tag:theconversation.com,2011:/fr/topics/h7n9-6165/articlesH7N9 – The Conversation2021-09-06T15:14:22Ztag:theconversation.com,2011:article/1669752021-09-06T15:14:22Z2021-09-06T15:14:22ZMessenger RNA: how it works in nature and in making vaccines<figure><img src="https://images.theconversation.com/files/419555/original/file-20210906-21-yivy2j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">mRNA technologies for vaccine production is gaining more prominence </span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/general-view-at-aspen-pharmacare-sterile-manufacturing-news-photo/1232178759?adppopup=true">Lulama Zenzile/Die Burger/Gallo Images via Getty Images</a></span></figcaption></figure><p>Vaccines <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5402432/">have long been</a> an integral part of public health programmes around the world, reducing the spread and severity of infectious diseases. The success of <a href="https://www.nicd.ac.za/wp-content/uploads/2017/08/NICD_Vaccine_Booklet_D132_FINAL.pdf">immunisation strategies</a> to protect children from diseases like polio, hepatitis B, and measles, and adults from influenza and pneumococcal disease, can be seen <a href="https://www.nature.com/articles/s41586-019-1656-7">globally</a>. </p>
<p>The COVID-19 pandemic created an urgent need for an effective vaccine. This is where messenger RNA (mRNA) vaccines, which are <a href="https://www.nature.com/articles/s41563-020-0746-0">classified</a> as a next-generation technology, gained prominence. Decades of research and clinical development into synthetic mRNA platforms for cancer treatments and vaccines for infectious diseases like influenza, malaria, and rabies, finally paid off as both <a href="https://www.who.int/news-room/feature-stories/detail/the-moderna-covid-19-mrna-1273-vaccine-what-you-need-to-know?gclid=CjwKCAjw092IBhAwEiwAxR1lRlM_GHRHjP6lh-AiQPV-RcVuE7gTd1KvD1Rbd2-cRaGHzeIjXf0LjhoCtSQQAvD_BwE">Moderna</a> and <a href="https://www.who.int/news-room/feature-stories/detail/who-can-take-the-pfizer-biontech-covid-19--vaccine">Pfizer/BioNTech’s</a> COVID-19 mRNA vaccines received emergency use authorisation. As a result, mRNA technologies have been catapulted into the public spotlight. </p>
<h2>Developing synthetic mRNA into vaccines</h2>
<p>Ribonucleic acid (RNA) is a natural molecule found in all our cells. There are many types of RNA, each with distinct functions. As the name implies, <a href="https://www.yourgenome.org/video/from-dna-to-protein">mRNA acts as an important messenger in human cells</a>. These molecules carry unique codes that tell our cells which proteins to make and when to make them. The code is copied from a strand of DNA in the nucleus of the cell, in a process called transcription. The mRNA is then transported into the cytoplasm (the solution contained in the cell) where the message is ‘read’ and translated by the cell’s protein production machinery. The result is an important protein, such as an enzyme, antibody, hormone, or structural component of the cell. </p>
<p>Nearly 40 years ago scientists <a href="https://www.nature.com/articles/nrd4278">found</a> that they could mimic transcription and produce synthetic mRNA without a cell. The process, known as in-vitro transcription, can generate many mRNA molecules from a strand of DNA in a test tube. This requires an enzyme (called RNA polymerase) and nucleotides (the molecules that are the building blocks of DNA and RNA). When mixed together, the polymerase reads the strand of DNA and converts the code into a strand of mRNA, by linking different nucleotides together in the correct order. </p>
<p>When in vitro transcribed mRNA is introduced into a cell, it is ‘read’ by the cell’s protein production machinery in a similar manner to how natural mRNA functions. In principle, the process can be used to generate synthetic mRNA that codes for any protein of interest. In the case of vaccines, the mRNA codes for a piece of a viral protein known as an antigen. Once translated, the antigen triggers an immune response to help confer protection against the virus. mRNA is short-lived and does not change the cell’s DNA. So it is safe for the development of vaccines and therapies. </p>
<p>A major advantage of in vitro transcription is that it does not require cells to produce the mRNA. It has certain manufacturing advantages over other vaccine technologies – rapid turnaround times and reduced biological safety risks, for example. It took only <a href="https://www.modernatx.com/modernas-work-potential-vaccine-against-covid-19">25 days</a> to manufacture a clinical batch of Moderna’s lipid nanoparticle mRNA vaccine candidate, which in March 2020 became the first COVID-19 vaccine to enter human clinical trials. </p>
<p>Importantly, as in vitro transcription is cell-free, the manufacturing pipeline for synthetic mRNAs is flexible and new vaccines or therapies can be streamlined into existing facilities. By replacing the DNA code, facilities can easily switch from producing one kind of mRNA vaccine to another. This not only future-proofs existing mRNA production facilities but could prove vital for rapid vaccine responses to new pandemics and emerging disease outbreaks.</p>
<h2>How do mRNA vaccines work?</h2>
<p>The mRNA vaccines we are familiar with today have benefited from many years of research, design and optimisation. Understanding how synthetic RNA is recognised in cells has proven essential in developing effective vaccines. Typically, the mRNA codes for a known viral antigen. In the case of COVID-19 mRNA vaccines, sequences coding for the SARS-CoV-2 spike protein or the receptor-binding domain have been used. These antigen-encoding mRNA molecules are incorporated into very small particles made primarily of lipids (fats). The lipid particle has two main functions: it protects the mRNA from degradation and helps deliver it into the cell. Once in the cytoplasm, the mRNA is translated into the antigen which triggers an immune response. </p>
