Targeted SAP Domain Mutations Weaken Foot-and-Mouth Disease Virus Virulence

Foot-and-mouth disease virus, or FMDV, remains one of the most consequential transboundary pathogens in veterinary medicine, capable of paralyzing livestock trade and triggering emergency vaccination campaigns whenever outbreaks occur. At the heart of its ability to overwhelm infected animals lies a small but potent viral enzyme known as the leader proteinase, Lpro, which dismantles the host’s first line of antiviral defense. A team of researchers at the ICAR-Indian Veterinary Research Institute in Bengaluru, India, has now shown that carefully engineered mutations within a specific structural region of this enzyme, the SAF-A/B, acinus, and PIAS domain, can strip the virus of much of its pathogenic punch without crippling its ability to replicate in cell culture. The findings, published in Applied Microbiology and Biotechnology, offer a rational strategy for designing attenuated FMDV strains and illuminate how a single viral protein coordinates immune evasion.

The SAP domain is a compact protein module originally described in host nuclear proteins, where it mediates binding to RNA and chromatin-associated structures. In FMDV Lpro, this region sits within the N-terminal portion of the enzyme and has been implicated in the proteinase’s ability to interfere with host cell functions, including the transcriptional activation of interferon-stimulated genes. Lpro carries out a dual life for the virus: on one hand it cleaves the host translation initiation factor eIF4G to shut down capped host mRNA translation while leaving viral RNA translation intact, and on the other hand it proteolytically removes or disrupts host factors that would otherwise signal the presence of a viral invader. By suppressing the innate immune response, Lpro buys the virus the window of time it needs to replicate to high titers, spread through the animal, and produce the characteristic vesicular lesions of foot-and-mouth disease. The Indian team hypothesized that introducing stable mutations into the SAP domain would blunt this immune suppression, thereby derepressing innate immune genes while ideally preserving the replication functions needed for vaccine production.

To test this hypothesis, the researchers built on an infectious cDNA clone derived from the wild-type FMDV strain Asia1/IND/63/1972, referred to as pAsia-WT. Using reverse genetics, they introduced targeted mutations into the codons encoding the SAP domain of Lpro and attempted to rescue viable virus from transfected cells. Two mutant viruses emerged from this effort, designated pAsia-SAP1 and pAsia-SAP2. The ability to rescue both mutants demonstrated that the SAP domain tolerates a degree of amino acid substitution without abolishing the viral life cycle, an encouraging sign for any attenuation strategy, since a vaccine candidate must replicate sufficiently to elicit immunity.

A central concern with any live attenuated virus is whether the engineered mutations compromise replication in a way that would undermine immunogenicity, or alternatively revert to wild type during propagation. The team therefore compared the growth kinetics and plaque morphology of the two mutants with those of the parental pAsia-WT virus in vitro. Both mutants replicated with kinetics statistically indistinguishable from the wild-type clone, with no significant differences observed across the measured time points, and their plaque phenotypes were likewise comparable. This result indicates that the SAP domain mutations do not impose a measurable fitness cost in cell culture, fulfilling one of the key prerequisites for a practical live vaccine platform.

Genetic stability proved to be the discriminating factor between the two mutants. When the viruses were serially passaged, sequencing revealed that pAsia-SAP2 retained its engineered mutations through twenty passages, a reassuring indicator of genomic stability. pAsia-SAP1, in contrast, showed partial reversion: the mutation at position 56, G56, was preserved, but the engineered change at position 55 reverted back to the wild-type serine, S55. This differential behavior underscores a broader principle of attenuated virus design, namely that the identity of the substituted amino acid and the precise position within the functional domain jointly determine whether an attenuation phenotype will be maintained over successive replication cycles. A mutation that is stable is not merely preferable; it is essential, since reversion during vaccine manufacture or in a vaccinated animal could restore virulence.

The immunological consequences of the mutations were assessed by measuring the transcripts of innate immune response genes in infected cell cultures. Here the two mutants diverged in a revealing way. Cells infected with pAsia-SAP1 showed a pronounced upregulation of innate immune response gene expression compared with the wild-type virus, consistent with the idea that an impaired SAP domain can no longer fully suppress the host’s antiviral transcriptional program. pAsia-SAP2, by contrast, did not produce the same degree of immune gene derepression in vitro. On the surface, this might suggest that SAP1 is the stronger candidate for immune activation, but the in vivo data told a different and more nuanced story.

