Pneumococci consist of a family of Gram positive bacteria with over 90 different types of capsular polysaccharide. The capsule is a virulence factor for the bacterium, which alters opsonisation by phagocytes. The capsular polysaccharides are T-cell independent, and only weekly immunogenic; immunity to them develops gradually during childhood. Antibodies against the capsular polysaccharides are protective and prevent pneumococcal disease. The classification of Streptococcus pneumoniae into serogroups, types and subtypes is done according to the capsular swelling reaction, which is mostly detected microscopically by naked eye as the quellung reaction or alternatively by counter immunoelectrophoresis or latex agglutination.
The first documented vaccines against Streptococcus pneumoniae used either the whole bacterial cell or the capsular polysaccharides (Wright et al., 1914) as the starting point. During the 1970's and 1980's more purified forms of the multivalent polysaccharide vaccines (PPV) were tested against a wide array of pneumococcal infections, but with disappointingly little success in young children (Fedson et al., 2004). In second generation of pneumococcal vaccines, the capsular polysaccharide is linked via a chemical conjugation process to a carrier protein such as an outer membrane protein of Neisseria meningitidis, tetanus or diphtheria toxoid, or a 37 kD protein of Haemophilus influenzae, thus making the polysaccharide antigen T-cell dependent. These vaccines induce immunological memory at an early stage, i.e. before 2 years of age, unlike the PPVs which are poorly immunogenic for most pneumococcal serotypes at this stage (Eskola et al., 2004).
Pneumococci have several surface proteins such as PspA and PsaA as well as endotoxins such as pneumolysin excreted by the bacterium at the different phases of the host-pathogen life cycle. The third generation of Pnc vaccines, which are in their early phase of clinical development utilize these common Pnc proteins alone without pneumococcal polysaccharides (see Chapter 10).
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