Effect of the purinergic inhibitor oxidized ATP in a model of islet allograft rejection

P2X7 receptor knockout prevents streptozotocin-induced type 1 diabetes in mice

Type 1 diabetes (T1D) is caused by autoimmune destruction of islet of Langerhans β-cells. P2X7 receptors (P2X7R) modulate proinflammatory immune responses by binding extracellular ATP, a classic 'danger signal'. Here, we evaluated whether the P2X7R has a role in T1D development. P2X7(-/-) mice are resistant to TD1 induction by streptozotocin (STZ) treatment, with no increase in blood glucose, decrease in insulin-positive cells, and pancreatic islet reduction, compared to WT (C57BL/6) mice. Also, the levels of proinflammatory mediators (IL-1β, IFN-γ and NO) did not increase after STZ treatment in P2X7(-/-) animals, with reduced infiltration of CD4(+), CD8(+), B220(+), CD11b(+) and CD11c(+) cells in the pancreatic lymph nodes. Treatment with a P2X7 antagonist mimicked the effect of P2X7 knockout, preventing STZ-induced diabetes. Our results show that the absence of the P2X7R provides resistance in the induction of diabetes in this model, and suggest that therapy targeting the P2X7R may be useful against clinical T1D.

P2X7 Receptor as a Therapeutic Target

P2X7 receptor is an ATP-gated cation channel that upon agonist interaction leads to cellular influx of Na(+) and Ca(2+) and efflux of K(+). P2X7 is expressed by a wide variety of cells and its activation mediates a large number of biological processes like inflammation, neuromodulation, cell death or cell proliferation and it has been associated to related pathological conditions including infectious, inflammatory, autoimmune, neurological, and musculoskeletal disorders and, in the last years, to cancer. This chapter describes structural features of P2X7, chemical properties of its agonist, antagonist, and allosteric modulators and summarizes recent advances on P2X7 receptor as therapeutic target in the aforementioned diseases. We also give an overview on recent literature suggesting that P2X7 single-nucleotide polymorphisms could be exploited as diagnostic biomarkers for the development of tailored therapies.

Ion efflux and influenza infection trigger NLRP3 inflammasome signaling in human dendritic cells

The nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome, a multiprotein complex, is an essential intracellular mediator of antiviral immunity. In murine dendritic cells, this complex responds to a wide array of signals, including ion efflux and influenza A virus infection, to activate caspase-1-mediated proteolysis of IL-1β and IL-18 into biologically active cytokines. However, the presence and function of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome in human dendritic cells, in response to various triggers, including viral infection, has not been defined clearly. Here, we delineate the contribution of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome to the secretion of IL-1β, IL-18, and IL-1α by human dendritic cells (monocyte-derived and primary conventional dendritic cells). Activation of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome in human dendritic cells by various synthetic activators resulted in the secretion of bioactive IL-1β, IL-18, and IL-1α and induction of pyroptotic cell death. Cellular IL-1β release depended on potassium efflux and the activity of proteins nucleotide-binding oligomerization domain-like receptor protein 3 and caspase-1. Likewise, influenza A virus infection of dendritic cells resulted in priming and activation of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome and secretion of IL-1β and IL-18 in an M2- and nucleotide-binding oligomerization domain-like receptor protein 3-dependent manner. The magnitude of priming by influenza A virus varied among different strains and inversely corresponded to type I IFN production. To our knowledge, this is the first report describing the existence and function of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome in human dendritic cells and the ability of influenza A virus to prime and activate this pathway in human dendritic cells, with important implications for antiviral immunity and pathogenesis.

