Hostile, hypoxia-A2-adenosinergic tumor biology as the next barrier to overcome for tumor immunologists

Targeting the adenosine 2A receptor enhances chimeric antigen receptor T cell efficacy

Chimeric antigen receptor (CAR) T cells have been highly successful in treating hematological malignancies, including acute and chronic lymphoblastic leukemia. However, treatment of solid tumors using CAR T cells has been largely unsuccessful to date, partly because of tumor-induced immunosuppressive mechanisms, including adenosine production. Previous studies have shown that adenosine generated by tumor cells potently inhibits endogenous antitumor T cell responses through activation of adenosine 2A receptors (A2ARs). Herein, we have observed that CAR activation resulted in increased A2AR expression and suppression of both murine and human CAR T cells. This was reversible using either A2AR antagonists or genetic targeting of A2AR using shRNA. In 2 syngeneic HER2+ self-antigen tumor models, we found that either genetic or pharmacological targeting of the A2AR profoundly increased CAR T cell efficacy, particularly when combined with PD-1 blockade. Mechanistically, this was associated with increased cytokine production of CD8+ CAR T cells and increased activation of both CD8+ and CD4+ CAR T cells. Given the known clinical relevance of the CD73/adenosine pathway in several solid tumor types, and the initiation of phase I trials for A2AR antagonists in oncology, this approach has high translational potential to enhance CAR T cell efficacy in several cancer types.

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

Potassium channels of T lymphocytes take center stage in the fight against cancer

A recent study by Eil at al. published in Nature in September 2016 provides evidence that alterations of the K+ homeostasis of tumor infiltrating lymphocytes (TILs) in necrotic areas of the tumor microenvironment (TME) suppress the function of effector T cells. Furthermore, they establish that overexpression of K+ channels in T lymphocytes counterbalances this negative effect of the TME and restores the ability of TILs to function, ultimately leading to increased survival of tumor bearing mice. Thus, K+ channels in T lymphocytes become interesting new targets for novel immunotherapies in cancer. This Commentary discusses Eil's finding in the context of the central role that K+ channels play in the suppressed state of TILs as they mediate the immunosuppressive effects of multiple conditions of the TME including hypoxia and adenosine.

Prostaglandin E2-mediated adenosinergic effects on CD14+ cells: Self-amplifying immunosuppression in cancer

CD39 and CD73 are surface-expressed ectonucleotidases that hydrolyze ATP in a highly regulated, serial manner into ADP, AMP and adenosine. The end product, adenosine, has both tumor-promoting and immunosuppressive effects. The aim of this study was to determine CD73 expression on immune cells in pleural effusion (PE) in order to have a better understanding of the immune environment in mesothelioma. PE- or blood-derived CD14⁺ cells of mesothelioma patients and healthy donors were analyzed by flow cytometry for the expression of CD39 and CD73. CD73-induction was studied by exposure of CD14⁺ cells to the soluble fraction of PE (sPE), while the signaling mechanism, responsible for CD73 induction, by phosphoflow cytometry and receptor-inhibition studies. We observed CD73 expression on CD14⁺ cells in PE but not peripheral blood of mesothelioma patients or healthy donors. CD73 expression was inducible on CD14⁺ cells with sPE, cyclic-AMP (cAMP)-inducers (forskolin and prostaglandin-E₂ (PGE₂)) and adenosine. Inhibition of PGE₂ receptors or adenosine A₂ receptors blocked CD73-induction by sPE. sPE treatment triggered protein kinase A and p38 activation. However, signal-transducer and activator of transcription 3 (STAT3)-blocking led to enhanced CD73 expression, demonstrating a hitherto unknown negative control of purinergic signaling by STAT3 in CD14⁺ cells. TNFα production by CD73⁺ CD14⁺ cells was significantly impaired in the presence of AMP, confirming immunosuppressive function. Taken together, CD73 expression can be induced by PGE₂, cAMP or adenosine on human CD14⁺ cells. We suggest that targeting this autocrine loop is a valid therapeutic approach in mesothelioma that may also enhance immunotherapy.

