The complex world of polysaccharides edited by Desiree Nedra Karunaratne

 

The Future of Synthetic Carbohydrate Vaccines: Immunological Studies on Streptococcus pneumoniae Type 14  619 

the respiratory tract [14, 16]. Recently, new S. pneumoniae serotypes have been identified, e.g. 

serotype 6C [17], 6D [18, 19], and 11E [20]. Capsular polysaccharides are large polymers (0.5-

2x10

6

 Da), composed of multiple repeating units of up to eight sugar residues [14]. The 

capsular polysaccharides are generally synthesized by the Wzx/Wzy-dependent pathway, 

except for type 3 and 37 which are synthesized by the synthase pathway [21, 22] (Fig. 2). In 

the synthase pathway capsule is produced through processive transferase activity [23, 24].  

 

Figure 1.

 Schematic structure of S. pneumoniae. StrA=sortase A. Hyl=hyluronate lyase. 

PavA=pneumococcal adhesion and virulence. Eno=enolase. NanA=neuraminidase. PsrP=pneumococcal 

serine-rich repeat protein. LytA=autolysin. LTA=lipoteichoic acid. PspA=pneumococcal surface protein 

A. PspC=pneumococcal surface protein C. PiaA/PiuA=pneumococcal iron acquisition and uptake. 

PsaA=pneumococcal surface antigen A. (Adopted from van der Poll, T. and Opal, S.M. [15] and de 

Velasco, E.A. et al [14]) 

Many studies have demonstrated that antibodies directed against the capsular 

polysaccharide are essential for protection against pneumococcal disease [25-27]. However, 

the native capsular polysaccharides are well-known thymus-independent type-2 (TI-2) 

antigens that lack T-helper epitopes and therefore mainly induce IgM antibodies, and to a 

lesser degree IgG [28]. The TI-2 characteristics of polysaccharides can be altered by 

conjugation of polysaccharide to a protein carrier (glycoconjugate) resulting in a switch to 

an anti-polysaccharide antibody response with characteristics of a T-cell-dependent 

response. This is reflected by the generation of memory B and T cells and the induction of 

high titers of anti-polysccharide IgG antibodies after booster immunization [29]. 

---

 

The Complex World of Polysaccharides 

620 

It should be noted that not all polysaccharides behave as TI-2 antigens. Zwitterionic 

polysaccharides such as S. pneumoniea type 1 polysaccharide: [3)--AATGal-(14)--D-

GalpA-(13)--D-GalpA-(1]

n

 with a right-handed helix with repeated zwitterionically 

charged grooves elicit potent T cell responses in vivo and in vitro [30, 31].  

 

Figure 2.

 Representation of the Wzx/Wzy-dependent pathway for biosynthesis of CPS 9A (Adopted 

from Bentley. S.D. et al [21]). Representation of the Wzx/Wzy-Dependent Pathway Pictured is a 

hypothetical model for capsule biosynthesis in S. pneumoniae based on a mixture of experimental 

evidence and speculation.  

1.  Non-housekeeping nucleotide sugar biosynthesis. 

2.  The initial transferase (WchA in this case) links the initial sugar as a sugar phosphate 

(Glc-P) to a membrane-associated lipid carrier (widely assumed to be undecaprenyl 

phosphate). 

3.  Glycosyl transferases sequentially link further sugars to generate repeat unit. 

4.  Wzx flippase transports the repeat unit across the cytoplasmic membrane. 

5.  Wzy polymerase links individual repeat units to form lipid-linked CPS. 

---

 

The Future of Synthetic Carbohydrate Vaccines: Immunological Studies on Streptococcus pneumoniae Type 14  621 

6.  Wzd/Wze complex translocates mature CPS to the cell surface and may be responsible 

for the attachment to peptidoglycan.  

4. Development of pneumococcal vaccines 

Although the first pneumococcal vaccines, including the application of the principle of 

conjugate vaccination, were already initiated in the beginning of the previous century, most 

of these developments stopped when antibiotics were introduced. Existing vaccines were 

even withdrawn from the market. By now, in many parts of the world, the antibiotic 

resistance of S. pneumoniae bacteria has increased: America [32, 33], Africa [34], Europe [35, 

36], Asia [37-39], and Australia [40]. This makes treatment of pneumococcal infections more 

difficult and stresses the importance of the development of effective vaccines as a strategy to 

reduce morbidity and mortality caused by S. pneumoniae infection worldwide. 

4.1. Pneumococcal polysaccharide-based vaccines. 

Currently two vaccine types against S. pneumoniae are commercially available: a 

pneumococcal polysaccharide vaccine (PPV) and a pneumococcal conjugate vaccine (PCV) 

[41]. The first multivalent pneumococcal polysaccharide vaccine (PPV) contains 23 purified 

capsular polysaccharides (25 µg of each capsule type; Pneumovax

®

, PPV23: 1, 2, 3, 4, 5, 6B, 7, 

8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 15F, 18C, 19A, 19F, 20, 22F, 23F, 33F ) which is licensed for 

use in adults and children older than 2 years of age [42]. This vaccine was shown to be 

moderately effective in young adults [43] but not in young children [44] and elderly [45] and 

also not in immunocompromised patients, e.g HIV infected people [46, 47].  

In early 2000, a polysaccharide-protein conjugate vaccine targeting seven pneumococcal 

serotypes was licensed in the United States for use in young children (Prevnar



, PCV7: 4, 6B, 

9V, 14, 18C, 19F, 23F). The polysaccharides are conjugated to the non-toxic cross reactive 

material from diphtheria toxin, CRM

197 

and each dose contains 2µg of each capsule type, 

except for 6B, for which 4 µg is included in every vaccine dose[48]. The PCV7 vaccine 

produces a significant effect regarding prevention of invasive pneumococcal disease in 

children younger than 24 months (based on a meta-analysis of published data from trials on 

pneumococcal vaccine) [49]. Large scale introduction of PCV7 has resulted in an overall 

decline in infectious pneumococcal disease (IPD). However, IPD caused by the non-vaccine 

serotypes serotypes 1, 19A, 3, 6A, and 7F has increased (replacement disease), highlighting 

the need for inclusion of these serotypes in future improved vaccine formulations [50]. 

Apart from the CRM

197

 based PCV7, several new candidate pneumococcal conjugate 

vaccines

 

have been developed to cover more serotypes with different protein carriers and 

most of them are in clinical trials, such as PCV10 vaccine (1, 4, 5, 6B, 7F, 9V, 14, 18C, 19F, 

23F) [51, 52] and PCV13 vaccine (1, 3, 4, 5, 6A, 6B, 7F, 9V, 14, 18C, 19A, 19F, 23F) [53]. 

4.2. Pneumococcal protein-based vaccines 

An alternative vaccine strategy focuses on the use of pneumococcal surface-associated 

proteins which are to be assumed to elicit protection in all age groups against all, or nearly 

---