Histamine H1- and H4-receptor expression in human colon-derived cell lines.

In previous studies, we demonstrated the involvement of H4R in inflammatory bowel disease (IBD) and IBD-associated colon cancer in mice and could ascribe H4R-mediated histamine function to colon epithelial cells. The transferability of obtained data to humans is however lacking. Functional expression of H4R on colon epithelial cells is a prerequisite to pursue the hypothesis of involvement of H4R in carcinogenesis. Thus, we here compared the expression of histamine receptor subtypes in a series of cell lines. Out of these, three colon-derived cell lines displaying different combinations of H1R and H4R expression were submitted to functional analyses. Human hematopoietic HMC-1, HL-60, and U937, lung-derived A549 and Calu-3, and colorectal LoVo, SW 480, Caco-2, HT-29, and HCT116 cells were included in the study. mRNA expression was quantified by RT-qPCR. For functional analyses, Caco-2, HT-29, and HCT116 cells were treated by incubation with 1 - 10 µM histamine in the presence or absence of selective histamine receptor antagonists. Calcium mobilization, cAMP accumulation, and cell proliferation were measured by fluorimetry, mass spectrometry, and real-time bioimpedance measurements, respectively. Histamine receptor expression was heterogeneous in the cell lines tested. In most cell lines, we detected H1R mRNA while H4R mRNAs were found only occasionally. The colon-derived epithelial cell lines LoVo, SW480, and HT-29 expressed H1R mRNA exclusively, while in HCT116 cells H1R and H4R mRNAs and in CaCo-2 H2R mRNA were detectable. Subsequent functional analyses in HT29, Caco-2, and HCT116 cells, however, indicated that only HT-29 responded to histamine stimulation, by means of H1R. For a detailed analysis of histamine receptor function, esp. that of H1R and H4R, in human colon-derived cell lines, the cell lines tested here are not fully convenient unless genetically modified.

The Function of the Histamine H4 Receptor in Inflammatory and Inflammation-Associated Diseases of the Gut.

Histamine is a pleiotropic mediator involved in a broad spectrum of (patho)-physiological processes, one of which is the regulation of inflammation. Compounds acting on three out of the four known histamine receptors are approved for clinical use. These approved compounds comprise histamine H1-receptor (H1R) antagonists, which are used to control allergic inflammation, antagonists at H2R, which therapeutically decrease gastric acid release, and an antagonist at H3R, which is indicated to treat narcolepsy. Ligands at H4R are still being tested pre-clinically and in clinical trials of inflammatory diseases, including rheumatoid arthritis, asthma, dermatitis, and psoriasis. These trials, however, documented only moderate beneficial effects of H4R ligands so far. Nevertheless, pre-clinically, H4R still is subject of ongoing research, analyzing various inflammatory, allergic, and autoimmune diseases. During inflammatory reactions in gut tissues, histamine concentrations rise in affected areas, indicating its possible biological effect. Indeed, in histamine-deficient mice experimentally induced inflammation of the gut is reduced in comparison to that in histamine-competent mice. However, antagonists at H1R, H2R, and H3R do not provide an effect on inflammation, supporting the idea that H4R is responsible for the histamine effects. In the present review, we discuss the involvement of histamine and H4R in inflammatory and inflammation-associated diseases of the gut.

Genetic Deficiency of the Histamine H4-Receptor Reduces Experimental Colorectal Carcinogenesis in Mice.

Colorectal cancer (CRC), a severe complication of inflammatory bowel diseases, is a common type of cancer and accounts for high mortality. CRC can be modeled in mice by application of the tumor promoter, azoxymethane (AOM), in combination with dextran sodium sulfate (DSS), which are able to induce colitis-like manifestations. Active colitis correlates with high mucosal concentrations of histamine, which, together with the histamine receptor subtype 4 (H4R), provide a pro-inflammatory function in a mouse colitis model. Here, we analyzed whether H4R is involved in the pathogenesis of AOM/DSS-induced CRC in mice. As compared to wild type (WT) mice, AOM/DSS-treated mice lacking H4R expression (TM) demonstrate ameliorated signs of CRC, i.e., significantly reduced loss of body weight, stiffer stool consistency, and less severe perianal bleeding. Importantly, numbers and diameters of tumors and the degree of colonic inflammation are dramatically reduced in TM mice as compared to WT mice. This is concomitant with a reduced colonic inflammatory response involving expression of cyclooxygenase 2 and the production of C-X-C motif chemokine ligand 1 (CXCL1) and CXCL2. We conclude that H4R is involved in the tumorigenesis of chemically-induced CRC in mice via cyclooxygenase 2 expression and, probably, CXCL1 and CXCL2 as effector molecules.

