Memory of extracellular adenosine A2A purinergic receptor-mediated signaling in murine T cells

Adenosinergic metabolism pathway: an emerging target for improving outcomes of solid organ transplantation

Extracellular nucleotides are widely recognized as crucial modulators of immune responses in peripheral tissues. Adenosine triphosphate (ATP) and adenosine are key components of extracellular nucleotides, the balance of which contributes to immune homeostasis. Under tissue injury, ATP exerts its pro-inflammatory function, while the adenosinergic pathway rapidly degrades ATP to immunosuppressive adenosine, thus inhibiting excessive and uncontrolled inflammatory responses. Previous reviews have explored the immunoregulatory role of extracellular adenosine in various pathological conditions, especially inflammation and malignancy. However, current knowledge regarding adenosine and adenosinergic metabolism in the context of solid organ transplantation remains fragmented. In this review, we summarize the latest information on adenosine metabolism and the mechanisms by which it suppresses the effector function of immune cells, as well as highlight the protective role of adenosine in all stages of solid organ transplantation, including reducing ischemia reperfusion injury during organ procurement, alleviating rejection, and promoting graft regeneration after transplantation. Finally, we discuss the potential for future clinical translation of adenosinergic pathway in solid organ transplantation.

Adenosine in cancer immunotherapy: Taking off on a new plane

As a new pillar of cancer therapy, tumor immunotherapy has brought irreplaceable durable responses in tumors. Considering its low response rate, additional immune regulatory mechanisms will be critical for the development of next-generation immune therapeutics. As a key regulatory mechanism, adenosine (ADO) protects tissues from excessive immune responses, but as a metabolite highly concentrated in tumor microenvironments, extracellular adenosine acts on adenosine receptors (mainly A2A receptors) expressed on MDSCs, Tregs, NK cells, effector T cells, DCs, and macrophages to promote tumor cell escape from immune surveillance by inhibiting the immune response. Amounting preclinical studies have demonstrated the adenosine pathway as a novel checkpoint for immunotherapy. Large number of adenosine pathway targeting clinical trials are now underway, including antibodies against CD39 and CD73 as well as A2A receptor inhibitors. There has been evidence of antitumor efficacy of these inhibitors in early clinical trials among a variety of tumors such as breast cancer, prostate cancer, non-small cell lung cancer, etc. As more clinical trial results are published, the combination of blockade of this pathway with immune checkpoint inhibitors, targeted drugs, traditional chemotherapy medications, radiotherapy and endocrine therapy will provide cancer patients with better clinical outcomes. We would elaborate on the role of CD39-CD73-A2AR pathway in the contribution of tumor microenvironment and the targeting of the adenosinergic pathway for cancer therapy in the review.

The Endogenous Dual Retinoid Receptor Agonist Alitretinoin Exhibits Immunoregulatory Functions on Antigen-Presenting Cells

Retinoids are a frequently used class of drugs in the treatment of inflammatory as well as malignant skin diseases. Retinoids have differential affinity for the retinoic acid receptor (RAR) and/or the retinoid X receptor (RXR). The endogenous dual RAR and RXR agonist alitretinoin (9-cis retinoic acid) demonstrated remarkable efficacy in the treatment of chronic hand eczema (CHE) patients; however, detailed information on the mechanisms of action remains elusive. Here, we used CHE as a model disease to unravel immunomodulatory pathways following retinoid receptor signaling. Transcriptome analyses of skin specimens from alitretinoin-responder CHE patients identified 231 significantly regulated genes. Bioinformatic analyses indicated keratinocytes as well as antigen presenting cells as cellular targets of alitretinoin. In keratinocytes, alitretinoin interfered with inflammation-associated barrier gene dysregulation as well as antimicrobial peptide induction while markedly inducing hyaluronan synthases without affecting hyaluronidase expression. In monocyte-derived dendritic cells, alitretinoin induced distinct morphological and phenotypic characteristics with low co-stimulatory molecule expression (CD80 and CD86), the increased secretion of IL-10 and the upregulation of the ecto-5'-nucleotidase CD73 mimicking immunomodulatory or tolerogenic dendritic cells. Indeed, alitretinoin-treated dendritic cells demonstrated a significantly reduced capacity to activate T cells in mixed leukocyte reactions. In a direct comparison, alitretinoin-mediated effects were significantly stronger than those observed for the RAR agonist acitretin. Moreover, longitudinal monitoring of alitretinoin-responder CHE patients could confirm in vitro findings. Taken together, we demonstrate that the dual RAR and RXR agonist alitretinoin targets epidermal dysregulation and demonstrates strong immunomodulatory effects on antigen presenting cell functions.

Adenosine, Schizophrenia and Cancer: Does the Purinergic System Offer a Pathway to Treatment?

For over a century, a complex relationship between schizophrenia diagnosis and development of many cancers has been observed. Findings from epidemiological studies are mixed, with reports of increased, reduced, or no difference in cancer incidence in schizophrenia patients. However, as risk factors for cancer, including elevated smoking rates and substance abuse, are commonly associated with this patient population, it is surprising that cancer incidence is not higher. Various factors may account for the proposed reduction in cancer incidence rates including pathophysiological changes associated with disease. Perturbations of the adenosine system are hypothesized to contribute to the neurobiology of schizophrenia. Conversely, hyperfunction of the adenosine system is found in the tumor microenvironment in cancer and targeting the adenosine system therapeutically is a promising area of research in this disease. We outline the current biochemical and pharmacological evidence for hypofunction of the adenosine system in schizophrenia, and the role of increased adenosine metabolism in the tumor microenvironment. In the context of the relatively limited literature on this patient population, we discuss whether hypofunction of this system in schizophrenia, may counteract the immunosuppressive role of adenosine in the tumor microenvironment. We also highlight the importance of studies examining the adenosine system in this subset of patients for the potential insight they may offer into these complex disorders.

Inversed Ratio of CD39/CD73 Expression on γδ T Cells in HIV Versus Healthy Controls Correlates With Immune Activation and Disease Progression

Background

γδ T cells are unconventional T cells that have been demonstrated to be crucial for the pathogenesis and potentially for the cure of HIV-1 infection. The ectonucleotidase CD39 is part of the purinergic pathway that regulates immune responses by degradation of pro-inflammatory ATP in concert with CD73. Few studies on the expression of the ectoenzymes CD73 and CD39 on human γδ T cells in HIV have been performed to date.

