Bacterial capsular polysaccharides--biochemistry, immunity and vaccine

The biochemical basis of immunogenicity to bacterial capsular polysaccharides (PSs) has been extensively studied. Antibody responses to PS antigens can be greatly affected by their physico-chemical properties, e.g. molecular size, specific determinants, conformation etc. At present, three bacterial PS vaccines, including meningococcal, pneumococcal, and H. influenzae type b, have been licensed in the U.S.A. Many other PSs, such as group B Streptococcus, Pseudomonas aeruginosa and Klebsiella, are still in various stages of vaccine development. Studies on the type distribution of pneumococcal isolates from Asian populations showed that the types included in the 23-valent pneumococcal vaccine were considerably fewer in Asia than that observed in the U.S.A.: the proportion of types was 62.9% in Taiwan, 72.9% in Japan and 87.9-92.8% in the U.S.A. A large number of pneumococcal types found in Taiwan and Japan, such as 15A, 23A, 24F and 34, are not present in the pneumococcal vaccine. Immunization with one type of group 9 or 19F PS induced a poor antibody response to the other type. All 9N, 9V, 19F and 19A PSs are required in the vaccine to induce sufficient antibodies against group 9 and 19 infection. Extensive cross-reactions have been found between pneumococcal PSs and PSs from other bacteria. Klebsiella K2 PS could enhance the magnitude of the antibody response to 19F PS. The structure of streptococcal 14636/74 was found to be identical to pneumococcal 19F PS. A difficult problem in the development of vaccines against bacterial diseases is the poor immune response of young children to purified PSs. Antibodies against the PSs can be elicited when PS antigens are conjugated to a protein carrier. In mice, the maternal immunization with pneumococcal PS during pregnancy or before mating, did not cause suppression or other observable harmful effect, rather it may provide sufficient antibodies for protection against infection during early life.