Genetic stability of Rift Valley fever virus MP-12 vaccine during serial passages in culture cells

Abstract

Rift Valley fever is a mosquito-borne zoonotic disease endemic to Africa, which affects both ruminants and humans. Rift Valley fever causes serious damage to the livestock industry and is also a threat to public health. The Rift Valley fever virus has a segmented negative-stranded RNA genome consisting of Large (L)-segment, Medium (M)-segment, and Small (S)-segment. The live-attenuated MP-12 vaccine is immunogenic in livestock and humans, and is conditionally licensed for veterinary use in the US. The MP-12 strain encodes 23 mutations (nine amino acid substitutions) and is attenuated through a combination of mutations in the L-segment, M-segment, and S-segment. Among them, the M-U795C, M-A3564G, and L-G3104A mutations contribute to viral attenuation through the L-segment and M-segment. The M-U795C, M-A3564G, L-U533C, and L-G3750A mutations are also independently responsible for temperature-sensitive phenotype. We hypothesized that a serial passage of the MP-12 vaccine in culture cells causes reversions of the MP-12 genome. The MP-12 vaccine and recombinant rMP12-ΔNSs16/198 were serially passaged 25 times. Droplet digital polymerase chain reaction analysis revealed that the reversion occurred at L-G3750A during passages of MP-12 in Vero or MRC-5 cells. The reversion also occurred at M-A3564G and L-U533C of rMP12-ΔNSs16/198 in Vero cells. Reversion mutations were not found in MP-12 or the variant, rMP12-TOSNSs, in the brains of mice with encephalitis. This study characterized genetic stability of the MP-12 vaccine and the potential risk of reversion mutation at the L-G3750A temperature-sensitive mutation after excessive viral passages in culture cells.

Rift Valley fever: Whole-virus vaccine alters the attenuation profile

A vaccine candidate for Rift Valley fever virus can undergo mutations and partially revert to parental pathogenic strain. Rift Valley fever is a prolific infection that affects livestock and humans in Africa. Tetsuro Ikegami and Nandadeva Lokugamage, of the University of Texas Medical Branch, United States, tested the genetic stability of vaccine candidate MP-12, an attenuated form of the virus, through ‘serial passaging’—culturing the virus, establishing a new culture from a sample, then repeating the procedure to determine how the virus mutates in response to successive new environments. The duo found that after 25 ‘passages’, MP-12 reverted a part of attenuation mutations back to its original, pathogenic sequence. Though MP-12 redundantly encodes stable attenuation mutations, it will be important to use a seed lot system to avoid the attenuation profile changes.