MINIMUM PROTECTIVE SERUM CONCENTRATIONS OF PNEUMOCOCCAL ANTI-CAPSULARANTIBODIES IN INFANT RATS

Infant rats were passively immunized to determine the protective capac ity of pneumococcal anticapsular antibodies. Animal-passaged strains o f Streptococcus pneumoniae serotypes 1, 4, 5, 6b, 7f, 9v, 14, 18c, 19f , and 23f were used as challenge inocula (1-1500 cfu) in a model of pu lmonary infection that resulted in bacteremia, meningitis, and death. From untreated control animals, histologic sections of lung demonstrat ed infiltrative pneumonia and lung homogenate cultures grew S. pneumon iae at concentrations of 10(3)-10(8) cfu per gram of lung tissue. A ty pe-specific anti-capsular antibody serum concentration of 0.1-1.15 mu g/mL resulted in a statistically significant reduction in mortality co mpared with the reduction in untreated controls, except for serotype 1 4, which required 2.32 mu ug/mL for a significant reduction in mortali ty. The serum antibody level that provided 50% reduction in mortality ranged from 0.1-3.5 mu ug/mL for all serotypes.