Design, Synthesis, and Pharmacological Characterization of 2-(2-Furanyl)thiazolo[5,4-d]pyrimidine-5,7-diamine Derivatives: New Highly Potent A2A Adenosine Receptor Inverse Agonists with Antinociceptive Activity

In this study, we describe the design and synthesis of new N⁵-substituted-2-(2-furanyl) thiazolo[5,4-d]pyrimidine-5,7-diamines (2-18) and their pharmacological characterization as A_2A adenosine receptor (AR) antagonists by using in vitro and in vivo assays. In competition binding experiments two derivatives (13 and 14) emerged as outstanding ligands showing two different affinity values (KH and KL) for the hA_2A receptor with the high affinity KH value in the femtomolar range. The in vitro functional activity assays, performed by using cyclic AMP experiments, assessed that they behave as potent inverse agonists at the hA_2A AR. Compounds 13 and 14 were evaluated for their antinociceptive activity in acute experimental models of pain showing an effect equal to or greater than that of morphine. Overall, these novel inverse agonists might represent potential drug candidates for an alternative approach to the management of pain.

Myeloid-derived suppressor cells contribute to A2B adenosine receptor-induced VEGF production and angiogenesis in a mouse melanoma model

Vascular endothelial growth factor (VEGF) is an angiogenic factor critically involved in tumor progression. Adenosine A2B receptor plays a pivotal role in promoting tumor growth. The aim of this study was to investigate the role of myeloid-derived suppressor cells (MDSCs) in the pro-angiogenic effects of A2B and to determine whether A2B blockade could enhance the effectiveness of anti-VEGF treatment. Mice treated with Bay60-6583, a selective A2B receptor agonist, showed enhanced tumor VEGF-A expression and vessel density. This effect was associated with accelerated tumor growth, which could be reversed with anti-VEGF treatment. Bay60-6583 increased the accumulation of tumor CD11b+Gr1+ cells. Depletion of MDSCs in mice significantly reduced A2B-induced VEGF production. However, A2B receptor stimulation did not directly regulate VEGF expression in isolated tumor myeloid cells. Mechanistically, Bay60-6583-treated melanoma tissues showed increased STAT3 activation. Inhibition of STAT3 significantly decreased the pro-tumor activity of Bay60-6583 and reduced tumor VEGF expression.

Pharmacological blockade of A2B receptor with PSB1115 significantly reduced tumor growth by inhibiting tumor angiogenesis and increasing T cells numbers within the tumor microenvironment. These effects are, at least in part, dependent on impaired tumor accumulation of Gr1+ cells upon A2B receptor blockade. PSB1115 increased the effectiveness of anti-VEGF treatment.

A2A adenosine receptor antagonists to weaken the hypoxia-HIF-1α driven immunosuppression and improve immunotherapies of cancer

Hypoxic and adenosine rich tumor microenvironments represent an important barrier that must be overcome to enable T and NK cells to reject tumors. The A2A adenosine receptor (A2AR) on activated immune cells was identified as a critical and non-redundant mediator of physiological immunosuppression. Observations showing that tumor-protecting A2AR also suppress and redirect the anti-tumor immune response pointed to the importance of inhibiting this pathway to improve cancer immunotherapy. We advocated (i) blocking immunosuppressive adenosine-A2AR-cAMP-mediated intracellular signaling by A2AR antagonists and (ii) weakening hypoxia-HIF-1α-mediated accumulation of extracellular adenosine by oxygenation agents that also inhibits CD39/CD73 adenosine-generating enzymes. In view of commencing clinical trials of synthetic A2AR antagonists in combination with cancer immunotherapies, we discuss their promise and exclusion criteria.

