Oral subunit SARS-CoV-2 vaccine induces systemic neutralizing IgG, IgA and cellular immune responses and can boost neutralizing antibody responses primed by an injected vaccine

Effect of Fusion to the LTB Carrier Protein on Coronavirus Spike Protein Vaccine Candidates Produced in Maize

Coronaviruses continue to disrupt health and economic productivity worldwide. Porcine epidemic diarrhea virus (PEDV) is a devastating swine disease and SARS-CoV-2 is the latest coronavirus to infect the human population. Both viruses display a similar spike protein on the surface that is a target of vaccine development. Despite the availability of commercial vaccines for both viruses, there is still a high occurrence of infections and a great need for enhanced efficacy and lower costs. We previously produced the PEDV spike protein (S) using transgenic maize, enabling a low-cost supply of the vaccine candidate. In this study, we (1) test orally delivered PEDV vaccine candidates in pigs to optimize the mucosal immune response; (2) generate the SARS-CoV-2 S1 protein in maize; and (3) perform structural characterization of the S1 protein for PEDV and SARS-CoV-2. We demonstrated high expression levels in maize of the S1 subunit of the SARS-CoV-2 spike protein, both with and without fusion to the heat-labile enterotoxin B (LTB) subunit. We found that the LTB fusion protein from both coronaviruses preferentially assembles into higher molecular weight multimers, consistent with the formation of trimers. For PEDV, administering the spike protein fused to LTB to young pigs elicited a higher level of mucosal IgAs compared to maize grain containing the S1 protein alone or controls. This suggests that fusing the coronavirus spike protein with LTB may provide better protection.

An experimental chimeric hepatitis E virus vaccine elicits both local and systemic immune responses

Introduction

The development of a hepatitis E virus (HEV) vaccine is critical, with ORF2 capsid protein as the main target. We previously demonstrated that oral coadministration of recombinant ORF2 with immunomodulatory bacterium-like-particles (IBLP) induces a specific immune response in mice, particularly using IBLP derived from Lacticaseibacillus rhamnosus IBL027 (IBLP027), which was effective in eliciting a local humoral response. IBLP are non-live bacteria with adjuvant and carrier properties, serving as a platform for exposing proteins or antigens fused to LysM (lysine motif) domains, protein modules that bind to cell wall polysaccharides like peptidoglycan.

Materials

We cloned the most immunogenic domain of ORF2 (O2P2) fused to five LysM domains (LysM5O2P2) and displayed this chimeric protein on the surface of IBLP027 to create a prototype vaccine (IBLP027-LysM5O2P2). We evaluated its capacity to induce an immune response in vivo by immunizing mice with three doses of either the experimental vaccine or the chimeric protein alone, using an oral or a combined schedule with subcutaneous priming followed by oral boosting. Control groups received IBLP027. Sera and small intestine fluid were analyzed for humoral response, while Peyer's patches and spleen immune cells were used for ex vivo stimulation with capsid protein to assess cellular response.

Results

The oral scheme failed to elicit an IgG response, but this was overcome by a subcutaneous priming dose followed by oral boosters, which led to increasing IgG titers in the combined scheme. The highest IgG titers were seen in the vaccine prototype group. Most groups produced significantly higher IgA levels in intestinal fluid, especially in those that received the oral scheme. Cellular response studies showed increased tumor necrosis factor (TNF)-α, interferon (IFN)-γ interleukin (IL)-4, and IL-17 levels in groups receiving the chimeric protein via oral or combined schedules.

Conclusion

Further and continuous research is needed to better understand both the needs and expectations of students and supervisors in different academic realities, including in Veterinary Medicine schools, from which the information available on the subject is scarce.

