Alveolar echinococcosis (AE) is a severe parasitic zoonosis caused by the larval stage of Echinococcus multilocularis. The identification of the antigens eliciting acquired immunity during infection is important for vaccine development against Echinococcus infection. Here, we identified that E. multilocularis calreticulin (EmCRT), a ubiquitous protein with a Ca2+-binding ability, could be recognized by the sera of mice infected with E. multilocularis. The native EmCRT was expressed on the surface of E. multilocularis larvae as well as in the secreted products of metacestode vesicles and protoscoleces (PSCs). The coding DNA for EmCRT was cloned from the mRNA of the E. multilocularis metacestode vesicles and a recombinant EmCRT protein (rEmCRT) was expressed in E. coli. Mice immunized with soluble rEmCRT formulated with Freund’s adjuvant (FA) produced a 43.16% larval vesicle weight reduction against the challenge of E. multilocularis PSCs compared to those that received the PBS control associated with a high titer of IgG, IgG1 and IgG2a antibody responses as well as high levels of Th1 cytokines (IFN-γ and IL-2) and Th2 cytokines (IL-4, IL-5 and IL-10), produced by splenocytes. Our results suggest that EmCRT is an immunodominant protein secreted by E. multilocularis larvae and a vaccine candidate that induces partial protective immunity in vaccinated mice against Echinococcus infection.
As a zoonotic disease caused by Echinococcus multilocularis larvae, alveolar echinococcosis (AE) is one of the most severe forms of parasitic infection. Over a long evolutional process E. multilocularis 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 E. multilocularis calreticulin (EmCRT), a highly conserved Ca2+-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 EmCRTs bound to human C1q, and the interaction of recombinant EmCRT (rEmCRT) inhibited C1q binding to IgM. Consequently, rEmCRT inhibited classical complement activation manifested as decreasing C4/C3 depositions and antibody-sensitized cell lysis. Moreover, rEmCRT binding to C1q suppressed C1q binding to human mast cell, HMC-1, resulting in reduced C1q-induced mast cell chemotaxis. According to these results, E. multilocularis expresses EmCRT 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.
Background Alveolar echinococcosis (AE) is a severe parasitic zoonosis, caused by the larval stage of Echinococcus multilocularis. Identification of the antigens eliciting acquired immunity during infection is important for vaccine development against Echinococcus infection. Methods First of all, we predicted the physical and chemical properties and protein structure characteristics of E. multilocularis calreticulin (EmCRT), and then constructed the recombinant plasmid pET28a-EmCRT. Secondly, the constructed recombinant plasmid was transformed into E. coli BL21 and induced to produce recombinant EmCRT (rEmCRT). Then the antigen specificity and the calcium-binding property of rEmCRT was detected. Thirdly, the expression level and distribution of Emcrt gene in PSCs and vesicles were analysed. And the protective immunity induced by rEmCRT immunized mice and the corresponding mechanismwas investigated in the end. Results We identified that EmCRT, a ubiquitous protein with Ca2+-binding ability, elicited protective immunity against E. multilocularis infection. EmCRT was found to be expressed on the surface of E. multilocularis larvae as well as in the secreted products of metacestode vesicles and protoscoleces (PSCs). Recombinant EmCRT formulated with Freund’s adjuvant (FA) elicited both IgG1 and IgG2a antibodies, with a higher IgG1/IgG2a ratio indicating a predominant Th2 response in vaccinated mice, and stimulated mouse lymphocytes from the spleen to produce high levels of Th1 (IFN-γ, IL-2) and Th2 (IL-4, 5, 10) cytokines. The protection was around 43.7% after challenged with E. multilocularis PSCs. Conclusions Our results suggest that EmCRT is an immunodominant protein secreted by E. multilocularis larvae and contributes to the induction of partial protective immunity in vaccinated mice against Echinococcus infection. Therefore, EmCRT could be a novel and potential candidate vaccine against AE.
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