Figure 1. Protection against mucosal colonization by systemic antibody.

(A) Mice were passively immunized (IP) with rabbit antisera raised against S. pneumoniae either 4 hours before (Ab→Sp) or 24 hours after (Sp→Ab) pneumococcal challenge with a type 23F isolate. Increasing doses of serotype 23F capsular polysaccharide–specific antibody were tested for effects on colonization, which was determined by quantitative culture of upper respiratory tract lavages. Mice were challenged intranasally (IN) with either a low dose (10⁴) or high dose (10⁶) of S. pneumoniae. CFU/animal was determined 24 hours post immunization with type-specific antisera (closed symbols) or control (open symbols). n≥3 mice per group. ns, not significant. * P<0.05, ** P<0.01. (B) Bacterial load was confirmed in the nasal lavages of mice passively immunized 4 hours before challenge with low dose (10⁴) S. pneumoniae by qPCR. Genomic equivalence per animal ± SEM was determined based on a standard curve of DNA of known bacterial quantity. n≥6 nasal lavage samples per group. ** P<0.01.