Comparative Immunomodulatory Evaluation of the Receptor Binding Domain of the SARS-CoV-2 Spike Protein; a Potential Vaccine Candidate Which Imparts Potent Humoral and Th1 Type Immune Response in a Mouse Model

Immunogenicity studies of recombinant RBD SARS-CoV-2 as a COVID-19 vaccine candidate produced in Escherichia coli

The severe acute respiratory syndrome coronavirus 2 -related global COVID-19 pandemic has been impacting millions of people since its outbreak in 2020. COVID-19 vaccination has proven highly efficient in reducing illness severity and preventing infection-related fatalities. The World Health Organization has granted emergency use approval to multiple, including protein subunit technology-based, COVID-19 vaccines. Foreseeably, additional COVID-19 subunit vaccine development would be essential to meet the accessible and growing demand for effective vaccines, especially for Low-Middle-Income Countries (LMIC). The SARS-CoV-2 spike protein receptor binding domain (RBD), as the primary target for neutralizing antibodies, holds significant potential for future COVID-19 subunit vaccine development. In this study, we developed a recombinant Escherichia coli-expressed RBD (rRBD) as a vaccine candidate and evaluated its immunogenicity and preliminary toxicity in BALB/c mice. The rRBD induced humoral immune response from day 7 post-vaccination and, following the booster doses, the IgG levels increased dramatically in mice. Interestingly, our vaccine candidate also significantly induced cellular immune response, indicated by the incrased IFN-ɣ-producing cell numbers. We observed no adverse effect or local reactogenicity either in control or treated mice. Taken together, our discoveries could potentially support efficient and cost-effective vaccine antigen production, from which LMICs could particularly benefit.

Pre-Clinical Safety and Immunogenicity Study of a Coronavirus Protein-Based Subunit Vaccine for COVID-19

Creating an effective and safe vaccine is critical to fighting the coronavirus infection successfully. Several types of COVID-19 vaccines exist, including inactivated, live attenuated, recombinant, synthetic peptide, virus-like particle-based, DNA and mRNA-based, and sub-unit vaccines containing purified immunogenic viral proteins. However, the scale and speed at which COVID-19 is spreading demonstrate a global public demand for an effective prophylaxis that must be supplied more. The developed products promise a bright future for SARS-CoV-2 prevention; however, evidence of safety and immunogenicity is mandatory before any vaccine can be produced. In this paper, we report on the results of our work examining the safety, toxicity, immunizing dose choice, and immunogenicity of QazCoVac-P, a Kazakhstan-made sub-unit vaccine for COVID-19. First, we looked into the product's safety profile by assessing its pyrogenicity in vaccinated rabbit models and using the LAL (limulus amebocyte lysate) test. We examined the vaccine's acute and sub-chronic toxicity on BALB/c mice and rats. The vaccine did not cause clinically significant toxicity-related changes or symptoms in our toxicity experiments. Finally, we performed a double immunization of mice, ferrets, Syrian hamsters, and rhesus macaques (Macaca mulatta). We used ELISA to measure antibody titers with the maximum mean geometric titer of antibodies in the animals' blood sera totaling approximately 8 log2. The results of this and other studies warrant recommending the QazCoVac-P vaccine for clinical trials.

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.

Antigenic epitope screening and functional modification of mannose enhance the efficacy of largemouth bass virus subunit vaccines

Major capsid protein (MCP) can be used as a subunit vaccine against largemouth bass virus (LMBV). However, subunit vaccines usually have low immunogenicity. Here, to identify the major immunogenicity determinant region of the MCP gene, we truncated the MCP of the LMBV gene into four parts (MCP-1, MCP-2, MCP-3 and MCP-4). Enzyme-linked immunosorbent assay (ELISA) was used to identify the antigenicity of these four truncated MCP proteins. Then, the highly antigenic truncated protein was modified with mannose and connected with functionalized single-walled carbon nanotubes (SWCNTs) as carriers. Largemouth basses were immunized by bath immersion, challenged with LMBV on the 28th day after immunization and evaluated for related immune indicators. The results indicated that the MCP-2 protein could induce a higher antibody titre than the other truncated MCP proteins. We found that the levels of immune-related genes (TNF-α, CD40, IgM, IFNγ and IL-10) in the spleen and kidney were significantly increased in the MCP-2 and MCP-2-Man groups. ELISA results showed that the antibody content in the serum increased significantly in the MCP-2 group 7 days post-vaccination and increased with days in all the vaccinated groups, with the highest observed on the 21st day. Notably, the MCP-2-Man vaccine (10 mg L^-1 ) showed durability of immunoprotection efficacy that could protect largemouth basses from LMBV challenge, and the immune protection rate reached 78.94%. These results suggest that MCP-2 might be the major immunogenicity determinant region of LMBV and that the mannose-modified MCP-2 vaccine can induce stronger adaptive immune responses.