Methods

PBMC of n=86 HIV-1-infected patients were compared to PBMC of n=26 healthy individuals using 16-color flow cytometry determining the surface expression of CD39 and CD73 on Vδ1 and Vδ2 T cells in association with differentiation (CD45RA, CD28, CD27), activation and exhaustion (TIGIT, PD-1, CD38, and HLA-DR), and assessing the intracellular production of pro- and anti-inflammatory cytokines (IL-2, TGF-ß, TNF-α, Granzyme B, IL-10, IFN-γ) after in vitro stimulation with PMA/ionomycin.

Results

CD39 and CD73 expression on γδ T cells were inversed in HIV infection which correlated with HIV disease progression and immune activation. CD39, but not CD73 expression on γδ T cells of ART-treated patients returned to levels comparable with those of healthy individuals. Only a small subset (<1%) of γδ T cells co-expressed CD39 and CD73 in healthy or HIV-infected individuals. There were significantly more exhausted and terminally differentiated CD39+ Vδ1 T cells regardless of the disease status. Functionally, IL-10 was only detectable in CD39+ γδ T cells after in vitro stimulation in all groups studied. Viremic HIV-infected patients showed the highest levels of IL-10 production. The highest percentage of IL-10+ cells was found in the small CD39/CD73 co-expressing γδ T-cell population, both in healthy and HIV-infected individuals. Also, CD39+ Vδ2 T cells produced IL-10 more frequently than their CD39+ Vδ1 counterparts in all individuals regardless of the HIV status.

Conclusions

Our results point towards a potential immunomodulatory role of CD39+ and CD73+ γδ T cells in the pathogenesis of chronic HIV infection that needs further investigation.

Extracellular ectonucleotidases are differentially regulated in murine tissues and human polymorphonuclear leukocytes during sepsis and inflammation

Sepsis is life-threatening organ dysfunction caused by a dysregulated inflammatory and immune response to infection. Sepsis involves the combination of exaggerated inflammation and immune suppression. During systemic infection and sepsis, the liver works as a lymphoid organ with key functions in regulating the immune response. Extracellular nucleotides are considered damage-associated molecular patterns and are involved in the control of inflammation. Their levels are finely tuned by the membrane-associated ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) enzyme family. Although previous studies have addressed the role of NTPDase1 (CD39), the role of the other extracellular NTPDases, NTPDase2, -3, and -8, in sepsis is unclear. In the present studies we identified NTPDase8 as a top downregulated gene in the liver of mice submitted to cecal ligation-induced sepsis. Immunohistochemical analysis confirmed the decrease of NTPDase8 expression at the protein level. In vitro mechanistic studies using HepG2 hepatoma cells demonstrated that IL-6 but not TNF, IL-1β, bacteria, or lipopolysaccharide are able to suppress NTPDase8 gene expression. NTPDase8, as well as NTPDase2 and NTPDase3 mRNA was downregulated, whereas NTPDase1 (CD39) mRNA was upregulated in polymorphonuclear leukocytes from both inflamed and septic patients compared to healthy controls. Although the host's inflammatory response of polymicrobial septic NTPDase8 deficient mice was no different from that of wild-type mice, IL-6 levels in NTPDase8 deficient mice were higher than IL-6 levels in wild-type mice with pneumonia. Altogether, the present data indicate that extracellular NTPDases are differentially regulated during sepsis.

A2 adenosine receptor contributes to the development of cow's milk protein allergy via regulating regulatory T cells

Objective(s):

A2 adenosine receptor (A2AR) is a novel promising target for the treatment of inflammatory and allergic diseases. However, its role in the development of cow’s milk protein allergy (CMPA) has not been elucidated. The present study was designed to investigate the function of A2AR in CMPA development.

Materials and Methods:

BALB/c mice were sensitized and challenged with ovalbumin (OVA) to induce allergic responses. The model was assessed by detecting allergic responses and plasma-specific IgE levels. The levels of A2AR were measured by PCR and flow cytometry. The subpopulation of Treg cells was analysed.

Results:

The mice sensitized and challenged with OVA showed classic allergic symptoms, such as acute allergic skin responses, increased anaphylactic shock symptom scores, and higher levels of total IgE, OVA-specific IgE, IgG1 and IgG2a. OVA-sensitized mice and CMPA patients showed decreased levels of A2AR and Treg cells. Interestingly, we observed a positive correlation between A2AR expression and Treg levels in CMPA patients. Further study showed that the A2AR agonist CGS21680 blocked OVA-induced allergic reactions, and the A2AR antagonist KW-6002 amplified allergic responses. Interestingly, CGS21680 not only activated the A2AR-mediated signalling pathway but also caused an increase in the population of Treg cells. In contrast, KW-6002 therapy decreased the levels of Tregs in allergic mice.

Conclusion:

A2AR expression is downregulated in CMPA. The A2AR-mediated pathway negatively regulates the development of CMPA, at least in part, by amplifying the differentiation of Tregs.

Bacteria elevate extracellular adenosine to exploit host signaling for blood-brain barrier disruption

Bacterial meningitis remains a substantial cause of mortality worldwide and survivors may have severe lifelong disability. Although we know that meningeal bacterial pathogens must cross blood-central nervous system (CNS) barriers, the mechanisms which facilitate the virulence of these pathogens are poorly understood. Here, we show that adenosine from a surface enzyme (Ssads) of Streptococcus suis facilitates this pathogen’s entry into mouse brains. Monolayer translocation assays (from the human cerebrovascular endothelium) and experiments using diverse inhibitors and agonists together demonstrate that activation of the A1 adenosine receptor signaling cascade in hosts, as well as attendant cytoskeleton remodeling, promote S. suis penetration across blood-CNS barriers. Importantly, our additional findings showing that Ssads orthologs from other bacterial species also promote their translocation across barriers suggest that exploitation of A1 AR signaling may be a general mechanism of bacterial virulence.

CRISPR/Cas9 mediated deletion of the adenosine A2A receptor enhances CAR T cell efficacy

Adenosine is an immunosuppressive factor that limits anti-tumor immunity through the suppression of multiple immune subsets including T cells via activation of the adenosine A_2A receptor (A_2AR). Using both murine and human chimeric antigen receptor (CAR) T cells, here we show that targeting A_2AR with a clinically relevant CRISPR/Cas9 strategy significantly enhances their in vivo efficacy, leading to improved survival of mice. Effects evoked by CRISPR/Cas9 mediated gene deletion of A_2AR are superior to shRNA mediated knockdown or pharmacological blockade of A_2AR. Mechanistically, human A_2AR-edited CAR T cells are significantly resistant to adenosine-mediated transcriptional changes, resulting in enhanced production of cytokines including IFNγ and TNF, and increased expression of JAK-STAT signaling pathway associated genes. A_2AR deficient CAR T cells are well tolerated and do not induce overt pathologies in mice, supporting the use of CRISPR/Cas9 to target A_2AR for the improvement of CAR T cell function in the clinic.