Beyond COVID-19: the promise of next-generation coronavirus vaccines

Coronaviruses (CoVs) have caused three global outbreaks: severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) in 2003, Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012, and SARS-CoV-2 in 2019, with significant mortality and morbidity. The impact of coronavirus disease 2019 (COVID-19) raised serious concerns about the global preparedness for a pandemic. Furthermore, the changing antigenic landscape of SARS-CoV-2 led to new variants with increased transmissibility and immune evasion. Thus, the development of broad-spectrum vaccines against current and future emerging variants of CoVs will be an essential tool in pandemic preparedness. Distinct phylogenetic features within CoVs complicate and limit the process of generating a pan-CoV vaccine capable of targeting the entire Coronaviridae family. In this review, we aim to provide a detailed overview of the features of CoVs, their phylogeny, current vaccines against various CoVs, the efforts in developing broad-spectrum coronavirus vaccines, and the future.

Current Progress, Challenges and Prospects in the Development of COVID-19 Vaccines

The COVID-19 pandemic has resulted in over 772 million confirmed cases, including nearly 7 million deaths, according to the World Health Organization (WHO). Leveraging rapid development, accelerated vaccine approval processes, and large-scale production of various COVID-19 vaccines using different technical platforms, the WHO declared an end to the global health emergency of COVID-19 on May 5, 2023. Current COVID-19 vaccines encompass inactivated, live attenuated, viral vector, protein subunit, nucleic acid (DNA and RNA), and virus-like particle (VLP) vaccines. However, the efficacy of these vaccines is diminishing due to the constant mutation of SARS-CoV-2 and the heightened immune evasion abilities of emerging variants. This review examines the impact of the COVID-19 pandemic, the biological characteristics of the virus, and its diverse variants. Moreover, the review underscores the effectiveness, advantages, and disadvantages of authorized COVID-19 vaccines. Additionally, it analyzes the challenges, strategies, and future prospects of developing a safe, broad-spectrum vaccine that confers sufficient and sustainable immune protection against new variants of SARS-CoV-2. These discussions not only offer insight for the development of next-generation COVID-19 vaccines but also summarize experiences for combating future emerging viruses.

Oral administration of a recombinant modified RBD antigen of SARS-CoV-2 as a possible immunostimulant for the care of COVID-19

BACKGROUND

Developing effective vaccines against SARS-CoV-2 that consider manufacturing limitations, equitable access, and acceptance is necessary for developing platforms to produce antigens that can be efficiently presented for generating neutralizing antibodies and as a model for new vaccines.

RESULTS

This work presents the development of an applicable technology through the oral administration of the SARS-CoV-2 RBD antigen fused with a peptide to improve its antigenic presentation. We focused on the development and production of the recombinant receptor binding domain (RBD) produced in E. coli modified with the addition of amino acids extension designed to improve antigen presentation. The production was carried out in shake flask and bioreactor cultures, obtaining around 200 mg/L of the antigen. The peptide-fused RBD and peptide-free RBD proteins were characterized and compared using SDS-PAGE gel, high-performance chromatography, and circular dichroism. The peptide-fused RBD was formulated in an oil-in-water emulsion for oral mice immunization. The peptide-fused RBD, compared to RBD, induced robust IgG production in mice, capable of recognizing the recombinant RBD in Enzyme-linked immunosorbent assays. In addition, the peptide-fused RBD generated neutralizing antibodies in the sera of the dosed mice. The formulation showed no reactive episodes and no changes in temperature or vomiting.

CONCLUSIONS

Our study demonstrated the effectiveness of the designed peptide added to the RBD to improve antigen immunostimulation by oral administration.

