Activation of histamine H2 receptors ameliorates experimental allergic encephalomyelitis

Postpartum hepatitis and host immunity in pregnant women with chronic HBV infection

In order to develop immune tolerant to the fetal, maternal immune system will have some modification comparing to the time before pregnancy. Immune tolerance starts and develops at the maternal placental interface. In innate immunity, decidual natural killer (dNK) cells, macrophages and dendritic cells play a key role in immue tolerance. In adaptive immunity, a moderate increase of number and immune inhibition function of regulatory T cells (Treg) are essential for immune tolerance. The trophoblast cells and immune cells expressing indoleamine 2,3-dioxygenase (IDO), the trophoblast cells expressing HLA-G, and Th1/Th2 shifting to Th2 dominant and Th17/Treg shifting to Treg domiant are in favor of maternal fetal immune tolerance. Steroids (estrogen and progesterone) and human chorionic gonadotropin (HCG) also participate in immune tolerance by inducing Treg cells or upregulating immunosuppressive cytokines. Most of the patients with chronic HBV infection are in the "HBV immune tolerance period" before pregnancy, and the liver disease is relatively stable during pregnancy. In chronic HBV infection women, after delivery, the relative immunosuppression in vivo is reversed, and Th1 is dominant in Th1/Th2 and Th17 is dominant in Th17/Treg balance. After delivery, the number of Treg decrease and NK cells increase in quantity and cytotoxicity in peripheral blood. Liver NK cells may cause liver inflammation through a non-antigen specific mechanism. After delivery, the number of CD8+ T cells will increase and HBV specific T cell response recovers from the disfunction in pregnancy. Under the background of postpartum inflammation, the rapid decrease of cortisol after delivery, and especially the enhancement of HBV specific T cell response induced by HBV DNA and cytokines, are the main reasons for postpartum hepatitis. HBeAg positive, especially HBeAg<700 S/CO, and HBV DNA>3-5Log10IU/ml are risk factors for postpartum hepatitis. Antiviral treatment in late pregnancy can reduce the incidence of mother to child transmission (MTCT) in chronic HBV infection women. Chronic HBV infection women have hepatitis both during pregnancy and more often in 12 weeks postpartum. It is generally agreed that postpartum hepatitis is mild symptoms and self-limited. Delaying drug withdrawal to 48 weeks can increase the seroconversion rate of HBeAg in delivery women with elevated alanine aminotransferase (ALT) in pregnancy.

The Histamine and Multiple Sclerosis Alliance: Pleiotropic Actions and Functional Validation

Multiple sclerosis (MS) is a disease with a resilient inflammatory component caused by accumulation into the CNS of inflammatory infiltrates and macrophage/microglia contributing to severe demyelination and neurodegeneration. While the causes are still in part unclear, key pathogenic mechanisms are the direct loss of myelin-producing cells and/or their impairment caused by the immune system. Proposed etiology includes genetic and environmental factors triggered by viral infections. Although several diagnostic methods and new treatments are under development, there is no curative but only palliative care against the relapsing-remitting or progressive forms of MS. In recent times, there has been a boost of awareness on the role of histamine signaling in physiological and pathological functions of the nervous system. Particularly in MS, evidence is raising that histamine might be directly implicated in the disease by acting at different cellular and molecular levels. For instance, constitutively active histamine regulates the differentiation of oligodendrocyte precursors, thus playing a central role in the remyelination process; histamine reduces the ability of myelin-autoreactive T cells to adhere to inflamed brain vessels, a crucial step in the development of MS; histamine levels are found increased in the cerebrospinal fluid of MS patients. The aim of the present work is to present further proofs about the alliance of histamine with MS and to introduce the most recent and innovative histamine paradigms for therapy. We will report on how a long-standing molecule with previously recognized immunomodulatory and neuroprotective functions, histamine, might still provide a renewed and far-reaching role in MS.