Targeting Hypoxia-A2A Adenosinergic Immunosuppression of Antitumor T Cells During Cancer Immunotherapy

The blockade of immunological negative regulators offered a novel therapeutic approach that revolutionized the immunotherapy of cancer. Still, a significant portion of patients fail to respond to anti-PD-1/PD-L1 and/or anti-CTLA-4 therapy or experience significant adverse effects. We propose that one of the major reasons that many patients do not respond to this form of therapy is due to the powerful physiological suppression mediated by hypoxia-adenosinergic signaling. Indeed, both inflamed and cancerous tissues are hypoxic and rich in extracellular adenosine, in part due to stabilization of the transcription factor hypoxia-inducible factor 1 alpha (HIF-1α). Adenosine signals through adenosine A2A receptors (A2AR) to suppress anti-tumor and anti-pathogen immune responses. Several classes of anti-hypoxia-A2AR therapeutics have been offered to refractory cancer patients, with A2AR blockers, inhibitors of adenosine-generating enzymes such as CD39 and CD73, and hypoxia-targeting drugs now reaching the clinical stage. Clinical results have confirmed preclinical observations that blockade of the hypoxia-adenosine-A2AR axis synergizes with inhibitors of immune checkpoints to induce tumor rejection. Thus, A2AR blockers provide a new hope for the majority of patients who are nonresponsive to current immunotherapeutic approaches including checkpoint blockade. Here, we discuss the discoveries that firmly implicate the A2AR as a critical and non-redundant biochemical negative regulator of the immune response and highlight the importance of targeting the hypoxia-adenosine-A2AR axis to manipulate anti-pathogen and anti-tumor immune responses.

Tumor Infiltrating Regulatory T Cells in Sporadic and Colitis-Associated Colorectal Cancer: The Red Little Riding Hood and the Wolf

Regulatory T cells represent a class of specialized T lymphocytes that suppress unwanted immune responses and size the activation of the immune system whereby limiting collateral damages in tissues involved by inflammation. In cancer, the accumulation of Tregs is generally associated with poor prognosis. Many lines of evidence indicate that Tregs accumulation in the tumor microenvironment (TME) suppresses the immune response against tumor-associated antigens (TAA), thus promoting tumor progression in non-small cell lung carcinoma (NSLC), breast carcinoma and melanoma. In colorectal cancer (CRC) the effect of Tregs accumulation is debated. Some reports describe the association of high number of Tregs in CRC stroma with a better prognosis while others failed to find any association. These discordant results stem from the heterogeneity of the immune environment generated in CRC in which anticancer immune response may coexists with tumor promoting inflammation. Moreover, different subsets of Tregs have been identified that may exert different effects on cancer progression depending on tumor stage and their location within the tumor mass. Finally, Tregs phenotypic plasticity may be induced by cytokines released in the TME by dysplastic and other tumor-infiltrating cells thus affecting their functional role in the tumor. Here, we reviewed the recent literature about the role of Tregs in CRC and in colitis-associated colorectal cancer (CAC), where inflammation is the main driver of tumor initiation and progression. We tried to explain when and how Tregs can be considered to be the “good” or the “bad” in the colon carcinogenesis process on the basis of the available data concluding that the final effect of Tregs on sporadic CRC and CAC depends on their localization within the tumor, the subtype of Tregs involved and their phenotypic plasticity.

Rethinking the adenosine-A2AR checkpoint: implications for enhancing anti-tumor immunotherapy

Adenosine signaling through A_2AR serves as a negative regulator of the immune system. Unique to this suppressive pathway is its ability to impact numerous stromal and immune cells. Additionally, tumors exhibit elevated concentrations of adenosine further advancing the pathway's potential as a powerful target for activating anti-tumor immunity. The promise of this therapeutic strategy has been repeatedly demonstrated in mice, but has so far only yielded limited success in the clinic. Nonetheless, it is notable that many of these observed clinical responses have been in individuals resistant to prior immunotherapy. These observations suggest this pathway is indeed involved in tumor immune evasion. Thus, identifying the disparities between the translational and clinical implementation of this therapy becomes necessary. To this end, this review will revisit how and where adenosine-A_2AR signaling regulates the immune system and anti-tumor immunity so as to reveal opportunities for improving the translational success of this immunotherapy.

Regulatory T cells: Master thieves of the immune system

Treg cells are the immune system's in-house combatants against pathological immune activation. Because they are vital to maintenance of peripheral tolerance, it is important to understand how they perform their functions. To this end, various mechanisms have been proposed for Treg-mediated immune inhibition. A major group of mechanisms picture Treg cells as skilled thieves stealing a plethora of molecules that would otherwise promote immune effector functions. This suggests that several million years of evolution have endowed Treg cells with efficient ways to deprive immune effectors of activating stimuli to prevent immunopathology for survival of the host. Although we are still long way from deciphering their complete set of tricks, this review will focus on the types of "crimes" committed by these master thieves in both secondary lymphoid organs and non-lymphoid tissue.

Pharmacological preconditioning with adenosine A1 receptor agonist induces immunosuppression and improves graft survival in novel allogeneic transplantation models

Adenosine is widely known as a potent modulator of innate and acquired immunity. It is released during transplants, and acts on four subtype receptors. In previous studies, we demonstrated that pharmacological preconditioning (PPC), pre-administration of the selective A1 receptor (A1R) agonist led to A1R desensitization, is followed by upregulation of the adenosine A2A receptor. This immunosuppressive effect resulted in lymphopenia, and it reduced T-cell reactivity. The aim of the current study was to challenge the immunosuppressive effects of A1R-PPC in models of allogeneic grafts. PPC mice were treated by intraperitoneal injection using specific adenosine A1R agonist 24 h and 12 h before starting any procedure. We challenged our method in novel allogeneic muscle and skin grafts models. Mice and grafts were assessed by complete blood counts, MLR from PPC splenocytes, and pathological evaluation. We found a significant reduction in WBC and lymphocyte counts in PPC-treated mice. Two-way MLR with splenocytes from PPC grafted mice showed decreased proliferation and anergy. Histology of PPC allogeneic grafts revealed profoundly less infiltration and even less muscle necrosis compared to vehicle treated allografts. Similar results observed in PPC skin transplantation. To conclude, PPC moderated graft rejection in separate allogeneic challenges, and reduced lymphocytes infiltration and ischemic damage.

