Adenosine A2A receptor activation limits graft-versus-host disease after allogenic hematopoietic stem cell transplantation

Purinergic signalling in graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation is used to treat blood cancers, but often results in lethal graft-versus-host disease (GVHD). GVHD is an inflammatory disorder mediated by donor leukocytes that damage host tissues. Purinergic signalling plays important roles in GVHD development in mice but studies of these pathways in human GVHD remain limited. P2X7 receptor activation by ATP on host antigen presenting cells contributes to the induction of GVHD, while activation of this receptor on regulatory T cells, myeloid-derived suppressor cells and possibly type 3 innate lymphoid cells results in their loss to promote GVHD progression. In contrast, A_2A receptor activation by adenosine on donor T cells serves to restrict GVHD development. These and other purinergic signalling molecules remain potential biomarkers and therapeutic targets in GVHD.

Resolution of Inflammation in Acute Graft-Versus-Host-Disease: Advances and Perspectives

Inflammation is an essential reaction of the immune system to infections and sterile tissue injury. However, uncontrolled or unresolved inflammation can cause tissue damage and contribute to the pathogenesis of various inflammatory diseases. Resolution of inflammation is driven by endogenous molecules, known as pro-resolving mediators, that contribute to dampening inflammatory responses, promoting the resolution of inflammation and the recovery of tissue homeostasis. These mediators have been shown to be useful to decrease inflammatory responses and tissue damage in various models of inflammatory diseases. Graft-versus-host disease (GVHD) is a major unwanted reaction following allogeneic hematopoietic stem cell transplantation (allo-HSCT) and is characterized by an exacerbated inflammatory response provoked by antigen disparities between transplant recipient and donor. There is no fully effective treatment or prophylaxis for GVHD. This review explores the effects of several pro-resolving mediators and discusses their potential use as novel therapies in the context of GVHD.

Purinergic Signalling in Allogeneic Haematopoietic Stem Cell Transplantation and Graft-versus-Host Disease

Allogeneic haematopoietic stem cell transplantation (allo-HSCT) is a curative therapy for blood cancers and other haematological disorders. However, allo-HSCT leads to graft-versus-host disease (GVHD), a severe and often lethal immunological response, in the majority of transplant recipients. Current therapies for GVHD are limited and often reduce the effectiveness of allo-HSCT. Therefore, pro- and anti-inflammatory factors contributing to disease need to be explored in order to identify new treatment targets. Purinergic signalling plays important roles in haematopoiesis, inflammation and immunity, and recent evidence suggests that it can also affect haematopoietic stem cell transplantation and GVHD development. This review provides a detailed assessment of the emerging roles of purinergic receptors, most notably P2X7, P2Y2 and A2A receptors, and ectoenzymes, CD39 and CD73, in GVHD.

Conversion of ATP to adenosine by CD39 and CD73 in multiple myeloma can be successfully targeted together with adenosine receptor A2A blockade

Background

PD1/PDL1-directed therapies have been unsuccessful for multiple myeloma (MM), an incurable cancer of plasma cells in the bone marrow (BM). Therefore, other immune checkpoints such as extracellular adenosine and its immunosuppressive receptor should be considered. CD39 and CD73 convert extracellular ATP to adenosine, which inhibits T-cell effector functions via the adenosine receptor A2A (A2AR). We set out to investigate whether blocking the adenosine pathway could be a therapy for MM.

Methods

Expression of CD39 and CD73 on BM cells from patients and T-cell proliferation were determined by flow cytometry and adenosine production by Liquid chromatograpy-mass spectrometry (HPCL/MS). ENTPD1 (CD39) mRNA expression was determined on myeloma cells from patients enrolled in the publicly available CoMMpass study. Transplantable 5T33MM myeloma cells were used to determine the effect of inhibiting CD39, CD73 and A2AR in mice in vivo.

Results

Elevated level of adenosine was found in BM plasma of MM patients. Myeloma cells from patients expressed CD39, and high gene expression indicated reduced survival. CD73 was found on leukocytes and stromal cells in the BM. A CD39 inhibitor, POM-1, and an anti-CD73 antibody inhibited adenosine production and reduced T-cell suppression in vitro in coculture of myeloma and stromal cells. Blocking the adenosine pathway in vivo with a combination of Sodium polyoxotungstate (POM-1), anti-CD73, and the A2AR antagonist AZD4635 activated immune cells, increased interferon gamma production, and reduced the tumor load in a murine model of MM.

Conclusions

Our data suggest that the adenosine pathway can be successfully targeted in MM and blocking this pathway could be an alternative to PD1/PDL1 inhibition for MM and other hematological cancers. Inhibitors of the adenosine pathway are available. Some are in clinical trials and they could thus reach MM patients fairly rapidly.

The adenosinergic pathway in mesenchymal stem cell fate and functions

Mesenchymal stem cells (MSCs) play an important role in tissue homeostasis and damage repair through their ability to differentiate into cells of different tissues, trophic support, and immunomodulation. These properties made them attractive for clinical applications in regenerative medicine, immune disorders, and cell transplantation. However, despite multiple preclinical and clinical studies demonstrating beneficial effects of MSCs, their native identity and mechanisms of action remain inconclusive. Since its discovery, the CD73/ecto-5'-nucleotidase is known as a classic marker for MSCs, but its role goes far beyond a phenotypic characterization antigen. CD73 contributes to adenosine production, therefore, is an essential component of purinergic signaling, a pathway composed of different nucleotides and nucleosides, which concentrations are finely regulated by the ectoenzymes and receptors. Thus, purinergic signaling controls pathophysiological functions such as proliferation, migration, cell fate, and immune responses. Despite the remarkable progress already achieved in considering adenosinergic pathway as a therapeutic target in different pathologies, its role is not fully explored in the context of the therapeutic functions of MSCs. Therefore, in this review, we provide an overview of the role of CD73 and adenosine-mediated signaling in the functions ascribed to MSCs, such as homing and proliferation, cell differentiation, and immunomodulation. Additionally, we will discuss the pathophysiological role of MSCs, via CD73 and adenosine, in different diseases, as well as in tumor development and progression. A better understanding of the adenosinergic pathway in the regulation of MSCs functions will help to provide improved therapeutic strategies applicable in regenerative medicine.

