Adenosine, through the A1 receptor, inhibits vesicular MHC class I cross-presentation by resting DC

The role of adenosine A1 receptor on immune cells

BACKGROUND

Adenosine, acting as a regulator by mediating the activation of G protein-coupled adenosine receptor families (A₁, A₂A, A₂B, and A₃), plays an important role under physiological and pathological conditions. As the receptor with the highest affinity for adenosine, the role of adenosine A₁ receptor (A₁R)-mediated adenosine signaling pathway in the central nervous system has been well addressed. However, functions of A₁R on immune cells are less summarized. Considering that some immune cells express multiple types of adenosine receptors with distinct effects and varied density, exogenous adenosine of different concentrations may induce divergent immune cell functions.

MATERIALS AND METHODS

The literatures about the expression of A₁R and its regulation on immune cells and how it regulates the function of immune cells were searched on PubMed and Google Scholar.

CONCLUSION

In this review, we discussed the effects of A₁R on immune cells, including monocytes, macrophages, neutrophils, dendritic cells, and microglia, and focused on the role of A₁R in regulating immune cells in diseases, which may facilitate our understanding of the mechanisms by which adenosine affects immune cells through A₁R.

The immunomodulatory function of adenosine in sepsis

Sepsis is an unsolved clinical condition with a substantial mortality rate in the hospital. Despite decades of research, no effective treatments for sepsis exists. The role of adenosine in the pathogenesis of sepsis is discussed in this paper. Adenosine is an essential endogenous molecule that activates the A1, A2a, A2b, and A3 adenosine receptors to regulate tissue function. These receptors are found on a wide range of immune cells and bind adenosine, which helps to control the immune response to inflammation. The adenosine receptors have many regulatory activities that determine the onset and progression of the disease, which have been discovered via the use of animal models. A greater understanding of the role of adenosine in modulating the immune system has sparked hope that an adenosine receptor-targeted treatment may be used one day to treat sepsis.

A novel definition and treatment of hyperinflammation in COVID-19 based on purinergic signalling

Hyperinflammation plays an important role in severe and critical COVID-19. Using inconsistent criteria, many researchers define hyperinflammation as a form of very severe inflammation with cytokine storm. Therefore, COVID-19 patients are treated with anti-inflammatory drugs. These drugs appear to be less efficacious than expected and are sometimes accompanied by serious adverse effects. SARS-CoV-2 promotes cellular ATP release. Increased levels of extracellular ATP activate the purinergic receptors of the immune cells initiating the physiologic pro-inflammatory immune response. Persisting viral infection drives the ATP release even further leading to the activation of the P2X7 purinergic receptors (P2X7Rs) and a severe yet physiologic inflammation. Disease progression promotes prolonged vigorous activation of the P2X7R causing cell death and uncontrolled ATP release leading to cytokine storm and desensitisation of all other purinergic receptors of the immune cells. This results in immune paralysis with co-infections or secondary infections. We refer to this pathologic condition as hyperinflammation. The readily available and affordable P2X7R antagonist lidocaine can abrogate hyperinflammation and restore the normal immune function. The issue is that the half-maximal effective concentration for P2X7R inhibition of lidocaine is much higher than the maximal tolerable plasma concentration where adverse effects start to develop. To overcome this, we selectively inhibit the P2X7Rs of the immune cells of the lymphatic system inducing clonal expansion of Tregs in local lymph nodes. Subsequently, these Tregs migrate throughout the body exerting anti-inflammatory activities suppressing systemic and (distant) local hyperinflammation. We illustrate this with six critically ill COVID-19 patients treated with lidocaine.

Electroacupuncture improves myocardial ischemia injury via activation of adenosine receptors

Electroacupuncture (EA) can improve myocardial ischemia (MI) injury; nevertheless, the mechanism is not entirely clear. And there were disagreements about whether the effect of EA at acupoint in disease-affected meridian is better than EA at acupoint in non-affected meridian and sham acupoint. Here, we showed that the effect of EA at Neiguan (PC6) is better than EA at Hegu (LI4) and sham acupoint in affecting RPP and ECG, increasing ATP and ADO production, decreasing AMP production, and upregulating the mRNA expression levels of A1AR, A2aAR, and A2bAR; knockdown of A1AR or A2bAR reversed the effect of EA at PC6 in alleviating MI injury; knockdown of A2aAR had no influence on the cardiac protection of EA at PC6; thus, the cardioprotective effect of EA at PC6 needs A1AR and A2bAR, instead of A2aAR; considering that the cardio protection of adenosine receptor needs activation of other adenosine receptors, one of the reasons may be that after silence of A1AR or A2bAR, EA at PC6 could not impact the expression levels of the other two adenosine receptors, and after silence of A2aAR, EA at PC6 could impact the expression levels of A1AR and A2bAR. These results suggested that EA at PC6 may be a potential and effective treatment for MI by activation of A1AR and A2bAR.

