Protective Immunity against SARS Subunit Vaccine Candidates Based on Spike Protein: Lessons for Coronavirus Vaccine Development

Protein subunit vaccines: Promising frontiers against COVID-19

The emergence of COVID-19 has posed an unprecedented global health crisis, challenging the healthcare systems worldwide. Amidst the rapid development of several vaccine formulations, protein subunit vaccines have emerged as a promising approach. This article provides an in-depth evaluation of the role of protein subunit vaccines in the management of COVID-19. Leveraging viral protein fragments, particularly the spike protein from SARS-CoV-2, these vaccines elicit a targeted immune response without the risk of inducing disease. Notably, the robust safety profile of protein subunit vaccines makes them a compelling candidate in the management of COVID-19. Various innovative approaches, including reverse vaccinology, virus like particles, and recombinant modifications are incorporated to develop protein subunit vaccines. In addition, the utilization of advanced manufacturing techniques facilitates large-scale production, ensuring widespread distribution. Despite these advancements, challenges persist, such as the requirement for cold-chain storage and the necessity for booster doses. This article evaluates the formulation and applications of protein subunit vaccines, providing a comprehensive overview of their clinical development and approvals in the context of COVID-19. By addressing the current status and challenges, this review aims to contribute to the ongoing discourse on optimizing protein subunit vaccines for effective pandemic control.

Discovery and development of COVID-19 vaccine from laboratory to clinic

The world has recently experienced one of the biggest and most severe public health disasters with severe acute respiratory syndrome coronavirus (SARS-CoV-2). SARS-CoV-2 is responsible for the coronavirus disease of 2019 (COVID-19) which is one of the most widespread and powerful infections affecting human lungs. Current figures show that the epidemic had reached 216 nations, where it had killed about 6,438,926 individuals and infected 590,405,710. WHO proclaimed the outbreak of the Ebola virus disease (EVD), in 2014 that killed hundreds of people in West Africa. The development of vaccines for SARS-CoV-2 becomes more difficult due to the viral mutation in its non-structural proteins (NSPs) especially NSP2 and NSP3, S protein, and RNA-dependent RNA polymerase (RdRp). Continuous monitoring of SARS-CoV-2, dynamics of the genomic sequence, and spike protein mutations are very important for the successful development of vaccines with good efficacy. Hence, the vaccine development for SARS-CoV-2 faces specific challenges starting from viral mutation. The requirement of long-term immunity development, safety, efficacy, stability, vaccine allocation, distribution, and finally, its cost is discussed in detail. Currently, 169 vaccines are in the clinical development stage, while 198 vaccines are in the preclinical development stage. The majority of these vaccines belong to the Ps-Protein subunit type which has 54, and the minor BacAg-SPV (Bacterial antigen-spore expression vector) type, at least 1 vaccination. The use of computational methods and models for vaccine development has revolutionized the traditional methods of vaccine development. Further, this updated review highlights the upcoming vaccine development strategies in response to the current pandemic and post-pandemic era, in the field of vaccine development.

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.

Characterization of two linear epitopes SARS CoV-2 spike protein formulated in tandem repeat

The vital roles of diagnostic tools and vaccines are prominent in controlling COVID-19. Spike protein of the SARS CoV-2, specifically the epitopes in that protein, are the critical components of the vaccines and immunological diagnostic tools. Two epitopes in the spike protein, the S14P5 and S21P2, identified previously are of great interest because they are linear and elicit neutralizing antibodies. The present study formulated each epitope in the tandem-repeat structure to increase their immunogenicity and facilitate their production. The tandem repeats (TR) were expressed efficiently in E. coli, yielding 58 mg and 46 mg per liter culture for TR-S14P5 and TR-S212, respectively. ELISA using either one of the repeating epitopes can be used as a serological test to identify individuals infected by the SARS-CoV-2 virus. The area under curves (AUC), based on testing 157 serum samples from COVID-19 patients and 26 from COVID-19-free individuals, were 0.806 and 0.889 for TR-S14P5 and TR-S21P2-based ELISAs, respectively. For 100% diagnostic specificity, the sensitivity was only 70%. The low sensitivity supposedly resulted from some samples being from early infection prior to antibody conversion. Both recombinant epitopes were highly immunogenic in rabbits, and the immune sera recognized inactivated SARS CoV-2 virus in dot-blot assays. These antibodies should be useful as a reagent for detecting SARS-CoV-2 antigens. Furthermore, the TR-S14P5 and TR-S21P2, being conserved and denaturation-resistant, are envisaged to be ideal for intra-nasal vaccines, which are required to complement current COVID-19 to overcome rapidly mutated SARS CoV-2.

