June 30, 2022

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Immunology: Design strategy for COVID-19 vaccines

Immunology: Design strategy for COVID-19 vaccines



Immunology: Design strategy for COVID-19 vaccines. The first outbreak of the coronavirus SARS-Cov-2 was in Wuhan, China at the end of 2019. It has now spread to 216 countries. The outbreak of COVID-19 has caused the world to stagnate. Medical staff, the elderly, and people with underlying health conditions face extreme risks in this environment.

In response to this emergency, a vaccine is urgently needed, which has reduced the vaccine development time that originally took 10-15 years to 1-2 years. However, people lack a clear understanding of what may happen next, how to construct a safe and effective COVID-19 vaccine, how to define the end of success in the vaccine testing process, and what challenges we face in the future? This article will give a brief overview.


★ Natural immunity and vaccine-induced immunity

Due to the body’s immune evasion strategy against viruses, vaccine-induced immunity is different from natural immunity. This is why the elderly are more susceptible to SARS-Cov-2 than young people.

● Innate immune response

The latest data show that the immune response of SARS-Cov-2 to the virus is similar to SARS and MERS-Cov in many respects. They will first suppress the innate immune system response, including dendritic cells, and inhibit anti-virus type 1 and type 2 interference. Prime response. It can be explained as the incubation period of SRAS-Cov-2. The incubation period of 2-12 days for SRAS-Cov-2 is equivalent to 1-2 days for influenza.

In the early stage of SRAS-Cov-2 infection, if it is not controlled, it may lead to high viral burden, dysregulation, potentially fatal inflammation, and immune diseases including acute respiratory distress syndrome. Recently, the concept of “training immunity as a potential COVID-19 vaccine strategy” was also proposed in the early stages of infection.

The most researched BCG vaccine against tuberculosis. After vaccination, the performance of the bone marrow pre-cells in the bone marrow is genetically and metabolically reorganized, so that circulating monocytes have training immune characteristics, and these monocytes enhance the resistance to heterologous infections (including respiratory infections). protection of. The number of BCG vaccination is negatively related to the number of COVID-19 deaths.


● Antibody reaction

SARA-CoV and SARS-CoV-2 have 88% homology to the S protein, and both bind to ACE2 with high affinity. Therefore, monoclonal antibodies and polyclonal antibodies to the S protein of SARS-CoV can neutralize the new coronavirus. Studies have shown that the amount of neutralizing antibodies is positively correlated with the severity of the disease, but the antibodies disappear after a few weeks of infection. Therefore, it is not clear whether the titers of neutralizing antibodies are sufficient to prevent infection, so establishing such a correlation is essential to guide the development of effective COVID-19 vaccines.


● T cell-mediated immunity

A study found that 100% of patients recovering from COVID-19 have S protein-specific CD4+ T cells in their circulatory system, and 70% of their cells have S protein-specific CD8+ T cells in their circulation; It can be seen from the above that T cell-mediated immunity is more reliable than antibody titers, so T cell-mediated methods can be considered in vaccine design.


● Pre-existing cross-reactive immunity

Studies have shown that there are four human coronaviruses (229E, NI63, OC43 and Hku1) that account for 15% of human common colds. Adults will be infected every 2-3 years. Among these antibodies, there may be some resistance to SARS-CoV-2. Antigen cross-reactive immunity. Therefore, considering the enhancement of the cross-reaction of the COVID-19 vaccine, it is also very important for the protective immunity induced by the vaccine.


● Increased dependence of disease on antibodies

The safety of a vaccine generally depends on the nature of the vaccine platform, the choice of adjuvant, the management mode of the vaccine, the route of vaccination, the age of vaccination and the existing vaccine immunity status. At present, it is not clear about the correlation between antibody titers and the severity of infection. How to define the titers of protective neutralizing antibodies to ensure that the COVID-19 vaccine can reach these titers and avoid frequent booster immunizations to reduce the antibodies The level of neutralization is essential to minimize the possibility of ADE. Therefore, adding neutralizing antibodies and powerful T cell-mediated immune induction in the vaccine design may reduce the risk of ADE.


★ Vaccine Design

Vaccine design involves the choice of antigen, vaccine platform, and vaccination route and plan.

● Select SARS-Cov-2 antigen

Infecting viruses, there are affinity structures including S protein, N protein, matrix (M) protein and envelope protein (E). Since the N protein wraps the large positive-strand RNA genome, the positive-strand RNA is wrapped in the lipid envelope of the host cell membrane, and the other three proteins (S, M, and E) are also inserted into it. In this case, SARS-CoV-2 has proved that only antibodies directed to the S protein can neutralize the virus and prevent infection.


● Vaccine Platform

Generally speaking, vaccine platforms are divided into six categories: live attenuated viruses, recombinant viral vectors (expressing target pathogen antibodies in vivo through bioengineering), inactivated viruses, protein subunits, virus-like particles (VLPs) and nucleic acid substrates (DNA or RNA). Broadly speaking, vaccines need two components: antigens of the target pathogen or generated by the vaccine recipient, and infection signals (such as pathogen-related molecular patterns or destruction of related molecular patterns) to alert and activate the host’s immune system.
Generally, non-viral vaccines require multiple vaccinations to induce protective immunity, while virus-based live vaccines have the ability to provide “one time” immunity.


