Porphyromonas gingivalis, identified as an aetiological agent in severe forms of adult periodontitis, is a prominent component of the oral microbiota. This gram-negative anaerobe can exist in commensal harmony with the host and can inhibit inflammatory responses due to other oral pathogens, but can also contribute to inflammation. Epithelial cells that line the gingival crevice function as a physical barrier but can also respond to the presence of bacteria following stimulation of pattern-recognition receptors (PRRs), resulting in secretion of inflammatory cytokines. Consequently, gingival epithelial cells (GECs) are an important component of the innate immune response to oral bacteria. Little is known about signaling events in epithelial cells following stimulation with intact P. gingivalis or pathogen-associated molecular patterns (PAMPs) from the bacteria. Infected or stressed cells also release danger signals (DSs) that interact with danger signal receptors (DSRs) on neighboring cells to induce/alter the host response. We will therefore test the hypothesis that ligation of a DSR for extracellular ATP, P2X7, may stimulate processing and secretion of the pro-inflammatory cytokines, IL-1beta and IL- 18, following activation of an "inflammasome" in GECs infected with P. gingivalis. We have found that GECs express both P2X7 and inflammasome components. PAMPs such as lipopolysaccharide promote synthesis and intracellular accumulation of pro-IL-1beta and pro-IL-18, but a second stimulus, provided by a DS such as ATP, activates the inflammasome, allowing processing and secretion of the mature cytokines. Furthermore, we have shown that P. gingivalis secretes an ATP-scavenging enzyme which inhibits P2X7-dependent signaling. Since the ATP-scavenging enzyme secreted by P. gingivalis should deplete ATP in the oral cavity regardless of the cause of ATP release, the P. gingivalis enzyme should inhibit P2X7-dependent cytokine secretion by both cells infected with P. gingivalis and neighboring cells infected with other oral pathogens. Thus, our second objective will be to test the hypothesis that the ATP-scavenging enzyme is a novel anti-inflammatory factor secreted by P. gingivalis.
Investigations of the proposed aims will enhance our understanding of the development of the innate immune response to oral pathogens. These studies will thus provide valuable data needed to define more targeted approaches to control inflammation and infection associated with periodontal disease. In addition the results will contribute to a conceptual framework for understanding the health and disease related outcomes of the interaction between P. gingivalis and host cells.
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