Virus-Like Particles of SARS-CoV-2 as Virus Surrogates: Morphology, Immunogenicity, and Internalization in Neuronal Cells

The engineering of virus-like particles (VLPs) is a viable strategy for the development of vaccines and for the identification of therapeutic targets without using live viruses. Here, we report the generation and characterization of quadruple-antigen SARS-CoV-2 VLPs. VLPs were generated by transient transfection of two expression cassettes in adherent HEK293T cells─one cassette containing Mpro for processing of three structural proteins (M, E, and N), and the second cassette expressing the Spike protein. Further characterization revealed that the VLPs retain close morphological and antigenic similarity with the native virus and also bind strongly to the SARS-CoV-2 receptor hACE-2 in an in vitro binding assay. Interestingly, the VLPs were found to internalize into U87-MG cells through cholesterol-rich domains in a dynamin-dependent process. Finally, our results showed that mice immunized with VLPs induce robust humoral and cellular immune responses mediated by enhanced levels of IL-4, IL-17, and IFNγ. Taken together, our results demonstrate that VLPs mimic the native virus and induce a strong immune response, indicating the possible use of these particles as an alternative vaccine candidate against SARS-CoV-2. VLPs can also be effective in mapping the initial stages of virus entry and screening inhibitors.

Potential mouse models of coronavirus-related immune injury

Basic research for prevention and treatment of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), continues worldwide. In particular, multiple newly reported cases of autoimmune-related diseases after COVID-19 require further research on coronavirus-related immune injury. However, owing to the strong infectivity of SARS-CoV-2 and the high mortality rate, it is difficult to perform relevant research in humans. Here, we reviewed animal models, specifically mice with coronavirus-related immune disorders and immune damage, considering aspects of coronavirus replacement, viral modification, spike protein, and gene fragments. The evaluation of mouse models of coronavirus-related immune injury may help establish a standardised animal model that could be employed in various areas of research, such as disease occurrence and development processes, vaccine effectiveness assessment, and treatments for coronavirus-related immune disorders. COVID-19 is a complex disease and animal models cannot comprehensively summarise the disease process. The application of genetic technology may change this status.

Squalene in oil-based adjuvant improves the immunogenicity of SARS-CoV-2 RBD and confirms safety in animal models

COVID-19 pandemic has accelerated the development of vaccines against its etiologic agent, SARS-CoV-2. However, the emergence of new variants of the virus lead to the generation of new alternatives to improve the current sub-unit vaccines in development. In the present report, the immunogenicity of the Spike RBD of SARS-CoV-2 formulated with an oil-in-water emulsion and a water-in-oil emulsion with squalene was evaluated in mice and hamsters. The RBD protein was expressed in insect cells and purified by chromatography until >95% purity. The protein was shown to have the appropriate folding as determined by ELISA and flow cytometry binding assays to its receptor, as well as by its detection by hamster immune anti-S1 sera under non-reducing conditions. In immunization assays, although the cellular immune response elicited by both adjuvants were similar, the formulation based in water-in-oil emulsion and squalene generated an earlier humoral response as determined by ELISA. Similarly, this formulation was able to stimulate neutralizing antibodies in hamsters. The vaccine candidate was shown to be safe, as demonstrated by the histopathological analysis in lungs, liver and kidney. These results have shown the potential of this formulation vaccine to be evaluated in a challenge against SARS-CoV-2 and determine its ability to confer protection.

