The role of histamine H1 and H4 receptors in allergic inflammation: the search for new antihistamines

Therapeutic manipulation of immune tolerance in allergic disease

Immune tolerance - the adaptation of the immune system to external antigens or allergens - might be therapeutically manipulated to restore normal immunity in conditions such as allergy, asthma and autoimmune diseases. The field of allergen-specific immunotherapy is experiencing exciting and novel developments for the treatment of allergic and autoimmune diseases, and recent insights into the reciprocal regulation and counter-balance between different T-cell subsets is foreseen to facilitate new strategies for immunointervention. This Review highlights current knowledge of immunomodulatory therapies for the manipulation of immune tolerance and highlights recent approaches to improve allergen-specific immunotherapy for the treatment of allergic diseases.

Immune tolerance in allergy

Research on the mechanisms of immune regulation in allergy and asthma has shown substantial progress in recent years and has led to a variety of allergen-specific therapeutic and preventive approaches. Studies on the area of allergen-specific immunotherapy (allergen-SIT) have provided substantial knowledge on the mechanisms of allergic disease with novel developments for treatment and prevention. Several studies have demonstrated that increased numbers and the activation of allergen-specific T regulatory cells correlate with successful allergen-SIT. Particularly, targeting of the molecular mechanisms of immune tolerance and reciprocal regulation of effector and regulatory T-cell subsets are foreseen for the development of new strategies for immune intervention. This review focuses on current knowledge of immune tolerance to allergens in healthy immune response to allergens and allergen-SIT.

Inhibition of histamine-induced bronchoconstriction in Guinea pig and Swine by pulsed electrical vagus nerve stimulation

Objective. Smooth muscle help regulate the diameter of the airways and their constriction can contribute to the pathology of acute asthma attacks. This study sought to determine if applying a specific electrical signal to the vagus nerve (VN) could minimize histamine-induced bronchoconstriction. Methods. Sixteen guinea pigs and three swine were anesthetized and had bipolar electrodes positioned on the cervical VNs. After the animals stabilized, i.v. histamine was titrated to elicit a moderate 2-4 cm H(2) O increase in pulmonary inflation pressure (Ppi). Histamine was then dosed with or without concurrent low voltage VN stimulation. Results. The peak change in Ppi following a histamine challenge was reduced in the guinea pig by VN stimulation (3.4 ± 0.4 vs. 2.1 ± 0.2 cm H(2) O, p < 0.001). The results were confirmed in a limited study in swine and indicate VN treatment is applicable to larger animals. Conclusion. This study suggests that VN stimulation can reduce bronchoconstriction and may prove useful as a rescue therapy in the treatment of acute asthma.

Histamine: metabolism, physiology, and pathophysiology with applications in veterinary medicine

OBJECTIVE

To review the human and veterinary literature on histamine physiology and pathophysiology and potential applications for clinical use in veterinary critical care.

DATA SOURCES

Human and veterinary clinical studies, reviews, texts, and recent research in histamine receptor and antagonist therapy.

HUMAN DATA SYNTHESIS

Recent progress in molecular biology has led to a more complete understanding of the enzymes involved in histamine metabolism and histamine receptor physiology. The past decade of research has confirmed the role of histamine in the classical functions (contraction of smooth muscle, increase in vascular permeability, and stimulation of gastric acid secretion) and has also elucidated newer ones that are now under investigation. Data on the roles of histamine in angiogenesis, circadian rhythm, bone marrow regeneration, bacterial eradication, and cancer are emerging in the literature. Newer histamine antagonists are currently in drug trials and are expected to advance the clinical field in treatment of allergic, gastrointestinal, and cognitive disorders.

VETERINARY DATA SYNTHESIS

Veterinary histamine research is directed at identifying the effects of certain pharmacological agents on blood histamine concentrations and establishing the relevance in clinical disease states. Research demonstrates important species differences in regards to histamine receptor physiology and tissue response. Studies in the area of trauma, sepsis, anaphylaxis, allergy, and gastrointestinal disorders have direct applications to clinical veterinary medicine.