<p>This process is essentially a training exercise for your immune system, and it normally takes a few weeks for your adaptive immunity to mature and synchronise. mRNA vaccines have been <a href="https://pubmed.ncbi.nlm.nih.gov/33477534/">shown</a> to stimulate both arms of the adaptive immune response, which are important for establishing protection. Humoral (B cell) immunity produces antibodies while cellular (T cell) immunity helps to detect infected cells. The current mRNA COVID-19 vaccine schedule uses a two dose (prime-boost) approach, which aims to strengthen your adaptive immune response towards the SARS-CoV-2 virus. </p>
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<a href="https://theconversation.com/covid-19-vaccines-produce-t-cell-immunity-that-lasts-and-works-against-virus-variants-166757">COVID-19 vaccines produce T-cell immunity that lasts and works against virus variants</a>
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<p>Another type of mRNA vaccine, referred to as <a href="https://www.nature.com/articles/s41434-020-00204-y">self-amplifying RNA</a>, may only require a single low dose to achieve the same level of protection. In a cell, these self-amplifying RNA vaccines can copy the mRNA code. This means that more antigen can be produced from less RNA. Several <a href="https://www.who.int/publications/m/item/draft-landscape-of-covid-19-candidate-vaccines">COVID-19 RNA vaccines</a> currently in clinical trials are exploring self-amplifying RNA technologies. </p>
<h2>mRNA vaccines beyond COVID-19</h2>
<p>It is an exciting time for mRNA technologies. Thanks to the collaborative efforts of governments, funding agencies, academia, biotech and pharmaceutical companies, large-scale manufacturing of mRNA drug products is becoming a reality. The success of <a href="https://www.nejm.org/doi/full/10.1056/nejmoa2035389">Moderna</a> and <a href="https://www.nejm.org/doi/full/10.1056/nejmoa2034577">Pfizer/BioNTech’s</a> COVID-19 vaccines has helped re-energise ongoing mRNA research. </p>
<p>Both mRNA and self-amplifying RNA have shown potential as vaccines for multiple infectious diseases including influenza, respiratory syncytial virus, rabies, Ebola, malaria and HIV-1. Coupled with therapeutic applications, most notably as <a href="https://www.pennmedicine.org/news/news-blog/2021/june/how-mrna-vaccines-help-fight-cancer-tumors-too">immunotherapy</a> for the treatment of cancers, mRNA technologies will continue to improve and expand, forming an integral part of future drug development.</p><img src="https://counter.theconversation.com/content/166975/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>At the time of writing Kristie Bloom does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed the following funding associated with her academic appointment: The National Research Foundation (NRF), The Poliomyelitis Research Foundation (PRF), and The South African Medical Research Council (SAMRC).</span></em></p>Thanks to the collaborative efforts of governments, funding agencies, academia, biotech and pharmaceutical companies, large-scale manufacturing of mRNA drug products is becoming a reality.Kristie Bloom, Group Leader: Next-generation Vaccines, Antiviral Gene Therapy Research Unit, University of the WitwatersrandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/139202013-08-22T05:47:50Z2013-08-22T05:47:50ZAvian flu may have moved between humans but we’re still far from a pandemic<figure><img src="https://images.theconversation.com/files/28864/original/zkskzmny-1375889677.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C1024%2C691&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Viral workload: how did H7N9 make the human-to-human leap?</span> <span class="attribution"><span class="source">James Gathany</span></span></figcaption></figure><p>The H7N9 virus is thought <a href="http://www.telegraph.co.uk/health/flu/10226810/Deadly-avian-flu-spreads-person-to-person-for-first-time.html">to have been transmitted between</a> a 60-year-old man in China and his 32-year-old daughter, who cared for him. Experts said she had been previously healthy and, unlike her father, had no known exposure to live poultry before falling ill. Both were reportedly treated in intensive care but died of multiple organ failure.</p>
<p>The new H7N9 influenza virus emerged in China in spring 2013. In the first month, the virus infected more than 100 people and was lethal in a fifth of cases. By July, <a href="http://www.who.int/csr/don/2013_07_20/en/index.html">134 cases and 43 deaths</a> had been reported.</p>
<p>Chickens, pigeons and environmental samples taken from live poultry markets in the affected area tested positive, which suggested them as a source of infection. H7N9 had never before circulated in humans but all 7bn or so of us are fully susceptible to it. Until now, however, the virus didn’t transmit between humans because it hadn’t acquired the right adaptive mutations. </p>
<p>It’s a worrying development, <a href="http://bit.ly/17340RR">but experts have said</a> the virus’ ability to transmit itself is “limited and non-sustainable”. </p>
<h2>Making a leap</h2>
<p>Viruses are tiny microbiological entities that are not independently alive but rely on gaining access to a living cell for survival. They interact intimately with cellular machinery, subverting it for their own ends to amplify hundreds of copies of their viral genetic material and then use virally encoded protein shells called capsids to move their genomes onto the next host. </p>