Mécanisme d'action du méthotrexate dans le traitement de la polyarthrite rhumatoïde

Le méthotrexate est utilisé dans le traitement de la polyarthrite rhumatoïde (PR) depuis les années 1980 et est souvent à ce jour le médicament de première intention pour le traitement de la PR. Dans cette revue, nous examinons plusieurs hypothèses pour expliquer le mécanisme à l'origine de l'efficacité du méthotrexate dans la PR. Celles-ci comprennent l'antagonisme du folate, la signalisation par l'adénosine, la génération d'espèces réactives de l'oxygène (ROS), la diminution des molécules d'adhérence, la modification des profils cytokiniques et l'inhibition des polyamines, entre autres. Actuellement, la signalisation par l'adénosine est probablement l'explication la plus largement acceptée du mécanisme du méthotrexate dans la PR, car le méthotrexate augmente les taux d'adénosine et suite à l'engagement de l'adénosine avec ses récepteurs extracellulaires, une cascade intracellulaire est activée et favorise un état antiinflammatoire global. Outre ces hypothèses, nous examinons le mécanisme du méthotrexate dans la PR sous l'angle de ses effets indésirables et considérons certains des nouveaux marqueurs génétiques de l'efficacité et de la toxicité du méthotrexate dans la PR. Enfin, nous discutons brièvement du mécanisme du méthotrexate en association avec un traitement de la PR par un inhibiteur du TNF-. En fin de compte, en trouvant une explication claire de la voie et du mécanisme conduisant à l'efficacité du méthotrexate dans la PR, il pourrait exister un moyen de formuler des thérapies plus puissantes avec moins d'effets secondaires.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Adenosine Metabolism: Emerging Concepts for Cancer Therapy

Adenosine is a key metabolic and immune-checkpoint regulator implicated in the tumor escape from the host immune system. Major gaps in knowledge that impede the development of effective adenosine-based therapeutics include: (1) lack of consideration of redundant pathways controlling ATP and adenosine levels; (2) lack of distinction between receptor-dependent and -independent effects of adenosine, and (3) focus on extracellular adenosine without consideration of intracellular metabolism and compartmentalization. In light of current clinical trials, we provide an overview of adenosine metabolism and point out the need for a more careful evaluation of the entire purinome in emerging cancer therapies.

Deficiency of CD73 activity promotes protective cardiac immunity against Trypanosoma cruzi infection but permissive environment in visceral adipose tissue

Damaged cells release the pro-inflammatory signal ATP, which is degraded by the ectonucleotidases CD39 and CD73 to the anti-inflammatory mediator adenosine (ADO). The balance between ATP/ADO is known to determine the outcome of inflammation/infection. However, modulation of the local immune response in different tissues due to changes in the balance of purinergic metabolites has yet to be investigated. Here, we explored the contribution of CD73-derived ADO on the acute immune response against Trypanosoma cruzi parasite, which invades and proliferates within different target tissues. Deficiency of CD73 activity led to an enhanced cardiac microbicidal immune response with an augmented frequency of macrophages with inflammatory phenotype and increased CD8+ T cell effector functions. The increment of local inducible nitric oxide (NO) synthase (iNOS)⁺ macrophages and the consequent rise of myocardial NO production in association with reduced ADO levels induced protection against T. cruzi infection as observed by the diminished cardiac parasite burden compared to their wild-type (WT) counterpart. Unexpectedly, parasitemia was substantially raised in CD73KO mice in comparison with WT mice, suggesting the existence of tissue reservoir/s outside myocardium. Indeed, CD73KO liver and visceral adipose tissue (VAT) showed increased parasite burden associated with a reduced ATP/ADO ratio and the lack of substantial microbicidal immune response. These data reveal that the purinergic system has a tissue-dependent impact on the host immune response against T. cruzi infection.

PET Imaging of Adenosine Receptors in Diseases

Adenosine receptors (ARs) are a class of purinergic G-protein-coupled receptors (GPCRs). Extracellular adenosine is a pivotal regulation molecule that adjusts physiological function through the interaction with four ARs: A1R, A2AR, A2BR, and A3R. Alterations of ARs function and expression have been studied in neurological diseases (epilepsy, Alzheimer's disease, and Parkinson's disease), cardiovascular diseases, cancer, and inflammation and autoimmune diseases. A series of Positron Emission Tomography (PET) probes for imaging ARs have been developed. The PET imaging probes have provided valuable information for diagnosis and therapy of diseases related to alterations of ARs expression. This review presents a concise overview of various ARs-targeted radioligands for PET imaging in diseases. The most recent advances in PET imaging studies by using ARs-targeted probes are briefly summarized.

CD73; a key ectonucleotidase in the development of breast cancer: Recent advances and perspectives

Tumor cell invasion and metastasis are the definitive cause of mortality in breast cancer (BC). Hypoxia and pro-inflammatory cytokines upregulate the CD73 gene in the tumor microenvironment. Subsequently, CD73 triggers molecular and cellular signaling pathways by both enzymatic and nonenzymatic pathways, which finally leads to breast tumor progression and development. In this paper, we summarize current advances in the understanding of CD73-driven mechanisms that promote BC development and mortality. Furthermore, we evaluate the therapeutic potential of CD73 targeting in BC.

Baseline adenosine receptor mRNA expression in blood as predictor of response to methotrexate therapy in patients with rheumatoid arthritis

Methotrexate (MTX) reduces inflammation by increasing extracellular adenosine levels in rheumatoid arthritis (RA) patients. Adenosine acts via G-protein coupled receptors; ADORA1, ADORA2a, ADORA2b and ADORA3. We studied if baseline expression of whole blood adenosine receptors can predict response to MTX. RA patients [American College of Rheumatology/European-League-Against-Rheumatism (EULAR) 2010 criteria], Disease modifying anti-rheumatic drug (DMARD) naïve with active disease [Disease Activity Score 28 (DAS28) > 3.2] were enrolled. Blood samples were collected at baseline (n = 100) and at 4 months after therapy (n = 50). Patients were treated with MTX monotherapy. Based on EULAR response, patients were categorized into three groups i.e. good, moderate and non-responders. Adenosine receptors gene expression (ADORA1, ADORA2a, ADORA2b and ADORA3) in whole-blood RNA was measured using real-time PCR. HPRT1 was used as housekeeping gene. Receptor expression at baseline was correlated with response to MTX. All values are expressed as median (interquartile range). Hundred patients [87% females; age 40 (18) years]; duration of disease 24 (24.75) months; DAS28 4.7 (1.25) were enrolled. Fifty-one were classified as good, 28 moderate and 21 as non-responders. No expression of ADORA1 and ADORA2b was detected. Significant difference was observed in the expression levels of ADORA3 between good vs non-responder (P = 0.03) and moderate vs non-responder (P = 0.002). On ROC curve analysis, ADORA3 with cut-off value of less than - 0.60 (ΔCt) predicted non-response to MTX treatment (AUC: 0.7, P = 0.006). ADORA3 mRNA levels in whole blood may serve as a biomarker of response to MTX.

