Conjugate vaccines composed of polysaccharides or oligosaccharides covalently linked to immunogenic proteins successfully overcome the limitations of pure polysaccharide antigens, which are classical T-cell-independent antigens.[1] To date, the use of oligosaccharides to provide the recognition element of such conjugate vaccines has largely been limited to experimental, proof of concept investigations, whereas polysaccharide conjugates are successfully employed in efficacious vaccination strategies against life-threatening bacterial infections. [1][2][3] Intensive efforts are directed toward the synthesis and evaluation of glycopeptide and glycolipid vaccines that present tumour-associated carbohydrate antigens, [4][5][6] the epitopes of which may contain glycosidic linkages that are potentially susceptible to endogenous glycosyl hydrolases, for example, ganglioside-specific plasma-membrane sialidase. [7][8][9] Such structural features include, but are not limited to, terminal sialyl residues, the loss of which would destroy crucial recognition elements. A method for the formation of glycosidic linkages that are resistant to enzymatic or acid hydrolysis in vivo has been proposed; C-glycosides are one class of compound that has received attention.[10-12] Herein we present data for four distinct antigen systems that show, for the first time, conjugate vaccines constructed from oligosaccharides that contain a thioglycosidic linkage function as well as antigens that induce antibodies specific for O-linked oligosaccharides.The choice of sulfur (over atoms such as carbon) to replace oxygen at the glycosidic center was based on three considerations: the ease of synthesis, [13] the existence of welldocumented examples with a similar conformational preference about the thioglycosidic and aglyconic bonds both when in solution and when complexed with a protein, [14][15][16][17] and significantly lower susceptibility to enzymatic and acid hydrolysis. [18,19] Although the sulfur-carbon bond is longer than the carbon-oxygen single bond, the CÀSÀC bond angle is significantly smaller than the CÀOÀC angle, which often results in relatively small differences between the position of the carbon atoms of the glycosidic linkage.[20] However, the longer bonds and weaker stereoelectronic effect that result when oxygen is replaced with sulfur allow substantially greater flexibility.This flexibility creates a potential obstacle to the use of thioglycoside immunogens. S-Glycosidic bonds are less constrained than the corresponding O-glycosides and can access the higher energy anti conformation in the unbound and bound state more readily. In the search for effective protective carbohydrate epitopes, it remains an open question as to whether such conformational flexibility would preclude the effective use of antigenic determinants that contain metabolically stable S-linkages. Although the structural similarity of O-and S-linked oligosaccharides is critical to this concept, their differences may also identify S-glycosides as non-self antigens ...