Inactivated whole-virion vaccine BBV152/Covaxin elicits robust cellular immune memory to SARS-CoV-2 and variants of concern

BBV152 is a whole-virion inactivated vaccine based on the Asp614Gly variant. BBV152 is the first alum-imidazoquinolin-adjuvanted vaccine authorized for use in large populations. Here we characterized the magnitude, quality and persistence of cellular and humoral memory responses up to 6 months post vaccination. We report that the magnitude of vaccine-induced spike and nucleoprotein antibodies was comparable with that produced after infection. Receptor binding domain-specific antibodies declined against variants in the order of Alpha (B.1.1.7; 3-fold), Delta (B.1.617.2; 7-fold) and Beta (B.1.351; 10-fold). However, pseudovirus neutralizing antibodies declined up to 2-fold against the Delta followed by the Beta variant (1.7-fold). Vaccine-induced memory B cells were also affected by the Delta and Beta variants. The SARS-CoV-2-specific multicytokine-expressing CD4⁺ T cells were found in ~85% of vaccinated individuals. Only a ~1.3-fold reduction in efficacy was observed in CD4⁺ T cells against the Beta variant. We found that antigen-specific CD4⁺ T cells were present in the central memory compartment and persisted for at least up to 6 months post vaccination. Vaccine-induced CD8⁺ T cells were detected in ~50% of individuals. Importantly, the vaccine was capable of inducing follicular T helper cells that exhibited B-cell help potential. These findings show that inactivated vaccine BBV152 induces robust immune memory to SARS-CoV-2 and variants of concern that persists for at least 6 months after vaccination.

Antigen-adjuvant interactions, stability, and immunogenicity profiles of a SARS-CoV-2 receptor-binding domain (RBD) antigen formulated with aluminum salt and CpG adjuvants

Low-cost, refrigerator-stable COVID-19 vaccines will facilitate global access and improve vaccine coverage in low- and middle-income countries. To this end, subunit-based approaches targeting the receptor-binding domain (RBD) of SARS-CoV-2 Spike protein remain attractive. Antibodies against RBD neutralize SARS-CoV-2 by blocking viral attachment to the host cell receptor, ACE2. Here, a yeast-produced recombinant RBD antigen (RBD-L452K-F490W or RBD-J) was formulated with various combinations of aluminum-salt (Alhydrogel®, AH; AdjuPhos®, AP) and CpG 1018 adjuvants. We assessed the effect of antigen-adjuvant interactions on the stability and mouse immunogenicity of various RBD-J preparations. While RBD-J was 50% adsorbed to AH and <15% to AP, addition of CpG resulted in complete AH binding, yet no improvement in AP adsorption. ACE2 competition ELISA analyses of formulated RBD-J stored at varying temperatures (4, 25, 37°C) revealed that RBD-J was destabilized by AH, an effect exacerbated by CpG. DSC studies demonstrated that aluminum-salt and CpG adjuvants decrease the conformational stability of RBD-J and suggest a direct CpG-RBD-J interaction. Although AH+CpG-adjuvanted RBD-J was the least stable in vitro, the formulation was most potent at eliciting SARS-CoV-2 pseudovirus neutralizing antibodies in mice. In contrast, RBD-J formulated with AP+CpG showed minimal antigen-adjuvant interactions, a better stability profile, but suboptimal immune responses. Interestingly, the loss of in vivo potency associated with heat-stressed RBD-J formulated with AH+CpG after one dose was abrogated by a booster. Our findings highlight the importance of elucidating the key interrelationships between antigen-adjuvant interactions, storage stability, and in vivo performance to enable successful formulation development of stable and efficacious subunit vaccines.

Effectiveness of ChAdOx1 nCoV-19 vaccine against SARS-CoV-2 infection during the delta (B.1.617.2) variant surge in India: a test-negative, case-control study and a mechanistic study of post-vaccination immune responses

Background

SARS-CoV-2 variants of concern (VOCs) have threatened COVID-19 vaccine effectiveness. We aimed to assess the effectiveness of the ChAdOx1 nCoV-19 vaccine, predominantly against the delta (B.1.617.2) variant, in addition to the cellular immune response to vaccination.

