This study provides evidence of increased mast cell numbers along with mast cell activation in the jejunum of diarrhoea‐predominant IBS patients. These novel findings extend those of previous studies showing mast cell hyperplasia in terminal ileum and colon of IBS patients, indicating that mucosal inflammation in D‐IBS patients is not limited to the lower gut. Moreover, activation of jejunal mast cells suggests that local mast cell‐mediated inflammatory events contribute to functional disturbances and clinical manifestations of D‐IBS.
Despite the increasing prevalence and importance of IBS, there is no single distinctive biological marker and the diagnosis is based on clinical criteria and exclusion of other gastrointestinal disorders.24
In recent years, the conventional view of IBS as a non‐organic disorder has been challenged by reports of low‐grade mucosal inflammation as a relevant pathological substrate in some IBS patients (reviewed in Bercik et al2
). In particular, increased numbers of inflammatory cells and chemical mediators have been identified in mucosa specimens from the ileum and colon of D‐IBS patients which were apparently not hypercellular on routine histology.29,30
In agreement with those studies, we have found that the vast majority of jejunal biopsies from D‐IBS patients were reported by the pathologist as being within normal limits or showing mild, usually considered as non‐specific, mucosal inflammation. Moreover, specific counts of IELs, based on CD3 immunohistochemistry, revealed a 1.5‐fold increase in D‐IBS jejunum further supporting the presence of low grade epithelial inflammation and making unlikely the existence of more severe inflammatory processes such as lymphocytic enteritis or coeliac disease.31
In addition, the young age of our patients also argues against microscopic colitis as an alternative diagnosis since most of these patients are in their 70s at the time of diagnosis. Although quantitative differences in inflammatory cells may be present in different studies, this can be explained by patient selection, and segmental or aetiological differences in IBS subsets.2,29,30
Whether these findings may serve to differentiate aetiological (post‐infectious, post‐stress) variants of IBS or may be related to clinical severity still remains unknown.
Mast cells have been frequently considered in the context of allergic and parasite inflammation, but growing and convincing evidence indicates that they also participate in a wide variety of physiological and pathological processes32
including the regulation of epithelial barrier, mucosal immune function, motility and gut visceral sensitivity.13,33
Such abnormalities in gut function have been reported in IBS and could be partly responsible for the clinical findings in these patients, especially those with predominant diarrhoea, since patients with increased intestinal mast cells,34
and up to 70% of those with systemic mastocytosis,35
develop diarrhoea and abdominal discomfort.
The quantitative analysis of mucosal mast cells was based on CD117 positive immunohistochemistry which has shown high correlation with tryptase staining, is not altered by massive degranulation and can be regarded as specific for mast cells in the gut mucosa.36
Our results indicate that jejunal mastocytosis (>20 MC/hpf) is a constant feature in the mucosa of this selected group of patients with D‐IBS. Our D‐IBS patient group was quite homogeneous and the average profile would be a naive individual experiencing active and mostly severe bowel disturbances, not previously diagnosed or treated, and suffering from moderate psychological stress but no coincident allergic disorders. Increased mast cell numbers and mast cell biological products have been previously described in the terminal ileum and proximal and distal colon of IBS patients.17,18,19,29,30
Moreover, increased numbers of mucosal mast cells have been recently described in the duodenal mucosa of patients with diarrhoea, with some of these individuals probably belonging to the D‐IBS subgroup.37
We cannot exclude the fact that small bowel mast cell hyperplasia may represent an epiphenomenon linked to low‐grade mucosal inflammation since there have been conflicting reports describing increased or decreased mast cell numbers in the upper small intestine in disorders such as chronic urticaria,38
and coeliac disease.40
Biopsies of the upper small bowel are often obtained as part of the clinical evaluation of diarrhoea and, based on routine histopathology, reported as normal. D‐IBS may be easily overlooked by the physician following a conventional approach and we suggest that on account of the relative ease of jejunal capsule biopsy sampling and the very high sensitivity and positive predictive values of CD‐117 analysis (over 90% in our study), biopsy sampling as a complementary criterion for the positive diagnosis of D‐IBS deserves further evaluation.