Human ectoenzyme-expressing ILC3: immunosuppressive innate cells that are depleted in graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is often associated with chemotherapy- and radiotherapy-induced host tissue damage, leading to graft-versus-host disease (GVHD). Innate lymphoid cells (ILC) have an essential role in tissue homeostasis and tissue repair via their production of interleukin (IL)-22, which acts on intestinal stem cells. The tissue healing capacities of ILC via IL-22 in the context of allo-HSCT and GVHD has previously been demonstrated in a mouse model for acute GVHD. We investigated potential other ways of ILC-mediated tissue protection against GVHD. Tissue injury leads to the release of danger-associated molecular patterns (DAMPs). DAMPs interact with purinergic receptors and ectoenzymes on immune cells and induce pleiotropic effects, including activation of proinflammatory antigen-presenting cells and immunosuppressive effects via the generation of adenosine. Here, we report a novel subset of human ILC3 that coexpress the ectoenzymes CD39 and CD73 (ecto+ ILC3). Ecto+ ILC3 express RORγt and were present in the oral-gastrointestinal tract and bone marrow. ILC3 ectoenzyme expression is modulated by the proinflammatory cytokine IL-1β. Extracellular adenosine triphosphate (eATP) stimulated ecto+ ILC3 to produce IL-22 and adenosine. Activated ecto+ ILC3 suppressed autologous T-cell proliferation in coculture experiments via the production of adenosine. In allo-HSCT recipients, intestinal GVHD was associated with reduced proportions of ecto+ ILC3 and decreased levels of adenosine and its metabolite inosine. Taken together, ecto+ ILC3 have immunosuppressive properties, but in patients with GVHD, ecto+ ILC3 are depleted. A lack of ecto+ ILC3 and subsequent reduced capacity to neutralize DAMPs may contribute to the development of GVHD.

Defibrotide inhibits donor leucocyte-endothelial interactions and protects against acute graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation (allo‐HCT) is an effective therapy for the treatment of high‐risk haematological malignant disorders and other life‐threatening haematological and genetic diseases. Acute graft‐versus‐host disease (aGvHD) remains the most frequent cause of non‐relapse mortality following allo‐HCT and limits its extensive clinical application. Current pharmacologic agents used for prophylaxis and treatment of aGvHD are not uniformly successful and have serious secondary side effects. Therefore, more effective and safe prophylaxis and therapy for aGvHD are an unmet clinical need. Defibrotide is a multi‐target drug successfully employed for prophylaxis and treatment of veno‐occlusive disease/sinusoidal obstruction syndrome. Recent preliminary clinical data have suggested some efficacy of defibrotide in the prevention of aGvHD after allo‐HCT. Using a fully MHC‐mismatched murine model of allo‐HCT, we report here that defibrotide, either in prophylaxis or treatment, is effective in preventing T cell and neutrophil infiltration and aGvHD‐associated tissue injury, thus reducing aGvHD incidence and severity, with significantly improved survival after allo‐HCT. Moreover, we performed in vitro mechanistic studies using human cells revealing that defibrotide inhibits leucocyte‐endothelial interactions by down‐regulating expression of key endothelial adhesion molecules involved in leucocyte trafficking. Together, these findings provide evidence that defibrotide may represent an effective and safe clinical alternative for both prophylaxis and treatment of aGvHD after allo‐HCT, paving the way for new therapeutic approaches.

Innate immunity orchestrates the mobilization and homing of hematopoietic stem/progenitor cells by engaging purinergic signaling-an update

Bone marrow (BM) as an active hematopoietic organ is highly sensitive to changes in body microenvironments and responds to external physical stimuli from the surrounding environment. In particular, BM tissue responds to several cues related to infections, strenuous exercise, tissue/organ damage, circadian rhythms, and physical challenges such as irradiation. These multiple stimuli affect BM cells to a large degree through a coordinated response of the innate immunity network as an important guardian for maintaining homeostasis of the body. In this review, we will foc++us on the role of purinergic signaling and innate immunity in the trafficking of hematopoietic stem/progenitor cells (HSPCs) during their egression from the BM into peripheral blood (PB), as seen along pharmacological mobilization, and in the process of homing and subsequent engraftment into BM after hematopoietic transplantation. Innate immunity mediates these processes by engaging, in addition to certain peptide-based factors, other important non-peptide mediators, including bioactive phosphosphingolipids and extracellular nucleotides, as the main topic of this review. Elucidation of these mechanisms will allow development of more efficient stem cell mobilization protocols to harvest the required number of HSPCs for transplantation and to accelerate hematopoietic reconstitution in transplanted patients.

