoV-RBD protein in the insect baculovirus expression system. The purified PDCoV-RBD protein was obtained and fully emulsified with CPG2395 adjuvant, aqueous adjuvant and Al(OH)3 adjuvant, respectively, to develop vaccines. The humoral and cellular immune responses were assessed on mice. The results showed that both the RBD/CPG2395 and RBD/aqueous adjuvant could induce stronger immune responses in mice than that of RBD/Al(OH)3. In addition, the PDCoV challenge infection was conducted and the RBD/CPG2395 could provide better protection against PDCoV in mice. Our study showed that the RBD protein has good antigenicity and can be used as a protective antigen, which provided a basis for the development of the PDCoV vaccine.

Combined detection of SARS-CoV-2 neutralizing antibodies and specific IgG in plasma based on SERS magnetic sensor

In this study, a surface-enhanced Raman spectroscopy (SERS) magnetic sensor is established based on SERS principle and magnetic separation technology, and a highly sensitive, simple and fast method for quantitative detection of neutralizing antibodies (nABs) and specific IgG of SARS-CoV-2 in plasma is established combined with immunoassay. Two kinds of Raman nanospheres (RNPs) with different characteristic Raman shifts are used as signal sources and coupled to ACE2 and anti-IgG (FC) antibodies respectively, and magnetic beads are coupled to RBD. The competitive relationship between ACE2 and nABs, the binding relationship between specific IgG and anti-IgG (FC) antibodies are determined. The results show that the concentrations of nABs and specific IgG in the range of 10-2000 ng ml-1are well correlated with SERS response intensity, and the recoveries are both between 90%-110%, with good precision. Bilirubin and common anticoagulants have no interference on the detection results. This method is accurate, reliable, sensitive and does not require complex pre-treatment, and is expected to be used for simultaneous detection of nABs and specific IgG in plasma of SARS-CoV-2. It has guiding significance for the development and evaluation of vaccines and the formulation of individualized vaccination schedule.

Large-Scale Purification and Characterization of Recombinant Receptor-Binding Domain (RBD) of SARS-CoV-2 Spike Protein Expressed in Yeast

SARS-CoV-2 spike protein is an essential component of numerous protein-based vaccines for COVID-19. The receptor-binding domain of this spike protein is a promising antigen with ease of expression in microbial hosts and scalability at comparatively low production costs. This study describes the production, purification, and characterization of RBD of SARS-CoV-2 protein, which is currently in clinical trials, from a commercialization perspective. The protein was expressed in Pichia pastoris in a large-scale bioreactor of 1200 L capacity. Protein capture and purification are conducted through mixed-mode chromatography followed by hydrophobic interaction chromatography. This two-step purification process produced RBD with an overall productivity of ~21 mg/L at >99% purity. The protein's primary, secondary, and tertiary structures were also verified using LCMS-based peptide mapping, circular dichroism, and fluorescence spectroscopy, respectively. The glycoprotein was further characterized for quality attributes such as glycosylation, molecular weight, purity, di-sulfide bonding, etc. Through structural analysis, it was confirmed that the product maintained a consistent quality across different batches during the large-scale production process. The binding capacity of RBD of spike protein was also assessed using human angiotensin-converting enzyme 2 receptor. A low binding constant range of KD values, ranging between 3.63 × 10-8 to 6.67 × 10-8, demonstrated a high affinity for the ACE2 receptor, revealing this protein as a promising candidate to prevent the entry of COVID-19 virus.

Saccharomyces cerevisiae as a research tool for RNA-mediated human disease

The budding yeast, Saccharomyces cerevisiae, has been used for decades as a powerful genetic tool to study a broad spectrum of biological topics. With its ease of use, economic utility, well-studied genome, and a highly conserved proteome across eukaryotes, it has become one of the most used model organisms. Due to these advantages, it has been used to study an array of complex human diseases. From broad, complex pathological conditions such as aging and neurodegenerative disease to newer uses such as SARS-CoV-2, yeast continues to offer new insights into how cellular processes are affected by disease and how affected pathways might be targeted in therapeutic settings. At the same time, the roles of RNA and RNA-based processes have become increasingly prominent in the pathology of many of these same human diseases, and yeast has been utilized to investigate these mechanisms, from aberrant RNA-binding proteins in amyotrophic lateral sclerosis to translation regulation in cancer. Here we review some of the important insights that yeast models have yielded into the molecular pathology of complex, RNA-based human diseases. This article is categorized under: RNA in Disease and Development > RNA in Disease.

