Polyosides (encapsulated bacteria)

The polysaccharide capsule which surrounds bacterial species Such as Haemop hilus influenzae, Streptococcus pneumoniae, Neisseria meningitidis and Salm onella typhi is a potent virulence factor by protecting the bacteria from p hagocytosis. The host responds with antibody production and specific antibo dies plus complement binding to the capsule facilitate opsonization of the micro-organism, which is phagocytized and eliminated. Purified capsular pol ysaccharides elicit T-independent antibody responses without a memory funct ion, but are often poorly immunogenic in infants where much of the invasive H. influenzae type b (Hib) and pneumococcal infections is seen. Therefore purified polysaccharides have found limited use as vaccines. However, coval ent linkage of the capsular polysaccharide, or fractions thereof, to immuno genic carrier proteins creates glycoconjugates which are T-dependent antige ns and which elicit antibodies also in infants and which prime for boosting either with the glycoconjugate or the capsular polysaccharide. In the last decade Hib glycoconjugate vaccines have been successfully introduced and i n countries with very high immunization coverage the disease has been virtu ally eliminated and a decline of over 95% has been seen in countries with s lightly lower vaccine rates. World-wide use of Hib glycoconjugate vaccines offers the possibility of elimination of invasive Hib disease. Pneumococcal (11 serotypes with coverage of approximately 85% of invasive disease), men ingococcal (A, C, W 135, Y but not B) and S. typhi glycoconjugates are in a dvanced development and offer the prospect of being as successful as the Hi b glycoconjugates. (C) 1999 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.