A single-nucleotide polymorphism in the human ENTPD1 gene encoding CD39 is associated with worsened graft-versus-host disease in a humanized mouse model

Regulatory T cells (Tregs) protect against graft-versus-host disease (GVHD), a life-threatening complication of allogeneic hematopoietic stem cell transplantation. The ectoenzyme CD39 is important for increasing the immunosuppressive function of Tregs. The rs10748643 (A → G) single-nucleotide polymorphism (SNP) in intron 1 of the human ENTPD1 gene is associated with increased proportions of CD39⁺ Tregs. This study aimed to determine whether the rs10748643 SNP corresponded to increased proportions of CD39⁺ Tregs in an Australian donor population, and whether this SNP influences clinical GVHD in a humanized mouse model. Donors were genotyped for the rs10748643 SNP by Sanger sequencing, and the proportion of CD39⁺ T cells in donor peripheral blood was determined by flow cytometry. Donors encoding the G allele (donors^AG/GG ) demonstrated higher proportions of CD39⁺ CD3⁺ CD4⁺ CD25⁺ CD127^lo Tregs, but not CD39⁺ CD3⁺ CD8⁺ T cells or CD39⁺ CD3⁺ CD4⁺ conventional T cells, compared with donors homozygous for the A allele (donors^AA ). NOD-SCID-IL2Rγ^null mice were injected with human peripheral blood mononuclear cells from either donors^AA (hCD39^AA mice) or donors^AG/GG (hCD39^AG/GG mice). hCD39^AG/GG mice demonstrated significantly greater weight loss and GVHD clinical scores, and significantly reduced survival, compared with hCD39^AA mice. hCD39^AG/GG mice showed significantly higher hCD4⁺ :hCD8⁺ T-cell ratios than hCD39^AA mice, but displayed similar proportions of CD3⁺ hCD4⁺ hCD25⁺ hCD127^lo Tregs and hCD39⁺ Tregs. However, the proportion of human Tregs corresponded to survival in hCD39^AA mice, but not in hCD39^AG/GG mice. This study demonstrates that donors encoding the G allele show higher percentages of CD39⁺ Tregs, but cause worsened GVHD in humanized mice compared with donors homozygous for the A allele.

Special Issue: G Protein-Coupled Adenosine Receptors: Molecular Aspects and Beyond

Adenosine is a purine nucleoside present in all human cells where it plays many different physiological roles: From being a building block for nucleic acids to a key constituent of the biological energy currency ATP [...].

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.

Involvement of A2B Receptor in DNA Damage Response and Radiosensitizing Effect of A2B Receptor Antagonists on Mouse B16 Melanoma

It is therapeutically important to elucidate the factors involved in the radiation resistance of tumors. We previously showed that ATP is released from mouse melanoma B16 cells in response to γ-irradiation, but the role of adenosine, a metabolite of ATP, is still unclear. Here, we show that the adenosine A2B receptor is involved in DNA damage repair and radioresistance in mouse melanoma B16 cells. The DNA damage response after γ-irradiation was attenuated by pretreatment with A2B receptor antagonists, such as PSB603, while it was enhanced by pretreatment with A2B receptor agonists, such as BAY60-6583. γ-Irradiation decreased the cell survival rate, and pretreatment with PSB603 further reduced the survival rate. On the other hand, pretreatment with BAY60-6583 increased the cell survival rate after irradiation. The DNA damage response and the cell survival rate after γ-irradiation were both decreased in A2B-knockdown cells. In vivo experiments in mice confirmed that tumor growth was suppressed and delayed in the irradiated group pretreated with PSB603, compared with the irradiation-alone group. Our results indicate that adenosine A2B receptor contributes to radioresistance, and could be a new target for the development of agents to increase the efficacy of radiotherapy.

The Biological and Clinical Relevance of G Protein-Coupled Receptors to the Outcomes of Hematopoietic Stem Cell Transplantation: A Systematized Review

Hematopoietic stem cell transplantation (HSCT) remains the only curative treatment for several malignant and non-malignant diseases at the cost of serious treatment-related toxicities (TRTs). Recent research on extending the benefits of HSCT to more patients and indications has focused on limiting TRTs and improving immunological effects following proper mobilization and engraftment. Increasing numbers of studies report associations between HSCT outcomes and the expression or the manipulation of G protein-coupled receptors (GPCRs). This large family of cell surface receptors is involved in various human diseases. With ever-better knowledge of their crystal structures and signaling dynamics, GPCRs are already the targets for one third of the current therapeutic arsenal. The present paper assesses the current status of animal and human research on GPCRs in the context of selected HSCT outcomes via a systematized survey and analysis of the literature.

The abnormal function of CD39+ regulatory T cells could be corrected by high-dose dexamethasone in patients with primary immune thrombocytopenia

Primary immune thrombocytopenia is an autoimmune disease, characterized with decreased platelet and increased risk of bleeding. Recent studies have shown the reduction and dysfunction of regulatory T (Treg) cells in ITP patients. CD39 is highly expressed on the surface of Treg cells. It degrades ATP to AMP and CD73 dephosphorylates AMP into adenosine. Then adenosine binds with adenosine receptor and suppresses immune response by activating Treg cells and inhibiting the release of inflammatory cytokines from effector T (Teff) cells. Adenosine receptor has several subtypes and adenosine A2A receptor (A2AR) plays a crucial role especially within lymphocytes. The CD39⁺ Treg cells and the expression of A2AR showed abnormality in some autoimmune disease. But knowledge of CD39⁺ Treg cells and A2AR which are crucial in the adenosine immunosuppressive pathway is still limited in ITP. Thirty-one adult patients with newly diagnosed ITP were enrolled in this study. CD39 and A2AR expression was measured by flow cytometry and RT-PCR. The function of CD39 was reflected by the change of ATP concentration detected by CellTiter-Glo Luminescent Cell Viability Assay. CD39 expression within CD4⁺CD25⁺ Treg cells in ITP patients was decreased compared to normal controls. After high-dose dexamethasone therapy, response (R) group showed increased CD39 expression within Treg cells while non-response (NR) group did not show any difference in contrast to those before treatment. The expression of A2AR in CD4⁺CD25^- Teff and CD4⁺CD25⁺ Treg cells was both lower in ITP patients than that of normal controls. After therapy, CD4⁺CD25^- Teff cells had higher A2AR expression while CD4⁺CD25⁺ Treg cells did not show any difference in comparison to that before treatment. The enzymatic activity of CD39 was damaged in ITP patients and improved after high-dose dexamethasone therapy. In ITP, there was not only numerical decrease but also impaired enzymatic activity in CD39⁺ Treg cells. After high-dose dexamethasone treatment, these two defects could be reversed. Our results also suggested that ITP patients had reduced A2AR expression in both CD4⁺CD25⁺ Treg cells and CD4⁺CD25^- Teff cells. CD4⁺CD25^- Teff cells had increased A2AR expression after treatment.

