Purinergic signaling and purine base metabolism at the crossroads between immunity, metabolism, and cancer: A review

In addition to its universally known role in transferring genetic material, DNA nucleotides and nucleosides are regarded as the most ancient form of extracellular signaling molecules. This unique signaling pathway was first reported by Dr. Albert Szent‐Györgyi in 1937 and established by Dr. Geoffrey Burnstock in 1972, who coined the term “purinergic signaling.” The significance of purinergic signaling is now recognized in various cellular processes, including immune responses. With an increased understanding of how changes in immunity impact cancer progression, the groundbreaking successes of immunotherapies, and emerging challenges facing patients receiving these treatments, in this review we revisit the history of purinergic signaling, provide a comprehensive summary of its impact on immune cells, and discuss the therapeutic potential of targeting this pathway for cancer treatment.

by purine nucleotides and nucleosides, is the oldest documented form of extracellular communication, highlighting the pleiotropic nature of these macromolecules in addition to their role in storing and transmitting genetic codes.This signaling pathway was first reported in 1929 by the Hungarian physiologist and Nobel Prize in Physiology or Medicine recipient (1937), Dr. Albert Szent-Györgyi, who observed that purified adenine compounds temporarily reduced heart rates in animals when injected. 3

The adenosine and purinergic hypotheses
Subsequent studies in the field of cardiac biology further contributed to our understanding of purinergic signaling and were conducted in large part by Robert M. Berne and his team in the early 1960s.
They first demonstrated that injections of adenosine into isolated cat hearts increased coronary blood flow 4 and spent the next few years deciphering the metabolism and breakdown products of adenosine nucleotides in mammalian hearts. 5Berne's seminal publication in 1963 demonstrated that significant amounts of the adenosine derivatives inosine and hypoxanthine were released under hypoxic conditions in isolated cat hearts and intact dog hearts.These molecules were hypothesized to be derived by adenosine deamination; therefore, adenosine was proposed to be a vasoactive substance with a key role in regulating coronary blood flow in response to hypoxia, a proposition known as the "adenosine hypothesis." 6As the adenosine hypothesis gained traction in the medical and scientific communities, several independent research groups sought to understand the role of adenosine and adenosine derivatives in modulating metabolic and circulatory functions within other organ systems.Adenosine was identified as a key regulator of contractility in mammalian muscle 7 and other forms of adenosine, including adenosine triphosphate (ATP), were soon being interrogated for extracellular signaling effects as well.
The field of purinergic signaling took another major step forward through the work of Dr. Geoffrey Burnstock, in 1972, who eventually coined the term purinergic signaling. 8Together with his students Max Bennett and Graham Campbell, they prepared innervated smooth muscle, taenia coli, of a guinea pig using the sucrose-gap technique for recording the correlation between mechanical and electrical activity. 9periments were performed at Melbourne University, in which neurotransmission mediated by the classical autonomic neurotransmitters acetylcholine (Ach) and noradrenaline (NA) were blocked by atropine and bretylium. 10Burnstock and his team expected to observe depolarization and contraction of the smooth muscle; however, they instead witnessed hyperpolarization and relaxation to single pulses.To characterize the mechanism behind the unexpected response, Burnstock and his team used tetrodotoxin, a biotoxin known to block nerve conduction but not affect smooth muscle activity.From this experiment they determined that tetrodotoxin completely blocked the hyperpolarization, indicating inhibitory junction potentials in response to non-adrenergic, noncholinergic (NANC) neurotransmission. 11It was subsequently shown that the response to NANC nerve stimulation was mediated by intrinsic inhibitory enteric neurons that were controlled by vagal and sacral parasympathetic nerves. 