Shaping immune responses through the activation of dendritic cells' P2 receptors

Progress in cancer research on the regulator of phagocytosis CD47, which determines the fate of tumor cells (Review)

Cluster of differentiation 47 (CD47) is a transmembrane protein that is widely and moderately expressed on the surface of various cells and can have an essential role in mediating cell proliferation, migration, phagocytosis, apoptosis, immune homeostasis and other related responses by binding to its ligands, integrins, thrombospondin-1 and signal regulatory protein α. The poor prognosis of cancer patients is closely associated with high expression of CD47 in glioblastoma, ovarian cancer, breast cancer, bladder cancer, colon cancer and hepatocellular carcinoma. Upregulation of CD47 expression facilitates the growth of numerous types of tumor cells, while downregulation of its expression promotes phagocytosis of tumor cells by macrophages, thereby limiting tumor growth. In addition, blocking CD47 activates the cyclic GMP-AMP (cGAMP) synthase/cGAMP/interferon gene stimulating factor signaling pathway and initiates an adaptive immune response that kills tumor cells. The present review describes the structure, function and interactions of CD47 with its ligands, as well as its regulation of phagocytosis and tumor cell fate. It summarizes the therapeutics, mechanisms of action, research advances and challenges of targeting CD47. In addition, this paper provides an overview of the latest therapeutic options for targeting CD47, such as chimeric antigen receptor (CAR) T-cells, CAR macrophages and nanotechnology-based delivery systems, which are essential for future clinical research on targeting CD47.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

CD47 as a promising therapeutic target in oncology

CD47 is ubiquitously expressed on the surface of cells and plays a critical role in self-recognition. By interacting with SIRPα, TSP-1 and integrins, CD47 modulates cellular phagocytosis by macrophages, determines life span of individual erythrocytes, regulates activation of immune cells, and manipulates synaptic pruning during neuronal development. As such, CD47 has recently be regarded as one of novel innate checkpoint receptor targets for cancer immunotherapy. In this review, we will discuss increasing awareness about the diverse functions of CD47 and its role in immune system homeostasis. Then, we will discuss its potential therapeutic roles against cancer and outlines, the possible future research directions of CD47- based therapeutics against cancer.

Mechanisms of innate events during skin reaction following intradermal injection of seasonal influenza vaccine

The skin plays a crucial role in host defences against microbial attack and the innate cells must provide the immune system with sufficient information to organize these defences. This unique feature makes the skin a promising site for vaccine administration. Although cellular innate immune events during vaccination have been widely studied, initial events remain poorly understood. Our aim is to determine molecular biomarkers of skin innate reaction after intradermal (i.d.) immunization. Using an ex vivo human explant model from healthy donors, we investigated by NanoLC-MS/MS analysis and MALDI-MSI imaging, to detect innate molecular events (lipids, metabolites, proteins) few hours after i.d. administration of seasonal trivalent influenza vaccine (TIV). This multimodel approach allowed to identify early molecules differentially expressed in dermal and epidermal layers at 4 and 18 h after TIV immunization compared with control PBS. In the dermis, the most relevant network of proteins upregulated were related to cell-to-cell signalling and cell trafficking. The molecular signatures detected were associated with chemokines such as CXCL8, a chemoattractant of neutrophils. In the epidermis, the most relevant networks were associated with activation of antigen-presenting cells and related to CXCL10. Our study proposes a novel step-forward approach to identify biomarkers of skin innate reaction. SIGNIFICANCE: To our knowledge, there is no study analyzing innate molecular reaction to vaccines at the site of skin immunization. What is known on skin reaction is based on macroscopic (erythema, redness…), microscopic (epidermal and dermal tissues) and cellular events (inflammatory cell infiltrate). Therefore, we propose a multimodal approach to analyze molecular events at the site of vaccine injection on skin tissue. We identified early molecular networks involved biological functions such cell migration, cell-to-cell interaction and antigen presentation, validated by chemokine expression, in the epidermis and dermis, then could be used as early indicator of success in immunization.

