Characterization of the alpha antigen of the c proteins of group B streptococci (GBS) using a murine monoclonal antibody.

A murine monoclonal antibody raised against the alpha antigen of the group B streptococcal c proteins was analysed by immunofluorescence and whole-cell ELISA against a collection of 22 c protein-producing GBS. All the strains showing fluorescence and reactivity in ELISA turned out to be alpha antigen-carrying strains as defined by polyclonal rabbit antisera, while none of the strains producing only the beta antigen was positive. Western blot analyses of the alpha antigen released into the culture medium of growing bacteria suggest that the alpha antigen is present as distinct proteins of variable molecular weights. The upper limit of the molecular weights varies considerably from one strain to another, from approximately 200 kD to 70 kD. With all strains, the bands seen by the MAb occurred at regularly spaced intervals of about 10 kD throughout the gel. Some strains gave rise to 15-16 bands, while others gave rise to only one or two bands. The present investigation suggests that alpha antigens include several, probably identical, repeating subunits of approximately 10 kD. The epitope recognized by the MAb seems to be located on a 10-kD fragment, and in addition, it appears to be surface located, making the MAb a suitable tool in serodiagnostic work.