12though this was an exciting discovery, further investigation was needed to identify the transmitter released during NANC inhibitory transmission in the gut and by NANC excitatory transmission in the urinary bladder.To be established as a neurotransmitter, several criteria must be satisfied as proposed by Jack Eccles in the mid-20th century: release by a Ca 2+ -dependent mechanism, mimicry of nerve-mediated responses by the exogenously applied transmitter, inactivation by ectoenzymes and/or neuronal uptake, and parallel block or potentiation of responses to stimulation by nerves and exogenously applied transmitter. 13Burnstock considered many biomolecules such as neuropeptides, monoamines, and amino acids, but none of them satisfied these criteria.Previous literature included work by Dr. Szent-Györgyi which showed robust extracellular actions by purines on heart and blood vessels, 3 by Feldberg and Hebb which showed extracellular actions of ATP on autonomic ganglia, 14 and by Pamela Holton which demonstrated release of ATP during antidromic stimulation of artery sensory nerves. 15Collectively, these previous findings led Burnstock and his colleagues to try ATP, which satisfied all the criteria needed to establish it as a transmitter involved in NANC neurotransmission [16][17][18] ; subsequently, they coined the term "purinergic" and developed the purinergic hypothesis. 8rnstock's purinergic hypothesis centered around nerves utilizing ATP as the principal transmitter, and he proposed a model of the storage, release, and inactivation of ATP during purinergic transmission. 8is model states that ATP, stored in vesicles in nerve varicosities, is released by exocytosis to act on postjunctional receptors for ATP on smooth muscle (neuromuscular junction signaling).ATP is broken down extracellularly by ATPases and 5′-nucleotidase to adenosine, which is taken up by varicosities to be resynthesized and reincorporated into vesicles.Adenosine is broken down further by adenosine deaminase (ADA) to inosine and hypoxanthine and removed by the circulation. 8A later study showed that ATP was not only released from NANC nerves in the taenia coli but also from sympathetic nerves supplying smooth muscle. 19These studies supported the concept of co-transmission of ATP with NA from sympathetic nerves. 20The co-transmitter concept was initially resisted by the scientific community, but it is now wellestablished that every nerve in both the peripheral and central nervous systems utilizes ATP as a co-transmitter. 21

Purinergic signaling in physiology
Purinergic signaling has been identified in many biological processes, diseases, and disorders.In 1978, the P1, P2X, and P2Y purinergic receptors, or purinoceptors, for adenosine (ADO), ATP, and adenosine diphosphate (ADP), respectively, were first postulated to exist based on their pharmacological profiles. 22[25] Subsequent work expanded upon this discovery, particularly a study in which P2X purinoceptors were identified in primitive animals, such as F I G U R E 1 Purinergic receptor expression and their impact on immune cells.This figure highlights the expression of purinergic receptors on innate and adaptive immune cells and their responses when stimulated by extracellular nucleotides.
amoeba and Schistosoma, that have comparable molecular structures to those identified in mammals.This indicates that ATP was likely to be one of the earliest forms of extracellular signaling. 26Many studies have further investigated the role of purinergic signaling in pathophysiology and sought to harness its therapeutic potential. 27In certain bladder disorders, such as interstitial cystitis, out-flow obstruction, and neurogenic bladder, it was found that the purinergic component of the parasympathetic co-transmission supplying the bladder increases up to 40% and therefore is a target for therapeutic treatment. 28In cardiovascular diseases, ADP acting on P2Y receptors mediates platelet aggregation; P2Y receptor antagonists are therefore widely used for the treatment of thrombosis and stroke. 29,30Targeting purinergic signaling is being explored as possible treatments for diseases and conditions such as streptozotocin-induced diabetes 31,32 HIV/AIDS, 33 kidney failure, 34 osteoporosis, 35 and cancer. 36

PURINERGIC SIGNALING AND IMMUNITY
Though the field of purinergic signaling was initially identified in the context of the cardiovascular and nervous systems, purines were subsequently characterized to play a large role in immunity (Figure 1).