Targeting Adenosine in Cancer Immunotherapy to Enhance T-Cell Function

T cells play a critical role in cancer control, but a range of potent immunosuppressive mechanisms can be upregulated in the tumor microenvironment (TME) to abrogate their activity. While various immunotherapies (IMTs) aiming at re-invigorating the T-cell-mediated anti-tumor response, such as immune checkpoint blockade (ICB), and the adoptive cell transfer (ACT) of natural or gene-engineered ex vivo expanded tumor-specific T cells, have led to unprecedented clinical responses, only a small proportion of cancer patients benefit from these treatments. Important research efforts are thus underway to identify biomarkers of response, as well as to develop personalized combinatorial approaches that can target other inhibitory mechanisms at play in the TME. In recent years, adenosinergic signaling has emerged as a powerful immuno-metabolic checkpoint in tumors. Like several other barriers in the TME, such as the PD-1/PDL-1 axis, CTLA-4, and indoleamine 2,3-dioxygenase (IDO-1), adenosine plays important physiologic roles, but has been co-opted by tumors to promote their growth and impair immunity. Several agents counteracting the adenosine axis have been developed, and pre-clinical studies have demonstrated important anti-tumor activity, alone and in combination with other IMTs including ICB and ACT. Here we review the regulation of adenosine levels and mechanisms by which it promotes tumor growth and broadly suppresses protective immunity, with extra focus on the attenuation of T cell function. Finally, we present an overview of promising pre-clinical and clinical approaches being explored for blocking the adenosine axis for enhanced control of solid tumors.

Tinkering with targeting nucleotide signaling for control of intracellular Leishmania parasites

Nucleotides are one of the most primitive extracellular signalling molecules across all phyla and regulate a multitude of responses. The biological effects of extracellular nucleotides/sides are mediated via the specific purinergic receptors present on the cell surface. In mammalian system, adenine nucleotides are the predominant nucleotides found in the extracellular milieu and mediate a constellation of physiological functions. In the context of host-pathogen interaction, extracellular ATP is recognized as a danger signal and potentiates the release of pro-inflammatory mediators from activated immune cells, on the other hand, its breakdown product adenosine exerts potential anti-inflammatory and immunosuppressive actions. Therefore, it is increasingly apparent that the interplay between extracellular ATP/adenosine ratios has a significant role in coordinating the regulation of the immune system in health and diseases. Several pathogens express ectonucleotidases on their surface and exploit the purinergic signalling as one of the mechanisms to modulate the host immune response. Leishmania pathogens are one of the most successful intracellular pathogens which survive within host macrophages and manipulate protective Th1 response into disease promoting Th2 response. In this review, we discuss the regulation of extracellular ATP and adenosine levels, the role of ATP/adenosine counter signalling in regulating the inflammation and immune responses during infection and how Leishmania parasites exploit the purinergic signalling to manipulate host response. We also discuss the challenges and opportunities in targeting purinergic signalling and the future prospects.

Adenosine A2A Receptor Stimulation Inhibits TCR-Induced Notch1 Activation in CD8+T-Cells

Notch receptors signaling is required for optimal T-cell activation and function. T-cell receptor (TCR) engagement can activate Notch receptors in T-cells in a ligand-independent fashion. In this study, we examined the role of adenosine A2A receptor (A2AR) signaling pathway in regulating the activity of Notch1 induced by TCR stimulation in CD8+T-cells. A selective A2AR agonist decreased Notch1 protein expression and Notch1 cleavage, and reduced transcripts of Notch1-target genes Hes1 and Myc in activated CD8+T-cells. Inhibition of TCR-induced Notch1 expression by an A2AR agonist was accompanied by increased cAMP concentration and mimicked by forskolin. This effect was associated with reduced IFN-γ and granzyme B production. The effect of an A2AR agonist was abrogated by a selective A2AR antagonist and absent in CD8+T-cells harvested from A2AR-/- mice. Stimulation of A2AR reduced Notch1 receptor levels by inhibiting upstream TCR signals, including ZAP70 phosphorylation, in turn impairing the generation of the active Notch1 intracellular domain (N1ICD). Direct activation of PKC with PMA and ionomycin bypassed A2AR-induced Notch1 inhibition. Overexpression of N1ICD in CD8+T-cells prevented the suppressive effects of an A2AR agonist on proliferation and cytokine release during activation. Our results identify the A2AR signaling pathway as an important regulator of TCR-induced Notch1 receptor activation in CD8+T-cells, and Notch as an important target of the immune suppressive effects of A2AR. We propose a mechanism whereby A2AR impairs CD8 T-cells function through inhibition of Notch1 receptor activation.

Methotrexate mechanism in treatment of rheumatoid arthritis

Methotrexate has been used in treatment of rheumatoid arthritis (RA) since the 1980s and to this day is often the first line medication for RA treatment. In this review, we examine multiple hypotheses to explain the mechanism of methotrexate efficacy in RA. These include folate antagonism, adenosine signaling, generation of reactive oxygen species (ROS), decrease in adhesion molecules, alteration of cytokine profiles, and polyamine inhibition amongst some others. Currently, adenosine signaling is probably the most widely accepted explanation for the methotrexate mechanism in RA given that methotrexate increases adenosine levels and on engagement of adenosine with its extracellular receptors an intracellular cascade is activated promoting an overall anti-inflammatory state. In addition to these hypotheses, we examine the mechanism of methotrexate in RA from the perspective of its adverse effects and consider some of the newer genetic markers of methotrexate efficacy and toxicity in RA. Lastly, we briefly discuss the mechanism of additive methotrexate in the setting of TNF-α inhibitor treatment of RA. Ultimately, finding a clear explanation for the pathway and mechanism leading to methotrexate efficacy in RA, there may be a way to formulate more potent therapies with fewer side effects.

Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages

Group 2 innate lymphoid cells (ILC2s) represent a rapid source of type 2 cytokines, such as IL-5 and IL-13, and play an important role in orchestrating type 2 immune response. Adenosine is an endogenous purine nucleoside, a catabolite of ATP that binds and activates ≥1 of 4 transmembrane G protein-coupled cell-surface adenosine receptors (ARs)-A1, A2A, A2B, and A3. Here, we studied the role of ARs in the regulation of cytokine production by ILC2s. We found that A2BARs suppress the production of both IL-5 and IL-13 by ILC2s, whereas A2AARs augment IL-5 production and fail to affect IL-13 release. Combined stimulation of all ARs led to the suppression of both IL-5 and IL-13 production, which indicated that A2BARs dominate A2AARs. Both pre- and post-transcriptional processes may be involved in the AR modulation of ILC2 IL-5 and IL-13 production. Thus, we identify adenosine as a novel negative regulator of ILC2 activation.-Csóka, B., Németh, Z. H., Duerr, C. U., Fritz, J. H., Pacher, P., Haskó, G. Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages.

Cervical cancer cells suppress effector functions of cytotoxic T cells through the adenosinergic pathway

BACKGROUND

The expression of CD73 in tumor cells plays a significant role in the production of adenosine (Ado) that suppresses antitumor effector cells.

METHODS

In this study we analyzed the capability of HPV-positive (HPV+) cervical cancer (CeCa) cell lines CaSki, SiHa, HeLa, and RoVa; and HPV-negative (HPV-) cell lines C33A and ViBo to produce Ado and inhibit effector functions of CD8+ T cells.

RESULTS

HPV+ CeCa cells expressed significantly higher levels of CD73 in the membrane (p<0.01) than HPV- CeCa cells and this expression was associated with the production of larger amounts of Ado (>400μM) compared to HPV-CeCa cells (<200μM) in the presence of AMP, as well asa stronger inhibition of (>50%) proliferation, activation, and cytotoxic activity of CD8+ T cells via interaction with A2A adenosine receptor. We also provide evidence that silenced E6/E7 expression in CeCa cells, strongly reduced its CD73 expression level and its capability to generate Ado.

CONCLUSION

This results suggest that HPV infection, which is associated with more than 99% of CeCa cases, may present an increased constitutive expression of CD73 in cervical neoplasia to contribute to the suppression of the immune response mediated by the production of large amounts of Ado.

Role of ATP as a Key Signaling Molecule Mediating Radiation-Induced Biological Effects

Adenosine triphosphate (ATP) serves as a signaling molecule for adaptive responses to a variety of cytotoxic agents and plays an important role in mediating the radiation stress-induced responses that serve to mitigate or repair the injurious effects of γ radiation on the body. Indeed, low doses of radiation may have a net beneficial effect by activating a variety of protective mechanisms, including antitumor immune responses. On the other hand, ATP signaling may be involved in the radiation resistance of cancer cells. Here, focusing on our previous work, we review the evidence that low-dose γ irradiation (0.25-0.5 Gy) induces release of extracellular ATP, and that the released ATP mediates multiple radiation-induced responses, including increased intracellular antioxidant synthesis, cell-mediated immune responses, induction of DNA damage repair systems, and differentiation of regulatory T cells.

Adenosine and adenosine receptors in the pathogenesis and treatment of rheumatic diseases

Adenosine, a nucleoside derived primarily from the extracellular hydrolysis of adenine nucleotides, is a potent regulator of inflammation. Adenosine mediates its effects on inflammatory cells by engaging one or more cell-surface receptors. The expression and function of adenosine receptors on different cell types change during the course of rheumatic diseases, such as rheumatoid arthritis (RA). Targeting adenosine receptors directly for the treatment of rheumatic diseases is currently under study; however, indirect targeting of adenosine receptors by enhancing adenosine levels at inflamed sites accounts for most of the anti-inflammatory effects of methotrexate, the anchor drug for the treatment of RA. In this Review, we discuss the regulation of extracellular adenosine levels and the role of adenosine in regulating the inflammatory and immune responses in rheumatic diseases such as RA, psoriasis and other types of inflammatory arthritis. In addition, adenosine and its receptors are involved in promoting fibrous matrix production in the skin and other organs, and the role of adenosine in fibrosis and fibrosing diseases is also discussed.

Structure-Activity Relationships of the Sustained Effects of Adenosine A2A Receptor Agonists Driven by Slow Dissociation Kinetics

The duration of action of adenosine A_2A receptor (A2A) agonists is critical for their clinical efficacy, and we sought to better understand how this can be optimized. The in vitro temporal response profiles of a panel of A2A agonists were studied using cAMP assays in recombinantly (CHO) and endogenously (SH-SY5Y) expressing cells. Some agonists (e.g., 3cd; UK-432,097) but not others (e.g., 3ac; CGS-21680) demonstrated sustained wash-resistant agonism, where residual receptor activation continued after washout. The ability of an antagonist to reverse pre-established agonist responses was used as a surrogate read-out for agonist dissociation kinetics, and together with radioligand binding studies suggested a role for slow off-rate in driving sustained effects. One compound, 3ch, showed particularly marked sustained effects, with a reversal t_1/2 > 6 hours and close to maximal effects that remained for at least 5 hours after washing. Based on the structure-activity relationship of these compounds, we suggest that lipophilic N6 and bulky C2 substituents can promote stable and long-lived binding events leading to sustained agonist responses, although a high compound logD is not necessary. This provides new insight into the binding interactions of these ligands and we anticipate that this information could facilitate the rational design of novel long-acting A2A agonists with improved clinical efficacy.

Mesenchymal stromal cells derived from cervical cancer produce high amounts of adenosine to suppress cytotoxic T lymphocyte functions

BACKGROUND

In recent years, immunomodulatory mechanisms of mesenchymal stem/stromal cells (MSCs) from bone marrow and other "classic" sources have been described. However, the phenotypic and functional properties of tumor MSCs are poorly understood. The aim of this study was to analyze the immunosuppressive capacity of cervical cancer-derived MSCs (CeCa-MSCs) on effector T lymphocytes through the purinergic pathway.

METHODS

We determined the expression and functional activity of the membrane-associated ectonucleotidases CD39 and CD73 on CeCa-MSCs and normal cervical tissue-derived MSCs (NCx-MSCs). We also analyzed their immunosuppressive capacity to decrease proliferation, activation and effector cytotoxic T (CD8+) lymphocyte function through the generation of adenosine (Ado).

