Expression of histamine receptors H2R and H4R are predominantly regulated via the IL-4/IL-13 receptor type II on human M2 macrophages

Novel insights on the biology and immunologic effects of histamine: A road map for allergists and mast cell biologists

Histamine (C5H9N3, molecular weight 111.15 g/mol) is a well-studied endogenous biogenic amine composed of an imidazole ring attached to an ethylamine side chain. It has a limited half-life of a few minutes within tissues and in circulation. Several cell types including mast cells (MCs), basophils, platelets, histaminergic neurons, and enterochromaffin cells produce varying amounts of histamine using histidine decarboxylase. However, only MCs and basophils have complex mechanisms to pack and store histamine in granules along with other mediators using serglycin and its carried glycosaminoglycan side chains. Relatively low granule pH (∼5.5) supports the binding of stored histamine to heparin, whereas exposure to neutral pH after degranulation weakens the binding and histamine becomes liberated. Histamine exerts multifaceted regulatory biofunctions by engaging its 4 types of heptahelical G protein-coupled receptors (H1R-H4R), which have different expression profiles and functions. MCs express H1R, H2R, and H4R, which gives them a dual role in histamine biology as producers and responsive target cells. Histamine plays a role in a variety of physiologic and pathologic processes such as cell proliferation, differentiation, hematopoiesis, vascular permeability, embryogenesis, tissue regeneration, and wound healing. The emergence of histamine receptor-deficient mouse models and the development of multiple histamine receptor agonists and antagonists have helped researchers better understand these physiologic and pathogenic functions of histamine. We review the biology of histamine with a focus on immunologic aspects and the role of histamine in allergy and MC biology.

Cryo-EM Structures and AlphaFold3 Models of Histamine Receptors Reveal Diverse Ligand Binding and G Protein Bias

Background: The four subtypes of G protein-coupled receptors (GPCRs) regulated by histamine play critical roles in various physiological and pathological processes, such as allergy, gastric acid secretion, cognitive and sleep disorders, and inflammation. Previous experimental structures of histamine receptors (HRs) with agonists and antagonists exhibited multiple conformations for the ligands and G protein binding. However, the structural basis for HR regulation and signaling remains elusive. Methods: We determined the cryo-electron microscopy (cryo-EM) structure of the H4R-histamine-Gi complex at 2.9 Å resolution, and predicted the models for all four HRs in the ligand-free apo and G protein subtype binding states using AlphaFold3 (AF3). Results: By comparing our H4R structure with the experimental HR structures and the computational AF3 models, we elucidated the distinct histamine binding modes and G protein interfaces, and proposed the essential roles of Y6.51 and Q7.42 in receptor activation and the intracellular loop 2 (ICL2) in G protein bias. Conclusions: Our findings deciphered the molecular mechanisms underlying the regulation of different HRs, from the extracellular ligand-binding pockets and transmembrane motifs to the intracellular G protein coupling interfaces. These insights are expected to facilitate selective drug discovery targeting HRs for diverse therapeutic purposes.

Opportunities and challenges in the therapeutic exploitation of histamine and histamine receptor pharmacology in inflammation-driven disorders

Inflammation-driven diseases encompass a wide array of pathological conditions characterised by immune system dysregulation leading to tissue damage and dysfunction. Among the myriad of mediators involved in the regulation of inflammation, histamine has emerged as a key modulatory player. Histamine elicits its actions through four rhodopsin-like G-protein-coupled receptors (GPCRs), named chronologically in order of discovery as histamine H1, H2, H3 and H4 receptors (H1-4R). The relatively low affinity H1R and H2R play pivotal roles in mediating allergic inflammation and gastric acid secretion, respectively, whereas the high affinity H3R and H4R are primarily linked to neurotransmission and immunomodulation, respectively. Importantly, however, besides the H4R, both H1R and H2R are also crucial in driving immune responses, the H2R tending to promote yet ill-defined and unexploited suppressive, protective and/or resolving processes. The modulatory action of histamine via its receptors on inflammatory cells is described in detail. The potential therapeutic value of the most recently discovered H4R in inflammatory disorders is illustrated via a selection of preclinical models. The clinical trials with antagonists of this receptor are discussed and possible reasons for their lack of success described.

