Streptococcus agalactiae capsular polysaccharide serotype Ib (GBS CPS Ib)

Streptococcus agalactiae capsular polysaccharide (CPS) serotype Ib is a major virulence factor located on the surface of Group B Streptococcus (GBS). It is composed of a repeating pentasaccharide unit that includes a disaccharide backbone and a trisaccharide side chain terminated by an alpha-2,3-linked sialic acid residue. This sialic acid component is biologically significant because it mimics host cell surface glycans, allowing the bacteria to avoid detection by the host's innate immune system and inhibit the alternative complement pathway by preventing C3 deposition. This evasion mechanism facilitates the survival and systemic spread of the bacteria, leading to severe conditions such as neonatal sepsis, meningitis, and pneumonia. From a therapeutic perspective, CPS Ib is a critical target for the development of glycoconjugate vaccines, which aim to provide protection against GBS infections. By conjugating the polysaccharide to a carrier protein like CRM197, the vaccine induces high-affinity, serotype-specific IgG antibodies. These antibodies are essential for opsonophagocytic killing of the bacteria. In clinical strategy, maternal immunization with these vaccines is designed to transfer protective antibodies to the fetus, thereby preventing early-onset and late-onset GBS disease in infants. Several multivalent vaccine candidates, including Pfizer's hexavalent GBS6, specifically incorporate CPS Ib to ensure broad coverage against the most prevalent pathogenic serotypes.

Other names

Group B Streptococcus serotype Ib capsular polysaccharideGBS type Ib polysaccharideStreptococcus agalactiae serotype Ib CPSType Ib GBS capsular polysaccharide

As a vaccine antigen, it induces the production of serotype-specific opsonophagocytic IgG antibodies. These antibodies, when induced in pregnant women, are transferred across the placenta to the fetus, providing passive immunity to the newborn and facilitating bacterial clearance by host phagocytes through opsonization.