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.
The biogenic amine histamine (2-[4-imidazolyl]ethylamine), which was discovered a century ago (Dale and Laidlaw, 1910, 1911), is involved in a series of physiologic and pathophysiological processes. Histamine is formed from the amino acid l-histidine by action of the enzyme l-histidine decarboxylase (HDC), the expression of which is regulated by entities that include glucocorticoids, intracellular cAMP, calcium ions, and protein kinase C activity, as well as Raf- and extracellular signal-regulated kinase-related signaling pathways (Höcker et al., 1996, Höcker et al., 1997a,b; Ichikawa et al., 2010). In mast cells, basophils, enterochromaffin-like cells of the gastrointestinal tract, and histaminergic neurons, histamine is stored, probably heparin-bound, in granules and is released in response to a specific stimulus (Barnes et al., 1988; Rabenstein et al., 1998). However, HDC is expressed not only in these “classically” known histamine-producing cells but also in monocyte-derived cells (László et al., 2000; Dunford et al., 2006; Xu et al., 2012; Alcañiz et al., 2013). As a consequence, one can speculate that HDC activity is present, at least after induction, in virtually every other cell type as well. These “nonclassic” histamine-producing cells likely are not able to store histamine but release it immediately upon its synthesis, giving rise to a rather delayed kinetic (Dunford et al., 2006). Therefore, one can speculate that local histamine concentrations upon release due to degranulation are substantially higher than those achieved by immediate release upon synthesis. However, experimental data on this question are not yet available.
The diverse functions of histamine, which include neurotransmission, regulation of gastric acid production, vasodilation, and smooth muscle contraction, are mediated on the cellular level by four specific histamine receptor subtypes, referred to as histamine H1-receptor (H1R), H2R, H3R, and H4R (de Esch et al., 2005; Thurmond et al., 2008; Leurs et al., 2009; Seifert et al., 2013). Histamine receptors belong to the family of 7-transmembrane G protein–coupled receptors, which constitute the largest group of clinically relevant drug targets (Kobilka and Deupi, 2007; Katritch et al., 2012; Venkatakrishnan et al., 2013). Of all histamine receptor subtypes, H1R, H2R, and H3R are addressed by clinically used pharmacotherapies. While H1R is used mainly as a target for antiallergic drugs (Simons and Simons, 2011), H2R-antagonists are used for the therapy of acid-related gastrointestinal diseases (Ichikawa et al., 2009). However, today H2R-antagonists have been clinically mostly replaced by proton pump inhibitors (Hrelja and Zerem, 2011; Alhazzani et al., 2013). Recently, another indication for the clinical use of histamine as H2R agonist evolved, based on the finding that histamine ameliorates the course of acute myeloid leukemia (Berry et al., 2011; Buyse et al., 2011; Aurelius et al., 2012). H3R antagonists are currently used as orphan drugs for the therapy of narcolepsy (Tiligada et al., 2011; Inocente et al., 2012). H4R antagonists have not yet been implemented in clinical routine but have been submitted to clinical trials addressing neurologic and inflammatory diseases, respectively (Tiligada et al., 2011; Walter et al., 2011). Of these reported clinical trials, however, only one can be found (September 6, 2013) in the common online database clinicaltrials.gov. This trial was completed in March 2011, but resulting data are still unpublished. Finally, agonists and antagonists for all histamine receptor subtypes are used experimentally in basic and preclinical research (Strasser et al., 2013).
Nevertheless, the use of histamine receptor subtype–selective ligands has to be interpreted with caution because the specificity, efficacy, and potency are not always as unproblematic as originally assumed (Deml et al., 2009; Neumann et al., 2010; Reher et al., 2012a; Seifert et al., 2013). Therefore, to unambiguously investigate the role of a specific histamine receptor subtype, the use of genetic knockout models is important. To the best of our knowledge, clearly defined human diseases based on a deficiency of HDC or histamine receptor expression have not been reported. However, mice with genetic deletions for each of the four histamine receptor subtypes are available, as well as mice devoid of the histamine-generating enzyme HDC (Table 1). In this Perspectives article, we summarize the main findings obtained by and problems associated with the use of these histamine receptor subtype–selective knockout mice, focusing on inflammation (Table 2). Because of space limitations, the effects on behavior and neuronal physiology are not covered in this article.