Interleukin-4 administration improves muscle function, adult myogenesis, and lifespan of colon carcinoma-bearing mice.

BACKGROUND: Anorexia, body wasting, inflammation, muscle, and adipose tissue loss are hallmarks of cancer cachexia, a syndrome that affects the majority of cancer patients, impairing their ability to endure chemotherapeutic therapies and reducing their lifespan. In the last 10 years, alterations of protein turnover and impairment of adult myogenesis have been proposed as major contributing factors. METHODS: Muscle stem cells, including satellite cells, mesoangioblasts, and fibroadipogenic progenitors, were isolated and characterized from C26 colon carcinoma-bearing (C26) mice. Circulating levels of interleukin-4/13 (IL4/IL13) were analysed by ELISA, and the effects of IL4 on muscle mass and function, protein synthesis, muscle regeneration, and myogenic progenitor cell number were analysed at both functional (treadmill and grip test) and molecular levels (qRT-PCR, immunofluorescence analysis, surface sensing of translation, and western blot). The Kaplan-Meier test was used to analyse the survival curve of IL4-treated and IL4-untreated C26 mice. RESULTS: The administration of IL4 to C26 mice rescued muscle mass by increasing protein synthesis. The IL4 treatment improved performances and prolonged survival of C26 mice. IL4 administration re-established both number and function of satellite cells and fibroadipogenic progenitors without affecting mesoangioblasts in C26 mice, rescuing myogenesis. Upon IL4 treatment, a high number of cytotoxic lymphocytes and type II macrophages were observed with a subsequent increase in necrotic areas of C26 tumours. CONCLUSIONS: The results here presented shed new light on IL4 signalling during muscle wasting and early stages of muscle regeneration that explain the beneficial effect observed in IL4-treated C26 mice. These findings might aid to develop therapeutic approaches to improve mobility and quality of life in cachectic patients.

Mouse Colonic Epithelial Cells Functionally Express the Histamine H4 Receptor.

We hypothesized that, in mice, histamine via the histamine receptor subtype 4 (H4R) on colon epithelial cells affects epithelial barrier integrity, perturbing physiologic function of the colonic mucosa and thus aggravating the severity of colitis. To test this hypothesis, bone marrow-chimeric mice were generated from H4R knockout (H4R-/-) and wild-type (WT) BALB/cJ mice and subjected to the dextrane sodium sulfate (DSS)-induced acute colitis model. Clinical symptoms and pathohistological derangements were scored. Additionally, total RNA was extracted from either mouse whole-colon homogenates or primary cell preparations enriched for epithelial cells, and gene expression was analyzed by real-time quantitative polymerase chain reaction. The impact of the H4R on epithelial barrier function was assessed by measurement of transepithelial electrical resistence of organoid-derived two-dimensional monolayers from H4R-/- and WT mice using chopstick electrodes. Bone marrow-chimeric mice with genetic depletion of the H4R in nonhematopoietic cells exhibited less severe DSS-induced acute colitis symptoms compared with WT mice, indicating a functional proinflammatory expression of H4R in nonimmune cells of the colon. Analysis of H4R expression revealed the presence of H4R mRNA in colon epithelial cells. This expression could be confirmed and complemented by functional analyses in organoid-derived epithelial cell monolayers. Thus, we conclude that the H4R is functionally expressed in mouse colon epithelial cells, potentially modulating mucosal barrier integrity and intestinal inflammatory reactions, as was demonstrated in the DSS-induced colitis model, in which presence of the H4R on nonhematopoietic cells aggravated the inflammatory phenotype. SIGNIFICANCE STATEMENT: The histamine H4 receptor (H4R) is functionally expressed on mouse colon epithelial cells, thereby aggravating dextrane sodium sulfate-induced colitis in BALB/cJ mice. Histamine via the H4R reduces transepithelial electrical resistance of colon epithelial monolayers, indicating a function of H4R in regulation of epithelial barrier integrity.

In vivo Evidence for Partial Activation of Eosinophils via the Histamine H4-Receptor: Adoptive Transfer Experiments Using Eosinophils From H4R-/- and H4R+/+ Mice.

