Calreticulin Interferes with C1q-Mediated Complement Activation.

As a zoonotic disease caused by larvae, alveolar echinococcosis (AE) is one of the most severe forms of parasitic infection. Over a long evolutional process has developed complex strategies to escape host immune attack and survive within a host. However, the mechanisms underlying immune evasion remain unclear. Here we investigated the binding activity of calreticulin (CRT), a highly conserved Ca-binding protein, to human complement C1q and its ability to inhibit classical complement activation. ELISA, Far Western blotting and immunoprecipitation results demonstrated that both recombinant and natural CRTs bound to human C1q, and the interaction of recombinant CRT (rCRT) inhibited C1q binding to IgM. Consequently, rCRT inhibited classical complement activation manifested as decreasing C4/C3 depositions and antibody-sensitized cell lysis. Moreover, rCRT binding to C1q suppressed C1q binding to human mast cell, HMC-1, resulting in reduced C1q-induced mast cell chemotaxis. According to these results, expresses CRT to interfere with C1q-mediated complement activation and C1q-dependent non-complement activation of immune cells, possibly as an immune evasion strategy of the parasite in the host.