Targeting of G-protein coupled receptors in sepsis

The Third International Consensus Definitions (Sepsis-3) define sepsis as life-threatening multi-organ dysfunction caused by a dysregulated host response to infection. Sepsis can progress to septic shock-an even more lethal condition associated with profound circulatory, cellular and metabolic abnormalities. Septic shock remains a leading cause of death in intensive care units and carries a mortality of almost 25%. Despite significant advances in our understanding of the pathobiology of sepsis, therapeutic interventions have not translated into tangible differences in the overall outcome for patients. Clinical trials of antagonists of various pro-inflammatory mediators in sepsis have been largely unsuccessful in the past. Given the diverse physiologic roles played by G-protein coupled receptors (GPCR), modulation of GPCR signaling for the treatment of sepsis has also been explored. Traditional pharmacologic approaches have mainly focused on ligands targeting the extracellular domains of GPCR. However, novel techniques aimed at modulating GPCR intracellularly through aptamers, pepducins and intrabodies have opened a fresh avenue of therapeutic possibilities. In this review, we summarize the diverse roles played by various subfamilies of GPCR in the pathogenesis of sepsis and identify potential targets for pharmacotherapy through these novel approaches.

Expression of CD73 slows down migration of skin dendritic cells, affecting the sensitization phase of contact hypersensitivity reactions in mice

BACKGROUND

Application of haptens to the skin induces release of immune stimulatory ATP into the extracellular space. This "danger" signal can be converted to immunosuppressive adenosine (ADO) by the action of the ectonucleotidases CD39 and CD73, expressed by skin and immune cells. Thus, the expression and regulation of CD73 by skin derived cells may have crucial influence on the outcome of contact hypersensitivity (CHS) reactions.

OBJECTIVE

To investigate the role of CD73 expression during 2,4,6-trinitrochlorobenzene (TNCB) induced CHS reactions.

METHODS

Wild type (wt) and CD73 deficient mice were subjected to TNCB induced CHS. In the different mouse strains the resulting ear swelling reaction was recorded along with a detailed phenotypic analysis of the skin migrating subsets of dendritic cells (DC).

RESULTS

In CD73 deficient animals the motility of DC was higher as compared to wt animals and in particular after sensitization we found increased migration of Langerin⁺ DC from skin to draining lymph nodes (LN). In the TNCB model this led to a stronger sensitization as indicated by increased frequency of interferon-γ producing T cells in the LN and an increased ear thickness after challenge.

CONCLUSION

CD73 derived ADO production slows down migration of Langerin⁺ DC from skin to LN. This may be a crucial mechanism to avoid over boarding immune reactions against haptens.

Purinergic signalling links mechanical breath profile and alveolar mechanics with the pro-inflammatory innate immune response causing ventilation-induced lung injury

Severe pulmonary infection or vigorous cyclic deformation of the alveolar epithelial type I (AT I) cells by mechanical ventilation leads to massive extracellular ATP release. High levels of extracellular ATP saturate the ATP hydrolysis enzymes CD39 and CD73 resulting in persistent high ATP levels despite the conversion to adenosine. Above a certain level, extracellular ATP molecules act as danger-associated molecular patterns (DAMPs) and activate the pro-inflammatory response of the innate immunity through purinergic receptors on the surface of the immune cells. This results in lung tissue inflammation, capillary leakage, interstitial and alveolar oedema and lung injury reducing the production of surfactant by the damaged AT II cells and deactivating the surfactant function by the concomitant extravasated serum proteins through capillary leakage followed by a substantial increase in alveolar surface tension and alveolar collapse. The resulting inhomogeneous ventilation of the lungs is an important mechanism in the development of ventilation-induced lung injury. The high levels of extracellular ATP and the upregulation of ecto-enzymes and soluble enzymes that hydrolyse ATP to adenosine (CD39 and CD73) increase the extracellular adenosine levels that inhibit the innate and adaptive immune responses rendering the host susceptible to infection by invading microorganisms. Moreover, high levels of extracellular adenosine increase the expression, the production and the activation of pro-fibrotic proteins (such as TGF-β, α-SMA, etc.) followed by the establishment of lung fibrosis.