A Novel Function for P2Y2 in Myeloid Recipient-Derived Cells during Graft-versus-Host Disease

Acute graft-versus-host disease (GvHD) is a life-threatening complication of allogeneic hematopoietic cell transplantation. During the initiation phase of acute GvHD, endogenous danger signals such as ATP are released and inform the innate immune system via activation of the purinergic receptor P2X7 that a noninfectious damage has occurred. A second ATP-activated purinergic receptor involved in inflammatory diseases is P2Y2. In this study, we used P2y2(-/-) mice to test the role of this receptor in GvHD. P2y2(-/-) recipients experienced reduced GvHD-related mortality, IL-6 levels, enterocyte apoptosis, and histopathology scores. Chimeric mice with P2y2 deficiency restricted to hematopoietic tissues survived longer after GvHD induction than did wild-type mice. P2y2 deficiency of the recipient was connected to lower levels of myeloperoxidase in the intestinal tract of mice developing GvHD and a reduced myeloid cell signature. Selective deficiency of P2Y2 in inflammatory monocytes decreased GvHD severity. Mechanistically, P2y2(-/-) inflammatory monocytes displayed defective ERK activation and reactive oxygen species production. Compatible with a role of P2Y2 in human GvHD, the frequency of P2Y2(+) cells in inflamed GvHD lesions correlated with histopathological GvHD severity. Our findings indicate a novel function for P2Y2 in ATP-activated recipient myeloid cells during GvHD, which could be exploited when targeting danger signals to prevent GvHD.

Enhancement of Muscle T Regulatory Cells and Improvement of Muscular Dystrophic Process in mdx Mice by Blockade of Extracellular ATP/P2X Axis

Infiltration of immune cells and chronic inflammation substantially affect skeletal and cardiac muscle degeneration in Duchenne muscular dystrophy. In the immune system, extracellular adenosine triphosphate (ATP) released by dying cells is sensed as a danger associated molecular pattern through P2 purinergic receptors. Specifically, the P2X7 subtype has a prominent role in regulating immune system physiology and contributes to inflammasome activation also in muscle cells. Here, we show that in vivo blockade of the extracellular ATP/P2X purinergic signaling pathway by periodate-oxidized ATP delayed the progression of the dystrophic phenotype and dampened the local inflammatory response in mdx mice, a spontaneous mouse model of dystrophin deficiency. Reduced infiltration of leukocytes and macrophages and decreased expression of IL-6 were revealed in the muscles of periodate-oxidized ATP-treated mdx mice. Concomitantly, an increase in Foxp3(+) immunosuppressive regulatory T cells was observed and correlated with enhanced myofiber regeneration. Moreover, we detected reduced concentrations of profibrotic cytokines, including transforming growth factor-β and connective tissue growth factor, in muscles of periodate-oxidized ATP-treated mdx mice. The improvement of inflammatory features was associated with increased strength and reduced necrosis, thus suggesting that pharmacologic purinergic antagonism altering the adaptive immune component in the muscle infiltrates might represent a promising therapeutic approach in Duchenne muscular dystrophy.

Co-transplantation of autologous MSCs delays islet allograft rejection and generates a local immunoprivileged site

AIMS

Mesenchymal stem cells (MSCs) are multipotent cells with immunomodulatory properties. We tested the ability of MSCs to delay islet allograft rejection.

METHODS

Mesenchymal stem cells were generated in vitro from C57BL/6 and BALB/c mice bone marrow, and their immunomodulatory properties were tested in vitro. We then tested the effect of a local or systemic administration of heterologous and autologous MSCs on graft survival in a fully allogeneic model of islet transplantation (BALB/c islets into C57BL/6 mice).

RESULTS

In vitro, autologous, but not heterologous, MSCs abrogated immune cell proliferation in response to alloantigens and skewed the immune response toward a Th2 profile. A single dose of autologous MSCs co-transplanted under the kidney capsule with allogeneic islets delayed islet rejection, reduced graft infiltration, and induced long-term graft function in 30 % of recipients. Based on ex vivo analysis of recipient splenocytes, the use of autologous MSCs did not appear to have any systemic effect on the immune response toward graft alloantigens. The systemic injection of autologous MSCs or the local injection of heterologous MSCs failed to delay islet graft rejection.

CONCLUSION

Autologous, but not heterologous, MSCs showed multiple immunoregulatory properties in vitro and delayed allograft rejection in vivo when co-transplanted with islets; however, they failed to prevent rejection when injected systemically. Autologous MSCs thus appear to produce a local immunoprivileged site, which promotes graft survival.

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.