CD47: role in the immune system and application to cancer therapy

BACKGROUND

CD47 is a widely expressed cellular receptor well known for its immunoregulatory functions. By interacting with its ligands, including thrombospondin-1 (TSP-1), signal regulatory protein α (SIRPα), integrins, and SH2-domain bearing protein tyrosine phosphatase substrate-1 (SHPS-1), it modulates cellular phagocytosis by macrophages, transmigration of neutrophils and activation of dendritic cells, T cells and B cells. Ample studies have shown that various types of cancer express high levels of CD47 to escape from the immune system. Based on this observation, CD47 is currently considered as a prominent target in cancer therapy.

CONCLUSIONS

Here, we review the role of CD47 in the maintenance of immune system homeostasis. We also depict three emerging CD47-targeting strategies for cancer therapy, including the use of mimicry peptides, antibodies, and gene silencing strategies. Among these approaches, the most advanced one is the use of anti-CD47 antibodies, which enhances cancer cell phagocytosis via inhibition of the CD47-SIRPα axis. These antibodies can also achieve higher anti-cancer efficacies when combined with chemotherapy and immunotherapy and hold promise for improving the survival of patients with cancer.

P2X4 receptors, immunity, and sepsis

Sepsis is life-threatening systemic organ dysfunction caused by a deregulated host response to an infectious insult. Currently, the treatment of sepsis is limited to the use of antibiotics, fluids, and cardiovascular/respiratory support. Despite these interventions, septic mortality remains high, with reduced life quality in survivors. For this reason, the identification of novel drug targets is a pressing task of modern pharmacology. According to a recent research, it appears that P2 purinergic receptors, which can regulate the host's response to infections, have been identified as potential targets for the treatment of sepsis. Among P2 receptors, the P2X₄ receptor has recently captured the attention of the research community owing to its role in protecting against infections, inflammation, and organ injury. The present review provides an outline of the role played by P2X₄ receptors in the modulation of the host's response to sepsis and the promise that targeting this receptor holds in the treatment of sepsis.

ATP and Its Metabolite Adenosine as Regulators of Dendritic Cell Activity

Adenosine (Ado) is a well-studied neurotransmitter, but it also exerts profound immune regulatory functions. Ado can (i) actively be released by various cells into the tissue environment and can (ii) be produced through the degradation of extracellular ATP by the concerted action of CD39 and CD73. In this sequence of events, the ectoenzyme CD39 degrades ATP into ADP and AMP, respectively, and CD73 catalyzes the last step leading to the production of Ado. Extracellular ATP acts as a “danger” signal and stimulates immune responses, i.e. by inflammasome activation. Its degradation product Ado on the other hand acts rather anti-inflammatory, as it down regulates functions of dendritic cells (DCs) and dampens T cell activation and cytokine secretion. Thus, the balance of proinflammatory ATP and anti-inflammatory Ado that is regulated by CD39⁺/CD73⁺ immune cells, is important for decision making on whether tolerance or immunity ensues. DCs express both ectoenzymes, enabling them to produce Ado from extracellular ATP by activity of CD73 and CD39 and thus allow dampening of the proinflammatory activity of adjacent leukocytes in the tissue. On the other hand, as most DCs express at least one out of four so far known Ado receptors (AdoR), DC derived Ado can also act back onto the DCs in an autocrine manner. This leads to suppression of DC functions that are normally involved in stimulating immune responses. Moreover, ATP and Ado production thereof acts as “find me” signal that guides cellular interactions of leukocytes during immune responses. In this review we will state the means by which Ado producing DCs are able to suppress immune responses and how extracellular Ado conditions DCs for their tolerizing properties.

