Genetic and phenotypic characterisation of selected african foot-and-mouth disease virus isolates

Abstract

Foot-and-mouth disease (FMD) is a contagious viral disease that affects cloven hoofed animals and can be economically devastating. However, vaccination can reduce the incidence of the disease when used in conjunction with other control methods. Studies described in this thesis address three aspects beneficial to the selection of vaccine seed viruses as well as improving the design of chimeric vaccines customized for use in Africa. Molecular characterization of the non-structural proteins of African FMD viruses (FMDV) can facilitate the selection of vaccine seed strains with a fitness advantage. In addition, common mutations acquired by South African Territory (SAT) serotypes on the virus surface capsid upon cell culture adaptation may be of value to the design of recombinant constructs for chimeric viruses optimized for vaccine production. Moreover, the antigenicity of a chimeric virus vZIM14-SAT2 compared to its parental virus, SAT2/ZIM/14/90, in cattle and additionally in vaccine matching assays evaluated its compatibility for future vaccine studies. Genome comparisons of the non-structural proteins for 79 African FMDV isolates from different serotypes and topotypes, spatially and temporally dispersed, showed that critical functional motifs were conserved and variation occurred away from enzymatic active sites. The southern African FMDV clustered separately from the other African viruses on phylogenetic analysis. This increased the molecular understanding of FMDV, aiding in the selection of vaccine seed strains. The BHK-21 cell line (preferred for vaccine production) was used to serially passage eleven SAT1 and four SAT2 FMDV. This resulted in Arg or Lys mutations in the VP1 capsid protein surrounding the 5-fold axis, at positions 84-85 and 111-112 respectively. Arg at position 112 was favoured in the binding of a heparin moiety onto a SAT1 model using molecular docking simulations. This knowledge can be applied to recombinant constructs for the generation of chimeric viruses that adapt rapidly to BHK-21 cell culture for vaccine production. Vaccines derived from chimeric viruses present a good prospect in the control of FMD. The antigenic nature of an intra-serotype chimeric virus, vZIM14-SAT2, consisting of the capsid-coding region of SAT2/ZIM/14/90 in a genome-length clone pSAT2, was compared to that of its parental virus. In cattle both chimeric and parental viruses elicited similar neutralizing antibody responses and provided protection against clinical disease following homologous virus challenge. In addition, antigenic matching resulted in comparable cross-reaction results for both chimeric and parental viruses. Therefore, when used as vaccines, the antigenic profiles of chimeric viruses against SAT2 isolates did not differ from parental viruses and could be used as an alternative in vaccine production combined with the advantage of including suitable mutations for cell culture adaptation. These findings increase our knowledge on the genome structure and replication of the FMDV in BHK-21 cells. The data can be used to facilitate selection of vaccine seed viruses specific to serotype and topotype. Additionally, the information can supplement the design of recombinant constructs for chimeric vaccines for future application. Taken together, these new data broaden our options to control FMD in Africa using tailor made chimeric vaccines.