Tomato-made edible COVID-19 vaccine TOMAVAC induces neutralizing IgGs in the blood sera of mice and humans

Plant-based edible vaccines that provide two-layered protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outweigh the currently used parenteral types of vaccines, which predominantly cause a systemic immune response. Here, we engineered and selected a transgenic tomato genotype (TOMAVAC) that stably synthesized an antigenic S1 protein of SARS-CoV-2. Two-course spaced force-feeding of mice with ≈5.4 μg/ml TOMAVAC increased up to 16-fold the synthesis of RBD-specific NAbs in blood serum and the significant induction of S-IgA in intestinal lavage fluid. In a surrogate virus neutralization test, TOMAVAC-induced NAbs had 15-25% viral neutralizing activity. The results suggested early evidence of the immunogenicity and protectivity of TOMAVAC against the coronavirus disease 2019 (COVID-19) infection. Furthermore, we observed a positive trend of statistically significant 1.2-fold (average of +42.28 BAU/ml) weekly increase in NAbs in the volunteers' serum relative to the initial day. No severe side effects were observed, preliminarily supporting the safety of TOMAVAC. With the completion of future large-scale studies, higher-generation TOMAVAC should be a cost-effective, ecologically friendly, and widely applicable novel-generation COVID-19 vaccine, providing two-layered protection against SARS-CoV-2.

Induction of neutralizing antibodies and mucosal IgA through intranasal immunization with the receptor binding domain of SARS-CoV-2 spike protein fused with the type IIb E. coli heat-labile enterotoxin A subunit

The outbreak of SARS-CoV-2 infections had led to the COVID-19 pandemic which has a significant impact on global public health and the economy. The spike (S) protein of SARS-CoV-2 contains the receptor binding domain (RBD) which binds to human angiotensin-converting enzyme 2 receptor. Numerous RBD-based vaccines have been developed and recently focused on the induction of neutralizing antibodies against the immune evasive Omicron BQ.1.1 and XBB.1.5 subvariants. In this preclinical study, we reported the use of a direct fusion of the type IIb Escherichia coli heat-labile enterotoxin A subunit with SARS CoV-2 RBD protein (RBD-LTA) as an intranasal vaccine candidate. The results showed that intranasal immunization with the RBD-LTA fusion protein in BALB/c mice elicited potent neutralizing antibodies against the Wuhan-Hu-1 and several SARS-CoV-2 variants as well as the production of IgA antibodies in bronchoalveolar lavage fluids (BALFs). Furthermore, the heterologous RBD representing the same strains used in the bivalent mRNA vaccine were used as a second-dose RBD-LTA/RBD protein booster after bivalent mRNA vaccination. The results showed that the neutralizing antibody titers elicited by the intranasal bivalent RBD-LTA/RBD protein booster were similar to the intramuscular bivalent mRNA booster, but the RBD-specific IgA titers in sera and BALFs significantly increased. Overall, this preclinical study suggests that the RBD-LTA fusion protein could be a promising candidate as a mucosal booster COVID-19 vaccine.

An updated review on oral protein-based antigen vaccines efficiency and delivery approaches: a special attention to infectious diseases

Various infectious agents affect human health via the oral entrance. The majority of pathogens lack approved vaccines. Oral vaccination is a convenient, safe and cost-effective approach with the potential of provoking mucosal and systemic immunity and maintaining individual satisfaction. However, vaccines should overcome the intricate environment of the gastrointestinal tract (GIT). Oral protein-based antigen vaccines (OPAVs) are easier to administer than injectable vaccines and do not require trained healthcare professionals. Additionally, the risk of needle-related injuries, pain, and discomfort is eliminated. However, OPAVs stability at environmental and GIT conditions should be considered to enhance their stability and facilitate their transport and storage. These vaccines elicit the local immunity, protecting GIT, genital tract and respiratory epithelial surfaces, where numerous pathogens penetrate the body. OPAVs can also be manipulated (such as using specific incorporated ligand and receptors) to elicit targeted immune response. However, low bioavailability of OPAVs necessitates development of proper protein carriers and formulations to enhance their stability and efficacy. There are several strategies to improve their efficacy or protective effects, such as incorporation of adjuvants, enzyme inhibitors, mucoadhesive or penetrating devices and permeation enhancers. Hence, efficient delivery of OPAVs into GIT require proper delivery systems mainly including smart target systems, probiotics, muco-adhesive carriers, lipid- and plant-based delivery systems and nano- and microparticles.

