Heat shock response of Streptococcus pneumoniae: identification of immunoreactive stress proteins

In order to investigate whether pneumococcal heat shock proteins (HSPs) were major immunogens of humoral immune response, we first characterized the heat shock response of S. pneumoniae. Three HSPs, HSP62, HSP72 and HSP80, having an apparent molecular mass of 62, 72, and 80 kDa, respectively, were detected by labelling proteins synthesized with [35S]methionine after a shift from 37 degrees C to 45 degrees C and fluorography of SDS-polyacrylamide gels. Radioimmunoprecipitation and immunoblot analyses with mouse anti-pneumococcal sera revealed that HSP72 was a major immunogen. S. pneumoniae HSP62 was another antigen which was precipitated by some immune sera. Anti-HSP72 antibodies appeared after the first immunization with S. pneumoniae antigens and subsequent immunization elicited a booster response. Monoclonal antibodies (MAbs) to pneumococcal HSP72 were produced and their specificities defined. The epitopes reactive with four MAbs are highly conserved in S. pneumoniae since 20 out of 20 different strains were recognized by each individual MAb. Western blot analysis revealed cross-reactivities with few non-pneumococcal strains. By N-terminal sequence analysis, the S. pneumoniae HSP72 was found to belong to the heat shock protein 70 family. That HSP72 is an important highly conserved antigen in S. pneumoniae should provide a basis for further investigation of its physiological and potential pathogenic role.