Structurally Enabled Discovery of Adenosine A2A Receptor Antagonists

Over the past decade there has been a revolution in the field of G protein-coupled receptor (GPCR) structural biology. Many years of innovative research from different areas have come together to fuel this significant change in the fortunes of this field, which for many years was characterized by the paucity of high-resolution structures. The determination to succeed has been in part due to the recognized importance of these proteins as drug targets, and although the pharmaceutical industry has been focusing on these receptors, it can be justifiably argued and demonstrated that many of the approved and commercially successful GPCR drugs can be significantly improved to increase efficacy and/or reduce undesired side effects. In addition, many validated targets in this class remain to be drugged. It is widely recognized that application of structure-based drug design approaches can help medicinal chemists a long way toward discovering better drugs. The achievement of structural biologists in providing high-resolution insight is beginning to transform drug discovery efforts, and there are a number of GPCR drugs that have been discovered by use of structural information that are in clinical development. This review aims to highlight the key developments that have brought success to GPCR structure resolution efforts and exemplify the practical application of structural information for the discovery of adenosine A_2A receptor antagonists that have potential to treat multiple conditions.

Hypoxia-inducible factors as molecular targets for liver diseases

Liver disease is a growing global health problem, as deaths from end-stage liver cirrhosis and cancer are rising across the world. At present, pharmacologic approaches to effectively treat or prevent liver disease are extremely limited. Hypoxia-inducible factor (HIF) is a transcription factor that regulates diverse signaling pathways enabling adaptive cellular responses to perturbations of the tissue microenvironment. HIF activation through hypoxia-dependent and hypoxia-independent signals have been reported in liver disease of diverse etiologies, from ischemia-reperfusion-induced acute liver injury to chronic liver diseases caused by viral infection, excessive alcohol consumption, or metabolic disorders. This review summarizes the evidence for HIF stabilization in liver disease, discusses the mechanistic involvement of HIFs in disease development, and explores the potential of pharmacological HIF modifiers in the treatment of liver disease.

Immunosuppressive activities of adenosine in cancer

Multiple immunosuppressive mechanisms impede anti-tumor immunity. Among them, the accumulation of extracellular adenosine is a potent and widespread strategy exploited by tumors to escape immunosurveillance through the activation of purinergic receptors. In the immune system, engagement of A2a and A2b adenosine receptors is a critical regulatory mechanism that protects tissues against excessive immune reactions. In tumors, this pathway is hijacked and hinders anti-tumor immunity, promoting cancer progression. Different groups have highlighted the therapeutic potential of blocking CD73-dependent adenosine-mediated immunosuppression to reinstate anti-tumor immunity. Phase clinical trials evaluating anti-CD73 antibodies and A2a receptor antagonists in cancer patients are currently ongoing. We here review the recent literature on the immunosuppressive effects of extracellular adenosine and discuss the development of adenosine inhibitors.

Adenosine as a Multi-Signalling Guardian Angel in Human Diseases: When, Where and How Does it Exert its Protective Effects?

The importance of adenosine for human health cannot be overstated. Indeed, this ubiquitous nucleoside is an integral component of ATP, and regulates the function of every tissue and organ in the body. Acting via receptor-dependent and -independent mechanisms [the former mediated via four G-protein-coupled receptors (GPCRs), A1, A2A, A2B, and A3,], it has a significant role in protecting against cell damage in areas of increased tissue metabolism, and combating organ dysfunction in numerous pathological states. Accordingly, raised levels of adenosine have been demonstrated in epilepsy, ischaemia, pain, inflammation, and cancer, in which its behaviour can be likened to that of a guardian angel, even though there are instances in which overproduction of adenosine is pathological. In this review, we condense the current body of knowledge on the issue, highlighting when, where, and how adenosine exerts its protective effects in both the brain and the periphery.

Cancer immunotherapy out of the gate: the 22nd annual Cancer Research Institute International Immunotherapy Symposium

The 22nd annual Cancer Research Institute (CRI) International Immunotherapy Symposium was held from October 5-8, 2014, in New York City. Titled "Cancer Immunotherapy: Out of the Gate," the symposium began with a Cancer Immunotherapy Consortium satellite meeting focused on issues in immunotherapy drug development, followed by five speaker sessions and a poster session devoted to basic and clinical cancer immunology research. The second annual William B. Coley lecture was delivered by Lieping Chen, one of the four recipients of the 2014 William B. Coley Award for Distinguished Research in Tumor Immunology; the other three recipients were Gordon Freeman, Tasuku Honjo, and Arlene Sharpe. Prominent themes of the conference were the use of genomic technologies to identify neoantigens and the emergence of new immune modulatory molecules, beyond CTLA-4 and PD-1/PD-L1, as new therapeutic targets for immunotherapy.

