Robin L. Thurmond

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

Histamine has a key role in allergic inflammatory conditions. The inflammatory responses resulting from the liberation of histamine have long been thought to be mediated by the histamine H1 receptor, and H1-receptor antagonists — commonly known as antihistamines — have been used to treat allergies for many years. However, the importance of histamine in the pathology of conditions such as asthma and chronic pruritus may have been underestimated. Here, we review accumulating evidence suggesting that histamine indeed has roles in inflammation and immune function modulation in such diseases. In particular, the discovery of a fourth histamine receptor (H4) and its expression on numerous immune and inflammatory cells has prompted a re-evaluation of the actions of histamine, suggesting a new potential for H4-receptor antagonists and a possible synergy between H1 and H4-receptor antagonists in targeting various inflammatory conditions.

Histamine H4 receptor mediates eosinophil chemotaxis with cell shape change and adhesion molecule upregulation

During mast cell degranulation, histamine is released in large quantities. Human eosinophils were found to express histamine H4 but not H3 receptors. The possible effects of histamine on eosinophils and the receptor mediating these effects were investigated in our studies. Histamine (0.01–30 μM) induced a rapid and transient cell shape change in human eosinophils, but had no effects on neutrophils. The maximal shape change was at 0.3 μM histamine with EC50 at 19 nM. After 60 min incubation with 1 μM histamine, eosinophils were desensitized and were refractory to shape change response upon histamine restimulation. Histamine (0.01–1 μM) also enhanced the eosinophil shape change induced by other chemokines. Histamine‐induced eosinophil shape change was mediated by the H4 receptor. This effect was completely inhibited by H4 receptor‐specific antagonist JNJ 7777120 (IC50 0.3 μM) and H3/H4 receptor antagonist thioperamide (IC50 1.4 μM), but not by selective H1, H2 or H3 receptor antagonists. H4 receptor agonists imetit (EC50 25 nM) and clobenpropit (EC50 72 nM) could mimic histamine effect in inducing eosinophil shape change. Histamine (0.01–100 μM) induced upregulation of adhesion molecules CD11b/CD18 (Mac‐1) and CD54 (ICAM‐1) on eosinophils. This effect was mediated by the H4 receptor and could be blocked by H4 receptor antagonists JNJ 7777120 and thioperamide. Histamine (0.01–10 μM) induced eosinophil chemotaxis with an EC50 of 83 nM. This effect was mediated by the H4 receptor and could be blocked by H4 receptor antagonists JNJ 7777120 (IC50 86 nM) and thioperamide (IC50 519 nM). Histamine (0.5 μM) also enhanced the eosinophil shape change induced by other chemokines. In conclusion, we have demonstrated a new mechanism of eosinophil recruitment driven by mast cells via the release of histamine. Using specific histamine receptor ligands, we have provided a definitive proof that the H4 receptor mediates eosinophil chemotaxis, cell shape change and upregulation of adhesion molecules. The effect of H4 receptor antagonists in blocking eosinophil infiltration could be valuable for the treatment of allergic diseases. The histamine‐induced shape change and upregulation of adhesion molecules on eosinophils can serve as biomarkers for clinical studies of H4 receptor antagonists.

The Histamine H4 Receptor Mediates Allergic Airway Inflammation by Regulating the Activation of CD4+ T Cells

Histamine is an important inflammatory mediator that is released in airways during an asthmatic response. However, current antihistamine drugs are not effective in controlling the disease. The discovery of the histamine H4 receptor (H4R) prompted us to reinvestigate the role of histamine in pulmonary allergic responses. H4R-deficient mice and mice treated with H4R antagonists exhibited decreased allergic lung inflammation, with decreases in infiltrating lung eosinophils and lymphocytes and decreases in Th2 responses. Ex vivo restimulation of T cells showed decreases in IL-4, IL-5, IL-13, IL-6, and IL-17 levels, suggesting that T cell functions were disrupted. In vitro studies indicated that blockade of the H4R on dendritic cells leads to decreases in cytokine and chemokine production and limits their ability to induce Th2 responses in T cells. This work suggests that the H4R can modulate allergic responses via its influence on T cell activation. The study expands the known influences of histamine on the immune system and highlights the therapeutic potential of H4R antagonists in allergic conditions.

