Part 4: Application of mRNA Vaccine

 

Anti-COVID-19 mRNA vaccine

Like other human coronaviruses including SARS and Middle East Respiratory Syndrome, SARS-CoV-2 is an enveloped, positive-stranded single-stranded RNA virus
.

Its genomic RNA encodes non-structural polyproteins and structural proteins, including spines, crown (S), envelope (E), membrane (M) and nucleocapsid (N) proteins

The S glycoprotein has a total length of 1273 amino acids and is composed of an N-terminal signal peptide, an extracellular domain, a transmembrane domain, and an intracellular domain
.

It is functionally divided into S1 and S2 subunits (Figure 1A)

The SARS-CoV-2 S trimer structure in the fusion conformation shows that after a large number of structural rearrangements, HR1 forms an unusually long central triple helix with the central helix (CH) (Figure 1C)
.

In addition, it has been reported that the SARS-CoV-2 S protein spontaneously transitions to the post-fusion state and has nothing to do with the target cell

The high instability of S protein in the pre-fusion conformation is undoubtedly the main obstacle to the development of basic vaccines
.

Fortunately, the introduction of two consecutive proline residues (2P) at the beginning of the CH is a general strategy to maintain the conformation of the β-coronavirus S protein before fusion

In addition, the cryo-EM structure of the SARS-CoV-2 S-2P mutant showed that the 2P substitution did not change the conformation of the S protein (Figure 1D)
.

At the same time, various structures such as RBD-hACE2 and RBD-monoclonal antibody are also reported

 

Figure 1 COVID-19 mRNA vaccine antigen

A: Schematic diagram of the full-length SARS-CoV-2S primary color structure
.

SS, signal peptide; NTD, N-terminal domain; RBD, receptor binding domain; CTD, C-terminal domain; FP, fusion peptide; HR1, heptapeptide repeat 1; CH, central helix; CD, connector domain ; HR2, heptapeptide repeat 2; TM, transmembrane domain; CT, cytoplasmic tail

The mRNA vaccine contains the S protein coding region, with optimized 5'-and 3'-UTR and polyA tails on both sides, synthesized by IVT, and then capped with a 5'-cap mimic at the 5'end, and encapsulated with LNP.
IM injection (intramuscular injection) (Figure 2 step 1)
.

Such as dendritic cells or macrophages, through endocytosis (step 2)

 

Figure 2 Delivery and working mechanism of COVID-19 mRNA vaccine

 

With its versatility and rapid development advantages, two COVID-19 mRNA vaccines (mRNA-1273 and BNT162b2) have been approved for marketing, one drug candidate is undergoing phase III clinical trials, and the other three drug candidates are currently undergoing phase I Or Phase II clinical evaluation
.

Table 1 lists all COVID-19 mRNA vaccines or candidate vaccines in clinical trials based on published preclinical experiments or clinical trial data, and summarizes their safety, neutralizing antibody response and protective effects

 

Table 1 Clinical application of COVID-19 mRNA vaccine

 

BNT162b2 is a lipid nanoparticle preparation, a nucleoside modified mRNA vaccine, used to prevent new coronavirus 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection
.
The expression of the SARS-CoV-2 S protein encoded by BNT162b2 can induce the receptor's immune response to the antigen
.
BNT162b2 was administered intramuscularly in double doses
.
The protection rate can reach 95%
.

BNT162b2 needs to be refrigerated (this is a logistical challenge for vaccine distribution) and must be thawed and diluted before use
.
After dilution, each bottle contains 6 30μg doses
.
The intramuscular injection of BNT162b2 is divided into two courses, 30μg each time (injection into the deltoid muscle under ideal conditions), and an interval of 21 days is recommended
.
Seven days after the second injection, COVID-19 may not be completely prevented
.
Like other vaccines, BNT162b2 may not protect every recipient
.

 

 

Mechanism

BNT162b2 is composed of nucleoside-modified mRNA, wrapped in lipid nanoparticles, encoding a membrane-anchored SARS-CoV-19 full-length S protein, and contains a mutation that stabilizes the S protein in an antigen-preferred pre-fusion conformation
.
Lipid nanoparticles protect non-replicating RNA from degradation and allow it to enter host cells after intramuscular injection
.
Once in the host cell, the mRNA is translated into SARS-CoV-2 S protein, which is expressed on the surface of the host cell and induces neutralizing antibodies and cellular immune responses (Figure 3)
.

