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Functional Gastrointestinal Disorders and the Potential Role of Eosinophils
Gastroenterol Clin N Am 37 (2008) 383–395
GASTROENTEROLOGY CLINICS OF NORTH AMERICA
Functional Gastrointestinal Disorders and the Potential Role of Eosinophils Marjorie M. Walker, BMedSci, BMBS, FRCPatha,*, Nicholas J. Talley, MD, PhD, FRACP, FRCP, FACPb a
Department of Histopathology, St. Mary’s Campus, Imperial College London, Norfolk Place, London W2 1PG, United Kingdom b Department of Internal Medicine, Mayo Clinic, 4500 San Pablo Road, Jacksonville, FL 32224, USA
FUNCTIONAL GASTROINTESTINAL DISORDERS The Rome III classification of the functional gastrointestinal disorders (FGIDs) defines 28 adult and 17 pediatric symptom-based gut conditions that have no established pathology [1]. These disorders place a considerable burden on individual patients, because quality of life is poorer in patients who have FGIDs compared with health and is comparable to organic GI disease [2]. There is also a major societal burden because of high direct and indirect costs [3]. Currently, diagnosis is based on symptoms or by exclusion, because there is no objective diagnostic test for these disorders; by definition it is necessary to exclude other diseases, specifically evidence of an inflammatory, anatomic, metabolic, or neoplastic process that explains the patient’s symptoms [1]. Abdominal pain, disordered defecation, and meal-related discomfort are common symptomatic themes for a range of FGIDs. Treatment remains empirically based on relieving symptoms rather than treating abnormal pathophysiology. The current accepted conceptual model of disease reflects an interaction between psychosocial factors, including stress and anxiety, that impinge on gut physiology by way of the brain–gut axis to produce visceral hypersensitivity or dysmotility. These signals interact by way of the gut enteric nervous system (ENS) and track back to the central nervous system (CNS) [1]. It is possible that this viscious cycle originates in gut mucosa, where an insult triggers a feedback loop, although how often this is the case remains uncertain. Key barriers in attempting to define an organic pathology for FGIDs include a lack of understanding of how symptoms are triggered and how any feedback loop is maintained. The pathophysiology of the FGIDs has been studied extensively to try to identify a common link for symptom expression. It is currently assumed that *Corresponding author. E-mail address: [email protected] (M.M. Walker). 0889-8553/08/$ – see front matter doi:10.1016/j.gtc.2008.02.007
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there is not a simple or universal pathway to these differing outcomes, but a complex interaction of genetic predisposition, visceral hypersensitivity, and psychosocial distress in a primed central and enteric nervous system [4,5]. EOSINOPHILS AND FUNCTIONAL GASTROINTESTINAL DISORDERS The gut is an active immune organ that is the first line of defense for many insults, such as allergens in food, drugs, viruses, bacteria, and parasites. The authors surmise that altered immune function, by way of infection or hypersensitivity on a background of genetic predisposition and environment, triggers symptoms (Fig. 1). This hypothesis suggests that components of the hypersensitivity immune cascade may play a vital role in FGIDs. A key component of this pathway is the eosinophil, and this review explores a potential role for eosinophils and associated components of this pathway in FGIDs, specifically irritable bowel syndrome (IBS) and functional dyspepsia (FD), which have been principally studied. EOSINOPHILS IN THE GASTROINTESTINAL TRACT Eosinophils are a normal constituent throughout the gastrointestinal tract (GIT), except in the squamous mucosa of the esophagus. These cells have a defined role in physiologic and pathologic function. In the gut mature eosinophils originate from hemopoietic stem cells in the bone marrow; recruitment is regulated by interleukin (IL)–5, IL-3, and granulocyte macrophage colony-stimulating factor (GM-CSF) to induce migration to the gut. Eosinophils are effector cells, which release secretory granules and have an immunoregulatory function by way of cytokines and release and presentation of antigens [6]. WHAT IS THE NORMAL EOSINOPHIL COUNT IN THE GUT? Eosinophils have a characteristic morphology with a bright eosinophilic cytoplasm and a bilobed nucleus. They are easily recognizable in biopsies and
Fig. 1. A role for eosinophils in FGIDs? The interaction of psychosocial distress, environmental factors, and a genetic predisposition with a hypersensitive immune system in the gut may be maintained by eosinophil accumulation in the gut, by way of signaling to the ENS and then the CNS in functional bowel disorders.
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therefore counts of eosinophils can be performed on routine hematoxylin and eosin stained sections. Counting of eosinophils is critical at all sites to discriminate normal from a pathologic state. In the esophagus, eosinophils are evoked in acid reflux and to distinguish this from eosinophilic esophagitis, the cutoff level is conventionally considered to be at least 15 per high power field (HPF), although variable cutoffs have been used [7]. In gastric mucosa, eosinophilia is often seen in Helicobacter pylori infection but typically not in functional dyspepsia [8]. In H pylori–negative adult subjects, the mean gastric eosinophil counts were 10/5HPF in antral and body mucosa [8]. In the duodenum, eosinophilia has been noted in children and adults who have functional dyspepsia, defined as greater than 10/HPF for children [9] and greater than 22/5HPF with or without clusters of eosinophils at the base of glands for adults [8]. Normal duodenal counts were defined by less than 10/HPF in children and 19/5HPF in adults in these studies based on control values [8,9]. In the cecum, eosinophils are more numerous in the normal state, and up to 40/HPF have been suggested to be normal at this site [10]. Colonic eosinophilia has also been associated with IBS, inflammatory bowel disease (IBD), and food allergy in children, and mean eosinophil numbers per crypt (standard deviation) have been reported to be 34.8 (17.1) in IBD, 21.3 (8.8) in IBS, and 25.4 (16.7) in food allergy [11]. In this study there was no variation between colonic sites. Geographic variation in colonic eosinophilia in routine biopsies, in which eosinophils in the intercryptal space were counted, has been noted in the United States, where those in southern states had a higher count than the northern states [12]. Few studies have consistently counted in the same manner in normal or diseased subjects so establishing true normal values is difficult. There are relatively few studies to adequately address the normal counts throughout the gut, particularly the upper gut. Because subtle changes in numbers may be significant [13], it is important therefore for histopathologists to count eosinophils in the gastrointestinal sites and have data on local normal controls to define abnormality. EOSINOPHILS, ATOPY, ALLERGY, AND FUNCTIONAL GASTROINTESTINAL DISORDERS It has been recognized that gastrointestinal symptoms often occur in children who have allergy and atopy [14,15], and IBS is significantly more prevalent in adult patients who have allergic diseases, such as asthma and allergic rhinitis [16]. Key cells in the allergy cascade are eosinophils. Small bowel biopsies from patients who have asthma and allergic rhinitis show accumulation of eosinophils, T-cells, mast cells, macrophages, and increased expression of the proallergic cytokines IL-4 and IL-5, which are also observed in the inflammatory reaction in the airways [17–19]. Allergy to foodstuffs is a common patient perception. In a study from Germany, 32% of patients attending outpatients complained of adverse reactions to food as a cause of their abdominal symptoms. In 14%, the diagnosis
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of intestinal food allergy was suspected by an elevated total IgE, specific IgE against food antigens, peripheral eosinophilia, responsiveness to cromoglycate, and clinical signs of atopic disease. In only 3% (one tenth of the original cohort), however, was the diagnosis confirmed by endoscopic allergen provocation or elimination diet and re-challenge [20]. Eosinophils in the lower intestine of patients who have documented food allergy show characteristic features with regard to morphology, distribution, and functional behavior to IgE receptor stimulation. By immunohistochemical analysis of eosinophil peroxidase, there was a highly significant increase of eosinophils throughout the lower GI tract in samples obtained from patients who had food hypersensitivity–related diarrhea compared with control subjects [21]. Although there is reasonable evidence for a role for food allergy in provoking GI symptoms in eosinophilic gastroenteritides, there is emerging evidence that food allergy may be also important in FGIDs. A recent systematic review of this subject concluded that food allergy may be a factor contributing to the development of some FGIDs and food-specific IgE antibodies can mediate hypersensitivity reactions in patients who have IBS by sensitizing mucosal mast cells [22]. It has also been shown in clinical trials with exclusion diets that there is the potential to reduce symptoms in a subgroup of patients who have IBS with a clear history of adverse food reactions. These specific foods included wheat, beef, pork, and lamb in a study from the United Kingdom [23]. A second study from Australia of patients who had IBS and FD suggested wheat, crab, and shrimp allergy in IBS, whereas patients who had FD had high antibody titers to soybean and egg [24]. It was concluded that these findings did not contribute to the pathogenesis of FD and IBS; however, the method did not include biopsy of the GI tract to evaluate eosinophils and mast cells. EOSINOPHILS, INFECTION, AND FUNCTIONAL GASTROINTESTINAL DISORDERS Bacterial infections have been implicated as a trigger of FGIDs, particularly in the development of IBS, in which a specific postinfectious type is best recognized [25]. Postinfectious FD also seems probable, because acute onset of FD has been described following Salmonella infection in Spain [26]. Another study noted that 17% of patients who had acute-onset FD describe a preceding infective episode [27]. The bacterial species responsible for postinfectious IBS include Salmonella, Shigella, and Campylobacter jejuni [28]. These patients have an initial response with an increase in the CD3 and CD8 T lymphocyte population and a striking increase in serotonin containing enterochromaffin cells [29]. Although these changes may decline in most subjects, those who have persistent symptoms may continue to have persistent but subtle cellular changes. Patients who develop postinfectious IBS when assessed up to 3 months following infection still had a 20% increase in enterochromaffin cells, which contain serotonin [29].