Methods

We did a test-negative, case-control study at two medical research centres in Faridabad, India. All individuals who had a positive RT-PCR test for SARS-CoV-2 infection between April 1, 2021, and May 31, 2021, were included as cases and individuals who had a negative RT-PCR test were included as controls after matching with cases on calendar week of RT-PCR test. The primary outcome was effectiveness of complete vaccination with the ChAdOx1 nCoV-19 vaccine against laboratory-confirmed SARS-CoV-2 infection. The secondary outcomes were effectiveness of a single dose against SARS-CoV-2 infection and effectiveness of a single dose and complete vaccination against moderate-to-severe disease among infected individuals. Additionally, we tested in-vitro live-virus neutralisation and T-cell immune responses to the spike protein of the wild-type SARS-CoV-2 and VOCs among healthy (anti-nucleocapsid antibody negative) recipients of the ChAdOx1 nCoV-19 vaccine.

Findings

Of 2379 cases of confirmed SARS-CoV-2 infection, 85 (3·6%) were fully vaccinated compared with 168 (8·5%) of 1981 controls (adjusted OR [aOR] 0·37 [95% CI 0·28–0·48]), giving a vaccine effectiveness against SARS-CoV-2 infection of 63·1% (95% CI 51·5–72·1). 157 (6·4%) of 2451 of cases and 181 (9·1%) of 1994) controls had received a single dose of the ChAdOx1 nCoV-19 vaccine (aOR 0·54 [95% CI 0·42–0·68]), thus vaccine effectiveness of a single dose against SARS-CoV-2 infection was 46·2% (95% CI 31·6–57·7). One of 84 cases with moderate-to-severe COVID-19 was fully vaccinated compared with 84 of 2295 cases with mild COVID-19 (aOR 0·19 [95% CI 0·01–0·90]), giving a vaccine effectiveness of complete vaccination against moderate-to-severe disease of 81·5% (95% CI 9·9–99·0). The effectiveness of a single dose against moderate-to-severe disease was 79·2% (95% CI 46·1–94·0); four of 87 individuals with moderate-to-severe COVID-19 had received a single dose compared with 153 of 2364 participants with mild disease (aOR 0·20 [95% CI 0·06–0·54]). Among 49 healthy, fully vaccinated individuals, neutralising antibody responses were lower against the alpha (B.1.1.7; geometric mean titre 244·7 [95% CI 151·8–394·4]), beta (B.1.351; 97·6 [61·2–155·8]), kappa (B.1.617.1; 112·8 [72·7–175·0]), and delta (88·4 [61·2–127·8]) variants than against wild-type SARS-CoV-2 (599·4 [376·9–953·2]). However, the antigen-specific CD4 and CD8 T-cell responses were conserved against both the delta variant and wild-type SARS-CoV-2.

Interpretation

The ChAdOx1 nCoV-19 vaccine remained effective against moderate-to-severe COVID-19, even during a surge that was dominated by the highly transmissible delta variant of SARS-CoV-2. Spike-specific T-cell responses were maintained against the delta variant. Such cellular immune protection might compensate for waning humoral immunity.

Funding

Department of Biotechnology India, Council of Scientific and Industrial Research India, and Fondation Botnar.

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.

Designed SARS‐CoV‐2 receptor binding domain variants form stable monomers

The receptor binding domain (RBD) of the SARS‐CoV‐2 spike (S)‐protein is a prime target of virus‐neutralizing antibodies present in convalescent sera of COVID‐19 patients and thus is considered a key antigen for immunosurveillance studies and vaccine development. Although recombinant expression of RBD has been achieved in several eukaryotic systems, mammalian cells have proven particularly useful. The authors aimed to optimize RBD produced in HEK293‐6E cells towards a stable homogeneous preparation and addressed its O‐glycosylation as well as the unpaired cysteine residue 538 in the widely used RBD (319‐541) sequence. The authors found that an intact O‐glycosylation site at T323 is highly relevant for the expression and maintenance of RBD as a monomer. Furthermore, it was shown that deletion or substitution of the unpaired cysteine residue C538 reduces the intrinsic propensity of RBD to form oligomeric aggregates, concomitant with an increased yield of the monomeric form of the protein. Bead‐based and enzyme‐linked immunosorbent assays utilizing these optimized RBD variants displayed excellent performance with respect to the specific detection of even low levels of SARS‐CoV‐2 antibodies in convalescent sera. Hence, these RBD variants could be instrumental for the further development of serological SARS‐CoV‐2 tests and inform the design of RBD‐based vaccine candidates.

