Synthetic oligosaccharides are designed and used to identify immunodominant epitopes of bacterial polysaccharide antigens. Conjugates of the most promising oligosaccharides are developed as potential vaccines. Focus is on diarrheal diseases.
Shigellosis – bacillary dysentery – is on the top list of diarrhoeal diseases for which the development of a broadly available vaccine is of high priority. We have designed SF2a-TT15, the first promising synthetic carbohydrate-based vacccine against endemic bacillary dysentery. The neoglycoprotein of interest targets S. flexneri 2a, the most prevalent Shigella serotype. The key component of SF2a-TT15 is a synthetic pentadecasaccharide. It was conceived to act as a functional mimic of the S. flexneri 2a somatic antigen, which is the major target of protection against homologous infection. Based on in vivo data, supported by structural, physicochemical and immunochemical information, a phase I clinical trial for this vaccine prototype is planned. Towards this aim, the synthesis of a clinical lot of SF2a-TT15 is in progress in the frame of Stopenterics, a european project.
Understanding Shigella serotype-specificity and broadening vaccine coverage in the context of endemic bacillary dysentery is our present goal. Thus, we focus on identifying functional mimics of selected Shigella somatic antigens. In particular, we have developed efficient, multistep, chemical syntheses of fragments from various S. flexneri O-SP by taking into account their non stoichiometric O-acetylation pattern -D-glucosyl side chains, and/or acidic nature. In the case of serotype 6, preliminary biological investigations with frame-shifted di-, tri-, and tetrasaccharides located on the natural surface antigen and larger fragments thereof, suggested the presence of an immunodominant epitope. On another aspect, our understanding that homologous protective antibodies recognize sets of S. flexneri serotype 3a synthetic oligosaccharides, albeit with poor affinity, supported the synthesis of larger fragments. Optimization of a novel iterative glycosylation strategy involving di- and trisaccharide building blocks paved the way to oligomers of the S. flexneri 3a O-SP repeating unit.
Chemo-enzymatic synthesis of bacterial oligosaccharides: engineered transglycosidases in action (col. I. Andre, P. Monsan, M. Remaud-Siméon, LISBP Toulouse) To contribute towards innovation in the synthesis of complex microbial cell-surface oligosaccharides, we also explored in vitro chemo-enzymatic pathways that take advantage of enzyme engineering. Transglucosidases were strategically designed to produce intermediates compatible with programmed chemical elongation from non-natural, conveniently-protected substrates. Starting from a disaccharide acceptor, an alternative synthesis of protected penta-, deca- and pentadecasaccharide haptens specific for the O-SP from S. flexneri serotype 2a was developed.
Glycopeptides as potential cancer vaccines (col. R. Lo-Man, C. Leclerc, H. Lafolly, C. Artaud, A. Dubois (IP)) Within the framework of developing a carbohydrate-based vaccine against cancer, we designed MAG-Tn3, a synthetic immunogen based on the tumor-associated Tn antigen. The MAG-Tn3 associates Tn clusters with a CD4+ T cell epitope, on a tetravalent backbone. It is a promising therapeutic vaccine against adenocarcinomas (breast, lung, and prostate cancer, among others). Based on the results of in vivo experiments in mice and non-human primates, a phase I clinical trial is under organization for this vaccine candidate and is scheduled to start late 2014. Towards this aim, we have successfully completed the key step of obtaining the MAG-Tn3 clinical batch, which complies with GMP requirements.