Type 2 Inflammatory Responses in Autoimmune Demyelination of the Central Nervous System: Recent Advances

Type 2 immunity has long been confined to a restricted spectrum of responses, mostly including allergic reactions to innocuous environmental triggers. However, growing evidence suggests that cells and mediators typically associated with type 2 inflammation are involved in several physiopathological conditions, such as defense against toxic substances, anticancer immunity, and autoimmune diseases. In neuromyelitis optica, an autoimmune demyelinating disorder of the spinal cord and optic nerve, eosinophils extensively infiltrate lesions in the central nervous system (CNS) and promote tissue pathology in experimental models of this disease. Next-generation sequencing of CD4⁺ T cells isolated from a specific subtype of multiple sclerosis plaque has uncovered an unexpectedly Th2 profile of these cells. Even mast cells and other allergic mediators have been implicated in the modulation and/or effector mechanisms of autoimmune reactions against the CNS. In this review article, the most recent developments showing the involvement of type 2 inflammatory components in CNS autoimmunity are summarised and possible lines of further investigation are discussed.

Immune Regulation in Pregnancy: A Matter of Perspective?

The maternal immune system is complex and governed by multiple hormonal and metabolic factors, including those provided to the mother via the fetus. Understanding of the balance between maternal tolerance and protection of the fetus may require thinking from multiple theoretical approaches to the general problem of immune activation and tolerance. This article provides a brief review of the immune system, with aspects relevant to pregnancy. The references include reviews that expand on the elements discussed. The article also uses different models of immune system activation and tolerance to provide a theoretical understanding of the problem of maternal tolerance.

Immune Regulation in Pregnancy: A Matter of Perspective?

The maternal immune system is complex and governed by multiple hormonal and metabolic factors, including those provided to the mother via the fetus. Understanding of the balance between maternal tolerance and protection of the fetus may require thinking from multiple theoretical approaches to the general problem of immune activation and tolerance. This article provides a brief review of the immune system, with aspects relevant to pregnancy. The references include reviews that expand on the elements discussed. The article also uses different models of immune system activation and tolerance to provide a theoretical understanding of the problem of maternal tolerance.

Albumin and multiple sclerosis

Leakage of the blood-brain barrier (BBB) is a common pathological feature in multiple sclerosis (MS). Following a breach of the BBB, albumin, the most abundant protein in plasma, gains access to CNS tissue where it is exposed to an inflammatory milieu and tissue damage, e.g., demyelination. Once in the CNS, albumin can participate in protective mechanisms. For example, due to its high concentration and molecular properties, albumin becomes a target for oxidation and nitration reactions. Furthermore, albumin binds metals and heme thereby limiting their ability to produce reactive oxygen and reactive nitrogen species. Albumin also has the potential to worsen disease. Similar to pathogenic processes that occur during epilepsy, extravasated albumin could induce the expression of proinflammatory cytokines and affect the ability of astrocytes to maintain potassium homeostasis thereby possibly making neurons more vulnerable to glutamate exicitotoxicity, which is thought to be a pathogenic mechanism in MS. The albumin quotient, albumin in cerebrospinal fluid (CSF)/albumin in serum, is used as a measure of blood-CSF barrier dysfunction in MS, but it may be inaccurate since albumin levels in the CSF can be influenced by multiple factors including: 1) albumin becomes proteolytically cleaved during disease, 2) extravasated albumin is taken up by macrophages, microglia, and astrocytes, and 3) the location of BBB damage affects the entry of extravasated albumin into ventricular CSF. A discussion of the roles that albumin performs during MS is put forth.

Analysis of histamine receptor knockout mice in models of inflammation

The diverse functions of histamine are mediated by four specific histamine receptor subtypes, which belong to the family of G-protein-coupled receptors. Here, we summarize data obtained with histamine-deficient L-histidine decarboxylase knockout and histamine receptor subtype knockout mice in inflammation models. Advantages and disadvantages of the knockout approaches compared with pharmacologic approaches are discussed critically. Due to many controversial data it is very difficult to draw clear-cut conclusions from the data provided in the literature. Thus, the published studies highlight the complexity of histamine function in inflammation and the need for much more systematic experimental work.