Because histamine directly or indirectly modulates many T-cell functions (Jutel et al., 2009), a short introduction on the immune system, especially on T-cell polarization, will be given. The immune system recognizes and combats potentially harmful agents, antigens, by specialized cells. Basically, the immune system can be subdivided into the innate and the adaptive arms. The first line of defense is mounted by the innate arm of the immune system, which acts very rapidly but rather unspecifically. This process is also referred to as acute inflammation and relies on the activity of macrophages, dendritic cells, mast cells, natural killer cells, and granulocytes (Akira et al., 2006; Kapetanovic and Cavaillon, 2007; dos Santos et al., 2012; Spaggiari and Moretta, 2013).
Subsequently, the adaptive arm of the immune system is activated, resulting in the amplification of innate defense mechanisms and the development of immunologic memory (Sallusto and Lanzavecchia, 2001; Lanzavecchia and Sallusto, 2009). Adaptive immunity, when directed for the first time against a specific antigen, develops rather slowly, but once established it is highly antigen-specific. T cells and B cells mount this specific immune response. Mature peripheral T cells are identified by the expression of a T-cell receptor, which is associated with the signal-transducing complex CD3. All receptors of an individual T cell are of the same specificity but different from those of probably all other T-cells. Engagement of a specific T-cell receptor by its cognate antigen activates the naïve T cell, resulting in its proliferation, a process called clonal expansion. In parallel, the expanding cells of the clone pass through a developmental process called polarization (Wahl et al., 2004; de Jong et al., 2005), which is strictly dependent on the cytokine concentrations active on the activated T cells.
Figure 1 comprehensively summarizes the process of T-cell activation followed by polarization into the helper T cells (Th)-1, Th2, and the combinations of tumor growth factor-β (TGFβ) plus IL-6 or IL-21 plus IL-23, the polarization of Th17 cells. TGFβ alone mediates the polarization into regulatory T cells (Treg). These diverse Th-cell subsets can be distinguished by their cytokine expression profiles. Th1 cells primarily produce interferon-γ; Th2 cells IL-4, IL-5, and IL-13; Th17 cells IL-17A and F, as well as IL-21 and IL-22; and Treg cells TGFβ and IL-10. Possible interactions between histamine and the T-cell compartments are depicted in Fig. 1. Because of the cytokines produced, the different Th-cell subsets are associated with different immune functions. Th1 and Th17 cells are involved in the immune reaction against intra- and extracellular pathogens, such as bacteria and viruses, whereas Th2 cells usually drive the defense against extracellular parasites but also allergic inflammation. Treg cells finally provide regulatory mechanisms enabling a timely and locally tightly regulated immune response (Ozdemir et al., 2010; Muranski and Restifo, 2013).
Therefore, by interfering in the balance of Th1/Th2 polarization, it would be possible to modulate the outcome of an inflammatory disease; for example, a Th2-driven allergic response could be reduced by shifting the Th1/Th2 bias toward Th1 polarization. By the same principle, Th1-driven autoimmune diseases would benefit from shifting Th-cell polarization toward Th2. Unfortunately, these mechanisms, however, are not that simplistic as depicted above, but much more complex, as described in the following sections.
Mice deficient in HDC expression (HDC−/−) were generated more than a decade ago. Histamine and HDC are virtually absent in the organs analyzed, however, with the exception of the brain. Here, although HDC-activity is absent, substantial amounts of histamine are detectable, which may be either diet-derived or generated by a brain-specific, yet unidentified enzyme other than HDC. Both possible explanations give rise to three still-unanswered questions. First, how is dietary histamine taken up and transported to the brain? Plasma of HDC−/− mice virtually lacks histamine (Ohtsu et al., 2001). This may be a consequence of low sensitivity of the high performance liquid chromatography detection method, which would imply the reanalysis of histamine concentrations in HDC−/− tissues using highly sensitive methods such as high-performance liquid chromatography with tandem mass spectroscopy (Zimmermann et al., 2011). Alternatively, dietary histamine may be taken up by and transported in blood cell vesicles. Basophil granulocytes may have this function; however, this issue has still to be answered experimentally. A second question is: why is dietary histamine transported specifically to the brain? Finally, we can ask: what is the nature of the putative alternative histamine-generating enzyme?
