Chemical Synthesis of Oligosaccharides Derived from Serotype 35B and D to Investigate Binding of Complement and Serum Factors.

The wide use of capsular polysaccharide (CPS) conjugate vaccines is causing serotype replacement and the emergence of serotype 35B is concerning because of its multidrug resistance. CPS of 35B is composed of pentasaccharide repeating units that are linked through phosphodiester linkages. One of the galactofuranose residues of the pentasaccharide is acetylated, which distinguishes it from the invasive serotype 35D lacking the acetyl ester. Here, we describe a synthetic approach that can provide oligosaccharides derived of CPS 35B and 35D composed of up to 15 monosaccharides using a pentasaccharide building block equipped with four orthogonal protecting groups. The synthetic compounds were used to examine binding properties of ficolin-2, which is a protein that can activate the lectin pathway of the complement system. It was found that -acetylation is essential for recognition by ficolin-2 requiring at least two repeating units. The data provides a rationale why 35D may escape immune detection and be more invasive. The synthetic oligosaccharides were also investigated for binding to pneumococcal serum factor 35a and 29b, which are employed for serotype identification. It indicates that immunization with 35B CPS will not provide protection against 35D and thus inclusion of 35B in vaccines may result in serotype replacement by 35D. On the other hand, antibodies that can bind 35D can recognize 35B and thus 35D CPS may provide cross protection. Our findings have direct implementation for the development of the next generation pneumococcal vaccine and provide an understanding of disease severity by the emerging serotype 35B and 35D.