Novel therapeutic and diagnostic management of heart transplant patients

Heart transplantation was performed for the first time 40 years ago and it is now universally considered the "gold standard" treatment for individuals suffering from end-stage heart failure. The increased understanding of the molecular mechanisms and of the role of the immune system in allograft rejection led to an overall improvement of graft survival, which is now around 10 years. The introduction of novel immunosuppressive drugs reduced the rate of acute allograft rejection but did not improve significantly the long-term graft survival. In addition, adverse effects (e.g. infections, cancer and renal failure) associated with immunosuppressive drugs are increasing over time and may affect post-transplantation outcomes. An immunosuppression-free protocol based on tolerance induction is the Holy Grail for heart transplant recipients, but it is still far beyond our reach. In this review, we discuss the landscape of immunological challenges that heart transplanted individuals face and we critically review the novel immunological approaches available to overcome these remaining issues. Some of the novel approaches, successfully tested in preclinical and clinical models, may lead to a prolongation of patient's and heart allograft survival.

Ion channels in innate and adaptive immunity

Ion channels and transporters mediate the transport of charged ions across hydrophobic lipid membranes. In immune cells, divalent cations such as calcium, magnesium, and zinc have important roles as second messengers to regulate intracellular signaling pathways. By contrast, monovalent cations such as sodium and potassium mainly regulate the membrane potential, which indirectly controls the influx of calcium and immune cell signaling. Studies investigating human patients with mutations in ion channels and transporters, analysis of gene-targeted mice, or pharmacological experiments with ion channel inhibitors have revealed important roles of ionic signals in lymphocyte development and in innate and adaptive immune responses. We here review the mechanisms underlying the function of ion channels and transporters in lymphocytes and innate immune cells and discuss their roles in lymphocyte development, adaptive and innate immune responses, and autoimmunity, as well as recent efforts to develop pharmacological inhibitors of ion channels for immunomodulatory therapy.

Accelerated tumor progression in mice lacking the ATP receptor P2X7

The ATP receptor P2X7 (P2X7R or P2RX7) has a key role in inflammation and immunity, but its possible roles in cancer are not firmly established. In the present study, we investigated the effect of host genetic deletion of P2X7R in the mouse on the growth of B16 melanoma or CT26 colon carcinoma cells. Tumor size and metastatic dissemination were assessed by in vivo calliper and luciferase luminescence emission measurements along with postmortem examination. In P2X7R-deficient mice, tumor growth and metastatic spreading were accelerated strongly, compared with wild-type (wt) mice. Intratumoral IL-1β and VEGF release were drastically reduced, and inflammatory cell infiltration was abrogated nearly completely. Similarly, tumor growth was also greatly accelerated in wt chimeric mice implanted with P2X7R-deficient bone marrow cells, defining hematopoietic cells as a sufficient site of P2X7R action. Finally, dendritic cells from P2X7R-deficient mice were unresponsive to stimulation with tumor cells, and chemotaxis of P2X7R-less cells was impaired. Overall, our results showed that host P2X7R expression was critical to support an antitumor immune response, and to restrict tumor growth and metastatic diffusion.

The dark side of extracellular ATP in kidney diseases

Intracellular ATP is the most vital source of cellular energy for biologic systems, whereas extracellular ATP is a multifaceted mediator of several cell functions via its interaction, in an autocrine or paracrine manner, with P2 purinergic receptors expressed on the cell surface. These ionotropic and metabotropic P2 purinergic receptors modulate a variety of physiologic events upon the maintenance of a highly sensitive "set point," the derangement of which may lead to the development of key pathogenic mechanisms during acute and chronic diseases. Growing evidence suggests that extracellular ATP signaling via P2 purinergic receptors may be involved in different renal pathologic conditions. For these reasons, investigators and pharmaceutical companies are actively exploring novel strategies to antagonize or block these receptors with the goal of reducing extracellular ATP production or accelerating extracellular ATP clearance. Targeting extracellular ATP signaling, particularly through the P2X7 receptor, has considerable translational potential, given that novel P2X7-receptor inhibitors are already available for clinical use (e.g., CE224,535, AZD9056, and GSK1482160). This review summarizes the current evidence regarding the involvement of extracellular ATP and its P2 purinergic receptor-mediated signaling in physiologic and pathologic processes in the kidney; potential therapeutic options targeting extracellular ATP purinergic receptors are analyzed as well.