Expression and role of gap junction protein connexin43 in immune challenge-induced extracellular ATP release in Japanese flounder (Paralichthys olivaceus)

Connexin43 (Cx43) is the best characterized gap junction protein that allows the direct exchange of signaling molecules during cell-to-cell communications. The immunological functions and ATP permeable properties of Cx43 have been insensitively examined in mammals. The similar biological significance of Cx43 in lower vertebrates, however, is not yet understood. In the present study we identified and characterized a Cx43 ortholog (termed PoCx43) from Japanese flounder (Paralichthys olivaceus) and investigated its role in immune challenge-induced extracellular ATP release. PoCx43 mRNA transcripts are widely distributed in all tested normal tissues and cells with predominant expression in the brain, and are significantly up-regulated by LPS, poly(I:C) and zymosan challenges and Edwardsiella tarda infections as well, suggesting that PoCx43 expression was modulated by the inflammatory stresses. In addition, cyclic AMP (cAMP), an essential second messenger, also plays an important role in regulating PoCx43 gene expression, by which the PoCx43-mediated gap junctional communication may be regulated. Furthermore, overexpression of PoCx43 in Japanese flounder FG-9307 cells significantly potentiates the LPS- and poly(I:C)-induced extracellular ATP release and this enhanced ATP release was attenuated by pre-incubation with Cx43 inhibitor carbenoxolone. In a complementary experiment, down-regulation of PoCx43 endogenous expression in FG-9307 cells with small interfering RNA also significantly reduced the PAMP-induced extracellular ATP release, suggesting that PoCx43 is an important ATP release conduit under the immune challenge conditions. Finally, we showed that extracellular ATP stimulation led to an increased PoCx43 expression which probably provides a feedback mechanism in regulating PoCx43 expression at the transcriptional level. These findings suggest that PoCx43 is an inducible immune response gene and an important conduit for immune challenge-induced extracellular ATP release in fish.

Identification and characterization of ATP-gated P2X2 receptor gene dominantly expressed in the Japanese flounder (Paralichthys olivaceus) head kidney macrophages

P2X2 receptor (P2X2R) belongs to the family of purinergic receptors that have been shown to play important roles in regulating host innate immune response. Although the immunologic significance of P2X2R has been studied in mammals, the presence and immune relevance of P2X2R in fish remains unclear. In this study we extended our previous observations by identifying and characterizing a P2X2R ortholog (termed PoP2X2R) from Japanese flounder (Paralichthys olivaceus). Quantitative real-time PCR analysis revealed that PoP2X2R mRNA transcripts are widely distributed in all examined normal tissues and are dominantly expressed in hepatopancreas tissue. In addition, we for the first time showed that multiple P2XR subtypes, including P2X2R, P2X4R and P2X7R are co-expressed in the Japanese flounder head kidney macrophages (HKMs) and peripheral blood lymphocytes (PBLs), indicating that they may assemble into hetero-receptor complex or interact in the form of homotrimers to trigger diverse purinergic signaling in the Japanese flounder immune cells. Compared with the known Japanese flounder P2X4 and P2X7 receptors, however, PoP2X2R is much more abundantly expressed in the Japanese flounder HKM cells, suggesting that PoP2X2R may play an important role in this type of immune cells. Glycosylation and immunohistochemistry analyses revealed that PoP2X2R is a glycoprotein expressed on the plasma membrane. Immune challenges experiments showed that PoP2X2R was significantly induced by LPS, poly(I:C) and zymosan stimulations in the HKM and PBL cells, and by Edwardsiella tarda infections in spleen and gill tissues as well. Taken together, we have identified and characterized a new P2X2R member that is involved in fish innate immune response.

Purinergic Signaling and the Immune Response in Sepsis: A Review

PURPOSE

Sepsis remains an unresolved clinical problem with high in-hospital mortality. Despite intensive research over decades, no treatments for sepsis have become available. Here we explore the role of ATP in the pathophysiology of sepsis. ATP is not only a universal energy carrier but it also acts as an extracellular signaling molecule that regulates immune function. ATP stimulates a large family of purinergic receptors found on the cell surface of virtually all mammalian cells. In severe sepsis and septic shock, ATP is released in large amounts into the extracellular space where it acts as a "danger" signal. In this review, we focus on the roles of ATP as a key regulator of immune cell function and as a disruptive signal that contributes to immune dysfunction in sepsis.

METHODS

We summarized the current understanding of the pathophysiology of sepsis, with special emphasis on the emerging role of systemic ATP as a disruptive force that promotes morbidity and mortality in sepsis.

