Antigenic refocusing of a SAT2 foot-and-mouth disease vaccine seed virus

Abstract

The majority of the world’s most widespread and problematic pathogens evade host immune responses by inducing strain-specific immunity. The host immune system seems to induce a vigorous immune response towards hypervariable epitopes, seemingly attracting less attention to more highly conserved vital regions. The South African Territory (SAT)-2 foot-and-mouth disease virus (FMDV) is the most prevalent and antigenic diverse of the SAT serotypes with the occurrence of multiple antigenic and genetic subtypes. Identification of the fine antigenic structure of the capsid of these viruses remains essential in the design and engineering of a vaccine seed strain that confers cross-protection against intra-typic viruses. Towards refocusing the antigenicity of SAT2/ZIM/07/83 virus, two strategies were utilised, (1) replacement of predicted antigenic determinants to corresponding sites of the antigenic distant SAT2/EGY/09/12 virus and (2) charge-dampening of previously identified epitope regions with alanine residues. The antigenic distance of refocused mutants was evaluated by (1) virus neutralisation assays using parental and heterologous convalescent bovine sera and (2) through antigenic profiling with non-neutralising SAT2-specific murine monoclonal antibodies (mAbs). One antigenic site on VP1 (Site 3) was identified using bovine polyclonal antibodies, whereas an additional three epitope regions were elucidated using the murine mAbs. Furthermore, the cell culture-adapted vSAT2 was shown to utilise a third FMDV alternate receptor to infect integrin- and heparin sulphate-deficient cell lines. Comprehensive knowledge on the antigenic structure of these viruses will assist in the fundamental design of engineered vaccines by incorporating critical antigenic sites that confer increased antigenicity and cross-protective immune response against myriad SAT2 field strains. Furthermore, this information will not only improve design of vaccine seed viruses, but will also contribute towards novel vaccine constructs or even empty nanoparticles as a vaccine strategy in the future.