RHAMM, a receptor for hyaluronan-mediated motility, compensates for CD44 in inflamed CD44-knockout mice: A different interpretation of redundancy

We report here that joint inflammation in collagen-induced arthritis is more aggravated in CD44-knockout mice than in WT mice, and we provide evidence for molecular redundancy as a causal factor. Furthermore, we show that under the inflammatory cascade, RHAMM (receptor for hyaluronan-mediated motility), a hyaluronan receptor distinct from CD44, compensates for the loss of CD44 in binding hyaluronic acid, supporting cell migration, up-regulating genes involved with inflammation (as assessed by microarrays containing 13,000 cDNA clones), and exacerbating collagen-induced arthritis. Interestingly, we further found that the compensation for loss of the CD44 gene does not occur because of enhanced expression of the redundant gene (RHAMM), but rather because the loss of CD44 allows increased accumulation of the hyaluronic acid substrate, with which both CD44 and RHAMM engage, thus enabling augmented signaling through RHAMM. This model enlightens several aspects of molecular redundancy, which is widely discussed in many scientific circles, but the processes are still ill defined.

The Histamine H4 Receptor Mediates Inflammation and Pruritus in Th2-Dependent Dermal Inflammation

The role of histamine H(4) receptor (H(4)R) was investigated in a T-helper type 2 (Th2)-cell-mediated mouse skin inflammation model that mimics several of the features of atopic dermatitis. Treatment with two specific H(4)R antagonists before challenge with FITC led to a significant reduction in ear edema, inflammation, mast cell, and eosinophil infiltration. This was accompanied by a reduction in the levels of several cytokines and chemokines in the ear tissue. Upon ex vivo antigen stimulation of lymph nodes, H(4)R antagonism reduced lymphocyte proliferation and IL-4, IL-5, and IL-17 levels. One explanation for this finding is that lymph nodes from animals dosed with the H(4)R antagonist, JNJ 7777120, contained a lower number of FITC-positive dendritic cells. The effect of H(4)R antagonism on dendritic cell migration in vivo may be an indirect result of the reduction in tissue cytokines and chemokines or a direct effect on chemotaxis. In addition to anti-inflammatory effects, JNJ 7777120 also significantly inhibited the pruritus shown in the model. Therefore, the dual effects of H(4)R antagonists on pruritus and Th2-cell-mediated inflammation point to their therapeutic potential for the treatment of Th2-mediated skin disorders, including atopic dermatitis.

Histamine H4 receptor antagonism diminishes existing airway inflammation and dysfunction via modulation of Th2 cytokines

BackgroundAirway remodeling and dysfunction are characteristic features of asthma thought to be caused by aberrant production of Th2 cytokines. Histamine H4 receptor (H4R) perturbation has previously been shown to modify acute inflammation and Th2 cytokine production in a murine model of asthma. We examined the ability of H4R antagonists to therapeutically modify the effects of Th2 cytokine production such as goblet cell hyperplasia (GCH), and collagen deposition in a sub-chronic model of asthma. In addition, effects on Th2 mediated lung dysfunction were also determined.MethodsMice were sensitized to ovalbumin (OVA) followed by repeated airway challenge with OVA. After inflammation was established mice were dosed with the H4R antagonist, JNJ 7777120, or anti-IL-13 antibody for comparison. Airway hyperreactivity (AHR) was measured, lungs lavaged and tissues collected for analysis.ResultsTherapeutic H4R antagonism inhibited T cell infiltration in to the lung and decreased Th2 cytokines IL-13 and IL-5. IL-13 dependent remodeling parameters such as GCH and lung collagen were reduced. Intervention with H4R antagonist also improved measures of central and peripheral airway dysfunction.ConclusionsThese data demonstrate that therapeutic H4R antagonism can significantly ameliorate allergen induced, Th2 cytokine driven pathologies such as lung remodeling and airway dysfunction. The ability of H4R antagonists to affect these key manifestations of asthma suggests their potential as novel human therapeutics.