COVID-19 Vaccines over Three Years after the Outbreak of the COVID-19 Epidemic

The outbreak of COVID-19 started in December 2019 and spread rapidly all over the world. It became clear that the development of an effective vaccine was the only way to stop the pandemic. It was the first time in the history of infectious diseases that the process of the development of a new vaccine was conducted on such a large scale and accelerated so rapidly. At the end of 2020, the first COVID-19 vaccines were approved for marketing. At the end of March 2023, over three years after the outbreak of the COVID-19 pandemic, 199 vaccines were in pre-clinical development and 183 in clinical development. The candidate vaccines in the clinical phase are based on the following platforms: protein subunit, DNA, RNA, non-replication viral vector, replicating viral vector, inactivated virus, virus-like particles, live attenuated virus, replicating viral vector combined with an antigen-presenting cell, non-replication viral vector combined with an antigen-presenting cell, and bacterial antigen-spore expression vector. Some of the new vaccine platforms have been approved for the first time for human application. This review presents COVID-19 vaccines currently available in the world, procedures for assurance of the quality and safety of the vaccines, the vaccinated population, as well as future perspectives for the new vaccine platforms in drug and therapy development for infectious and non-infectious diseases.

Designing and developing a sensitive and specific SARS-CoV-2 RBD IgG detection kit for identifying positive human samples

BACKGROUND

More than 3 y into the coronavirus 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to undergo mutations. In this context, the Receptor Binding Domain (RBD) is the most antigenic region among the SARS-CoV-2 Spike protein and has emerged as a promising candidate for immunological development. We designed an IgG-based indirect enzyme-linked immunoassay (ELISA) kit based on recombinant RBD, which was produced from the laboratory to 10 L industry scales in Pichia pastoris.

METHODS

A recombinant-RBD comprising 283 residues (31 kDa) was constructed after epitope analyses. The target gene was initially cloned into an Escherichia coli TOP10 genotype and transformed into Pichia pastoris CBS7435 muts for protein production. Production was scaled up in a 10 L fermenter after a 1 L shake-flask cultivation. The product was ultrafiltered and purified using ion-exchange chromatography. IgG-positive human sera for SARS-CoV-2 were employed by an ELISA test to evaluate the antigenicity and specific binding of the produced protein.

RESULTS

Bioreactor cultivation yielded 4 g/l of the target protein after 160 h of fermentation, and ion-exchange chromatography indicated a purity > 95%. A human serum ELISA test was performed in 4 parts, and the ROC area under the curve (AUC) was > 0.96 for each part. The mean specificity and sensitivity of each part was 100% and 91.5%, respectively.

CONCLUSION

A highly specific and sensitive IgG-based serologic kit was developed for improved diagnostic purposes in patients with COVID-19 after generating an RBD antigen in Pichia pastoris at laboratory and 10 L fermentation scales.

Advanced Vaccine Design Strategies against SARS-CoV-2 and Emerging Variants

Vaccination is the most cost-effective means in the fight against infectious diseases. Various kinds of vaccines have been developed since the outbreak of COVID-19, some of which have been approved for clinical application. Though vaccines available achieved partial success in protecting vaccinated subjects from infection or hospitalization, numerous efforts are still needed to end the global pandemic, especially in the case of emerging new variants. Safe and efficient vaccines are the key elements to stop the pandemic from attacking the world now; novel and evolving vaccine technologies are urged in the course of fighting (re)-emerging infectious diseases. Advances in biotechnology offered the progress of vaccinology in the past few years, and lots of innovative approaches have been applied to the vaccine design during the ongoing pandemic. In this review, we summarize the state-of-the-art vaccine strategies involved in controlling the transmission of SARS-CoV-2 and its variants. In addition, challenges and future directions for rational vaccine design are discussed.

Advances in SARS-CoV-2 receptor-binding domain-based COVID-19 vaccines

INTRODUCTION

The Coronavirus Disease 2019 (COVID-19) pandemic has caused devastating human and economic costs. Vaccination is an important step in controlling the pandemic. Severe acute respiratory coronavirus-2 (SARS-CoV-2), the causative agent of COVID-19, infects cells by binding a cellular receptor through the receptor-binding domain (RBD) within the S1 subunit of the spike (S) protein. Viral entry and membrane fusion are mediated by the S2 subunit.

