Vaccines against polysaccharide antigens

Encapsulated bacteria such as Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae serogroup B (Hib) are a major cause of disease worldwide. Vaccine development against these organisms has targeted their capsular polysaccharides (CPS), as anti-capsular antibodies often protect against disease. The capsular polysaccharide vaccines that have been available against these organisms are neither immunogenic nor protective in young children and certain immunocompromised individuals. In general, polysaccharide (PS) antigens elicit a T-independent immune response, characterized by lack of memory, and poor immunogenicity at the extremes of life. Efforts to overcome the poor immunogenicity of CPS vaccines have led to development of conjugate vaccines. By conjugating CPS to carrier proteins it is possible to induce a T-dependent immune response against these antigens. Although conjugate vaccines have been successful against Hib disease, their applicability to multi-serotype/serogroup pathogens like the pneumococcus or the meningococcus is questioned. As a result, alternative vaccines including (1) surface proteins conserved across serotypes/serogroups, (2) peptides that mimic PS antigens and (3) DNA vaccines are presently under investigation. This review will highlight the potential and limitations of both CPS and CPS-conjugate vaccines against encapsulated bacteria as well as alternative strategies against PS antigens.