Some studies have shown a potential influence of clinical history or age on mucosal mast cell numbers.17,24,41
Although our data do not support a correlation between age or gender with mast cell numbers, we acknowledge the relatively low number of participants as a limitation to properly evaluating this aspect. Some questions remain to be answered such as the influence of the clinical course (active or remission), the length of clinical history and the relationship of clinical severity with mucosal mast cell numbers or mast cell phenotype. Again, further studies are warranted to address these and other emerging concerns.
Tryptase is an abundant specific neutral protease of human mast cells that can be measured in various biological fluids and may serve as a useful marker of mast cell activation.42
Our study is unique in reporting the association of jejunal mast cell hyperplasia with, only local, in vivo mast cell activation, as disclosed by elevated levels of jejunal luminal tryptase but not serum tryptase. Although mast cells are the only significant source of tryptase in the intestinal mucosa, no correlation between the number of mucosal mast cells and the levels of luminal tryptase was detected. One plausible explanation for these findings is that mast cell activation and secondary release of tryptase may not be a continuous process. In fact, mast cell activation in IBS seems to be more a piecemeal‐like phenomenon where slow and selective release of mediators occurs and increased luminal release takes place only in specific settings. Indeed, ultrastructural signs of piecemeal degranulation and in vitro release of mast cell products such as tryptase and histamine have been shown in colonic and ileal biopsies of IBS patients17,18,19
and in some cases of duodenal samples, in patients with chronic diarrhoea.43
Although tryptase, via activation of protease‐activated receptor‐2 or other unrelated mechanisms, is a good candidate to explain some of the pro‐secretory and pro‐inflammatory effects of mast cells,44,45
other mast cell mediators may be also involved.
The mechanisms, mediators and pathways that may account for mast cell activation and hyperplasia in the jejunum of D‐IBS patients remain to be fully characterised. The list of potential candidates is large and growing although the known ability of molecules such as stem cell factor,46
transforming growth factor‐β1
to modulate crucial aspects of human mast cell physiology such as secretory activity, growth and maturation, phenotype or migration could make them initial, but not exclusive, favourites.
We have found that D‐IBS patients were experiencing higher levels of psychological stress than the control group. Although we did not find a positive correlation between stress levels and mast cell numbers or tryptase release, a suggestive trend was apparent. However, it is well recognised that IBS is a stress‐sensitive disorder, where life events are strong predictors of clinical exacerbation49
and the existence of distorted autonomic patterns along with neuroendocrine abnormalities in the hypothalamic‐hypophyseal‐adrenal axis50
seem to be related to changes that result in the predominant bowel habit (diarrhoea/constipation) and intestinal visceral hyperalgesia.51
These observations, consistent with the participation of neurohumoral mediators of stress in the initiation and development of such pathophysiological abnormalities,11,52
are being substantiated by experimental and clinical studies showing that both stress and corticotropin‐releasing hormone regulate intestinal epithelial and immune function via mast cell activation.12,15,22,53
Finally, epidemiologic studies suggest that IBS and functional dyspepsia overlap to a greater extent than would be expected by chance alone.20,21
In agreement with these proposals, we have found that 70% of our D‐IBS patients fulfilled Rome II criteria for functional dyspepsia. Although extension of mucosal mast cell involvement from the distal colon to the upper gut might be a helpful explanation for the frequent overlap, we did not observe any difference in jejunal mast cell numbers between dyspeptic and non‐dyspeptic IBS patients. Although not thoroughly evaluated, others have shown that mast cells were increased in the antrum and corpus of patients with Helicobacter pylori
‐negative functional dyspepsia54
compared to controls. Thus, another key unanswered question is whether patients with dyspepsia and IBS share a similar pathogenesis but different clinical expression.
In conclusion, jejunal mast cell hyperplasia and tryptase release may be frequent and useful findings in non‐treated D‐IBS patients. Their validity as biological markers for D‐IBS and usefulness for developing mast cell‐related treatment strategies in these patients should be established by further studies.