Immunology: Design strategy for COVID-19 vaccinesFigure 1 | Global CoVID-19 vaccine field


The six main candidate types (live attenuated virus, recombinant virus vector, inactivated virus, protein subunit, virus-like particle and nucleic acid-based) of vaccines against the 2019 coronavirus disease (coronavirus-19) show that the current The number of ongoing vaccine candidates. Nucleic acid-based platforms include: mRNA vaccines (6 clinical vaccines and 16 preclinical vaccines) and plasmid DNA vaccines (4 clinical vaccines and data obtained from the literature)


● Vaccination route and plan

The best treatment opportunity for SARS-CoV-2 control and clearance is in the asymptomatic or early stage of COVID-19 (2-14 days), which may require all immune components to be in the mucosa before the virus enters. Inactivated viruses, protein subunits and nucleic acid vaccines cannot be injected through the respiratory mucosal route because they require potentially unsafe immune adjuvants and repeated administration. In contrast, recombinant viral vector vaccines, especially those using human serotype 5 adenovirus (Ad5) or chimpanzee-derived adenovirus (ChAd), are safe and effective for inoculation to the respiratory mucosa.

● Major COVID-19 vaccine candidates

As of July 31, 2020, there are 27 COVID-19 vaccine candidates in clinical evaluation and 139 vaccines in the preclinical development stage. Traditionally, the safety, immunogenicity, and protective effects of experimental vaccines have been rigorously evaluated and established in animal models before clinical trials. However, in the context of pandemic vaccine development, the development of preclinical and clinical stage vaccines has been compressed and moved forward in parallel.
Table 1 | Immune characteristics of CoVID-19 main candidate vaccine platform


● Live attenuated virus vaccine

The virus is rationally designed to attenuate disease strains through targeted mutation or deletion of virulence genes, so that they can replicate to a limited extent in the host cell, but lose the ability to cause disease. Coronavirus has several genes that are not needed for replication, and they can be deleted, causing its toxicity in the body to attenuate. Deleting various non-structural proteins, as well as structural E proteins, has been used as a strategy to design vaccine strains for some zoonotic coronaviruses.

Another method of virus attenuation is called codon deoptimization, which is to modify the nucleic acid sequence to use suboptimal codons to encode the wild-type amino acid sequence, which greatly slows down the translation of viral proteins during infection. Currently, the vaccines developed by this method by Mehmet Ali Aydinlar University in Turkey, Codagenix and Serum Institute of India, India Immunology Co., Ltd. and Griffith University are in preclinical development.


● Recombinant virus vector vaccine

Recombinant viral vector vaccines are built on a replication-deficient virus backbone or an attenuated virus backbone with replication capability, which is bioengineered to express antigens derived from target pathogens. At present, this vaccine has been approved for human infection control, such as Ebola. The vaccine has a good track record of infectious diseases and cancer. In view of its heredity, safety and induction of strong T cell response, it is designed No adjuvant counseling is required.

Therefore, recombinant viral vectors are the most common platform for the second COVID-19 vaccine development. At present, there are 3 candidate vaccines in clinical phase III (Table 2 Ad5-nCoV developed by CanSino Biosciences, Oxford University and AstraZeneca jointly developed ChAdOx1 nCov-19 (AZD-1222) COVID- developed by Merck with VSV-S 19 vaccines), 38 vaccines are in the preclinical development stage. Viral vectors with replication capabilities are mainly based on vaccine strains of other human pathogens (such as measles or influenza virus) or veterinary pathogens (vesicular stomatitis virus (VSV)).

However, it is important to consider whether humans have pre-existing immunity to the virus backbone (Table 1). Pre-existing antibodies may weaken the ability of such vaccines to function with the immune system. Using a virus backbone like ChAd (humans have almost no pre-existing immunity to ChAd) can avoid this problem.


● Inactivated virus vaccine

Utilizing perfect infrastructure and methods, inactivated viruses can be rapidly produced and scaled up under pandemic conditions. Unlike live attenuated vaccines, inactivated virus vaccines have few safety issues, and they can express viral antigens extensively, but at the same time retain surface antigens in epitope conformations, which can cause conformation-dependent antibody responses . Currently, there are 5 early clinical trials for evaluating the SARS-CoV-2 inactivated vaccine (Table 2), and 9 other vaccine candidates are in preclinical development.

Inactivated virus vaccines usually require adjuvants and repeated injections to be effective. As an adjuvant, alum is not suitable for respiratory mucosal administration. The cytotoxicity of inactivated virus vaccines is a poor inducer for CD8+ T cells, and studies on inactivated SARS-CoV and respiratory syncytial virus vaccines have reported that the vaccines lead to related disease enhancement, which may involve TH2 cell response and pulmonary eosinophil Increase, which may worsen in elderly hosts, so other adjuvants such as CpG can be used to avoid this problem.