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In FD, meta-analyses of trials show a small benefit of eradication of the gastric bacteria H pylori. In dyspepsia in the absence of alarm features and in FD, a policy of test and treat for H pylori is generally recommended; the number needed to successfully treat FD is 15, which suggests that this is not the sole cause of FD [30] and no consistent disturbance of upper GI motility or sensory disturbances have been found to be linked to H pylori infection [31]. Few studies have addressed the immune pathways in FGIDs. Notably, in eosinophil infiltration can be a major component after bacterial infection. Gastric eosinophil infiltration has been reported to be significantly higher in patients who have serum CagA positivity for H pylori [32]. A study of subjects who have FD showed gastric eosinophilia was predominant only in H pylori infection [8]. In colonic and rectal biopsies of patients who had IBS, eosinophils were not noted to be increased compared with controls, although these were not typed as to whether these were postinfection cases [33]. Although not all patients who have FGIDs have overt atopy, allergy, or history of infection, we have postulated that this type of inflammatory mechanism, and in particular the eosinophil, may provide potential answers to the pathophysiology of certain FGIDs [8]. EOSINOPHILS IN THE DUODENUM IN FUNCTIONAL DYSPEPSIA AND IRRITABLE BOWEL SYNDROME It is postulated that the pathophysiology of FD is linked to duodenal dysmotility with distension [34] and sensitivity to luminal contents [35]. Biopsy of the duodenum to ascertain pathology is, however, not routine practice and there are few studies that have focused on duodenal histology as a potential target for investigation. A study of pediatric patients who had dyspepsia with duodenal biopsies found that in 71% had duodenal eosinophilia. Following treatment with H1 and H2 antagonist therapy, on repeat biopsy this was reduced and abdominal pain decreased or disappeared [9]. We have shown that duodenal eosinophilia is associated with FD in adults [8]. In our population-based study of 1000 normal subjects, not actively seeking health care, the prevalence of reflux symptoms, dyspeptic symptoms, and irritable bowel symptoms was 40%, 38%, and 30% respectively [36]. There was a significant duodenal eosinophilia in subjects who had FD compared with controls and it was also noted in subjects who had FD that eosinophils formed clusters at the base of duodenal mucosal crypts [8]. These clusters of eosinophils in the duodenal bulb and second part were present more often in subjects who had FD compared with controls (respectively 51% versus 21%; 62% versus 12.5%). Furthermore, degranulation of eosinophils was seen only in subjects who had FD in a subset analysis (7 of 15 cases, 47% versus none in the 5 controls). In subjects who had IBS from the same cohort, however, duodenal eosinophilia was not observed, suggesting that duodenal eosinophilia is linked to FD but not IBS [37]. EOSINOPHIL FUNCTION IN THE GUT Eosinophils store active immunologic mediators in two intracellular compartments, large granules and small secretory granules. These active mediators
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are cationic proteins, growth factors, cytokines, chemokines, lipid mediators, and neuromodulators. The large granules contain four primary cationic proteins: eosinophil peroxidase (EPO), major basic protein (MBP), eosinophil cationic protein (ECP), and eosinophil-derived neurotoxin (EDN); these are cytotoxic secretory products [38]. The ability of eosinophils to show diversity in disease, for example by differing modes of degranulation [38], may be key to a role in the FGIDs. In a study of FD, eosinophils accumulated at the base of crypts and degranulation of MBP was shown by immunocytochemistry in those who had dyspepsia, but not in control subjects [8]. The interaction of eosinophils with other cells in the hypersensitivity cascade may be important in their role in FGIDs. In allergic inflammation, a combination of Th2 cell and cytokines (IL-4, IL-5, and IL13) and mast cell activity seems to be responsible for the initiation of eosinophil activity [39]. This response is sustained because it has been shown that tissue eosinophils can survive for 2 weeks in vitro [40]. IL5 and eotaxin selectively regulate eosinophil trafficking [41]. The Th2 cytokines (tumor necrosis factor, IL4, and IL13) up-regulate eotaxin production [42]. Eotaxin is produced locally and recruits eosinophils to the intestine with accumulation in Peyer patches [43]. Although eotaxin-1 has been noted to play a role in eosinophilic esophagitis in mice, eotaxin-1–deficient mice only developed modestly attenuated disease [44,45]. Esophageal eotaxin-3 mRNA and protein levels strongly correlated with tissue eosinophilia and mastocytosis in eosinophilic esophagitis in human subjects who have characteristic gene arrays, and a single-nucleotide polymorphism in the human eotaxin-3 gene was associated with disease susceptibility [46]. This susceptibility to eosinophil accumulation because of a genetic predisposition may provide a link to a genetic susceptibility to FGIDs. Twin studies support a genetic predisposition to FGIDs, although FD has not been adequately evaluated [47–49]. An association between the CC genotype of GNb3 and functional dyspepsia has been observed in two independent studies, which may reflect an altered immunophenotype in subjects prone to dyspepsia [50–52]. Eosinophils also secrete growth factors, including nerve growth factor (NGF), which maintains sympathetic neurons and promotes mast cell survival and activation. NGF is also produced by mast cells and has a role in resolution of tissue healing. [53]. Other major mediators secreted by eosinophils are the lipid mediators, leukotrienes (LTC) and platelet-activating factors (PAF), chemokines, and cytokines [54]. EOSINOPHIL INTERACTIONS IN GUT MUCOSA Eosinophils and T Cells Eosinophils communicate with T cells in a bidirectional manner and activate T cells by serving as antigen-presenting cells (APCs). Eosinophils can also regulate T-cell polarization through synthesis of indoleamine 2,3-dioxygenase (IDO). IDO catalyzes the oxidative metabolism of L-tryptophan to N-formylkynurenine (KYN), a regulator of Th1/Th2 balance [54]. KYN is subsequently