Thus, when analyzing histamine and its effects using HDC−/− mice, not only endogenously synthesized histamine but also exogenously supplied histamine has to be taken into consideration (Ohtsu et al., 2002). Therefore, the animals should be kept on a histamine-free diet and preferably also under germ-free conditions to ensure that the mice are as histamine-free as possible. This was, at least partly, performed in some studies (Ohtsu et al., 2002; Fitzpatrick et al., 2003). However, since the commercially available “histamine-free” diet still contains residual histamine, in reality, it is a low-concentration histamine-containing (<1 nmol/g) diet. Moreover, mice were fed this chow for only 7 days prior to experiments, not excluding the possibility that histamine originating from the normal diet and from commensal bacteria still resided in the animals. Nonetheless, although not completely histamine-free, HDC−/− mice exhibit dramatically reduced histamine concentrations in their tissues and a significantly reduced number of mast cells, which moreover exhibit a reduced granular content (Ohtsu et al., 2001). Thus, histamine seems to be not only a content molecule of mast cell granules but also necessary for their structural integrity.
Histamine, via the H2R, promotes inflammatory angiogenesis, demonstrated by the use of HDC−/− mice and the H2R agonist dimaprit (Ghosh et al., 2002). In this model, the source for histamine is not mast cells but probably infiltrating macrophages, another indication for the formerly mentioned assumption that histamine is produced in other than the classically known cells. HDC−/− mice were further analyzed in a variety of models for inflammatory conditions, such as arthritis (Rajasekaran et al., 2009), colitis (Bene et al., 2004), and acute allergic asthma (Koarai et al., 2003; Kozma et al., 2003; Yamauchi et al., 2008, 2009), (the study by Yamauchi et al., 2009 is a reproduction of identical data presented by the same group in 2008 (Yamauchi et al., 2008), however, without reference to the 2008 article). Most of these studies point to a proinflammatory role of histamine, mediated by its regulation of the activity of immune cells. These findings have been reviewed recently (Ohtsu, 2011), and thus will not be discussed in more detail here.
In contrast to the observed proinflammatory role of histamine are results of studies in models of acute contact dermatitis (Garaczi et al., 2004) and experimentally induced peritonitis (Hori et al., 2002). In these models, histamine deficiency accelerates neutrophil and macrophage accumulation into dermis and peritoneum, respectively, indicating an anti-inflammatory effect of histamine, which can be attributed to the functional expression of H2R in these cells (Hasturk et al., 2006; Cho et al., 2011; Reher et al., 2012a). Neutrophils and mast cells also produce histamine upon proper stimulation [i.e., activation of Toll-like receptor 4 (TLR4) or TLR7], in a phosphatidylinositol 3-kinase-dependent manner (Smuda et al., 2011). In contrast to mast cells, neutrophils do not store histamine in granules, and histamine is not necessary for granule integrity. Thus, in these models, neutrophil-derived histamine may serve as an autocrine factor that reduces or delays neutrophil inflammation; however, this has not been formally proven so far. Despite its reported anti-inflammatory role in contact dermatitis (Garaczi et al., 2004), histamine seems to mediate scratching behavior in diphenylcyclopropenone-induced contact dermatitis. Scratching activity was dramatically reduced in HDC-deficient mice compared with wild-type controls (Seike et al., 2005).