CONCLUSIONS

Histamine plays a key role in the morbidity and mortality associated with allergy, asthma, gastric ulcers, anaphylaxis, sepsis, hemorrhagic shock, anesthesia, surgery, cardiovascular disease, cancer, CNS disorders, and immune-mediated disease. Histamine antagonism has been in common use to block its adverse effects. With recent advances in the understanding of histamine receptor physiology, pharmaceutical agents targeting these receptors have increased the therapeutic options.

In vitro and in vivo characterization of A-940894: a potent histamine H4 receptor antagonist with anti-inflammatory properties

BACKGROUND AND PURPOSE

The histamine H4 receptor is widely expressed in cells of immune origin and has been shown to play a role in a variety of inflammatory processes mediated by histamine. In this report, we describe the in vitro and in vivo anti-inflammatory properties of a potent histamine H4 receptor antagonist, A-940894 (4-piperazin-1-yl-6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-d]pyrimidin-2-ylamine).

EXPERIMENTAL APPROACH

We have analysed the pharmacological profile of A-940894 at mouse native, rat recombinant and human recombinant and native, histamine H4 receptors by radioligand binding, calcium mobilization, mast cell shape change, eosinophil chemotaxis assays and in the mouse model of zymosan-induced peritonitis.

KEY RESULTS

A-940894 potently binds to both human and rat histamine H4 receptors and exhibits considerably lower affinity for the human histamine H1, H2 or H3 receptors. It potently blocked histamine-evoked calcium mobilization in the fluorometric imaging plate reader assays and inhibited histamine-induced shape change of mouse bone marrow-derived mast cells and chemotaxis of human eosinophils in vitro. In a mouse mast cell-dependent model of zymosan-induced peritonitis, A-940894 significantly blocked neutrophil influx and reduced intraperitoneal prostaglandin D2 levels. Finally, A-940894 has good pharmacokinetic properties, including half-life and oral bioavailability in rats and mice.

CONCLUSIONS AND IMPLICATIONS

These data suggest that A-940894 is a potent and selective histamine H4 receptor antagonist with pharmacokinetic properties suitable for long-term in vivo testing and could serve as a useful tool for the further characterization of histamine H4 receptor pharmacology.

Molecular and biochemical pharmacology of the histamine H4 receptor

The elucidation of the human genome has had a major impact on histamine receptor research. The identification of the human H4 receptor by several groups has been instrumental for a new appreciation of the role of histamine in the modulation of immune function. In this review, we summarize the historical developments and the molecular and biochemical pharmacology of the H4 receptor.

Major advances in the development of histamine H4 receptor ligands

The search for new and potent histamine H4 receptor ligands is leading to a steadily increasing number of scientific publications and patent applications. Several interesting and structurally diverse compounds have been found, but fierce IP competition for a preferred 2-aminopyrimidine scaffold is becoming apparent. Recent investigations into the role of the histamine H(4)R in (patho)physiology and the use of H4R ligands in in vivo disease models reveal enormous potential in the field of inflammation and allergy, among others. The development of ligands that display activity at two or more histamine receptor (HR) subtypes is another clinical opportunity that is currently being explored. Taken together, the histamine H4R field is gearing up for clinical studies and has the potential to deliver another generation of blockbuster drugs.

Structural features of mammalian histidine decarboxylase reveal the basis for specific inhibition

For a long time the structural and molecular features of mammalian histidine decarboxylase (EC 4.1.1.22), the enzyme that produces histamine, have evaded characterization. We overcome the experimental problems for the study of this enzyme by using a computer-based modelling and simulation approach, and have now the conditions to use histidine decarboxylase as a target in histamine pharmacology. In this review, we present the recent (last 5 years) advances in the structure-function relationship of histidine decarboxylase and the strategy for the discovery of new drugs.