Adenosine production: a common path for mesenchymal stem-cell and regulatory T-cell-mediated immunosuppression

Adenosine is an important molecule that exerts control on the immune system, by signaling through receptors lying on the surface of immune cells. This nucleotide is produced, in part, by the action of the ectoenzymes CD39 and CD73. Interestingly, these proteins are expressed on the cell surface of regulatory T-cells (Tregs) and mesenchymal stromal cells (MSCs)-two cell populations that have emerged as potential therapeutic tools in the field of cell therapy. In fact, the production of adenosine constitutes a mechanism used by both cell types to control the immune response. Recently, great scientific progress was obtained regarding the role of adenosine in the inflammatory environment. In this context, the present review focuses on the advances related to the impact of adenosine production over the immune modulatory activity of Tregs and MSCs, and how this nucleotide controls the biological functions of these cells. Finally, we mention the main challenges and hurdles to bring such molecule to clinical settings.

Regulatory T cell-derived adenosine induces dendritic cell migration through the Epac-Rap1 pathway

Dendritic cells (DC) are one target for immune suppression by regulatory T cells (Treg), because their interaction results in reduced T cell stimulatory capacity and secretion of inhibitory cytokines in DC. We show that DC in the presence of Treg are more mobile as compared with cocultures with conventional CD4(+) T cells and form DC-Treg aggregates within 2 h of culture. The migration of DC was specifically directed toward Treg, as Treg, but not CD4(+) T cells, attracted DC in Boyden chambers. Treg deficient for the ectonucleotidase CD39 were unable to attract DC. Likewise, addition of antagonists for A2A adenosine receptors abolished the formation of DC-Treg clusters, indicating a role for adenosine in guiding DC-Treg interactions. Analysis of the signal transduction events in DC after contact to Treg revealed increased levels of cAMP, followed by activation of Epac1 and the GTPase Rap1. Subsequently activated Rap1 localized to the subcortical actin cytoskeleton in DC, providing a means by which directed locomotion of DC toward Treg is facilitated. In aggregate, these data show that Treg degrade ATP to adenosine via CD39, attracting DC by activating Epac1-Rap1-dependent pathways. As a consequence, DC-Treg clusters are formed and DC are rendered less stimulatory. This adenosine-mediated attraction of DC may therefore act as one mechanism by which Treg regulate the induction of immune responses by DC.

Overview of pleiotropic effects of platelet P2Y12 receptor inhibitors

Dual antiplatelet therapy consisting of one of the P2Y12 receptor inhibitors in conjunction with aspirin is the mainstay of treatment for patients with acute coronary syndromes (ACS) and those undergoing percutaneous coronary interventions (PCI). In recent years, multiple extra-platelet features of P2Y12 receptor antagonists have been reported in numerous clinical trials. The aim of this review is to summarise reported pleiotropic effects of clopidogrel, prasugrel, ticagrelor and other P2Y12 receptor blockers. We included observations made both in human and in animal models, together with proposed mechanisms of action for described features. If confirmed in randomised studies and properly applied to everyday practice, the observed extra-platelet actions could enable us to improve efficacy of ACS and post-PCI treatment, as well as to confine mortality and occurrence rate of cardiovascular events.