Genome-wide associations between genetic and epigenetic variation influence mRNA expression and insulin secretion in human pancreatic islets

Genetic and epigenetic mechanisms may interact and together affect biological processes and disease development. However, most previous studies have investigated genetic and epigenetic mechanisms independently, and studies examining their interactions throughout the human genome are lacking. To identify genetic loci that interact with the epigenome, we performed the first genome-wide DNA methylation quantitative trait locus (mQTL) analysis in human pancreatic islets. We related 574,553 single nucleotide polymorphisms (SNPs) with genome-wide DNA methylation data of 468,787 CpG sites targeting 99% of RefSeq genes in islets from 89 donors. We identified 67,438 SNP-CpG pairs in cis, corresponding to 36,783 SNPs (6.4% of tested SNPs) and 11,735 CpG sites (2.5% of tested CpGs), and 2,562 significant SNP-CpG pairs in trans, corresponding to 1,465 SNPs (0.3% of tested SNPs) and 383 CpG sites (0.08% of tested CpGs), showing significant associations after correction for multiple testing. These include reported diabetes loci, e.g. ADCY5, KCNJ11, HLA-DQA1, INS, PDX1 and GRB10. CpGs of significant cis-mQTLs were overrepresented in the gene body and outside of CpG islands. Follow-up analyses further identified mQTLs associated with gene expression and insulin secretion in human islets. Causal inference test (CIT) identified SNP-CpG pairs where DNA methylation in human islets is the potential mediator of the genetic association with gene expression or insulin secretion. Functional analyses further demonstrated that identified candidate genes (GPX7, GSTT1 and SNX19) directly affect key biological processes such as proliferation and apoptosis in pancreatic β-cells. Finally, we found direct correlations between DNA methylation of 22,773 (4.9%) CpGs with mRNA expression of 4,876 genes, where 90% of the correlations were negative when CpGs were located in the region surrounding transcription start site. Our study demonstrates for the first time how genome-wide genetic and epigenetic variation interacts to influence gene expression, islet function and potential diabetes risk in humans.

Inter-individual variation in genetics and epigenetics affects biological processes and disease susceptibility. However, most studies have investigated genetic and epigenetic mechanisms independently and to uncover novel mechanisms affecting disease susceptibility there is a highlighted need to study interactions between these factors on a genome-wide scale. To identify novel loci affecting islet function and potentially diabetes, we performed the first genome-wide methylation quantitative trait locus (mQTL) analysis in human pancreatic islets including DNA methylation of 468,787 CpG sites located throughout the genome. Our results showed that DNA methylation of 11,735 CpGs in 4,504 unique genes is regulated by genetic factors located in cis (67,438 SNP-CpG pairs). Furthermore, significant mQTLs cover previously reported diabetes loci including KCNJ11, INS, HLA, PDX1 and GRB10. We also found mQTLs associated with gene expression and insulin secretion in human islets. By performing causality inference tests (CIT), we identified CpGs where DNA methylation potentially mediates the genetic impact on gene expression and insulin secretion. Our functional follow-up experiments further demonstrated that identified mQTLs/genes (GPX7, GSTT1 and SNX19) directly affect pancreatic β-cell function. Together, our study provides a detailed map of genome-wide associations between genetic and epigenetic variation, which affect gene expression and insulin secretion in human pancreatic islets.

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.