Histamine pharmacogenomics

Genetic polymorphisms for histamine-metabolizing enzymes are responsible for interindividual variation in histamine metabolism and are associated with diverse diseases. Initial reports on polymorphisms of histamine-related genes including those coding for the enzymes histidine decarboxylase (HDC), diamine oxidase (ABP1) and histamine N-methyltransferase (HNMT), as well as histamine receptor genes, often have pointed to polymorphisms that occur with extremely low frequencies or that could not be verified by later studies. In contrast, common and functionally significant polymorphisms recently described have been omitted in many association studies. In this review we analyze allele frequencies, functional and clinical impact and interethnic variability on histamine-related polymorphisms. The most relevant nonsynonymous polymorphisms for the HDC gene are rs17740607 Met31Thr, rs16963486 Leu553Phe and rs2073440 Asp644Glu. For ABP1 the most relevant polymorphisms are rs10156191 Thr16Met, rs1049742 Ser332Phe, and particularly because of its functional effect, rs1049793 His645Asp. In addition the ABP1 polymorphisms rs45558339 Ile479Met and rs35070995 His659Asn are relevant to Asian and African subjects, respectively. For HNMT the only nonsynonymous polymorphism present with a relevant frequency is rs1801105 Thr105Ile. For HRH1 the polymorphism rs7651620 Glu270Gly is relevant to African subjects only. The HRH2 rs2067474 polymorphism, located in an enhancer element of the gene promoter, is common in all populations. No common nonsynonymous SNPs were observed in the HRH3 gene and two SNPs were observed with a significant frequency in the HRH4 gene: rs11665084 Ala138Val and rs11662595 His206Arg. This review summarizes relevant polymorphisms, discusses controversial findings on association of histamine-related polymorphisms and allergies and other diseases, and identifies topics requiring further investigation.

The protective effects of ultraviolet A1 irradiation on spontaneous lupus erythematosus-like skin lesions in MRL/lpr mice

We investigated the effects of ultraviolet A1 (UVA1) irradiation on spontaneous lupus erythematosus- (LE-) like skin lesions of MRL/lpr mice, using a disease prevention model. UVA1 irradiation significantly inhibited the development of LE-like skin lesions, without obvious changes of the disease including renal disease and serum antinuclear antibody levels. Besides the massive infiltration of mast cells in the LE-like skin lesions, in the nonlesional skins, more mast cells infiltrated in the UVA1-irradiated group compared with the nonirradiated group. Although apoptotic cells were remarkably seen in the dermis of UVA1-irradiated mice, those cells were hardly detectable in the dermis of the nonirradiated mice without skin lesions. Further analysis showed that some of those apoptotic cells were mast cells. Thus, UVA1 might exert its effects, at least in part, through the induction of the apoptosis of pathogenic mast cells. Our results supported the clinical efficacy of UVA1 irradiation for skin lesions of lupus patients.

A common variant on chromosome 11q13 is associated with atopic dermatitis

We conducted a genome-wide association study in 939 individuals with atopic dermatitis and 975 controls as well as 270 complete nuclear families with two affected siblings. SNPs consistently associated with atopic dermatitis in both discovery sets were then investigated in two additional independent replication sets totalling 2,637 cases and 3,957 controls. Highly significant association was found with allele A of rs7927894 on chromosome 11q13.5, located 38 kb downstream of C11orf30 (P(combined) = 7.6 x 10(-10)). Approximately 13% of individuals of European origin are homozygous for rs7927894[A], and their risk of developing atopic dermatitis is 1.47 times that of noncarriers.

The role of histamine H4 receptor in immune and inflammatory disorders

Since its discovery at the beginning of the 20th century, histamine has been established to play a pathophysiological regulatory role in cellular events through binding to four types of G-protein-coupled histamine receptors that are differentially expressed in various cell types. The discovery, at the turn of the millennium, that the histamine H4 receptor is largely expressed in haemopoietic cells as well as its chemotactic properties designate its regulatory role in the immune system. H4 receptors modulate eosinophil migration and selective recruitment of mast cells leading to amplification of histamine-mediated immune responses and eventually to chronic inflammation. H4 receptor involvement in dendritic cell activation and T cell differentiation documents its immunomodulatory function. The characterization of the H4 as the immune system histamine receptor directed growing attention towards its therapeutic exploitation in inflammatory disorders, such as allergy, asthma, chronic pruritus and autoimmune diseases. The efficacy of a number of H4 receptor ligands has been evaluated in in vivo and in vitro animal models of disease and in human biological samples. However, before reaching decisive conclusions on H4 receptor pathophysiological functions and therapeutic exploitation, identification of genetic polymorphisms and interspecies differences in its relative actions and pharmacological profile need to be addressed and taken into consideration. Despite certain variations in the reported findings, the available data strongly point to the H4 receptor as a novel target for the pharmacological modulation of histamine-transferred immune signals and offer an optimistic perspective for the therapeutic exploitation of this promising new drug target in inflammatory disorders.