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converted to a range of metabolites, including xanthurenic acid and the neurotoxin quinolinic acid. IDO activity is induced in response to interferon-c and requires heme as a cofactor. Recent studies have suggested that, in vivo, IDO expression by APCs is responsible for inhibiting T-cell proliferation, and may be involved in immune tolerance [55]. In a study of subjects who had FD, the number of duodenal intraepithelial lymphocytes (IELs) was similar in patients who had FD and healthy controls. A careful analysis of activation markers on IELs showed CD95/Fas and HLADR were expressed by a significantly smaller percentage of duodenal CD3þ IELs in H pylori–negative patients who had FD defined according to Rome II criteria [56]. These lower ratios of CD95/Fas and HLA-DR expressing duodenal IELs in dyspeptic patients may reflect the presence of an altered population of primed lymphocytes [57]. Possibly these may be regulated by the increased eosinophils as seen in FD [8]. Eosinophils and Mast Cells Mast cells are bone marrow–derived granulocytes, also normally present in the small bowel, which contribute to a variety of immune responses. Mast cells are activated in stress [58,59], without allergic degranulation [60]. This finding may provide an all-important link with stress-related symptoms of FGIDs. Mast cells have been studied in FGIDs, particularly IBS, wherein they are increased in the jejunum [61], ileum [62], and cecum [33], and lie in close proximity to enteric nerves in colonic and rectal mucosa [63]. In FD, gastric mast cells may be increased in both H pylori–negative and –positive patients [64]. Mast cells can be activated through eosinophil-derived MBP by way of a similar pathway to the neuropeptide substance P mast cell activation pathway. MBP and substance P may both directly activate the mast cell Gi3 protein that controls degranulation [65]. Eosinophils express various growth factors, including stem cell factor (SCF), which promote mast cell survival, proliferation, maturation, and mediator release. It is speculated that the production of SCF by eosinophils is stimulated by mast cell chymase [66]; eosinophil-derived NGF further influences mast cell growth and survival by way of a specific TrkA NGF receptor expressed by mast cells [67]. Mast cells and eosinophils are thus codependent; initially mast cells can induce eosinophils into the mucosa, and these in turn activate mast cells by way of mediators and growth factors, causing proliferation, maturation, and degranulation. Mast cells in the presence of eosinophil-derived growth factors demonstrate resilience to death induced by death receptors [68]. The mast cell has been principally studied in IBS. The mast cells are not only increased but show greater proximity to nerves, and this has been positively correlated with the severity and frequency of patients’ abdominal pain [69]. Mast cells express receptors for the endocrine and paracrine messengers released from associated nerve endings and also have the ability to produce, store, metabolize, and release relevant molecules that modulate neural responses [69]. Substance P, released from nerve endings, activates mast cells.
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The mast cells release a mediator, which in turn results in neural activation, promoting intestinal secretion [70]. Nonactivated human mast cells that have not formed synaptic associations with nerve fibers do not respond to neuropeptides, such as substance P [71]. An interaction between mast cells and the CNS in rats has been demonstrated by pairing an audiovisual cue with an antigen challenge, thereby degranulating mast cell protease II. This interaction occurred predominantly in the intestinal mucosal mast cells [72]. In mast cell hyperplasia and inflammation, afferent vagal nerves penetrate jejunal mucosa and synapse with intestinal mucosal mast cells [73]. If truncal vagotomy is performed, the concentration of mucosal mast cells decreases and, conversely, stimulation of the cervical vagus nerve leads to an increase in mast cell concentration and histamine content of jejunal mucosa [74]. This finding provides important evidence for a CNS link to hypersensitivity (Fig. 2). Eosinophils and Serotonin The serotonin (5-hydroxytryptamine) signaling system is also implicated in functional bowel disorders [75] and the interplay of serotonin and enteric nerves may induce symptoms of IBS. Serotonin is the predominant neurotransmitter of the ENS and facilitates the effector components: muscle, epithelial secretion, enterochromaffin cells, and vasculature. It is also the principal communicator molecule between the ENS and the CNS. The gut secretes 95% of serotonin within the body, compared with 3% in the brain and 2% taken up
Fig. 2. Eosinophil function. Eosinophils produce lipid mediators, platelet-activating factor and leukotrienes (PAF, LTC), cytokines and chemokines (CXC), cytotoxic secretory products, eosinophil peroxidase (EPO), major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), nerve growth factor (NGF), and stem cell factor (SCF). MBP, SCF, and NGF regulate mast cell interaction. Eosinophils activate T cells by serving as antigen-presenting cells (APC) and regulate T cells through synthesis of indoleamine 2,3dioxygenase (IDO) by way of metabolism of L-tryptophan to N-formylkynurenine (KYN), a regulator of Th1/Th2 balance. The interaction of mast cells and eosinophils with neural pathways may produce pain and dysmotility.
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by platelets. [76]. Enterochromaffin cells in the gut epithelium from duodenum to rectum secrete serotonin. Serotonin is a potent eosinophil-active chemoattractant that can function additively with eotaxin [77]. Serotonin also induces mast cell adhesion and migration [78] and mast cells also secrete serotonin [79]. It is therefore likely that serotonin secreted by mast cells modulates dysmotility. On release, serotonin binds to receptors on the intrinsic afferent neurons in the mucosa leading to peristalsis and epithelial secretion [80]. Binding to 5HT3 receptors on efferent sensory neurons increases pain (and nausea) perception, which occurs by way of the vagus nerve, thus acting as another coordinating link between the CNS and the ENS (Fig. 3). EOSINOPHILS IN GASTROINTESTINAL DISEASE Eosinophil accumulation in the gastrointestinal tract is commonly seen in parasite infestation, drug reactions, specific eosinophilic syndromes, IBD, gastroesophageal reflux disease, and celiac disease [81]. A specific group, the eosinophilic syndromes of the gastrointestinal tract (eosinophilic esophagitis, eosinophilic gastritis, and gastroenteritis) are hypersensitivity disorders in which allergy is a prominent feature, and they often are associated with other allergy states, such as atopy [81,82]. These syndromes are independent of blood eosinophilia. In eosinophilic syndromes large numbers of eosinophils in the mucosa cause damage, and it is the accrual of cells at mucosal sites that is key to maintaining the balance between good and harm. There may be an overlap between eosinophilic syndromes and FGIDs that depends on eosinophil load. The evidence from studies of functional dyspepsia indicating a subtle elevation of duodenal eosinophils [8,9] suggests that this functional disorder in some cases may be another eosinophil-mediated disease.
Fig. 3. Interaction of serotonin, eosinophils, and mast cells in the gut. Serotonin is a potent eosinophil-active chemoattractant and induces mast cell adhesion and migration. Mast cells also secrete serotonin. Serotonin secreted by mast cells increases gut dysmotility and binds to receptors on afferent neurons leading to pain, by way of the vagus nerve, acting as a coordinating link between the CNS and the ENS.
