Cannabinoids are a group of compounds that mediate their effects through cannabinoid receptors. The discovery of Δ9-tetrahydrocannabinol (THC) as the major psychoactive principle in marijuana, as well as the identification of cannabinoid receptors and their endogenous ligands, has led to a significant growth in research aimed at understanding the physiological functions of cannabinoids. Cannabinoid receptors include CB1, which is predominantly expressed in the brain, and CB2, which is primarily found on the cells of the immune system. The fact that both CB1 and CB2 receptors have been found on immune cells suggests that cannabinoids play an important role in the regulation of the immune system. Recent studies demonstrated that administration of THC into mice triggered marked apoptosis in T cells and dendritic cells, resulting in immunosuppression. In addition, several studies showed that cannabinoids downregulate cytokine and chemokine production and, in some models, upregulate T-regulatory cells (Tregs) as a mechanism to suppress inflammatory responses. The endocannabinoid system is also involved in immunoregulation. For example, administration of endocannabinoids or use of inhibitors of enzymes that break down the endocannabinoids, led to immunosuppression and recovery from immune-mediated injury to organs such as the liver. Manipulation of endocannabinoids and/or use of exogenous cannabinoids in vivo can constitute a potent treatment modality against inflammatory disorders. This review will focus on the potential use of cannabinoids as a new class of anti-inflammatory agents against a number of inflammatory and autoimmune diseases that are primarily triggered by activated T cells or other cellular immune components.

Cannabinoid pharmacology has made important advances in recent years after the discovery of the cannabinoid receptors. These discoveries have added to our understanding of exogenous and endogenous cannabinoid signaling along with exploring the various pathways of their biosynthesis, molecular structure, inactivation, and anatomical distribution of their receptors throughout the body. The endocannabinoid system is involved in immunoregulation and neuroprotection. In this article, we have reviewed the possible mechanisms of the regulation of the immune response by endocannabinoids which include modulation of immune response in different cell types, effect on cytokine network, induction of apoptosis in immune cells and downregulation of innate and adaptive immune response. Studies from our laboratory have suggested that administration of endocannabinoids or use of inhibitors of enzymes that breakdown the endocannabinoids, leads to immunosuppression and recovery from immune-mediated injury to organs such as the liver. Thus, manipulation of endocannabinoids in vivo may constitute a novel treatment modality against inflammatory disorders.

Allogeneic hematopoietic cell transplantation (HCT) is widely used to treat patients with life-threatening malignant and nonmalignant hematological diseases. However, allogeneic HCT often is accompanied by severe and lethal complications from graft-versus-host disease (GVHD), in which activated donor T cells recognize histocompatibility antigenic mismatches and cause significant toxicity in the recipient. In the current study, we tested the hypothesis that activation of cannabinoid receptors on donor-derived T cells may prevent GVHD. We tested the effect of ⌬ 9 -tetrahydrocannabinol (THC) in an acute model of GVHD that was induced by transferring parental C57BL/6 (B6) spleen cells into (C57BL/6 ϫ DBA/2) F 1 (BDF1) mice. Transfer of B6 cells into BDF1 mice produced severe acute GVHD in the recipient, characterized by lymphoid hyperplasia, weight loss, T helper l cytokine production and mortality. THC administration led to early recovery from body weight loss, reduced tissue injury in the liver and intestine, as well as complete survival. THC treatment reduced the expansion of donor-derived effector T cells and blocked the killing of host-derived immune cells while promoting Foxp3 ϩ regulatory T cells. Impaired hematopoiesis seen during GVHD was rescued by treatment with THC. The ability of THC to reduce the clinical GVHD was reversed, at least in part, by administration of cannabinoid receptor (CB) 1 and CB2 antagonists, thereby demonstrating that THC-mediated amelioration of GVHD was cannabinoid receptor-dependent. Our results demonstrate for the first time that targeting cannabinoid receptors may constitute a novel treatment modality against acute GVHD.

Autoimmune hepatitis is a severe immune mediated chronic liver disease with a prevalence range between 50 and 200 cases per million in Western Europe and North America and mortality rates of up to 80% in untreated patients. The induction of CB1 and CB2 cannabinoid receptors during liver injury and the potential involvement of endocannabinoids in the regulation of this process have sparked significant interest in further evaluating the role of cannabinoid systems during hepatic disease. Cannabinoids have been shown to possess significant immunosuppressive and anti-inflammatory properties. Cannabinoid abuse has been shown to exacerbate liver fibrogenesis in patients with chronic hepatitis C infection involving CB1 receptor. Nonetheless, CB2 receptor activation may play a protective role during chronic liver diseases. Thus, differential targeting of cannabinoid receptors may provide novel therapeutic modality against autoimmune hepatitis. In this review, we summarize current knowledge on the role of endocannabinoids and exocannabinoids in the regulation of autoimmune hepatitis.

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