Continued work in this field has revealed key functional differences between structurally similar purine molecules.Adenosine, as the most ubiquitous purine signaling molecule, binds the entire P1 family of purinoceptors or adenosine receptors (A 1 , A 2A , A 2B , and A 3 ). 37Extracellular purine and pyrimidine molecules can collectively induce intracellular signaling through two primary classes of purinoceptors, ligandgated ionotropic P2X receptors and G-protein coupled metabotropic P2Y receptors, with widely variable effects. 38rhaps the most widely recognized purinergic receptor is P2 × 7, which plays a key role in activating the NLRP3 inflammasome in immune cells. 39This individual receptor was initially identified as belonging to a separate class of purinergic receptors and termed "P2Z," but was cloned and fully identified by Annmarie Surprenant et al in 1996, where it was found to exhibit both ATP-dependent macrophage lysis and cation-dependent synaptic transmission. 40Though these mechanisms occur through an incompletely specified mechanism of non-selective pore opening, another key feature of P2 × 7 activation is inflammasome activation resulting in downstream release of caspase-1 and activation of interleukin (IL)-1β and IL-18. 41The caspase-interleukin cascade is initiated by the activation of different NLRP3 inflammasome subunits, which oligomerize and bind caspase recruitment and activation domains (CARDs) that cleave and activate procaspase-1 to induce cytokine release. 42[46][47]

Purine metabolism in the myeloid lineage
ADA was the first purine metabolic enzyme to be documented with a role in immune function.It was discovered in the late 1970s that its activity positively correlated with the maturation of monocytes to macrophages. 48While levels of three intracellular adenosinecatabolizing enzymes-ADA, purine nucleoside phosphorylase (PNP), and adenine phosphoribosyl transferase (APRT)-were increased in activated murine macrophages, the adenosine-generating enzyme known as ecto-5′-nucleotidase 49 (also called 5′NT or CD73) on the cell surface was decreased upon macrophage activation, 50 indicating that mature macrophages exhibit greater adenosine catabolism and reduced anabolism compared to naïve monocytes.In the HL-60 promyelocytic leukemia cell line, researchers identified reduced intracellular guanylate, but not adenylate, pools and inhibited guanosine derivative synthesis upon HL-60 maturation.This guanylate-specific reduction in both de novo and salvage purine synthesis represented an increased adenylate-to-guanylate ratio during the induction of maturation. 51Furthermore, maturation was demonstrated to be induced by the inhibition of inosine monophosphate (IMP) dehydrogenase (IMPDH or IDH), confirming that guanine metabolism must be halted in order for monocyte maturation to proceed. 52 addition to influencing the activation status of macrophages, purine metabolism also appears to dictate their pro-inflammatory or anti-inflammatory status. 53Extracellular ATP accumulation drives macrophages toward a pro-inflammatory M1 phenotype, [54][55][56] while extracellular adenosine augments the anti-inflammatory M2 phenotype of macrophages and decreases their ability to produce inflammatory cytokines such as IL-1β and TNF-α. 57This relationship was exploited therapeutically when a group found that delivering targeted methotrexate (MTX) to macrophages positive for the cell surface purine ectoenzymes, CD39 and CD73, alleviated M1-driven inflammation in a murine arthritis model. 58Another study investigated the role of the adenine nucleobase in modulating bacterial lipopolysaccharide (LPS)-stimulated immune responses, and found that adenine inhibited the inflammatory response by decreasing the production of pro-inflammatory mediators TNF-α and IL-6 and decreasing NF-κB and Akt phosphorylation in both macrophages and mast cells. 59Similar results have been observed in the context of other innate immune cells, in which adenine suppresses mast cell activation in response to allergens. 59,60Collectively, adenosine and adenine nucleotides demonstrate strong capability to suppress immune responses in myeloid cells, while ATP exerts an opposing effect and stimulates these immune cells to mount a pro-inflammatory response.