A2A adenosine receptor signaling in lymphocytes and the central nervous system regulates inflammation during experimental autoimmune encephalomyelitis

Extracellular adenosine has an important role in regulating the severity of inflammation during an immune response. Although there are four adenosine receptor (AR) subtypes, the A2AAR is both highly expressed on lymphocytes and known as a prime mediator of adenosine's anti-inflammatory effects. To define the importance of A2AAR signaling during neuroinflammatory disease progression, we used the experimental autoimmune encephalomyelitis (EAE) animal model for multiple sclerosis. In EAE induction experiments, A2AAR antagonist treatment protected mice from disease development and its associated CNS lymphocyte infiltration. However, A2AAR(-/-) mice developed a more severe acute EAE phenotype characterized by more proinflammatory lymphocytes and activated microglia/macrophages. Interestingly, very high levels of A2AAR were expressed on the choroid plexus, a well-established CNS lymphocyte entry point. To determine the contribution of A2AAR signaling in lymphocytes and the CNS during EAE, we used bone marrow chimeric mice. Remarkably, A2AAR(-/-) donor hematopoietic cells potentiated severe EAE, whereas lack of A2AAR expression on nonhematopoietic cells protected against disease development. Although no defect in the suppressive ability of A2AAR(-/-) regulatory T cells was observed, A2AAR(-/-) lymphocytes were shown to proliferate more and produced more IFN-γ following stimulation. Despite this more proinflammatory phenotype, A2AAR antagonist treatment still protected against EAE when A2AAR(-/-) lymphocytes were adoptively transferred to T cell-deficient A2AAR(+/+) mice. These results indicate that A2AAR expression on nonimmune cells (likely in the CNS) is required for efficient EAE development, while A2AAR lymphocyte expression is essential for limiting the severity of the inflammatory response.

Adenosine release: a potential explanation for the benefits of ticagrelor in the PLATelet inhibition and clinical outcomes trial?

OBJECTIVE

the objective of the study was to hypothesize on the potential mechanism explaining the surprising mortality benefit of ticagrelor in the PLATO trial.

BACKGROUND

in PLATO, the mortality reduction (107 deaths) numerically exceeds the myocardial infarction prevention benefit (89 events), making it a hitherto unmatchable achievement of ticagrelor over active comparator. If confirmed, such an impressive mortality advantage will be critical for the further success of ticagrelor to compensate for its otherwise unfavorable safety profile. In fact, such an impressive survival represents an entirely unexpected benefit, which will serve as a key point in the drug approval process and subsequent use in clinical practice.

METHODS

The potential association of ticagrelor as a promoter of blood adenosine serving as adenosine agonist is assessed.

RESULTS

multiple properties of adenosine, which can be closely matched with both clinical benefits and adverse events after ticagrelor, suggest that this novel pyrimidine is not a pure antiplatelet agent. Unquestionably, ticagrelor potently inhibits platelet activity via established mechanism of P2Y12 receptor blockade, probably chronically increasing blood adenosine levels and ultimately contributing to the vascular outcome benefit observed in PLATO.

CONCLUSIONS

future randomized trials of ticagrelor in acute heart failure, sudden death prevention, and treatment of atrial fibrillation are warranted and will expand our understanding of the potential role of adenosine in the outcome benefit after pyrimidines.

Hypoxic tumors and their effect on immune cells and cancer therapy

The abnormal decrease or the lack of oxygen supply to cells and tissues is called hypoxia. This condition is commonly seen in various diseases such as rheumatoid arthritis and atherosclerosis, also in solid cancers. Pre-clinical and clinical studies have shown that hypoxic cancers are extremely aggressive, resistant to standard therapies (chemotherapy and radiotherapy), and thus very difficult to eradicate. Hypoxia affects both the tumor and the immune cells via various pathways. This review summarizes the most common effects of hypoxia on immune cells that play a key role in the anti-tumor response, the limitation of current therapies, and the potential solutions that were developed for hypoxic malignancies.

TLR9 activation increases TAP-independent vesicular MHC class I processing in vivo

Cross-presentation of soluble protein antigens on major histocompatibility complex (MHC) class I by dendritic cells (DC) can occur in vesicular, endolysosomal compartments and be either dependent or independent of TAP peptide transporters. Here we investigate if an immunostimulatory CpG oligodeoxynucleotide can increase the activity in a TAP-independent endolysosomal vesicular pathway (el-VP) in vivo as we have earlier found in in vitro cultured DC. We use the in vivo response of CFSE labelled OT-1 T cells, transgenic for a T-cell receptor (TCR) that recognizes an ovalbumin (OVA)-derived peptide (SIINFEKL) presented by H-2K(b), transferred into TAP1(-/-) mice, as a functional read-out for activity in the el-VP. We have found a poor OT-1 T-cell response to soluble OVA which, however, could be strongly enhanced by the simultaneous administration of CpG. This increased responsiveness required both the endolysosomal cathepsin S (CatS) and Toll like receptor (TLR)9, the CpG receptor, both of which are present in the el-VP. Confocal microscopy demonstrated a co-localization of H-2K(b)/SIINFEKL and the endolysosomal marker LAMP1 in CD11c positive DC which was markedly increased by CpG administration. No complexes were found in the ER and cis-Golgi compartments in TAP1(-/-) mice, indicating the lack of classical MHC-I processing. In DC isolated from CatS(-/-) mice the opposite was found, complexes were present in the ER but not in the el-VP. We conclude that in vivo activation of TLR9 by CpG increases the efficiency of TAP independent el-VP and that this might contribute to the potent adjuvant activity of this type of compound. The cellular mechanisms remain to be established.