Our previous in vitro studies revealed that histamine via histamine the H4-receptors (H4R), as compared to other stimuli, such as eotaxin or formylpeptides, rather partially activates eosinophilic granulocytes (eosinophils). In order to evaluate the H4R-mediated activation of eosinophils in vivo, we employed dextran sodium sulfate (DSS)-induced colitis in mice, closely resembling human ulcerative colitis (UC), which is largely characterized by a local eosinophilic infiltration of the colon. IL-5-deficient BALB/c mice served as a model with reduced endogenous numbers of eosinophils, in which wild-type (H4R+/+) or H4R-deficient (H4R-/-) eosinophils were adoptively transferred during the course of DSS-induced colitis. During the 1-week observation period, transfer of eosinophils transiently reversed the acute clinical colitis-like phenotype (body weight loss, perianal bleeding, soft stool consistency) resulting from IL-5-deficiency. This reversion was significantly more pronounced upon transfer of eosinophils from H4R+/+ mice as compared to those from H4R-/- mice. Already at the end of the observation period, the clinical effects of the transfer of H4R+/+ and H4R-/- eosinophils became similar, as were the results of the histological examination of the cola and the analyses of cytokine production in cola and in re-stimulated lymph node cells performed at this time. Thus, analyzing clinical and pathological parameters representative of colitis in this model, we demonstrate that as well as in vitro, also in vivo histamine via the H4R only partially activates eosinophils.

Lack of Histamine H4-Receptor Expression Aggravates TNBS-Induced Acute Colitis Symptoms in Mice.

Inflammatory bowel diseases (IBD) are a growing health problem worldwide, severely affecting patients' life qualities and life expectancies. Therapeutic options, which are rare and focus on symptoms associated with the disease, suffer from increasing numbers of patients refractory to the established strategies. Thus, in order to generate new therapeutic regimens, the detailed understanding of the pathogenic mechanisms causing IBD is necessary. Histamine is an inflammatory mediator associated with IBD. Four histamine receptors are currently known of which the histamine H4-receptor (H4R) has been shown to possess a pro-inflammatory function in several experimental models of inflammatory diseases, including dextran sodium sulfate (DSS)-induced colitis in mice. No single model reflects the complexity of human IBD, but each model provides valuable information on specific aspects of IBD pathogenesis. While DSS-induced colitis mostly relies on innate immune mechanisms, trinitrobenzene sulfonic acid (TNBS)-induced colitis rather reflects T-cell mechanisms. Consequently, an observation made in a single model has to be verified in at least one other model. Therefore, in the present study we investigated the effect of genetic blockade of H4R-signaling in mice subjected to the model of TNBS-induced acute colitis. We analyzed severity and progression of clinical signs of colitis, as well as histopathologic alterations in the colon and local cytokine production. Genetic ablation of H4R expression worsened clinical signs of acute colitis and histological appearance of colon inflammation after TNBS application. Moreover, TNBS instillation enhanced local synthesis of inflammatory mediators associated with a neutrophilic response, i.e., CXCL1, CXCL2, and interleukin-6. Lastly, also myeloperoxidase concentration, indicative for the presence of neutrophils, was elevated in cola of TNBS-treated mice due to the absence of H4R expression. Our results indicate an anti-inflammatory role of histamine via H4R in TNBS-induced acute neutrophilic colitis in mice, thus questioning the strategy of pharmacological H4R blocked as new therapeutic option for patients suffering from IBD.

Role of the Histamine H4-Receptor in Bronchial Asthma.

Histamine is a pro-inflammatory mediator with a prominent role in allergic diseases. Antagonists at the histamine receptor subtype 1 are central in anti-allergic therapies, with the exception of allergic asthma, where they are clinically without effect. The latest identified histamine receptor subtype 4, which is expressed mainly in hematopoietic cells, now provides a reasonable target for new therapeutic strategies inhibiting histamine function. The pathophysiology of allergy, esp. allergic asthma, and in its context the effects of antagonists at the histamine receptor subtype 4 in preclinical and clinical settings are discussed in this chapter.

Histamine regulates murine primary dendritic cell functions.