Designing and Expression of Recombinant Chimeric Spike Protein from SARS-CoV-2 in Escherichia coli and Its Immunogenicity Assessment

Since December 2019, the world has been grappling with an ongoing global COVID-19 pandemic. Various virus variants have emerged over the past two years, each posing a greater threat than its predecessors. The recent appearance of the omicron variant (B.1.1.529) has raised significant alarm within the field of epidemiology due to its highly contagious nature and rapid transmission rate. The omicron variant possessed mutations in the key receptor-binding domain (RBD) region, the S region, and these modifications have shown a notable impact on the strain's susceptibility to neutralizing antibodies. Developing safe and efficient vaccines to prevent a future severe acute respiratory outbreak of coronavirus syndrome 2 (SARS-CoV-2) is significant. Viral surface spike proteins are ideal targets for vaccines. This study aimed to find a multi-subunit chimeric vaccine. After conducting bioinformatics analysis, the recombinant spike (RS) protein of SARS-CoV-2 was deliberately designed and subsequently produced using E. coli expression systems. The immunogenicity of RS and neutralizing antibody responses were evaluated on immunized BALB/c mice. There was a significant difference in antibody titers between RS-immunized mice and control groups. The endpoint of the serum antibody titer of mice immunized with our chimeric protein was 2.5 times higher than that of the negative control. The chimeric construct could present multiple antigens simultaneously, influentially affecting immunization. Sera from mice vaccinated by RS could recognize the SARS-CoV-2 virus and neutralize antibodies. Our chimeric peptide could bind to antibodies in the serum of patients infected with different serotypes of the SARS-CoV-2 virus, such as alpha, delta, and omicron variants. The results indicated that the RS protein would be a potential novel antigenic candidate for subunit vaccine development and could be used as a useful alternative to generate diagnostic serological tests for SARS-CoV-2 infection.

An overview of viral mutagenesis and the impact on pathogenesis of SARS-CoV-2 variants

Viruses are submicroscopic, obligate intracellular parasites that carry either DNA or RNA as their genome, protected by a capsid. Viruses are genetic entities that propagate by using the metabolic and biosynthetic machinery of their hosts and many of them cause sickness in the host. The ability of viruses to adapt to different hosts and settings mainly relies on their ability to create de novo variety in a short interval of time. The size and chemical composition of the viral genome have been recognized as important factors affecting the rate of mutations. Coronavirus disease 2019 (Covid-19) is a novel viral disease that has quickly become one of the world's leading causes of mortality, making it one of the most serious public health problems in recent decades. The discovery of new medications to cope with Covid-19 is a difficult and time-consuming procedure, as new mutations represent a serious threat to the efficacy of recently developed vaccines. The current article discusses viral mutations and their impact on the pathogenicity of newly developed variants with a special emphasis on Covid-19. The biology of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), its mutations, pathogenesis, and treatment strategies are discussed in detail along with the statistical data.

Recent developments in SARS-CoV-2 vaccines: A systematic review of the current studies