Inflammation-associated skin and colon carcinogenesis are counteracted by histamine (Yang et al., 2011). This effect, which probably relies on the histamine-mediated maturation and accumulation of myeloid cells at the site of inflammatory cancer tissue, also seems to be auto-regulatory, because cancer-associated immature myeloid cells produce histamine (Yang et al., 2011). In contrast, in a model of implanted mammary adenocarcinoma, the absence of histamine reduced tumor growth and was accompanied by enhanced Th1 (activating cellular immunomechanisms, such as macrophages) (Fig. 1) and reduced Th2-type (activating humoral immunomechanisms, such as antibody production) (Fig. 1) and regulatory T-cell (Treg; Fig. 1) immunoreactivity (Hegyesi et al., 2007). Thus, in carcinogenesis, histamine seems to affect both the innate and the adaptive arms of the immune system but with contrasting effects. Whether the differential induction of these effects is dependent on the type of carcinoma, however, has still to be elucidated.
Skeletal and heart muscle cells can be induced to express HDC as well (He et al., 2012; Niijima-Yaoita et al., 2012), and the absence of histamine, probably in muscle, reduces prolonged walking endurance, indicating a protective effect of histamine on exercise-induced exhaustion (Niijima-Yaoita et al., 2012). Whether this applies also for the heart muscle, and therefore, could be addressed therapeutically in heart failure, needs to be examined.
Lastly, it should be taken into consideration that all the above-mentioned experiments were performed using the HDC−/− genotype on differing genetic backgrounds, one of which is the intrinsically very heterogeneous 129/Sv strain (Threadgill et al., 1997). Because the results may be strongly affected by the genetics of the background strains, a comparison of individual studies is problematic. Moreover, at least the recombinantly expressed human H3R and H4R display a very high ligand-independent (constitutive) activity (Schneider et al., 2009). Whether all murine histamine receptors lack such constitutive activity, as is the case with the murine H4R (Schnell et al., 2011), is not clear at present. In addition, whether histamine receptors are constitutively active in vivo and whether this bears a possible physiologic function is unknown as well. Thus, in comparison with HDC−/− mice, data obtained by the use of histamine receptor knockout mice are easier to interpret. However, analysis of the HDC−/− mouse under truly histamine-free conditions could constitute an excellent model system for the analysis of constitutive G protein–coupled receptor activity in vivo, a still unresolved highly important issue (Seifert and Wenzel-Seifert, 2002).
The H1R is expressed ubiquitously, and H1R antagonists are clinically mostly associated with allergy and sedation (Simons, 2004; Simons and Simons, 2011). Effects of the H1R on the immune system are a well-elaborated topic. By use of selectively stimulated T cells in vitro and by use of immunization to an experimental antigen, ovalbumin, in vivo, T cells were demonstrated to shift their bias toward Th2-directed polarization due to the absence of the H1R, while absence of the H2R results in amplification of both Th1 and Th2 responses (Jutel et al., 2001; Noubade et al., 2007). These data would imply an accelerated allergic response in H1R−/− mice, which, however, could not be detected, as discussed below. The effect of histamine on proliferation of lymphocytes had been analyzed in a previous study (Banu and Watanabe, 1999), demonstrating that in spleen cells of H1R−/− mice, antigen receptor–induced proliferation of T cells and B cells is reduced in comparison with the corresponding wild-type cells. Interestingly, this study also analyzed the isotype usage of antigen-specific antibodies generated in response to experimental immunization with ovalbumin, which is indicative for polarization of Th cells, but could not detect a difference between wild-type and H1R−/− mice. However, it is obvious that the H1R is involved in the immune response; thus, its function was evaluated in several models of inflammatory diseases. In models of nasal allergy (Kayasuga et al., 2002a,b) and of Th2-driven allergic asthma (Bryce et al., 2006; Miyamoto et al., 2006), the absence of H1R led to significantly reduced symptoms. In the asthma model, this was accompanied by a shift of T-cell polarization toward enhanced Th2-type activity, as had already been detected (Jutel et al., 2001). This latter fact would imply an accelerated allergic response (type 1 hypersensitivity), the development of which, however, was hampered by the impaired capacity of H1R−/− T cells, both CD4+ and CD8+, to migrate to the site of inflammation (Bryce et al., 2006). In addition, cells other than T cells also express the H1R and regulate T-cell polarization, for example, dendritic cells (DCs); however, studies examining the role of the H1R on DCs in the mouse model of allergic asthma are still missing. Thus, the cellular and molecular mechanisms explaining the reduced asthmatic phenotype in H1R−/− mice are not yet well understood.
