Attenuation without protective immunogenicity: effects of codon-pair deoptimization on foot-and-mouth disease virus serotype O in swine

Foot-and-mouth-disease (FMD), caused by the FMD virus, is a contagious disease affecting cloven-hoofed animals. Since outbreaks often cause significant economic impact, control and prevention measures against FMDV, such as vaccines, are urgently needed especially in endemic regions. The use of inactivated whole-virus vaccines has led to the eradication of FMD in many industrialized nations. Nonetheless, recent outbreaks in Europe have highlighted the virus's potential for rapid spread and its capacity to inflict substantial economic losses. Codon pair deoptimization is a novel strategy for viral attenuation. In this study, we report the first application of this approach to the P1 capsid coding region of FMDV serotype O. A recombinant virus carrying the deoptimized P1 region was attenuated in vitro, as evidenced by reduced replication capacity in porcine LFBK-αvβ6 cells. In vivo evaluations further demonstrated that a substantially higher dose was required to induce clinical disease in pigs than with wild-type viruses. However, immunization with lower and innocuous doses did not result in protective immunity. These findings underscore both the potential and the challenges of utilizing codon pair deoptimization for developing safe and effective FMDV vaccine candidates, emphasizing that further optimization is necessary to achieve a desirable balance between attenuation and immunogenicity.