Designing and manufacturing efficient vaccines against coronavirus disease 2019 (COVID-19) is a major objective. In this systematic review, we aimed to evaluate the most important vaccines under construction worldwide, their efficiencies and clinical results in healthy individuals and in those with specific underlying diseases. We conducted a comprehensive search in PubMed, Scopus, EMBASE, and Web of Sciences by 1 December 2021 to identify published research studies. The inclusion criteria were publications that evaluated the immune responses and safety of COVID-19 vaccines in healthy individuals and in those with pre-existing diseases. We also searched the VAERS database to estimate the incidence of adverse events of special interest (AESI) post COVID-19 vaccination. Almost all investigated vaccines were well tolerated and developed good levels of both humoural and cellular responses. A protective and efficient humoural immune response develops after the second or third dose of vaccine and a longer interval (about 28 days) between the first and second injections of vaccine could induce higher antibody responses. The vaccines were less immunogenic in immunocompromised patients, particularly those with haematological malignancies. In addition, we found that venous and arterial thrombotic events, Bell's palsy, and myocarditis/pericarditis were the most common AESI. The results showed the potency of the SARS-CoV-2 vaccines to protect subjects against disease. The provision of further effective and safe vaccines is necessary in order to reach a high coverage of immunisation programs across the globe and to provide protection against infection itself.

A Spike Protein-Based Subunit SARS-CoV-2 Vaccine for Pets: Safety, Immunogenicity, and Protective Efficacy in Juvenile Cats

Whereas, multiple vaccine types have been developed to curb the spread of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) among humans, there are very few vaccines being developed for animals including pets. To combat the threat of human-to-animal, animal-to-animal, and animal-to-human transmission and the generation of new virus variants, we developed a subunit SARS-CoV-2 vaccine which is based on the recombinant spike protein extracellular domain expressed in insect cells and then formulated with appropriate adjuvants. Sixteen 8–12-week-old outbred female and male kittens (n = 4 per group) were randomly assigned into four treatment groups: spike protein alone; spike plus ESSAI oil-in-water (O/W) 1849102 adjuvant; spike plus aluminum hydroxide adjuvant; and a PBS control. All animals were vaccinated intramuscularly twice, 2 weeks apart, with 5 μg of spike protein in a volume of 0.5 ml. On days 0 and 28, serum samples were collected to evaluate anti-spike IgG, antibody inhibition of spike binding to angiotensin-converting enzyme 2 (ACE-2), neutralizing antibodies against wild-type and delta variant viruses, and hematology studies. At day 28, all groups were challenged with SARS-CoV-2 wild-type virus 10⁶ TCID₅₀ intranasally. On day 31, tissue samples (lung, heart, and nasal turbinates) were collected for viral RNA detection, and virus titration. After two immunizations, both vaccines induced high titers of serum anti-spike IgG that inhibited spike ACE-2 binding and neutralized both wild-type and delta variant virus. Both adjuvanted vaccine formulations protected juvenile cats against virus shedding from the upper respiratory tract and viral replication in the lower respiratory tract and hearts. These promising data warrant ongoing evaluation of the vaccine's ability to protect cats against SARS-CoV-2 infection and in particular to prevent transmission.

A two-dose optimum for recombinant S1 protein-based COVID-19 vaccination

BACKGROUND

Recombinant protein subunit vaccination is considered to be a safe, fast and reliable technique when combating emerging and re-emerging diseases such as coronavirus disease 2019 (COVID-19). Typically, such subunit vaccines require the addition of adjuvants to attain adequate immunogenicity. AS01, which contains adjuvants MPL and saponin QS21, is a liposome-based vaccine adjuvant system that is one of the leading candidates. However, the adjuvant effect of AS01 in COVID-19 vaccines is not well described yet.

METHODS

In this study, we utilized a mixture of AS01 as the adjuvant for an S1 protein-based COVID-19 vaccine.

RESULTS

The adjuvanted vaccine induced robust immunoglobulin G (IgG) binding antibody and virus-neutralizing antibody responses. Importantly, two doses induced similar levels of IgG binding antibody and neutralizing antibody responses compared with three doses and the antibody responses weakened only slightly over time up to six weeks after immunization.

CONCLUSION

These results suggested that two doses may be enough for a clinical vaccine strategy design using MPL & QS21 adjuvanted recombinant protein, especially in consideration of the limited production capacity of COVID-19 vaccine in a public health emergency.