Five-year follow-up of patients with type 1 diabetes transplanted with allogeneic islets: the UIC experience

This report summarizes a 5-year phase 1/2 allogeneic islet transplantation clinical trial conducted at the University of Illinois at Chicago (UIC). Ten patients were enrolled in this single center, open label, and prospective trial in which patients received 1-3 transplants. The first four subjects underwent islet transplantation with the Edmonton immunosuppressive regimen and the remaining six subjects received the UIC immunosuppressive protocol (Edmonton plus etanercept and exenatide). All 10 patients achieved insulin independence after 1-3 transplants. At 5 years of follow-up, 6 of the initial 10 patients were free of exogenous insulin. During the follow-up period, 7 of the 10 patients maintained positive C-peptide levels and a composite hypoglycemic score of 0. Most patients maintained HbA1c levels <6.0 % (42.1 mmol/mol) and a significantly improved β-score. In conclusion, this study demonstrated long-term islet graft function without using T cell depleting induction, with an encouraging outcome that includes 60 % of patients remaining insulin independent after 5 years of initial transplantation.

Inhibition of the purinergic pathway prolongs mouse lung allograft survival

Lung transplantation has limited survival with current immunosuppression. ATP is released from activated T cells, which act as costimulatory molecules through binding to the purinergic receptor P2XR7. We investigated the role of blocking the ATP/purinergic pathway, primarily P2XR7, using its inhibitor oxidized ATP (oATP) in modulating rejection of mouse lung allografts. Mouse lung transplants were performed using mice with major histocompatibility complex mismatch, BALB/c to C57BL6. Recipients received suramin or oATP, and lung allografts were evaluated 15 to ≥ 60 days after transplantation. Recipients were also treated with oATP after the onset of moderate to severe rejection to determine its ability to rescue lung allografts. Outcomes measures included lung function, histology, thoracic imaging, and allo-immune responses. Blocking purinergic receptors with the nonselective inhibitor suramin or with the P2XR7-selective inhibitor oATP reduced acute rejection and prolonged lung allograft survival for ≥ 60 days with no progression in severity. There were fewer inflammatory cells within lung allografts, less rejection, and improved lung function, which was maintained over time. CD4 and CD8 T cells were reduced within lung allografts with impaired activation with prolonged impairment of CD8 responses. In vitro, oATP reduced CD8 activation of Th1 inflammatory cytokines IFN-γ and TNF-α and cytolytic machinery, granzyme B. Cotreatment with immunosuppressive agents, cyclosporine, rapamycin, or CTLA-4Ig resulted in no additive benefits, and oATP alone resulted in better outcomes than cyclosporine alone. This study illustrates a potential new pathway to target in hopes of prolonging survival of lung transplant recipients.

The rise, fall, and resurgence of immunotherapy in type 1 diabetes

Despite considerable effort to halt or delay destruction of β-cells in autoimmune type 1 diabetes (T1D), success remains elusive. Over the last decade, we have seen a proliferation of knowledge on the pathogenesis of T1D that emerged from studies performed in non-obese diabetic (NOD) mice. However, while results of these preclinical studies appeared to hold great promise and boosted patients' hopes, none of these approaches, once tested in clinical settings, induced remission of autoimmune diabetes in individuals with T1D. The primary obstacles to translation reside in the differences between the human and murine autoimmune responses and in the contribution of many environmental factors associated with the onset of disease. Moreover, inaccurate dosing as well as inappropriate timing and uncertain length of drug exposure have played a central role in the negative outcomes of such therapeutic interventions. In this review, we summarize the most important approaches tested thus far in T1D, beginning with the most successful preclinical studies in NOD mice and ending with the latest disappointing clinical trials in humans. Finally, we highlight recent stem cell-based trials, for which expectations in the scientific community and among individuals with T1D are high.

Transdisciplinary approach to restore pancreatic islet function

The focus of our research is on islet immunobiology. We are exploring novel strategies that could be of assistance in the treatment and prevention of type 1 diabetes, as well as in the restoration of metabolic control via transplantation of insulin producing cells (i.e., islet cells). The multiple facets of diabetes and β-cell replacement encompass different complementary disciplines, such as immunology, cell biology, pharmacology, and bioengineering, among others. Through their interaction and integration, a transdisciplinary dimension is needed in order to address and overcome all aspects of the complex puzzle toward a successful clinical translation of a biological cure for diabetes.