Histamine H₂ receptor signaling × environment interactions determine susceptibility to experimental allergic encephalomyelitis

Histamine and its receptors are important in both multiple sclerosis and experimental allergic encephalomyelitis (EAE). C57BL/6J (B6) mice deficient for the histamine H2 receptor (H2RKO) are less susceptible to EAE and exhibit blunted Th1 responses. However, whether decreased antigen-specific T-cell effector responses in H2RKO mice were due to a lack of H2R signaling in CD4(+) T cells or antigen-presenting cells has remained unclear. We generated transgenic mice expressing H2R specifically in T cells on the H2RKO background, and, using wild-type B6 and H2RKO mice as controls, induced EAE either in the presence or absence of the ancillary adjuvant pertussis toxin (PTX), which models the effects of infectious inflammatory stimuli on autoimmune disease. We monitored the mice for clinical signs of EAE and neuropathology, as well as effector T-cell responses using flow cytometry. EAE severity and neuropathology in H2RKO mice expressing H2R exclusively in T cells become equal to those in wild-type B6 mice only when PTX is used to elicit disease. EAE complementation was associated with frequencies of CD4(+)IFN-γ(+) and CD4(+)IL-17(+) cells that are equal to or greater than those in wild-type B6, respectively. Thus, the regulation of encephalitogenic T-cell responses and EAE susceptibility by H2R signaling in CD4(+) T cells is dependent on gene × environment interactions.

Neuroendocrine immunoregulation in multiple sclerosis

Currently, it is generally accepted that multiple sclerosis (MS) is a complex multifactorial disease involving genetic and environmental factors affecting the autoreactive immune responses that lead to damage of myelin. In this respect, intrinsic or extrinsic factors such as emotional, psychological, traumatic, or inflammatory stress as well as a variety of other lifestyle interventions can influence the neuroendocrine system. On its turn, it has been demonstrated that the neuroendocrine system has immunomodulatory potential. Moreover, the neuroendocrine and immune systems communicate bidirectionally via shared receptors and shared messenger molecules, variously called hormones, neurotransmitters, or cytokines. Discrepancies at any level can therefore lead to changes in susceptibility and to severity of several autoimmune and inflammatory diseases. Here we provide an overview of the complex system of crosstalk between the neuroendocrine and immune system as well as reported dysfunctions involved in the pathogenesis of autoimmunity, including MS. Finally, possible strategies to intervene with the neuroendocrine-immune system for MS patient management will be discussed. Ultimately, a better understanding of the interactions between the neuroendocrine system and the immune system can open up new therapeutic approaches for the treatment of MS as well as other autoimmune diseases.

Systemic lack of canonical histamine receptor signaling results in increased resistance to autoimmune encephalomyelitis

Histamine (HA) is a key regulator of experimental allergic encephalomyelitis (EAE), the autoimmune model of multiple sclerosis. HA exerts its effects through four known G-protein-coupled receptors: H1, H2, H3, and H4 (histamine receptors; H(1-4)R). Using HR-deficient mice, our laboratory has demonstrated that H1R, H2R, H3R, and H4R play important roles in EAE pathogenesis, by regulating encephalitogenic T cell responses, cytokine production by APCs, blood-brain barrier permeability, and T regulatory cell activity, respectively. Histidine decarboxylase-deficient mice (HDCKO), which lack systemic HA, exhibit more severe EAE and increased Th1 effector cytokine production by splenocytes in response to myelin oligodendrocyte gp35-55. In an inverse approach, we tested the effect of depleting systemic canonical HA signaling on susceptibility to EAE by generating mice lacking all four known G-protein-coupled-HRs (H(1-4)RKO mice). In this article, we report that in contrast to HDCKO mice, H(1-4)RKO mice develop less severe EAE compared with wild-type animals. Furthermore, splenocytes from immunized H(1-4)RKO mice, compared with wild-type mice, produce a lower amount of Th1/Th17 effector cytokines. The opposing results seen between HDCKO and H1-4RKO mice suggest that HA may signal independently of H1-4R and support the existence of an alternative HAergic pathway in regulating EAE resistance. Understanding and exploiting this pathway has the potential to lead to new disease-modifying therapies in multiple sclerosis and other autoimmune and allergic diseases.

Elevated CSF histamine levels in multiple sclerosis patients

Background

Histamine is an ubiquitous inflammatory mediator of numerous physiological processes. Histamine and its receptors have been implicated in multiple sclerosis (MS) disease pathogenesis. We prospectively enrolled 36 MS patients and 19 age and gender-matched healthy volunteers for cerebrospinal fluid (CSF) histamine analysis.

