No abstract

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-1β, 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-1β secretion.

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.

Bacterial infections of the mucosal epithelium are a major cause of human disease. The prolonged presence of microbial pathogens stimulates inflammation of the local tissues, which leads to changes in the molecular composition of the extracellular milieu. A well-characterized molecule that is released to the extracellular milieu by stressed or infected cells is extracellular ATP and its ecto-enzymatic degradation products, which function as signaling molecules through ligation of purinergic receptors. There has been little information, however, on the effects of the extracellular metabolites on bacterial growth in inflamed tissues. Millimolar concentrations of ATP have been previously shown to inhibit irreversibly bacterial infection through ligation of P2X 7 receptors. We show here that the proinflammatory mediator, ATP, is released from Chlamydia trachomatis-infected epithelial cells. Moreover, further stimulation of the infected cells with micromolar extracellular ADP or ATP significantly impairs the growth of the bacteria, with a profile characteristic of the involvement of P2X 4 receptors. A specific role for P2X 4 was confirmed using cells overexpressing P2X 4 . The chlamydiae remain viable and return to normal growth kinetics after removal of the extracellular stimulus, similar to responses previously described for persistence of chlamydial infection. Danger signals comprise a varied group of extracellular molecules which indicate a potentially harmful physiological state and for which specific sensory mechanisms exist (38,40,51,55,56). Well-known danger signals include extracellular ATP, adenosine, uric acid crystals, the chromatin component HMGB1, and heat shock proteins. However, the level of danger posed elicits different cellular responses. For example, ATP is released from resting cells (nanomolar range) (5, 37), stressed and dying cells (micromolar range) (23), and physically compromised cells (millimolar range), although actual concentrations depend heavily on cell type and environment. Cells in the adjacent tissue must then decide whether to modify their own behavior in response to the extracellular signal, and whether to amplify the response by secreting other signaling molecules such as cytokines.The cells sense extracellular ATP and other nucleotides and nucleosides via a family of membrane receptors called purinergic receptors. These receptors are subdivided by function and homology into three classes: P2X are ligand-gated ion channels sensitive to ATP, P2Y are G-protein-coupled receptors stimulated by adenine and uracil nucleotides, and P1 receptors are also G protein coupled but are sensitive to adenosine (11,45). The tissue distribution and sensitivity to the ligands vary significantly within each family (12), such that slightly different concentrations of ligand can have different effects at a single cell type, and the same concentration of ligand can have contrasting effects on different cell types.While purinergic receptors have been best characterized in the central nervous system, the...