Mutations in the SAF-A/B, acinus, and PIAS domain reduce the virulence of foot-and-mouth disease virus

Foot-and-mouth disease (FMD) is a transboundary viral disease affecting the animal trade. The leader proteinase (Lpro) of the FMD virus (FMDV) plays a critical role in virulence by suppressing the innate immune response of the host. We hypothesized that stable mutations within the SAF-A/B, acinus, and PIAS (SAP) domain of Lpro would attenuate the pathogenicity while derepressing the innate immune genes. Accordingly, SAP domain mutants were generated using the FMDV Asia1/IND/63/1972 wild-type (pAsia-WT) infectious clone. Two mutant viruses, pAsia-SAP1 and pAsia-SAP2, were rescued. Replication kinetics and plaque morphology of both the mutants were comparable with that of the pAsia-WT (p > 0.05). Sequencing revealed that the pAsia-SAP2 retained the mutations till 20 passages. However, the pAsia-SAP1 retained the G56, but reverted at S55. The pAsia-SAP1 upregulated the transcripts of innate immune response genes in vitro as compared to the pAsia-SAP2. In contrast, pAsia-SAP2 showed reduced pathogenicity as evidenced by an increased survival and delayed mortality in suckling mice. Further, it failed to induce viremia in guinea pigs. It was concluded that mutations in the SAP domain of Lpro derepress the innate immune responses and reduce the pathogenicity depending on the sites of mutation. Key points • SAP domain mutations attenuate FMDV without affecting replication in vitro. • pAsia-SAP2 double mutant is genetically stable. • The site of mutation and type of amino acids influence the phenotype of mutant.