SARS-CoV-2 and Variant Diagnostic Testing Approaches in the United States

Purpose of Review Given the rapid development of diagnostic approaches to test for and diagnose infection with SARS-CoV-2 and its associated variants including Omicron (B.1.1.529), many options are available to diagnose infection. Multiple established diagnostic companies are now providing testing platforms whereas initially, testing was being performed with simple PCR-based tests using standard laboratory reagents. Recent Findings Additional testing platforms continue to be developed, including those to detect specific variants, but challenges with testing, including obtaining testing reagents and other related supplies, are frequently encountered. With time, the testing supply chain has improved, and more established companies are providing materials to support these testing efforts. In the United States (U.S.), the need for rapid assay development and subsequent approval through the attainment of emergency use authorization (EUA) has superseded the traditional arduous diagnostic testing approval workflow mandated by the FDA. Through these efforts, the U.S. has been able to continue to significantly increase its testing capabilities to address this pandemic; however, challenges still remain due to the diversity of the performance characteristics of tests being utilized and newly discovered viral variants. Summary This review provides an overview of the current diagnostic testing landscape, with pertinent information related to SARS-CoV-2 virology, variants and antibody responses that are available to diagnose infection in the U.S.

Diagnostic Testing for SARS-CoV-2 Infection

PURPOSE OF REVIEW

Given the rapid development of diagnostic approaches to test for and diagnose infection with SARS-CoV-2, many options are available to assess infection. Multiple established diagnostic companies are now providing testing platforms whereas initially, testing was being performed with simple PCR-based tests using standard laboratory reagents.

RECENT FINDINGS

Additional testing platforms continue to be developed but challenges with testing, including obtaining testing reagents and other related supplies, are frequently encountered. With time, the testing supply chain will improve and more companies will be providing materials to support these testing efforts. In the USA, the need for rapid assay development and subsequent approval through attainment of emergency use authorization (EUA) has superseded the traditional arduous diagnostic testing approval workflow mandated by the FDA. It is anticipated that the USA will be able to continue to significantly increase its testing capabilities to address this pandemic; however, challenges remain due to the diversity of the performance characteristics of tests being utilized.

SUMMARY

This review provides an overview of the current diagnostic testing landscape, with pertinent information related to SARS-CoV-2 virology and antibody responses, that is available to diagnose infection.

The impact of spike mutated variants of SARS-CoV2 [Alpha, Beta, Gamma, Delta, and Lambda] on the efficacy of subunit recombinant vaccines

Since the first described human infection with SARS-CoV-2 in December of 2019 many subunit protein vaccines have been proposed for use in humans. Subunit vaccines use one or more antigens suitable for eliciting a robust immune response. However, the major concern is the efficacy of subunit vaccines and elicited antibodies to neutralize the variants of SARS-CoV-2 like B.1.1.7 (Alpha), B.1.351 (Beta) and P1 (Gamma), B.1.617 (Delta) and C.37 (Lambda). The Spike protein (S) is a potential fragment for use as an antigen in vaccine development. This protein plays a crucial role in the first step of the infection process, as it binds to Angiotensin-Converting Enzyme 2 (ACE2) receptor and enters the host cell after binding. Immunization-induced specific antibodies against the receptor binding domain (RBD) may block and effectively prevent virus invasion. The focus of this review is the impact of spike mutated variants of SARS-CoV2 (Alpha, Beta, Gamma, Delta, and Lambda) on the efficacy of subunit recombinant vaccines. To date, a low or no significant impact on vaccine efficacy against Alpha and Delta variants has been reported. Such an impact on vaccine efficacy for Beta, Delta, Gamma, and Lambda variants may be even greater compared to the Alpha variant. Nonetheless, more comprehensive analyses are needed to assess the real impact on vaccine efficacy brought about by SARS-CoV-2 variants.

DNA vaccination induced protective immunity against SARS CoV-2 infection in hamsterss