Findings

CSF histamine levels in MS patient samples were significantly higher (median: 35.6 pg/ml) than in controls (median: 5.5 pg/ml; Beta = 0.525, p < 0.001). In addition, histamine increased with age (Pearson’s correlation, p < 0.003).

Conclusions

Histamine may be an important factor for both the initiation and maintenance of chronic inflammatory diseases of the central nervous system. Our observation encourages a deeper investigation of the role of histamine in MS.

Mast cell regulation of the immune response

Mast cells are well known as principle effector cells of type I hypersensitivity responses. Beyond this role in allergic disease, these cells are now appreciated as playing an important role in many inflammatory conditions. This review summarizes the support for mast cell involvement in resisting bacterial infection, exacerbating autoimmunity and atherosclerosis, and promoting cancer progression. A commonality in these conditions is the ability of mast cells to elicit migration of many cell types, often through the production of inflammatory cytokines such as tumor necrosis factor. However, recent data also demonstrates that mast cells can suppress the immune response through interleukin-10 production. The data encourage those working in this field to expand their view of how mast cells contribute to immune homeostasis.

Sphingolipids: a potential molecular approach to treat allergic inflammation

Allergic inflammation is an immune response to foreign antigens, which begins within minutes of exposure to the allergen followed by a late phase leading to chronic inflammation. Prolonged allergic inflammation manifests in diseases such as urticaria and rhino-conjunctivitis, as well as chronic asthma and life-threatening anaphylaxis. The prevalence of allergic diseases is profound with 25% of the worldwide population affected and a rising trend across all ages, gender, and racial groups. The identification and avoidance of allergens can manage this disease, but this is not always possible with triggers being common foods, prevalent air-borne particles and only extremely low levels of allergen exposure required for sensitization. Patients who are sensitive to multiple allergens require prophylactic and symptomatic treatments. Current treatments are often suboptimal and associated with adverse effects, such as the interruption of cognition, sleep cycles, and endocrine homeostasis, all of which affect quality of life and are a financial burden to society. Clearly, a better therapeutic approach for allergic diseases is required. Herein, we review the current knowledge of allergic inflammation and discuss the role of sphingolipids as potential targets to regulate inflammatory development in vivo and in humans. We also discuss the benefits and risks of using sphingolipid inhibitors.

Combinatorial roles for histamine H1-H2 and H3-H4 receptors in autoimmune inflammatory disease of the central nervous system

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system in which histamine (HA) and its receptors have been implicated in disease pathogenesis. HA exerts its effects through four different G protein-coupled receptors designated H(1)-H(4). We previously examined the effects of traditional single HA receptor (HR) knockouts (KOs) in experimental allergic encephalomyelitis (EAE), the autoimmune model of MS. Our results revealed that H(1) R and H(2) R are propathogenic, while H(3) R and H(4) R are antipathogenic. This suggests that combinatorial targeting of HRs may be an effective disease-modifying therapy (DMT) in MS. To test this hypothesis, we generated H(1) H(2) RKO and H(3) H(4) RKO mice and studied them for susceptibility to EAE. Compared with wild-type (WT) mice, H(1) H(2) RKO mice developed a less severe clinical disease course, whereas the disease course of H(3) H(4) RKO mice was more severe. H(1) H(2) RKO mice also developed less neuropathology and disrupted blood brain barrier permeability compared with WT and H(3) H(4) RKO mice. Additionally, splenocytes from immunized H(1) H(2) RKO mice produced less interferon(IFN)-γ and interleukin(IL)-17. These findings support the concept that combined pharmacological targeting of HRs may be an appropriate ancillary DMT in MS and other immunopathologic diseases.

Potential immunological consequences of pharmacological suppression of gastric acid production in patients with multiple sclerosis