While many of the well-documented effects of purines occur in macrophages, purinergic signaling can also modulate immune activity in myeloid-derived suppressor cells (MDSCs).MDSCs are composed of granulocytic and monocytic subsets, both of which can suppress T-cells through direct cell-cell contact. 61One example of the purinergic modulation of MDSC function was documented in a study of neuroblastoma.The tumor microenvironment was found to express high levels of ATP, which were correlated with increased expression of the P2 × 7 receptor on MDSCs and increased MDSC functionality and immunosuppression as documented by the increased production of arginase-1, transforming growth factor-b1, and reactive oxygen species (ROS). 62nversely, in the setting of acute graft-versus-host disease (GVHD) in which immunosuppressive MDSCs would be beneficial to the host, the stimulation of MDSCs by ATP binding to the P2 × 7 receptor induced prolonged NLRP3 inflammasome activation and resulted in MDSC dysfunction that decreased survival in mice. 63

Purine metabolism in the lymphoid lineage
Adenine nucleotides and ATP derivatives can stimulate or inhibit T-cells through purinoceptors, depending on the cellular origin and concentration of the nucleotides, by altering intracellular cyclic AMP (cAMP) levels.Normal cellular concentrations of ATP range from 1 to 10 mmol/L, 68 while concentrations are between 1077.0 and 3152.0 uM in human blood samples 69 and 1290 to 1790 uM in human cellular cytoplasm. 70Adenosine concentrations in humans are between 0.0425 uM 71 to 5.66 uM 72 in blood and 0.90 uM to 1.50 uM 70 in cytoplasm; physiological ranges of adenosine activity exist in such a range so that nanomolar or micromolar concentrations positively activate cell surface purinoceptors, while adenosine concentrations at millimolar ranges interfere with intracellular nucleotide pool homeostasis. 73,74Abnormal concentrations of ATP have only been detected in cerebrospinal fluid during neurological disturbances (0.23-1.09 uM 75 ), while adenosine was detected in the range of 0.126 ± 0.018 uM during septic shock. 72P and hydrolysis-resistant derivatives of ATP inhibit the activation of human peripheral CD4 + T-cells through the P2Y 11 purinoceptor, increasing cAMP independent of the actions of adenosine or its A 2 A receptor.76 Extracellular ATP also induces cell death in T-cells via activation of the P2X 7 receptor, which requires concentrations of ATP greater than 100 uM for half maximal receptor stimulation, 77 through a maturation-dependent mechanism as splenocytes are more sensitive to this induction of cell death than thymocytes in the same model.78 Adenosine binding to A 2 AR can suppress chemotactic cytokine release and extravasation of activated lymphocytes through the vasculature, preventing proper activation and migration of T-cells to sites of inflammation or injury.79 Moreover, ATP, ADP, and AMP also reduce  53 Another theory posits that the presence of the two ectoenzymes on the T-cell surface may correlate with stemness, as CD73 appears to decrease with differentiation but CD39 increases.81 To add further complexity to the regulation of the purinergic axis in T-cells, the inhibitory capacity of several purine nucleosides is cell culture media dependent; extracellular purines lost their repressive activity if cells were cultured in Xvivo15, which caused a downregulation of CD39 and CD73, instead of supplemented RPMI.82 Current literature identifies ATP as a danger-sensing agent or "alarmin," released by injured cells to stimulate the immune response, thus explaining its pro-inflammatory function, while adenosine functions as an immunosuppressant in lymphoid as well as myeloid cells.83 Though guanine signaling remains largely unexplored, one group has identified that treatment with guanosine or inosine derivatives can suppress cytokine release from stimulated T-cells.84 Though the field of purinergic signaling has traditionally been concerned with the extracellular signaling cascades initiated by adenosine or ATP binding to cell-surface P1 or P2 receptors, newer studies have documented independent signaling roles for other purine molecules such as the adenine nucleobase 59,60 and guanosine and inosine nucleotides.84 Because of the rapid shifts in metabolism that immune cells must undergo to alter their proliferative capacity in response to stimuli, and because of the increasingly blurred boundaries between extracellular purinergic signaling and intracellular purine metabolism, we thus find it important to discuss the role of purine metabolism in immunity generally and in T-cells specifically. On recent study identified that T-cells, like cancer cells, can adapt to glucose starvation by using purine nucleoside phosphorylase (PNP) to metabolize inosine into hypoxanthine and phosphorylated ribose, which can be used as precursors for other biosynthetic molecules such as ATP.85 Another study found that activation of the T-cell receptor (TCR) triggers the filamentous assembly of the enzymes inosine-5′monophosphate dehydrogenase 1 and 2 (collectively referred to as IMPDH), a process which is negatively regulated by the levels of guanine nucleotides and is regulated by the downstream TCR mediators mTOR and STIM1.86 Another regulator of intracellular purine metabolism was also found to be crucial for regulating T-cell function, differentiation, and identity; a 2022 study demonstrated the essential nature of the one-carbon metabolism enzyme methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) in T-cells, where its inhibition results in impaired de novo purine synthesis that suppresses mTORC1 activity, increases oxidative phosphorylation, and shifts Th17 cells towards adopting a Treg-like metabolism and phenotypic state.87 Given the conserved nature and importance of purine signaling and metabolism in every immune cell type, it is unsurprising that these signaling mediators contribute to both normal and pathogenic physiology.

