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We read the recent article by Houghton et al and found the results very interesting (Gut 2003;52:663–70). Their observations included higher platelet concentrations of 5-hydroxytryptamine among patients with diarrhoea predominant irritable bowel syndrome (d-IBS) compared with controls. It is interesting that a small but significant subgroup of IBS patients report onset of their symptoms after an episode of acute gastroenteritis and a role of subclinical inflammatory aetiology has been suggested for the condition.1 The role of platelets in various inflammatory conditions has previously been demonstrated but their importance in IBS remains largely unknown.2–7 We recently looked at the possibility of platelet activation in IBS patients by determining surface expression of the activation markers at baseline and after stimulation. Stimulation involved the use of thrombin receptor activating peptide (TRAP), activation markers P-selectin (CD62) and glycoprotein 53 (CD63), and glycoprotein (GP) receptors GPIb-IX and GPIIb/GPIIIa, using whole blood flow cytometric analysis (Becton Dickenson Flow Cytometer).8,9
Twenty consecutive IBS patients (18 females), mean age 29 years (20–62), fulfilling the Rome II criteria (90% d-IBS) and 15 healthy controls (11 females), mean age 28 years (22–49), were included. Raised inflammatory markers, previous bowel disease or surgery, diverticulosis, and current or recent (past four weeks) use of non-steroidal anti-inflammatory drugs were exclusion criteria.
Standard venepuncture precautions were observed for sample collection and final analysis.8 A fluorescein isothiocynate (FITC) conjugated GP1b specific antibody was used to gate around the platelet population and list mode data on 10 000 platelets acquired. Mean fluorescence intensity (MFI) was used to quantify FITC labelled GPIIb/GPIIIa and GPIb-IX specific antibody binding. Binding of P-selectin and GP53 to a phycoerythrin labelled monoclonal antibody was expressed as the percentage of platelets positive for that antibody (% fluorescence). We tested varying strengths of TRAP, ranging from 110 to 670 mm, in five controls and found maximal reactivity of circulating platelets at a concentration of 223 mM (concentration used for activation studies). Differences between groups (p) were assessed using the Mann-Whitney U test for unpaired data. All analyses were performed using the Minitab statistical software and SPSS for windows (10.0.5).
Baseline expression of P-selectin was significantly increased in the IBS group (median 5.9 (interquartile range (IQR) 4.4–8.9)) compared with healthy controls (median 4.1 (IQR 3.2–5.9)) (p = 0.03), all values representing per cent expression. Baseline expression of GP53 was higher in the IBS group (median 3.0 (IQR 1.9–4.0)) compared with normal controls (median 2.3 (IQR 1.9–2.8)) but failed to reach clinical significance. TRAP stimulation resulted in increased expression of P-selectin and GP53 in both groups. Glycoprotein reactivity post stimulation was significantly lower in the IBS group compared with normal controls (p<0.05).
The numbers of GPIIb/IIIa and GPIb-IX receptor sites on the platelet surface for each group were calculated using a calibration curve where MFI and the corresponding number of antibody sites of multiple bead populations were plotted using a log log scale. The results in the two groups were comparable.
In IBS patients with normal routine inflammatory markers, we demonstrated a significant increase in surface expression of baseline P-selectin. The observed changes in baseline and reactive expression of platelet activation markers may support the theory of an ongoing subclinical inflammatory process in IBS. Reduced glycoprotein reactivity following TRAP stimulation in IBS may possibly signify a continuous low level platelet activation and degranulation with consequent platelet “exhaustion” and reduced expression of antigens. Precise interpretation of our results remains unclear due to the small number of included patients. Future studies involving a wider IBS population with possible subdivision based on the various disease characteristics, including determination of the possible disease triggering event, particularly a past history of gastroenteritis, may help to further clarify these observations.
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