Role of histamine H(4) receptor in allergic conjunctivitis in mice

We investigated the character of histamine H(1) receptor and H(4) receptor in allergic conjunctivitis. Histamine is the most important mediator in allergic conjunctivitis. We measured eye scratching behavior and allergic-like symptoms score, that is, hyperemia and edema in ICR mice, and examined which receptors intimately involved in allergic conjunctivitis. Histamine caused a dose-dependent eye scratching behavior and allergic-like symptoms. Histamine H(1) receptor antagonist (levocabastine) and H(4) receptor antagonist (JNJ7777120) inhibited eye scratching behavior and histamine H(1) receptor antagonist inhibited allergic-like symptoms induced by histamine. Additionally, combination of levocabastine and JNJ7777120 caused more potent inhibition in allergic conjunctivitis. On the other hand, both selective histamine H(1) receptor agonist (HTMT) and selective H(4) receptor agonist (4-methylhistamine) induced a dose-dependent eye scratching behavior and allergic-like symptoms. JNJ7777120 inhibited the effect of HTMT. However, levocabastine caused no inhibition on the response of 4-methylhistamine. H(4) receptor was closely related with allergic conjunctivitis. H(4) receptor antagonists may be effective in allergic conjunctivitis which showed no inhibition by histamine H(1) receptor antagonists.

Association of the histamine N-methyltransferase C314T (Thr105Ile) polymorphism with atopic dermatitis in Caucasian children

STUDY OBJECTIVE

To investigate potential associations between the histamine N-methyltransferase (HNMT) gene, HNMT, C314T (Thr105Ile) polymorphism and atopic dermatitis in a cohort of Caucasian children.

DESIGN

Prospective, multicenter, genotype-association study.

SETTING

Four academic, tertiary care medical centers within the Pediatric Pharmacology Research Unit network.

PARTICIPANTS

Two hundred forty-nine Caucasian children aged 6 months-5 years with atopic dermatitis (127 patients) or without (122 control subjects).

INTERVENTION

Buccal swabs (one swab/cheek) were performed to obtain epithelial cells for extraction of genomic DNA.

MEASUREMENTS AND MAIN RESULTS

Data were collected on severity of atopic dermatitis, oral antihistamine treatment, and treatment response through parental report. The HNMT genotypes were successfully obtained in 116 control subjects and 122 patients with atopic dermatitis. Frequencies of the T314 variant allele (0.12 vs 0.06, p=0.04) and combined CT/TT genotype (0.24 vs 0.12, p=0.02) were significantly higher in children with atopic dermatitis compared with control subjects. Children with genotypes conferring reduced HNMT activity were 2 times more likely to have atopic dermatitis than those who were homozygous for the C314 reference allele.

CONCLUSION

Increased histamine levels in patients with atopic dermatitis may result, at least in part, from reduced enzymatic inactivation via HNMT. Genetically associated reduction in histamine biotransformation may therefore contribute to the pathogenesis, persistence, and progression of atopic dermatitis. If confirmed, these data indicate that HNMT genotype might represent a common risk factor for development of atopic dermatitis, asthma, and allergic rhinitis and may be useful in identifying individuals who are candidates for early preventive pharmacotherapeutic intervention. Additional longitudinal studies will be required to assess the relationship between genotype, disease severity, and antihistamine response.

Acute regulation of tight junction ion selectivity in human airway epithelia

Electrolyte transport through and between airway epithelial cells controls the quantity and composition of the overlying liquid. Many studies have shown acute regulation of transcellular ion transport in airway epithelia. However, whether ion transport through tight junctions can also be acutely regulated is poorly understood both in airway and other epithelia. To investigate the paracellular pathway, we used primary cultures of differentiated human airway epithelia and assessed expression of claudins, the primary determinants of paracellular permeability, and measured transepithelial electrical properties, ion fluxes, and La(3+) movement. Like many other tissues, airway epithelia expressed multiple claudins. Moreover, different cell types in the epithelium expressed the same pattern of claudins. To evaluate tight junction regulation, we examined the response to histamine, an acute regulator of airway function. Histamine stimulated a rapid and transient increase in the paracellular Na(+) conductance, with a smaller increase in Cl(-) conductance. The increase was mediated by histamine H(1) receptors and depended on an increase in intracellular Ca(2+) concentration. These results suggest that ion flow through the paracellular pathway can be acutely regulated. Such regulation could facilitate coupling of the passive flow of counter ions to active transcellular transport, thereby controlling net transepithelial salt and water transport.