The importance of the H1R in allergic diseases was underscored in the model of atopic dermatitis (Vanbervliet et al., 2011). In this study, in which bone marrow–derived dendritic cells were used as a model for DCs, it was found that histamine via the H1R increases their function to promote cytotoxic CD8 T-cell generation. Whether the H1R also has a direct effect on T cells, as suggested above (Banu and Watanabe, 1999; Jutel et al., 2001; Bryce et al., 2006), was not examined in this study. However, in the model of Th1-driven experimental autoimmune encephalomyelitis, a proinflammatory function for the H1R expressed on CD4+ T cells, but not on CD11b+ cells (i.e., monocytes, macrophages, DCs, neutrophils, and microglia), was demonstrated also in vivo (Ma et al., 2002; Noubade et al., 2007; Saligrama et al., 2012b). As already seen in the asthma model (Bryce et al., 2006), the absence of the H1R shifted the immune reaction toward a Th2 phenotype. This function of the H1R was opposed in a model of infection, the coxsackievirus B3–induced myocarditis in mice (Case et al., 2012). Here, H1R−/− mice have enhanced numbers of interferon-γ-producing CD4+ T cells—thus, Th1 cells in the periphery—and, moreover, also enhanced numbers of Vγ4 T cells and Tregs. This study adds to previous findings that histamine via the H1R is active not only on T cells in terms of directing Th-cell polarization but also on γ/δT cells and probably others. The involvement of the H1R in histamine-induced γ/δT-cell activation was contradicted by a study using human peripheral γ/δT cells (Truta-Feles et al., 2010) in which only the H2R and the H4R were active. Whether this discrepancy is species-dependent or other factors are responsible has not been addressed so far.
Lastly, using H1R-deficient mice, an involvement of the H1R was also demonstrated in models of atherosclerosis and nonalcoholic steatohepatitis (Rozenberg et al., 2010; Wang et al., 2010), indicating the function of H1R on vascular cells, as well as in the nervous system and pancreatic tissue, respectively. Such H1R functions were already suggested when analyzing the models of allergy and atopy (Nakahara et al., 2000; Rossbach et al., 2009; Rossbach et al., 2011) and provide a plausible potential mechanism for itching and sneezing. However, dilatory effects of histamine on endothelial cells via the H1R were excluded by Lu et al., who demonstrated a probably opposing effect in a model of experimental autoimmune encephalomyelitis (Lu et al., 2010).
In summary, the results discussed above suggest the use of H1R antagonists to treat inflammatory and allergic diseases. Indeed, such therapeutics have been successfully introduced in the clinic to control allergic inflammation, such as rhinitis and conjunctivitis. Nevertheless, for the treatment of other inflammatory diseases, such as allergic asthma, H1R antagonists lack effectiveness (Simons and Simons, 2011). Especially for the treatment of asthma, recent data, however, indicate that H1R antagonists may serve as comedication to modulate the effect of other anti-inflammatory drugs (Beermann et al., 2012a; Bartho and Benko, 2013). However, one should always keep in mind that chronic use of H1R antagonists can lead to unfavorable side effects, such as obesity (Masaki and Yoshimatsu, 2010) and sedation (Simons and Simons, 2011).