<p>Viruses are usually specialists at infecting a particular type of cell inside a specific host, because they have co-evolved with that species and the viral proteins are a good fit with those of the host. Take influenza as an example; natural host species are wild aquatic birds. </p>
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<img alt="" src="https://images.theconversation.com/files/28865/original/v6cmgx8v-1375890122.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/28865/original/v6cmgx8v-1375890122.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=766&fit=crop&dpr=1 600w, https://images.theconversation.com/files/28865/original/v6cmgx8v-1375890122.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=766&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/28865/original/v6cmgx8v-1375890122.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=766&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/28865/original/v6cmgx8v-1375890122.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=962&fit=crop&dpr=1 754w, https://images.theconversation.com/files/28865/original/v6cmgx8v-1375890122.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=962&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/28865/original/v6cmgx8v-1375890122.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=962&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Not so cute now.</span>
<span class="attribution"><span class="source">David Monniaux</span></span>
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<p>Occasionally, an influenza virus passes from ducks to poultry, or to other domesticated animals such as pigs or horses, or even to humans. This is called zoonosis. </p>
<p>But if the zoonotic virus manages to undergo some replication, mutations that work better with the new host can adapt the virus to its new environment. This basic Darwinian evolution happens very quickly for viruses compared to larger organisms because viruses copy vast numbers of genomes and often replicate their genetic material without proof reading the results first.</p>
<h2>Breaking through barriers</h2>
<p>Research accumulated over a number of years has suggested at least two barriers that restrict avian influenza viruses in humans: the avian flu viruses bind poorly to human cells, and the virus would have to “switch” to do be able to go from human to human. </p>
<p>Viruses that caused influenza pandemics in 1957 and 1968 were able to make two mutations in order to switch. Interestingly, the current H7N9 viruses have one but not both of these changes. This may explain the ability of the virus to more frequently infect exposed humans than previous H7 avian viruses. </p>
<p>That the H7N9 virus doesn’t have two of these mutations may be why it still won’t efficiently transmit between humans, and likely why the Chinese researchers believe the virus that infected the father and daughter is “limited and non-sustainable”.</p>
<p>The second barrier avian influenza viruses must overcome to infect human cells is the poor performance of a virally encoded enzyme whose job it is to interact with the nucleus of the infected cell and direct the replication of the virus’ genome. However, it’s readily overcome if the genes that encode the enzyme mutate. One gene in particular, PB2, has already been shown to have mutated in humans but not birds.</p>
<p>Taken together, the H7N9 mutations place it one step closer to human-to-human transmission <a href="https://theconversation.com/severity-of-h7n9-compared-to-other-flu-outbreaks-15487">than the notorious H5</a> virus around since 2003.</p>
<h2>Final piece of evolution?</h2>
<p>In research into H5N1, in which artificial viruses were created and transmitted between ferrets, it was discovered that one other mutation in the HA protein enabled the virus to transmit through the air, by making it more resistant to heat or low pH. </p>
<p>It’s conceivable that this is the final piece of evolution required to achieve transmission in humans; a mutation that enhances survival so that the virus reaches the new host intact, despite the harsh environmental exposure it undergoes during the transmission process. And how readily H7 or any other emerging influenza virus can acquire this property may be what determines the next influenza pandemic.</p>
<p>This research may also reveal important clues about the likelihood that other emerging pathogens <a href="http://www.bbc.co.uk/news/health-23179564">such as the new coronavirus</a> could cause new epidemics. Work involving poliovirus, a completely different virus that enters through the mouth, has recently identified that stability of the virus particle is enhanced by interaction with bacteria in the gut and this is key to its onwards transmission. </p>
<p>In the end a virus can only continue to exist if it can pass from one host to the next. Emergence of new pandemics is largely driven by transmissibility and understanding this process is now, quite rightly, an intense area of research.</p><img src="https://counter.theconversation.com/content/13920/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Wendy Barclay's laboratory is funded by the Medical Research Council, the Biotechnology and Biological Sciences Research Council, the Wellcome Trust and commercial bodies.</span></em></p>The H7N9 virus is thought to have been transmitted between a 60-year-old man in China and his 32-year-old daughter, who cared for him. Experts said she had been previously healthy and, unlike her father…Wendy Barclay, Professor of Virology , Imperial College LondonLicensed as Creative Commons – attribution, no derivatives.