by Alexis Hubaud
figures by Anna Maurer

Vaccination is key to preventing disease and has been a major advance in public health to eradicate epidemics like smallpox or polio. Vaccines work by mimicking an infectious agent, and by doing so, train our bodies to respond more rapidly and effectively against them. A new class of vaccines, “RNA vaccines”, has recently been developed. RNA vaccines rely on a different way to mimic infection. Compared to previous vaccines, this method is more robust, more versatile, and yet, equally efficient. Therefore, the RNA vaccine technology holds great promise to prevent and treat a wide range of diseases, such as influenza or cancer.

Have you heard about RNA vaccines? This technology recently made the news when the Bill & Melinda Gates Foundation invested $53 million in the German company, CureVac, which specializes in the development of these vaccines [1]. In this article, we will discuss how RNA vaccines work, their main advantages compared to traditional vaccines, and their applications in diseases such as influenza and cancer.

How do RNA-based vaccines work?

Vaccination is the process in which substances called antigens are introduced artificially into the body to stimulate the immune system, the set of cells that protects the body against infections [2,3]. Those antigens are generally infectious agents – pathogens – that have been inactivated by heat or chemical treatment so that they will not cause disease, or they can also be purified proteins from the pathogens. Exposing the body to antigens leads to the production of molecules specifically directed against them, called antibodies. Antibodies create a memory of a specific pathogen (“acquired immunity”) and enable a more rapid and efficient response to a real infection with an active pathogen.

Vaccination has been central in diminishing or eradicating multiple infectious diseases, such as smallpox or polio. However, producing vaccines is a long and complex process, and it has been difficult to implement vaccines against certain pathogens. Thus, designing new vaccines remains a major challenge for public health. To answer this challenge, there have been many improvements to designing vaccines, such as using computational prediction. Development of nucleotide vaccines based on DNA, and the related molecule RNA, is another promising area of progress in the field [4].

In each cell of a living organism, DNA is the molecule that contains the genetic information of the organism [5]. It is composed of a series of four building blocks, whose sequence gives the instructions to fabricate proteins. This process requires a transient intermediary called messenger RNA that carries the genetic information to the cell machinery responsible for protein synthesis. As an analogy, one can see the DNA as a cook book in a library: the recipe is stored here but cannot be used. The commis, or chef’s assistant, first makes a copy (the RNA) of a specific recipe and brings it to the kitchen. The information is now ready-to-use by the chef, who can add the ingredients in the order specified by the recipe and create a cake (the protein).

Figure 1: RNA vaccine technology. An RNA is injected in the body (left). This RNA encodes the information to produce the antigen, which is a protein from a pathogen, that will stimulate the immune system. Inside the cells, the RNA is used to synthesize the antigen, which is exposed to the cell surface (middle). Then, a subset of immune system cells recognizes the antigen and trigger an immune response (direct response and long-term memory) (right).

For a classical vaccine, the antigen is introduced in the body to produce an immune response. However, in the case of DNA- or RNA-based vaccines, no antigen is introduced, only the RNA or DNA containing the genetic information to produce the antigen. That is, for this specific class of vaccines, introduction of DNA and RNA provides the instructions to the body to produce the antigen itself (Figure 1). They can be injected in various ways (under the skin, in the vein or in lymph nodes) and then they can enter our body’s cells. Those cells will use the RNA sequence of the antigen to synthesize the protein [2,6]. After this step, the mechanism is similar to classical vaccines: the antigen is presented at the surface of a subset of cells and triggers the activation of specific cells of the immune system (Figure 2).

The ways in which DNA and RNA vaccines work are similar in many ways, and some of the common steps are described above. However, RNA vaccines have some distinct advantages. One is that RNA-based vaccines appear to perform better than DNA-based vaccines. Another is that they are also safer, as injection of RNA presents no risk of disrupting the cell’s natural DNA sequence. To continue our kitchen analogy, disruption from DNA is like inserting a foreign ingredient in an existing recipe, which can change the resulting dish [2].  Injecting RNA, on the other hand, is like temporarily adding a new recipe in the cook book while keeping old ones untouched, and therefore will not result in surprising changes to existing recipes.

Figure 2: Disease prevention. Vaccination with RNA induces a primary response (top) by instructing the body’s cells to produce an antigen that is presented to the immune system. This activates specific cells, which create a memory for this antigen. Later, when the real pathogen is present (bottom), those cells recognize the same antigen and react rapidly and strongly against the infectious agent (secondary response).

How are they produced?

With the considerable progress in DNA sequencing, it has become relatively easy to determine the genome sequence of pathogens. RNA can thus be produced in vitro, i.e. outside the cells, using a DNA template containing the sequence of a specific antigen. Creating a RNA vaccine also requires some engineering of the RNA to achieve a strong expression of the antigen [4,6].

This is a much simpler process than the culture of virus in eggs. Egg cultures, the more common way of producing vaccines, can provoke allergic reactions; the in vitro production of RNA avoids this possibility. Producing RNA vaccines is also less expensive than producing the full antigen protein [4,6,7].

Another advantage is that the production of RNA-based vaccines is more rapid compared to production of traditional vaccines. This rapid production could be a major advantage in face of sudden pandemics. Moreover, RNA-based vaccines may be effective against pandemics because they also provide more flexibility to prevent or treat pathogens that are rapidly evolving [8,9]. For instance, influenza vaccines have to be tailored each year to specific strains that are most likely to cause disease in the coming season. However, these forecasts have not always been accurate, such as during the winter of 2014-2015, making the influenza vaccine less protective. The World Health Organization estimates it takes approximately five to six months to produce an influenza vaccine, whereas the company CureVac claims that RNA-based vaccines could be manufactured in less than two months at a lower production cost, making it possible to respond to epidemics even as they develop. Therefore, RNA-based vaccines offer a comparatively simple and rapid solution to unpredictable, rapidly evolving pathogens.

