Mutations that disabled accessory genes 3, 4a, 4b and 5 did not seem to hinder the virus: mutant viruses had similar growth rates as the wild-type virus, indicating that the mutations do not disable the virus enough to deploy the mutants in a vaccine. Mutations in the envelope protein (E protein), on the other hand, enabled the virus to replicate its genetic material, but prevented the virus from propagating, or infecting nearby cells.
A large amount of the rMERS-CoV-ΔE virus would be needed for a live attenuated MERS vaccine. A virus that can't propagate itself would be unable to grow the volume needed without help. Enjuanes says they provided the virus with a supplemental form E protein.
"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," says Enjuanes. "When the virus in administered to a person for vaccination, this person will not be able to provide the E protein to the defective virus," so the virus will die off after producing antigens to train the human immune system to fight a MERS-CoV infection.
Enjuanes says rMERS-CoV-ΔE is a very promising vaccine candidate, but more work remains before they can start clinical trials. He says the mutation in the E protein that prevents the virus from propagating represents one safe guard, but the US Food and Drug Administration requires that a recombinant live attenuated vaccine strains include at least three safe guards to ensure the virus doesn't revert easily back to its virulent form. His group is currently working on introducing other disabling
|Contact: Jim Sliwa|
American Society for Microbiology