Abstract <p>The E2 subunit vaccine has been considered a promising alternative to an attenuated classical swine fever (CSF) vaccine. However, it fails to induce a good cellular immune response. Given that immunogenic adjuvants can regulate the cellular immunity to achieve a maximum efficacy against antigens, immunostimulatory effects of porcine IL-28B on the CSF virus (CSFV) E2 subunit vaccine were evaluated in the present study. We expressed recombinant proteins E2-IL28B, E2, and IL-28B using CHO-S mammalian cells as an antigen expression platform, and three types of CSFV E2 subunit vaccines based on antigens E2-IL28B, E2 + IL-28B, and E2 were prepared, respectively. We found that both E2-IL28B and E2 + IL-28B antigens exhibited superior immunogenicity with dramatically induced antibody titers and neutralizing antibody levels than the E2 alone. Moreover, E2-IL28B or E2 + IL-28B, instead of E2, boosted cellular immune responses via obviously increasing the percentages of CD3⁺CD4⁺ T lymphocytes, promoting the lymphocyte proliferations, and enhancing the release of Th1-type cytokines. All results revealed that the inclusion of IL-28B, whether fused or mixed with E2, significantly elevated E2-induced immune potencies, suggesting that IL-28B could be used as a molecular adjuvant to optimize the design of E2 subunit vaccine for more effective controls of the CSF disease.</p> Key points <p><i>• New CSF E2 subunit vaccine candidates were developed in which IL-28B was an immunoadjuvant</i></p> <p><i>• IL-28B significantly elevated the E2-induced immune potency whether it was fused or mixed with E2</i></p> <p><i>• This study provided novel insights into the immunoregulatory properties of IL-28B used for the optimized subunit vaccine design</i></p>