The A2A receptor agonist CGS 21680 has beneficial and adverse effects on disease development in a humanised mouse model of graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative method for blood cancers and other blood disorders, but is limited by the development of graft-versus-host disease (GVHD). GVHD results in inflammatory damage to the host liver, gastrointestinal tract and skin, resulting in high rates of morbidity and mortality in HSCT recipients. Activation of the A_2A receptor has been previously demonstrated to reduce disease in allogeneic mouse models of GVHD. This study aimed to investigate the effect of A_2A activation on disease development in a humanised mouse model of GVHD. Immunodeficient non-obese diabetic-severe combined immunodeficiency-interleukin (IL)-2 receptor γ^null (NSG) mice injected with human (h) peripheral blood mononuclear cells (hPBMCs), were treated with either the A_2A agonist CGS 21680 or control vehicle. Contrary to the beneficial effect of A_2A activation in allogeneic mouse models, CGS 21680 increased weight loss, and failed to reduce the clinical score or increase survival in this humanised mouse model of GVHD. Moreover, CGS 21680 reduced T regulatory cells and increased serum human IL-6 concentrations. Conversely, CGS 21680 reduced serum human tumour necrosis factor (TNF)-α concentrations and leukocyte infiltration into the liver, indicating that A_2A activation can, in part, reduce molecular and histological GVHD in this model. Notably, CGS 21680 also prevented healthy weight gain in NSG mice not engrafted with hPBMCs suggesting that this compound may be suppressing appetite or metabolism. Therefore, the potential benefits of A_2A activation in reducing GVHD in HSCT recipients may be limited and confounded by adverse impacts on weight, decreased T regulatory cell frequency and increased IL-6 production.

Pharmacological blockade of the CD39/CD73 pathway but not adenosine receptors augments disease in a humanized mouse model of graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation is a curative therapy for a number of hematological malignancies, but is limited by the development of graft-versus-host disease (GVHD). CD39 and CD73 form an ectoenzymatic pathway that hydrolyzes extracellular adenosine 5'-triphosphate (ATP) to adenosine, which respectively exacerbate or alleviate disease in allogeneic mouse models of GVHD. The current study aimed to explore the role of the CD39/CD73 pathway and adenosine receptor (AR) blockade in a humanized mouse model of GVHD. Immunodeficient nonobese diabetic-severe combined immunodeficiency-IL-2 receptor γ^null mice were injected with human peripheral blood mononuclear cells, and subsequently injected with the CD39/CD73 antagonist αβ-methylene-ADP (APCP) (50 mg kg^-1 ) or saline for 7 days, or the AR antagonist caffeine (10 mg kg^-1 ) or saline for 14 days. Mice predominantly engrafted human CD4⁺ and CD8⁺ T cells, with smaller proportions of human regulatory T cells, invariant natural killer T cells, monocytes and dendritic cells. Neither APCP nor caffeine altered engraftment of these human leukocyte subsets. APCP (CD39/CD73 blockade) augmented GVHD as shown through increased weight loss and worsened liver histology, including increased leukocyte and human T-cell infiltration, and increased apoptosis. This treatment also increased serum human IL-2 concentrations and decreased the frequency of human CD39^-  CD73^-  CD4⁺ T cells. In contrast, caffeine (AR blockade) did not alter GVHD severity or human serum cytokine concentrations (IL-2, IL-6, IL-10 or tumor necrosis factor-α). In conclusion, blockade of CD39/CD73 but not ARs augments disease in a humanized mouse model of GVHD. These results indicate that CD39/CD73 blockade maintains sufficient extracellular ATP concentrations to promote GVHD in this model.

ATP and T-cell-mediated rejection

PURPOSE OF REVIEW

Purine nucleosides and nucleotides are released in the extracellular space following cell injury and act as paracrine mediators through a number of dedicated membrane receptors. In particular, extracellular ATP (eATP) significantly influences T-lymphocyte activation and phenotype. The purpose of this review is to discuss the role of ATP signaling in the T-cell-mediated alloimmune response.

RECENT FINDINGS

In various animal models of solid transplantation, the purinergic axis has been targeted to prevent acute rejection and to promote long-term graft tolerance. The inhibition of ATP-gated P2X receptors has been shown to halt lymphocyte activation, to downregulate both Th1 and Th17 responses and to promote T-regulatory (Treg) cell differentiation. Similarly, the inhibition of ATP signaling attenuated graft-versus-host disease in mice undergoing hematopoietic cell transplantation. Significantly, different drugs targeting the purinergic system have been recently approved for human use and may be a viable therapeutic option for transplant patients.

SUMMARY

The inhibition of eATP signaling downregulates the alloimmune response, expands Treg cells and promotes graft survival. This robust preclinical evidence and the recent advances in pharmacological research may lead to intriguing clinical applications.

