Antibodies to Pneumococcal Proteins PhtD, CbpA, and LytC in Filipino Pregnant Women and Their Infants in Relation to Pneumococcal Carriage

This study focuses on the immunogenicity of the following three pneumococcal vaccine candidate proteins in Filipino infants, all inducing protection in animal models: pneumococcal histidine triad protein D (PhtD), choline binding protein A (CbpA), and the lysozyme LytC. The immunoglobulin G antibody concentrations to PhtD, its putative, protective, and exposed C-terminal fragment (PhtD C), CbpA, and LytC were measured by enzyme immunoassay in 52 serum samples from pregnant women, 39 cord blood samples, and consecutive serum samples (n = 263) from 52 newborns between 6 weeks and 10 months of age scheduled to be taken at six time points. A nasopharyngeal swab to detect pneumococcal carriage was taken parallel to the serum samples. The antibody concentrations in the cord blood samples were similar to those in the samples from the mothers. In infant sera, the geometric mean antibody concentrations (GMCs) for all three proteins decreased until the age of 18 weeks and started to increase after that age, suggesting that the infants' own antibody production started close to the age of 4 to 5 months. The increase in GMCs by age, most clear-cut for CbpA, was associated with pneumococcal carriage. Anti-PhtD concentrations were higher than anti-PhtD C concentrations but correlated well (r of 0.89 at 10.5 months), suggesting that antibodies are directed to the supposedly exposed and protective C-terminal part of PhtD. Our results show that young children are able to develop an antibody response to PhtD, CbpA, and LytC and encourage the development of pneumococcal protein vaccines for this age group.