Immunogenic proteins in cell-free culture supernatants of Haemophilus influenzae type b.

Cell-free culture supernatant (CFCS) prepared from Haemophilus influenzae type b (Hib) was examined for the presence of soluble Hib proteins. Two proteins with apparent molecular weights of 100,000 (100K) and 116K were predominant in the CFCS, and antibodies directed against these proteins could be detected by radioimmunoprecipitation or Western blot analyses of serum from adult rats immunized with Hib. Radioimmunoprecipitation analyses and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of these two proteins demonstrated that the 100K CFCS protein was also present on the cell surface of Hib, whereas the 116K CFCS protein was only detectable in culture supernatants. Both the 100K and 116K CFCS proteins were immunogenic in human infants with Hib meningitis and in infant rats systemically infected with Hib. In addition, the first detectable antibodies produced in these Hib-infected rats against Hib proteins were specific for the 100K protein in both its CFCS and cell-associated forms. These two CFCS proteins were also immunogenic in rats immunized with CFCS in the absence of Hib infection. Monoclonal antibody directed against the 100K protein reacted with 34 of 55 Hib strains examined by using a colony blot radioimmunoassay. The immunogenicity of the 100K and 116K CFCS proteins suggests that one or both of these proteins may have potential for vaccine development, either by themselves or covalently coupled to Hib capsular polysaccharide.