Altered homeostatic regulation of innate and adaptive immunity in lower gastrointestinal tract GVHD pathogenesis

Lower gastrointestinal (GI) tract graft-versus-host disease (GVHD) is the predominant cause of morbidity and mortality from GVHD after allogeneic stem cell transplantation. Recent data indicate that lower GI tract GVHD is a complicated process mediated by donor/host antigenic disparities. This process is exacerbated by significant changes to the microbiome, and innate and adaptive immune responses that are critical to the induction of disease, persistence of inflammation, and a lack of response to therapy. Here, we discuss new insights into the biology of lower GI tract GVHD and focus on intrinsic pathways and regulatory mechanisms crucial to normal intestinal function. We then describe multiple instances in which these homeostatic mechanisms are altered by donor T cells or conditioning therapy, resulting in exacerbation of GVHD. We also discuss data suggesting that some of these mechanisms produce biomarkers that could be informative as to the severity of GVHD and its response to therapy. Finally, novel therapies that might restore homeostasis in the GI tract during GVHD are highlighted.

Genetic susceptibility to severe asthma with fungal sensitization

Severe asthma is problematic and its pathogenesis poorly understood. Fungal sensitization is common, and many patients with severe asthma with fungal sensitization (SAFS), used to denote this subgroup of asthma, respond to antifungal therapy. We have investigated 325 haplotype-tagging SNPs in 22 candidate genes previously associated with aspergillosis in patients with SAFS, with comparisons in atopic asthmatics and healthy control patients, of whom 47 SAFS, 279 healthy and 152 atopic asthmatic subjects were genotyped successfully. Significant associations with SAFS compared with atopic asthma included Toll-like receptor 3 (TLR3) (p = .009), TLR9 (p = .025), C-type lectin domain family seven member A (dectin-1) (p = .043), interleukin-10 (IL-10) (p = .0010), mannose-binding lectin (MBL2) (p = .007), CC-chemokine ligand 2 (CCL2) (2 SNPs, p = .025 and .041), CCL17 (p = .002), plasminogen (p = .049) and adenosine A2a receptor (p = .024). These associations differ from those found in ABPA in asthma, indicative of contrasting disease processes. Additional and broader genetic association studies in SAFS, combined with experimental work, are likely to contribute to our understanding of different phenotypes of problematic asthma.

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.

Danger Signals and Graft-versus-host Disease: Current Understanding and Future Perspectives

Graft-versus-host response after allogeneic hematopoietic stem cell transplantation (allo-HCT) represents one of the most intense inflammatory responses observed in humans. Host conditioning facilitates engraftment of donor cells, but the tissue injury caused from it primes the critical first steps in the development of acute graft-versus-host disease (GVHD). Tissue injuries release pro-inflammatory cytokines (such as TNF-α, IL-1β, and IL-6) through widespread stimulation of pattern recognition receptors (PRRs) by the release of danger stimuli, such as damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). DAMPs and PAMPs function as potent stimulators for host and donor-derived antigen presenting cells (APCs) that in turn activate and amplify the responses of alloreactive donor T cells. Emerging data also point towards a role for suppression of DAMP induced inflammation by the APCs and donor T cells in mitigating GVHD severity. In this review, we summarize the current understanding on the role of danger stimuli, such as the DAMPs and PAMPs, in GVHD.

The Role of Purine Metabolites as DAMPs in Acute Graft-versus-Host Disease

Acute graft-versus-host disease (GvHD) causes high mortality in patients undergoing allogeneic hematopoietic cell transplantation. An early event in the classical pathogenesis of acute GvHD is tissue damage caused by the conditioning treatment or infection that consecutively leads to translocation of bacterial products [pathogen-associated molecular patterns (PAMPs)] into blood or lymphoid tissue, as well as danger-associated molecular patterns (DAMPs), mostly intracellular components that act as pro-inflammatory agents, once they are released into the extracellular space. A subtype of DAMPs is nucleotides, such as adenosine triphosphate released from dying cells that can activate the innate and adaptive immune system by binding to purinergic receptors. Binding to certain purinergic receptors leads to a pro-inflammatory microenvironment and promotes allogeneic T cell priming. After priming, T cells migrate to the acute GvHD target organs, mainly skin, liver, and the gastrointestinal tract and induce cell damage that further amplifies the release of intracellular components. This review summarizes the role of different purinergic receptors in particular P2X7 and P2Y2 as well as nucleotides in the pathogenesis of GvHD.

Unlocking the Potential of Purinergic Signaling in Transplantation

Purinergic signaling has been recognized as playing an important role in inflammation, angiogenesis, malignancy, diabetes and neural transmission. Activation of signaling pathways downstream from purinergic receptors may also be implicated in transplantation and related vascular injury. Following transplantation, the proinflammatory "danger signal" adenosine triphosphate (ATP) is released from damaged cells and promotes proliferation and activation of a variety of immune cells. Targeting purinergic signaling pathways may promote immunosuppression and ameliorate inflammation. Under pathophysiological conditions, nucleotide-scavenging ectonucleotidases CD39 and CD73 hydrolyze ATP, ultimately, to the anti-inflammatory mediator adenosine. Adenosine suppresses proinflammatory cytokine production and is associated with improved graft survival and decreased severity of graft-versus-host disease. Furthermore, purinergic signaling is involved both directly and indirectly in the mechanism of action of several existing immunosuppressive drugs, such as calcineurin inhibitors and mammalian target of rapamycin inhibitors. Targeting of purinergic receptor pathways, particularly in the setting of combination therapies, could become a valuable immunosuppressive strategy in transplantation. This review focuses on the role of the purinergic signaling pathway in transplantation and immunosuppression and explores possible future applications in clinical practice.

The role of danger signals and ectonucleotidases in acute graft-versus-host disease

Allogeneic hematopoietic cell transplantation (allo-HCT) represents the only curative treatment approach for many patients with benign or malignant diseases of the hematopoietic system. However, post-transplant morbidity and mortality are significantly increased by the development of acute graft-versus-host disease (GvHD). While alloreactive T cells act as the main cellular mediator of the GvH reaction, recent evidence suggests a critical role of the innate immune system in the early stages of GvHD initiation. Danger-associated molecular patterns released from the intracellular space as well as from the extracellular matrix activate antigen-presenting cells and set pro-inflammatory pathways in motion. This review gives an overview about danger signals representing therapeutic targets with a clinical perspective with a particular focus on extracellular nucleotides and ectonucleotidases.

