Interactions of Porphyromonas gingivalis with oral epithelial cells

Interactions between pathogenic bacteria and the epithelial lining of host surfaces have been shown to be important in the course of mucosal infections. However, relatively little is known on the bacterial-epithelial interplay involved in the early steps of the pathogenesis of periodontal infections. This series of investigations deals with interactions between the invasive periodontal pathogen Porphyromonas gingivalis and oral epithelial cells in vitro. The studies focus on (i) the internalization process of P. gingivalis, (ii) P. gingivalis intracellular fate, (iii) the impact of P. gingivalis infection on neutrophil transepithelial migration and (iv) the ability of P. gingivalis to induce an epithelial cytokine response.The experiments involved the oral epithelial cell line KB, the cervix epithelial cell line HeLa and primary cultures of pocket epithelium harvested in conjunction with periodontal surgery. The bacteria used included a collection of laboratory and clinical strains of P. gingivalis with different adhesive/invasive potential, the oral invasive pathogen Actinobacillus actinomycetemcomitans, a strain of enterotoxigenic Escherichia coli (ETEC) and an invasive strain of Salmonella typhi.The findings demonstrated that P. gingivalis enters into oral epithelial cells by a clathrin-dependent receptor mediated endocytosis process (RME), involving tyrosine phosphorylation of eukaryotic proteins and rearrangements of cytoskeletal microtubuli. We further showed that a number of enzyme-linked receptor ligands and nutritive/transport proteins known to become internalized through RME significantly enhance the invasive ability of P. gingivalis, suggesting that RME-internalized host molecules may stimulate the uptake of bacteria that utilize the same mode of entry. Similarly to other invasive pathogens, P. gingivalis is able to multiply and persist within oral epithelial cells. Infection of oral epithelial cells with P. gingivalis does not trigger neutrophil transepithelial migration, in a two compartment (Transwell®) in vitro system. On the contrary, P. gingivalis significantly inhibits neutrophil transmigration induced by the chemoattractants N-formyl-methionyl-leusyl-phenylalanin (fMLP) and interleukin-8 (IL-8) as well as by the intestinal pathogen ETEC. The ability to block PMN transmigration is strongly associated with P. gingivalis adhesive/invasive properties. In addition, P. gingivalis conspicuously suppresses ICAM-1 and IL-8 expression by epithelial cells. In situ hybridization analysis revealed that P. gingivalis induces an interleukin-1b response by oral epithelial cells, a property also related to its adhesive/invasive capacity. Hence, it appears that P. gingivalis may trigger an inflammatory response and at the same time evade neutrophil clearance. These properties may entail important virulent factors with bearing on the pathogenesis of periodontal infections