Design, preparation and characterization of octopus-like self-releasing intracellular protein transporter LEB5 based on Escherichia coli heat-labile enterotoxin

Despite the great potential of protein drugs as intracellular therapeutic agents, the unmet challenge in breaking through the cell membrane barrier and delivering them to intracellular targets remains. Therefore, developing safe and effective delivery vehicles is critical for fundamental biomedical research and clinical applications. In this study, we designed an octopus-like self-releasing intracellular protein transporter, the LEB5, based on the heat-labile enterotoxin. This carrier comprises five identical units, each of which has three main components: a linker, a self-releasing enzyme sensitivity loop, and the LTB transport domain. The LEB5 comprises five purified monomers that self-assemble to create a pentamer with ganglioside GM1 binding capacity. The fluorescent protein EGFP was used as a reporter system to identify the LEB5 features. The high-purity fusion protein ELEB monomer was produced from modified bacteria carrying pET24a(+)-eleb recombinant plasmids. EGFP protein could effectively detach from LEB5 by low dosage trypsin, according to electrophoresis analysis. The transmission electron microscopy results indicate that both LEB5 and ELEB5 pentamers exhibit a relatively regularly spherical shape, and the differential scanning calorimetry measurements further suggest that these proteins possess excellent thermal stability. Fluorescence microscopy revealed that LEB5 translocated EGFP into different cell types. Flow cytometry showed cellular differences in the transport capacity of LEB5. According to the confocal microscopy, fluorescence analysis and western blotting data, EGFP was transferred to the endoplasmic reticulum by the LEB5 carrier, detached from LEB5 by cleavage of the enzyme-sensitive loop, and released into the cytoplasm. Within the dosage range of LEB5 10-80 μg/mL, cell counting kit-8 assay revealed no significant changes in cell viability. These results demonstrated that LEB5 is a safe and effective intracellular self-releasing delivery vehicle capable of transporting and releasing protein medicines into cells.

An overview of the vaccine platforms to combat COVID-19 with a focus on the subunit vaccines

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an emerging virus that has caused the recent coronavirus disease (COVID-19) global pandemic. The current approved COVID-19 vaccines have shown considerable efficiency against hospitalization and death. However, the continuation of the pandemic for more than two years and the likelihood of new strain emergence despite the global rollout of vaccination highlight the immediate need for the development and improvement of vaccines. mRNA, viral vector, and inactivated virus vaccine platforms were the first members of the worldwide approved vaccine list. Subunit vaccines. which are vaccines based on synthetic peptides or recombinant proteins, have been used in lower numbers and limited countries. The unavoidable advantages of this platform, including safety and precise immune targeting, make it a promising vaccine with wider global use in the near future. This review article summarizes the current knowledge on different vaccine platforms, focusing on the subunit vaccines and their clinical trial advancements against COVID-19.

Current status of mucosal vaccines against SARS-CoV2: a hope for protective immunity

INTRODUCTION

The current vaccines used to fight against COVID-19 are effective, however the induction of protective immunity is a pending goal required to prevent viral transmission, prevent the generation of new variants, and ultimately eradicate SARS-CoV-2. Mucosal immunization stands as a promising approach to achieve protective immunity against SARS-CoV-2; therefore, it is imperative to innovate the current vaccines by developing mucosal candidates, focusing not only on their ability to prevent severe COVID-19 but to neutralize the virus before invasion of the respiratory system and other mucosal compartments.

AREAS COVERED

This review covers the current advances on the development of anti-COVID-19 mucosal vaccines. Biomedical literature, including PubMed and clinicaltrials.gov website, was analyzed to identify the state of the art for this field. The achievements in preclinical and clinical evaluations are presented and critically analyzed.