Similar to the H1R, the H2R is expressed ubiquitously (Jutel et al., 2009). However, its clinical relevance has been limited to acid-related gastrointestinal diseases (Ichikawa et al., 2009) but was recently extended to the M4/M5 forms of acute myeloid leukemia (Aurelius et al., 2012). In parietal cells of the gastric mucosa, histamine via the H2R stimulates the production of protons (Hersey and Sachs, 1995). Although histamine is only one of several mediators exhibiting this function, its physiologic relevance is demonstrated by pharmacologic studies using selective antagonists (Shamburek and Schubert, 1993). Mice lacking the H2R were generated in 2000 (Kobayashi et al., 2000) and were found to have a normal gastric pH, indicating a compensation of the permanently absent H2R function by muscarinic M1 receptors, which may also point to problems associated with chronic therapy with H2R antagonists (Kobayashi et al., 2000). However, although basal gastric acid secretion was unaffected, in comparison with wild-type mice the histologic appearance of the oxyntic mucosa in H2R−/− mice was altered. Moreover, histamine appears to be essential for the gastrin-induced proton secretion only, and not for that induced by the muscarinic receptor agonist carbachol, a finding confirmed in HDC−/− mice (Tanaka et al., 2002).
In addition to studies of the regulation of proton secretion, H2R−/− mice have been used to explore the involvement of the H2R in inflammatory and immunologic reactions. In the model of experimental autoimmune encephalomyelitis, which is Th1-driven, acute symptoms are reduced in H2R−/− mice in comparison with wild-type controls. This is accompanied by a reduction of the Th1-type immune response (Fig. 1), mediated by reduced production of IL-12 and IL-6 and enhanced synthesis of monocyte chemotactic protein–1 in antigen-presenting cells (Teuscher et al., 2004). The finding that H2R signaling is necessary for in vitro histamine-induced IL-18 synthesis (Takahashi et al., 2006) supports the notion that the H2R promotes an effective Th1-type polarization of the immune response. These data, obtained in the experimental autoimmune encephalomyelitis-model using H2R−/− mice, are in direct contrast to those obtained by pharmacologic intervention at the H2R. The inverse H2R-agonist cimetidine, although readily crossing the blood-brain-barrier (Pan et al., 1994; Abbott, 2000), does not prevent the induction of experimental autoimmune encephalomyelitis (Babington and Wedeking, 1971) but rather increases severity (Staykova et al., 1988), whereas the H2R-agonist dimaprit ameliorates the disease (Emerson et al., 2002). This again is a good example that gene knockout models do not necessarily provide reliable models for predicting pharmacologic intervention, because they do not reflect possible off-target effects, as demonstrated for cimetidine (Cotton et al., 2013). The knockout model always reflects both the function of the disrupted gene during ontogenesis and in the experimental disease. Moreover, as discussed above, because of its absence during ontogenesis, compensatory mechanisms can be established that are absent in gene-competent animals. These limitations, not only with regard to the lastly mentioned study but in general, must at least be carefully discussed or, even better, addressed experimentally (e.g., by using inducible knockout-models).
Basically, histamine via the H2R regulates an immune response; for example, as described above, by its inducing effect on IL-18 production (Takahashi et al., 2006). However, another study indicates that in a hepatitis model, histamine via the H2R in vivo reduces the expression of IL-18, thereby preventing excessive inflammation (Yokoyama et al., 2004). Although the protective role of the H2R in hepatitis was confirmed in another model (Masaki et al., 2005), still a discrepancy concerning the data on IL-18 production exists. Certainly, the data were generated in differing models, and the presence or absence of other cytokines, such as IL-12, and also kinetic differences have to be taken into consideration. However, the observation of such diametrically opposing data is noteworthy, inasmuch as the data were published by the same authors but without an explanation of the discrepancy. Moreover, Jutel et al., 2001 demonstrated that the absence of H2R signaling in T cells enhances both Th1- and Th2-type cytokine production. This group observed that in vivo, H2R−/− mice, in comparison with wild-type mice, develop partly reduced Th1- and Th2-type immune responses after immunization, as demonstrated by the reduced production of immunogen-specific immunoglobulin (Ig) G3 and IgE antibodies, respectively (Jutel et al., 2001). Finally, in the model of IL-4-induced lung inflammation, a Th2-associated pathology, the absence of the H2R led to markedly reduced symptoms, again indicating that H2R signaling is necessary for a proper Th2-type immune response (Swartzendruber et al., 2012). These studies, in combination with those discussed above (Teuscher et al., 2004), indicate that in pathologic models, H2R signaling regulates both Th1- and Th2-type immune responses (Jutel et al., 2001).