While injection of simple RNA can elicit an immune response, RNAs in this form are prone to a rapid degradation. Current vaccines are fragile and can lose their efficiency when exposed at freezing or high temperatures, and must be stored at 35-45°F (2-8°C)[4,6,10]. Thus, preserving the cold chain is a major hurdle for the implementation of vaccine campaign. Fortunately, scientists have found ways to combat this RNA degradation. For instance, they can change the sequence of RNA to make it much easier to store. Furthermore, other molecules can be added to bind the RNA and protect it. Such engineering enables the storage of RNA vaccines at room temperature for at least 18 months. This feature precludes the necessity of maintaining the cold chain, making RNA vaccines particularly practical for developing countries.

What is the current state of the research?

This new exciting technology could be applied to many diseases, and pharmaceutical companies are making major investments in that area. RNA vaccines are still at the pre-clinical or clinical stage, but have yielded promising results. Below, we will explore two examples with the most advanced results: RNA vaccines to treat cancer and RNA vaccines to prevent influenza.

In the field of cancer immunotherapy, “cancer vaccines” take advantage of the expression of specific markers by cancer cells to direct the immune response and attack the tumor. RNA vaccines against prostate cancer, melanoma, and lung cancer (non-small cell lung cancer) are currently in clinical trials. For instance, six different RNAs against proteins produced in excess in tumor cells were used to formulate a vaccine against lung cancer. By taking advantage of the flexibility of RNA vaccine production, scientists can thus produce a vaccine with different antigens which is consequently better at targeting the tumor cells [11].  In the case of the prostate cancer vaccine, a preliminary study showed that injection of those RNAs foster an immune response in most of the patients. Whether this production of antibodies is sufficient to slow down the tumor progression remain to be determined.

Interestingly, because of the versatility of RNA vaccines, they could be tailored to fit the antigen repertoire of each patient tumor. Tumor cells are very different between patients, and this variability is an ongoing an issue for cancer treatment.  An ongoing clinical trial is testing whether RNA vaccines may be effective for addressing variability in melanoma patients: in the trial, each tumor was first sequenced to identify its unique antigen repertoire, and then, a RNA vaccine is tailored to each tumor (Figure 3). This study shows that RNA vaccines could play a major role in this growing field of “personalized medicine” [7]. Moreover, these tailored, on-demand vaccines are practical – the company BioNTech claims that it could be manufactured in 5 months [12]).

Figure 3: Disease treatment (example of personalized cancer immunotherapy). The DNA from the tumor cells is first analyzed (top) to identify antigens specific to the patient’s tumor (Antigens A,B,C). Secondly (middle), a personalized vaccine comprising the specific RNAs for those antigens found in the analysis is injected to direct the attack of the immune system against the tumor (bottom).

RNA vaccines are also being developed to prevent infectious diseases. A vaccine against rabies is currently in clinical trials, while vaccines against influenza, HIV or tuberculosis are still at the research stage. Published results with the influenza vaccine [9] showed promising protection in mice. Indeed, injection of RNA coding for different proteins of the influenza virus induced the production of antibodies, and when the mice were later exposed to the virus, all survived. Similar immune response was observed in ferrets and pigs. All these observations in animals point to a potential use in humans.

The field of RNA vaccines is still nascent. However, their production is flexible and rapid, and recent studies indicate they could be effective against a wide range of infectious diseases and cancers. While their clinical potential in humans remains to be firmly established, RNA vaccines appear to be a promising technology worth watching out for.

Alexis Hubaud is a PhD student in Developmental Biology working at the Brigham and Women’s Hospital / Harvard Medical School

References

[1] Press statement from the Bill and Melinda Gates Foundation and CureVac
http://www.gatesfoundation.org/Media-Center/Press-Releases/2015/03/CureVac-Collaboration
[2] Introductory video about vaccination http://www.pbs.org/wgbh/nova/body/immunity-and-vaccines.html
[3] Vaccination ingredients from the NHS (UK National Health Service) website http://www.nhs.uk/conditions/vaccinations/pages/vaccine-ingredients.aspx
[4] Review about RNA vaccines- Schlake et al. RNA Biology (2012) 9(11):1319-1330
[5] Introductory video about synthesis of proteins from DNA and RNA http://www.pbs.org/wgbh/nova/body/cellular-factory.html
[6] Review about the CureVac vaccine – Kallen et al., Human Vaccines and Immunotherapeutics (2013) 9(10):2263-2276
[7] Review about RNA-based therapies – Sahin et al., Nat Rev Drug Disc (2014) 13 :759-780
[8] News article about the use of RNA vaccine against Influenza
Making a Flu Vaccine Without the Virus – http://news.sciencemag.org/2012/11/making-flu-vaccine-without-virus
[9] Scientific article on a RNA vaccine against influenza
Petsch et al. Nat Biotech (2012) 30(12):1210-1216
[10] Website from the company CureVac, which specializes in RNA vaccine http://www.curevac.com/
[11] Scientific article on a RNA vaccine against non-small cell lung cancer – Sebastian et al., BMC Cancer (2014) 14 :748
[12] Website from the company BioNTech, which specializes in RNA vaccine http://www.biontech.de/