Chemical Synthesis Elucidates the Key Antigenic Epitope of the Autism-Related BacteriumClostridium bolteaeCapsular Octadecasaccharide

The gut pathogenClostridium bolteaehas been associated with the onset of autism spectrum disorder (ASD). To create vaccines againstC. bolteae, it is important to identify exact protective epitopes of the immunologically active capsular polysaccharide (CPS). Here, a series ofC. bolteaeCPS glycans, up to an octadecasaccharide, was prepared. Key to achieving the total syntheses is a [2+2] coupling strategy based on a beta-d-Rhap-(1 -> 3)-alpha-d-Manprepeating unit that in turn was accessed by a stereoselective beta-d-rhamnosylation. The 4,6-O-benzylidene-induced conformational locking is a powerful strategy for forming a beta-d-mannose-type glycoside. An indirect strategy based on C2 epimerization of beta-d-quinovoside was efficiently achieved by Swern oxidation and borohydride reduction. Sequential glycosylation, and regioselective and global deprotection produced the disaccharide and tetrasaccharide, up to the octadecasaccharide. Glycan microarray analysis of sera from rabbits immunized with inactivatedC. bolteaebacteria revealed a humoral immune response to the di- and tetrasaccharide, but none of the longer sequences. The tetrasaccharide may be a key motif for designing glycoconjugate vaccines againstC. bolteae.