J Neuroimmunol. Sep;() Cannabinoids and the immune system: potential for the treatment of inflammatory diseases? Croxford JL(1). Allergic asthma is a complex inflammatory that cannabinoids, such as THC, facilitate a Th1. The treatment of cannabis use disorders has recently been reviewed. . Its concentrations are significantly increased in three different inflammatory and.
& other inflammatory diseases Cannabinoids
This review discusses the endocannabinoid system and TLR family and evaluates the interaction between them with emphasis on the innate immune system. Cannabinoid receptors encompass multiple subtypes reviewed in [ 24 — 26 ]. Endogenous cannabinoids also bind Transient Receptor Potential Vanilloid 1 receptor, a capsaicin receptor, which is structurally different from CB1 and CB2 receptors [ 24 — 26 ]. The orphan receptor GPR55 may be another receptor subtype, although it has low homology to the other cannabinoid receptors [ 24 — 26 ].
Other candidate receptors have been implicated by pharmacological and functional studies [ 24 — 26 ]. CB1 and CB2 receptors greatly differ in their tissue distribution. CB1 receptor, originally identified in rat cerebral cortex, is primarily expressed in the central nervous system [ 27 , 28 ]. This receptor subtype is also expressed in various peripheral tissues, such as testis, vascular endothelium, and small intestine [ 27 , 28 ].
Its expression is heterogeneous within the nervous system and is mainly responsible for cannabinoid psychoactive properties. In contrast, CB2 receptor was originally identified in the promyelocytic leukemic cell line HL60 and is prevalent within the immune system [ 29 , 30 ]. All lineages of immune cells express the CB2 receptor, although its expression level varies among the cell types.
In rank order, B cells express the highest level followed by natural killer cells, macrophages, monocytes, polymorphonuclear cells, and T cells [ 31 ]. CB2 receptor expression in healthy brains is limited to a few neurons in the brain stem [ 32 , 33 ].
Some immune cells, including monocytes, also express the CB1 receptor [ 35 — 37 ]. When both receptor subtypes are present in immune cells, the CB2 receptor is usually expressed at a significantly higher level than the CB1 receptor [ 35 — 37 ]. Unlike CB1 receptor-mediated cell activation, signal transduction through the CB2 receptor lacks psychotropic effects [ 38 , 39 ] making it an attractive target for immunotherapy.
Cannabis is a complex mixture of over cannabinoids along with other classes of compounds that have pharmacological and biological activities. Numerous synthetic analogues have been produced based on structure-activity relationship studies [ 24 , 25 ].
Many synthetic cannabinoid analogues have biological effects similar to their natural counterparts [ 40 , 41 ]. Cannabimimetic compounds are lipophilic molecules and are classified into four main groups based on their chemical structure [ 24 , 25 ].
The classical group consists of dibenzopyran derivatives, and the phytocannabinoids fall into this group [ 24 , 25 ]. Another notable member is cannabidiol that is a nonpsychoactive ingredient of Sativex. Cannabidiol does not activate cannabinoid receptors and yet has biological activity, including immune suppression.
Instead, cannabidiol appears to behave as a potent antagonist or inverse agonist [ 24 , 25 ]. Other members are synthetic analogues of phytocannabinoids, and some synthetic compounds are selective receptor agonists with higher affinity for one or other cannabinoid receptor subtype.
These synthetic compounds lack a pyran ring. Several are selective cannabinoid receptor agonists. Cannabinoids belonging to the aminoalkylindole group are very structurally different from classical and nonclassical compounds [ 24 , 25 ].
They were initially developed as potential analogues of nonsteroidal anti-inflammatory drugs [ 42 ]. Eicosanoids are the endogenous compounds and are oxidized derivatives of carbon fatty acids [ 24 , 25 ]. Arachidonoyl-ethanolamide also called anandamide was the first one isolated from porcine brain and was found to have cannabimimetic activity [ 43 ]. Anandamide is an arachidonic acid derivative that is highly sensitive to oxidation and hydrolysis.
