Low-grade inflammation or immune activation of the gut has been proposed to have a role in irritable bowel syndrome (IBS) pathophysiology (1
), and specifically in the development of visceral hypersensitivity and epithelial dysfunction (5
). Reported findings in post-infectious IBS (PI-IBS) as well as in non-PI-IBS or unselected patients generally support this hypothesis. The strongest evidence is from studies of patients with PI-IBS, which have shown increased number colonic lamina propria T lymphocytes (8
), and increased mRNA levels of the pro-inflammatory cytokine inter-leukin-1β (IL-1β) in the rectal mucosa compared with patients who do not develop IBS symptoms after a gastroenteritis (9
). However, findings have been less consistent in non-PI-IBS patients and unselected patients, particularly in regards to possible correlations of such findings with IBS symptoms. This may be in part related to sample size and increased heterogeneity. Additionally, although IBS occurs in both men and women, most of the studies conducted in IBS have been performed in female patients without a matched sex distribution in the control groups. However, a female predisposition to PI-IBS (10
) and sex differences in the physiologic disturbances in IBS, including colonic mucosal mast cell numbers of IBS (11
), suggest that sex differences in immune activation may exist in IBS.
In unselected IBS patients, studies have reported increased activated T cells in the blood (12
) and increased T lymphocytes (3
) mast cells (2
), and toll-like receptor expression (17
) within the colonic mucosa. Barbara et al.
) found that the number of mast cells in close proximity to sensory neurons positively correlated with severity and frequency of abdominal pain/discomfort. However, other studies have found similar or lower mast cell numbers in colonic or rectal mucosa in IBS patients compared with healthy controls (3
). In one of these studies, IBS patients had fewer mast cells in the rectum and descending colon and decreased tryptase release regardless of the presence of visceral hypersensitivity in comparison to control subjects (18
Similar to studies of inflammatory cells in non-PI-IBS and unselected patients, levels of serum cytokines have generally been increased but specific cytokines studied and positive findings have differed (4
). There is a lack of evidence to support higher mucosal mRNA or protein levels of pro-inflammatory cytokines in non-PI-IBS (23
). In female IBS patients, Macsharry et al.
) demonstrated lower or normal mRNA expression and protein levels of pro-inflammatory cytokines, including IL-1β and IL-6, in colonic tissue from IBS but decreased expression of the anti-inflammatory cytokines IL-10 and TGF-β (transforming growth factor-β)compared with healthy females. In contrast, patients with active inflammatory bowel disease (IBD) had increased chemokines and pro-inflammatory cytokines. Furthermore, we have previously shown lower mRNA levels of IL-2, IL-6, and IL-10 from sigmoid colon biopsies in female IBS patients with diarrhea (IBS-D) compared with healthy females although the two groups were relatively small in size (24
Studies in IBD have demonstrated that active inflammation is associated with upregulation of mucosal signaling systems, including the corticotropin releasing factor (CRF) (25
), neurokinin (NK) (26
), and serotonin (5-HT) systems located in enterochromaffin (EC) cells (27
). Several studies indicate increased expression of the CRF ligand –receptor system in the colonic mucosa of IBD patients (28
). Increased expression of NK-1R and altered serotonin signaling have been reported in patients with IBD and in animal models of colitis (26
). For example, in severe UC (ulcerative colitis), EC cell counts, mucosal 5-HT content, and mRNA expression of tryptophan hydroxylase 1 and SERT (serotonin reuptake transporter) were decreased (32
). In contrast, increased EC cell counts have been demonstrated in rectal biopsies of PI-IBS patients (8
), while normal counts have been found in unspecified or non-PI-IBS (14
). EC cell counts were found to be either normal (non-severe) or decreased (severe) in ulcerative colitis (32
). However, mucosal 5-HT content and mRNA expression of tryptophan hydroxylase 1 and the serotonin transporter (SERT) were decreased. It has been hypothesized that SERT expression decreases after intestinal inflammation, resulting in amplification of serotonergic signaling that exerts an increased inflammatory response (27
Despite the large body of published data, uncertainty remains regarding the following questions: (i) Is the colonic mucosa the source of reported immune activation? (ii) Are IBS symptoms correlated with mucosal immune activation? (iii) Are reported findings related to coexistent psychological symptoms? (iv) Do the reported findings differ between men and women? The current study, performed in a well-characterized sample of male and female IBS patients and matching healthy controls, was aimed to address these questions. A comprehensive set of immune markers, and markers previously shown to be upregulated in association with mucosal inflammation were measured in serum and mucosal biopsies to test the following hypotheses: (i) IBS patients show increased mucosal or systemic expression of pro-inflammatory cytokines, and increased numbers of mucosal lymphocytes and mast cells compared with controls; (ii) IBS patients show altered expression of neuropeptide and neuroendocrine markers (CRF and NK signaling systems, EC and enteroendocrine (EE) cells), which are often associated with mucosal inflammation; and (iii) these peripheral changes correlate with subjective symptoms of IBS.