RESULTS

We detected that CeCa-MSCs express higher levels of CD39 and CD73 ectonucleotidases in cell membranes compared to NCx-MSCs, and that this feature was associated with the ability to strongly suppress the proliferation, activation and effector functions of cytotoxic T-cells through the generation of large amounts of Ado from the hydrolysis of ATP, ADP and AMP nucleotides.

CONCLUSIONS

This study suggests that CeCa-MSCs play an important role in the suppression of the anti-tumor immune response in CeCa through the purinergic pathway.

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.

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.

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

Hypoxia-driven, A2A adenosine receptor (A2AR)-mediated (hypoxia-A2-adenosinergic), T-cell-autonomous immunosuppression was first recognized as critical and nonredundant in protecting normal tissues from inflammatory damage and autoimmunity. However, this immunosuppressive mechanism can be highjacked by bacteria and tumors to provide misguided protection for pathogens and cancerous tissues. Inhibitors of the hypoxia-A2-adenosinergic pathway represent a conceptually novel type of immunologic coadjuvants that could be combined with cancer vaccines, adoptive cell transfer, and/or blockade of negative immunologic regulators to further prolong patient survival and to minimize treatment-related side effects. In support of this approach are preclinical studies and findings that some human cancers are resistant to chemotherapies and immunotherapies due to the tumor-generated extracellular adenosine and A2AR on antitumor T and natural killer (NK) cells. Among the coadjuvants are (i) antagonists of A2AR, (ii) extracellular adenosine-degrading drugs, (iii) inhibitors of adenosine generation by CD39/CD73 ectoenzymes, and (iv) inhibitors of hypoxia-HIF-1α signaling. Combining these coadjuvants with CTLA-4 and/or PD-1 blockade is expected to have additive or even synergistic effects of targeting two different antitumor protective mechanisms. It is expected that even after multicombinatorial blockade of negative immunologic regulators, the antitumor T and NK cells would still be vulnerable to inhibition by hypoxia and A2AR. Yet to be tested is the potential capacity of coadjuvants to minimize the side effects of CTLA-4 and/or PD-1 blockade by decreasing the dose of blocking antibodies or by eliminating the need for dual blockade.

Phenotypic switch of CD8(+) T cells reactivated under hypoxia toward IL-10 secreting, poorly proliferative effector cells

CD8(+) T cells controlling pathogens or tumors must function at sites where oxygen tension is frequently low, and never as high as under atmospheric culture conditions. However, T-cell function in vivo is generally analyzed indirectly, or is extrapolated from in vitro studies under nonphysiologic oxygen tensions. In this study, we delineate the role of physiologic and pathologic oxygen tension in vitro during reactivation and differentiation of tumor-specific CD8(+) T cells. Using CD8(+) T cells from pmel-1 mice, we observed that the generation of CTLs under 5% O2, which corresponds to physioxia in lymph nodes, gave rise to a higher effector signature than those generated under atmospheric oxygen fractions (21% O2). Hypoxia (1% O2) did not modify cytotoxicity, but decreasing O2 tensions during CTL and CD8(+) tumor-infiltrating lymphocyte reactivation dose-dependently decreased proliferation, induced secretion of the immunosuppressive cytokine IL-10, and upregulated the expression of CD137 (4-1BB) and CD25. Overall, our data indicate that oxygen tension is a key regulator of CD8(+) T-cell function and fate and suggest that IL-10 release may be an unanticipated component of CD8(+) T cell-mediated immune responses in most in vivo microenvironments.

Hypoxia-Inducible Factor-2α Limits Natural Killer T Cell Cytotoxicity in Renal Ischemia/Reperfusion Injury

Natural killer T (NKT) cells are the major early-acting immune cell type and fundamental immune modulators in ischemia-reperfusion injury (IRI). Because lymphocytes are exposed to various oxygen tensions under pathophysiologic conditions, we hypothesize that hypoxia-inducible factors (HIFs) have roles in NKT cell activation, and thus determine the final outcome of renal IRI. In this study, we used Lck-Cre transgenic mice to specifically disrupt HIF-2α in T/NKT cells and found that HIF-2α knockout led to upregulated Fas ligand expression on peripheral NKT cells, but not on conventional T cells. HIF-2α knockout promoted infiltration of NKT cells into ischemic kidneys and exacerbated IRI, which could be mitigated by in vivo NK1.1(+) cell depletion or Fas ligand blockade. Compared with wild-type NKT cells, HIF-2α(-/-) NKT cells adoptively transferred to Rag1-knockout mice elicited more severe renal injury, and these mice were not protected by CGS21680, an adenosine A2A receptor agonist. Mechanistically, hypoxia-induced expression of adenosine A2A receptor in NKT cells and CGS21680-induced cAMP production in thymocytes were HIF-2α-dependent. Hydrogen peroxide-induced Fas ligand expression on thymic wild-type NKT cells was significantly attenuated by CGS21680 treatment, but this effect was lost in HIF-2α(-/-) NKT cells. Finally, CGS21680 and LPS, an inducer of HIF-2α in endothelium, synergistically reduced renal IRI substantially, but this effect was absent in Mx1-Cre-induced global HIF-2α-knockout mice. Taken together, our results reveal a hypoxia/HIF-2α/adenosine A2A receptor axis that restricts NKT cell activation when confronted with oxidative stress and thus protects against renal IRI.

Adenosine signalling in diabetes mellitus--pathophysiology and therapeutic considerations

Adenosine is a key extracellular signalling molecule that regulates several aspects of tissue function by activating four G-protein-coupled receptors, A1, A2A, A2B and A1 adenosine receptors. Accumulating evidence highlights a critical role for the adenosine system in the regulation of glucose homeostasis and the pathophysiology of type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). Although adenosine signalling is known to affect insulin secretion, new data indicate that adenosine signalling also contributes to the regulation of β-cell homeostasis and activity by controlling the proliferation and regeneration of these cells as well as the survival of β cells in inflammatory microenvironments. Furthermore, adenosine is emerging as a major regulator of insulin responsiveness by controlling insulin signalling in adipose tissue, muscle and liver; adenosine also indirectly mediates effects on inflammatory and/or immune cells in these tissues. This Review critically discusses the role of the adenosine-adenosine receptor system in regulating both the onset and progression of T1DM and T2DM, and the potential of pharmacological manipulation of the adenosinergic system as an approach to manage T1DM, T2DM and their associated complications.