Corticosteroids are standard treatment for patients with multiple sclerosis experiencing acute relapse. Because dyspeptic pain is a common side effect of this intervention, patients can be given a histamine receptor-2 antagonist, proton pump inhibitor or antacid to prevent or ameliorate this disturbance. Additionally, patients with multiple sclerosis may be taking these medications independent of corticosteroid treatment. Interventions for gastric disturbances can influence the activation state of the immune system, a principal mediator of pathology in multiple sclerosis. Although histamine release promotes inflammation, activation of the histamine receptor-2 can suppress a proinflammatory immune response, and blocking histamine receptor-2 with an antagonist could shift the balance more towards immune stimulation. Studies utilizing an animal model of multiple sclerosis indicate that histamine receptor-2 antagonists potentially augment disease activity in patients with multiple sclerosis. In contrast, proton pump inhibitors appear to favor immune suppression, but have not been studied in models of multiple sclerosis. Antacids, histamine receptor-2 antagonists and proton pump inhibitors also could alter the intestinal microflora, which may indirectly lead to immune stimulation. Additionally, elevated gastric pH can promote the vitamin B12 deficiency that patients with multiple sclerosis are at risk of developing. Here, we review possible roles of gastric acid inhibitors on immunopathogenic mechanisms associated with multiple sclerosis.

Histamine and neuroinflammation: insights from murine experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a chronic inflammatory, neurodegenerative disease of the CNS whose pathogenesis remains largely unknown, and available therapies are rarely successful in reversing neurological deficits or stopping disease progression. Ongoing studies on MS and the widely used murine model of experimental autoimmune encephalomyelitis (EAE) are focused on the many components of this complex and heterogeneous neurodegenerative disease in the hope of providing a mechanism-based characterization of MS that will afford successful strategies to limit and repair the neuronal damage. Recently, histamine has been postulated to have a key regulatory role in EAE and MS pathogenesis. Histamine is a mediator of inflammation and immune responses, exerting its many actions through four G protein-coupled receptors (H_1,2,3,4R) that signal through distinct intracellular pathways and have different therapeutic potentials as they vary in expression, isoform distribution, signaling properties, and function. Immune cells involved in MS/EAE, including dendritic cells (DCs) and T lymphocytes, express H₁R, H₂R and H₄R, and histamine may have varying and counteracting effects on a particular cell type, depending on the receptor subtypes being activated. Here, we review evidence of the complex and controversial role of histamine in the pathogenesis of MS and EAE and evaluate the therapeutic potential of histaminergic ligands in the treatment of autoimmune diseases.

Combination treatment of mice with CRx-153 (nortriptyline and desloratadine) decreases the severity of experimental autoimmune encephalomyelitis

Pro-inflammatory CD4(+) T cell-mediated autoimmune diseases, such as multiple sclerosis, are hypothesized to be initiated and maintained by self-reactive interferon-gamma (IFN-γ) and interleukin-17 (IL-17) producing CD4(+) T cells. Previous studies have shown moderate to significant alterations in inflammatory T cell responses and potentially treatment of autoimmune disease by administration of antihistamine or tricyclic antidepressants alone. The goal of the present study was to determine if treatment of PLP(139-151)-induced relapsing-remitting experimental autoimmune encephalomyelitis (R-EAE) in SJL/J mice with a combination of two FDA approved drugs for other indications could decrease R-EAE disease. The findings show that combination treatment with desloratadine and nortriptyline decreases the mean clinical score, disease relapse frequency, and number of CD4(+) T cells infiltrating into the CNS. In addition, combination treatment of PLP(139-151) primed mice decreases the level of IFN-γ and IL-17 secreted via a decrease in both the number of cells secreting and the amount of cytokine secreted per cell following PLP(139-151) reactivation ex vivo. This is in contrast to an increase in the level of IL-4 produced and the number of IL-4 secreting cells. The data also show that combination treatment with desloratadine and nortriptyline inhibits the production of IFN-γ and IL-17 produced by naive CD4(+) T cells activated in the presence of Th1 cell- and Th17 cell-promoting conditions, while increasing the level of IL-4 produced by naive CD4(+) T cells activated in the presence of Th2 cell-promoting conditions. The present findings suggest a novel method for the development of a putative autoimmune therapy.

Histamine regulates autoreactive T cell activation and adhesiveness in inflamed brain microcirculation

Histamine may contribute to the pathology of MS and its animal model EAE. We explored the effects of histamine and specific HR agonists on activation and migratory capacity of myelin-autoreactive T cells. We show that histamine in vitro inhibits proliferation and IFN-γ production of mouse T cells activated against PLP(139-151). These effects were mimicked by the H1R agonist HTMT and the H2R agonist dimaprit and were associated with reduced activation of ERK½ kinase and with increased levels of cell cycle inhibitor p27Kip-1, both involved in T cell proliferation and anergy. H1R and H2R agonists reduced spontaneous and chemokine-induced adhesion of autoreactive T cells to ICAM-1 in vitro and blocked firm adhesion of these cells in inflamed brain microcirculation in vivo. Thus histamine, through H1R and H2R, inhibits activation of myelin-autoreactive T cells and their ability to traffic through the inflamed BBB. Strategies aimed at interfering with the histamine axis might have relevance in the therapy of autoimmune disease of the CNS.