It behaves as a partial agonist and has lower intrinsic activity for CB2 than for CB1 receptor. Another prominent member is 2-arachionoyl glycerol, a monoglyceride, which is more potent than anandamide. Initially, 2-arachionoyl glycerol was isolated from intestine and brain [ 44 , 45 ], and its concentration in the brain is approximately fold higher than that of anandamide [ 46 ].
Because 2-arachionoyl glycerol favors the CB2 receptor, it is viewed as the main endocannabinoid to modulate immune functions. Several synthetic analogues of anandamide have been produced to achieve higher potency and efficacy. Most cannabinoids exhibit stereoselectivity in pharmacological assays due to chiral centers in the molecules. Frequently, stereoselectivity is also observed in biological assays. Classical and nonclassical cannabinoids and aminoalkylindoles are far more active than their corresponding enantiomer or stereoisomer [ 24 , 25 ].
Stereoselectivity is one criterion for receptor-mediated actions. Cannabinoid receptors are seven-transmembrane-spanning G protein-coupled receptors [ 27 — 30 ]. The first evidence that cannabinoid receptors are protein-coupled receptors was cannabinoid-induced inhibition of adenylate cyclase leading to decreased intracellular cAMP level [ 47 ]. Pertussis toxin inhibition of a cannabinoid effect confirms protein-mediated signal transduction [ 48 ].
In turn, cAMP-dependent protein kinase A activity diminishes leading to less active transcription factor cAMP response element-binding protein affecting gene expression [ 40 , 41 ]. In contrast, stimulation of rat microglial cells with 2-arachionoyl glycerol causes MAPK activation that is dependent on protein kinase C, not PI-3K [ 51 ].
Rapamycin, a mTOR inhibitor, blocks cannabinoid-induced neural progenitor cell proliferation and cannabinoid-enhanced oligodendrocyte differentiation [ 53 , 54 ]. Furthermore, stimulation of promyelocytic HL60 cells with 2-arachionoyl glycerol or other cannabinoid agonists does not activate Akt or mTOR [ 40 ].
While CB1 and CB2 receptors share many signaling steps, distinct differences have been identified in the signaling pathways of theses receptors. Most notably, CB1 receptor signaling may increase intracellular cAMP levels and cAMP-dependent protein kinase A activity due to the receptor associating with proteins [ 40 ].
Furthermore, the CB1 receptor may associate with proteins leading to phospholipase D activation [ 40 ]. While the CB1 receptor may couple to or proteins, the CB2 receptor does not [ 41 ].
Within the central nervous system, anandamide and 2-arachionoyl glycerol are not preformed molecules stored in vesicles but rather are synthetized when cells are stimulated, such as depolarization of neurons [ 56 , 57 ]. The critical precursors of 2-arachionoyl glycerol are sn acylarachidonoylgylerols, and multiple enzymatic pathways generate 2-arachionoyl precursors [ 63 ].
The main pathway involves phosphoinositide-selective phospholipase C or similar phospholipases followed by two sn selective diacylglycerol lipase isoenzymes reviewed in [ 62 — 65 ]. Unlike anandamide, 2-arachionoyl glycerol is an important precursor of other molecules [ 63 ], which may explain their differential resting levels. In most cases, enhanced anandamide and 2-arachionoyl glycerol biosynthesis is limited in time and location, and, thus, their release affects only cells in the nearby vicinity.
After release, endocannabinoids are rapidly internalized into cells by an undefined mechanism, and a proposed transporter has not been definitely identified. Intracellular anandamide is principally hydrolyzed by fatty acid amide hydrolase, an integral plasma membrane protein [ 66 ]. Similarly, internalized 2-arachionoyl glycerol is primarily hydrolyzed by monoacylglycerol lipase associated with the plasma membrane [ 67 ]. Reaction products do not activate cannabinoid receptors and may recycle back into their respective biosynthetic pathways [ 63 ].
One hydrolysis product, arachidonic acid, may be metabolized by various enzymes to produce prostaglandins, thromboxanes, leukotrienes, and other biologically active compounds, which are potent inflammatory mediators or immune suppressors. Anandamide and 2-arachionoyl glycerol themselves may be oxidized by cytochrome P, cyclooxygenase-2, and 5- and lipoxygenases [ 63 , 65 ].
