Porphyromonas gingivalis, a keystone pathogen in
periodontitis (PD), produces
cysteine proteases named
gingipains (RgpA, RgpB, and Kgp), which strongly affect the host immune system. The range of action of
gingipains is extended by their release as components of outer membrane vesicles, which efficiently diffuse into surrounding gingival tissues. However, away from the anaerobic environment of
periodontal pockets, increased
oxygen levels lead to oxidation of the catalytic
cysteine residues of
gingipains, inactivating their proteolytic activity. In this context, the influence of catalytically inactive
gingipains on periodontal tissues is of significant interest. Here, we show that proteolytically inactive RgpA induced a proinflammatory response in both gingival keratinocytes and dendritic cells. Inactive RgpA is bound to the cell surface of gingival keratinocytes in the region of
lipid rafts, and using affinity chromatography, we identified RgpA-interacting
proteins, including
epidermal growth factor receptor (EGFR). Next, we showed that EGFR interaction with inactive RgpA stimulated the expression of inflammatory
cytokines. The response was mediated via the EGFR-
phosphatidylinositol 3-kinase (PI3K)-protein
kinase B (AKT) signaling pathway, which when activated in the gingival tissue rich in dendritic cells in the proximity of the alveolar bone, may significantly contribute to
bone resorption and the progress of PD. Taken together, these findings broaden our understanding of the biological role of
gingipains, which in acting as proinflammatory factors in the gingival tissue, create a favorable milieu for the growth of inflammophilic pathobionts. IMPORTANCE
Gingipain cysteine proteases are essential
virulence factors of Porphyromonas gingivalis, an oral bacterium implicated in development of
periodontitis.
Gingipains diffusing from anaerobic
periodontal pockets lose proteolytic activity in the oxygenated environment of gingival tissues. We found that despite the loss of activity,
gingipains still elicit a strong inflammatory response, which may contribute to the progression of
periodontitis and
bone resorption. Moreover, we identified the host molecules utilized by the pathogen as receptors for proteolytically inactivated
gingipains. The broad distribution of those receptors in human tissue suggests their involvement in systemic diseases associated with periodontal pathogens.