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Classical swine fever (CSF) remains one of the most important highly contagious and fatal viral disease of swine with high morbidity and mortality. CSF is caused by classical swine fever virus (CSFV), a small, enveloped RNA virus of the genus Pestivirus. The aim of this study was to construct the a novel CSFV Fc-fusion recombinant protein and evaluate the efficacy as a vaccine against CSFV. Here, we obtained a novel subunit vaccine expressing CSFV E2 recombinant fusion protein in CHO-S cells. Functional analysis revealed that CSFV Fc-fusion recombinant protein (CSFV-E2-Fc) could bind to FcγRI on antigen-presenting cells (APCs) and significantly increase IgA levels in serum and feces, inducing stronger mucosal immune response in swine. Additionally, CSFV-E2-Fc immunization enhanced CSFV-specific T cell immune response with a Th1-like pattern of cytokine secretion, remarkably stimulated the Th1-biased cellular immune response and humoral immune response. Further, the protective effects of CSFV-E2-Fc subunit vaccines were confirmed. The data suggest that CSFV E2-Fc recombinant fusion protein may be a promising candidate subunit vaccine to elicit immune response and protect against CSFV.

The glycoprotein E2 of classical swine fever virus (CSFV) is a major immunogenic protein that induces neutralizing antibodies and protective immunity. Thus, E2 is a suitable target antigen for the development of genetically engineered CSFV vaccines. However, these vaccines cannot generate complete protective immunity in their hosts, thereby limiting the scope of applications under field conditions. IFN-γ is an immune adjuvant that has been shown to enhance antigen immune response in various experimental models. In this study, porcine IFN-γ was used to improve the immunogenicity of the CSFV E2 subunit vaccine in pigs. Pigs were immunized with E2 subunit vaccine alone or in combination with IFN-γ. Results demonstrated that porcine IFN-γ did not enhance the CSFV-specific antibody and neutralizing antibody titers compared with the E2 subunit vaccine alone. However, co-administration of the E2 and IFN-γ subunit vaccines significantly enhanced the CSFV-specific IFN-γ expression. These findings indicated that porcine IFN-γ can increase cellular immune responses to E2 protein in pigs. Furthermore, co-immunization with E2 + IFN-γ subunit vaccine and C-strain conferred complete protection against CSFV. In contrast, E2 subunit vaccines provided incomplete protection in pigs. These results indicated that using IFN-γ as an adjuvant with CSFV E2 subunit vaccines can enhance the specific protective immune response. Therefore, E2 + IFN-γ subunit vaccine is a promising marker vaccine candidate for the control and eradication of CSF.