Cancer Immunotherapy: Selected Targets and Small-Molecule Modulators

There is a significant amount of excitement in the scientific community around cancer immunotherapy, as this approach has renewed hope for many cancer patients owing to some recent successes in the clinic. Currently available immuno-oncology therapeutics under clinical development and on the market are mostly biologics (antibodies, proteins, engineered cells, and oncolytic viruses). However, modulation of the immune system with small molecules offers several advantages that may be complementary and potentially synergistic to the use of large biologicals. Therefore, the discovery and development of novel small-molecule modulators is a rapidly growing research area for medicinal chemists working in cancer immunotherapy. This review provides a brief introduction into recent trends related to selected targets and pathways for cancer immunotherapy and their small-molecule pharmacological modulators.

CD73-adenosine: a next-generation target in immuno-oncology

Cancer immunotherapy has entered in a new era with the development of first-generation immune checkpoint inhibitors targeting the PD1/PD-L1 and CTLA-4 pathways. In this context, considerable research effort is being deployed to find the next generation of cancer immunotherapeutics. The CD73-adenosine axis constitutes one of the most promising pathways in immuno-oncology. We and others have demonstrated the immunosuppressive role of CD73-adenosine in cancer and established proof-of-concept that the targeted blockade of CD73 or adenosine receptors could effectively promote anti-tumor immunity and enhance the activity of first-generation immune checkpoint blockers. With Phase I clinical trials now underway evaluating anti-CD73 or anti-A2A therapies in cancer patients, we here discuss the fundamental, preclinical and clinical findings related to the role of the CD73-adenosinergic pathway in tumor immunity.

αβ TCR-mediated recognition: relevance to tumor-antigen discovery and cancer immunotherapy

αβ T lymphocytes sense perturbations in host cellular body components induced by infectious pathogens, oncogenic transformation, or chemical or physical damage. Millions to billions of these lymphocytes are generated through T-lineage development in the thymus, each endowed with a clonally restricted surface T-cell receptor (TCR). An individual TCR has the capacity to recognize a distinct "foreign" peptide among the myriad of antigens that the mammalian host must be capable of detecting. TCRs explicitly distinguish foreign from self-peptides bound to major histocompatibility complex (MHC) molecules. This is a daunting challenge, given that the MHC-linked peptidome consists of thousands of distinct peptides with a relevant nonself target antigen often embedded at low number, among orders of magnitude higher frequency self-peptides. In this Masters of Immunology article, I review how TCR structure and attendant mechanobiology involving nonlinear responses affect sensitivity as well as specificity to meet this requirement. Assessment of human tumor-cell display using state-of-the-art mass spectrometry physical detection methods that quantify epitope copy number can help to provide information about requisite T-cell functional avidity affording protection and/or therapeutic immunity. Future rational CD8 cytotoxic T-cell-based vaccines may follow, targeting virally induced cancers, other nonviral immunogenic tumors, and potentially even nonimmunogenic tumors whose peptide display can be purposely altered by MHC-binding drugs to stimulate immune attack.

Immunological mechanisms of the antitumor effects of supplemental oxygenation

Antitumor T cells either avoid or are inhibited in hypoxic and extracellular adenosine-rich tumor microenvironments (TMEs) by A2A adenosine receptors. This may limit further advances in cancer immunotherapy. There is a need for readily available and safe treatments that weaken the hypoxia-A2-adenosinergic immunosuppression in the TME. Recently, we reported that respiratory hyperoxia decreases intratumoral hypoxia and concentrations of extracellular adenosine. We show that it also reverses the hypoxia-adenosinergic immunosuppression in the TME. This, in turn, stimulates (i) enhanced intratumoral infiltration and reduced inhibition of endogenously developed or adoptively transfered tumor-reactive CD8 T cells, (ii) increased proinflammatory cytokines and decreased immunosuppressive molecules, such as transforming growth factor-β (TGF-β), (iii) weakened immunosuppression by regulatory T cells, and (iv) improved lung tumor regression and long-term survival in mice. Respiratory hyperoxia also promoted the regression of spontaneous metastasis from orthotopically grown breast tumors. These effects are entirely T cell- and natural killer cell-dependent, thereby justifying the testing of supplemental oxygen as an immunological coadjuvant to combine with existing immunotherapies for cancer.

