Pneumococcal infections cause millions of deaths among children and the elderly worldwide, despite the widespread use of a blockbuster glycoconjugate vaccine Prevnar13®.[1] This vaccine induces an immune response against bacterial cell-wall glycans which are not present on human cells.[2] Although it has dramatically reduced mortality caused by invasive pneumococcal diseases (IPD), several issues prevail, such as insufficient serotype coverage, troublesome purification from isolated capsular polysaccharides (CPSs) and lack of efficacy against certain serotypes.[2] Chemically defined synthetic oligosaccharides containing the protective glycan epitope (glycotope) are an attractive alternative to isolated CPSs. They induce immune response to the native CPSs and avoid problems common for CPSs, such as contamination with pneumococcal cell wall polysaccharide, various chain lengths and multiple coupling sites.
The CPS of S. pneumoniae serotype 3 (ST3) has a disaccharide repeating unit (1) consisting of β-linked glucuronic acid and glucose (cellobiuronic acid) (Fig. 1). It is known to cause invasive pneumococcal infections both in children and adults. Although it is covered by highly successful pneumococcal conjugate vaccine Prevnar13®, it has limited efficacy and remains the main cause of invasive pneumococcal disease in Europe.[3] Recently, a tetrasaccharide 2-CRM197 conjugate has been identified as a synthetic vaccine candidate.[4] In order to gain deeper understanding of the role of the terminal sugar as well as to study antibody binding, two trisaccharides 3 and 4 representing frameshifts of the repeating unit were designed and prepared (Fig. 1).
Streptococcus pneumoniae serotype 7F (ST7F) is the second major cause of IPD in Europe, infecting mainly adult population.[3] Its repeating unit (RU) structure was assigned in 1988.[6] The branched heptasaccharide ST7F CPS RU 5 is composed of a linear sequence of D-galactose (D-Gal), 2-O-acetyl-L-rhamnose (L-RhaOAc), D-glucose (D-Glc) and N-acetyl-D-galactoseamine (D-GalNAc). A two-sugar side chain branching from D-GalNAc contains L-rhamnose (L-Rha) and N-acetyl-D-glucoseamine (D-GlcNAc). A second branch leads from D-Gal and is formed by another D-Gal unit (Fig 1). In 1992, Kamerling et al. reported the synthesis of O-deacetylated L-Rha-D-Glc-D-GalNAc trisaccharide.[7] To the best of our knowledge, there are no other reports concerning the synthesis or immunology of ST7F oligosaccharide antigens.
In order to identify the minimal glycotope that can elicit a robust immune response to the CPS, we designed a series of oligosaccharides related to the RU of ST7F CPS. The effect of branching, length and the role of deacetylation on overall immunogenicity was evaluated. Each compound was equipped with a reducing-end C5 linker to enable printing on glycan arrays and conjugation to a carrier protein.
References
[1] Nat. Rev. Micro, 2009, 7, 838-838.
[2] B. Schumann, C. Anish, C. L. Pereira and P. H. Seeberger, in Biotherapeutics: Recent Developments using Chemical and Molecular Biology, The Royal Society of Chemistry, 2013, DOI: 10.1039/9781849737159-00068 pp. 68-104.
[3] ECDC Report, 2014.
[4] S. G. Parameswarappa, K. Reppe, A. Geissner, P. Ménová, S. Govindan, Adam D. J. Calow, A. Wahlbrink, Markus W. Weishaupt, Bopanna P. Monnanda, Roland L. Bell, L.-A. Pirofski, N. Suttorp, Leif E. Sander, M. Witzenrath, Claney L. Pereira, C. Anish and Peter H. Seeberger, Cell Chem. Biol., 2016, 23, 1407-1416.
[5] X. Wu, C.-C. Ling and D. R. Bundle, Organic Letters, 2004, 6, 4407-4410.
[6] M. Moreau, J. C. Richards, M. B. Perry and P. J. Kniskern, Carb. Res., 1988, 182, 79-99.
[7] A. M. P. van Steijn, J. P. Kamerling and J. F. G. Vliegenthart, J. Carb. Chem., 1992, 11, 665-689.