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SUMMARY We propose that the neural–mast cell–eosinophil interaction may cause abdominal pain or meal-related symptoms characteristic of functional disease. The trigger is likely a pathogen, food, infection, or other allergen in the gut mucosa. This trigger evokes eosinophils, mast cells, and other components to cascade to up-regulate serotonin and modulation of the ENS directly and thus the CNS. Further study of the putative role of eosinophils in FGIDs by molecular techniques and histopathology is worth pursuing. References [1] Drossman DA. The functional gastrointestinal disorders and the Rome III process. Gastroenterology 2006;130:1377–90. [2] Simren M, Svedlund J, Posserud I, et al. Health-related quality of life in patients attending a gastroenterology outpatient clinic: functional disorders versus organic diseases. Clin Gastroenterol Hepatol 2006;4:187–95. [3] Nyrop KA, Palsson OS, Levy RL, et al. Costs of health care for irritable bowel syndrome, chronic constipation, functional diarrhoea and functional abdominal pain. Aliment Pharmacol Ther 2007;26:237–48. [4] Mayer EA, Naliboff BD, Craig AD. Neuroimaging of the brain-gut axis: from basic understanding to treatment of functional GI disorders. Gastroenterology 2006;131:1925–42. [5] Jones MP, Dilley JB, Drossman D, et al. Brain-gut connections in functional GI disorders: anatomic and physiologic relationships. Neurogastroenterol Motil 2006;18:91–103. [6] Goldstein RA, Paul WE, Metcalfe DD, et al. NIH conference. Asthma. Ann Intern Med 1994;121:698–708. [7] Rothenberg ME, Mishra A, Collins MH, et al. Pathogenesis and clinical features of eosinophilic esophagitis. J Allergy Clin Immunol 2001;108:891–4. [8] Talley NJ, Walker MM, Aro P, et al. Non-ulcer dyspepsia and duodenal eosinophilia: an adult endoscopic population-based case-control study. Clin Gastroenterol Hepatol 2007;5:1175–83. [9] Friesen CA, Sandridge L, Andre L, et al. Mucosal eosinophilia and response to H1/H2 antagonist and cromolyn therapy in pediatric dyspepsia. Clin Pediatr (Phila) 2006;45: 143–7. [10] Lowichik A, Weinberg A. A quantitative evaluation of mucosal eosinophils in the pediatric gastrointestinal tract. Mod Pathol 1996;9:110–4. [11] Pensabene L, Brundler MA, Bank JM, et al. Evaluation of mucosal eosinophils in the pediatric colon. Dig Dis Sci 2005;50:221–9. [12] Pascal RR, Gramlich TL, Parker KM, et al. Geographic variations in eosinophil concentration in normal colonic mucosa. Mod Pathol 1997;10:363–5. [13] Rothenberg ME, Cohen MB. An eosinophil hypothesis for functional dyspepsia. Clin Gastroenterol Hepatol 2007;5:1147–8. [14] Caffarelli C, Deriu FM, Terzi V, et al. Gastrointestinal symptoms in patients with asthma. Arch Dis Child 2000;82:131–5. [15] Caffarelli C, Cavagni G, Deriu FM, et al. Gastrointestinal symptoms in atopic eczema. Arch Dis Child 1998;78:230–4. [16] Powell N, Huntley B, Beech T, et al. Increased prevalence of gastrointestinal symptoms in patients with allergic disease. Postgrad Med J 2007;83:182–6. [17] Powell N, Huntley B, Knight W, et al. Dyspepsia is associated with allergic rhinitis. J Allergy Clin Immunol 2002;109(Suppl 1):S100. [18] Wallaert B, Desreumaux P, Coplin MC, et al. Immunoreactivity for interleukin 3 and 5 granulocyte/macrophage colony-stimulating factor of intestinal mucosa in bronchial asthma. J Exp Med 1995;182:1897–904.
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[42] Zimmermann N, Hershey GK, Foster PS, et al. Chemokines in asthma: cooperative interaction between chemokines and IL-13. J Allergy Clin Immunol 2003;111:227–42. [43] Hogan SP, Mishra A, Brandt EB, et al. A pathological function for eotaxin and eosinophils in eosinophilic gastrointestinal inflammation. Nat Immunol 2001;2:353–60. [44] Mishra A, Hogan S, Brandt EB, et al. An etiological role for aeroallergens and eosinophils in experimental esophagitis. J Clin Invest 2001;107:83–90. [45] Mishra A, Rothenberg ME. Intratracheal IL-13 induces eosinophilic esophagitis by an IL-5, eotaxin-1, and STAT6-dependent mechanism. Gastroenterology 2003;125:1419–27. [46] Blanchard C, Wang N, Stringer KF, et al. Eotaxin-3 and a uniquely conserved geneexpression profile in eosinophilic esophagitis. J Clin Invest 2006;116:536–47. [47] Morris-Yates A, Talley NJ, Boyce PM, et al. Evidence of a genetic contribution to functional bowel disorder. Am J Gastroenterol 1998;93:1311–7. [48] Levy RL, Jones KR, Whitehead WE, et al. Irritable bowel syndrome in twins: heredity and social learning both contribute to etiology. Gastroenterology 2001;121:799–804. [49] Lembo A, Zaman M, Jones M, et al. Influence of genetics on irritable bowel syndrome, gastro-oesophageal reflux and dyspepsia: a twin study. Aliment Pharmacol Ther 2007;1(25):1343–50. [50] Camilleri CE, Carlson PJ, Camilleri M, et al. A study of candidate genotypes associated with dyspepsia in a U.S. community. Am J Gastroenterol 2006;101:581–92. [51] Holtmann G, Siffert W, Haag S, et al. G-protein beta 3 subunit 825 CC genotype is associated with unexplained (functional) dyspepsia. Gastroenterology 2004;126:971–9. [52] Holtmann G, Talley NJ. Hypothesis driven research and molecular mechanisms in functional dyspepsia: the beginning of a beautiful friendship in research and practice? Am J Gastroenterol 2006;101:593–5. [53] Micera A, Puxeddu I, Aloe L, et al. New insights on the involvement of nerve growth factor in allergic inflammation and fibrosis. Cytokine Growth Factor Rev 2003;14:369–74. [54] Rothenberg ME, Hogan SP. The eosinophil. Annu Rev Immunol 2006;24:147–74. [55] von Bubnoff D, Hanau D, Wenzel J, et al. Indoleamine 2,3-dioxygenase-expressing antigenpresenting cells and peripheral T-cell tolerance: another piece to the atopic puzzle? J Allergy Clin Immunol 2003;112:854–60. [56] Gargala G, Lecleire S, Franc¸ois A, et al. Duodenal intraepithelial T lymphocytes in patients with functional dyspepsia. World J Gastroenterol 2007;13:2333–8. [57] Morimoto Y, Hizuta A, Ding EX, et al. Functional expression of Fas and Fas ligand on human intestinal intraepithelial lymphocytes. Clin Exp Immunol 1999;116:84–9. [58] Eutamene H, Theodorou V, Fioramonti J, et al. Acute stress modulates the histamine content of mast cells in the gastrointestinal tract through interleukin-1 and corticotropin-releasing factor release in rats. J Physiol 2003;553:959–66. [59] Santos J, Guilarte M, Alonso C, et al. Review—Pathogenesis of irritable bowel syndrome: the mast cell connection. Scand J Gastroenterol 2005;40:129–40. [60] Theoharides TC, Cochrane DE. Critical role of mast cells in inflammatory diseases and the effect of acute stress. J Neuroimmunol 2004;146:1–12. [61] Guilarte M, Santos J, de Torres I, et al. Diarrhoea-predominant IBS patients show mast cell activation and hyperplasia in the jejunum. Gut 2007;56:203–9. [62] Weston AP, Biddle WL, Bhatia PS, et al. Terminal ileal mucosal mast cells in irritable bowel syndrome. Dig Dis Sci 1993;38:1590–5. [63] Barbara G, Stanghellini V, De Giorgio R, et al. Activated mast cells in proximity to colonic nerves correlate with abdominal pain in irritable bowel syndrome. Gastroenterology 2004;126:693–702. [64] Hall W, Buckley M, Crotty P, et al. Gastric mucosal mast cells are increased in Helicobacter pylori-negative functional dyspepsia. Clin Gastroenterol Hepatol 2003;1:363–9. [65] Aridor M, Rajmilevich G, Beaven MA, et al. Activation of exocytosis by the heterotrimeric G-protein Gi3. Science 1993;262:1569–72.