OBJECTIVE AND DESIGN: The modulation of antigen uptake and activation of dendritic cells (DCs) by histamine may function as a regulator of inflammation. Therefore, we sought to determine the impact of histamine on antigen uptake by and activation of murine DCs. MATERIAL AND METHODS: DCs from spleen and lung were either identified by flow cytometry or were immunomagnetically enriched. Cells were stimulated with histamine, and the regulation of MHC-II and co-stimulatory molecule expression (CD80, CD86, and ICOS-L) and antigen uptake were quantified by flow cytometry. Individual contributions of the histamine receptor subtypes were determined by using the antagonists mepyramine (histamine H1-receptor: H1R), famotidine (H2R), and JNJ 7777120 (H4R). RESULTS: Histamine accelerated the uptake of soluble antigen via the H1R, H2R, and H4R in splenic DCs. Co-stimulatory molecule expression was enhanced already by enrichment procedures, thus, the analyses were performed in unseparated cell populations. Histamine enhanced the expression of CD86 and ICOS-L while expression of CD80 was unaffected. Antagonism at H1R, H2R, and H4R and at H1R and H4R reduced the histamine-induced enhanced expression of CD86 and ICOS-L, respectively. CONCLUSIONS: Histamine contributes to the regulation of the immunological synapse by stimulation of antigen uptake and activation of DCs via H1R, H2R, and H4R.

High constitutive Akt2 activity in U937 promonocytes: effective reduction of Akt2 phosphorylation by the histamine H2-receptor and the ß2-adrenergic receptor.

Histamine (HA) is approved for the treatment of acute myeloid leukemia (AML). Its antileukemic activity is related to histamine H2-receptor (H2R)-mediated inhibition of reactive oxygen species (ROS) production in myeloid cells facilitating survival of antineoplastic lymphocytes. The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway, which plays a crucial role in cell survival and proliferation, is constitutively activated in leukemic cells of most AML patients resulting in poor survival prognosis. In a proof-of-principle experiment using a human phosphorylated mitogen-activated protein kinase (MAPK) array, we found high phosphorylation levels of Akt2 in U937 promonocytes that was abrogated by HA or selective H2R agonists. The H2R and the ß2-adrenergic receptor (ß2AR) are Gs-protein-coupled receptors. Stimulation results in adenylyl cyclase activation followed by generation of the second messenger adenosine 3',5'-cyclic monophosphate (cAMP). In our present study, we evaluated the pharmacological profile of the H2R and the ß2AR regarding Akt2 phosphorylation at Ser474 via western blot analysis and ELISA and cAMP accumulation via HPLC-MS/MS in U937 promonocytes. H2R and ß2AR agonists concentration-dependently decreased Akt2 phosphorylation at Ser474. Deviations of potencies and efficacies of agonists in Akt2 phosphorylation and cAMP accumulation assays indicated participation of cAMP-independent signaling in GPCR-induced reduction of Akt2 phosphorylation. Accordingly, our study supports the concept of functional selectivity of the H2R and the ß2AR in U937 promonocytes. In summary, we extended the antileukemic mechanism of HA via H2R and revealed the potential of ß2AR agonists, which are already approved in the treatment of bronchial asthma and chronic obstructive pulmonary disease, as antileukemic drugs.

Deletion of IL-18 Expression Ameliorates Spontaneous Kidney Failure in MRLlpr Mice.

The role of IL-18 in the pathogenesis of systemic lupus erythematosus is still not definitively solved. In this study, we generated MRLlpr mice, which develop a disease resembling systemic lupus erythematosus, genetically devoid of IL-18 expression. These mice in comparison to IL-18-competent MRLlpr mice show reduced signs of renal pathogenesis, while other parameters such as mean survival time, lymphadenopathy, constitutive interferon-γ production, and frequency of CD3+B220+ abnormal T cells were without differences. We conclude that in the systemic lupus erythematosus syndrom IL-18 is involved specifically in the renal pathogenesis.

Proinflammatory role of the histamine H4 receptor in dextrane sodium sulfate-induced acute colitis.

Millions of people worldwide are suffering from inflammatory bowel disease (IBD), which severely affects patients' life qualities and even life expectancies. The cause of the ailment is unknown and a profound understanding of the underlying pathogenetic mechanisms is still lacking. The biogenic amine histamine is one of several inflammatory mediators, to which a pathogenetic role in IBD has been attributed. Out of the four known histamine receptors, the histamine H4 receptor (H4R) has been demonstrated to act proinflammatory in experimental models of several inflammatory diseases. In order to evaluate a potential involvement of H4R in IBD we investigated the effect of genetic or pharmacological blockade of H4R-signaling in the model of dextran sodium sulfate (DSS)-induced colitis in mice. We analysed severity and progression of clinical signs of colitis, as well as histopathologic alterations in the colons and systemic or local cytokine concentrations. Both genetic deficiency and pharmacological blockade of H4R with the selective antagonist JNJ7777120 improved clinical and histological signs of colitis and dampened the inflammatory cytokine response. Our results indicate a proinflammatory role of histamine via H4R in IBD, thus extending the current pathophysiological understanding of IBD and demonstrating the therapeutic potential of selective H4R-antagonists for patients suffering from IBD.

