Classical swine fever (CSF), caused by classical swine fever virus (CSFV), continues to threaten the global swine industry. Current E2-based subunit vaccines are limited by their slow adaptation to emerging viral strains and insufficient cell-mediated immunity. To address these challenges, we developed mRNA-lipid nanoparticle (LNP) vaccines encoding modified forms of the CSFV E2 glycoprotein. In this study, we constructed three novel mRNA-LNP vaccines encoding engineered E2 proteins: E2-mRNA-LNP, XCL1-E2-mRNA-LNP containing the X-C motif chemokine ligand 1 (XCL1) to target dendritic cells, and Ub-E2-mRNA-LNP containing ubiquitin (Ub) to enhance antigen presentation through Ub-mediated degradation. These vaccines were evaluated in rabbits and piglets. Following challenge with the live attenuated CSFV C-strain, rabbits immunized with XCL1-E2-mRNA-LNP or Ub-E2-mRNA-LNP exhibited neither fever nor detectable viral RNA in the spleens. Notably, immunization with Ub-E2-mRNA-LNP conferred complete protection against challenge with the highly virulent CSFV Shimen strain in piglets, outperforming the E2-based subunit vaccine, the unmodified E2-mRNA-LNP, and XCL1-E2-mRNA-LNP. Further analysis revealed that Ub-E2-mRNA-LNP elicited more potent antibody and cell-mediated immune responses. Our study demonstrates that Ub-E2-mRNA-LNP is a promising vaccine candidate for CSF prevention.IMPORTANCECSF remains a major threat to the global swine industry. Recently, a moderately virulent subgenotype CSFV 2.1c strain was isolated and reported in China. Given that the mRNA platform enables rapid vaccine development to address emerging pathogens, we constructed three nucleoside-modified mRNA-LNP vaccines expressing engineered E2 glycoproteins of CSFV, including E2-mRNA-LNP, XCL1-E2-mRNA-LNP (targeting dendritic cells), and Ub-E2-mRNA-LNP (enhancing proteasomal degradation). Evaluated in rabbits, XCL1-E2- and Ub-E2-mRNA-LNPs conferred strong protection against CSFV C-strain challenge, with no fever or detectable virus in tissues. Both induced stronger antibody responses than E2-mRNA-LNP. Notably, Ub-E2-mRNA-LNP provided complete protection in piglets, superior to the E2-based subunit vaccine, E2-mRNA-LNP, or XCL1-E2-mRNA-LNP groups and eliciting robust humoral and cellular immunity. These results highlight Ub-E2-mRNA-LNP as a highly promising candidate for an effective CSF vaccine.

Keywords: E2 protein; antigen modification; classical swine fever virus; mRNA vaccine; ubiquitination.