MRNA vaccine
An mRNA 'vaccine' is a type of vaccine that uses a copy of a molecule called messenger RNA to produce an immune response. The vaccine delivers molecules of antigen-encoding mRNA into cells, which use the designed mRNA as a blueprint to build foreign protein that would normally be produced by a pathogen or by a cancer cell. These protein molecules stimulate an adaptive immune response that teaches the body to identify and destroy the corresponding pathogen or cancer cells. The mRNA is delivered by a co-formulation of the RNA encapsulated in lipid nanoparticles that protect the RNA strands and help their absorption into the cells.
Reactogenicity, the tendency of a vaccine to produce adverse reactions, is similar to that of conventional non-RNA vaccines. People susceptible to an autoimmune response may have an adverse reaction to messenger RNA vaccines. The advantages of mRNA vaccines over traditional vaccines are ease of design, speed and lower cost of production, the induction of both cellular and humoral immunity, and lack of interaction with the genomic DNA. While some messenger RNA vaccines, such as the Pfizer–BioNTech COVID-19 vaccine, have the disadvantage of requiring ultracold storage before distribution, other mRNA vaccines, such as the Moderna vaccine, do not have such requirements.
In RNA therapeutics, messenger RNA vaccines have attracted considerable interest as COVID-19 vaccines. In December 2020, Pfizer–BioNTech and Moderna obtained authorization for their mRNA-based COVID-19 vaccines. On 2 December, the UK Medicines and Healthcare products Regulatory Agency became the first medicines regulator to approve an mRNA vaccine, authorizing the Pfizer–BioNTech vaccine for widespread use. On 11 December, the US Food and Drug Administration issued an emergency use authorization for the Pfizer–BioNTech vaccine and a week later similarly authorized the Moderna vaccine. In 2023 the Nobel Prize in Physiology or Medicine was awarded to Katalin Karikó and Drew Weissman for their discoveries concerning modified nucleosides that enabled the development of effective mRNA vaccines against COVID-19.
History
Early research
The first successful transfection of designed mRNA packaged within a liposomal nanoparticle into a cell was published in 1989. "Naked" lab-made mRNA was injected a year later into the muscle of mice. These studies were the first evidence that in vitro transcribed mRNA with a chosen gene was able to deliver the genetic information to produce a desired protein within living cell tissue and led to the concept proposal of messenger RNA vaccines.Liposome-encapsulated mRNA encoding a viral antigen was shown in 1993 to stimulate T cells in mice. The following year self-amplifying mRNA was developed by including both a viral antigen and replicase encoding gene. The method was used in mice to elicit both a humoral and cellular immune response against a viral pathogen. The next year mRNA encoding a tumor antigen was shown to elicit a similar immune response against cancer cells in mice.
Development
The first human clinical trial using ex vivo dendritic cells transfected with mRNA encoding tumor antigens was started in 2001. Four years later, the successful use of modified nucleosides as a method to transport mRNA inside cells without setting off the body's defense system was reported. Clinical trial results of an mRNA vaccine directly injected into the body against cancer cells were reported in 2008.BioNTech in 2008, and Moderna in 2010, were founded to develop mRNA biotechnologies. The US research agency DARPA launched at this time the biotechnology research program ADEPT to develop emerging technologies for the US military. The agency recognized the potential of nucleic acid technology for defense against pandemics and began to invest in the field. DARPA grants were seen as a vote of confidence that in turn encouraged other government agencies and private investors to invest in mRNA technology. DARPA awarded at the time a $25 million grant to Moderna.
The first human clinical trials using an mRNA vaccine against an infectious agent began in 2013. Over the next few years, clinical trials of mRNA vaccines for a number of other viruses were started. mRNA vaccines for human use were studied for infectious agents such as influenza, Zika virus, cytomegalovirus, and Chikungunya virus.
Acceleration
The COVID-19 pandemic, and sequencing of the causative virus SARS-CoV-2 at the beginning of 2020, led to the rapid development of the first approved mRNA vaccines. BioNTech and Moderna in December of the same year obtained approval for their mRNA-based COVID-19 vaccines. On 2 December, seven days after its final eight-week trial, the UK Medicines and Healthcare products Regulatory Agency became the first global medicines regulator in history to approve an mRNA vaccine, granting emergency authorization for Pfizer–BioNTech's BNT162b2 COVID-19 vaccine for widespread use. On 11 December, the FDA gave emergency use authorization for the Pfizer–BioNTech COVID-19 vaccine and a week later similar approval for the Moderna COVID-19 vaccine.Mechanism
The goal of a vaccine is to stimulate the adaptive immune system to create antibodies that precisely target that particular pathogen. The markers on the pathogen that the antibodies target are called antigens.Traditional vaccines stimulate an antibody response by injecting either antigens, an attenuated virus, an inactivated virus, or a recombinant antigen-encoding viral vector into the body. These antigens and viruses are prepared and grown outside the body.