Cutting Edge: Histamine Receptor H4 Activation Positively Regulates In Vivo IL-4 and IFN-γ Production by Invariant NKT Cells

Histamine (HA) is a biogenic amine with multiple activities in the immune system. In this study we demonstrate that histamine-free histidine decarboxylase-deficient (HDC−/−) mice present a numerical and functional deficit in invariant NK T (iNKT) cells as evidenced by a drastic decrease of IL-4 and IFN-γ production. This deficiency was established both by measuring cytokine levels in the serum and intracellularly among gated iNKT cells. It resulted from the lack of HA, because a single injection of this amine into HDC−/− mice sufficed to restore normal IL-4 and IFN-γ production. HA-induced functional recovery was mediated mainly through the H4 histamine receptor (H4R), as assessed by its abrogation after a single injection of a selective H4R antagonist and the demonstration of a similar iNKT cell deficit in H4R−/− mice. Our findings identify a novel function of HA through its H4R and suggest that it might become instrumental in modulating iNKT cell functions.

Molecular Determinants of Ligand Binding to H4R Species Variants

The histamine H4 receptor (H4R) is the latest identified histamine receptor to emerge as a potential drug target for inflammatory diseases. Animal models are employed to validate this potential drug target. Concomitantly, various H4R orthologs have been cloned, including the human, mouse, rat, guinea pig, monkey, pig, and dog H4Rs. In this article, we expressed all these H4R orthologs in human embryonic kidney 293T cells and compared their interactions with currently used standard H4R ligands, including the H4R agonists histamine, 4-methylhistamine, guanidinylethyl isothiourea (VUF 8430), the H4R antagonists 1-[(5-chloro-1H-indol-2-yl)carbonyl]-4-methylpiperazine (JNJ 7777120) and [(5-chloro-1H-benzimidazol-2-yl)carbonyl]-4-methylpiperazine (VUF 6002), and the inverse H4R agonist thioperamide. Most of the evaluated ligands display significantly different affinities at the different H4R orthologs. These “natural mutants” of H4R were used to study ligand-receptor interactions by using chimeric human-pig-human and pig-human-pig H4R proteins and site-directed mutagenesis. Our results are a useful reference for ligand selection for studies in animal models of diseases and offer new insights in the understanding of H4R-ligand receptor interactions.

The histamine H4 receptor in autoimmune disease

Histamine exerts its actions through four known receptors. The recently cloned histamine receptor, H4R, has been shown to have a role in chemotaxis and mediator release in various types of immune cells including mast cells, eosinophils, dendritic cells and T cells. H4R antagonists have been shown to have anti-inflammatory properties and efficacy in a number of disease models, such as those for asthma and colitis in vivo. Recently, H4R antagonists have been developed with high receptor affinity and specificity, which make them good tools for further characterisation of the receptor in animal models and, eventually, in humans. Histamine and the cells that produce it, such as mast cells and basophils, have long been thought to be involved in allergic conditions but there has recently been recognition that they may also play a role in various autoimmune diseases. Given this and the fact that the H4R has function in mast cells, dendritic cells and T cells, antagonists for the receptor may be useful in treating autoimmune diseases in addition to allergy.

Histamine H4 Receptor Optimizes T Regulatory Cell Frequency and Facilitates Anti-Inflammatory Responses within the Central Nervous System

Histamine is a biogenic amine that mediates multiple physiological processes, including immunomodulatory effects in allergic and inflammatory reactions, and also plays a key regulatory role in experimental allergic encephalomyelitis, the autoimmune model of multiple sclerosis. The pleiotropic effects of histamine are mediated by four G protein-coupled receptors, as follows: Hrh1/H1R, Hrh2/H2R, Hrh3/H3R, and Hrh4/H4R. H4R expression is primarily restricted to hematopoietic cells, and its role in autoimmune inflammatory demyelinating disease of the CNS has not been studied. In this study, we show that, compared with wild-type mice, animals with a disrupted Hrh4 (H4RKO) develop more severe myelin oligodendrocyte glycoprotein (MOG)35\x{2013}55-induced experimental allergic encephalomyelitis. Mechanistically, we also show that H4R plays a role in determining the frequency of T regulatory (TR) cells in secondary lymphoid tissues, and regulates TR cell chemotaxis and suppressor activity. Moreover, the lack of H4R leads to an impairment of an anti-inflammatory response due to fewer TR cells in the CNS during the acute phase of the disease and an increase in the proportion of Th17 cells.