AREAS COVERED

SARS-CoV-2 S protein, particularly RBD, serves as an important target for vaccines. Here we review the structure and function of SARS-CoV-2 S protein and its RBD, summarize current COVID-19 vaccines targeting the RBD, and outline potential strategies for improving RBD-based vaccines. Overall, this review provides important information that will facilitate rational design and development of safer and more effective COVID-19 vaccines.

EXPERT OPINION

The S protein of SARS-CoV-2 harbors numerous mutations, mostly in the RBD, resulting in multiple variant strains. Although many COVID-19 vaccines targeting the RBD of original virus strain (and previous variants) can prevent infection of these strains, their ability against recent dominant variants, particularly Omicron and its offspring, is significantly reduced. Collective efforts are needed to develop effective broad-spectrum vaccines to control current and future variants that have pandemic potential.

An engineered SARS-CoV-2 receptor-binding domain produced in Pichia pastoris as a candidate vaccine antigen

Developing affordable and easily manufactured SARS-CoV-2 vaccines will be essential to achieve worldwide vaccine coverage and long-term control of the COVID-19 pandemic. Here the development is reported of a vaccine based on the SARS-CoV-2 receptor-binding domain (RBD), produced in the yeast Pichia pastoris. The RBD was modified by adding flexible N- and C-terminal amino acid extensions that modulate protein/protein interactions and facilitate protein purification. A fed-batch methanol fermentation with a yeast extract-based culture medium in a 50 L fermenter and an immobilized metal ion affinity chromatography-based downstream purification process yielded 30-40 mg/L of RBD. Correct folding of the purified protein was demonstrated by mass spectrometry, circular dichroism, and determinations of binding affinity to the angiotensin-converting enzyme 2 (ACE2) receptor. The RBD antigen also exhibited high reactivity with sera from convalescent individuals and Pfizer-BioNTech or Sputnik V vaccinees. Immunization of mice and non-human primates with 50 µg of the recombinant RBD adjuvanted with alum induced high levels of binding antibodies as assessed by ELISA with RBD produced in HEK293T cells, and which inhibited RBD binding to ACE2 and neutralized infection of VeroE6 cells by SARS-CoV-2. Additionally, the RBD protein stimulated IFNγ, IL-2, IL-6, IL-4 and TNFα secretion in splenocytes and lung CD3⁺-enriched cells of immunized mice. The data suggest that the RBD recombinant protein produced in yeast P. pastoris is suitable as a vaccine candidate against COVID-19.

A vaccine based on the yeast-expressed receptor-binding domain (RBD) elicits broad immune responses against SARS-CoV-2 variants

Development of safe and efficient vaccines is still necessary to deal with the COVID-19 pandemic. Herein, we reported that yeast-expressed recombinant RBD proteins either from wild-type or Delta SARS-CoV-2 were able to elicit immune responses against SARS-CoV-2 and its variants. The wild-type RBD (wtRBD) protein was overexpressed in Pichia pastoris, and the purified protein was used as the antigen to immunize mice after formulating an aluminium hydroxide (Alum) adjuvant. Three immunization programs with different intervals were compared. It was found that the immunization with an interval of 28 days exhibited the strongest immune response to SARS-CoV-2 than the one with an interval of 14 or 42 days based on binding antibody and the neutralizing antibody (NAb) analyses. The antisera from the mice immunized with wtRBD were able to neutralize the Beta variant with a similar efficiency but the Delta variant with 2~2.5-fold decreased efficiency. However, more NAbs to the Delta variant were produced when the Delta RBD protein was used to immunize mice. Interestingly, the NAbs may cross react with the Omicron variant. To increase the production of NAbs, the adjuvant combination of Alum and CpG oligonucleotides was used. Compared with the Alum adjuvant alone, the NAbs elicited by the combined adjuvants exhibited an approximate 10-fold increase for the Delta and a more than 53-fold increase for the Omicron variant. This study suggested that yeast-derived Delta RBD is a scalable and an effective vaccine candidate for SARS-CoV-2 and its variants.

Recombinant expression of SARS-CoV-2 receptor binding domain (RBD) in Escherichia coli and its immunogenicity in mice

Objectives

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), giving rise to the coronavirus disease 2019 (COVID-19), has become a danger to wellbeing worldwide. Thus, finding efficient and safe vaccines for COVID-19 is of great importance. As a basic step amid contamination, SARS-CoV-2 employs the receptor-binding domain (RBD) of the spike protein to lock in with the receptor angiotensin-converting enzyme 2 (ACE2) on host cells. SARS-CoV-2 receptor-binding domain (RBD) is the main human antibody target for developing vaccines and virus inhibitors, as well as neutralizing antibodies. A bacterial procedure was developed for the expression and purification of the SARS-CoV-2 spike protein receptor-binding domain.

