Antibody to capsular polysaccharide of Streptococcus pneumoniae at the time of hospital admission for pneumococcal pneumonia

IgG to capsular polysaccharide (CPS) of Streptococcus pneumoniae is thought to provide the greatest degree of protection against pneumococcal disease. Serum obtained at hospital admission from 14 (27%) of 51 patients with bac teremic pneumococcal pneumonia and 11 (37%) of 30 with nonbacteremic pneumo coccal pneumonia contained Ige to CPS of the infecting serotype; these perc entages are similar to the prevalence of IgG to CPS in a control population , However, when compared with antibody from healthy adults, this IgG had fa r less capacity to opsonize the infecting pneumococcal serotype for phagocy tosis in vitro by normal human polymorphonuclear leukocytes or to protect m ice against experimental challenge, Failure to opsonize correlated closely with failure to protect mice, and each of these parameters correlated well with poor avidity for CPS, Future vaccine studies may need to examine the f unctional capacity of antibodies as a surrogate for infection, in addition to measuring their concentration in serum.