Streptococcus pneumoniae serotype-specific capsular polysaccharide (CPS)

Streptococcus pneumoniae serotype-specific capsular polysaccharides (CPS) are complex carbohydrate polymers that form the outermost layer of the bacterium and serve as its primary virulence factor. These polysaccharides shield the pathogen from the host's innate immune system by inhibiting complement deposition and preventing opsonophagocytosis by neutrophils and macrophages. There are currently over 100 recognized serotypes of S. pneumoniae, each characterized by a biochemically distinct CPS structure that determines its immunogenicity and invasive potential. Because of their surface exposure and critical role in pathogenesis, purified CPS molecules are the central components of both polysaccharide and conjugate vaccines. These vaccines work by inducing serotype-specific antibodies that neutralize the capsule's protective effects, allowing the immune system to identify and destroy the bacteria. However, the high diversity of CPS structures presents a therapeutic challenge known as serotype replacement, where non-vaccine serotypes increase in prevalence following the suppression of targeted strains. Monitoring these polysaccharides is essential for the ongoing development of broader-spectrum vaccines to combat invasive pneumococcal disease.

Other names

Pneumococcal capsular polysaccharideS. pneumoniae CPSBacterial capsulePnCP

Induction of opsonophagocytic antibodies (IgG and IgM) that facilitate bacterial clearance by neutrophils and macrophages; conjugate vaccines additionally recruit T-helper cells to induce B-cell memory and mucosal immunity.