The Interaction Between Echinococcus multilocularis Calreticulin S-Domain and Human Complement C1q Inhibits...

EmCRT is a calreticulin secreted by Echinococcus multilocularis during its infection in host, playing an important role in evading host immune attack as a survival strategy. Our previous study has demonstrated that recombinant EmCRT (rEmCRT) was able to bind to C1q and lectin to interfere with host classical and lectin complement activation pathway, respectively. However, the C1q-binding site on EmCRT and the associated immune evasion mechanism remain unknown. In this study, the C1q-binding site on EmCRT was determined through molecular docking analysis and fragment expression to be localized to the S-domain (EmCRT-S) between Lys¹⁴⁰ at the N-domain and Gln²⁹² at the P-domain. The recombinant EmCRT-S protein (rEmCRT-S) was subsequently expressed in bacteria. Functional analysis confirmed that rEmCRT-S was able to bind to human C1q and inhibit C1q-initiated complement activation at the similar level to the full-length rEmCRT, resulting in the reduction in C4b/C3b deposition and antibody-sensitized sheep red blood cell hemolysis. Furthermore, rEmCRT-S binding to C1q suppressed THP-1-derived macrophage chemotaxis and ROS generation. Given that the identified functional domain EmCRT-S provides similar complement regulatory functions to the full-length EmCRT, this domain is a more feasible and practical target for vaccine development against E. multilocularis infection or for inflammatory and autoimmune diseases.