Characterization of the Porphyromonas gingivalis antigen recognized by a monoclonal antibody which prevents colonization by the organism.

Monoclonal antibody (MAb) 61BG1.3 prevented recolonization of deep pockets by Porphyromonas gingivalis in patients with periodontitis. The aim of this work was to identify the antigen recognized by the MAb. This was carried out by dose-dependent inhibition with materials extracted from P. gingivalis and assessed by a radioimmunoassay. A protease preparation and a capsular extract inhibited about 95% of the binding activity, whereas LPS or fimbriae had no effect. However, about 125 times greater concentration of the capsular than the protease material was needed to inhibit 50% of the antibody activity, suggesting that the MAb recognizes the protease preparation and that the capsular extract contained some protease. Western blotting of MAb 61BG1.3 with recombinant prpR1 protein expressed in Escherichia coli confirmed that MAb 61BG1.3 recognizes the haemagglutinating protease and mapped its epitope to residues 748-1130 of the beta component of the polyprotein. Three major bands of M(r) 45,000, 38,300 and 31,400 were detected in native whole cells of the virulent P. gingivalis strain W50 by Western blotting with MAb 61BG1.3. The MAb inhibited haemagglutination of human red blood cells by P. gingivalis or by a native protease extract. Blocking adhesion of P. gingivalis to the receptors on erythrocytes might be a mechanism by which the MAb inhibits recolonization by the microorganism.