Novel immunological strategies for islet transplantation

Islet transplantation has been demonstrated to improve glycometabolic control, to reduce hypoglycemic episodes and to halt the progression of diabetic complications. However, the exhaustion of islet function and the side effects related to chronic immunosuppression limit the spread of this technique. Consequently, new immunoregulatory protocols have been developed, with the aim to avoid the use of a life-time immunosuppression. Several approaches have been tested in preclinical models, and some are now under clinical evaluation. The development of new small molecules and new monoclonal or polyclonal antibodies is continuous and raises the possibility of targeting new costimulatory pathways or depleting particular cell types. The use of stem cells and regulatory T cells is underway to take advantage of their immunological properties and to induce tolerance. Xenograft islet transplantation, although having severe problems in terms of immunological compatibility, could theoretically provide an unlimited source of donors; using pigs carrying human immune antigens has showed indeed promising results. A completely different approach, the use of encapsulated islets, has been developed; synthetic structures are used to hide islet alloantigen from the immune system, thus preserving islet endocrine function. Once one of these strategies is demonstrated safe and effective, it will be possible to establish clinical islet transplantation as a treatment for patients with type 1 diabetes long before the onset of diabetic-related complications.

Differential requirement for P2X7R function in IL-17 dependent vs. IL-17 independent cellular immune responses

IL17-dependent autoimmunity to collagen type V (Col V) has been associated with lung transplant obliterative bronchiolitis. Unlike the T helper 1 (Th1)-dependent immune responses to Tetanus Toxoid (TT), the Th17 response to Col V in lung transplant patients and its Th1/17 variant observed in coronary artery disease patients requires IL-1β, tumor necrosis factor α and CD14(+) cells. Given the involvement of the P2X7 receptor (P2X7R) in monocyte IL-1β responses, we investigated its role in Th17-, Th1/17- and Th1-mediated proinflammatory responses. Transfer of antigen-pulsed peripheral blood mononucleated cells (PBMCs) from Col V-reactive patients into SCID mouse footpads along with P2X7R antagonists revealed a selective inhibition of Col V-, but not TT-specific swelling responses. P2X7R inhibitors blocked IL-1β induction from monocytes, including both Col V-α1 peptide-induced (T-dependent), as well as native Col V-induced (T-independent) responses. Significantly higher P2X7R expression was found on CXCR3(neg) CCR4(+)/6(+) CD4(+) [Th17] versus CXCR3(+)CCR4/6(neg) CD4(+) [Th1] subsets in PBMCs, suggesting that the paradigm of selective dependence on P2X7R might extend beyond Col V autoimmunity. Indeed, P2X7R inhibitors suppressed not only anti-Col V, but also Th1/17-mediated alloimmunity, in a heart transplant patient without affecting anti-viral Epstein-Barr virus responses. These results suggest that agents targeting the P2X7R might effectively treat Th17-related transplant pathologies, while maintaining Th1-immunity to infection.

The P2X7 receptor channel: recent developments and the use of P2X7 antagonists in models of disease

The P2X7 receptor is a trimeric ATP-gated cation channel found predominantly, but not exclusively, on immune cells. P2X7 activation results in a number of downstream events, including the release of proinflammatory mediators and cell death and proliferation. As such, P2X7 plays important roles in various inflammatory, immune, neurologic and musculoskeletal disorders. This review focuses on the use of P2X7 antagonists in rodent models of neurologic disease and injury, inflammation, and musculoskeletal and other disorders. The cloning and characterization of human, rat, mouse, guinea pig, dog, and Rhesus macaque P2X7, as well as recent observations regarding the gating and permeability of P2X7, are discussed. Furthermore, this review discusses polymorphic and splice variants of P2X7, as well as the generation and use of P2X7 knockout mice. Recent evidence for emerging signaling pathways downstream of P2X7 activation and the growing list of negative and positive modulators of P2X7 activation and expression are also described. In addition, the use of P2X7 antagonists in numerous rodent models of disease is extensively summarized. Finally, the use of P2X7 antagonists in clinical trials in humans and future directions exploring P2X7 as a therapeutic target are described.

