Characterization of the secreted cathepsin B cysteine proteases family of the carcinogenic liver fluke Clonorchis sinensis.

Clonorchis sinensis excretory/secretory products (ESP) have gained high attentions because of their potential to be vaccine candidates and drug targets in C. sinensis prevention. In this study, we extensively profiled the characteristics of four C. sinensis cathepsin B cysteine proteases (CsCB1, CsCB2, CsCB3, and CsCB4). Bioinformatics analysis showed all CsCBs contained signal peptides at the N-terminal. Functional domains and residues were found in CsCB sequences. We expressed four CsCBs and profiled immune responses followed by vaccine trials. Recombinant CsCBs could induce high IgG titers, indicating high immunogenicity of CsCB family. Additionally, ELISA results showed that both IgG1 and IgG2a levels apparently increased post-immunization with all four CsCBs, showing that combined Th1/Th2 immune responses were triggered by CsCB family. Both Real-time polymerase chain reaction (RT-PCR) and Western blotting confirmed that four CsCBs have distinct expression patterns in C. sinensis life stages. More importantly, we validated our hypothesis that CsCBs were C. sinensis excretory/secretory products. CsCBs could be recognized by C. sinensis-infected sera throughout the infection period, indicating that secreted CsCBs are immune triggers during C. sinensis infection. The protective effect was assessed by comparing the worm burden and egg per gram (EPG) between CsCB group and control group, showing that worm burden (P < 0.01) and EPG (P < 0.01) in CsCB2 and CsCB3 groups were significantly lower than in control group. In conclusion, we profiled secreted cathepsin B cysteine proteases family for the first time and demonstrated that all CsCB family were C. sinensis excretory/secretory products that may regulate host immune responses.