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[66] Piliponsky AM, Gleich GJ, Bar I, et al. Review—Effects of eosinophils on mast cells: a new pathway for the perpetuation of allergic inflammation. Mol Immunol 2001;38:1369–72. [67] Horigome K, Bullock ED, Johnson EM Jr. Effect of nerve growth factor on rat peritoneal mast cells, survival promotion and immediate- early gene induction. J Biol Chem 1994;269: 2695–702. [68] Piliponsky AM, Daigle I, Simon H-U, et al. Expression and functionality of death receptors on the human mast cell line HMC-1. J Allergy Clin Immunol 2000;105:S63. [69] Spiller R, Campbell E. Post-infectious irritable bowel syndrome. Curr Opin Gastroenterol 2006;22:13–7. [70] Suzuki R, Furuno T, McKay DM, et al. Direct neurite–mast cell communication in vitro occurs via the neuropeptide substance P. J Immunol 1999;163:2410–5. [71] Bischoff SC, Schwengberg S, Lorentz A, et al. Substance P and other neuropeptides do not induce mediator release in isolated human intestinal mast cells. Neurogastroenterol Motil 2004;16:185–93. [72] Stead RH, Colley EC, Wang B, et al. Vagal influences over mast cells. Auton Neurosci 2006;125:53–61. [73] Williams RM, Berthoud HR, Stead RH. Vagal afferent nerve fibres contact mast cells in rat small intestinal mucosa. Neuroimmunomodulation 1997;4:266–70. [74] Gottwald T, Lhotak S, Stead RH. Effect of truncal vagotomy and capsaicin on mast cells and IgA- positive plasma cells in rat cells in rat jejunal mucosa. Neurogastroenterol Motil 1997;9:25–32. [75] Gershon MD, Tack J. The serotonin signaling system: from basic understanding to drug development for functional GI disorders. Gastroenterology 2007;132:397–414. [76] Gershon MD. Serotonin and its implication for the management of irritable bowel syndrome. Rev Gastroenterol Disord 2003;3:S25–34. [77] Boehme SA, Lio FM, Sikora L, et al. Cutting edge: serotonin is a chemotactic factor for eosinophils and functions additively with eotaxin. J Immunol 2004;173:3599–603. [78] Kushnir-Sukhov NM, Gilfillan AM, Coleman JW, et al. 5-hydroxytryptamine induces mast cell adhesion and migration. J Immunol 2006;177:6422–32. [79] Kushnir-Sukhov NM, Brown JM, et al. Human mast cells are capable of serotonin synthesis and release. J Allergy Clin Immunol 2007;119:498–9. [80] Gill RK, Saksena S, Tyagi S, et al. Serotonin inhibits Naþ/Hþ exchange activity via 5-HT4 receptors and activation of PKC alpha in human intestinal epithelial cells. Gastroenterology 2005;128:962–74. [81] Rothenberg ME. Eosinophilic gastrointestinal disorders (EGID). J Allergy Clin Immunol 2004;113:11–28. [82] Liacouras CA, Spergel JM, Ruchelli E, et al. Eosinophilic esophagitis: a 10-year experience in 381 children. Clin Gastroenterol Hepatol 2005;3:1198–206.
GASTROENTEROLOGY CLINICS OF NORTH AMERICA
Functional Gastrointestinal Disorders and the Potential Role of Eosinophils Marjorie M. Walker, BMedSci, BMBS, FRCPatha,*, Nicholas J. Talley, MD, PhD, FRACP, FRCP, FACPb a
Department of Histopathology, St. Mary’s Campus, Imperial College London, Norfolk Place, London W2 1PG, United Kingdom b Department of Internal Medicine, Mayo Clinic, 4500 San Pablo Road, Jacksonville, FL 32224, USA
FUNCTIONAL GASTROINTESTINAL DISORDERS The Rome III classification of the functional gastrointestinal disorders (FGIDs) defines 28 adult and 17 pediatric symptom-based gut conditions that have no established pathology [1]. These disorders place a considerable burden on individual patients, because quality of life is poorer in patients who have FGIDs compared with health and is comparable to organic GI disease [2]. There is also a major societal burden because of high direct and indirect costs [3]. Currently, diagnosis is based on symptoms or by exclusion, because there is no objective diagnostic test for these disorders; by definition it is necessary to exclude other diseases, specifically evidence of an inflammatory, anatomic, metabolic, or neoplastic process that explains the patient’s symptoms [1]. Abdominal pain, disordered defecation, and meal-related discomfort are common symptomatic themes for a range of FGIDs. Treatment remains empirically based on relieving symptoms rather than treating abnormal pathophysiology. The current accepted conceptual model of disease reflects an interaction between psychosocial factors, including stress and anxiety, that impinge on gut physiology by way of the brain–gut axis to produce visceral hypersensitivity or dysmotility. These signals interact by way of the gut enteric nervous system (ENS) and track back to the central nervous system (CNS) [1]. It is possible that this viscious cycle originates in gut mucosa, where an insult triggers a feedback loop, although how often this is the case remains uncertain. Key barriers in attempting to define an organic pathology for FGIDs include a lack of understanding of how symptoms are triggered and how any feedback loop is maintained. The pathophysiology of the FGIDs has been studied extensively to try to identify a common link for symptom expression. It is currently assumed that *Corresponding author. E-mail address: [email protected] (M.M. Walker). 0889-8553/08/$ – see front matter doi:10.1016/j.gtc.2008.02.007
ª 2008 Elsevier Inc. All rights reserved. gastro.theclinics.com
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there is not a simple or universal pathway to these differing outcomes, but a complex interaction of genetic predisposition, visceral hypersensitivity, and psychosocial distress in a primed central and enteric nervous system [4,5]. EOSINOPHILS AND FUNCTIONAL GASTROINTESTINAL DISORDERS The gut is an active immune organ that is the first line of defense for many insults, such as allergens in food, drugs, viruses, bacteria, and parasites. The authors surmise that altered immune function, by way of infection or hypersensitivity on a background of genetic predisposition and environment, triggers symptoms (Fig. 1). This hypothesis suggests that components of the hypersensitivity immune cascade may play a vital role in FGIDs. A key component of this pathway is the eosinophil, and this review explores a potential role for eosinophils and associated components of this pathway in FGIDs, specifically irritable bowel syndrome (IBS) and functional dyspepsia (FD), which have been principally studied. EOSINOPHILS IN THE GASTROINTESTINAL TRACT Eosinophils are a normal constituent throughout the gastrointestinal tract (GIT), except in the squamous mucosa of the esophagus. These cells have a defined role in physiologic and pathologic function. In the gut mature eosinophils originate from hemopoietic stem cells in the bone marrow; recruitment is regulated by interleukin (IL)–5, IL-3, and granulocyte macrophage colony-stimulating factor (GM-CSF) to induce migration to the gut. Eosinophils are effector cells, which release secretory granules and have an immunoregulatory function by way of cytokines and release and presentation of antigens [6]. WHAT IS THE NORMAL EOSINOPHIL COUNT IN THE GUT? Eosinophils have a characteristic morphology with a bright eosinophilic cytoplasm and a bilobed nucleus. They are easily recognizable in biopsies and
Fig. 1. A role for eosinophils in FGIDs? The interaction of psychosocial distress, environmental factors, and a genetic predisposition with a hypersensitive immune system in the gut may be maintained by eosinophil accumulation in the gut, by way of signaling to the ENS and then the CNS in functional bowel disorders.