In contrast, mRNA vaccines introduce a short-lived synthetically created fragment of the RNA sequence of a virus into the individual being vaccinated. These mRNA fragments are taken up by dendritic cells through phagocytosis. The dendritic cells use their internal machinery to read the mRNA and produce the viral antigens that the mRNA encodes. The body degrades the mRNA fragments within a few days of introduction. Although non-immune cells can potentially also absorb vaccine mRNA, produce antigens, and display the antigens on their surfaces, dendritic cells absorb the mRNA globules much more readily. The mRNA fragments are translated in the cytoplasm and do not affect the body's genomic DNA, located separately in the cell nucleus.
Once the viral antigens are produced by the host cell, the normal adaptive immune system processes are followed. Antigens are broken down by proteasomes. Class I and class II MHC molecules then attach to the antigen and transport it to the cellular membrane, "activating" the dendritic cell. Once activated, dendritic cells migrate to lymph nodes, where they present the antigen to T cells and B cells. This triggers the production of antibodies specifically targeted to the antigen, ultimately resulting in immunity.
mRNA
The central component of a mRNA vaccine is its mRNA construct. The in vitro transcribed mRNA is generated from an engineered plasmid DNA, which has an RNA polymerase promoter and sequence which corresponds to the mRNA construct. By combining T7 phage RNA polymerase and the plasmid DNA, the mRNA can be transcribed in the lab. Efficacy of the vaccine is dependent on the stability and structure of the designed mRNA.The in vitro transcribed mRNA has the same structural components as natural mRNA in eukaryotic cells. It has a 5' cap, a 5'-untranslated region and 3'-UTR, an open reading frame, which encodes the relevant antigen, and a 3'-poly tail. By modifying these different components of the synthetic mRNA, the stability and translational ability of the mRNA can be enhanced, and in turn, the efficacy of the vaccine improved.
The mRNA can be improved by using synthetic 5'-cap analogues which enhance the stability and increase protein translation. Similarly, regulatory elements in the 5'-untranslated region and the 3'-untranslated region can be altered, and the length of the poly tail optimized, to stabilize the mRNA and increase protein production. The mRNA nucleotides can be modified to both decrease innate immune activation and increase the mRNA's half-life in the host cell. The nucleic acid sequence and codon usage impacts protein translation. Enriching the sequence with guanine-cytosine content improves mRNA stability and half-life and, in turn, protein production. Replacing rare codons with synonymous codons frequently used by the host cell also enhances protein production.
Delivery
For a vaccine to be successful, sufficient mRNA must enter the host cell cytoplasm to stimulate production of the specific antigens. Entry of mRNA molecules, however, faces a number of difficulties. Not only are mRNA molecules too large to cross the cell membrane by simple diffusion, they are also negatively charged like the cell membrane, which causes a mutual electrostatic repulsion. Additionally, mRNA is easily degraded by RNAases in skin and blood.Various methods have been developed to overcome these delivery hurdles. The method of vaccine delivery can be broadly classified by whether mRNA transfer into cells occurs within or outside the organism.
''Ex vivo''
s display antigens on their surfaces, leading to interactions with T cells to initiate an immune response. Dendritic cells can be collected from patients and programmed with the desired mRNA, then administered back into patients to create an immune response.The simplest way that ex vivo dendritic cells take up mRNA molecules is through endocytosis, a fairly inefficient pathway in the laboratory setting that can be significantly improved through electroporation.
''In vivo''
Since the discovery that the direct administration of in vitro transcribed mRNA leads to the expression of antigens in the body, in vivo approaches have been investigated. They offer some advantages over ex vivo methods, particularly by avoiding the cost of harvesting and adapting dendritic cells from patients and by imitating a regular infection.Different routes of injection, such as into the skin, blood, or muscles, result in varying levels of mRNA uptake, making the choice of administration route a critical aspect of in vivo delivery. One study showed, in comparing different routes, that lymph node injection leads to the largest T-cell response.