EXPERT OPINION

There is a significant advance on the development of mucosal vaccines against SARSCoV-2, which is a promise to increase the efficacy of immunization against this pathogen. Both preclinical and clinical evaluation for several candidates have been performed. The challenges in this road (e.g. low immunogenicity, a reduced number of adjuvants available, and inaccurate dosage) are identified and also critical perspectives for the field are provided.

Plant-Produced Mouse-Specific Zona Pellucida 3 Peptide Induces Immune Responses in Mice

Contraceptive vaccines are designed to stimulate autoimmune responses to molecules involved in the reproductive process. A mouse-specific peptide from zona pellucida 3 (mZP3) has been proposed as a target epitope. Here, we employed a plant expression system for the production of glycosylated mZP3 and evaluated the immunogenicity of plant-produced mZP3-based antigens in a female BALB/c mouse model. In the mZP3-1 antigen, mZP3 fused with a T-cell epitope of tetanus toxoid, a histidine tag, and a SEKDEL sequence. A fusion antigen (GFP-mZP3-1) and a polypeptide antigen containing three repeats of mZP3 (mZP3-3) were also examined. Glycosylation of mZP3 should be achieved by targeting proteins to the endoplasmic reticulum. Agrobacterium-mediated transient expression of antigens resulted in successful production of mZP3 in Nicotiana benthamiana. Compared with mZP3-1, GFP-mZP3-1 and mZP3-3 increased the production of the mZP3 peptide by more than 20 and 25 times, respectively. The glycosylation of the proteins was indicated by their size and their binding to a carbohydrate-binding protein. Both plant-produced GFP-mZP3-1 and mZP3-3 antigens were immunogenic in mice; however, mZP3-3 generated significantly higher levels of serum antibodies against mZP3. Induced antibodies recognized native zona pellucida of wild mouse, and specific binding of antibodies to the oocytes was observed in immunohistochemical studies. Therefore, these preliminary results indicated that the plants can be an efficient system for the production of immunogenic mZP3 peptide, which may affect the fertility of wild mice.

Mucosal vaccination: onward and upward

INTRODUCTION

The unique mucosal immune system allows the generation of robust protective immune responses at the front line of pathogen encounters. The needle-free delivery route and cold chain-free logistic requirements also provide additional advantages in ease and economy. However, the development of mucosal vaccines faces several challenges, and only a handful of mucosal vaccines are currently licensed. These vaccines are all in the form of live attenuated or inactivated whole organisms, whereas no subunit-based mucosal vaccine is available.

AREAS COVERED

The selection of antigen, delivery vehicle, route and adjuvants for mucosal vaccination are highly important. This is particularly crucial for subunit vaccines, as they often fail to elicit strong immune responses. Emerging research is providing new insights into the biological and immunological uniqueness of mucosal tissues. However, many aspects of the mucosal immunology still await to be investigated.

EXPERT OPINION

This article provides an overview of the current understanding of mucosal vaccination and discusses the remaining knowledge gaps. We emphasize that because of the potential benefits mucosal vaccines can bring from the biomedical, social and economic standpoints, the unmet goal to achieve mucosal vaccine success is worth the effort.

Heterologous SARS-CoV-2 IgA neutralising antibody responses in convalescent plasma

Objectives

Following infection with SARS-CoV-2, virus-specific antibodies are generated, which can both neutralise virions and clear infection via Fc effector functions. The importance of IgG antibodies for protection and control of SARS-CoV-2 has been extensively reported. By comparison, other antibody isotypes including IgA have been poorly characterised.

Methods

Here, we characterised plasma IgA from 41 early convalescent COVID-19 subjects for neutralisation and Fc effector functions.