FINDINGS

Over the past two decades, the discovery that regulated ATP release and purinergic signaling represent a novel regulatory mechanism in immune cell physiology has opened up new possibilities in the treatment of sepsis. Immune cells respond to stimulation with the release of cellular ATP, which regulates cell functions in autocrine and paracrine fashions. In sepsis, large amounts of systemic ATP produced by tissue damage and inflammation disrupt these regulatory purinergic signaling mechanisms, leading to immune dysfunction that promotes the pathophysiologic processes involved in sepsis.

IMPLICATIONS

The knowledge of these ATP-dependent signaling processes is likely to reveal exciting new avenues in the treatment of the unresolved clinical problem of sepsis.

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.

Regulation of innate immunity by extracellular nucleotides

Extracellular ATP (eATP) is the most abundant among extracellular nucleotides and is commonly considered as a classical danger signal, which stimulates immune responses in the presence of tissue injury. In fact, increased nucleotide concentration in the extracellular space is generally closely associated with tissue stress or damage. However non-lytic nucleotide release may also occur in many cell types under a variety of conditions. Extracellular nucleotides are sensed by a class of plasma membrane receptors called P2 purinergic receptors (P2Rs). P2 receptors are expressed by all immunological cells and their activation elicits different responses. Extracellular ATP can act as an initiator or terminator of immune responses being able to induce different effects on immune cells depending on the pattern of P2 receptors engaged, the duration of the stimulus and its concentration in the extracellular milieu. Millimolar (high) concentrations of extracellular ATP, induce predominantly proinflammatory effects, while micromolar (low) doses exert mainly tolerogenic/immunosuppressive action. Moreover small, but significant differences in the pattern of P2 receptor expression in mice and humans confer diverse capacities of ATP in regulating the immune response.

Immunoregulation through extracellular nucleotides

Extracellular ATP (eATP), the most abundant among nucleotides, can act as a mediator during inflammatory responses by binding to plasmamembrane P2 purinergic receptors, which are widely expressed on cells of the immune system. eATP is generally considered as a classical danger signal, which stimulates immune responses in the presence of tissue damage. Converging evidence from several studies using murine models of chronic inflammation have supported this hypothesis; however, the role of eATP in the regulation of human immune function appears to be more complex. Chronic stimulation with micromolar eATP concentrations inhibits the proliferation of T and NK lymphocytes and enhances the capacity of dendritic cells to promote tolerance. The effect of eATP depends on multiple factors, such as the extent of stimulation, eATP concentration, presence/absence of other mediators in the microenvironment, and pattern of P2 receptor engagement. Small but significant differences in the pattern of P2 receptor expression in mice and humans confer the diverse capacities of ATP in regulating the immune response. Such diversity, which is often overlooked, should therefore be carefully considered when evaluating the role of eATP in human inflammatory and autoimmune diseases.

Murine epidermal Langerhans cells and keratinocytes express functional P2X7 receptors

Extracellular ATP via the activation of purinergic P2 receptors has an emerging role in cutaneous biology; however, the distribution of these receptors in mouse skin is poorly defined. This study investigated whether murine epidermal cell subpopulations express functional purinergic P2X(7) receptors. P2X(7) expression was examined by immunoblotting and immunofluorescence staining of epidermal cells from C57Bl/6 mice. P2X(7) function was evaluated by nucleotide-induced ethidium(+) uptake measurements in epidermal cells from C57Bl/6 mice, and from P2X(7) deficient mice and wild-type littermate controls. P2X(7) was detected in whole epidermal cell preparations, and specifically on Langerhans cells (LCs) and keratinocytes (KCs). ATP induced ethidium(+) uptake into LCs and KCs, with EC(50) values of 503 and 482 microm, respectively. BzATP, and to a lesser extent ATPgammaS and ADP, also induced ethidium(+) uptake; while UTP, alphabeta-meth-ATP and NAD were ineffective. ATP-induced ethidium(+) uptake was impaired by Na(+) and Mg(2+), and the P2X(7) antagonist, A-438079 and was absent in LCs and KCs from P2X(7) deficient mice. These results demonstrate that murine LCs and KCs express functional P2X(7), and support a role for this receptor in cutaneous biology.