Streptococcus pneumoniae capsular polysaccharide serotype 4 (ST4 CPS)

The Streptococcus pneumoniae capsular polysaccharide serotype 4 (ST4 CPS) is a high-molecular-weight polymer composed of repeating tetrasaccharide units, including a distinctive cyclic trans-(2,3) pyruvate ketal modification, anchored directly via a 1,6 glycosidic bond from its reducing-end glucose to the β-D-N-acetylglucosamine of peptidoglycan in the bacterial cell wall. This capsule serves as the primary virulence factor for S. pneumoniae, a leading cause of bacterial pneumonia, meningitis, and otitis media worldwide, by forming a protective shield that inhibits opsonophagocytic clearance by host neutrophils and complement-mediated lysis. Serotype 4 CPS contributes to immune evasion, adhesion, and biofilm formation, though it is relatively thin compared to more prevalent serotypes, correlating with moderate resistance to neutrophil killing. As a key target of pneumococcal vaccines like PCV13, PPSV23, PCV15, PCV20, and PCV21, it elicits serotype-specific antibodies that promote phagocytosis, but protection does not cross-react with other serotypes. Structural studies highlight its role in defining antigenic specificity, aiding vaccine design against this pathogen responsible for significant global morbidity and mortality.

Other names

ST4 CPSPneumococcal capsular polysaccharide serotype 4Serotype 4 capsule

Induction of serotype-specific opsonophagocytic antibodies; Neutralization of capsule to enable phagocytosis