Histamine and Th2 cytokines independently and synergistically upregulate MMP12 expression in human M2 macrophages

Beyond Th2 cells and various immune cells, M2 macrophages have been identified in lesional skin of atopic dermatitis (AD) indicating their involvement in the disease's underlying mechanisms. MMP12, a matrix-degrading enzyme, which is predominantly produced by macrophages, is increased in skin lesions of AD patients. In this study we investigated the expression of MMP12 mRNA in lesional AD skin at single cell level through RNA sequencing (scRNA-seq) and the expression of MMP12 in M2 macrophages from healthy individuals and AD patients in response to Th2 cytokines and histamine using quantitative PCR and ELISA. Additionally, we analyzed macrophages from dupilumab-treated AD patients using the same methods to assess the influence of Th2 cytokines on MMP12 expression ex-vivo. ScRNA-seq identified macrophages as the primary producers of MMP12 in lesional AD skin. In-vitro, both MMP12 mRNA and protein expression were significantly increased in monocytes during differentiation to M2 macrophages in the presence of histamine, of Th2 cytokines or of Th2 cytokines in combination with histamine. In M2 macrophages obtained from dupilumab-treated AD patients, the upregulation of MMP12 expression by IL-4 and IL-13 was attenuated. Our findings unveil a novel mechanism whereby Th2 cytokines and histamine regulate MMP12 expression, potentially impacting skin barrier homeostasis in AD.

The stimulation of TH2 cells results in increased IL-5 and IL-13 production via the H4 receptor

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory skin disease resulting in decreased quality of life. Histamine and specifically the H4 receptor play a key role in the inflammatory process in AD and serve as targets for novel therapeutic approaches.

OBJECTIVE

In the present study we aimed to elucidate the immunopathological mechanisms with which the H4 receptor impacts TH2 cells and contributes to AD pathophysiology.

METHODS

Total CD4+ T cells obtained from healthy or AD individuals and in vitro differentiated TH2 cells were cultured under different conditions and the mRNA expression or protein production of target molecules were determined using quantitative real-time PCR and ELISA.

RESULTS

H4 receptor mRNA expression was upregulated concentration dependent upon IL-4 stimulation in in vitro differentiated TH2 cells progressively during the differentiation. Transcriptomic analysis of in vitro differentiated TH2 versus TH1 cells revealed that the H4 receptor among other genes represents one of the highly upregulated genes in TH2 cells. Most importantly, increased amounts of IL-5 and IL-13 mRNA expression were detected in in vitro differentiated TH2 cells as well as protein secretion in the presence of histamine or of the H4 receptor-selective-agonist when compared to the untreated control.

CONCLUSION

We show for the first time an H4 receptor dependent upregulation of the pro-inflammatory cytokines IL-5 and IL-13 in human TH2 cells by histamine. This suggests that the blockade of the H4 receptor may lead to downregulation of these cytokines and amelioration of AD symptoms as reported in first clinical studies.

The injectable hydrogel loading cannabidiol to regulate macrophage polarization in vitro for the treatment of chronic enteritis

OBJECTIVE

Chronic bowel disease has the characteristics of high recurrence rate, prolonged and non-healing, and the incidence has increased year by year in recent years. Cannabidiol (CBD) has significant anti-inflammatory and antioxidant activities, but it is limited by its characteristics of fat solubility and low bioavailability. This study aims to treat chronic inflammatory bowel disease by preparing a CBD-loaded hydrogel system (GelMA + CBD) that can deliver CBD in situ and improve its bioavailability through slow release.

METHOD

The study designed and constructed GelMA + CBD, and its surface morphology was observed by scanning electron microscopy, and its pore size, swelling rate and release rate were evaluated to evaluate its bioactivity and biosafety. The expression of various inflammatory factors was detected by ELISA, and the expression of protein and reactive oxygen species were observed by laser confocal microscopy to evaluate their anti-inflammatory and antioxidant properties.

RESULTS

Our study found that GelMA + CBD with biosafety, could make CBD be slowly released, and effectively inhibit the M1-type polarization of macrophages in vitro, and promote the M2-type polarization. In addition, GelMA + CBD can also reduce the expression of pro-inflammatory factors (such as iNOS) in macrophages, and increase the expression of anti-inflammatory factors (such as Arg-1), clear intracellular reactive oxygen species (ROS), and relieve oxidative stress.

CONCLUSION

The vitro experiments have confirmed that the CBD-loaded hydrogel system has good biosafety, and can alleviate inflammation by regulating the polarization direction of macrophages, and then inhibiting the secretion of pro-inflammatory factors, laying a strong foundation for the treatment of chronic enteritis.