When purinergic signaling goes awry
The first purine-centered immunological discovery, which is still the best understood to date, is that adenosine plays a key role in modulating immune cell function.This understanding evolved upon the observation of absent ADA enzymatic activity in two unrelated individuals with impaired B-and T-cell activity, a disease later identified as ADA-severe combined immunodeficiency (ADA-SCID). 880][91][92] Further characterization of purine metabolism in immunity revealed that PNP deficiency inhibits T-cell function, 93,94 while defective CD73 results in impaired B-cell activity. 95Accumulation of extracellular adenosine via defects in any of its catabolic enzymes therefore results in a highly immunosuppressive microenvironment.
Though the definitive role of guanine as an independent immunomodulatory molecule remains to be documented, the impact of defective guanine metabolism on immunity has been well char- • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4215161/• https://www.merckmanuals.com/professional/pediatrics/inherited-disorders-ofmetabolism/overviewof-purine-and-pyrimidine-metabolisstimulated in vitro in the absence of a bicarbonate buffer system or in the presence of methotrexate, both of which exploit the dependency of HGPRT-deficient cells on de novo purine synthesis. 96Similarly, a heterozygous knockout of the inosine 5′-monophosphate dehydrogenase (IMPDH) II gene decreased guanylate pool synthesis and reduced responsiveness of T-cells to αCD3/αCD38 (compromised proliferation and cytokine production). 97Defects in guanine metabolism are currently being investigated for their role in promoting immunological homeostasis or disease (Table 1).
Defects in purine signaling and metabolism have been wellcharacterized in several inflammatory diseases including lung disorders such as asthma [98][99][100][101][102] or chronic obstructive pulmonary disease (COPD), [103][104][105][106][107] graft-versus-host-disease (GVHD), [108][109][110][111] ischemiareperfusion injury, [112][113][114][115][116] gastrointestinal (GI) inflammation or inflammatory bowel disorder (IBD) [117][118][119][120][121][122] multiple sclerosis (MS), [123][124][125][126][127] and myasthenia gravis, 128,129 among others.Inflammatory disorders in the airway are mediated in part by both enhanced levels of adenosine in the bronchoalveolar lavage fluid and by enhanced hyperresponsiveness to adenosine challenge. 130A prospective clinical study in GVHD found that levels of the CD73 ectonucleotidase in pre-transplant patients were inversely correlated with occurrence and grade of chronic GVHD, demonstrating an essential and prognostically useful role for CD73 in dampening the immune response in transplants. 108Other groups who interrogated the role of purines in GVHD found that transplant regimens commonly induced the release of danger-associated molecular patterns (DAMPs) such as ATP, and that ATP can bind to purinergic receptors on T-cells to promote a pro-inflammatory microenvironment that exacerbates GVHD; however, the concentration of ATP and the activation of different purinergic receptors will dictate outcomes. 109The purine metabolism enzyme xanthine oxidase (XO) generates ROS that cause the cellular damage and inflammation associated with ischemia-reperfusion injury, [112][113][114] while CD39, CD73, P1 receptors, and P2 receptors have all been implicated in the development of GI inflammation and IBD. 117Progression of the autoimmune demyelinating disease, MS, is associated with differential expression of three purinergic receptors on microglial cells; P2Y12 is increased in control cells and decreases over the course of disease as chemotaxis is reduced, while P2 × 7 and P2 × 4 are both increased at the site of MS lesions and promote detrimental inflammasome activation and beneficial myelin phagocytosis, respectively. 123Another study found that adenosine receptor expression was altered in a murine model of experimental autoimmune myasthenia gravis; lymphocyte expression of the A 1 and A 2A receptors were significantly decreased while A 2B was insignificantly increased, with no changes in A 3 as compared to controls. 128In humans, the expression of a different purine receptor, P2 × 7, was positively correlated with disease severity and with serum levels of the cytokines IL-1β, IL-6, IL-17, and IL-21. 129The complex interplay between purinergic signaling, purine metabolism, and immunity provides a wide range of opportunities for developing therapies to treat these and other inflammatory conditions.

