P2X7 Receptor in Bone Marrow-Derived Cells Aggravates Tuberculosis Caused by Hypervirulent Mycobacterium bovis

T cell-specific P2RX7 favors lung parenchymal CD4+ T cell accumulation in response to severe lung infections

CD4+ T cells are key components of the immune response during lung infections and can mediate protection against tuberculosis (TB) or influenza. However, CD4+ T cells can also promote lung pathology during these infections, making it unclear how these cells control such discrepant effects. Using mouse models of hypervirulent TB and influenza, we observe that exaggerated accumulation of parenchymal CD4+ T cells promotes lung damage. Low numbers of lung CD4+ T cells, in contrast, are sufficient to protect against hypervirulent TB. In both situations, lung CD4+ T cell accumulation is mediated by CD4+ T cell-specific expression of the extracellular ATP (eATP) receptor P2RX7. P2RX7 upregulation in lung CD4+ T cells promotes expression of the chemokine receptor CXCR3, favoring parenchymal CD4+ T cell accumulation. Our findings suggest that direct sensing of lung eATP by CD4+ T cells is critical to induce tissue CD4+ T cell accumulation and pathology during lung infections.

Animal Models for the Investigation of P2X7 Receptors

The P2X7 receptor is a trimeric ligand-gated cation channel activated by extracellular adenosine 5'-triphosphate. The study of animals has greatly advanced the investigation of P2X7 and helped to establish the numerous physiological and pathophysiological roles of this receptor in human health and disease. Following a short overview of the P2X7 distribution, roles and functional properties, this article discusses how animal models have contributed to the generation of P2X7-specific antibodies and nanobodies (including biologics), recombinant receptors and radioligands to study P2X7 as well as to the pharmacokinetic testing of P2X7 antagonists. This article then outlines how mouse and rat models have been used to study P2X7. These sections include discussions on preclinical disease models, polymorphic P2X7 variants, P2X7 knockout mice (including bone marrow chimeras and conditional knockouts), P2X7 reporter mice, humanized P2X7 mice and P2X7 knockout rats. Finally, this article reviews the limited number of studies involving guinea pigs, rabbits, monkeys (rhesus macaques), dogs, cats, zebrafish, and other fish species (seabream, ayu sweetfish, rainbow trout and Japanese flounder) to study P2X7.

Purinergic modulation of the immune response to infections

Infectious diseases are caused by the invasion of pathogenic microorganisms such as fungi, bacteria, viruses, and parasites. After infection, disease progression relies on the complex interplay between the host immune response and the microorganism evasion strategies. The host's survival depends on its ability to mount an efficient protective anti-microbial response to accomplish pathogen clearance while simultaneously preventing tissue injury by keeping under control the excessive inflammatory process. The purinergic system has the dual function of regulating the immune response and triggering effector antimicrobial mechanisms. This review provides an overview of the current knowledge of the modulation of innate and adaptive immunity driven by the purinergic system during parasitic, bacterial and viral infections.

Inherent P2X7 Receptors Regulate Macrophage Functions during Inflammatory Diseases

Macrophages are mononuclear phagocytes which derive either from blood-borne monocytes or reside as resident macrophages in peripheral (Kupffer cells of the liver, marginal zone macrophages of the spleen, alveolar macrophages of the lung) and central tissue (microglia). They occur as M1 (pro-inflammatory; classic) or M2 (anti-inflammatory; alternatively activated) phenotypes. Macrophages possess P2X7 receptors (Rs) which respond to high concentrations of extracellular ATP under pathological conditions by allowing the non-selective fluxes of cations (Na⁺, Ca²⁺, K⁺). Activation of P2X7Rs by still higher concentrations of ATP, especially after repetitive agonist application, leads to the opening of membrane pores permeable to ~900 Da molecules. For this effect an interaction of the P2X7R with a range of other membrane channels (e.g., P2X4R, transient receptor potential A1 [TRPA1], pannexin-1 hemichannel, ANO6 chloride channel) is required. Macrophage-localized P2X7Rs have to be co-activated with the lipopolysaccharide-sensitive toll-like receptor 4 (TLR4) in order to induce the formation of the inflammasome 3 (NLRP3), which then activates the pro-interleukin-1β (pro-IL-1β)-degrading caspase-1 to lead to IL-1β release. Moreover, inflammatory diseases (e.g., rheumatoid arthritis, Crohn’s disease, sepsis, etc.) are generated downstream of the P2X7R-induced upregulation of intracellular second messengers (e.g., phospholipase A2, p38 mitogen-activated kinase, and rho G proteins). In conclusion, P2X7Rs at macrophages appear to be important targets to preserve immune homeostasis with possible therapeutic consequences.

