Purinergic Signalling (2007) 3:83–90 viability of extracellular Leishmania promastigotes. In fact, some nucleotides, such as UTP, stimulate the proliferation of promastigotes. It is also worthwhile noting that the more infective forms of L. amazonensis express more magne- sium-dependent ecto-ATPase on their membranes than less virulent Leishmania, leading to the proposal that the ecto- ATPase should be viewed as a virulence factor of the parasite [16]. The cellular pathway allowing ATPe to decrease Leish- mania infection remains to be determined, but must be different from the PLD activation observed during Chla- mydia and Mycobacterium infection [41, 51], since both Chlamydia and Mycobacterium inhibit phagolysosome formation and acidification [4, 6, 8], while Leishmania survives well in acidic phagolysosomes [56]. Interestingly, Leishmania lipophosphoglycans, which promote parasite survival, act by perturbing MAPKinase signalling in macrophages to inhibit macrophage IL-1β [57]. This might be relevant for the involvement of P2 receptors in the escape mechanisms used by Leishmania, since several P2 receptors are connected to MAPKinase pathways [58]. The involvement of ATPe in Chagas’ disease Chagas’ disease is caused by the facultative intracellular protozoan pathogen, T. cruzi. The disease is a chronic inflammatory condition characterised by cardiomyopathy and digestive disorders [59, 60]. T. cruzi infection affects over 17 million people in endemic areas of Latin America, leading to 45 thousand deaths per year [61]. The involve- ment of ATPe signalling was recently examined during the acute phase of T. cruzi infection. Thymus atrophy occurs during the acute phase of infection but the thymus recovers weight and cellularity during the chronic phase [62]. These alterations do not appear to be associated with stress or glucocorticoid release [63]. It has recently been observed that thymocytes are sensitive to ATPe-induced membrane permeabilisation and host-cell death only during the atrophy phase of infection, with CD4+/CD8+ double- positive thymocytes being the most sensitive subpopulation of thymocytes [64] (Table 1). Since the phenomenon of ATP-induced permeabilisation can be blocked by the P2X7 inhibitors, oxidised ATP and Mg2+, and the P2X7 agonist, BzATP, was more potent than ATP, we proposed that the increased sensitivity to ATPe may be responsible, at least in part, for the thymocyte clearance and thymic atrophy observed during the acute phase of Chagas’s disease [64]. It is, therefore, reasonable to suppose that ATPe plays an important role in the T. cruzi infection cycle. In this context, it is worth noting that the ecto-ATPases produced by these parasites have been associated with strain virulence [17]. 87 P2X7 receptors and inflammation The stimulation of P2X7 by ATPe leads to caspase-1 activation, cleavage of pro-IL-1β and the secretion of mature IL-1β [66] (Fig. 1). The P2X7 receptor is also upregulated in macrophages by inflammatory cytokines, such as interferon-+ (IFN-+) and tumour necrosis factor-! (TNF-!), and lipopolysaccharide (LPS). LPS, a surface component of most Gram-negative bacteria, and IFN-+ act synergistically to upregulate P2X7 receptor function and P2X7 mRNA in the human monocyte cell line, THP-1, and human macrophages [34, 37]. In fact, P2X7 receptors contain several motifs that are homologous to motifs from other receptors known to be involved in protein–protein interactions and LPS binding [67]. The upregulation of P2X7 expression in monocytes by TNF-!, LPS and IFN-+ is consistent with the ability of these cytokines to act as inflammatory mediators, and these effects are markedly attenuated by coincubation with prostaglandin E2 (PGE2) or the membrane-permeable cAMP analogue, dibutyril cAMP [68]. It is tempting to speculate that the temporal sequence of macrophage exposure to pro-inflammatory activators and anti-inflammatory stimuli (such as PGE2) might regulate not only receptor expression, but also downstream signalling by the P2X7 receptors. It is known that macrophages express adenosine recep- tors, which are expressed during the differentiation of mono- cytes to macrophages and may influence phagocytosis [69]. Moreover, the treatment of macrophages with IFN-+ upregulates expression of the adenosine receptor, A2B, and the activation of A2B receptors is involved with the deac- tivation of macrophages, possibly through an increase of cAMP [70]. Therefore, the extracellular nucleotides may be involved with activation and a feedback mechanism for macrophage deactivation, depending on the timing and type of nucleotide released during infection. In this context, \studies with P2X7-deficient mice have reinforced the view that P2X7 receptors are involved in inflammation. Thus, disruption of the P2X7 receptor gene is associated with less severe disease in an arthritis model [71] and studies of chronic inflammation and neuropathic pain [72]. These results led to the hypothesis that the P2X7 receptor, through the regulation of IL-1β production and secretion, plays a common, early role in the development of pain of neuropathic and inflammatory origin [71, 72]. Additionally, a recent study demonstrates that the inhibition of the P2X7 receptor attenuates fever and cytokine responses induced by LPS in rats [73]. Finally, the P2X7 receptor is present and its expression is modulated by inflammation in sites of chronic inflammation [74]. All of these findings reinforce the conclusion that P2X7-dependent signalling plays a significant role in host