Purinergic signaling and tumor development
After several decades of interrogating the role of global purinergic signaling, researchers began to focus on understanding the impact of this signaling pathway on tumor development given that several classes of chemotherapies target metabolites.Adenosine was documented to be pro-angiogenic and pro-tumorigenic, acting through purinoceptors expressed on endothelial cells to increase blood vessel formation and vasculature development in solid tumors. 131Extracellular adenosine nucleotides have also been found to stimulate tumor cell proliferation directly by activating purinoceptors and ectoenzymes recently determined to be expressed at high levels in multiple types of cancer. 132,133wnstream regulation of tumor cell growth by purinergic signals may be mediated, in part, by controlling intracellular de novo purine metabolism, which is a limiting factor in rapid cell growth. 134,135A recent study demonstrated that purines are also correlated with radiation resistance in glioblastoma and purinergic nucleotide availability modulates DNA repair mechanisms, 136 highlighting the importance of purine metabolism in both tumorigenesis and responses therapeutic interventions.
In contrast to solid tumors, purinergic signaling in hematological malignancies has shown both tumor-suppressing and tumorpromoting effects.When THP-1, a human leukemia monocytic cell line, is treated with adenosine derivatives, there is a decrease in leukemia cell growth and survival. 137Conversely, the ATP/P2 × 7 axis increases acute myeloid leukemia (AML) leukemia burden which could be therapeutically targeted using a P2 × 7 receptor antagonist to reduce AML growth in mice. 138,139What is clear from published studies is that purinergic signaling impacts the development of blood cancers; however, we are just beginning to scratch the surface in our understanding of this relationship.Given that the pathogenesis of solid and hematological malignancies is impacted by purinergic signaling, we expect to see therapies that target this pathway emerging in the near future.The well-established role of intracellular purine metabolism in cancer has led to several anti-metabolite purine drugs that are currently in clinical use to treat specific cancer subtypes and other malignancies (Table 2).

Targeting the purinergic pathway for therapeutic intervention
Purine metabolites were targeted therapeutically long before extracellular mechanisms of purinergic signaling were established.Early antimetabolite chemotherapies inhibited nucleobase incorporation into DNA, preventing their intracellular functions without our knowledge of their extracellular role. 140Given our current understanding of the field of purinergic signaling, adenosine and its receptors are key therapeutic targets.CD73 is an interesting immunotherapy target due to its enzymatic role in generating adenosine from AMP, 141 while adenosine receptors 142,143 and purinoceptors 144 are being interrogated for their impact in tumor responses to both chemotherapies and immunotherapies.However, the focus in the field of purinergic signaling in cancer therapies appears to be shifting in the direction of immunomodulatory agents.Several groups are investigating the role of the ectoenzymes CD39 and CD73 in the immune response to cancer with published studies describing these molecules as "novel checkpoint inhibitors" in cancer immunotherapy. 145,146Furthermore, inhibition of purine synthesis appears to improve the response of T-cells to other checkpoint therapies such as anti-PD1. 147Expanding upon this work, recent findings demonstrate that the function of endogenous and chimeric antigen receptor (CAR) T-cells can be improved through the inhibition of purine metabolism via A 2A R knockout, 148,149 further indicating a key role for purinergic signaling in the multifaceted exploration of cancer immunotherapies.