Histamine H(1)- and H(4)-receptor signaling cooperatively regulate MAPK activation.

The histamine (HA) receptor subtype 1 (H1R) and H4R are expressed on immune cells and contribute to an inflammatory reaction. Both receptor subtypes individually enhance the intracellular concentrations of calcium and regulate the accumulation of cAMP, increase MAPK activity, and regulate expression of e.g., inflammatory genes. In a previous study we characterized and compared signaling pathways of the murine orthologs of the H1R and the H4R recombinantly expressed at comparable levels in HEK 293 cells. In the present study, we aimed at analyzing possible interactions of the signaling pathways emerging at the mH1R and the mH4R. Therefore, we co-expressed both receptor subtypes at comparable levels in HEK 293 cells and investigated HA-induced signaling parameters such as the concentrations of intracellular calcium and cAMP, phosphorylation of the MAPKs p38, ERK 1, and ERK 2, and of the transcription factor CREB, and expression of the immediate early gene EGR-1. We demonstrate that the intracellular concentrations of calcium and cAMP as well as the EGR-1 expression are regulated exclusively via the mH1R. In contrast, both receptor subtypes H1R and H4R synergize in HA-induced MAPK activation. This synergism most probably relies on signaling pathways independent of the second messenger calcium and cAMP. In summary, we provide evidence that the mH1R inhibits or dampens the function of the co-expressed mH4R regarding specific parameters, while other signaling events are regulated cooperatively by the mH1R and the mH4R.

Flow cytometric analysis with a fluorescently labeled formyl peptide receptor ligand as a new method to study the pharmacological profile of the histamine H2 receptor.

The histamine H2 receptor (H2R) is a Gs protein-coupled receptor. Its activation leads to increases in the second messenger adenosine-3',5'-cyclic monophosphate (cAMP). Presently, several systems are established to characterize the pharmacological profile of the H2R, mostly requiring radioactive material, animal models, or human blood cells. This prompted us to establish a flow cytometric analysis with a fluorescently labeled formyl peptide receptor (FPR) ligand in order to investigate the H2R functionally and pharmacologically. First, we stimulated U937 promonocytes, which mature in a cAMP-dependent fashion upon H2R activation, with histamine (HA) or selective H2R agonists and measured increases in cAMP concentrations by mass spectrometry. Next, indicative for the maturation of U937 promonocytes, we assessed the FPR expression upon incubation with HA or H2R agonists. FPR expression was measured either indirectly by formyl peptide-induced changes in intracellular calcium concentrations ([Ca(2+)]i) or directly with the fluorescein-labeled FPR ligand fNleLFNleYK-Fl. HA and H2R agonists concentration-dependently induced FPR expression, and potencies and efficacies of fMLP-induced increases in [Ca(2+)]i and FPR density correlated linearly. Accordingly, flow cytometric analysis of FPR expression constitutes a simple, inexpensive, sensitive, and reliable method to characterize the H2R pharmacologically. Furthermore, we evaluated FPR expression at the mRNA level. Generally, quantitative real-time polymerase chain reaction confirmed functional data. Additionally, our study supports the concept of functional selectivity of the H2R, since we observed dissociations in the efficacies of HA and H2R agonists in cAMP accumulation and FPR expression.

Human neutrophils are activated by a peptide fragment of Clostridium difficile toxin B presumably via formyl peptide receptor.

Clostridium difficile may induce antibiotic-associated diarrhoea and, in severe cases, pseudomembranous colitis characterized by tremendous neutrophil infiltration. All symptoms are caused by two exotoxins: TcdA and TcdB. We describe here the activation of isolated human blood neutrophils by TcdB and, moreover, by toxin fragments generated by limited proteolytical digestion. Kinetics and profiles of TcdB-induced rise in intracellular-free Ca(2+) and reactive oxygen species production were similar to that induced by fMLF, which activates the formyl peptide receptor (FPR) recognizing formylated bacterial peptide sequences. Transfection assays with the FPR-1 isoform hFPR26 in HEK293 cells, heterologous desensitization experiments and FPR inhibition via cyclosporine H strongly suggest activation of cells via FPR-1. Domain analyses revealed that the N-terminal glucosyltransferase domain of TcdB is a potent activator of FPR pointing towards an additional mechanism that might contribute to pathogenesis. This pro-inflammatory ligand effect can be triggered even by cleaved and, thus, non-cytotoxic toxin. In summary, we report (i) a ligand effect on neutrophils as completely new molecular mode of action, (ii) pathogenic potential of truncated or proteolytically cleaved 'non-cytotoxic' fragments and (iii) an interaction of the N-terminal glucosyltransferase domain instead of the C-terminal receptor binding domain of TcdB with target cells.

