Towards a synthetic glycoconjugate vaccine against Neisseria meningitidis A

Albumin conjugates of synthetic fragments of the capsular polysaccharide of the Gram-negative bacterium Neisseria meningitidis serogroup A were prepared. The fragments include monosaccharides 1 [alpha-D-ManpNAc(1--> 0)-(CH2)(2)NH2] and 2 [6-O-P(O)-(O-)(2)-alpha-D-ManpNAc-(1 --> O)-(CH2)(2)NH2] ,disaccharide 3 [alpha-D-ManpNAc[1-->O-P(O)(O-) --> 6]-alpha-D-ManpNAc(1 --> O)-(CH2)(2)NH2], and trisaccharide 4 [alpha-D-ManpNAc-[1 -->O-P(O)(O-)--> 6]-alpha-D-ManpNAc-[1 -->O-P(O)(O-)--> 6]-alpha-D-ManpNAc-(1 --> O)-(CH2)(2)NH2]. Two monosaccharide blocks were employed as key intermediates. The reduc-ing-end mannose unit featured the NHAc group at C-2, and contained the aminoethyl spacer as the aglycon for the final bioconjugation. The interresidual phosphodiester linkages were fashioned from an anomerically positioned H-phosphonate group in a 2-azido-man-nose building block. The spacer-linked saccharides 1-4 were N-acylated with hepta-4,6-dienoic acid and the resulting conjugated diene-equipped saccharides were subjected to Diels-Alder-type addition with maleimidobutyryl-group functionalized human serum albumin to form covalent conjugates containing up to 26 saccharide haptens per albumin molecule. Complete H-1, C-13, and P-31 NMR assignments for 1-4 are given. Antigenicity of the neoglycoconjugates containing 1-4 was demonstrated by a double immunodiffusion assay which indicated that a fragment as small as a monosaccharide is recognized by a polyclonal meningococcus group A antiserum and that the O-acetyl group(s) present in the natural capsular material is not essential for antigenicity.