Localization of histamine H4 receptors in the central nervous system of human and rat

Existing data on the expression of H(4) histamine receptor in the CNS are conflicting and inconclusive. In this report, we present the results of experiments that were conducted in order to elucidate H(4) receptor expression and localization in the brain, spinal cord, and dorsal root ganglia (DRG). Here we show that transcripts of H(4) receptor are present in all analyzed regions of the human CNS, including spinal cord, hippocampus, cortex, thalamus and amygdala, with the highest levels of H(4) mRNA detected in the spinal cord. In rat, H(4) mRNA was detected in cortex, cerebellum, brainstem, amygdala, thalamus and striatum. Very low levels of H(4) mRNA were detected in hypothalamus, and no H(4) signal was detected in the rat hippocampus. Fairly low levels of H(4) mRNA were detected in examined peripheral tissues including spleen and liver. Interestingly, strong expression of H(4) mRNA was detected in the rat DRG and spinal cord. Immunohistochemical analysis revealed expression of H(4) receptors on neurons in the rat lumbar DRG and in the lumbar spinal cord. Our observations provide evidence of the H(4) presence in both human and rodent CNS and offer some insight into possible role of H(4) in itch and pain.

In vivo switch to IL-10-secreting T regulatory cells in high dose allergen exposure

High dose bee venom exposure in beekeepers by natural bee stings represents a model to understand mechanisms of T cell tolerance to allergens in healthy individuals. Continuous exposure of nonallergic beekeepers to high doses of bee venom antigens induces diminished T cell-related cutaneous late-phase swelling to bee stings in parallel with suppressed allergen-specific T cell proliferation and T helper type 1 (Th1) and Th2 cytokine secretion. After multiple bee stings, venom antigen-specific Th1 and Th2 cells show a switch toward interleukin (IL) 10-secreting type 1 T regulatory (Tr1) cells. T cell regulation continues as long as antigen exposure persists and returns to initial levels within 2 to 3 mo after bee stings. Histamine receptor 2 up-regulated on specific Th2 cells displays a dual effect by directly suppressing allergen-stimulated T cells and increasing IL-10 production. In addition, cytotoxic T lymphocyte-associated antigen 4 and programmed death 1 play roles in allergen-specific T cell suppression. In contrast to its role in mucosal allergen tolerance, transforming growth factor beta does not seem to be an essential player in skin-related allergen tolerance. Thus, rapid switch and expansion of IL-10-producing Tr1 cells and the use of multiple suppressive factors represent essential mechanisms in immune tolerance to a high dose of allergens in nonallergic individuals.

Histamine H4 receptors modulate dendritic cell migration through skin--immunomodulatory role of histamine

BACKGROUND

Dendritic cells (DC) are the major antigen-presenting cells and play a key role in adaptive immunity as they are able to activate naive T cells. It was recently described, that the histamine H(4) receptor (H4R) is present on human monocyte-derived DC and that chemotaxis and T-helper (Th)1-Th2 polarization is mediated by this receptor. However, the distribution of histamine receptors on murine DC has not been studied yet.

METHODS

The histamine receptor expression on murine bone marrow (BM)-derived DC and effects of histamine and H4R agonism on DC migration through skin were studied. As it was demonstrated in scratching experiments that NMRI mice are more susceptible to H4R-mediated itch than BALB/c mice, DC function of NMRI and BALB/c mice was compared.

RESULTS

The mRNA of the H1R, H2R and H4R could be detected in murine BM-derived DC, while mRNA of the H3R was found to be low or undetectable. There were no distinct differences in mRNA expression and in H4R protein level (flow cytometry) between NMRI compared with BALB/c mice indicating, that a higher susceptibility is not associated with a generally higher H4R expression in all cell types. Histamine as well as the H4R agonist clobenpropit induced an enhanced chemotaxis in the skin DC migration assay. The enhanced chemotaxis was blocked by the H4R antagonist JNJ7777120. This finding was confirmed by in vitro migration experiments with BM-derived DC.