Therefore, from a clinical point of view, the use of H2R-antagonists would be indicated for both Th1- and Th2-driven diseases, such as autoimmune diseases and IgE-mediated allergies, respectively, and indeed, in principle a clinical effect of H2R antagonists, specifically ranitidine, in the atopic reaction has already been shown (Lin et al., 2000; Watson et al., 2000; Kupczyk et al., 2007). However, by use of the knockout mouse approaches, to date, new definite indications for the use of H1R and H2R antagonists in humans have yet to be elaborated.
The H3R is expressed mainly on presynaptic membranes of histaminergic neurons, where it autoregulates the release of histamine (Arrang et al., 1983) and thereby affects behavior. This issue, the effect of selective histamine subtype receptor deletion on behavioral parameters, is not subject of this article.
To the best of our knowledge, the only study aiming at analyzing the involvement of the H3R in inflammatory diseases using H3R−/− mice was performed in the model of experimental autoimmune encephalomyelitis (Teuscher et al., 2007). Lack of the H3R enhanced the onset of the disease, while it did not alter maximal severity. This alteration was accompanied by enhanced blood-brain barrier permeability and enhanced expression of chemokines by restimulated T cells, which support the infiltration of T cells into the brain. A satisfying mechanistic explanation for this observation, however, is missing.
The H4R is the latest identified and characterized member of the histamine receptor subtypes (Nakamura et al., 2000; Oda et al., 2000; Hough, 2001; Liu et al., 2001; Morse et al., 2001; Nguyen et al., 2001). H4R−/− mice were generated soon after and analyzed for mast cell functions (Hofstra et al., 2003). Histamine-induced calcium mobilization and chemotaxis, both occurring in wild-type mast cells, were absent in mast cells derived from H4R−/− mice, while degranulation remained unaffected. The capacity of histamine to induce chemotaxis via the H4R was demonstrated using genetic and pharmacologic approaches for other cell types, such as eosinophils and dendritic cells (Thurmond et al., 2004; Bäumer et al., 2008; Cowden et al., 2010b; Gschwandtner et al., 2010; Simon et al., 2011; Reher et al., 2012b). In adaptive immunity, effects of the H4R seem to be limited to dendritic cell activation, both in terms of migration and mediator release, while data supporting a direct effect on T cells are ambiguous (Dunford et al., 2006; del Rio et al., 2012). Because of its effect on dendritic cells, histamine via the H4R is involved in a series of experimental diseases, notably experimental asthma (Dunford et al., 2006), dermatitis (Dunford et al., 2007; Cowden et al., 2010b; Gschwandtner et al., 2010), and autoimmune encephalomyelitis (del Rio et al., 2012). However, while the H4R in experimental asthma and dermatitis is proinflammatory, in experimental autoimmune encephalomyelitis, it is anti-inflammatory. These results would render H4R antagonists as promising drugs to treat asthma or dermatitis (Dunford et al., 2007; Cowden et al., 2010a,b; Beermann et al., 2012a; Coruzzi et al., 2012). However, as expected from the knockout data, in the model of experimental autoimmune encephalomyelitis, the H4R antagonist JNJ 7777120 exacerbates the disease (Ballerini et al., 2013), indicating a considerable harmful potential of antagonism at the H4R. Whether this is specific for the compound JNJ 7777120 or a general effect of antagonism at the H4R has to be examined.
In addition, also in invariant natural killer T cells, cells, which in contrast to dendritic cells, recognize selective antigens out of the context of the major histocompatibility by complex; histamine via the H4R regulates production of Th-associated cytokine and, therefore, the adaptive immune response (Leite-de-Moraes et al., 2009). Thus, in summary, it appears that histamine regulates the adaptive immune response by direct interaction with the H4R expressed on cells bridging the innate and the adaptive immune system. Whether this could be translated into human clinics is unclear at present, because the availability of data obtained with H4R antagonists in clinical trials is very limited.