Histamine and histamine receptors in pathogenesis and treatment of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease associated with chronic inflammatory demyelination of the central nervous system (CNS). Due to disease complexity and heterogeneity, its pathogenesis remains unknown and despite extensive studies, specific effective treatments have not yet been developed. The factors behind the initiation of the inflammatory reactions in CNS have not been identified until now. MS is considered as a complex disease depending on genetic as well as environmental factors. Experimental autoimmune encephalomyelitis (EAE) is the preferential experimental rodent model for MS. Histamine [2-(4-imidazole) ethylamine] is a ubiquitous inflammatory mediator of diverse physiological processes including neurotransmission, secretion of pituitary hormones, and regulation of the gastrointestinal and circulatory systems which can modulate immune responses. Histamine functions are mediated through four G-protein coupled receptors that are named H1-H4 receptor. Histamine is implicated as an important factor in pathophysiology of MS and EAE. It has been shown that histamine can change the permeability of blood brain barrier, which leads to elevation of infiltrated cells in CNS and neuroinflammation. In contrast, there are evidence that show the protective role of histamine in MS and its animal model, EAE. In this review, we try to clarify the role of histamine in pathogenesis of MS, as well as we evaluate the efficacy of histamine receptors agonists and antagonists in treatment of this disease.

Histamine-N-methyl transferase polymorphism and risk for multiple sclerosis

BACKGROUND

Histamine N-methyltransferase (HNMT) is the main metabolizing enzyme of histamine (a mediator of inflammation implicated in the pathogenesis of multiple sclerosis-MS) in the CNS. We have investigated the possible association between a single nucleotide polymorphism of the HNMT (chromosome 2q22.1), that causes the amino acid substitution Thr105Ile (decreasing enzyme activity) and the risk for MS.

METHODS

We studied the frequency of the HNMT genotypes and allelic variants in 228 MS patients and 295 healthy controls using a PCR-RLFP method.

RESULTS

The frequencies of the HNMT genotypes and allelic variants did not differ significantly between MS patients and controls, and were unrelated with the age of onset of MS, gender, and course of MS.

CONCLUSION

The HNMT polymorphism is not related with the risk for MS.

Central histamine H3 receptor signaling negatively regulates susceptibility to autoimmune inflammatory disease of the CNS

Histamine (HA), a biogenic amine with a broad spectrum of activities in both physiological and pathological settings, plays a key regulatory role in experimental allergic encephalomyelitis, the autoimmune model of multiple sclerosis. HA exerts its effect through four G protein-coupled receptors designated HA receptor H1, H2, H3, and H4. We report here that, compared with wild-type animals, mice with a disrupted HA H3 receptor (H3RKO), the expression of which is normally confined to cells of the nervous system, develop more severe disease and neuroinflammation. We show that this effect is associated with dysregulation of blood-brain barrier permeability and increased expression of MIP-2, IP-10, and CXCR3 by peripheral T cells. Our data suggest that pharmacological targeting of the H3R may be useful in preventing the development and formation of new lesions in multiple sclerosis, thereby significantly limiting the progression of the disease.

A key regulatory role for histamine in experimental autoimmune encephalomyelitis: disease exacerbation in histidine decarboxylase-deficient mice