The histamine H4 -receptor (H4 R) regulates eosinophilic inflammation in ovalbumin-induced experimental allergic asthma in mice.

Via the histamine H4 -receptor (H4 R), histamine promotes the pathogenesis of experimental allergic asthma in mice. Application of H4 R antagonists during sensitization as well as during provocation reduces the severity of the disease. However, the specific cell types functionally expressing H4 R in experimental allergic asthma have not been well characterized in vivo. In this study, we identified the cell type(s) responsible for H4 R activity in experimental asthma and related physiological mechanisms. Using H4 R-deficient mice, we studied the role of H4 R in the sensitization and effector phase. DCs lacking H4 R expression during the in vitro sensitization reaction resulted in effector T cells unable to induce an entire eosinophilic inflammation in the lung upon adoptive transfer in vivo. Recipient mice lacking H4 R expression, which were adoptively transferred with H4 R(+/+) T cells polarized in the presence of H4 R(+/+) DCs, showed reduced signs of inflammation and ameliorated lung function. Here, we provide in vivo evidence that in experimental asthma in mice the H4 R specifically regulates activation of DCs during sensitization, while in the effector phase the H4 R is active in cells involved in the activation of eosinophils, and possibly other cells. A putative therapy targeting the H4 R may be an option for asthma patients developing IL-5-dependent eosinophilia.

No evidence for histamine H4 receptor in human monocytes.

The histamine H4 receptor (H4R) is a classic pertussis toxin-sensitive Gi protein-coupled receptor that mediates increases in intracellular calcium concentration ([Ca(2+)]i). The presence of H4R in human eosinophils has been rigorously documented by several independent groups. It has also been suggested that H4R is expressed in human monocytes, but this suggestion hinges in part on H4R antibodies with questionable specificity. This situation prompted us to reinvestigate H4R expression in human monocytes. As positive control, we studied human embryonic kidney 293T cells stably expressing the human H4R (hH4R). In these cells, histamine (HA) and the H4R agonist UR-PI376 (2-cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[(2-phenylthio)ethyl]guanidine) induced pertussis toxin-sensitive [Ca(2+)]i increases. However, in quantitative real-time polymerase chain reaction studies we failed to detect hH4R mRNA in human monocytes and U937 promonocytes. In human monocytes, ATP and N-formyl-l-methionyl-l-leucyl-l-phenylalanine increased [Ca(2+)]i, but HA, UR-PI376, and 5-methylhistamine (a dual H4R/H2 receptor agonist) did not. In U937 promonocytes and differentiated U937 cells, HA increased [Ca(2+)]i, but this increase was mediated via HA H1 receptor. In conclusion, there is no evidence for the presence of H4R in human monocytes.

Analysis of the histamine H2-receptor in human monocytes.

Histamine receptors are G-protein-coupled receptors (GPCRs). Canonically, the histamine H2-receptor (H2R) couples to Gs-proteins and activates adenylyl cyclases (ACs) with subsequent adenosine-3',5'-cyclic monophosphate (cAMP) generation. Recently, the classic two-state model describing GPCR activation has been extended to a multiple-state model implying that any ligand stabilizes a ligand-specific receptor conformation, also referred to as functional selectivity. In our present study we pharmacologically characterized the H2R in human monocytes. We found dissociations in the effects of histamine (HA) and H2R agonists on cAMP accumulation and inhibition of formyl peptide-induced production of reactive oxygen species (ROS). In addition, we excluded participation of protein kinase A (PKA) in HA-induced ROS inhibition. Comparison of the potencies and efficacies of H2R agonists in monocytes, neutrophils and eosinophils unmasked cell type-specific pharmacological profiles of individual ligands. Taken together, our data extend the concept of functional selectivity to the H2R in human monocytes and demonstrate striking cell-type specificity in the pharmacological profile of the H2R.