Serum samples from infants vaccinated with a pneumococcal conjugate vaccine, PncT, protect mice against invasive infection caused by Streptococcus pneumoniae serotypes 6A and 6B

Streptococcus pneumoniae serogroup 6 is an important cause of respiratory t ract disease worldwide. Vaccination with 6B polysaccharide induces antibody response to the cross-reacting serotype 6A, but the protective capacity of 6A antibodies induced in infants remains unknown. In this study, passive i mmunization with serum samples obtained from infants vaccinated with an oct avalent polysaccharide protein conjugate vaccine, PncT, protected mice agai nst bacteremia and/or lung infection caused by intranasal challenge with se rotypes 6B and 6A. Protective infant serum samples had significantly higher serotype-specific IgG levels and opsonic activity than did nonprotective s erum samples. The protective level to either serotype was similar to1 mug o f specific IgG antibodies injected per mouse (corresponding to similar to0. 3 mug/mL). The protection was strongly related to opsonophagocytic antibody levels measured in vitro. These results demonstrate that PncT induces anti bodies in infants that protect mice against invasive disease caused by the homologous serotype and by the cross-reacting serotype 6A.