 

Figure 3 The mechanism of action of intramuscular injection of BNT162b2

 

Although mRNA vaccines have shown clear advantages in the past few years, it was not until the SARS-CoV-2 pandemic significantly accelerated and accelerated clinical trials and reviews that humans approved the first mRNA vaccine
.
This is undoubtedly a milestone in the history of vaccination
.
If successful, mRNA-based vaccines may become an immediate “standard” solution for future pandemics, but they may also replace some traditional protein-based live attenuated vaccines
.
The mRNA platform may also be superior to other platforms, and can be modified and distributed the fastest to combat new mutant virus strains that emerge during a pandemic
.

Based on the major technological innovations and advancements of the mRNA vaccine platform in the past 10 years, the new coronary pneumonia mRNA vaccine has been successfully developed at an unprecedented speed
.
In clinical trials, compared with protein subunit vaccines and inactivated virus vaccines, the COVID-19 mRNA vaccine has a higher incidence of systemic adverse events such as fever and fatigue
.
Therefore, it is necessary to carry out long-term monitoring of the safety of the COVID-19 mRNA vaccine
.
Most mRNA vaccines are designed to produce neutralizing IgG antibodies, which can only effectively protect the lower respiratory tract through muscle immunity
.
However, IgA antibodies may be necessary for disinfection and immunity.
Its main role is to protect the upper respiratory tract.
Current clinical trials have not yet determined the level of IgA antibodies induced by mRNA vaccines
.
In addition, although the results of the mRNA-1273 study showed that after the first human vaccination, the high-level and antibody titers lasted at least 4 months, but the time for these mRNA vaccines to protect humans against COVID-19 needs further clarification
.

On the other hand, data is still needed to evaluate whether mRNA vaccines are suitable for everyone, including children, the elderly, immunosuppressed individuals, and patients with chronic diseases such as autoimmune diseases
.
Compatibility with different drugs also needs to be evaluated
.
Will the type I IFN response induced by mRNA vaccine become a problem for patients with various underlying diseases or type I IFN treatment? Some of these types of investigations have been started or are being planned
.
During the distribution of the approved SARS-CoV-2 mRNA vaccine, other issues with the mRNA vaccine caused great concern in the health care system, including cold chain and storage restrictions
.
Even in high-income countries, many clinics and vaccination sites cannot use low-temperature refrigerators to meet the needs of certain mRNA vaccines
.
This challenge will become more prominent in low-income countries
.
Therefore, there is an urgent need to improve and verify these issues
.

 

 

Other clinical applications of mRNA vaccines

Since Wolf et al.
demonstrated that proteins can be produced from in vitro transcribed mRNA in living tissues, mRNA vaccines have proven their effectiveness in many applications
.
The first clinical trial based on RNA pulse DC cancer vaccine using mRNA technology can be traced back to 2003
.
Today, more than 140 clinical trials can be found using mRNA to deal with different conditions, such as cancer or infectious diseases (Figure 4)
.

 

Figure 4 The breakdown of mRNA vaccine clinical trials submitted by disease type (left) and drug delivery system (right) each year

 

Cancer is currently the preferred target of mRNA technology
.
More than 50% of clinical trials focus on the treatment of melanoma, prostate cancer and brain cancer (Figure 5), and most trials are still in the early stages (I and II)
.
In addition to safety and immune response, the lack of standards for cancer treatment hinders the assessment of vaccine effectiveness
.
However, this is not the case for infectious diseases, because many traditional vaccines can be used as benchmarks for validating new mRNA vaccines
.
mRNA also shows other potential, not only for the treatment of cancer, but also as a therapeutic agent for protein expression in many other diseases, such as cardiovascular disease and type II diabetes
.

 

Figure 5 Application of mRNA vaccine

 

Tables 2 and 3 summarize the clinical application of mRNA vaccines to cancer and infectious diseases
.

 

Table 2 mRNA vaccine anti-tumor clinical trials

 

Table 3 Clinical trials of mRNA vaccines against viral diseases

 

 

Overview of the development of mRNA vaccines: part one

Overview of the development of mRNA vaccines: part two

Overview of the development of mRNA vaccines: part three

Overview of the development of mRNA vaccines: part four

Overview of the development of mRNA vaccines: Part 5

 

 

 

 

(Source: Internet, for reference only)

---