FUTURE DIRECTIONS
Dr. Geoffrey Burnstock, the pioneering scientist in the field of purinergic signaling who conceived the "purinergic hypothesis," was initially regarded as absurd for believing that such ubiquitous intracellular molecules-DNA nucleotides, of all things-could function as intercellular signals. 150In the 50 years since his landmark papers establishing extracellular release of purine molecules, 8 the field has evolved tremendously.Purinergic signaling is rapidly gaining attention as a key modulator of extracellular and intracellular processes throughout the human body.Moreover, it has been causally and correlatively associated with dozens of diseases and is at the forefront of several novel therapeutic approaches for numerous pathologies, including cancer.
The study of purinergic signaling is rapidly expanding and moving to the forefront of the fields of immunology and cancer biology, particularly in its exploration as novel immunotherapeutic options.
To move the field forward, scientists should focus on thoroughly defining the signaling cascade of each basic metabolite, as well as identifying the putative receptor(s) for guanine signaling.[153] been elucidated, our ability to target immunomodulatory processes (including those involved in metabolism) will be significantly enhanced.
Once these goals are achieved, we will then be able to effectively utilize this information to develop novel treatment options for cancer treatments, inflammatory conditions, and other immune-related diseases.
Though conflicting at face value, these studies suggest that MDSC function is modulated in large part by the precise concentration of ATP in the microenvironment and that MDSC dysfunction can result from either extreme end of the spectrum; the ATP/P2 × 7 balance, therefore, appears to be key for successful MDSC activity and immunosuppression.Dendritic cells (DCs), another important myeloid-derived antigenpresenting cell (APC), express several purinergic receptors on their surface and can respond to extracellular purine nucleotides.Adenosine receptor gene expression of lipopolysaccharide (LPS)-differentiated and immature human DCs was characterized in 2001; mRNA for A 1 , A 2A , and A 3 receptors were all present in immature DCs, while only A 2A was found in differentiated cells. 64Neither cell state demonstrated expression of the A 2B receptor, although another group in 2009 demonstrated that DCs derived from murine bone marrow expressed the A 2B receptor.The same group demonstrated that activation of A 2B with an adenosine receptor agonist inhibited the production of TNFa and IL-12 but increased the production of IL-10; furthermore, they found that these A 2B -stimulated cells expressed lower levels of MHC class II and CD86, resulting in the decreased promotion of CD4+ Tcell proliferation and IL-2 production. 65Another notable study found that ATP increased functionally active CXC chemokine receptor 4 (CXCR4) and CC chemokine receptor 7 (CCR7) on both immature and LPS-stimulated DCs, while decreasing CCR5 expression on immature DCs, indicating the ability of ATP to influence the migratory capacity of DCs.Stimulation of DCs with ATP also caused a shift in chemokine production such that DCs were less efficient in recruiting type 1 polarized T-cells but the recruitment of type 2 T-cells was not inhibited 66,67 ; thus, both adenosine and ATP can modulate the cytokine and chemokine production profiles of DCs to decrease their interactions with T-cells and alter the global immune cascade.
activation and proliferation of T-cells stimulated by concanavalin A, through mechanisms independent of both adenosine/A 2 AR and P2X 7 receptor signaling. 80In addition to dictating T-cell death or activation, purinergic signaling can also influence T-cell differentiation.CD39 and CD73 are increasingly recognized as general markers of T-regulatory cells (Tregs) and of Treg activation, generating adenosine to mediate the immunosuppressive functions of Tregs and reduce the proinflammatory activation of T-effector cells (Teffs).

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Competes with GTP to inhibit DNA synthesis • T-cell leukemias • T-cell lymphomas • Bone marrow suppression • Delirium Unknown/Drug currently being tested in clinical trials Adapted from: • https://www.ncbi.nlm.nih.gov/books/NBK548594/• https://go.drugbank.com/categories/DBCAT000253 Diseases associated with alterations in purine metabolism and purinergic signaling acterized.A severe defect in the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT) causes Lesch-Nyhan syndrome, which results in normal levels of circulating lymphocytes and relatively normal in vivo activity.However, T-cells collected from patients with Lesch-Nyhan syndrome demonstrated reduced activity when TA B L E 1