● Protein subunit vaccine

There are currently 7 COVID-19 subunit vaccines in clinical trials (Table 2), and the other 50 candidate vaccines are in the preclinical development stage, so protein subunit vaccines are the most common platform. Subunit vaccines mainly induce CD4+TH cell and antibody responses. Therefore, most of these vaccines contain complete SARS-CoV-2 protein or part of the protein, with the purpose of inducing neutralizing antibodies, which is similar to most SARS and MERS vaccines.

They have different levels of efficacy. Subunit vaccines can be designed to focus the immune response on neutralizing epitopes, thereby avoiding the production of non-neutralizing antibodies that may promote disease development. However, unlike vaccines based on nucleic acids or viral vectors, recombinant vaccines.

Unless it is produced in mammalian cells, the S protein in subunit vaccines may have incorrect heterogeneity. Simple proteins or polypeptides are poorly immunogenic, and usually require not only adjuvants, but also repeated administration. They are also poor activators of CD8+ T cell responses, and this platform is not suitable for respiratory mucosal vaccination and vaccination. Like inactivated virus vaccines, the use of unmodified alum as an adjuvant will distort the immune response to the TH2 cell-like response, which is detrimental to the host’s defense against SARS-CoV-2 and may have ADE effects.


●Virus-like particles

VLPs are highly structured protein particles assembled from single or multiple structural proteins of the virus. They are similar in morphology and structure to natural virus particles and have strong immunogenicity and biological activity. In the case of the coronavirus envelope, when the viral proteins S, M, and E (with or without N) are co-expressed in eukaryotic cells, VLPs are formed.

This results in VLPs producing cells that are structurally identical to infectious viruses, but lack viral nucleic acid and therefore have no ability to multiply. The presence of S protein on the surface of VLPs allows it to bind to and enter ACE2+ cells in the same way as the parent virus.

Unlike subunit vaccines, the S protein array on the surface of VLPs cross-links B cell receptors and directly activates B cells, but just like subunit and inactivated virus vaccines, VLPs usually require adjuvants and repeated administration. Nevertheless, VLP technology is very mature, and it is relatively simple to understand the biology and safety of coronavirus VLPs, and its large-scale production meets good manufacturing standards. Currently, only one VLP-based COVID-19 vaccine is in clinical trials (Table 2), and there are 12 more in pre-clinical development.


● Nucleic acid vaccine

Currently, there are 6 mRNA-based COVID-19 vaccines and 4 DNA vaccines in clinical trials (Table 2), and 27 vaccines (16 mRNA-based vaccines and 11 DNA-based vaccines) are in the preclinical development stage.

The antigen-encoding mRNA compounded with lipid nanoparticles and other carriers can be effectively delivered to the cytoplasm of host cells in vivo for protein translation and post-translational modification, so it has more advantages than recombinant protein subunit vaccines.

The mRNA vaccine is non-infectious, is synthesized through in vitro transcription, and does not contain microbial molecules. These beneficial features distinguish mRNA vaccines from live attenuated vaccines, inactivated virus vaccines, subunit vaccines and recombinant viral vector vaccines in terms of safety, effectiveness and antiviral immunity, enabling them to be produced quickly and cheaply And repeated vaccination.

American Moderna Biotechnology Company has experience in mRNA-based MERS vaccines. mRNA-1273 is an ideal stable vaccine that encodes SARS-CoV-2 protein encapsulated in lipid nanoparticles.

Pfizer and BioNTech are also evaluating the encoded mRNA-lipid nanoparticle vaccine for human S protein RBD (called BNT162b1). After two repeated intravenous injections, it produced powerful S protein specific antibodies and CD4+ and CD8+ T cell responses. . Plasmid DNA vaccines and mRNA vaccines have some common characteristics, including safety, ease of production and scalability, but their immunogenicity is poor, requiring multiple doses and adjuvants, and two clinical studies have shown that mRNA There are differences in the scale and life span of the vaccine-induced immune response.

Therefore, although mRNA-based COVID-19 vaccines have shown promise in early clinical trials, their protective effects on humans are still questionable. It is still unclear whether mRNA vaccines are suitable for respiratory mucosal transmission.


★Conclusions and prospects

The non-injectable COVID-19 vaccine strategy can induce a strong, long-lasting neutralizing antibody and T cell response and provide a significant level of protection. Almost all vaccines in the current human immunization program are vaccinated through the skin or muscle, and current COVID-19 vaccine strategies mostly focus on the parenteral route of vaccination.

We further speculate that the respiratory mucosal vaccine strategy that can directly induce these reactions in the respiratory mucosa will be the most effective in the early control or elimination of SARS-CoV-2, which is particularly relevant to the high-risk elderly population. The respiratory mucosal vaccine strategy for COVID-19 can learn from the successful experience of influenza, measles, pneumonia and other respiratory mucosal administration.

The advantage of respiratory mucosal vaccination is that it does not require injection, and the dose is much smaller than the injection route. However, compared with the injection route, there are fewer platforms for safe and effective respiratory mucosal vaccination. In addition, the use of inhalation devices for airway mucosal delivery may be a potential limiting factor.


(source:internet, reference only | Immunology: Design strategy for COVID-19 vaccines)


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