Passive protection of mice against Streptococcus pneumoniae challenge by naturally occurring and vaccine-induced human anti-PhtD antibodies

Currently marketed Streptococcus pneumoniae vaccines are based on polysaccharide capsular antigens from the most common strains. Pneumococcal histidine triad protein D (PhtD) is a conserved surface protein that is being evaluated as a candidate for a vaccine with improved serotype coverage. Here, we measured the functional activity of human anti-PhtD antibodies in a passive protection model wherein mice were challenged with a lethal dose of S. pneumoniae by intravenous injection. This functional activity was compared with anti-PhtD antibody concentrations measured by enzyme-linked immunosorbent assay (ELISA) to estimate the 50% protective dose (ED50). Anti-PhtD antibodies affinity purified from pooled normal human sera passively protected mice with an ED50 of 1679 ELISA units/ml (95% confidence interval, 1420–1946). Sera from subjects injected with aluminum-adjuvanted PhtD in a phase I trial had similar activity per unit of antibody (ED50 = 1331 ELISA units/ml [95% confidence interval, 762–2038]). Vaccine-induced activity in the passive protection model was blocked by pre-incubation with recombinant PhtD but not by a control S. pneumoniae antigen (LytB). These results show that human anti-PhtD antibodies, whether naturally acquired or induced by the PhtD candidate vaccine, are functional. This supports the development of the PhtD candidate as part of a broadly protective pneumococcal vaccine.

Synchrony in serum antibody response to conserved proteins of Streptococcus pneumoniae in young children

Conserved Streptococcus pneumoniae (Spn) proteins are currently under investigation as vaccine candidates. We recently identified a subset of children prone to frequent acute otitis media (AOM) that we refer to as stringently-defined otitis prone (sOP). We investigated the synchrony of serum antibody responses against 5 Spn protein vaccine antigens, PhtD, LytB, PcpA, PhtE, and PlyD1 resulting from nasopharyngeal colonization and AOM in sOP children (49 observations) and non-otitis prone (NOP) children (771 observations). Changes in serum IgG and IgM were quantitated with ELISA. IgG antibody concentrations against PhtD, PcpA, and PlyD1 rose in synchrony in sOP and NOP children; that is, the proteins appeared equally and highly immunogenic in children at age 6 to 15 months and then leveled off in their rise at 15 to 25 months. In contrast, rises in concentrations to PhtE and LytB were significantly slower and had not peaked in children even at 25 months of age, consistent with lower immunogenicity. Serum IgM responses against PhtD and PlyD1 were in synchrony in children at age 6-25 months old. PcpA did not induce a significant increase of serum IgM response in children, suggesting that primary responses to PcpA occurred prior to children attaining age 6 months old. PhtD, PcpA, and Ply elicit a synchronous natural acquisition of serum antibody in young children suggesting that a trivalent Spn protein vaccine combining PhtD, PcpA, and PlyD1 would be less likely to display antigen competition when administered as a combination vaccine in young children.