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therefore counts of eosinophils can be performed on routine hematoxylin and eosin stained sections. Counting of eosinophils is critical at all sites to discriminate normal from a pathologic state. In the esophagus, eosinophils are evoked in acid reflux and to distinguish this from eosinophilic esophagitis, the cutoff level is conventionally considered to be at least 15 per high power field (HPF), although variable cutoffs have been used [7]. In gastric mucosa, eosinophilia is often seen in Helicobacter pylori infection but typically not in functional dyspepsia [8]. In H pylori–negative adult subjects, the mean gastric eosinophil counts were 10/5HPF in antral and body mucosa [8]. In the duodenum, eosinophilia has been noted in children and adults who have functional dyspepsia, defined as greater than 10/HPF for children [9] and greater than 22/5HPF with or without clusters of eosinophils at the base of glands for adults [8]. Normal duodenal counts were defined by less than 10/HPF in children and 19/5HPF in adults in these studies based on control values [8,9]. In the cecum, eosinophils are more numerous in the normal state, and up to 40/HPF have been suggested to be normal at this site [10]. Colonic eosinophilia has also been associated with IBS, inflammatory bowel disease (IBD), and food allergy in children, and mean eosinophil numbers per crypt (standard deviation) have been reported to be 34.8 (17.1) in IBD, 21.3 (8.8) in IBS, and 25.4 (16.7) in food allergy [11]. In this study there was no variation between colonic sites. Geographic variation in colonic eosinophilia in routine biopsies, in which eosinophils in the intercryptal space were counted, has been noted in the United States, where those in southern states had a higher count than the northern states [12]. Few studies have consistently counted in the same manner in normal or diseased subjects so establishing true normal values is difficult. There are relatively few studies to adequately address the normal counts throughout the gut, particularly the upper gut. Because subtle changes in numbers may be significant [13], it is important therefore for histopathologists to count eosinophils in the gastrointestinal sites and have data on local normal controls to define abnormality. EOSINOPHILS, ATOPY, ALLERGY, AND FUNCTIONAL GASTROINTESTINAL DISORDERS It has been recognized that gastrointestinal symptoms often occur in children who have allergy and atopy [14,15], and IBS is significantly more prevalent in adult patients who have allergic diseases, such as asthma and allergic rhinitis [16]. Key cells in the allergy cascade are eosinophils. Small bowel biopsies from patients who have asthma and allergic rhinitis show accumulation of eosinophils, T-cells, mast cells, macrophages, and increased expression of the proallergic cytokines IL-4 and IL-5, which are also observed in the inflammatory reaction in the airways [17–19]. Allergy to foodstuffs is a common patient perception. In a study from Germany, 32% of patients attending outpatients complained of adverse reactions to food as a cause of their abdominal symptoms. In 14%, the diagnosis
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of intestinal food allergy was suspected by an elevated total IgE, specific IgE against food antigens, peripheral eosinophilia, responsiveness to cromoglycate, and clinical signs of atopic disease. In only 3% (one tenth of the original cohort), however, was the diagnosis confirmed by endoscopic allergen provocation or elimination diet and re-challenge [20]. Eosinophils in the lower intestine of patients who have documented food allergy show characteristic features with regard to morphology, distribution, and functional behavior to IgE receptor stimulation. By immunohistochemical analysis of eosinophil peroxidase, there was a highly significant increase of eosinophils throughout the lower GI tract in samples obtained from patients who had food hypersensitivity–related diarrhea compared with control subjects [21]. Although there is reasonable evidence for a role for food allergy in provoking GI symptoms in eosinophilic gastroenteritides, there is emerging evidence that food allergy may be also important in FGIDs. A recent systematic review of this subject concluded that food allergy may be a factor contributing to the development of some FGIDs and food-specific IgE antibodies can mediate hypersensitivity reactions in patients who have IBS by sensitizing mucosal mast cells [22]. It has also been shown in clinical trials with exclusion diets that there is the potential to reduce symptoms in a subgroup of patients who have IBS with a clear history of adverse food reactions. These specific foods included wheat, beef, pork, and lamb in a study from the United Kingdom [23]. A second study from Australia of patients who had IBS and FD suggested wheat, crab, and shrimp allergy in IBS, whereas patients who had FD had high antibody titers to soybean and egg [24]. It was concluded that these findings did not contribute to the pathogenesis of FD and IBS; however, the method did not include biopsy of the GI tract to evaluate eosinophils and mast cells. EOSINOPHILS, INFECTION, AND FUNCTIONAL GASTROINTESTINAL DISORDERS Bacterial infections have been implicated as a trigger of FGIDs, particularly in the development of IBS, in which a specific postinfectious type is best recognized [25]. Postinfectious FD also seems probable, because acute onset of FD has been described following Salmonella infection in Spain [26]. Another study noted that 17% of patients who had acute-onset FD describe a preceding infective episode [27]. The bacterial species responsible for postinfectious IBS include Salmonella, Shigella, and Campylobacter jejuni [28]. These patients have an initial response with an increase in the CD3 and CD8 T lymphocyte population and a striking increase in serotonin containing enterochromaffin cells [29]. Although these changes may decline in most subjects, those who have persistent symptoms may continue to have persistent but subtle cellular changes. Patients who develop postinfectious IBS when assessed up to 3 months following infection still had a 20% increase in enterochromaffin cells, which contain serotonin [29].