The role of P2X7 receptor in infection and metabolism: Based on inflammation and immunity

The P2X7 receptor (P2X7R) is a ligand-gated receptor belonging to the P2 receptor family. It is distributed in various tissues of the human body and is involved in regulating the physiological functions of tissues and cells to affect the occurrence and development of diseases. Unlike all other P2 receptors, the P2X7 receptor is mainly expressed in immune cells and can be activated not only by extracellular nucleotides but also by non-nucleotide substances which act as positive allosteric modulators. In this review, we comprehensively describe the role of the P2X7 receptor in infection and metabolism based on its role as an important regulator of inflammation and immunity, and briefly introduce the structure and general function of the P2X7 receptor. These provide a clear knowledge framework for the study of the P2X7 receptor in human health. Targeting the P2X7 receptor may be an effective method for the treatment of inflammatory and immune diseases. And its role in microbial infection and metabolism may be the main direction for in-depth research on the P2X7 receptor in the future.

P2x7 Receptor Signaling Blockade Reduces Lung Inflammation and Necrosis During Severe Experimental Tuberculosis

The risk of developing severe forms of tuberculosis has increased by the acquired immunodeficiency syndrome (AIDS) epidemic, lack of effective drugs to eliminate latent infection and the emergence of drug-resistant mycobacterial strains. Excessive inflammatory response and tissue damage associated with severe tuberculosis contribute to poor outcome of the disease. Our previous studies using mice deficient in the ATP-gated ionotropic P2X7 receptor suggested this molecule as a promising target for host-directed therapy in severe pulmonary tuberculosis. In this study, we assessed the effects of P2X7 pharmacological blockade on disease severity. First, we observed an increase in P2RX7 gene expression in the peripheral blood of tuberculosis patients compared to healthy donors. Lung leukocytes of mice infected with hypervirulent mycobacteria also showed increased expression of the P2X7 receptor. P2X7 blockade in mice with advanced tuberculosis recapitulated in many aspects the disease in P2X7-deficient mice. P2X7-directed therapy reduced body weight loss and the development of inflammatory and necrotic lung lesions, as well as delayed mycobacterial growth. Lower TNF-α production by lung cells and a substantial reduction in the lung GR-1⁺ myeloid cell population were observed after P2X7 inhibition. The effector CD4⁺ T cell population also decreased, but IFN-γ production by lung cells increased. The presence of a large population with characteristics of myeloid dendritic cells, as well as the increase in IL-6 production by lung cells, also indicate a qualitative improvement in the pulmonary immune response due to P2X7 inhibition. These findings support the use of drugs that target the P2X7 receptor as a therapeutic strategy to improve the outcome of pulmonary tuberculosis.

Harmful Effects of Granulocytic Myeloid-Derived Suppressor Cells on Tuberculosis Caused by Hypervirulent Mycobacteria

BACKGROUND

The role of myeloid-derived suppressor cells (MDSCs) in patients with severe tuberculosis who suffer from uncontrolled pulmonary inflammation caused by hypervirulent mycobacterial infection remains unclear.

METHODS

This issue was addressed using C57BL/6 mice infected with highly virulent Mycobacterium bovis strain MP287/03.

RESULTS

CD11b+GR1int population increased in the bone marrow, blood and lungs during advanced disease. Pulmonary CD11b+GR1int (Ly6GintLy6Cint) cells showed granularity similar to neutrophils and expressed immature myeloid cell markers. These immature neutrophils harbored intracellular bacilli and were preferentially located in the alveoli. T-cell suppression occurred concomitantly with CD11b+GR1int cell accumulation in the lungs. Furthermore, lung and bone marrow GR1+ cells suppressed both T-cell proliferation and interferon γ production in vitro. Anti-GR1 therapy given when MDSCs infiltrated the lungs prevented expansion and fusion of primary pulmonary lesions and the development of intragranulomatous caseous necrosis, along with increased mouse survival and partial recovery of T-cell function. Lung bacterial load was reduced by anti-GR1 treatment, but mycobacteria released from the depleted cells proliferated extracellularly in the alveoli, forming cords and clumps.

CONCLUSIONS

Granulocytic MDSCs massively infiltrate the lungs during infection with hypervirulent mycobacteria, promoting bacterial growth and the development of inflammatory and necrotic lesions, and are promising targets for host-directed therapies.