Histamine can modulate the cytokine network and influence Th1 and Th2 balance and Ab-isotype switching. Thus, pharmacological blockade or genetic deletion of specific histamine receptors has been shown to reduce the severity of experimental autoimmune encephalomyelitis (EAE), a prototypic Th1-mediated disease with similarities to human multiple sclerosis. To study the comprehensive contribution of endogenous histamine to the expression of EAE, we attempted to induce EAE in histidine decarboxylase-deficient mice, which are genetically unable to make histamine. In this study, we show that EAE is significantly more severe in HDC-/-, histamine-deficient mice, with diffuse inflammatory infiltrates, including a prevalent granulocytic component, in the brain and cerebellum. Unlike splenocytes from wild-type mice, splenocytes from HDC-/- mice do not produce histamine in response to the myelin Ag, whereas production of IFN-gamma, TNF, and leptin are increased in HDC-/- splenocytes in comparison to those from wild-type mice. Endogenous histamine thus appears to regulate importantly the autoimmune response against myelin and the expression of EAE, in this model, and to limit immune damage to the CNS. Understanding which receptor(s) for histamine is/are involved in regulating autoimmunity against the CNS might help in the development of new strategies of treatment for EAE and multiple sclerosis.

Histamine in allergic inflammation and immune modulation

Histamine, originally considered as a mediator of acute inflammatory and immediate hypersensitivity responses has also been demonstrated to affect chronic inflammation and regulate several essential events in the immune response. On the other hand, various cytokines control histamine synthesis, release and expression of histamine receptors (HRs). The cells involved in the regulation of immune response and hematopoiesis express HRs and also secrete histamine, which can selectively recruit the major effector cells into tissue sites and affect their maturation, activation, polarization and effector functions leading to chronic inflammation. Histamine, acting through its receptor type 2, positively interferes with the peripheral antigen tolerance induced by T regulatory cells in several pathways. Histamine also regulates antigen-specific Th1 and Th2 cells, as well as related antibody isotype responses. The diverse effects of histamine on immune regulation are due to differential expression and regulation of four HRs and their distinct intracellular signals. In addition, differences in affinities of these receptors are highly decisive on the biological effects of histamine and agents that target HRs. This article highlights the findings leading to a change of perspective in histamine immunobiology.

Histamine-cytokine connection in immunity and hematopoiesis

A number of recent studies have led to a reappraisal of the functional capacities of histamine in immunity and hematopoiesis. This change of perspective was provided by the following findings: (1) the evidence for multiple cellular sources of histamine, differing from mature basophils and mast cells by their ability to newly synthesize and liberate the mediator without prior storage, (2) the discovery of a novel histamine receptor (H4R), preferentially expressed on hematopoietic and immunocompetent cells, (3) the potential intracellular activity of histamine through cytochrome P450 and (4) the demonstration of a histamine-cytokine cross-talk. Indeed, cytokines not only modulate the degranulation process of histamine but also control its neosynthesis by the histamine-forming enzyme, histidine decarboxylase (HDC), at transcriptional and post-transcriptional levels. In turn, histamine intervenes in the intricate cytokine network, regulating cytokine production by immune cells through distinct receptors signaling distinct biological effects. This type of regulation is particularly relevant in the context of TH1/TH2 differentiation, autoimmunity and tumor immunotherapy.

Experimental allergic encephalomyelitis is exacerbated in mice deficient for 12/15-lipoxygenase or 5-lipoxygenase

12/15-Lipoxygenase (12/15-LO) produces 15-hydroxyeicosatetraenoic acid (15-HETE) and 13-hydroxyoctadecadienoic acid (13-HODE) which are agonists for peroxisome proliferator-activated receptor-gamma (PPARgamma). PPARgamma agonists reduce clinical severity of experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis. In contrast, 5-lipoxygenase (5-LO) produces the generally proinflammatory leukotrienes (LTs) which would be expected to worsen EAE. We tested the hypotheses that EAE severity would be exacerbated in 12/15-LO-deficient mice and attenuated in 5-LO-deficient mice. 12/15-LO deficiency conferred a significantly worse disease course, and surprisingly, 5-LO deficiency also caused significantly more severe EAE compared to control mice. These data suggest that PPARgamma-regulated gene expression and that 5-LO production of certain LTs have the ability to diminish EAE. Continued analysis will provide insight into the endogenous LO-generated effectors that assist in tempering EAE.