Some lipoxygenase products bind both CB1 and CB2 receptors [ 25 , 56 , 57 ]. Hence, results from metabolic enzyme inhibitors or enzyme deficient mice should not presume to be caused by only increased endocannabinoid levels. A fundamental characteristic of the immune system is the ability to distinguish between self and non-self-molecules or antigens. Immune cells are particularly adept at detecting microbial antigens, and the subsequent immune response can clear an infection and provide protection against a future infection.
Innate immunity represents the first line of defense against infectious diseases, and cells, such as neutrophils and macrophages, are the first responders. Inflammation is the initial immune response against infectious agents, and the inflammatory response promotes initiation of an adaptive immune response by antigen-specific T and B cells.
However, cells of innate immunity do not express antigen-specific receptors, unlike T and B cells of adaptive immunity. Decades ago, Janeway proposed a hypothesis that innate immune cells utilize germline-encoded receptors that are antigen-selective [ 68 ], and such receptors were eventually identified many years later. Innate immune cells express pattern-recognition receptors that bind conserved molecular patterns, and engagement of these receptors transduces a signal allowing cells to sense danger in the form of a pathogen or host cellular damage [ 69 , 70 ].
These receptors are present at the cell surface, in endocytic organelles, or in the cytoplasm permitting perception of both extracellular and intracellular dangers. The major receptor gene families based on protein domain homology include TLRs, retinoid acid-inducible genelike receptors, absent in melanoma-2 receptors, C-type lectin receptors, intracellular DNA sensors, and nucleotide-binding domain, leucine-rich repeat-containing receptors or nucleotide-binding, oligomerization domain-like receptors that are discussed in several comprehensive reviews [ 70 — 76 ].
This review focuses on the TLR family, which has been extensively investigated and was the first one discovered. Toll gene was first identified in Drosophila as essential in regulating embryonic development of dorsal-ventral polarity [ 77 , 78 ].
In addition, Toll has a critical role in resistance to fungal infections in adult flies [ 79 ]. This latter discovery was key to understanding innate inflammatory triggers and led to finding the mammalian homologues [ 80 , 81 ].
Although the number of family members varies among mammalian species, TLRs are evolutionarily conserved type I transmembrane proteins [ 70 — 76 ]. TLRs are predominantly expressed by innate immune cells, especially dendritic cells and macrophages, while adaptive immune B and T cells, and nonimmune cells, including fibroblasts and epithelial cells, have limited TLR expression [ 70 — 76 ].
Each TLR recognizes a distinct set of molecular patterns [ 70 — 76 ]. Promiscuous ligand recognition, which is determined by the leucine-rich repeat extracellular domain, is indispensable for the handful of TLRs to mediate effective immune defenses against an array of diverse pathogens.
Regarding mammalian host resistance, TLRs bind pathogen-associated molecular patterns PAMPs present within microbial molecules, but absent from mammalian molecules [ 70 — 75 ]. The broad gamut of ligand specificity ranges from hydrophobic lipids to hydrophilic nucleic acids. TLRs are divided into two subfamilies based on their cellular location, and the following discussion will focus on the TLR chains expressed in humans.
Interestingly, ligand recognition correlates with the cellular location of TLRs [ 70 — 75 ]. In contrast, human TLR10 is an orphan receptor without a known ligand.
Receptor compartmentalization influences the types of ligands accessible for cell activation. For example, endosomal TLRs engage when virulent intracellular pathogens infect cells, but do not signal when the pathogen remains outside the cell.
Although all TLRs possess a similar TIR domain, cell surface and intracellular receptors, in general, utilize different adaptor molecules to transduce a signal [ 70 — 75 ]. The different pairs of adaptor molecules activate distinct transcription factors, and, hence, influence the nature and outcome of the inflammatory response.
For many years, one puzzling aspect of innate immunity has been inflammatory responses in the absence of an infection that contribute to tissue damage during autoimmune diseases and chronic inflammatory diseases.
Matzinger proposed that the immune system does not distinguish between self and non-self-antigens per se but rather is designed to detect danger [ 82 ]. The apparent paradox of sterile inflammation is resolved by realization that pattern-recognition receptors, including TLRs, recognize endogenous ligands.