Results

Convalescent plasma IgA from > 60% of the cohort had the capacity to inhibit the interaction between wild-type RBD and ACE2. Furthermore, a third of the cohort induced stronger IgA-mediated ACE2 inhibition than matched IgG when tested at equivalent concentrations. Plasma IgA and IgG from this cohort broadly recognised similar RBD epitopes and had similar capacities to inhibit ACE2 from binding to 22 of the 23 prevalent RBD mutations assessed. However, plasma IgA was largely incapable of mediating antibody-dependent phagocytosis in comparison with plasma IgG.

Conclusion

Overall, convalescent plasma IgA contributed to the neutralising antibody response of wild-type SARS-CoV-2 RBD and various RBD mutations. However, this response displayed large heterogeneity and was less potent than IgG.

An Algae-Made RBD from SARS-CoV-2 Is Immunogenic in Mice

Despite the current advances in global vaccination against SARS-CoV-2, boosting is still required to sustain immunity in the population, and the induction of sterilizing immunity remains as a pending goal. Low-cost oral immunogens could be used as the basis for the design of affordable and easy-to-administer booster vaccines. Algae stand as promising platforms to produce immunogens at low cost, and it is possible to use them as oral delivery carriers since they are edible (not requiring complex purification and formulation processes). Herein, a Chlamydomonas-made SARS-CoV-2 RBD was evaluated as an oral immunogen in mice to explore the feasibility of developing an oral algae-based vaccine. The test immunogen was stable in freeze-dried algae biomass and able to induce, by the oral route, systemic and mucosal humoral responses against the spike protein at a similar magnitude to those induced by injected antigen plus alum adjuvant. IgG subclass analysis revealed a Th2-bias response which lasted over 4 months after the last immunization. The induced antibodies showed a similar reactivity against either Delta or Omicron variants. This study represents a step forward in the development of oral vaccines that could accelerate massive immunization.

Evaluation of the Immune Response Afforded by Combined Immunization with Orf Virus DNA and Subunit Vaccine in Mice

Contagious ecthyma (Orf) is a highly contagious disease caused by Orf virus (ORFV) infection. Orf is prevalent all over the world and, not only affects the healthy development of sheep husbandry, but also threatens human health. However, there are no safe and effective vaccines or drugs for the prevention and treatment of Orf at present. In this study, we constructed a DNA plasmid expressing ORFV B2L and F1L genes as a DNA vaccine candidate, with purified B2L full-length protein and F1L truncated protein as subunit vaccine candidates. BALB/c mice were immunized with the DNA vaccine, subunit vaccine, as well as DNA prime-protein boost strategies. The results showed that compared with the DNA vaccine and subunit vaccine alone, the DNA prime-protein boost immunization group had a higher level of specific antibodies, stronger lymphocyte proliferation, and higher expression of cytokines such as IL-2, IL-4, IL-6, IFN-γ, and TNF-α, which are considered to cause a Th1/Th2 mixed cytokine response. Our results demonstrated that the DNA prime-protein boost immunization strategy induced stronger humoral and cellular immune responses, which have potential advantages in preventing ORFV infection.

Gastrointestinal Involvement in SARS-CoV-2 Infection

SARS-CoV-2 has evolved into a virus that primarily results in mild or asymptomatic disease, making its transmission more challenging to control. In addition to the respiratory tract, SARS-CoV-2 also infects the digestive tract. Some gastrointestinal symptoms occur with or before respiratory symptoms in patients with COVID-19. Respiratory infections are known to cause intestinal immune impairment and gastrointestinal symptoms. When the intestine is inflamed, cytokines affect the lung immune response and inflammation through blood circulation. The gastrointestinal microbiome may be a modifiable factor in determining the risk of SARS-CoV-2 infection and disease severity. The development of oral SARS-CoV-2 vaccine candidates and the maintenance of gut microbiota profiles may contribute to the early control of COVID-19 outbreaks. To this end, this review summarizes information on the gastrointestinal complications caused by SARS-CoV-2, SARS-CoV-2 infection, the gastrointestinal–lung axis immune response, potential control strategies for oral vaccine candidates and maintaining intestinal microbiota homeostasis.