ATP-dependent activation of an inflammasome in primary gingival epithelial cells infected by Porphyromonas gingivalis.

Production of IL-1beta typically requires two-separate signals. The first signal, from a pathogen-associated molecular pattern, promotes intracellular production of immature cytokine. The second signal, derived from a danger signal such as extracellular ATP, results in assembly of an inflammasome, activation of caspase-1 and secretion of mature cytokine. The inflammasome component, Nalp3, plays a non-redundant role in caspase-1 activation in response to ATP binding to P2X(7) in macrophages. Gingival epithelial cells (GECs) are an important component of the innate-immune response to periodontal bacteria. We had shown that GECs express a functional P2X(7) receptor, but the ability of GECs to secrete IL-1beta during infection remained unknown. We find that GECs express a functional Nalp3 inflammasome. Treatment of GECs with LPS or infection with the periodontal pathogen, Porphyromonas gingivalis, induced expression of the il-1beta gene and intracellular accumulation of IL-1beta protein. However, IL-1beta was not secreted unless LPS-treated or infected cells were subsequently stimulated with ATP. Conversely, caspase-1 is activated in GECs following ATP treatment but not P. gingivalis infection. Furthermore, depletion of Nalp3 by siRNA abrogated the ability of ATP to induce IL-1beta secretion in infected cells. The Nalp3 inflammasome is therefore likely to be an important mediator of the inflammatory response in gingival epithelium.

The P2X(7) receptor and intracellular pathogens: a continuing struggle.

The purinergic receptor, P2X(7), has recently emerged as an important component of the innate immune response against microbial infections. Ligation of P2X(7) by ATP can stimulate inflammasome activation and secretion of proinflammatory cytokines, but it can also lead directly to killing of intracellular pathogens in infected macrophages and epithelial cells. Thus, while some intracellular pathogens evade host defense responses by modulating with membrane trafficking or cell signaling in the infected cells, the host cells have also developed mechanisms for inhibiting infection. This review will focus on the effects of P2X(7) on control of infection by intracellular pathogens, microbial virulence factors that interfere with P2X(7) activity, and recent evidence linking polymorphisms in human P2X(7) with susceptibility to infection.

Effect of the purinergic receptor P2X7 on Chlamydia infection in cervical epithelial cells and vaginally infected mice.

Ligation of the purinergic receptor, P2X7R, with its agonist ATP has been previously shown to inhibit intracellular infection by chlamydiae and mycobacteria in macrophages. The effect of P2X7R on chlamydial infection had never been investigated in the preferred target cells of chlamydiae, cervical epithelial cells, nor in vaginally infected mice. In this study, we show that treatment of epithelial cells with P2X7R agonists inhibits partially Chlamydia infection in epithelial cells. Chelation of ATP with magnesium or pretreatment with a P2X7R antagonist blocks the inhibitory effects of ATP. Similarly to previous results obtained with macrophages, ATP-mediated inhibition of infection in epithelial cells requires activation of host-cell phospholipase D. Vaginal infection was also more efficient in P2X7R-deficient mice, which also displayed a higher level of acute inflammation in the endocervix, oviduct, and mesosalpingeal tissues than in infected wild-type mice. However, secretion of IL-1beta, which requires P2X7R ligation during infection by other pathogens, was decreased mildly and only at short times of infection. Taken together, these results suggest that P2X7R affects Chlamydia infection by directly inhibiting infection in epithelial cells, rather than through the ability of P2X7R to modulate IL-1beta secretion.