The endogenous molecules called damage-associated molecular patterns DAMPs are created or released upon tissue injury or cell death [ 69 , 76 ]. Within months, necrotic cells were shown to induce proinflammatory gene expression and dendritic cell maturation via TLR2 [ 84 , 85 ]. Since then, several DAMPs have been identified, and some are intracellular molecules to which innate immune cells are not normally exposed.
These TLR2 heterodimers have different ligand specificities. When a ligand binds, the TIR domains oligomerize recruiting adaptor proteins that initiate downstream signaling events.
MD-2 is required for TLR4 dimerization. MD-2 complex concentrates within cholesterol-rich lipid rafts containing CD14, which is anchored to the plasma membrane by glycosylphosphatidylinositol.
Gioannini and Weiss estimate that LPS monomers extracted from one bacterium are sufficient to activate 1, macrophages [ 87 ]. Hence, sequential LPS transfer is thought to heighten sensitivity of innate immune cells, in particular macrophages.
These transcription factors induce nitric oxide and proinflammatory cytokine production. CD36 functions as a scavenger receptor in monocytes and macrophages and participates in phagocytosis and endocytosis [ 89 ]. Colocalization, time kinetics, and endocytosis inhibition studies indicate that TRAM engages after TLR4 internalization [ 70 , 72 — 75 , 86 ]. Hence, the MyDdependent and TRIF-dependent signaling pathways are sequestered from each other, and commence at distinct cellular locations.
Cell activation by TLR signals is tightly regulated on multiple levels. Critical regulatory mechanisms range from TLR trafficking and cleavage to protein modification of signaling molecules. Furthermore, negative regulators are important in preventing autoimmune and inflammatory diseases. On the flip side, abnormal TLR activation by PAMPs, mutations in TLR signaling molecules, and TLR activation by DAMPs are associated with the development and pathogenesis of numerous diseases, including autoimmune diseases, hypersensitivities, chronic inflammatory diseases, cancer, and cardiovascular diseases.
Hence, cannabinoid modulation of TLR signal transduction pathways may have beneficial or detrimental consequences on human health. Important innate immune cells expressing TLRs are monocytes, macrophages, microglial cells, and dendritic cells.
These myeloid cells are closely related. Monocytes circulate in the blood and mature into macrophages or dendritic cells depending on the stimulus. Microglial cells are resident macrophages in the brain.
All these cells express cannabinoid receptors, and cannabinoids influence their immune functions. This review discusses cannabinoid modulation of TLR ligand responses by innate immune cells, and vice versa. Cannabinoids display biphasic dose-response curves for cytokine secretion in some culture systems [ 96 , 97 ], which may account for this apparent discrepancy.
In addition, chronic marijuana use increases CB1 and CB2 receptor expression on peripheral blood monocytes [ 98 ], which may enhance cannabinoid sensitivity. Thus, exogenous cannabinoids may alter the endocannabinoid system leading to greater suppression of the LPS response. Cannabinoids directly impair TLR-induced cell activation in culture Table 1. These results indicate that cannabinoids directly interfere with TLR signal transduction. On the other hand, cannabinoids may also indirectly suppress in vitro cytokine production Table 1.
For example, a CB2 receptor-selective agonist prevents LPS-upregulated TLR4 expression on mouse bone marrow dendritic cells rendering the cells less LPS responsive [ ], although a similar cannabinoid effect upon TLR4 expression has not been reported for other innate immune cells.
Moreover, cannabinoids induce apoptosis that has been proposed as an immunosuppressive mechanism [ , ]. However, the majority of findings regarding cannabinoid inhibition of TLR-mediated responses cannot be attributed to apoptosis or cell toxicity. Lastly, cannabinoids may induce other immune suppressive processes. CP55, induces interleukin-1 receptor antagonist expression in LPS-stimulated microglial cells [ ], which interferes with interleukin-1 signals.
They were first described as psychotropic compounds; however, cannabinoids are also potent immunoregulatory agents. Cannabinoids can modulate neutrophil activity in sterile and infectious inflammatory diseases. Concerning sterile inflammatory diseases as arthritis, ischemic diseases, and colitis, the use of CB2 agonist impairs the intracellular signaling pathways involved in the production of inflammatory mediators and expression of adhesion molecules.
