Molecular and immunological characterisation of two vaccine dominant antigens of Schistosoma mansoni.

Abstract

Exposure to radiation attenuated cercariae of Schistosoma mansoni induces immunologically mediated protection against a challenge infection. One approach towards the selection of putative vaccine candidate molecules has therefore been the identification of ’vaccine dominant' antigens. Such molecules can be defined as those which are recognised by sera from animals vaccinated with irradiated parasites but not by sera raised in non-immune animals harbouring a single sex infection. This thesis describes the immunological and molecular characterisation of a 16 kDa vaccine dominant schistosomula surface antigen of S.mansoni and a 15 kDa antigen found on all stages of the parasite which also meets this criteria. The 16 kDa antigen extracted from mechanically transformed schistosomula and subsequently purified by immunoaffinity chromatography was shown to be a glycoprotein which incorporates both protein and carbohydrate epitopes. The latter, which include the target of a passively protective monoclonal antibody (Bickle et at., 1986), bind the lectins peanut and ricin agglutinin and are believed to incorporate the monosaccharide N- acetylgalactosamine O-linked to the peptide core. Attempts to determine the amino acid sequence of the antigen by gas phase NH2-terminal amino acid sequencing have also suggested that the protein moiety of the 16 kDa antigen is N-terminally blocked. The immunoaffinity purified 16 kDa antigen was subsequently used to immunise groups of mice in conjunction with a number of different adjuvants. Significant, albeit low, levels of resistance were achieved following immunisation with the 16 kDa molecule plus the nova some adjuvant formulation. Clones encoding the 15 kDa antigen were identified during the screening of a cDNA sporocyst library with sera specific for low molecular weight antigens. Sequence obtained for the antigen showed that it had some homology to members of a calcium binding protein superfamily, although the 15 kDa antigen itself was unable to bind calcium. The cDNA encoding this antigen was subsequently subcloned into a vector suitable for expression and the resulting fusion protein was used to immunise mice. Sera raised in these mice recognised the native 15 kDa antigen by Western blotting. However, the mice were not protected against a challenge infection.