Still, a general question regarding all mice with histamine subtype receptor–specific genetic deletion remains: does the absence of the expression of a certain receptor subtype lead to a compensatory reaction mediated by, for example, another receptor subtype? This issue was addressed in an article analyzing H1R/H2R−/− and H3R/H4R−/− double knockouts in a model of experimental autoimmune encephalomyelitis (Saligrama et al., 2012a). Disease symptoms in the double knockouts were reduced in H1R/H2R−/− and enhanced in H3R/H4R−/− mice. More importantly, the authors also quantified the mRNA expression of H1R, H2R, and H4R in CD4+ T cells derived from histamine receptor subtype single and double knockout mice. They found an upregulated expression of H4R mRNA in H1R/H2R−/− T cells and an upregulated expression of H1R and H2R mRNAs in H3R/H4R−/− T cells compared with the respective single-knockout T cells (Saligrama et al., 2012a). Whether these findings account for compensatory mechanisms is questionable, because in terms of disease activity, the absence of propathogenic signaling in H1R/H2R−/− mice enhances antipathogenic signaling via enhanced H4R expression and vice versa in H3R/H4R−/− mice, probably leading to additive or synergistic effects. Moreover, enhanced expression of mRNA does not necessarily lead to enhanced protein synthesis (de Sousa Abreu et al., 2009). Unfortunately, the analysis of histamine receptor protein expression is highly problematic because of the lack of reliable antibodies (Beermann et al., 2012b), and radioligand-binding studies are working well only for H1R and H3R (Strasser et al., 2013). Therefore, the issue of compensatory mechanisms within the family of histamine receptors has still to be elucidated.
HDC and histamine receptor subtype knockout mice are valuable tools for the examination of the function of histamine and a specific receptor subtype, which cannot be addressed unambiguously by the use of selective agonists and antagonists. The interpretation of data obtained by the use of HDC−/− mice, however, remains difficult because of residual amounts of histamine found in these mice and the inhomogeneous genetic backgrounds used in the studies. Therefore, whether histamine is pro- or anti-inflammatory in all cases is a still unanswered question.
In vivo, H1R−/− mice provide evidence for an inhibitory function of the H1R on Th2-cell polarization. Thus, H1R−/− mice would be expected to show enhanced allergic symptoms, but allergic inflammation is reduced. So far, this paradox is mechanistically not well understood. By use of H2R−/− mice, it is demonstrated that both Th1- and Th2-mediated immune responses are promoted by the H2R, however, with differing results depending on the model analyzed. Lastly, H4R−/− mice demonstrate a function of the H4R at the interface between innate and adaptive immunity with pro- or anti-inflammatory function, depending on the specific disease studied. Nevertheless, especially for the analysis of histamine receptors ex vivo on a specific cell population such as T cells, the histamine receptor knockout mice are indispensable.
Beyond these, no new clear-cut conclusions can be drawn from the studies available, since the data are contradictory. Factors which may be responsible for such controversial results include the plasticity of receptor systems (leading to the functional regulation of related receptors), the genetic background strain used, genetic differences between mice of the same background with the same gene deleted but generated separately, differences in the method of gene disruption, possible leakiness of the knockout genotype, genetic variability due to the development of individually inbred substrains and subtle differences in the basically identical experimental disease models or treatment schemes (Rohrer and Kobilka, 1998).
Wrote or contributed to writing of the manuscript: Neumann, Schneider, Seifert.
- Received July 9, 2013.
- Accepted October 7, 2013.
This work was supported by grants from the Deutsche Forschungsgemeinschaft [NE 647/8-1, GRK 1441, and GRK 760].
- dendritic cell
- histamine receptor subtypes 1–4
- l-histidine decarboxylase
- JNJ 7777120
- cytotoxic T cell
- tumor growth factor-β
- helper T cell
- Toll-like receptor
- regulatory T cell
- Copyright © 2013 by The American Society for Pharmacology and Experimental Therapeutics