Screening and Immune Efficacy Evaluation of Antigens with Protection Against Feline Calicivirus

Background: Feline calicivirus (FCV), a pathogen that causes upper respiratory tract diseases in felids, primarily leads to oral ulcers and various respiratory symptoms, which can be fatal in severe cases. Currently, FCV prevention and control rely primarily on vaccination; however, the existing vaccine types in China are mainly inactivated vaccines, leading to a single prevention and control method with suboptimal outcomes. Methods and Results: This study commences with a genetic evolution analysis of Chinese FCV isolates, confirming the presence of two major genotypes, GI and GII with GI emerging as the dominant form. We subsequently selected the broadly neutralizing vaccine candidate strain DL39 as the template for the truncation and expression of multiple recombinant proteins. Through serological assays, we successfully confirmed the optimal protective antigen region, which is designated CE39 (CDE). Further investigation revealed the location of the optimal protective antigen region within the CE region for both the GI and GII genotype strains. Capitalizing on this discovery, a bivalent recombinant protein, designated CE39-CEFB, was generated. Cat antisera generated against CE39 and CE39-CEFB proteins were used in cross-neutralization against various strains of different genotypes, yielding high neutralization titers ranging from 1:45 to 1:15 and from 1:48 to 1:29, respectively, which surpassed those induced by antisera from cats vaccinated with Mi-aosanduo (commercial vaccine, strain 255). Ultimately, in vivo challenge experiments were per-formed after immunizing cats with the CE39 and CE39-CEFB proteins, utilizing Miaosanduo as a control for comparison. The results demonstrated that immunization with both proteins effectively made cats less susceptible to FCV GI, GII, and VSD strains infection, resulting in superior immune efficacy compared with that in the Miaosanduo group. Conclusion: These results indicate that this study successfully identified the antigen CE39, which has broad-spectrum antigenicity, through in vivo and in vitro experiments. These findings pre-liminarily demonstrate that the optimal protective antigen region of FCV strains is the CE region, laying a theoretical foundation for the development of novel broad-spectrum vaccines against FCV disease.