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In FD, meta-analyses of trials show a small benefit of eradication of the gastric bacteria H pylori. In dyspepsia in the absence of alarm features and in FD, a policy of test and treat for H pylori is generally recommended; the number needed to successfully treat FD is 15, which suggests that this is not the sole cause of FD [30] and no consistent disturbance of upper GI motility or sensory disturbances have been found to be linked to H pylori infection [31]. Few studies have addressed the immune pathways in FGIDs. Notably, in eosinophil infiltration can be a major component after bacterial infection. Gastric eosinophil infiltration has been reported to be significantly higher in patients who have serum CagA positivity for H pylori [32]. A study of subjects who have FD showed gastric eosinophilia was predominant only in H pylori infection [8]. In colonic and rectal biopsies of patients who had IBS, eosinophils were not noted to be increased compared with controls, although these were not typed as to whether these were postinfection cases [33]. Although not all patients who have FGIDs have overt atopy, allergy, or history of infection, we have postulated that this type of inflammatory mechanism, and in particular the eosinophil, may provide potential answers to the pathophysiology of certain FGIDs [8]. EOSINOPHILS IN THE DUODENUM IN FUNCTIONAL DYSPEPSIA AND IRRITABLE BOWEL SYNDROME It is postulated that the pathophysiology of FD is linked to duodenal dysmotility with distension [34] and sensitivity to luminal contents [35]. Biopsy of the duodenum to ascertain pathology is, however, not routine practice and there are few studies that have focused on duodenal histology as a potential target for investigation. A study of pediatric patients who had dyspepsia with duodenal biopsies found that in 71% had duodenal eosinophilia. Following treatment with H1 and H2 antagonist therapy, on repeat biopsy this was reduced and abdominal pain decreased or disappeared [9]. We have shown that duodenal eosinophilia is associated with FD in adults [8]. In our population-based study of 1000 normal subjects, not actively seeking health care, the prevalence of reflux symptoms, dyspeptic symptoms, and irritable bowel symptoms was 40%, 38%, and 30% respectively [36]. There was a significant duodenal eosinophilia in subjects who had FD compared with controls and it was also noted in subjects who had FD that eosinophils formed clusters at the base of duodenal mucosal crypts [8]. These clusters of eosinophils in the duodenal bulb and second part were present more often in subjects who had FD compared with controls (respectively 51% versus 21%; 62% versus 12.5%). Furthermore, degranulation of eosinophils was seen only in subjects who had FD in a subset analysis (7 of 15 cases, 47% versus none in the 5 controls). In subjects who had IBS from the same cohort, however, duodenal eosinophilia was not observed, suggesting that duodenal eosinophilia is linked to FD but not IBS [37]. EOSINOPHIL FUNCTION IN THE GUT Eosinophils store active immunologic mediators in two intracellular compartments, large granules and small secretory granules. These active mediators
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are cationic proteins, growth factors, cytokines, chemokines, lipid mediators, and neuromodulators. The large granules contain four primary cationic proteins: eosinophil peroxidase (EPO), major basic protein (MBP), eosinophil cationic protein (ECP), and eosinophil-derived neurotoxin (EDN); these are cytotoxic secretory products [38]. The ability of eosinophils to show diversity in disease, for example by differing modes of degranulation [38], may be key to a role in the FGIDs. In a study of FD, eosinophils accumulated at the base of crypts and degranulation of MBP was shown by immunocytochemistry in those who had dyspepsia, but not in control subjects [8]. The interaction of eosinophils with other cells in the hypersensitivity cascade may be important in their role in FGIDs. In allergic inflammation, a combination of Th2 cell and cytokines (IL-4, IL-5, and IL13) and mast cell activity seems to be responsible for the initiation of eosinophil activity [39]. This response is sustained because it has been shown that tissue eosinophils can survive for 2 weeks in vitro [40]. IL5 and eotaxin selectively regulate eosinophil trafficking [41]. The Th2 cytokines (tumor necrosis factor, IL4, and IL13) up-regulate eotaxin production [42]. Eotaxin is produced locally and recruits eosinophils to the intestine with accumulation in Peyer patches [43]. Although eotaxin-1 has been noted to play a role in eosinophilic esophagitis in mice, eotaxin-1–deficient mice only developed modestly attenuated disease [44,45]. Esophageal eotaxin-3 mRNA and protein levels strongly correlated with tissue eosinophilia and mastocytosis in eosinophilic esophagitis in human subjects who have characteristic gene arrays, and a single-nucleotide polymorphism in the human eotaxin-3 gene was associated with disease susceptibility [46]. This susceptibility to eosinophil accumulation because of a genetic predisposition may provide a link to a genetic susceptibility to FGIDs. Twin studies support a genetic predisposition to FGIDs, although FD has not been adequately evaluated [47–49]. An association between the CC genotype of GNb3 and functional dyspepsia has been observed in two independent studies, which may reflect an altered immunophenotype in subjects prone to dyspepsia [50–52]. Eosinophils also secrete growth factors, including nerve growth factor (NGF), which maintains sympathetic neurons and promotes mast cell survival and activation. NGF is also produced by mast cells and has a role in resolution of tissue healing. [53]. Other major mediators secreted by eosinophils are the lipid mediators, leukotrienes (LTC) and platelet-activating factors (PAF), chemokines, and cytokines [54]. EOSINOPHIL INTERACTIONS IN GUT MUCOSA Eosinophils and T Cells Eosinophils communicate with T cells in a bidirectional manner and activate T cells by serving as antigen-presenting cells (APCs). Eosinophils can also regulate T-cell polarization through synthesis of indoleamine 2,3-dioxygenase (IDO). IDO catalyzes the oxidative metabolism of L-tryptophan to N-formylkynurenine (KYN), a regulator of Th1/Th2 balance [54]. KYN is subsequently
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converted to a range of metabolites, including xanthurenic acid and the neurotoxin quinolinic acid. IDO activity is induced in response to interferon-c and requires heme as a cofactor. Recent studies have suggested that, in vivo, IDO expression by APCs is responsible for inhibiting T-cell proliferation, and may be involved in immune tolerance [55]. In a study of subjects who had FD, the number of duodenal intraepithelial lymphocytes (IELs) was similar in patients who had FD and healthy controls. A careful analysis of activation markers on IELs showed CD95/Fas and HLADR were expressed by a significantly smaller percentage of duodenal CD3þ IELs in H pylori–negative patients who had FD defined according to Rome II criteria [56]. These lower ratios of CD95/Fas and HLA-DR expressing duodenal IELs in dyspeptic patients may reflect the presence of an altered population of primed lymphocytes [57]. Possibly these may be regulated by the increased eosinophils as seen in FD [8]. Eosinophils and Mast Cells Mast cells are bone marrow–derived granulocytes, also normally present in the small bowel, which contribute to a variety of immune responses. Mast cells are activated in stress [58,59], without allergic degranulation [60]. This finding may provide an all-important link with stress-related symptoms of FGIDs. Mast cells have been studied in FGIDs, particularly IBS, wherein they are increased in the jejunum [61], ileum [62], and cecum [33], and lie in close proximity to enteric nerves in colonic and rectal mucosa [63]. In FD, gastric mast cells may be increased in both H pylori–negative and –positive patients [64]. Mast cells can be activated through eosinophil-derived MBP by way of a similar pathway to the neuropeptide substance P mast cell activation pathway. MBP and substance P may both directly activate the mast cell Gi3 protein that controls degranulation [65]. Eosinophils express various growth factors, including stem cell factor (SCF), which promote mast cell survival, proliferation, maturation, and mediator release. It is speculated that the production of SCF by eosinophils is stimulated by mast cell chymase [66]; eosinophil-derived NGF further influences mast cell growth and survival by way of a specific TrkA NGF receptor expressed by mast cells [67]. Mast cells and eosinophils are thus codependent; initially mast cells can induce eosinophils into the mucosa, and these in turn activate mast cells by way of mediators and growth factors, causing proliferation, maturation, and degranulation. Mast cells in the presence of eosinophil-derived growth factors demonstrate resilience to death induced by death receptors [68]. The mast cell has been principally studied in IBS. The mast cells are not only increased but show greater proximity to nerves, and this has been positively correlated with the severity and frequency of patients’ abdominal pain [69]. Mast cells express receptors for the endocrine and paracrine messengers released from associated nerve endings and also have the ability to produce, store, metabolize, and release relevant molecules that modulate neural responses [69]. Substance P, released from nerve endings, activates mast cells.