Advances in the design and development of SARS-CoV-2 vaccines

Since the end of 2019, coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread worldwide. The RNA genome of SARS-CoV-2, which is highly infectious and prone to rapid mutation, encodes both structural and nonstructural proteins. Vaccination is currently the only effective method to prevent COVID-19, and structural proteins are critical targets for vaccine development. Currently, many vaccines are in clinical trials or are already on the market. This review highlights ongoing advances in the design of prophylactic or therapeutic vaccines against COVID-19, including viral vector vaccines, DNA vaccines, RNA vaccines, live-attenuated vaccines, inactivated virus vaccines, recombinant protein vaccines and bionic nanoparticle vaccines. In addition to traditional inactivated virus vaccines, some novel vaccines based on viral vectors, nanoscience and synthetic biology also play important roles in combating COVID-19. However, many challenges persist in ongoing clinical trials.

The SARS CoV-2 spike directed non-neutralizing polyclonal antibodies cross-react with Human immunodeficiency virus (HIV-1) gp41

Cross-reactivity among the two diverse viruses is believed to originate from the concept of antibodies recognizing similar epitopes on the two viral surfaces. Cross-reactive antibody responses have been seen in previous variants of SARS and SARS-CoV-2, but little is known about the cross reactivity with other similar RNA viruses like HIV-1. In the present study, we examined the reactivity the SARS-CoV-2 directed antibodies, via spike, immunized mice sera and demonstrated whether they conferred any cross-reactive neutralization against HIV-1. Our findings show that SARS-CoV-2 spike immunized mice antibodies cross-react with the HIV-1 Env protein. Cross-neutralization among the two viruses is uncommon, suggesting the presence of a non-neutralizing antibody response to conserved epitopes amongst the two viruses. Our results indicate, that SARS-CoV-2 spike antibody cross reactivity is targeted towards the gp41 region of the HIV-1 Env (gp160) protein. Overall, our investigation not only answers a crucial question about the understanding of cross-reactive epitopes of antibodies generated in different viral infections, but also provides critical evidence for developing vaccine immunogens and novel treatment strategies with enhanced efficacy capable of recognising diverse pathogens with similar antigenic features.

Non-neutralizing SARS CoV-2 directed polyclonal antibodies demonstrate cross-reactivity with the HA glycans of influenza virus

The spike protein of the SARS-CoV-2 virus is the foremost target for the designing of vaccines and therapeutic antibodies and also acts as a crucial antigen in the assessment of COVID-19 immune responses. The enveloped viruses; such as SARS-CoV-2, Human Immunodeficiency Virus-1 (HIV-1) and influenza, often hijack host-cell glycosylation pathways and influence pathobiology and immune selection. These glycan motifs can lead to either immune evasion or viral neutralization by the production of cross-reactive antibodies that can lead to antibody-dependent enhancement (ADE) of infection. Potential cross-protection from influenza vaccine has also been reported in COVID-19 infected individuals in several epidemiological studies recently; however, the scientific basis for these observations remains elusive. Herein, we show that the anti-SARS-CoV2 antibodies cross-reacts with the Hemagglutinin (HA) protein. This phenomenon is common to both the sera from convalescent SARS-CoV-2 donors and spike immunized mice, although these antibodies were unable to cross-neutralize, suggesting the presence of a non-neutralizing antibody response. Epitope mapping suggests that the cross-reactive antibodies are targeted towards glycan epitopes of the SARS-CoV-2 spike and HA. Overall, our findings address the cross-reactive responses, although non-neutralizing, elicited against RNA viruses and warrant further studies to investigate whether such non-neutralizing antibody responses can contribute to effector functions such as antibody-dependent cellular cytotoxicity (ADCC) or ADE.