Modulation of neuroimmunity by adenosine and its receptors: metabolism to mental illness

Adenosine is a pleiotropic bioactive with potent neuromodulatory properties. Due to its ability to easily cross the blood-brain barrier, it can act as a signaling molecule between the periphery and the brain. It functions through four (A1, A2A, A2B, and A3) cell surface G protein-coupled adenosine receptors (ARs) that are expressed in some combination on nearly all cells types within the CNS. By regulating the activity of adenylyl cyclase and changing the intracellular concentration of cAMP, adenosine can alter neuronal function and neurotransmission. A variety of illnesses related to metabolic dysregulation, such as type 1 diabetes and Alzheimer's disease, are associated with an elevated serum concentration of adenosine and a pathogenesis rooted in inflammation. This review describes the accepted physiologic function of adenosine in neurological disease and explores its new potential as a peripheral to central danger signal that can activate the neuroimmune system and contribute to symptoms of sickness and psychopathologies.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Extracellular adenosine metabolism in immune cells in melanoma

Malignant melanoma is characterized by the development of chronic inflammation in the tumor microenvironment, which leads to a strong immunosuppression associated with a rapid tumor progression. Adenosine is considered as one of the main immunosuppressive factors in the tumor environment. It is produced via enzymatic hydrolysis of extracellular ATP by ectonucleotidases CD39 and CD73 localized on cell surface. Using the ret transgenic mouse melanoma model that closely mimics human melanoma, we demonstrated an increased frequency of ectonucleotidase-positive myeloid-derived suppressor cells (MDSCs) in melanoma lesions and lymphoid organs. Furthermore, we observed that conventional CD4(+)FoxP3(-) and CD8(+) T cells infiltrating melanoma lesions of ret transgenic mice were distinctly enriched in the CD39(+)CD73(+) subpopulation that co-expressed also PD-1. Ectonucleotidase expression was also up-regulated in CD4(+) and CD8(+) T cells upon activation. In addition, these ectoenzymes were largely found to be expressed on memory T cell compartment (in particular, on effector memory cells). Our data suggest that extracellular adenosine produced by regulatory T cells (Tregs) and MDSCs can suppress T cell effector functions through paracrine signaling. Another mechanism involves its production also by effector T cells and an inhibition of their anti-tumor reactivity via autocrine signaling as a part of the negative feedback loop. This mode of adenosine signaling could be also used by Tregs and MDSCs to enhance their immunosuppressive activity.

Role of TGF-β signaling in generation of CD39+CD73+ myeloid cells in tumors

There is growing evidence that generation of adenosine from ATP, which is mediated by the CD39/CD73 enzyme pair, predetermines immunosuppressive and proangiogenic properties of myeloid cells. We have previously shown that the deletion of the TGF-β type II receptor gene (Tgfbr2) expression in myeloid cells is associated with decreased tumor growth, suggesting protumorigenic effect of TGF-β signaling. In this study, we tested the hypothesis that TGF-β drives differentiation of myeloid-derived suppressor cells into protumorigenic terminally differentiated myeloid mononuclear cells (TDMMCs) characterized by high levels of cell-surface CD39/CD73 expression. We found that TDMMCs represent a major cell subpopulation expressing high levels of both CD39 and CD73 in the tumor microenvironment. In tumors isolated from mice with spontaneous tumor formation of mammary gland and conditional deletion of the type II TGF-β receptor in mammary epithelium, an increased level of TGF-β protein was associated with further increase in number of CD39(+)CD73(+) TDMMCs compared with MMTV-PyMT/TGFβRII(WT) control tumors with intact TGF-β signaling. Using genetic and pharmacological approaches, we demonstrated that the TGF-β signaling mediates maturation of myeloid-derived suppressor cells into TDMMCs with high levels of cell surface CD39/CD73 expression and adenosine-generating capacity. Disruption of TGF-β signaling in myeloid cells resulted in decreased accumulation of TDMMCs, expressing CD39 and CD73, and was accompanied by increased infiltration of T lymphocytes, reduced density of blood vessels, and diminished progression of both Lewis lung carcinoma and spontaneous mammary carcinomas. We propose that TGF-β signaling can directly induce the generation of CD39(+)CD73(+) TDMMCs, thus contributing to the immunosuppressive, proangiogenic, and tumor-promoting effects of this pleiotropic effector in the tumor microenvironment.

Targeting CD73 and downstream adenosine receptor signaling in triple-negative breast cancer

INTRODUCTION

Despite significant improvements in diagnosis and therapy over the past 20 years, breast cancer remains a worldwide public health issue. In particular, triple negative breast cancer (TNBC), a subset of very aggressive breast tumors, is associated with a poor prognosis and has very few efficient therapeutic options. The ectonucleotidase CD73 has recently emerged as a promising new target for TNBC in preclinical models. Pharmacological targeting of CD73 and downstream adenosine A2A/A2B receptor signaling is currently an active field of research that could lead to the development of new cancer therapeutics, including options against TNBC.

AREAS COVERED

This article reviews the basic structural and molecular features of CD73 and its role in the development of cancer, with a particular focus on CD73's role in the biology of TNBC.

EXPERT OPINION

It was recently demonstrated that CD73 expression in TNBC is associated with worse clinical outcomes and increased resistance to anthracycline chemotherapy. Targeted blockade of the CD73/A2A axis has been shown to impair various aspects of tumorigenesis and displays synergism with other anti-cancer treatments in preclinical studies. Hence, we strongly argue for the development of CD73 inhibitors and for the repositioning of A2A antagonists in cancer.

Peripheral blood mononuclear cells as a laboratory to study dementia in the elderly

The steady and dramatic increase in the incidence of Alzheimer's disease (AD) and the lack of effective treatments have stimulated the search for strategies to prevent or delay its onset and/or progression. Since the diagnosis of dementia requires a number of established features that are present when the disease is fully developed, but not always in the early stages, the need for a biological marker has proven to be urgent, in terms of both diagnosis and monitoring of AD. AD has been shown to affect peripheral blood mononuclear cells (PBMCs) that are a critical component of the immune system which provide defence against infection. Although studies are continuously supplying additional data that emphasize the central role of inflammation in AD, PBMCs have not been sufficiently investigated in this context. Delineating biochemical alterations in AD blood constituents may prove valuable in identifying accessible footprints that reflect degenerative processes within the Central Nervous System (CNS). In this review, we address the role of biomarkers in AD with a focus on the notion that PBMCs may serve as a peripheral laboratory to find molecular signatures that could aid in differential diagnosis with other forms of dementia and in monitoring of disease progression.