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The mast cells release a mediator, which in turn results in neural activation, promoting intestinal secretion [70]. Nonactivated human mast cells that have not formed synaptic associations with nerve fibers do not respond to neuropeptides, such as substance P [71]. An interaction between mast cells and the CNS in rats has been demonstrated by pairing an audiovisual cue with an antigen challenge, thereby degranulating mast cell protease II. This interaction occurred predominantly in the intestinal mucosal mast cells [72]. In mast cell hyperplasia and inflammation, afferent vagal nerves penetrate jejunal mucosa and synapse with intestinal mucosal mast cells [73]. If truncal vagotomy is performed, the concentration of mucosal mast cells decreases and, conversely, stimulation of the cervical vagus nerve leads to an increase in mast cell concentration and histamine content of jejunal mucosa [74]. This finding provides important evidence for a CNS link to hypersensitivity (Fig. 2). Eosinophils and Serotonin The serotonin (5-hydroxytryptamine) signaling system is also implicated in functional bowel disorders [75] and the interplay of serotonin and enteric nerves may induce symptoms of IBS. Serotonin is the predominant neurotransmitter of the ENS and facilitates the effector components: muscle, epithelial secretion, enterochromaffin cells, and vasculature. It is also the principal communicator molecule between the ENS and the CNS. The gut secretes 95% of serotonin within the body, compared with 3% in the brain and 2% taken up
Fig. 2. Eosinophil function. Eosinophils produce lipid mediators, platelet-activating factor and leukotrienes (PAF, LTC), cytokines and chemokines (CXC), cytotoxic secretory products, eosinophil peroxidase (EPO), major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), nerve growth factor (NGF), and stem cell factor (SCF). MBP, SCF, and NGF regulate mast cell interaction. Eosinophils activate T cells by serving as antigen-presenting cells (APC) and regulate T cells through synthesis of indoleamine 2,3dioxygenase (IDO) by way of metabolism of L-tryptophan to N-formylkynurenine (KYN), a regulator of Th1/Th2 balance. The interaction of mast cells and eosinophils with neural pathways may produce pain and dysmotility.
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by platelets. [76]. Enterochromaffin cells in the gut epithelium from duodenum to rectum secrete serotonin. Serotonin is a potent eosinophil-active chemoattractant that can function additively with eotaxin [77]. Serotonin also induces mast cell adhesion and migration [78] and mast cells also secrete serotonin [79]. It is therefore likely that serotonin secreted by mast cells modulates dysmotility. On release, serotonin binds to receptors on the intrinsic afferent neurons in the mucosa leading to peristalsis and epithelial secretion [80]. Binding to 5HT3 receptors on efferent sensory neurons increases pain (and nausea) perception, which occurs by way of the vagus nerve, thus acting as another coordinating link between the CNS and the ENS (Fig. 3). EOSINOPHILS IN GASTROINTESTINAL DISEASE Eosinophil accumulation in the gastrointestinal tract is commonly seen in parasite infestation, drug reactions, specific eosinophilic syndromes, IBD, gastroesophageal reflux disease, and celiac disease [81]. A specific group, the eosinophilic syndromes of the gastrointestinal tract (eosinophilic esophagitis, eosinophilic gastritis, and gastroenteritis) are hypersensitivity disorders in which allergy is a prominent feature, and they often are associated with other allergy states, such as atopy [81,82]. These syndromes are independent of blood eosinophilia. In eosinophilic syndromes large numbers of eosinophils in the mucosa cause damage, and it is the accrual of cells at mucosal sites that is key to maintaining the balance between good and harm. There may be an overlap between eosinophilic syndromes and FGIDs that depends on eosinophil load. The evidence from studies of functional dyspepsia indicating a subtle elevation of duodenal eosinophils [8,9] suggests that this functional disorder in some cases may be another eosinophil-mediated disease.
Fig. 3. Interaction of serotonin, eosinophils, and mast cells in the gut. Serotonin is a potent eosinophil-active chemoattractant and induces mast cell adhesion and migration. Mast cells also secrete serotonin. Serotonin secreted by mast cells increases gut dysmotility and binds to receptors on afferent neurons leading to pain, by way of the vagus nerve, acting as a coordinating link between the CNS and the ENS.
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SUMMARY We propose that the neural–mast cell–eosinophil interaction may cause abdominal pain or meal-related symptoms characteristic of functional disease. The trigger is likely a pathogen, food, infection, or other allergen in the gut mucosa. This trigger evokes eosinophils, mast cells, and other components to cascade to up-regulate serotonin and modulation of the ENS directly and thus the CNS. Further study of the putative role of eosinophils in FGIDs by molecular techniques and histopathology is worth pursuing. References [1] Drossman DA. The functional gastrointestinal disorders and the Rome III process. Gastroenterology 2006;130:1377–90. [2] Simren M, Svedlund J, Posserud I, et al. Health-related quality of life in patients attending a gastroenterology outpatient clinic: functional disorders versus organic diseases. Clin Gastroenterol Hepatol 2006;4:187–95. [3] Nyrop KA, Palsson OS, Levy RL, et al. Costs of health care for irritable bowel syndrome, chronic constipation, functional diarrhoea and functional abdominal pain. Aliment Pharmacol Ther 2007;26:237–48. [4] Mayer EA, Naliboff BD, Craig AD. Neuroimaging of the brain-gut axis: from basic understanding to treatment of functional GI disorders. Gastroenterology 2006;131:1925–42. [5] Jones MP, Dilley JB, Drossman D, et al. Brain-gut connections in functional GI disorders: anatomic and physiologic relationships. Neurogastroenterol Motil 2006;18:91–103. [6] Goldstein RA, Paul WE, Metcalfe DD, et al. NIH conference. Asthma. Ann Intern Med 1994;121:698–708. [7] Rothenberg ME, Mishra A, Collins MH, et al. Pathogenesis and clinical features of eosinophilic esophagitis. J Allergy Clin Immunol 2001;108:891–4. [8] Talley NJ, Walker MM, Aro P, et al. Non-ulcer dyspepsia and duodenal eosinophilia: an adult endoscopic population-based case-control study. Clin Gastroenterol Hepatol 2007;5:1175–83. [9] Friesen CA, Sandridge L, Andre L, et al. Mucosal eosinophilia and response to H1/H2 antagonist and cromolyn therapy in pediatric dyspepsia. Clin Pediatr (Phila) 2006;45: 143–7. [10] Lowichik A, Weinberg A. A quantitative evaluation of mucosal eosinophils in the pediatric gastrointestinal tract. Mod Pathol 1996;9:110–4. [11] Pensabene L, Brundler MA, Bank JM, et al. Evaluation of mucosal eosinophils in the pediatric colon. Dig Dis Sci 2005;50:221–9. [12] Pascal RR, Gramlich TL, Parker KM, et al. Geographic variations in eosinophil concentration in normal colonic mucosa. Mod Pathol 1997;10:363–5. [13] Rothenberg ME, Cohen MB. An eosinophil hypothesis for functional dyspepsia. Clin Gastroenterol Hepatol 2007;5:1147–8. [14] Caffarelli C, Deriu FM, Terzi V, et al. Gastrointestinal symptoms in patients with asthma. Arch Dis Child 2000;82:131–5. [15] Caffarelli C, Cavagni G, Deriu FM, et al. Gastrointestinal symptoms in atopic eczema. Arch Dis Child 1998;78:230–4. [16] Powell N, Huntley B, Beech T, et al. Increased prevalence of gastrointestinal symptoms in patients with allergic disease. Postgrad Med J 2007;83:182–6. [17] Powell N, Huntley B, Knight W, et al. Dyspepsia is associated with allergic rhinitis. J Allergy Clin Immunol 2002;109(Suppl 1):S100. [18] Wallaert B, Desreumaux P, Coplin MC, et al. Immunoreactivity for interleukin 3 and 5 granulocyte/macrophage colony-stimulating factor of intestinal mucosa in bronchial asthma. J Exp Med 1995;182:1897–904.
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