Lack of cross-protection against invasive pneumonia caused by heterologous strains following murine Streptococcus pneumoniae nasopharyngeal colonisation despite whole cell ELISAs showing significant cross-reactive IgG.

Prior exposure to intact Streptococcus pneumoniae can induce a protective antibody response to proteins antigens, which prevents subsequent invasive disease. This may be achieved either by colonisation with live bacteria or by immunisation with killed cells. Such approaches could provide novel vaccine strategies that overcome the serotype restriction of conjugate vaccines, and would aim to prevent disease caused by all strains of S. pneumoniae. Serum antibody is required to prevent invasive disease, but which in vitro measure of antibody response correlates best with protective immunity has not been established for protein antigens. Using a model of homologous protection induced through D39 colonisation of CD1 mice, we investigate the potential for heterologous protection against two distinct serotype strains and its serological correlates. Serum IgG from colonised mice bound to heterologous strains in whole cell ELISA at titres similar to the homologous D39. However, no cross-protection was observed, correlating with lack of surface binding of IgG to whole bacteria as measured by flow cytometry. Serum antibody binding to pre-lysed and untreated bacteria in the whole cell ELISA was similar suggesting that ELISA does not discriminate between surface and subcapsular antigens, unlike the flow cytometric approach. Thus, flow cytometric binding to whole bacteria maybe a more reliable correlate of cross-protection for novel species-wide vaccines than whole cell ELISA.