In the suckling mouse model, a sensitive standard assay for FMDV virulence, pAsia-SAP2 displayed a markedly reduced pathogenicity. Animals inoculated with the SAP2 mutant survived in greater numbers, and those that did succumb did so with delayed mortality, indicating that the virus had lost a substantial portion of its capacity to cause lethal disease. The partial reversion at S55 in SAP1 may help explain why the less stable mutant did not show the same degree of attenuation in animals; restoration of even a single wild-type residue can partially restore the virulence-associated functions of Lpro. Taken together, the data support the authors’ conclusion that mutations in the SAP domain derepress innate immune responses and reduce pathogenicity in a manner that depends critically on the site of mutation and the nature of the substituted amino acids.

The attenuation phenotype was further corroborated in guinea pigs, an established laboratory host for FMDV research. When guinea pigs were inoculated with pAsia-SAP2, the mutant failed to induce detectable viremia, meaning the virus could not establish a systemic blood-borne infection in these animals. This absence of viremia is a strong indication that the SAP2 mutations substantially impair the virus’s ability to progress beyond local replication and disseminate within a mammalian host, a defining feature of FMDV pathogenesis. For a candidate vaccine strain, failure to produce viremia is doubly desirable, because it simultaneously suggests reduced disease potential and a lower probability that the vaccine virus will spread systemically and be shed by immunized animals.

The broader significance of this work lies in the strategy it validates. Rather than deleting entire genes or relying on serial passage to weaken the virus, the researchers targeted a defined virulence determinant with site-directed mutagenesis, preserving replication competence while dismantling immune evasion. Because Lpro sits at the interface of viral replication and innate immune antagonism, tuning this protein offers an unusually elegant attenuation mechanism: the crippled immune evasion means the host mounts an earlier and stronger antiviral response, while the intact replicative machinery ensures the virus still delivers antigen effectively. The demonstration that mutation site and amino acid identity govern both stability and phenotype gives vaccine developers a practical map for engineering the next generation of FMDV vaccine candidates. FMDV circulates as seven distinct serotypes with substantial antigenic diversity, and vaccines based on attenuated strains can, with appropriate matching, deliver broader and more durable immunity than conventional inactivated vaccines, which require high-containment production and provide incomplete cross-protection.

As foot-and-mouth disease continues to menace livestock economies across Asia, Africa, and beyond, the demand for safer, more stable, and more easily manufactured vaccines remains acute. This study, conducted with support from the Department of Biotechnology, Government of India, adds a molecularly precise tool to that effort. By showing that SAP domain mutations in Lpro can render an Asia1 serotype virus attenuated in vivo while preserving its growth properties in vitro and its genetic integrity over serial passage, the ICAR-IVRI team has provided both a candidate platform and a mechanistic insight: the same domain that helps FMDV hide from the immune system can, when disrupted at the right positions, become the virus’s undoing. Future work will need to test protective efficacy and cross-serotype applicability, but the foundational principle demonstrated here, that rational attenuation of a virulence factor can outperform empirical weakening, is likely to inform live attenuated vaccine design well beyond this single pathogen.

Subject of Research: SAP domain mutations in foot-and-mouth disease virus leader proteinase that reduce virulence

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

Article References: Pyatla, M. K. G., Shekhawat, I., Mundhe, S. S., Elango, S., Periyasamy, T. S. R., Nagargoje H, S., Mahadappa, P., Vijayapillai, U., Krishnaswamy, N., Chaudhuri, P., & Joyappa, D. H. (2026). Mutations in the SAF-A/B, acinus, and PIAS domain reduce the virulence of foot-and-mouth disease virus. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14034-2

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14034-2

Keywords: foot-and-mouth disease virus, FMDV, leader proteinase, SAP domain, virulence, attenuation, innate immunity, reverse genetics, live vaccine, viral pathogenesis, Asia1 serotype, genetic stability