Safety, efficacy, and DIVA feasibility on a novel live attenuated classical swine fever marker vaccine candidate

• A new CSF marker MLV candidate was successfully constructed based on the attenuation of the predominant 2.1 genotype strain by the deletion of two virulent associated functional residues in the Erns protein and mutation of a specific E2 epitope. • This CSF marker MLV showed genetic stable during in vitro passage and also showed safety to piglets at high dose inoculation. • Low dose of this CSF marker MLV showed full clinical protection and also prevented the virus shedding after challenge. • A accompany ELISA method was invented as serological DIVA method that showed feasibility to screened out the wild-type infection of pig while vaccinated pigs remain negative. Classical swine fever virus (CSFV) is the etiological agent of classical swine fever, a highly contagious disease that causes significant economic losses to the swine industry. Systemic prophylactic immunization with the live attenuated vaccine, the C-strain vaccine, is one of the effective measures for CSF control. However, one of the limitations of the C-strain vaccine is that the field strains-infected animals cannot be differentiated from the C-strain vaccinated herds by serological tests (DIVA). This constraint hampers the practical usage of the C-strain vaccine to eradicate the CSF in China. In the current study, a novel CSF modified live marker vaccine candidate was constructed based on the attenuation of the prevalent 2.1 genotype strain by the deletion of two virulence associated functional residues in the CSFV E rns , H79, and C171. Meanwhile, four residues S14, G22, E24, and E25 were identified specifically for the 6B8 mAb binding to the CSFV E2 as the novel conformational epitope. Then four substitutions of S14K, G22A, E24R, and G25D were further incorporated in the double deletion construct as a negative serological marker. Finally, the double-deletion marker MLV candidate GD18-ddErnHC-KARD was rescued, and its safety and efficacy profiles were evaluated in piglets. The safety study results indicated that the candidate did not induce fever, clinical signs, or pathological lesions with a high dose of 10 5.0 TCID 50 , and in addition, no virus shedding was detected until 21 days post-inoculation. Meanwhile, the efficacy study results demonstrated that at a low dose of 10 3.0 TCID 50 , it conferred complete clinical protection and no virus shedding was detected after the challenge with a highly virulent Shimen strain. Importantly, the infected animals were differentiated using the accompanied DIVA ELISA. These results constitute a proof-of-concept for rationally designing a CSF antigenically marked modified live vaccine candidate.