Streptococcus agalactiae capsular polysaccharide serotype III (GBS CPS III)

Streptococcus agalactiae capsular polysaccharide (CPS) serotype III is a critical surface antigen and primary virulence factor of Group B Streptococcus (GBS), a leading bacterial pathogen responsible for neonatal sepsis and meningitis. The CPS is a high-molecular-weight polymer consisting of repeating pentasaccharide units, notably featuring terminal N-acetylneuraminic acid (sialic acid) residues that mimic host cell surfaces. This structural similarity allows the bacteria to evade the host's innate immune system by inhibiting the alternative pathway of complement and preventing opsonophagocytic killing by leukocytes. Clinically, serotype III is the most prevalent serotype associated with late-onset neonatal disease and central nervous system infections. In drug development, this polysaccharide is the primary target for multivalent conjugate vaccines, such as Pfizer's hexavalent GBS6 candidate, which covalently link the CPS to carrier proteins like CRM197 to induce a robust T-cell dependent immune response. Therapeutic strategies focus on maternal immunization during the third trimester of pregnancy to elicit high levels of protective IgG antibodies, which are then transplacentally transferred to the fetus to provide passive immunity during the high-risk neonatal period.

Other names

Group B Streptococcus capsular polysaccharide serotype IIIGBS Serotype III capsular polysaccharideType III GBS polysaccharideS. agalactiae type III CPSSerotype III polysaccharide antigen

Induction of serotype-specific opsonophagocytic IgG antibodies following vaccination; these antibodies facilitate bacterial clearance via the opsonophagocytic pathway and provide passive protection to neonates through transplacental transfer.