Inhibiting cytokines of the interleukin-12 family: recent advances and novel challenges

Interleukin-12 (IL-12) and the more recently discovered IL-23 and IL-27 constitute a unique family of structurally related, heterodimeric cytokines that regulate cell-mediated immune responses and T helper 1 (Th1)-type inflammatory reactions. Not surprisingly, the potentiality of treating conditions such as multiple sclerosis (MS) and rheumatoid arthritis (RA) through pharmacological interference with IL-12 pathways has received widespread attention. In this review we have examined over 50 substances with reported IL-12 inhibitory effects. We demonstrate that a majority of these belong to a limited number of major functional classes, each of which targets discrete events in the IL-12 biological pathway. Thus, most IL-12 inhibitory substances appear to work either through inhibition of transcription factor NF-kappa B activation, up-regulation of intracellular cAMP, blockage of posttranslational processing or interference with signal transduction pathways. In addition, cyclophilin-binding drugs, and generic inhibitors of nuclear histone deacetylases, and of ion channels, pumps and antiporters are emerging as potential leads to novel targets for interference with IL-12 production. Many inhibitors of NF-kappa B and of IL-12 signal transduction have been proven effective in limiting or preventing disease in experimental autoimmune encephalomyelitis (EAE) models of MS. The sharing of the p40 subunit, the IL-12R beta 1 and components of the signal transduction pathways between IL-12 and IL-23 raises the question as to whether the beneficial effects of various drugs previously ascribed to inhibition of IL-12 may, in fact, have been due to concurrent blockage of both cytokines, or of IL-23, rather than IL-12. Moreover, the homodimeric beta(2)-form of IL-12, though originally considered to display only antagonistic effects, is now emerging as a pronounced agonist in a variety of inflammatory processes. Reassessment of IL-12 inhibitory compounds is therefore needed to scrutinize their effects on IL-12 alpha beta, beta(2) and IL-23 formation. This is likely to open exciting perspectives to the identification of drugs that target these cytokines either indiscriminately or selectively. The functional diversity of presently available inhibitors should facilitate an unprecedented flexibility in designing future trials for the treatment of IL-12- and IL-23-mediated disorders.

Attenuation of Th1 effector cell responses and susceptibility to experimental allergic encephalomyelitis in histamine H2 receptor knockout mice is due to dysregulation of cytokine production by antigen-presenting cells

Histamine, a biogenic amine with both neurotransmitter and vasoactive properties, is well recognized as an immunomodulatory agent in allergic and inflammatory reactions. It also plays a regulatory role in the development of antigen-specific immune responses. CD4+ T-cells from histamine H1 receptor (H1R)-deficient (H1RKO) mice produce significantly less interferon-gamma and more interleukin (IL)-4 in in vitro recall assays compared to wild-type controls. H1RKO mice are also less susceptible to acute early-phase experimental allergic encephalomyelitis indicating that H1R signaling in CD4+ T cells plays a central role in regulating pathogenic T-cell responses. In this study, we show that mice lacking histamine H2 receptor (H2RKO) are similar to H1RKO mice in that they develop encephalitogen-specific T-cell responses as assessed by proliferation and IL-2 production and present with less severe acute early-phase experimental allergic encephalomyelitis. However, unlike T cells from H1RKO mice, which exhibit a strong Th2 bias, T cells from H2RKO mice do not. Rather, they are uniquely characterized by a significant inhibition of Th1 effector cell responses. Given that both histamine and adjuvants such as pertussis toxin modulate antigen-presenting cell (APC) maturation and function, including T-cell-polarizing activity, we analyzed the cytokines/chemokines secreted by APCs from wild-type, H1RKO, and H2RKO mice. Significant differences in cytokine/chemokine production by APCs from unimmunized and immunized mice were delineated. APCs from H2RKO mice produce significantly less IL-12 and IL-6 and markedly greater amounts of MCP-1 compared to wild-type and H1RKO mice. Because MCP-1 is known to inhibit IL-12 production, the failure of H2RKO mice to generate encephalitogenic Th1 effector cell responses is consistent with inhibition of negative regulation of MCP-1 secretion by H2R signaling in APCs.

New concepts of histamine receptors and actions

Histamine and antihistamines are so deeply woven into the fabric of allergic diseases that it is sometimes difficult to see how this field could advance beyond our current, potent histamine H1-receptor drugs. Investigations of other actions of histamine and the identification of H2, H3, and now H4 receptors have suddenly reignited the search for new mono- and multi-receptor-specific agonists and antagonists. There is great excitement due to preliminary findings that H3 receptors act as neural inhibitory autoreceptors, and H4 receptors might modulate immune cell functions.