Materials and Methods

In this research study, RBD was expressed by Escherichia coli and purified with Ni-NTA chromatography. Then it was affirmed by the western blot test. The immunogenicity and protective efficacy of RBD recombinant protein were assessed on BALB/c mice. Additionally, RBD recombinant protein was tested by ELISA utilizing sera of COVID-19 healing patients contaminated with SARS-CoV-2 wild type and Delta variation.

Results

Indirect ELISA was able to detect the protein RBD in serum of the immunized mouse expressed in E. coli. The inactive SARS-CoV2 was detected by antibodies within the serum of immunized mice. Serum antibodies from individuals recovered from Covid19 reacted to the expressed protein.

Conclusion

Our findings showed that RBD is of great importance in vaccine design and it can be used to develop recombinant vaccines through induction of antibodies against RBD.

Direct capture of neutralized RBD enables rapid point-of-care assessment of SARS-CoV-2 neutralizing antibody titer

Neutralizing antibody (NAb) titer is a key biomarker of protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, but point-of-care methods for assessing NAb titer are not widely available. Here, we present a lateral flow assay that captures SARS-CoV-2 receptor-binding domain (RBD) that has been neutralized from binding angiotensin-converting enzyme 2 (ACE2). Quantification of neutralized RBD in this assay correlates with NAb titer from vaccinated and convalescent patients. This methodology demonstrated superior performance in assessing NAb titer compared with either measurement of total anti-spike immunoglobulin G titer or quantification of the absolute reduction in binding between ACE2 and RBD. Our testing platform has the potential for mass deployment to aid in determining at population scale the degree of protective immunity individuals may have following SARS-CoV-2 vaccination or infection and can enable simple at-home assessment of NAb titer.

Neutralizing Potency of Prototype and Omicron RBD mRNA Vaccines Against Omicron Variant

The newly emerged Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) contains more than 30 mutations on the spike protein, 15 of which are located within the receptor binding domain (RBD). Consequently, Omicron is able to extensively escape existing neutralizing antibodies and may therefore compromise the efficacy of current vaccines based on the original strain, highlighting the importance and urgency of developing effective vaccines against Omicron. Here we report the rapid generation and evaluation of an mRNA vaccine candidate specific to Omicron, and explore the feasibility of heterologous immunization with WT and Omicron RBD vaccines. This mRNA vaccine encodes the RBD of Omicron (designated as RBD-O) and is formulated with lipid nanoparticle. Two doses of the RBD-O mRNA vaccine efficiently induce neutralizing antibodies in mice; however, the antisera are effective only on the Omicron variant but not on the wildtype and Delta strains, indicating a narrow neutralization spectrum. It is noted that the neutralization profile of the RBD-O mRNA vaccine is opposite to that observed for the mRNA vaccine expressing the wildtype RBD (RBD-WT). Importantly, booster with RBD-O mRNA vaccine after two doses of RBD-WT mRNA vaccine can significantly increase neutralization titers against Omicron. Additionally, an obvious increase in IFN-γ, IL-2, and TNF-α-expressing RBD-specific CD4⁺ T cell responses was observed after immunization with the RBD-WT and/or RBD-O mRNA vaccine. Together, our work demonstrates the feasibility and potency of an RBD-based mRNA vaccine specific to Omicron, providing important information for further development of heterologous immunization program or bivalent/multivalent SARS-CoV-2 vaccines with broad-spectrum efficacy.

Effective protection of ZF2001 against the SARS-CoV-2 Delta variant in lethal K18-hACE2 mice

To investigate the protective efficacy and mechanism of ZF2001 (a protein subunit vaccine with conditional approval in China) to SARS-CoV-2 Delta variant-induced severe pneumonia, the lethal challenge model of K18-hACE2 transgenic mice was used in this study. An inactivated-virus vaccine at the research and development stage (abbreviated as RDINA) was compared to ZF2001. We found that ZF2001 and RDINA could provide the protective effect against Delta variant-induced severe cases, as measured by the improved survival rates, the reduced virus loads, the alleviated lung histopathology and the high neutralizing antibody geomean titers, compared to aluminum adjuvant group. To prevent and control Omicron or other variant epidemics, further improvements in vaccine design and compatibilities with the novel adjuvant are required to achieve better immunogenicity.