Immunomodulatory effects of P2X7 receptor in intracellular parasite infections

Adenosine triphosphate (ATP) is released from host cells during parasite infections and acts as a danger signal in the extracellular space by activating plasma membrane purinergic type 2 receptors-P2 receptors. The activation of these receptors has been described as a crucial step in immune cell activation, inflammation and parasite control. The P2X7 receptor is most involved in the activation of host microbicidal mechanisms, including production of reactive oxygen and nitrogen species, phagolysosomal fusion, acidification of parasitophorous vacuoles and release of cytokines and chemokines. The P2X7 receptor also modulates adaptive immune responses in various infectious diseases. Here, we discuss key points from the recent literature regarding P2X7 receptor activation during intracellular parasite infections.

Purinergic Regulation of Neutrophil Function

Purinergic signaling, which utilizes nucleotides (particularly ATP) and adenosine as transmitter molecules, plays an essential role in immune system. In the extracellular compartment, ATP predominantly functions as a pro-inflammatory molecule through activation of P2 receptors, whereas adenosine mostly functions as an anti-inflammatory molecule through activation of P1 receptors. Neutrophils are the most abundant immune cells in circulation and have emerged as an important component in orchestrating a complex series of events during inflammation. However, because of the destructive nature of neutrophil-derived inflammatory agents, neutrophil activation is fine-tuned, and purinergic signaling is intimately involved in this process. Indeed, shifting the balance between P2 and P1 signaling is critical for neutrophils to appropriately exert their immunologic activity. Here, we review the role of purinergic signaling in regulating neutrophil function, and discuss the potential of targeting purinergic signaling for the treatment of neutrophil-associated infectious and inflammatory diseases.

The P2X7 Receptor in Inflammatory Diseases: Angel or Demon?

Under physiological conditions, adenosine triphosphate (ATP) is present at low levels in the extracellular milieu, being massively released by stressed or dying cells. Once outside the cells, ATP and related nucleotides/nucleoside generated by ectonucleotidases mediate a high evolutionary conserved signaling system: the purinergic signaling, which is involved in a variety of pathological conditions, including inflammatory diseases. Extracellular ATP has been considered an endogenous adjuvant that can initiate inflammation by acting as a danger signal through the activation of purinergic type 2 receptors-P2 receptors (P2Y G-protein coupled receptors and P2X ligand-gated ion channels). Among the P2 receptors, the P2X7 receptor is the most extensively studied from an immunological perspective, being involved in both innate and adaptive immune responses. P2X7 receptor activation induces large-scale ATP release via its intrinsic ability to form a membrane pore or in association with pannexin hemichannels, boosting purinergic signaling. ATP acting via P2X7 receptor is the second signal to the inflammasome activation, inducing both maturation and release of pro-inflammatory cytokines, such as IL-1β and IL-18, and the production of reactive nitrogen and oxygen species. Furthermore, the P2X7 receptor is involved in caspases activation, as well as in apoptosis induction. During adaptive immune response, P2X7 receptor modulates the balance between the generation of T helper type 17 (Th17) and T regulatory (Treg) lymphocytes. Therefore, this receptor is involved in several inflammatory pathological conditions. In infectious diseases and cancer, P2X7 receptor can have different and contrasting effects, being an angel or a demon depending on its level of activation, cell studied, type of pathogen, and severity of infection. In neuroinflammatory and neurodegenerative diseases, P2X7 upregulation and function appears to contribute to disease progression. In this review, we deeply discuss P2X7 receptor dual function and its pharmacological modulation in the context of different pathologies, and we also highlight the P2X7 receptor as a potential target to treat inflammatory related diseases.

Purinergic Signaling: A Common Path in the Macrophage Response against Mycobacterium tuberculosis and Toxoplasma gondii

Immune responses are essential for the protection of the host against external dangers or infections and are normally efficient in the clearance of invading microbes. However, some intracellular pathogens have developed strategies to replicate and survive within host cells resulting in latent infection associated with strong inflammation. This excessive response can cause cell and tissue damage and lead to the release of the intracellular content, in particular the nucleotide pool, into the extracellular space. Over the last decade, new studies have implicated metabolites from the purinergic pathway in shaping the host immune response against intracellular pathogens and proved their importance in the outcome of the infection. This review aims to summarize how the immune system employs the purinergic system either to fight the pathogen, or to control collateral tissue damage. This will be achieved by focusing on the macrophage response against two intracellular pathogens, the human etiologic agent of tuberculosis, Mycobacterium tuberculosis and the protozoan parasite, Toxoplasma gondii.