Metabolite itaconate in host immunoregulation and defense

Metabolic states greatly influence functioning and differentiation of immune cells. Regulating the metabolism of immune cells can effectively modulate the host immune response. Itaconate, an intermediate metabolite derived from the tricarboxylic acid (TCA) cycle of immune cells, is produced through the decarboxylation of cis-aconitate by cis-aconitate decarboxylase in the mitochondria. The gene encoding cis-aconitate decarboxylase is known as immune response gene 1 (IRG1). In response to external proinflammatory stimulation, macrophages exhibit high IRG1 expression. IRG1/itaconate inhibits succinate dehydrogenase activity, thus influencing the metabolic status of macrophages. Therefore, itaconate serves as a link between macrophage metabolism, oxidative stress, and immune response, ultimately regulating macrophage function. Studies have demonstrated that itaconate acts on various signaling pathways, including Keap1-nuclear factor E2-related factor 2-ARE pathways, ATF3-IκBζ axis, and the stimulator of interferon genes (STING) pathway to exert antiinflammatory and antioxidant effects. Furthermore, several studies have reported that itaconate affects cancer occurrence and development through diverse signaling pathways. In this paper, we provide a comprehensive review of the role IRG1/itaconate and its derivatives in the regulation of macrophage metabolism and functions. By furthering our understanding of itaconate, we intend to shed light on its potential for treating inflammatory diseases and offer new insights in this field.

Advances in macrophage-targeting nanoparticles for the diagnosis and treatment of inflammatory bowel disease

The pathogenesis of inflammatory bowel disease (IBD) is not fully elucidated. However, it has been considered that inflammatory macrophages may be involved in the imbalance of the intestinal mucosal immunity to regulate several signaling pathways, leading to IBD progression. The ratio of M1 to M2 subtypes of activated macrophages tends to increase in the inflamed intestinal section. There are challenges in the diagnosis and treatment of IBD, such as unsatisfactory specificity of imaging findings, low drug accumulation in the intestinal lesions, unstable therapeutic efficacy, and drug-related systemic toxicity. Recently developed nanoparticles may provide a new approach for the diagnosis and treatment of IBD. Nanoparticles targeted to macrophages can be used as contrast agents to improve the imaging quality or used as a drug delivery vector to increase the therapeutic efficiency of IBD. This article reviews the research progress on macrophage-targeting nanoparticles for the diagnosis and treatment of IBD to provide a reference for further research and clinical application.

Basic mechanisms of itch

Pruritus (or itch) is an unpleasant sensation leading to a desire to scratch. In the epidermis, there are selective C or Aδ epidermal nerve endings that are pruriceptors. At their other ends, peripheral neurons form synapses with spinal neurons and interneurons. Many areas in the central nervous system are involved in itch processing. Although itch does not occur solely because of parasitic, allergic, or immunologic diseases, it is usually the consequence of neuroimmune interactions. Histamine is involved in a minority of itchy conditions, and many other mediators play a role: cytokines (eg, IL-4, IL-13, IL-31, IL-33, and thymic stromal lymphopoietin), neurotransmitters (eg, substance P, calcitonin gene-related peptide, vasoactive intestinal peptide, neuropeptide Y, NBNP, endothelin 1, and gastrin-releasing peptide), and neurotrophins (eg, nerve growth factor and brain-derived neurotrophic factor). Moreover, ion channels such as voltage-gated sodium channels, transient receptor potential vanilloid 1, transient receptor ankyrin, and transient receptor potential cation channel subfamily M (melastatin) member 8 play a crucial role. The main markers of nonhistaminergic pruriceptors are PAR-2 and MrgprX2. A notable phenomenon is the sensitization to pruritus, in which regardless of the initial cause of pruritus, there is an increased responsiveness of peripheral and central pruriceptive neurons to their normal or subthreshold afferent input in the context of chronic itch.

