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Chapter 24
© 2012 Snippe et al., licensee InTech. This is an open access chapter distributed under the terms of the
Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits
unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The Future of Synthetic Carbohydrate Vaccines:
Immunological Studies on Streptococcus
pneumoniae Type 14
Dodi Safari, Ger Rijkers and Harm Snippe
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/48326
1. Introduction
Studies on synthetic carbohydrates to be used as potential vaccine candidates for
polysaccharide encapsulated bacteria were started in the mid-1970s. They were the logical
follow-up to studies being performed at that time on the immunogenicity of antigens
composed of carrier proteins and synthetic hapten groups. Hapten-carrier complexes were
first introduced in immunology by Karl Landsteiner in the early 1900s [1]. He discovered
that (i) small organic molecules with a simple structure, such as phenyl arsonates and
nitrophenyls, do not provoke antibodies by themselves, but (ii) if those molecules are
attached covalently, by simple chemical reactions, to a protein carrier, then antibodies
against those small organic molecules are evoked. Since their introduction, these hapten-
carrier complexes have become excellent tools to elucidate the role of different antigen-
reactive cells in the immune response [2]. The key players in this immunological process are
thymus-derived T cells and bone marrow-derived B cells. The former group of lymphoid
cells is responsible for various phenomena of cell-mediated immunity, e.g. delayed
hypersensitivity, allograft-, and graft-versus-host reactions, and reacts with specific
determinants on the carrier protein (T cell epitopes). The latter group of lymphoid cells (B
cells) give rise to the precursors of antibody-secreting cells, and reacts with both the carrier
protein and the synthetic haptenic determinants. This results in antibody formation to both
the carrier and the hapten.
The reason to apply the above concepts and techniques to carbohydrate antigens was to
address an immunological problem: polysaccharide molecules are classified as so-called
thymus-independent (TI) antigens, because they do not require T cells to induce an immune
response of B cells. As a result, the antibodies formed are mainly of the IgM class and have a
The Complex World of Polysaccharides
618
low avidity. Moreover, no immunological memory is generated and the antigens are poorly
immunogenic in infants. Latter characteristic has major implications for development of
vaccines against polysaccharide encapsulated bacteria. It was hypothesized that by linking
small carbohydrates (oligosaccharides) to a carrier protein, the immunogenic behavior
would change to that of a thymus-dependent (TD) antigen. Therefore, the studies of both
Goebel [3, 4] and Campbell and Pappenheimer [5], who first isolated the antigenic
determinant of Streptococcus pneumoniae type 3, were combined and extended. The hapten-
inhibition studies by Mage and Kabat [6] demonstrated that the antibody-combining site of
type 3 pneumococcal polysaccharide consists of two to three cellobiuronic acid units. In the
dextran-anti-dextran system extensively studied by Kabat and colleagues [7] the upper size
limit of the antibody-combining site appeared to be a hexa- or heptasaccharide and the lower
limit was estimated to be somewhat larger than a monosaccharide. Snippe and colleagues [8]
proved in 1983 that small synthetic oligosaccharides (tetra- and hexasaccharides) of S.
pneumoniae type 3 could be transformed into TD antigens by conjugating them to a protein
carrier. This opened the way to explore the synthesis and immunogenicity of numerous
oligosaccharide-carrier protein conjugates of different pneumococcal serotypes. Those
studies culminated in 2004 in the large-scale synthesis and introduction of a synthetic
oligosaccharide vaccine for Haemophilus influenzae type b for use in humans in Cuba [9]. The
recent exploration of gold nanoclusters coated with synthetic oligosaccharides and peptides
as a vaccine are a promising platform towards the development of fully synthetic
carbohydrate-based vaccines [10].
2. Streptococcus pneumoniae
Streptococcus pneumoniae (
S. pneumoniae or pneumococcus) is a leading cause of bacterial
pneumonia, meningitis, and sepsis in children worldwide. It is estimated that 1.6 million
people die from these infections each year, of whom one million are children [11, 12]. S.
pneumoniae are lancet-shaped, gram-positive, and alpha-hemolytic bacteria that colonize the
mucosal surfaces of the upper respiratory tract [13]. Three major surface layers can be
distinguished from the inside to the outside: the plasma membrane, the cell wall, and the
capsule (Fig. 1) [14]. The cell wall consists of a triple-layered peptidoglycan backbone that
anchors the capsular polysaccharide, the cell wall polysaccharide, and also various proteins
such as pneumococcal surface protein A (pspA) and hyluronate lyase (Hyl) (Fig. 1). The
capsule is the thickest layer, completely concealing the inner structures of exponentially
growing S. pneumoniae bacteria.
3. Capsular polysaccharide
Capsular polysaccharides are well known as the major virulence factors of S. pneumoniae.
Today more than 92 serotypes have been identified based on the different chemical structures
of these polysaccharides [16, 17]. This diversity determines the ability of the serotypes to
survive in the bloodstream and very likely the ability to cause invasive disease, especially in