Immunity to schistosomiasis: glycans are potential antigenic targets for immune intervention

The major humoral immune responses in animals infected with Schistosoma mansoni are directed toward carbohydrate antigens. Among these antigens are complex-type N-glycans expressing LDN GalNAcbeta1-4GlcNAc-R], LDNF GalNAcbeta1-4(Fucalpha1-3)GlcNAc-R], and polymeric Lewis x (Lex) Galbeta1-4(Fucalpha1-3)GlcNAc]n-R epitopes. We have now evaluated the potential of the three glycan antigens as targets for immune-mediated intervention of infections and serodiagnosis. A variety of approaches were employed, including ELISA, Western blot, immunohistology, and in vitro complement lysis assays, to determine the immunogenicity of the glycans in infected humans, their localization on the parasites and their efficacy as targets for parasite lysis. Our results show that S. mansoni-infected patients, with either intestinal or hepatosplenic disease, generate predominantly IgM, but also IgG and IgA, antibodies to LDN, LDNF, and Lex. However, immune responses to Lex are generally lower than responses to LDN and LDNF and less specific to schistosome infections. Western blot analysis with monoclonal antibodies (mAb) to LDN, LDNF, and Lex determinants show that the glycan antigens occur on multiple glycoproteins from cercariae, 3-h, 48-h, and lung stage schistosomula, as well as adults and eggs. Immunohistological studies demonstrate that LDN, LDNF, and Lex are expressed on the parasite surface at all stages of development in the vertebrate host. Importantly, a mAb to LDN in the presence of complement efficiently kills schistosomula in vitro, as demonstrated by flow-cytometric assays that quantify cytolysis by propidium iodide uptake into damaged parasites. These findings raise the possibility that LDN and LDNF may be targets for vaccination and/or serodiagnosis of chronic schistosomiasis in humans.