The ability of fractionated sera from animals vaccinated with irradiated cercariae of schistosoma mansoni to transfer immunity to mice

To study the role of IgG and IgM isotypes found in sera of mice or rabbits immunized with irradiated cercariae in schistosome immunity, the respective sera were fractionated by protein A chromatography. Both the protein A-bound and unbound fractions of vaccinated mouse serum (VMS) showed reactivities in ELISA assay using NP-40 membrane extracts of 3-hr schistosomula as antigens and in indirect immunofluorescence assay (IIF) using live 3-hr schistosomula. Both the protein A-bound and unbound fractions possessed high levels (84% and 76%, respectively) of complement-mediated cytotoxicity against schistosomula in vitro. The IgG- and IgM-containing fractions each conferred passive protection (30% and 20%) against challenge infection, although at a lower level when compared to unfractionated VMS (42%). These data demonstrate that in the mouse model both IgG and IgM can recognize the epitopes on the surface of schistosomula, mediate cytotoxicity in vitro, and provide passive protection in vivo. Similarly, the protein A-bound and unbound fractions of vaccinated rabbit serum (VRS) were also shown to be positive in ELISA and IIF. The IgG- and IgM-containing fractions each possessed high levels (95% and 85%, respectively) of complement-dependent cytotoxicity against schistosomula in vitro. In contrast to VMS fractions, the IgG fraction of VRS conferred a similar level (28%) of in vivo protection as unfractioned VRS when injected into mice no later than 6 days after challenge. Moreover, the IgG fraction of VRS was still able to provide passive protection to mice when given as late as 15 days postinfection, but failed to confer protection when injected at 24 or 35 days postinfection. When VRS was fractionated with Sepharose 4B conjugated with schistosomula NP-40 membrane extracts, the bound fraction was IIF positive on both living and acetone-fixed schistosomula, whereas the unbound fraction was IIF positive only on acetone-fixed parasites. However, both fractions demonstrated high levels of complement-dependent cytotoxicity in vitro. These data suggest that epitopes present in and absent from NP-40 membrane extracts of schistosomula recognized by antibodies found in VRS might be involved in cytotoxicity in vitro.