Hypoxia-Driven Adenosine Accumulation: A Crucial Microenvironmental Factor Promoting Tumor Progression

Systematic studies on the oxygenation status of solid tumors have shown that the development of hypoxic/anoxic tissue subvolumes is a pathophysiological trait in a wide range of human malignancies. As a result of this characteristic property, adenosine (ADO) accumulation (range: 50-100 μM) occurs caused by intra- and extracellular generation of ADO. Extracellular nucleotide catabolism by hypoxia-/HIF-1α-sensitive, membrane-associated ecto-5'-nucleotidases most probably is the major source of ADO in the halo of cancer cells upon specific genetic alterations taking place during tumor growth. Extracellular ADO can act through autocrine and paracrine pathways following receptor-binding and involving different intracellular signalling cascades. Hypoxia-driven receptor activation can lead to a broad spectrum of strong immune-suppressive properties facilitating tumor escape from immune control (e.g., inhibition of CD4+, CD8+, NK and dendritic cells, stimulation of Treg cells). In addition, tumor growth and progression is promoted by ADO-driven direct stimulation of tumor cell proliferation, migration, invasion, metastatic dissemination, and an increase in the production of molecules stimulating tumor angiogenesis. Hypoxia- driven ADO accumulation in the tumor microenvironment thus plays a critical role in tumor growth and progression at multiple pathophysiological levels.

CAR T-cell immunotherapy: The path from the by-road to the freeway?

Chimeric antigen receptors are genetically encoded artificial fusion molecules that can re-program the specificity of peripheral blood polyclonal T-cells against a selected cell surface target. Unparallelled clinical efficacy has recently been demonstrated using this approach to treat patients with refractory B-cell malignancy. However, the approach is technically challenging and can elicit severe toxicity in patients. Moreover, solid tumours have largely proven refractory to this approach. In this review, we describe the important structural features of CARs and how this may influence function. Emerging clinical experience is summarized in both solid tumours and haematological malignancies. Finally, we consider the particular challenges imposed by solid tumours to the successful development of CAR T-cell immunotherapy, together with a number of innovative strategies that have been developed in an effort to reverse the balance in favour of therapeutic benefit.

Purinergic signaling in Parkinson's disease. Relevance for treatment

Purinergic signaling modulates dopaminergic neurotransmission in health and disease. Classically adenosine A1 and A2A receptors have been considered key for the fine tune control of dopamine actions in the striatum, the main CNS motor control center. The main adenosine signaling mechanism is via the cAMP pathway but the future will tell whether calcium signaling is relevant in adenosinergic control of striatal function. Very relevant is the recent approval in Japan of the adenosine A2A receptor antagonist, istradefylline, for use in Parkinson's disease patients. Purine nucleotides are also regulators of striatal dopamine neurotransmission via P2 purinergic receptors. In parallel to the alpha-synuclein hypothesis of Parkinson's disease etiology, purinergic P2X1 receptors have been identified as mediators of accumulation of the Lewy-body enriched protein alpha-synuclein. Of note is the expression in striatum of purinergic-receptor-containing heteromers that are potential targets of anti-Parkinson's disease therapies and should be taken into account in drug discovery programs. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

CD39 is a promising therapeutic antibody target for the treatment of soft tissue sarcoma

Soft tissue sarcoma (STS) is a heterogenous tumor arising from the embryonic mesoderm represented by approximately 50 histological subtypes. Effective therapeutic intervention is lacking for recurrent, late stage and metastatic disease. CD39, a cell-surface ectonucleotidase, has previously been shown to be upregulated in hematological malignancies and various epithelial tumors, but not in STS. Here, we show by mass spectrometry and immunohistochemistry that CD39 is highly expressed in primary patient sarcoma samples. Moreover, CD39 nucleotidase activity is enhanced in fibrosarcoma compared with normal control cells. We demonstrate that an inhibitory monoclonal anti-CD39 antibody, abrogates CD39 enzymatic activity significantly and prolongs survival in a lethal metastatic patient-derived sarcoma model. Taken together, the data suggest CD39 is a novel therapeutic target for the treatment of STS.

