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Scientists have developed a strain of the Middle East respiratory syndrome coronavirus (MERS-CoV) that could be used to develop a vaccine against the deadly pathogen. According to the World Health Organization, the virus has so far infected 114 and killed 54 people since September 2012.
The team, from the Autonomous University of Madrid ("la Autónoma") in Spain, reports the achievement in a study published online this week in mBio.
They engineered the lab strain, called rMERS-CoV-[delta]E, to have a mutation in its envelope protein that allows it to infect a limited number of cells and replicate itself but not spread into surrounding tissue and cause disease.
This is an important feature for a vaccine, because you need enough live virus to provoke an immune response and produce antibodies, but not so much that it overwhelms the body to cause illness.
The researchers say that with further modification, such as engineering more safeguards into the lab strain, it could offer a viable starting point for developing a safe and effective live-attenuated vaccine against MERS.
Co-author Professor Luis Enjuanes, who heads the Coronavirus Lab at la Autónoma, where he and his team have been studying the molecular biology of coronaviruses for over 30 years, describes their achievement as a "combination of synthetic biology and genetic engineering."
"The injected vaccine will only replicate in a reduced number of cells and produce enough antigen to immunize the host," he explains, adding that it cannot infect other people, even if they have close contact with a vaccinated person.
So far MERS, which was first identified in June 2012, has not infected many people, but the proportion who die from it is alarmingly high, and health authorities are deeply concerned about it.
If the virus mutates into a form that spreads easily among humans, it could lead to a widespread epidemic.
While much work has been done to improve diagnosis and treatment, there are as yet no reliable vaccines against the deadly virus.
The team at la Autónoma broke new ground on the virus development front. They synthesized an infectious clone of MERS using a published sequence of its genome and then mutated several of its genes, one at a time, to see what effect it would have on its ability to infect cultured human cells, replicate in them, and then re-infect them.
They found several gene mutations made little difference, but they also found mutations in the envelope protein (E protein) of the virus enabled it to replicate its genetic material but not infect neighboring cells.
Many viruses that infect animals cover themselves with a protective envelope that they make by robbing proteins from the membranes of the cells they infect (the host cells) and mixing with their own glycoproteins. The envelope helps the virus invade host cells.
However, while the team achieved an important step in finding a MERS mutation that could replicate but not propagate, it was not enough: to produce a live vaccine, you cannot have a virus that cannot propagate at all.
You would need huge volumes to inject into people to produce enough antibody response to create immunity.
So, the next step was to find a way to allow the virus to grow but not spread.
They discovered the solution was to provide another type of E protein in special cells that would allow the virus to borrow it to make more copies of itself, but not integrate it into its genetic material. This way they could control the number of cells in which the virus can propagate.
Prof. Enjuanes explains:
"To grow the virus, we create what are called 'packaging cells' that express the E protein missing in the virus. The gene to encode this protein is integrated in the cell chromosomes and will not mix with the viral genes. Therefore, in these cells, and only within them, the virus will grow by borrowing the E protein produced by the cell."
This means that when people are vaccinated with the defective virus, they will not be able to provide it with the E protein that would help it spread easily to other host cells. In effect, it would die off after producing antigens that train the immune system to fight off an infection by the MERS virus.
The team believes in developing their promising vaccine candidate, they have made a good start. But they say there is still a long way to go before it is ready for clinical trials.
Prof. Enjuanes says providing the E protein that stops the virus propagating is just one safeguard, but regulators such as the US Food and Drug Administration (FDA) require that live attenuated vaccines have at least three safeguards. This is to ensure they do not revert to virulent forms.
The team is already investigating other safe disabling MERS gene mutations.
Written by Catharine Paddock PhD
Copyright: Medical News Today
Not to be reproduced without the permission of Medical News Today.
Engineering a Replication-Competent, Propagation-Defective Middle East Respiratory Syndrome Coronavirus as a Vaccine Candidate; Fernando Almazán, Marta L. DeDiego, Isabel Sola, Sonia Zuñiga, Jose L. Nieto-Torres, Silvia Marquez-Jurado, German Andrés, Luis Enjuanes; mBio vol. 4 no. 5 e00650-13, Published online 10 September 2013; DOI:10.1128/mBio.00650-13; Abstract.
Additional source: American Society for Microbiology news release 10 September 2013; Middle East respiratory syndrome coronavirus (MERS-CoV) - update, WHO Global Alert and Response (GAR), 7 September 2013 .
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