A Novel Bacterial Protease Inhibitor Adjuvant in RBD-Based COVID-19 Vaccine Formulations Containing Alum Increases Neutralizing Antibodies, Specific Germinal Center B Cells and Confers Protection Against SARS-CoV-2 Infection in Mice

In this work, we evaluated recombinant receptor binding domain (RBD)-based vaccine formulation prototypes with potential for further clinical development. We assessed different formulations containing RBD plus alum, AddaS03, AddaVax, or the combination of alum and U-Omp19: a novel Brucella spp. protease inhibitor vaccine adjuvant. Results show that the vaccine formulation composed of U-Omp19 and alum as adjuvants has a better performance: it significantly increased mucosal and systemic neutralizing antibodies in comparison to antigen plus alum, AddaVax, or AddaS03. Antibodies induced with the formulation containing U-Omp19 and alum not only increased their neutralization capacity against the ancestral virus but also cross-neutralized alpha, lambda, and gamma variants with similar potency. Furthermore, the addition of U-Omp19 to alum vaccine formulation increased the frequency of RBD-specific geminal center B cells and plasmablasts. Additionally, U-Omp19+alum formulation induced RBD-specific Th1 and CD8⁺ T-cell responses in spleens and lungs. Finally, this vaccine formulation conferred protection against an intranasal severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) challenge of K18-hACE2 mice.

Diverse vaccine platforms safeguarding against SARS-CoV-2 and its variants

INTRODUCTION

Appearances of SARS-CoV-2 variants have created havoc and additional challenges for the ongoing vaccination drive against pandemic COVID-19. Interestingly, several vaccine platforms are showing great potential to produce successful vaccines against SARS-CoV-2 and its variants. Billions of COVID-19 vaccine doses have been administered worldwide. Mix-and-Match COVID-19 vaccines involving the mixing of the same platform vaccines and also two different vaccine platforms may provide greater protection against SARS-CoV-2 and its variants. COVID-19 vaccines have become one of the most important tools to mitigate the ongoing pandemic COVID-19.

AREAS COVERED

We describe SARS-Cov-2 variants, their impact on the population, COVID-19 vaccines, diverse vaccine platforms, doses of vaccines, the efficacy of vaccines against SARS-CoV-2 and its variants, mitigation of the COVID-19 transmission- alternatives to vaccines.

EXPERT OPINION

Diverse vaccine platforms may safeguard against ongoing, deadly SARS-CoV-2 and its infectious variants. The efficacies of COVID-19 vaccines are significantly high after the administration of the second dose. Further, it protects individuals including vulnerable patients with co-morbidities from SARS-CoV-2 and its variants. The hospitalizations and deaths of the individuals may be prevented by COVID-19 vaccines.

Cell fusion as a link between the SARS-CoV-2 spike protein, COVID-19 complications, and vaccine side effects

A distinctive feature of the SARS-CoV-2 spike protein is its ability to efficiently fuse cells, thus producing syncytia found in COVID-19 patients. This commentary proposes how this ability enables spike to cause COVID-19 complications as well as side effects of COVID-19 vaccines, and suggests how these effects can be prevented.

Scientific rationale for developing potent RBD-based vaccines targeting COVID-19

Vaccination of the global population against COVID-19 is a great scientific, logistical, and moral challenge. Despite the rapid development and authorization of several full-length Spike (S) protein vaccines, the global demand outweighs the current supply and there is a need for safe, potent, high-volume, affordable vaccines that can fill this gap, especially in low- and middle-income countries. Whether SARS-CoV-2 S-protein receptor-binding domain (RBD)-based vaccines could fill this gap has been debated, especially with regards to its suitability to protect against emerging viral variants of concern. Given a predominance for elicitation of neutralizing antibodies (nAbs) that target RBD following natural infection or vaccination, a key biomarker of protection, there is merit for selection of RBD as a sole vaccine immunogen. With its high-yielding production and manufacturing potential, RBD-based vaccines offer an abundance of temperature-stable doses at an affordable cost. In addition, as the RBD preferentially focuses the immune response to potent and recently recognized cross-protective determinants, this domain may be central to the development of future pan-sarbecovirus vaccines. In this study, we review the data supporting the non-inferiority of RBD as a vaccine immunogen compared to full-length S-protein vaccines with respect to humoral and cellular immune responses against both the prototype pandemic SARS-CoV-2 isolate and emerging variants of concern.