CONCLUSION

Referring to DC migration, blocking the H4R on inflammatory cells might be a promising anti-inflammatory, immunomodulatory strategy.

Expression, purification, crystallization and preliminary X-ray studies of histamine dehydrogenase from Nocardioides simplex

Histamine dehydrogenase (HADH) from Nocardioides simplex catalyzes the oxidative deamination of histamine to produce imidazole acetaldehyde and an ammonium ion. HADH is functionally related to trimethylamine dehydrogenase (TMADH), but HADH has strict substrate specificity towards histamine. HADH is a homodimer, with each 76 kDa subunit containing two redox cofactors: a [4Fe-4S] cluster and an unusual covalently bound flavin mononucleotide, 6-S-cysteinyl-FMN. In order to understand the substrate specificity of HADH, it was sought to determine its structure by X-ray crystallography. This enzyme has been expressed recombinantly in Escherichia coli and successfully crystallized in two forms. Diffraction data were collected to 2.7 A resolution at the SSRL synchrotron with 99.7% completeness. The crystals belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 101.14, b = 107.03, c = 153.35 A.

Histamine-induced itch and its relationship with pain

Itch is one of the major complications of skin diseases. Although there are various substances that induce itch or pruritus, it is evident that histamine is the best known endogenous agent that evokes itch. Even though histamine-induced itch has been studied for some time, the underlying mechanism of itch is just beginning to emerge. Although various downstream signaling pathways of histamine receptors have been revealed, more studies are required to determine the cause of histamine-induced itch. It appears that itch and pain involve different neuronal pathways. Pain generally inhibits itch, which indicates an inter-communication between the two. Complex interactions between itch and pain may be expected based on reports on disease states and opioids. In this review, we discuss the molecular mechanism and the pharmacological aspects of histamine-induced itch. Especially, the underlying mechanism of TRPV1 (an anti-pruritus target) has been determined to some extent.

Immune regulation by histamine H4 receptors in skin

The diverse effects of histamine on immune regulation appear to be due to differential expression and regulation of four types of histamine receptors and their distinct intracellular signals. The differences in cellular expression and affinities of these receptors for histamine determine the biological effects of histamine and the drugs that target histamine receptors. In this issue, Dijkstra et al., demonstrate the expression and some of the functions of histamine H(4) receptors on inflammatory dendritic cells in atopic dermatitis skin.

Evidence for the role of neurogenic inflammation components in trypsin-elicited scratching behaviour in mice

BACKGROUND AND PURPOSE

We investigated the mechanisms underlying the pruritogenic response induced by trypsin in mice, to assess the relevance of neurogenic inflammation components in this response.

EXPERIMENTAL APPROACH

Itching was induced by an intradermal injection of trypsin in the mouse neck. The animals were observed for 40 min and their scratching behaviour was quantified.

KEY RESULTS

Trypsin-induced itching was blocked by the lima bean trypsin inhibitor, the selective proteinase-activated receptor-2 (PAR-2) antagonist FSLLRY and PAR-2 receptor desensitization. An important involvement of mast cells was observed, as chronic pretreatment with the mast cell degranulator compound 48/80 or the mast cell stabilizer disodium cromoglycate prevented scratching. Also, trypsin response was inhibited by the selective COX-2 inhibitor celecoxib and by the selective kinin B2 (FR173657) and B1 (SSR240612) receptor antagonists. Moreover, an essential role for the mediators of neurogenic inflammation was established, as the selective NK1 (FK888), NK3 (SR142801) and calcitonin gene-related peptide (CGRP(8-37) fragment) receptor antagonists inhibited trypsin-induced itching. Similarly, blockade of transient receptor potential vanilloid 1 (TRPV1) receptors by the selective TRPV1 receptor antagonist SB366791, or by genetic deletion of TRPV1 receptor reduced this behaviour in mice. C-fibre desensitization showed a very similar result.

CONCLUSIONS AND IMPLICATIONS

Trypsin intradermal injection proved to be a reproducible model for the study of itching and the involvement of PAR-2 receptors. Also, trypsin-induced itching seems to be widely dependent on neurogenic inflammation, with a role for TRPV1 receptors. In addition, several other mediators located in the sensory nerves and skin also seem to contribute to this process.