Nanodelivery of Histamine H3/H4 Receptor Modulators BF-2649 and Clobenpropit with Antibodies to Amyloid Beta Peptide in Combination with Alpha Synuclein Reduces Brain Pathology in Parkinson's Disease

Parkinson's disease (PD) in military personnel engaged in combat operations is likely to develop in their later lives. In order to enhance the quality of lives of PD patients, exploration of novel therapy based on new research strategies is highly warranted. The hallmarks of PD include increased alpha synuclein (ASNC) and phosphorylated tau (p-tau) in the cerebrospinal fluid (CSF) leading to brain pathology. In addition, there are evidences showing increased histaminergic nerve fibers in substantia niagra pars compacta (SNpc), striatum (STr), and caudate putamen (CP) associated with upregulation of histamine H3 receptors and downregulation of H4 receptors in human brain. Previous studies from our group showed that modulation of potent histaminergic H3 receptor inverse agonist BF-2549 or clobenpropit (CLBPT) partial histamine H4 agonist with H3 receptor antagonist induces neuroprotection in PD brain pathology. Recent studies show that PD also enhances amyloid beta peptide (AβP) depositions in brain. Keeping these views in consideration in this review, nanowired delivery of monoclonal antibodies to AβP together with ASNC and H3/H4 modulator drugs on PD brain pathology is discussed based on our own observations. Our investigation shows that TiO₂ nanowired BF-2649 (1 mg/kg, i.p.) or CLBPT (1 mg/kg, i.p.) once daily for 1 week together with nanowired delivery of monoclonal antibodies (mAb) to AβP and ASNC induced superior neuroprotection in PD-induced brain pathology. These observations are the first to show the modulation of histaminergic receptors together with antibodies to AβP and ASNC induces superior neuroprotection in PD. These observations open new avenues for the development of novel drug therapies for clinical strategies in PD.

Neuro-immune communication regulating pruritus in atopic dermatitis

Atopic dermatitis (AD) is a common, chronic-relapsing inflammatory skin disease with significant disease burden. Genetic and environmental trigger factors contribute to AD, activating two of our largest organs, the nervous and immune system. Dysregulation of neuro-immune circuits plays a key role in the pathophysiology of AD causing inflammation, pruritus, pain, and barrier dysfunction. Sensory nerves can be activated by environmental or endogenous trigger factors transmitting itch stimuli to the brain. Upon stimulation, sensory nerve endings also release neuromediators into the skin contributing again to inflammation, barrier dysfunction and itch. Additionally, dysfunctional peripheral and central neuronal structures contribute to neuroinflammation, sensitization, nerve elongation, neuropathic itch, thus chronification and therapy-resistance. Consequently, neuro-immune circuits in skin and central nervous system may be targets to treat pruritus in AD. Cytokines, chemokines, proteases, lipids, opioids, ions excite/sensitize sensory nerve endings not only induce itch but further aggravate/perpetuate inflammation, skin barrier disruption, and pruritus. Thus, targeted therapies for neuro-immune circuits as well as pathway inhibitors (e.g., kinase inhibitors) may be beneficial to control pruritus in AD either in systemic and/or topical form. Understanding neuro-immune circuits and neuronal signaling will optimize our approach to control all pathological mechanisms in AD, inflammation, barrier dysfunction and pruritus.

Histamine Increases Th2 Cytokine-Induced CCL18 Expression in Human M2 Macrophages

The chemokine CCL18 is produced in cells of the myelomonocytic lineage and represents one of the most highly expressed chemokines in lesional skin and serum of atopic dermatitis patients. We investigated the role of histamine in CCL18 production in human monocyte-derived M2 macrophages differentiated in the presence of M-CSF and activated with IL-4, IL-13 or with IL-10. Since expression and regulation of histamine H1 receptor (H1R), H2R and H4R by IL-4 and IL-13 on human M2 macrophages were described, we analyzed expression of the histamine receptors in response to IL-10 stimulation by quantitative RT-PCR. IL-10 upregulated H2R and downregulated H4R mRNA expression by trend in M2 macrophages. IL-10, but in a more pronounced manner, IL-4 and IL-13, also upregulated CCL18. Histamine increased the cytokine-induced upregulation of CCL18 mRNA expression by stimulating the H2R. This effect was stronger in IL-10-stimulated M2 macrophages where the upregulation of CCL18 was confirmed at the protein level by ELISA using selective histamine receptor agonist and antagonists. The histamine-induced CCL18 upregulation in IL-10-activated M2 macrophages was almost similar in cells obtained from atopic dermatitis patients compared to cells from healthy control persons. In summary, our data stress a new function of histamine showing upregulation of the Th2 cells attracting chemokine CCL18 in human, activated M2 macrophages. This may have an impact on the course of atopic dermatitis and for the development of new therapeutic interventions.