Activation of Innate Immunity in Graft-versus-Host Disease: Implications for Novel Targets?

Acute graft-versus-host disease (GvHD) is mediated by alloreactive donor-derived T cells with a suitable T cell receptor recognizing recipient major histocompatibility complex or minor histocompatibility antigens. However, the process of T cell activation and tissue injury sensing is also dependent on innate immune cells and non-hematopoietic cells. Different cell types of the innate immune system have the ability to sense danger-associated and pathogen-associated molecular patterns via pattern recognition receptors which can be transmembrane Toll-like receptors or cytoplasmic nucleotide-binding oligomerization domain-like receptors. Infectious stimuli include bacterial, viral, and fungal components, while non-infectious stimuli can be components derived from damaged cells or extracellular matrix. A better understanding of the complex sensing and effector mechanisms of innate immune cells in GvHD may help to improve preventive and therapeutic strategies in GvHD.

Graft versus host disease: New insights into A2A receptor agonist therapy

Allogeneic transplantation can cure many disorders, including sickle cell disease, chronic granulomatous disease (CGD), severe combined immunodeficiency (SCID) and many types of cancers. However, there are several associated risks that can result in severe immunological reactions and, in some cases, death. Much of this morbidity is related to graft versus host disease (GVHD) [1]. GVHD is an immune mediated reaction in which donor T cells recognize the host as antigenically foreign, causing donor T cells to expand and attack host tissues. The current method of treating recent transplant patients with immunosuppressants to prevent this reaction has met with only partial success, emphasizing a need for new methods of GVHD treatment and prevention. Recently, a novel strategy has emerged targeting adenosine A_2A receptors (A_2AR) through the use of adenosine agonists. These agonists have been shown in vitro to increase the TGFβ-induced generation of FoxP3⁺ regulatory T cells (T_regs) and in vivo to improve weight gain and mortality as well as inhibit the release of pro-inflammatory cytokines in GVHD murine models [2,3]. Positive results involving A_2AR agonists in vitro and in vivo are promising, suggesting that A_2AR agonists should be a part of the management of clinical GvHD.

A delicate balance: CD73-generated adenosine limits the severity of graft vs. host disease but also constrains the allogeneic graft vs. tumor effect

The utility of allogeneic stem cell transplantation for treating hematologic malignancies is enhanced by the graft vs. tumor (GvT) effect, but limited by graft vs. host disease (GvHD). Studies involving the inhibition of CD73 by genetic or pharmacologic means suggest that the levels of CD73-generated adenosine may be manipulated to control GvHD, while maintaining the GvT effect.

[Extracellular adenosine is a therapeutic target for limiting graft-versus-host disease and enhancing the graft-versus-tumor effect against hematopoietic malignancy]

Allogeneic hematopoietic stem cell transplantation is performed in patients with hematologic malignancies refractory to chemotherapy. However, its efficacy is often limited by the development of graft-versus-host disease (GVHD) secondary to the allogeneic interaction of donor T cells with host dendritic cells. On the other hand, the antihost cytotoxicity of donor T cells enhances the graft-versus-tumor (GVT) effect. Extracellular adenosine generated by CD73/ecto-5'-nucleotidase from ATP via AMP plays pleiotropic roles under physiological and pathological conditions by engaging four adenosine receptors. One study recently demonstrated that ATP released from damaged cells exacerbates GVHD by activating the P2X7 receptor on host dendritic cells. In this review, we summarize our recent findings on the immunosuppressive role of extracellular adenosine in GVHD and the GVT effect. We have shown that in MHC-mismatched bone marrow transplantation, CD73 deficiency, particularly in the recipient, enhanced GVHD severity because of excessive donor T-cell expansion. Severe GVHD was enhanced by repeated administration of a CD73 inhibitor or an adenosine receptor antagonist. A competitive engraftment assay identified endogenous A2AAR signaling in donor T cells as part of a regulatory mechanism by CD73-generated adenosine. Pharmacological inhibition of CD73 enhanced the GVT effect against B-cell lymphoma and improved survival in tumor-relapsing mice after transplantation. Along with our findings, we herein introduce a novel concept that CD73-generated adenosine counteracts the ATP-evoked allogeneic immune reaction as a negative regulatory mechanism in GVHD. Pharmacological manipulation of CD73 activity could be a therapeutic strategy to limit GVHD and to preserve the GVT effect against hematopoietic malignancy.

Extracellular nucleotide and nucleoside signaling in vascular and blood disease

Nucleotides and nucleosides-such as adenosine triphosphate (ATP) and adenosine-are famous for their intracellular roles as building blocks for the genetic code or cellular energy currencies. In contrast, their function in the extracellular space is different. Here, they are primarily known as signaling molecules via activation of purinergic receptors, classified as P1 receptors for adenosine or P2 receptors for ATP. Because extracellular ATP is rapidly converted to adenosine by ectonucleotidase, nucleotide-phosphohydrolysis is important for controlling the balance between P2 and P1 signaling. Gene-targeted mice for P1, P2 receptors, or ectonucleotidase exhibit only very mild phenotypic manifestations at baseline. However, they demonstrate alterations in disease susceptibilities when exposed to a variety of vascular or blood diseases. Examples of phenotypic manifestations include vascular barrier dysfunction, graft-vs-host disease, platelet activation, ischemia, and reperfusion injury or sickle cell disease. Many of these studies highlight that purinergic signaling events can be targeted therapeutically.

