We have demonstrated that a mouse monoclonal IgE antibody, SJ18 epsilon.1, recognizes a 97 kDa surface molecule (Sj97) of Schistosoma japonicum larvae and that the antibody induces partial but significant protection against the skin to lung-stage of S. japonicum infection. The antibody stimulates eosinophil- and macrophage-mediated killing of schistosomula in vitro. In the present study, we isolated the putative full-length cDNA of Sj97 by screening a lambda gt11 cDNA library from S. japonicum adult worms with SJ18 epsilon.1. The predicted amino acid sequence of the cDNA showed highly significant homology to that of S. mansoni paramyosin, a potential vaccine candidate for schistosomiasis. The deletion mutants of S. japonicum paramyosin were expressed in Escherichia coli and the translation product of 443 amino acid residues of paramyosin was found to be recognized by the antibody. Moreover, we observed by immunoelectron microscopy the presence of paramyosin in the post-acetabular gland as well as in the tegument and muscle layers of the larvae. These results suggest that paramyosin is a secretory protein which may be incorporated into the tegument during the development of schistosomula, thus becoming a target for protective immunity during the migratory phase of the parasite.

Although the strain difference in protection of mice to Schistosoma mansoni infection has been described, limited information is available in the case of Schistosoma japonicum. In the present study, we compared the protective immunity to S. japonicum infection and cytokine production in various strains of mice vaccinated with gamma-irradiated cercariae. A significant reduction in worm recovery was observed in male and female mice of DBA/2 at a 6-week interval between vaccination and a challenge infection, whereas vaccinated mice of C57BL/6, C57BL/10, (C57BL/6 x DBA/2) F1 (B6D2F1) and (C57BL/10 x DBA/2) F1 (B10D2F1) showed no detectable level of protection. No sex-linked difference in development of resistance was observed in any of the strains so far examined. Vaccination with gamma-irradiated cercariae twice with a 3-week interval also induced significant protection against a challenge infection in DBA/2 but not in BALB/c or C57BL/6 strains. Further studies demonstrated that spleen cells of vaccinated C57BL/6 mice produced lower levels of IFN-gamma compared to the cells of vaccinated BALB/c and DBA/2. On the other hand, production of IL-10 by spleen cells was relatively higher in BALB/c mice than in the other two strains. Macrophages that had been stimulated with spleen cell culture supernatants derived from vaccinated DBA/2 damaged schistosomula more effectively than cells stimulated with supernatants derived from the other strains. These results suggest that different levels of protection observed among strains of mice depend on the balance of cytokine responses which consequently activate or suppress macrophage-mediated damage to schistosomula.

Human recombinant tumour necrosis factor (rTNF) enhanced monoclonal IgE-dependent eosinophil-mediated cytotoxicity to schistosomula of Schistosoma japonicum in a dose-dependent manner. The enhancing effect of rTNF was also observed for the antibody-dependent cytotoxicity of a human eosinophilic leukemia cell line, EoL-3, but not of another cell line, EoL-1. Observation by a slow-motion movie camera demonstrated that activated EoL-3 cells adhered to the surface of schistosomula by 6 h after incubation, which triggered intracellular movement of eosinophil granules. The granules were concentrated toward the surface of the larvae and then degranulation started. The cell membrane was left as a balloon-like remnant. Cell sorting analysis by FACStar indicated that the expression of receptors for C3bi (CR3) and low affinity FcR for IgE (Fc epsilon RII) increased on the surface of EoL-3 cells after stimulation with rTNF, while this was not observed for EoL-1 cells.

We have been able to produce a mouse monoclonal IgE antibody specific to an adult worm antigen extracted from Schistosoma japonicum (Sj). The antibody was able to elicit passive cutaneous anaphylaxis in the rat skin against Sj with the highest titer of 1:256,000 but did not cross-react with S. mansoni antigen. The antibody recognized a 97-kDa molecule expressed on the surface of mechanically transformed schistosoma of S. japonicum. Passive transfer of the antibody into mice in the early stage of challenge infection resulted in a partial but significant reduction of recovery of adult worms. Induction of eosinophilia by an oral administration of embryonated eggs of Toxocara canis prior to challenge infection enhanced the reduction.

Passive transfer of immunity using a mouse monoclonal IgE antibody against Schistosoma japonicum was found to be enhanced by heterologous helminth infections. BALB/c mice were infected with Toxocara canis or Nippostrongylus brasiliensis so as to induce eosinophilia prior to a challenge infection with S. japonicum. Recovery of adult schistosomes decreased in a group of mice that had been infected with T. canis and challenged with cercariae at the cutaneous site of sensitization with the IgE antibody as compared with that in mice that had been similarly treated with normal serum in the absence of T. canis infection. Histological examinations revealed a close association of polymorphonuclear cells, including eosinophils, with damaged schistosomula in the skin of T. canis-infected mice that had received the IgE antibody. An enhancement in worm reduction was also observed in mice harboring either of both nematodes when the monoclonal antibody had been injected intraperitoneally during the phase of migration of schistosomula from the skin to the lungs. In vitro studies on macrophage-mediated damage to schistosomula suggested that the enhancement in worm reduction was at least partly due to the activation of macrophages induced by the heterologous infections.

Participation of IgE in protective immunity against Schistosoma japonicum was examined by comparing congenital IgE-deficient SJA/9 and IgE-producing SJL/J mice. Mice immunized with 100 irradiated cercariae 7 weeks previously were infected with 50 live cercariae. In SJL/J mice at 40 days after infection, a 3-to 4-fold increase of total IgE levels and anti-S. japonicum egg IgE antibody production were observed with no significant difference between immunized and nonimmunized mice. IgE was not detected in SJA/9 mice throughout the experiments. Protective immunity evaluated by recovery of adult worms was found in SJA/9 mice and was comparable to that of SJL/J mice. These results suggest that acquired immunity in mice with irradiated cercariae of S. japonicum was not dependent on IgE in these strains of mice.

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