Porphyromonas gingivalis Fimbriae Bind to Cytokeratin of Epithelial Cells

The adherence of Porphyromonas gingivalis to host cells is likely a prerequisite step in the pathogenesis of P. gingivalis-induced periodontal disease. P. gingivalis binds to and invades epithelial cells, and fimbriae are shown to be involved in this process. Little is known regarding epithelial receptor(s) involved in binding of P. gingivalis fimbriae. Using an overlay assay with purified P. gingivalis fimbriae as a probe, two major epithelial cell proteins with masses of 50 and 40 kDa were identified by immunoblotting with fimbria-specific antibodies. Iodinated purified fimbriae also bound to the same two epithelial cell proteins. An affinity chromatography technique was utilized to isolate and purify the epithelial components to which P. gingivalis fimbriae bind. Purified fimbriae were coupled to CNBr-activated Sepharose-4B, and the solubilized epithelial cell extract proteins bound to the immobilized fimbriae were isolated from the column. A major 50-kDa component and a minor 40-kDa component were purified and could be digested with trypsin, suggesting that they were proteins. These affinity-eluted 50- and 40-kDa proteins were then subjected to amino-terminal sequencing, and no sequence could be determined, suggesting that these proteins have blocked amino-terminal residues. CNBr digestion of the 50-kDa component resulted in an internal sequence homologous to that of Keratin I molecules. Further evidence that P. gingivalis fimbriae bind to cytokeratin molecule(s) comes from studies showing that multicytokeratin rabbit polyclonal antibodies cross-react with the affinity-purified 50-kDa epithelial cell surface component. Also, binding of purified P. gingivalis fimbriae to epithelial components can be inhibited in an overlay assay by multicytokeratin rabbit polyclonal antibodies. Furthermore, we showed that biotinylated purified fimbriae bind to purified human epidermal keratin in an overlay assay. These studies suggest that the surface-accessible epithelial cytokeratins may act as receptor(s) for P. gingivalis fimbriae. We hypothesize that adherence of P. gingivalis fimbriae to cytokeratin may be important for colonization of oral mucous membranes and possibly also for activation of epithelial cells.