The plant hormone zeatin riboside inhibits T lymphocyte activity via adenosine A2A receptor activation

Cytokinins are plant hormones that play an integral role in multiple aspects of plant growth and development. The biological functions of cytokinins in mammalian systems are, however, largely uncharacterized. The naturally occurring cytokinin zeatin riboside has recently been demonstrated to activate the mammalian adenosine A(2A) receptor, which is broadly expressed by various cell types including immune system cells, with the activation of the A(2A)R playing a role in the regulation of cells involved in both innate and adaptive immunity. We show for the first time that zeatin riboside modulates mammalian immune system activity via an A(2A)R-dependent mechanism. Specifically, zeatin riboside treatment induces the production of cyclic adenosine monophosphate (cAMP) by T lymphocytes and inhibits the production by CD3(+)CD4(+) T cells of interferon (IFN)-γ, IL-2, tumor-necrosis factor (TNF)-α, IL-4 and IL-13, and the production by CD3(+)CD8(+) T cells of IFN-γ, IL-2 and TNF-α. Additionally, the upregulation of CD25, CD69 and CD40L by activated T lymphocytes is modulated by zeatin riboside. Zeatin riboside treatment also potently inhibits thioglycollate-induced peritoneal leukocytosis. The immunomodulatory activities of zeatin riboside are blocked by co-treatment with the selective A(2A)R antagonist ZM241385. These data suggest that zeatin riboside possesses therapeutic potential as a mammalian immunomodulatory agent.

Pathogenesis of graft-versus-host disease: innate immunity amplifying acute alloimmune responses

In addition to reduced-intensity conditioning, which has expanded the eligibility for hematopoietic cell transplantation (HCT) to older patients, increased availability of alternative donors, including HLA-mismatched unrelated donors, has increased access to allogeneic HCT for more patients. However, acute graft-versus-host disease (GVHD) remains a lethal complication, even in HLA-matched donor-recipient pairs. The pathophysiology of GVHD depends on aspects of adaptive immunity and interactions between donor T-cells and host dendritic cells (DCs). Recent work has revealed that the role of other immune cells and endothelial cells and components of the innate immune response are also important. Tissue damage caused by the conditioning regimen leads to the release of exogenous and endogenous "danger signals". Exogenous danger signals called pathogen-associated molecular patterns and endogenous noninfectious molecules known as damage-associated molecular patterns (DAMPs) are responsible for initiating or amplifying acute GVHD by enhancing DC maturation and alloreactive T-cell responses. A significant association of innate immune receptor polymorphisms with outcomes, including GVHD severity, was observed in patients receiving allogeneic HCT. Understanding of the role of innate immunity in acute GVHD might offer new therapeutic approaches.

Graft-versus-host disease is enhanced by selective CD73 blockade in mice

CD73 functions as an ecto-5′-nucleotidase to produce extracellular adenosine that has anti-inflammatory and immunosuppressive activity. We here demonstrate that CD73 helps control graft-versus-host disease (GVHD) in mouse models. Survival of wild-type (WT) recipients of either allogeneic donor naïve CD73 knock-out (KO) or WT T cells was similar suggesting that donor naïve T cell CD73 did not contribute to GVHD. By contrast, donor CD73 KO CD4⁺CD25⁺ regulatory T cells (Treg) had significantly impaired ability to mitigate GVHD mortality compared to WT Treg, suggesting that CD73 on Treg is critical for GVHD protection. However, compared to donor CD73, recipient CD73 is more effective in limiting GVHD. Pharmacological blockade of A2A receptor exacerbated GVHD in WT recipients, but not in CD73 KO recipients, suggesting that A2 receptor signaling is primarily implicated in CD73-mediated GVHD protection. Moreover, pharmacological blockade of CD73 enzymatic activity induced stronger alloreactive T cell activity, worsened GVHD and enhanced the graft-versus-leukemia (GVL) effect. These findings suggest that both donor and recipient CD73 protects against GVHD but also limits GVL effects. Thus, either enhancing or blocking CD73 activity has great potential clinical application in allogeneic bone marrow transplants.

Adenosine A₂A receptor agonist-mediated increase in donor-derived regulatory T cells suppresses development of graft-versus-host disease

Graft-versus-host disease (GVHD) remains a significant complication of allogeneic transplantation. We previously reported that the adenosine A(2A) receptor (A(2A)R) specific agonist, ATL146e, decreases the incidence and severity of GVHD in a mouse transplant model. There is increasing interest in treatments that increase CD4(+)CD25(high)Foxp3(+) regulatory T cells (Tregs) to suppress GVHD. Our current study found in vitro that A(2A)R selective agonists enhanced TGF-β-induced generation of mouse Tregs 2.3- to 3-fold. We demonstrated in vivo suppression of GVHD with specific A(2A)R agonists in two different murine GVHD transplant models associated with profound increases in both circulating and target tissue Tregs of donor origin. Three different A(2A)R agonists of differing potency, ATL146e, ATL370, and ATL1223, all significantly inhibited GVHD-associated weight loss and mortality. At the same time, Tregs shown to be of donor origin increased 5.1- to 7.4-fold in spleen, 2.7- to 4.6-fold in peripheral blood, 2.3- to 4.7-fold in colon, and 3.8- to 4.6-fold in skin. We conclude that specific activation of A(2A)R inhibits acute GVHD through an increase of donor-derived Tregs. Furthermore, the increased presence of Tregs in target tissues (colon and skin) of A(2A)R-specific agonist-treated mice is likely the mechanistic basis for the anti-inflammatory effect preventing acute GVHD.

