In the past decade RNA vaccine development has been possible due to major advances in vaccine technology – how do RNA vaccines work?
Conventional vaccines have been used to protect populations against various diseases for over a century. Their mechanism of action typically involves a needle injection introducing a weakened or inactivated version of the pathogen into the body. The injected pathogen has antigens – proteins that signal to immune cells that the pathogen is “foreign”. Specialized proteins called antibodies bind to the antigen, prompting a further immune response to destroy the pathogen. Antibodies store the memory of the specific pathogen, so that when exposed to the same infectious agent, it can quickly neutralize the pathogen before symptoms present.
Unfortunately, these traditional vaccines are not suitable for preventing all infectious diseases or for treating most non-infectious diseases, including cancer. Further, rapidly emerging viral diseases, such as the Ebola virus and novel coronavirus, require a faster vaccine development than conventional vaccines can provide. Nucleic acid vaccines – DNA- and RNA-based vaccines – were both developed in the 1990s as an alternative to conventional vaccines, however, they presented significant issues in their stability and delivery. Only in the past decade has RNA vaccine development overcome its previous shortcomings due to major advances in vaccine technology. An extensive body of preclinical studies and a growing number of human trials have emerged, showing promise.
So, how does an RNA vaccine work?
What is RNA?
Ribonuleic acid (RNA) is an information-carrying nucleic acid containing temporary instructions regarding which proteins must be produced in a cell. There are many forms of RNA, each with a specific function in the protein synthesis process which occurs in a cell’s cytoplasm. For example, mRNA is formed from DNA in order to carry genetic information needed for synthesizing proteins. It is this form of RNA that is typically used in vaccines.
RNA can be found in most living organisms, such as humans, and viruses. Viruses that contain RNA as their primary genetic material, such as coronaviruses, survive by invading a host (i.e., human) cell and replicating their RNA there. This can interfere with the host cell’s typical functions since the viral RNA is now directing its protein synthesis. In this way, RNA viruses have the capacity to be severely damaging to the healthy functioning of the human body.
What types of RNA vaccines exist?
Two classes of RNA are possible for use in vaccine development: non-replicating mRNA and self-amplifying RNA.
Non-replicating mRNA vaccines are developed in a test tube, generating an mRNA sequence that encodes the antigen(s) associated with the infectious agent or tumor of interest.
Self-amplifying RNA vaccines are based on the genetic material of a self-amplifying RNA virus, most commonly an alphavirus. These RNA viruses typically invade their host’s cells where they replicate their RNA to produce a higher level of viral proteins. To make the self-amplifying RNA vaccine, the infectious protein in the RNA virus is removed and replaced with the genes encoding for the antigen(s) of interest. Much like a virus, the RNA vaccine enters the host cell where it can replicate its RNA to increase the level of antigen synthesis. Self-replication strengthens the vaccine’s duration and effect in the body.
How do mRNA vaccines work?
The mRNA instructions for synthesizing the pathogen-specific antigen(s) are introduced into the body. The injection of the RNA-based vaccines prompts the lymph nodes – containing specialized immune cells – to send antigen-presenting cells and antibodies to the site of the injection. Once the vaccine is taken up by available antigen-presenting cells, the RNA sequence is translated into the desired antigen(s). The newly synthesized antigen(s) make their way to the cells’ surface where it attracts antibodies. This then prompts immune cells to react and form an immune response. .
RNA-based vaccines work differently depending on whether it is intended to produce immunity against an infectious agent or treat a non-infectious disease such as cancer. mRNA vaccines work similarly to conventional vaccines for protection against infectious diseases – antibodies store the memory of the pathogen-specific antigen(s) to trigger a rapid and effective immune response if an infection should occur. For cancer, mRNA vaccines are intended to be a treatment rather than a preventative approach to developing tumors. The recognition of tumor associated antigen(s) made in an antigen-presenting cell prompts immune cells to attack the tumor cells that have the same antigens on their cell surface.
How do RNA vaccines enter targeted cells?
Early research focused on injecting ‘naked’ non-replicating mRNA directly into the body, however, the mRNA often rapidly degraded and struggled to reach the inside of the cell where it would be necessary to work. So, various approaches to improve the cellular uptake of RNA vaccines were developed.
Physical approaches include gene gunning, electroporation or micro-injection. All physical approaches are focused on improving the insertion of the modified RNA into the targeted cell. These approaches can be used either in a patient’s body or in a test tube.
Viral vectors rely on modifying RNA viruses to replace infectious structural proteins with genes encoding the antigen(s) of interest. The vaccine is typically injected into the body where it can enter the targeted cell through the uptake methods that the virus normally uses to infect cells. This method is commonly used for delivering self-amplifying RNA vaccines.
Non-viral vectors encapsulate the modified RNA during transport. Lipid-based vectors are the most commonly used non-viral vectors based on their effective cellular uptake. Lipid nanoparticles form a casing around the RNA to help it enter the targeted cell. While its mechanism of delivery is not completely understood, the lipid nanoparticles are thought to be engulfed by the target cells.
Ex vivo loading of dendritic cells – specialized antigen-presenting cells – eliminates the need for cellular uptake of mRNA within the body. It begins with dendritic cells being removed from a patient’s blood and placed in a test tube where non-replicating mRNA is inserted into the cell’s cytoplasm. The mRNA loaded dendritic cells are then injected into the individual’s body. Ex vivo loading of dendritic cells has been primarily used in cancer treatment, however, it has potential to prevent or treat infectious agents.
In early pre-clinical and clinical trials, non-replicating mRNA vaccines were most commonly tested, with its two typical delivery methods being direct injection and ex vivo loading of dendritic cells. Preclinical mRNA vaccines have been tested primarily in animal models, demonstrating its safety, rapid design and production capabilities, and versatility for use for a variety of infectious agents.
Early results from the first few human trials reported more modest results than the numerous preclinical trials. This indicates that human trial data is important in determining the effectiveness of RNA-based vaccines.
Recently, self-amplifying mRNA vaccines have picked up interest, especially in the development of a covid-19 vaccine. One study testing the efficacy of self-amplifying mRNA encapsulated within lipid nanoparticles in mice, reported the vaccine provided protective immune responses against a respiratory syncytial virus. Current human trials of both self-amplifying RNA and non-replicating mRNA vaccines for protection against the novel coronavirus are currently underway with a few vaccines showing promising preliminary results, such as Moderna’s mRNA-1273 vaccine.
RNA vs DNA vaccines
Recent years have seen the impressive progression of RNA-based vaccine development. With improved RNA vaccine technology, it holds numerous advantages over DNA vaccines.
For instance, RNA-based vaccines are considered safer than DNA vaccines. mRNA vaccines are delivered to the cytoplasm preventing integration into the host cell’s genome. This ensures the cell’s normal DNA sequence will be unchanged by the vaccine. In contrast, DNA vaccines are delivered to the cell’s nucleus – where the cell’s genome is located – risking integration. Further, mRNA has a fairly short half life in the body and will be destroyed naturally by the body’s cellular processes .
Based on the mechanism of producing the necessary antigen(s) required for prompting an immune response, RNA-based vaccines are more effective than DNA vaccines. While modified mRNA is able to rapidly produce its encoded antigen(s) in a cell, DNA vaccines must first produce the mRNA needed to synthesize the antigen(s). Lastly, RNA-based vaccines are quick and inexpensive to manufacture in test tubes compared to DNA vaccine development.
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