Ana Carolina Morandini

The role of p2x7 receptor in infectious inflammatory diseases and the influence of ectonucleotidases

The purinergic receptor P2X ligand-gated ion channel 7 (P2X7) is ubiquitously expressed in almost all tissues and organs of the body with the highest distribution in the immune cells of monocyte-macrophage origin. Classically, P2X7 receptor is involved in apoptotic cell death, and it is well known that extracellular ATP ligation to this purinergic receptor serves as an important secondary stimulus, which is also considered as danger signal for the interleukin (IL)-1β cleavage and secretion from pro-inflammatory cells. More recently, however, there has been substantial evidence of additional roles for the P2X7 receptor, both in innate immune response and as an adaptive link, including T-cell activation in a chronic state of inflammation. Also, compelling evidences have revealed an important role for ectonucleotidases as ATP-consuming enzymes in the control and fine-tuning of P2X7 function by regulating the time, concentration, and availability of ATP during infection-driven inflammation. This review focuses on the current evidences for P2X7 receptor involvement in the initial stages of inflammation, as well as for its role in acute and chronic stages of infection. Here, we also highlight the role of ectonucleotidase family in the control of P2X7 function, including the initial and resolution phases of inflammation.

Porphyromonas gingivalis Fimbriae Dampen P2X7-Dependent Interleukin-1β Secretion

Porphyromonas gingivalis is a major contributor to the pathogenesis of periodontitis, an infection-driven inflammatory disease that leads to bone destruction. This pathogen stimulates pro-interleukin (IL)-1β synthesis but not mature IL-1β secretion, unless the P2X7 receptor is activated by extracellular ATP (eATP). Here, we investigated the role of P. gingivalis fimbriae in eATP-induced IL-1β release. Bone marrow-derived macrophages (BMDMs) from wild-type (WT) or P2X7-deficient mice were infected with P. gingivalis (381) or isogenic fimbria-deficient (DPG3) strain with or without subsequent eATP stimulation. DPG3 induced higher IL-1β secretion after eATP stimulation compared to 381 in WT BMDMs, but not in P2X7-deficient cells. This mechanism was dependent on K+ efflux and Ca2+-independent phospholipase A2 activity. Accordingly, non-fimbriated P. gingivalis failed to inhibit apoptosis via the eATP/P2X7 pathway. Furthermore, P. gingivalis-driven stimulation of IL-1β was Toll-like receptor 2 and MyD88 dependent, and not associated with fimbria expression. Fimbria-dependent down-modulation of IL-1β was selective, as levels of other cytokines remained unaffected by P2X7 deficiency. Confocal microscopy demonstrated the presence of discrete P2X7 expression in the absence of P. gingivalis stimulation, which was enhanced by 381-stimulated cells. Notably, DPG3-infected macrophages revealed a distinct pattern of P2X7 receptor expression with a marked focus formation. Collectively, these data demonstrate that eATP-induced IL-1β secretion is impaired by P. gingivalis fimbriae in a P2X7-dependent manner.

A dual role for P2X7 receptor during Porphyromonas gingivalis infection

Emerging evidence suggests a role for purinergic signaling in the activation of multiprotein intracellular complexes called inflammasomes, which control the release of potent inflammatory cytokines, such as interleukin (IL) -1β and -18. Porphyromonas gingivalis is intimately associated with periodontitis and is currently considered one of the pathogens that can subvert the immune system by limiting the activation of the NLRP3 inflammasome. We recently showed that P. gingivalis can dampen eATP-induced IL-1β secretion by means of its fimbriae in a purinergic P2X7 receptor–dependent manner. Here, we further explore the role of this purinergic receptor during eATP-induced IL-1β processing and secretion by P. gingivalis–infected macrophages. We found that NLRP3 was necessary for eATP-induced IL-1β secretion as well as for caspase 1 activation irrespective of P. gingivalis fimbriae. Additionally, although the secretion of IL-1β from P. gingivalis–infected macrophages was dependent on NLRP3, its adaptor protein ASC, or caspase 1, the cleavage of intracellular pro-IL-1β to the mature form was found to occur independently of NLRP3, its adaptor protein ASC, or caspase 1. Our in vitro findings revealed that P2X7 receptor has a dual role, being critical not only for eATP-induced IL-1β secretion but also for intracellular pro-IL-1β processing. These results were relevant in vivo since P2X7 receptor expression was upregulated in a P. gingivalis oral infection model, and reduced IFN-γ and IL-17 were detected in draining lymph node cells from P2rx7-/- mice. Furthermore, we demonstrated that P2X7 receptor and NLRP3 transcription were modulated in human chronic periodontitis. Overall, we conclude that the P2X7 receptor has a role in periodontal immunopathogenesis and suggest that targeting of the P2X7/NLRP3 pathway should be considered in future therapeutic interventions in periodontitis.