Construction and Immunogenicity Evaluation of a Digestive Protein-Based Chimeric Antigen Composed of Cathepsin L1, Cathepsin B1, and Saposin-like Protein 2 from Fasciola gigantica

Fasciolosis, caused by the liver fluke Fasciola gigantica, remains a major parasitic disease affecting livestock in tropical regions and results in substantial economic losses. Although anthelmintic drugs are widely used for disease control, increasing reports of drug resistance highlight the need for alternative strategies such as vaccination. In this study, a recombinant digestive protein-based chimeric antigen (rFgCHI-DP) composed of three F. gigantica antigens—cathepsin L1 (FgCL1), cathepsin B1 (FgCB1), and saposin-like protein 2 (FgSAP2)—was designed and expressed in Escherichia coli. The mature regions of these proteins were sequentially linked to form a single chimeric construct. The recombinant protein was successfully expressed and purified under denaturing conditions, producing a protein of approximately 62 kDa. To evaluate its immunogenicity, BALB/c mice were immunized with rFgCHI-DP formulated with Quil A adjuvant. Indirect ELISA revealed that immunization induced antigen-specific IgG responses. Antibody responses showed strong reactivity toward FgCL1 and FgCB1, whereas the response against FgSAP2 was comparatively lower. Western blot analysis further demonstrated that antibodies generated against rFgCHI-DP recognized native parasite antigens. Immunolocalization also revealed that the anti-rFgCHI-DP antibodies could detect targeted antigens in the cecal epithelium of the parasite. These findings indicate that the adult-stage chimeric protein rFgCHI-DP is immunogenic in mice and capable of inducing specific antibody responses against F. gigantica. The results support the potential of rFgCHI-DP as a candidate antigen for future fasciolosis vaccine development.