A STREPTOCOCCUS-AGALACTIAE R-PROTEIN ANALYZED BY POLYCLONAL AND MONOCLONAL-ANTIBODIES

Unexpected cross-reactivity between two Streptococcus agalactiae (GBS) isolates formed the basis for purification of a GBS protein called th e Ra antigen, and raising of murine monoclonal antibody (MAb) against Ra. The Ra protein was resistant to trypsin digestion, susceptible to pepsin digestion, formed a ladder-like pattern of lines with a periodi city of similar to 8 kD on immunoblotting, was surface-localized in GB S strains, and was variably expressed by GBS. These characteristics pr ovided evidence that the Ra antigen belonged to the R proteins of GBS. By testing of reference GBS isolates and antiserum, including an anti -lid, protein serum, cross-reactivity was recorded consistent with the assumption that Ra is a R4 protein. The Ra/R4 protein also showed cro ss-reactivity with a previously described GBS protein called protein R ib (J. Exp. Med. 177: 1593-1603, 1993). Several characteristics of the Ra/R4 protein were similar to those of the GBS protein c(alpha), but the two proteins showed no cross-reactivity. The anti-Ra/R4 MAb has pr oved useful in serosubtype determination of GBS of known serotype and should be a valuable tool for studying the immunobiological function o f antibodies targetting the surface-localized Ra/R4 protein.