A safe foot-and-mouth disease vaccine platform with two negative markers for differentiating infected from vaccinated animals.

Vaccination of domestic animals with chemically inactivated foot-and-mouth disease virus (FMDV) is widely practiced to control FMD. Currently, FMD vaccine manufacturing requires the growth of large volumes of virulent FMDV in biocontainment-level facilities. Here, two marker FMDV vaccine candidates (A(24)LL3D(YR) and A(24)LL3B(PVKV)3D(YR)) featuring the deletion of the leader coding region (L(pro)) and one of the 3B proteins were constructed and evaluated. These vaccine candidates also contain either one or two sets of mutations to create negative antigenic markers in the 3D polymerase (3D(pol)) and 3B nonstructural proteins. Two mutations in 3D(pol), H(27)Y and N(31)R, as well as RQKP(9-12)→PVKV substitutions, in 3B(2) abolish reactivity with monoclonal antibodies targeting the respective sequences in 3D(pol) and 3B. Infectious cDNA clones encoding the marker viruses also contain unique restriction endonuclease sites flanking the capsid-coding region that allow for easy derivation of custom designed vaccine candidates. In contrast to the parental A(24)WT virus, single A(24)LL3D(YR) and double A(24)LL3B(PVKV)3D(YR) mutant viruses were markedly attenuated upon inoculation of cattle using the natural aerosol or direct tongue inoculation. Likewise, pigs inoculated with live A(24)LL3D(YR) virus in the heel bulbs showed no clinical signs of disease, no fever, and no FMD transmission to in-contact animals. Immunization of cattle with chemically inactivated A(24)LL3D(YR) and A(24)LL3B(PVKV)3D(YR) vaccines provided 100% protection from challenge with parental wild-type virus. These attenuated, antigenically marked viruses provide a safe alternative to virulent strains for FMD vaccine manufacturing. In addition, a competitive enzyme-linked immunosorbent assay targeted to the negative markers provides a suitable companion test for differentiating infected from vaccinated animals.