Oxygenation to improve cancer vaccines, adoptive cell transfer and blockade of immunological negative regulators

Oxygenation of tumors weakens the tumor-protecting immunosuppressive signaling by A2A adenosine receptors in hypoxic and extracellular adenosine-rich microenvironments. This, in turn, unleashes the otherwise inhibited tumor-reactive T and natural killer (NK) cells. Oxygenation of tumors thus emerges as a novel checkpoint inhibitor of potential therapeutic value, but only in combination with cancer immunotherapies.

Recovery from experimental autoimmune uveitis promotes induction of antiuveitic inducible Tregs

The recovery of EAU, a mouse model of endogenous human autoimmune uveitis, is marked with the emergence of autoantigen-specific regulatory immunity in the spleen that protects the mice from recurrence of EAU. This regulatory immunity is mediated by a melanocortin-driven suppressor APC that presents autoantigen and uses adenosine to activate an antigen-specific CD4(+) Tregs through the A2Ar. These cells are highly effective in suppressing uveitis, and they appear to be inducible Tregs. In this study, we determined whether they are inducible or natural Tregs and identified the dependent mechanism for the function of these post-EAU Tregs. The post-EAU spleen CD25(+)CD4(+) T cells were sorted for NRP-1 expression and transferred to recipient mice immunized for EAU. The sorted NRP-1(-), but not the NRP-1(+), Tregs suppressed EAU. These NRP-1(-) Tregs coexpress PD-1 and PD-L1. Treatment of naive APCs with α-MSH promoted a regulatory APC that induced CD25(+) CD4(+) Tregs in a CD73-dependent manner. These Tregs were PD-L1(+) PD-1(+) NRP-1(-) FOXP3(+) HELIOS(-) and suppressed EAU when transferred to recipient mice. In contrast, PD-1(-) T cells did not suppress EAU, indicating that PD-1 is necessary for the suppressive activity of iTregs. Moreover, these Tregs did not suppress effector T cells when the PD/-1/PD-L1 pathway was blocked. These results demonstrate that post-EAU Tregs are inducible Tregs, which use a PD-1/PD-L1 mechanism to suppress disease.

A2aR antagonists: Next generation checkpoint blockade for cancer immunotherapy

The last several years have witnessed exciting progress in the development of immunotherapy for the treatment of cancer. This has been due in great part to the development of so-called checkpoint blockade. That is, antibodies that block inhibitory receptors such as CTLA-4 and PD-1 and thus unleash antigen-specific immune responses against tumors. It is clear that tumors evade the immune response by usurping pathways that play a role in negatively regulating normal immune responses. In this regard, adenosine in the immune microenvironment leading to the activation of the A2a receptor has been shown to represent one such negative feedback loop. Indeed, the tumor microenvironment has relatively high concentrations of adenosine. To this end, blocking A2a receptor activation has the potential to markedly enhance anti-tumor immunity in mouse models. This review will present data demonstrating the ability of A2a receptor blockade to enhance tumor vaccines, checkpoint blockade and adoptive T cell therapy. Also, as several recent studies have demonstrated that under certain conditions A2a receptor blockade can enhance tumor progression, we will also explore the complexities of adenosine signaling in the immune response. Despite important nuances to the A2a receptor pathway that require further elucidation, studies to date strongly support the development of A2a receptor antagonists (some of which have already been tested in phase III clinical trials for Parkinson Disease) as novel modalities in the immunotherapy armamentarium.

