Development of experimental carbohydrate-conjugate vaccines composed of Streptococcus pneumoniae capsular polysaccharides and the universal helper T-lymphocyte epitope (PADRE®)
Introduction
There are over 90 characterized serotypes of Streptococcus pneumoniae, differentiated by their polysaccharide chemical structure, and many are known to cause disease [1], [2]. These capsular polysaccharides are a natural target of the immune system and antibody responses to specific polysaccharides correlate well with protection [3], [4]. The large numbers of serotypes have required the development of vaccine products composed of polysaccharides from multiple serotypes, such as the 14-valent product licensed in 1977 [5], [6] and the 23-valent vaccine licensed subsequently in 1983 [7].
These vaccines proved to be efficacious in healthy adults but were significantly less effective in young children and the elderly [8], [9], [10], [11], [12]. Lack of good response rates can be attributed to the fact that carbohydrates of bacterial origin are often poorly immunogenic because they do not contain epitopes recognized by helper T-lymphocytes (HTL). Immunogenicity can be augmented using carbohydrate-protein conjugates wherein a ‘carrier’ protein is added as the source of HTL epitopes. Carrier proteins commonly used in commercialized and experimental vaccines include tetanus toxoid, diphtheria toxoid, CRM 197 and N. meningitides OMP [13], [14], [15], [16], [17], [18].
Synthetic peptides, representing HTL epitopes, provide an alternative to the large bacterial proteins with potential advantages for manufacturing, due to their small size and readily defined chemical properties. To address this possibility, we evaluated the non-natural Pan HLA-DR-binding Epitope (PADRE) as the carrier component in experimental conjugate vaccines based on S. pneumoniae polysaccharides derived from three serotypes, 9V, 6B and 14 [19], [20]. The results of this study are presented herein.
Section snippets
PADRE peptide synthesis and conjugation S. pneumoniae polysaccharides
The Pan HLA-DR-binding Epitope was designed to bind with moderate-high affinity to the most common HLA-DR allelic products with charged or bulky amino acids of the epitope as T cell receptor (TCR) contact points [19], [20]. PADRE binds to murine I-Ab molecules and is immunogenic in C57BL/6 mice; making is possible to evaluate conjugate vaccine immunogenicity in vivo in a well characterized animal model species. PADRE synthetic peptide, with the sequence of AKXVAAWTLKAAA where X
Immunogenicity of PADRE in experimental S. pneumoniae capsular polysaccharide conjugate vaccines
The integrity of the PADRE peptide following conjugation S. pneumoniae capsular polysaccharides was evaluated as a function or immunogenicity (Fig. 1). The CD4+ lymphocytes from mice immunized with the PADRE-Ps9V conjugate emulsified in CFA responded specifically and vigorously to both the conjugate vaccine and the PADRE peptide. These data support the conclusion that PADRE is functional following conjugation using adipic dihydrazide chemistry to the S. pneumoniae capsular polysaccharide.
Antibody responses to PADRE-S. pneumoniae polysaccharide conjugate vaccines
The
Discussion
Control of S. pneumoniae through the use of vaccines has proved to be effective in certain well-defined populations. Unfortunately, the unconjugated 23-valent vaccine fails to induce protective polysaccharide-specific antibody responses in children less than 2 years of age and the elderly [8], [9], [10], [11], [12] whereas the licensed 7-valent conjugate vaccine is immunogenic in infants but lacks appropriate serotype coverage to be effective in the elderly as well as developing nations [25].
Acknowledgements
We would like to thank Ms. Denise Porter for her expert editorial assistance.
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