G protein-coupled receptor connectivity to NF-kappaB in inflammation and cancer

Complex intracellular network interactions regulate gene expression and cellular behavior. Whether at the site of inflammation or within a tumor, individual cells are exposed to a plethora of signals. The transcription factor nuclear factor-kappaB (NF-kappaB) regulates genes that control key cellular activities involved in inflammatory diseases and cancer. NF-kappaB is regulated by several distinct signaling pathways that may be activated individually or simultaneously. Multiple ligands and heterologous cell-cell interactions have an impact on NF-kappaB activity. The G protein-coupled receptor (GPCR) superfamily makes up the largest class of transmembrane receptors in the human genome and has multiple molecularly distinct natural ligands. GPCRs regulate proliferation, differentiation, and chemotaxis and play a major role in inflammatory diseases and cancer. Both GPCRs and NF-kappaB have been, and continue to be, major targets for drug discovery. A clear understanding of network interactions between GPCR signaling pathways and those that control NF-kB may be valuable for the development of better drugs and drug combinations.

Adenosine receptors and inflammation

Extracellular adenosine is produced in a coordinated manner from cells following cellular challenge or tissue injury. Once produced, it serves as an autocrine- and paracrine-signaling molecule through its interactions with seven-membrane-spanning G-protein-coupled adenosine receptors. These signaling pathways have widespread physiological and pathophysiological functions. Immune cells express adenosine receptors and respond to adenosine or adenosine agonists in diverse manners. Extensive in vitro and in vivo studies have identified potent anti-inflammatory functions for all of the adenosine receptors on many different inflammatory cells and in various inflammatory disease processes. In addition, specific proinflammatory functions have also been ascribed to adenosine receptor activation. The potent effects of adenosine signaling on the regulation of inflammation suggest that targeting specific adenosine receptor activation or inactivation using selective agonists and antagonists could have important therapeutic implications in numerous diseases. This review is designed to summarize the current status of adenosine receptor signaling in various inflammatory cells and in models of inflammation, with an emphasis on the advancement of adenosine-based therapeutics to treat inflammatory disorders.

Brefeldin A inhibits vesicular MHC class I processing in resting but not in CpG- and disruption-activated DC

Dendritic cells (DC) can process exogenous proteins for presentation on MHC class I (MHC-I) in a vesicular pathway (VP) that is distinct from the classical, cytosolic MHC-I pathway. Here we investigate the sensitivity of this VP to Brefeldin A (BFA), a fungal metabolite which inhibits vesicular trafficking by preventing the activation of some ADP-ribosylation factor (ARF) proteins, in both resting and differently activated DC. The VP could be directly visualized in DC by the presence of OVA-derived H-2K(b)/SIINFEKL complexes in a LAMP1 positive, but EEA negative, compartment in wt but not in cathepsin S(-/-) mice as these are unable to process OVA into the SIINFEKL peptide in endolysosomes. BFA, which binds to specific ARF-GDP-sec7 sites, both in the Golgi and in endolysosomes, was found to bind to and inhibit the VP in resting DC. If the VP was selectively activated with an immunostimulatory CpG ODN, binding to endolysosomal TLR9 receptors, or by the mere mechanical disruption of clustered DC cells, BFA no longer had this effect. The activation of the VP with both CpG and cellular disruption was found to be dependent on the MyD88 adaptor protein. We conclude that vesicular MHC-I processing in DC occurs in ARF-regulated LAMP1 positive vesicles which, as a consequence of cellular activation, no longer can bind BFA and thus become resistant to the inhibitory effect of this drug.