As a consequence, neutrophils did not release metalloproteinases either to adhere to endothelial cells, resulting in reduced tissue damage. A similar anti-inflammatory CB2 agonist mechanism of action in sepsis and mycobacterial infection models is observed. However, it is not clear if inflammation resolution promoted by cannabinoid treatment during infection is also related to microbial viability.
Despite the growing literature showing the effects of cannabinoids on neutrophils, there are still some gaps that should be filled before proposing cannabinoid-based drugs to treat neutrophil-dependent diseases. Neutrophils have been classically recognized as the most relevant cell during acute inflammatory responses and, more recently, in chronic inflammation [ 1 ].
On the one hand, neutrophils produce bactericidal molecules and coordinate the accumulation of pro-resolving cells. On the other hand, neutrophil over activation leads to tissue damage. In this context, several approaches have been proposed to regulate the accumulation and the activity of neutrophils in pathological conditions.
In parallel, the findings concerning the importance of the endocannabinoid system as the endogenous immunoregulatory mechanism raise questions on how it could be therapeutically used to treat inflammatory diseases. In this chapter, we discuss how the endocannabinoid system can be used to modulate the activity of neutrophils in sterile and infectious inflammatory diseases.
Cannabis sativa marijuana is one of the oldest plants that produced psychoactive effects on humans. In addition, it has been used in medicine in controlling pain, convulsion, inflammation, and asthma [ 2 , 3 ]. Cannabinoids are a group of lipophilic and pharmacologically active compounds present in C. Cannabinoid receptor agonists are responsible for several biological effects, such as analgesic, antiemetic, antitumor, and anti-inflammatory [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ], and are classified into three groups based on their origin: Chemical structure of some representatives of the cannabinoid agonists.
Endocannabinoids eCBs are eicosanoids derived from polyunsaturated chain fatty acids, such as arachidonic acid, and comprise amides, esters, and ether [ 14 ]. Anandamide AEA was the first endocannabinoid described in the mammalian brain and other tissues [ 15 ], followed by 2-arachidonoylglycerol 2-AG [ 16 , 17 ].
Beyond the well-known psychotropic effects, the endocannabinoid system plays an essential immunomodulatory role by modulating the release of cytokines and on acute or chronical diseases through two main ways, neuro- and immunomodulation [ 19 , 20 ]. The group of phytocannabinoids consists of active substances initially extracted from Cannabis sativa , whose pharmacological activity is associated with the terpene phenolic class.
Phytocannabinoids are classified into two groups: From THC, it was possible to synthesize several compounds that have similar chemical structures with different levels of affinity for cannabinoid receptors [ 5 , 28 ]. Synthetic cannabinoids have been used as a pharmacological tool for in vivo or in vitro studies to explore the therapeutic potential of the cannabinoid system. However, it has already been described that metabolites form dipyrone and paracetamol exert its analgesic effects by inhibiting endocannabinoid biosynthesis and binding of cannabinoid receptors, respectively [ 29 ].
The cannabinoid receptors CB1 and CB2 are the main receptors of the cannabinoid system. By inhibiting AC, the reduction of the second messenger cyclic adenosine monophosphate cAMP leads to the opening of rectifying potassium channels.
Neutrophil activated expresses cannabinoid receptors CB receptors. CB1 is expressed in the central nervous system, especially by neurons [ 44 ] and modulates physiological processes, such as motor behavior, learning, memory and cognition, and pain perception [ 45 ]. This receptor is associated with the psychotropic effects of cannabinoid agonists, such as THC [ 46 , 47 ].
CB2 is the peripheral receptor for cannabinoid agonists. It is mainly expressed in immune tissues such as the spleen and thymus as well as in blood cell subpopulations such as CD4 and CD8 lymphocytes, neutrophils, monocytes, natural killer NK cells, and B lymphocytes [ 47 ].
Nevertheless, CB2 is also found at low levels in neuronal and nonneuronal cells of the brain, but it does not produce psychoactive effects [ 47 , 48 , 49 ].