Ectonucleotidases in solid organ and allogeneic hematopoietic cell transplantation

Extracellular nucleotides are ubiquitous signalling molecules which modulate distinct physiological and pathological processes. Nucleotide concentrations in the extracellular space are strictly regulated by cell surface enzymes, called ectonucleotidases, which hydrolyze nucleotides to the respective nucleosides. Recent studies suggest that ectonucleotidases play a significant role in inflammation by adjusting the balance between ATP, a widely distributed proinflammatory danger signal, and the anti-inflammatory mediator adenosine. There is increasing evidence for a central role of adenosine in alloantigen-mediated diseases such as solid organ graft rejection and acute graft-versus-host disease (GvHD). Solid organ and hematopoietic cell transplantation are established treatment modalities for a broad spectrum of benign and malignant diseases. Immunological complications based on the recognition of nonself-antigens between donor and recipient like transplant rejection and GvHD are still major challenges which limit the long-term success of transplantation. Studies in the past two decades indicate that purinergic signalling influences the severity of alloimmune responses. This paper focuses on the impact of ectonucleotidases, in particular, NTPDase1/CD39 and ecto-5'-nucleotidase/CD73, on allograft rejection, acute GvHD, and graft-versus-leukemia effect, and on possible clinical implications for the modulation of purinergic signalling after transplantation.

Safety and efficacy of defibrotide for the treatment of severe hepatic veno-occlusive disease

Hepatic veno-occlusive disease (VOD), also known as sinusoidal obstruction syndrome, is a potentially life-threatening complication of chemotherapeutic conditioning used in preparation for hematopoietic stem-cell transplantation (SCT). VOD may occur in up to 62% of patients undergoing SCT, with onset generally within the first month after SCT. In severe cases, 100-day mortality is in excess of 80%. Current management consists of best supportive care, with no agents to date approved for treatment in the USA or the EU. Defibrotide, a polydisperse oligonucleotide, has been shown in phase II and III trials to improve complete response and survival in patients undergoing SCT with severe VOD. This article reviews our current understanding of VOD, and examines recent clinical findings on defibrotide for the treatment and prophylaxis of VOD.

Deficiency of CD73/ecto-5'-nucleotidase in mice enhances acute graft-versus-host disease

Extracellular ATP and adenosine have immunoregulatory roles during inflammation. Elevated extracellular ATP is known to exacerbate GVHD, and the pharmacologic activation of the adenosine A2A receptor is protective. However, the role of endogenous adenosine is unknown. We used gene-targeted mice and a pharmacologic inhibitor to test the role of adenosine generated by CD73/ecto-5'-nucleotidase in GVHD. In allogeneic transplants, both donor and recipient CD73 were protective, with recipient CD73 playing the dominant role. CD73 deficiency led to enhanced T-cell expansion and IFN-γ and IL-6 production, and the migratory capacity of Cd73-/- T cells in vitro was increased. However, the number of regulatory T cells and expression of costimulatory molecules on antigen-presenting cells were unchanged. A2A receptor deficiency led to increased numbers of allogeneic T cells, suggesting that signaling through the A2A receptor via CD73-generated adenosine is a significant part of the mechanism by which CD73 limits the severity of GVHD. Pharmacologic blockade of CD73 also enhanced graft-versus-tumor activity. These data have clinical implications, as both the severity of GVHD and the strength of an alloimmune antitumor response could be manipulated by enhancing or blocking CD73 activity or adenosine receptor signaling depending on the clinical indication.

Mesenchymal stromal cells up-regulate CD39 and increase adenosine production to suppress activated T-lymphocytes

Mesenchymal stromal cells (MSCs) suppress T cell responses through mechanisms not completely understood. Adenosine is a strong immunosuppressant that acts mainly through its receptor A(2a) (ADORA2A). Extracellular adenosine levels are a net result of its production (mediated by CD39 and CD73), and of its conversion into inosine by Adenosine Deaminase (ADA). Here we investigated the involvement of ADO in the immunomodulation promoted by MSCs. Human T lymphocytes were activated and cultured with or without MSCs. Compared to lymphocytes cultured without MSCs, co-cultured lymphocytes were suppressed and expressed higher levels of ADORA2A and lower levels of ADA. In co-cultures, the percentage of MSCs expressing CD39, and of T lymphocytes expressing CD73, increased significantly and adenosine levels were higher. Incubation of MSCs with media conditioned by activated T lymphocytes induced the production of adenosine to levels similar to those observed in co-cultures, indicating that adenosine production was mainly derived from MSCs. Finally, blocking ADORA2A signaling raised lymphocyte proliferation significantly. Our results suggest that some of the immunomodulatory properties of MSCs may, in part, be mediated through the modulation of components related to adenosine signaling. These findings may open new avenues for the development of new treatments for GVHD and other inflammatory diseases.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Adenosine A(2A) receptor activation limits chronic granulomatous disease-induced hyperinflammation

Chronic granulomatous disease (CGD) is caused by defects in the NADPH oxidase complex and is characterized by an increased susceptibility to infection. Other significant complications of CGD include autoimmunity and non-infectious hyperinflammatory disorders. We show that a gp91(phox) deficiency leads to the development of phenotypically altered T lymphocytes in mice and that this abnormal, hyperactive phenotype can be modulated by activation of the adenosine A(2A) receptor. T cells isolated from CGD mice produce significantly higher levels of the pro-inflammatory cytokines IFN-γ, IL-2, TNF-α, IL-4 and IL-13 than do WT cells after TCR-mediated activation; treatment with the selective adenosine A(2A) receptor agonist, CGS21680, potently inhibits this response. Additionally, the over exuberant inflammatory response elicited by thioglycollate challenge in gp91(phox) deficient mice is attenuated by CGS21680. These data suggest that treatment with A(2A)R agonists may be an effective therapy by which to regulate the immune system hyperactivity that results from a gp91(phox) deficiency.