ANTIGEN HETEROGENEITY AMONG ISOLATES OF MYCOPLASMA-BOVIS IS GENERATEDBY HIGH-FREQUENCY VARIATION OF DIVERSE MEMBRANE-SURFACE PROTEINS

The protein and antigen profiles of 11 isolates of Mycoplasma bovis we re compared by sodium dodecyl sulfate-polyacrylamide gel electrophores is and immunoblot analysis of whole organisms. The isolates examined i ncluded the type strain PG45 and 10 other filter-cloned strains or pur ified isolates both from animals without clinical signs and from clini cal cases of bovine mastitis, arthritis, or pneumonia. While the overa ll protein patterns visualized by silver staining were very similar, m arked differences in the antigen banding profiles were detected by rab bit antiserum prepared against whole organisms from one of the strains analyzed. This antigenic heterogeneity was shown to be independent of the geographical origin, the type of clinical disease, and the site o f isolation and was also observed among serial isolates from a single animal. Antigen profiles were further monitored throughout sequentiall y subcloned populations of the PG45 strain. This clonal analysis revea led a high-frequency variation in the expression levels of several pro minent antigens. Ail of these variable antigens were defined by deterg ent-phase fractionation with Triton X-114 as amphiphilic integral memb rane proteins. A subset of different-sized membrane proteins was ident ified by a monoclonal antibody raised against a PG45 subclone expressi ng a 63- and a 46-kDa variant antigen within that set. The selective s usceptibility of these proteins to trypsin treatment of intact organis ms and their ability to bind the monoclonal antibody in colony immunob lots demonstrated that they were exposed on the cell surface. In addit ion, their preferential recognition by serum antibodies from individua l cattle with naturally induced M. bovis mastitis or arthritis confirm ed that they were major immunogens of this organism. These studies est ablish that the apparent antigenic heterogeneity among M. bovis isolat es reported here does not represent stable phenotypic strain differenc es generated from accumulated mutational events but reflects distinct expression patterns of diverse, highly variable membrane surface prote ins.