Prelude to oral microbes and chronic diseases: past, present and future

Associations between oral and systemic health are ancient. Oral opportunistic bacteria, particularly, Porphyromonas gingivalis and Fusobacterium nucleatum, have recently been deviated from their traditional roles as periodontal pathogens and arguably ascended to central players based on their participations in complex co-dependent mechanisms of diverse systemic chronic diseases risk and pathogenesis, including cancers, rheumatoid-arthritis, and diabetes.

Inhibition of CD39 enzymatic function at the surface of tumor cells alleviates their immunosuppressive activity

The ectonucleotidases CD39 and CD73 hydrolyze extracellular adenosine triphosphate (ATP) and adenosine diphosphate (ADP) to generate adenosine, which binds to adenosine receptors and inhibits T-cell and natural killer (NK)-cell responses, thereby suppressing the immune system. The generation of adenosine via the CD39/CD73 pathway is recognized as a major mechanism of regulatory T cell (Treg) immunosuppressive function. The number of CD39⁺ Tregs is increased in some human cancers, and the importance of CD39⁺ Tregs in promoting tumor growth and metastasis has been demonstrated using several in vivo models. Here, we addressed whether CD39 is expressed by tumor cells and whether CD39⁺ tumor cells mediate immunosuppression via the adenosine pathway. Immunohistochemical staining of normal and tumor tissues revealed that CD39 expression is significantly higher in several types of human cancer than in normal tissues. In cancer specimens, CD39 is expressed by infiltrating lymphocytes, the tumor stroma, and tumor cells. Furthermore, the expression of CD39 at the cell surface of tumor cells was directly demonstrated via flow cytometry of human cancer cell lines. CD39 in cancer cells displays ATPase activity and, together with CD73, generates adenosine. CD39⁺CD73⁺ cancer cells inhibited the proliferation of CD4 and CD8 T cells and the generation of cytotoxic effector CD8 T cells (CTL) in a CD39- and adenosine-dependent manner. Treatment with a CD39 inhibitor or blocking antibody alleviated the tumor-induced inhibition of CD4 and CD8 T-cell proliferation and increased CTL- and NK cell-mediated cytotoxicity. In conclusion, interfering with the CD39-adenosine pathway may represent a novel immunotherapeutic strategy for inhibiting tumor cell-mediated immunosuppression.

Alternative splicing of human NT5E in cirrhosis and hepatocellular carcinoma produces a negative regulator of ecto-5'-nucleotidase (CD73)

Alternative splicing of human NT5E generates CD73S, an endoplasmic reticulum–associated and dimerization-deficient glycoprotein that lacks enzymatic activity. CD73S functions as a negative regulator of canonical CD73 by promoting its proteasomal degradation, which may have significance in chronic liver disease and liver cancer.

Ecto-5′-nucleotidase (CD73), encoded by NT5E, is the major enzymatic source of extracellular adenosine. CD73 controls numerous pathophysiological responses and is a potential disease target, but its regulation is poorly understood. We examined NT5E regulation by alternative splicing. Genomic database analysis of human transcripts led us to identify NT5E-2, a novel splice variant that was expressed at low abundance in normal human tissues but was significantly up-regulated in cirrhosis and hepatocellular carcinoma (HCC). NT5E-2 encodes a shorter CD73 isoform we named CD73S. The presence of CD73S protein, which lacks 50 amino acids, was detected in HCC using an isoform-specific antibody. A noncanonical mouse mRNA, similar to human CD73S, was observed, but the corresponding protein was undetectable. The two human isoforms exhibited functional differences, such that ectopic expression of canonical CD73 (CD73L) in human HepG2 cells was associated with decreased expression of the proliferation marker Ki67, whereas CD73S expression did not have an effect on Ki67 expression. CD73S was glycosylated, catalytically inactive, unable to dimerize, and complexed intracellularly with the endoplasmic reticulum chaperone calnexin. Furthermore, CD73S complexed with CD73L and promoted proteasome-dependent CD73L degradation. The findings reveal species-specific CD73 regulation, with potential significance to cancer, fibrosis, and other diseases characterized by changes in CD73 expression and function.