The development of efficient vaccines against COVID-19 is an emergent need for global public health. The spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a major target for the COVID-19 vaccine. To quickly respond to the outbreak of the SARS-CoV-2 pandemic, a nucleic acid-based vaccine is a novel option, beyond the traditional inactivated virus vaccine or recombinant protein vaccine. Here, we report a DNA vaccine containing the spike gene for delivery via electroporation. The spike genes of SARS-CoV and SARS-CoV-2 were codon optimized for mammalian cell expression and then cloned into mammalian cell expression vectors, called pSARS-S and pSARS2-S, respectively. Spike protein expression was confirmed by immunoblotting after transient expression in HEK293T cells. After immunization, sera were collected for antigen-specific antibody and neutralizing antibody titer analyses. We found that both pSARS-S and pSARS2-S immunization induced similar levels of antibodies against S2 of SARS-CoV-2. In contrast, only pSARS2-S immunization induced antibodies against the receptor-binding domain of SARS-CoV-2. We further found that pSARS2-S immunization, but not pSARS-S immunization, could induce very high titers of neutralizing antibodies against SARS-CoV-2. We further analyzed SARS-CoV-2 S protein-specific T cell responses and found that the immune responses were biased toward Th1. Importantly, pSARS2-S immunization in hamsters could induce protective immunity against SARS-CoV-2 challenge in vivo. These data suggest that DNA vaccination could be a promising approach for protecting against COVID-19.

Mapping the technological landscape of SARS, MERS, and SARS-CoV-2 vaccines

PURPOSE

The last two decades have seen the emergence of several viral outbreaks. Some of them are the severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and severe acute respiratory syndrome 2 (SARS-CoV2) - the cause of the coronavirus disease 2019 (COVID-19). Ever, vaccines for emergency use have been authorized for the control and prevention of COVID-19. Currently, there is an urgent need to develop a vaccine for prophylaxis of COVID-19 and for other future epidemics.

METHODS

This review describes patented vaccines for SARS and MERS-CoV and vaccines developed and approved for emergency use against the new coronavirus (COVID-19). The European Patent Office and the World Intellectual Property Organization were the patent databases used using specific terms. In addition, another search was carried out in the Clinical Trials in search of ongoing clinical studies focused on the COVID-19 vaccine.

RESULTS

The patent search showed that most vaccines are based on viral vector platforms, nucleic acids, or protein subunits. The review also includes an overview of completed and ongoing clinical trials for SARS-CoV-2 in several countries.

CONCLUSION

The information provided here lists vaccines for other types of coronavirus that have been used in the development of vaccines for COVID-19.

Human lung-on-chips: Advanced systems for respiratory virus models and assessment of immune response

Respiratory viral infections are leading causes of death worldwide. A number of human respiratory viruses circulate in all age groups and adapt to person-to-person transmission. It is vital to understand how these viruses infect the host and how the host responds to prevent infection and onset of disease. Although animal models have been widely used to study disease states, incisive arguments related to poor prediction of patient responses have led to the development of microfluidic organ-on-chip models, which aim to recapitulate organ-level physiology. Over the past decade, human lung chips have been shown to mimic many aspects of the lung function and its complex microenvironment. In this review, we address immunological responses to viral infections and elaborate on human lung airway and alveolus chips reported to model respiratory viral infections and therapeutic interventions. Advances in the field will expedite the development of therapeutics and vaccines for human welfare.

Engineering a novel subunit vaccine against SARS-CoV-2 by exploring immunoinformatics approach

As the number of infections and deaths caused by the recent COVID-19 pandemic is increasing dramatically day-by-day, scientists are rushing towards developing possible countermeasures to fight the deadly virus, SARS-CoV-2. Although many efforts have already been put forward for developing potential vaccines; however, most of them are proved to possess negative consequences. Therefore, in this study, immunoinformatics methods were exploited to design a novel epitope-based subunit vaccine against the SARS-CoV-2, targeting four essential proteins of the virus i.e., spike glycoprotein, nucleocapsid phosphoprotein, membrane glycoprotein, and envelope protein. The highly antigenic, non-allergenic, non-toxic, non-human homolog, and 100% conserved (across other isolates from different regions of the world) epitopes were used for constructing the vaccine. In total, fourteen CTL epitopes and eighteen HTL epitopes were used to construct the vaccine. Thereafter, several in silico validations i.e., the molecular docking, molecular dynamics simulation (including the RMSF and RMSD studies), and immune simulation studies were also performed which predicted that the designed vaccine should be quite safe, effective, and stable within the biological environment. Finally, in silico cloning and codon adaptation studies were also conducted to design an effective mass production strategy of the vaccine. However, more in vitro and in vivo studies are required on the predicted vaccine to finally validate its safety and efficacy.