The CB2 expression intensity in immune cells depends on cell populations and activation state [ 50 , 51 ]. Many subsequent studies have shown that cannabinoids inhibited the production of cytokines in innate and adaptive immune responses, both in animal models and in human cell cultures to review [ 53 ]. In such a way, CB2 has become an important target, especially in inflammatory conditions. The CB2 receptor modulates immune cell functions, both in vitro and in animal models of inflammatory diseases.
In this context, some studies have reported that mice lacking the CB2 receptor have an exacerbated inflammatory phenotype to review [ 19 ].
Besides, CB2 agonists have an inhibitory effect on leukocyte migration and in the production of pro-inflammatory mediators in vivo and in vitro , showing a high anti-inflammatory potential [ 54 ]. Due to the lack of psychotropic effects, CB2 agonists are considered a promising therapeutic strategy for the treatment of chronic inflammatory diseases. Preclinical studies showed the action of CB2 agonists on different experimental models of inflammation, such as colitis, arthritis, cerebral ischemia, and sepsis [ 10 , 53 , 55 , 56 , 57 , 58 , 59 ], and in these studies, they showed that the action of CB2 agonists modulated the neutrophil activity.
Neutrophils play a crucial role in inflammatory processes, which are present in the pathology of different diseases. The neutrophil recruitment to the inflammatory site is an essential stage in the inflammatory responses; these cells are released from the bone marrow to the periphery immediately after the first signal of inflammation.
In this context, chemokines from inflamed foci might make their way to the bone marrow and modulate neutrophil egress. Thus, CXCL1 and CXCL2 can act locally by inducing neutrophil recruitment from blood to peripheral tissue and systemically by inducing neutrophil mobilization from the bone marrow to the bloodstream [ 61 ]. Once in the peripheral blood, neutrophils can be rapidly recruited into inflamed or infected tissues.
Neutrophils enter the tissue through surface molecules which interact with vascular endothelial cells. This process is accompanied by a regulated rearrangement of the cytoskeleton of neutrophils that lead to actin polymerization [ 64 ], which is mainly governed by members of the Rho family GTPases including RhoA, Rac1, and Cdc42 [ 65 ].
Concerning cannabinoid receptor expression, it was observed that neutrophil from healthy donors expresses low levels of CB2 [ 66 ]. In addition, it was shown that the CB2 receptor plays a crucial role in neutrophil differentiation, and it has been implicated in the development of leukemia [ 67 , 68 ]. The role of the cannabinoid system in neutrophil migration is controversial.
The activation of CB2 receptors by 2-AG did not induce polarization and migration of human blood neutrophils [ 69 ]. In contrast, McHugh and coworkers showed that 2-AG does not inhibit the migration of human neutrophils toward fMLP and does not show chemotactic effects by itself [ 70 ]. Furthermore, Balenga and coworkers showed that 2-AG induces migration of neutrophils toward inflammatory sites, through cross talk with activated GPR55 [ 62 ].
Despite the uncertainty regarding the involvement of CB2 agonist with the neutrophil migration and action, some studies show that there is a relationship between neutrophils and cannabinoid system in the pathogenesis of inflammatory or infection diseases Table 1. In the next section, we will discuss the cannabinoid system and its action on the neutrophil participation in inflammatory and infection conditions [ 54 , 62 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 ].
The action of CB2 agonists on their receptors impairs the secretion of pro-inflammatory cytokines and chemokines and reduces the recruitment of neutrophils. As discussed previously, neutrophils play a significant role in inflammatory diseases, including acute, chronic, autoimmune, infectious, and noninfectious conditions.
The relation between neutrophil and cannabinoid system in inflammatory disease was discussed in this section. Increased macro- and microscopic colon damage scores, a high number of macrophage and neutrophil and MPO activity, characterizes experimental colitis.
The activation of CB receptors by their ligands produces a protective effect in experimental colitis by decreasing prostaglandin, ROS and nitric oxide production, and reduction of leukocyte accumulation as neutrophils, resulting in diminished of colon tissue inflammation. Besides, mice lacking functional CB receptors are less resistant to colon inflammation than wild-type animals [ 53 , 86 ]. Thus, during inflammation, the CB2 receptor activation by endocannabinoids or synthetic cannabinoid provides a mechanism for the reestablishment of regular GI transit to review [ 89 ].
Mediators of Inflammation
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