Antigenic, structural, and functional relationships between fimbriae and the hemagglutinating adhesin HA-Ag2 of Porphyromonas gingivalis.

While the adhesive properties of Porphyromonas gingivalis are known to allow colonization of the subgingival tissues, the roles of fimbriae and adhesin molecules in hemagglutination remain unclear. The purpose of this study was to analyze the antigenic, structural, and functional relationships of these two components. Five populations of monoclonal antibodies were produced against (i) the hemagglutinating adhesin HA-Ag2 resolved by crossed immunoelectrophoresis (CIE), (ii) native fimbriae, and (iii) each of the three immunoprecipitates, Ag8a, Ag8b, and Ag8c, that define fimbriae by CIE. The tests used for characterization of the monoclonal antibodies included immunoblot reactivity, inhibition of hemagglutination, capacity to dissociate immunoprecipitates by CIE, localization of recognized epitopes by immunoelectron microscopy, and epitope mapping by competition enzyme-linked immunosorbent assay. The results from the different immunochemical tests clearly showed a close antigenic relationship between fimbriae and the hemagglutinating adhesin HA-Ag2. We were able to establish that the epitopic domain H1 of HA-Ag2 is hemagglutinin specific and that domain F2 is fimbria specific. Our data indicate that the polymeric structural unit of fimbriae must be complexed to HA-Ag2, the adhesin, to confer hemagglutination activity to the bacterial cells.