Localization of the Complement C1q-Binding Site on Echinococcus multilocularis Calreticulin Identified by Peptide Mapping.

Alveolar echinococcosis is a life-threatening zoonotic parasitic disease caused by infection of Echinococcus multilocularis larvae. To survive within the host's immune milieu, E. multilocularis has evolved sophisticated immune evasion strategies, including the expression of immunomodulatory proteins that regulate the host immune response. Our previous studies have demonstrated that E. multilocularis calreticulin (EmCRT) possessed strong binding ability to human complement component C1q to inhibit C1q-initiated complement activation and biological functions. To further elucidate the mechanism by which EmCRT mediates C1q inactivation and immune evasion, the precise C1q-binding site on EmCRT was identified and analyzed in this study through expression of overlapping fragments and synthesis of overlapping peptides covering the identified functional fragment. The fragment expression and functional assay narrowed down the C1q-binding site to the EmCRT-S1 fragment located between amino acids 140 and 204 of EmCRT. The precise binding site was further pinpointed to the P5 peptide (EmCRT160-174 aa) by testing the synthetic peptides covering this region. The binding of peptide P5 to C1q markedly suppressed the activation of the C1q-mediated classical complement pathway and C1q-induced neutrophil chemotaxis, production of reactive oxygen species, cathepsin G, and myeloperoxidase. These findings suggest that the C1q-binding P5 peptide of EmCRT may serve as a potential target for the development of vaccines against echinococcosis or therapeutic drugs for complement-associated inflammatory or autoimmune diseases.