TIM4 Regulates the Anti-Islet Th2 Alloimmune Response

The role of the novel costimulatory molecule TIM4 in anti-islet response is unknown. We explored TIM4 expression and targeting in Th1 (BALB/c islets into C57BL/6 mice) and Th2 (BALB/c islets into Tbet(-/-) C57BL/6 mice) models of anti-islet alloimmune response and in a model of anti-islet autoimmune response (diabetes onset in NOD mice). The targeting of TIM4, using the monoclonal antibody RMT4-53, promotes islet graft survival in a Th1 model, with 30% of the graft surviving in the long term; islet graft protection appears to be mediated by a Th1 to Th2 skewing of the immune response. Differently, in the Th2 model, TIM4 targeting precipitates graft rejection by further enhancing the Th2 response. The effect of anti-TIM4 treatment in preventing autoimmune diabetes was marginal with only minor Th1 to Th2 skewing. B-Cell depletion abolished the effect of TIM4 targeting. TIM4 is expressed on human B-cells and is upregulated in diabetic and islet-transplanted patients. Our data suggest a model in which TIM4 targeting promotes Th2 response over Th1 via B-cells. The targeting of TIM4 could become a component of an immunoregulatory protocol in clinical islet transplantation, aiming at redirecting the immune system toward a Th2 response.

Ex vivo expanded human regulatory T cells delay islet allograft rejection via inhibiting islet-derived monocyte chemoattractant protein-1 production in CD34+ stem cells-reconstituted NOD-scid IL2rγnull mice

Type 1 diabetes mellitus (T1DM) is an autoimmune disease caused by immune-mediated destruction of insulin-secreting β cells of the pancreas. Near complete dependence on exogenous insulin makes T1DM very difficult to control, with the result that patients are exposed to high blood glucose and risk of diabetic complications and/or intermittent low blood glucose that can cause unconsciousness, fits and even death. Allograft transplantation of pancreatic islets restores normoglycemia with a low risk of surgical complications. However, although successful immediately after transplantation, islets are progressively lost, with most of the patients requiring exogenous insulin within 2 years post-transplant. Therefore, there is an urgent requirement for the development of new strategies to prevent islet rejection. In this study, we explored the importance of human regulatory T cells in the control of islets allograft rejection. We developed a pre-clinical model of human islet transplantation by reconstituting NOD-scid IL2rγ^null mice with cord blood-derived human CD34⁺ stem cells and demonstrated that although the engrafted human immune system mediated the rejection of human islets, their survival was significantly prolonged following adoptive transfer of ex vivo expanded human T_regs. Mechanistically, T_regs inhibited the infiltration of innate immune cells and CD4⁺ T cells into the graft by down-regulating the islet graft-derived monocyte chemoattractant protein-1. Our findings might contribute to the development of clinical strategies for T_reg therapy to control human islet rejection. We also show for the first time that CD34⁺ cells-reconstituted NOD-scid IL2rγ^null mouse model could be beneficial for investigating human innate immunity in vivo.

The purinergic system in allotransplantation

The purine nucleotide adenosine triphosphate (ATP) is a universal source of energy for any intracellular reaction. Under specific physiological or pathological conditions, ATP can be released into extracellular spaces, where it binds and activates the purinergic receptors system (i.e. P2X, P2Y and P1 receptors). Extracellular ATP (eATP) binds to P2X or P2Y receptors in immune cells, where it mediates proliferation, chemotaxis, cytokine release, antigen presentation and cytotoxicity. eATP is then hydrolyzed by ectonucleotidases into adenosine diphosphate (ADP), which activates P2Y receptors. Ectonucleotidases also hydrolyze ADP to adenosine monophosphate and adenosine, which binds P1 receptors. In contrast to P2X and P2Y receptors, P1 receptors exert mainly an inhibitory effect on the immune response. In transplantation, a prominent role has been demonstrated for the eATP/P2X7R axis; the targeting of this pathway in fact is associated with long-term graft function and reduced graft versus host disease severity in murine models. Novel P2X receptor inhibitors are available for clinical use and are under assessment as immunomodulatory agents. In this review, we will focus on the relevance of the purinergic system and on the potential benefits of targeting this system in allograft rejection and tolerance.