The results of this study demonstrate a significant association between IL-10 genotypes and IBS, with fewer patients having the high producer genotype compared with healthy controls. The lower prevalence of the high producer genotype in IBS suggests that high production of IL-10 may have some protective role or, conversely, that individuals predisposed to produce lower amounts of this cytokine might be more likely to develop the condition. A genetic predisposition to lower anti-inflammatory cytokine production could mean that control of the inflammatory response may be compromised in some individuals and may help to explain why gastrointestinal infections, for example, can sometimes lead to continuing problems. It is possible that an inflammatory process is perpetuated by failure of downregulation secondary to an inadequate anti-inflammatory cytokine response.
There are a number of points however that need to be considered in the interpretation of these findings. IL-10 is only one of the anti-inflammatory cytokines involved in regulation of immune and inflammatory responses, and the possible involvement of other cytokines in the inflammatory process cannot be ruled out. However, examination of allelic and genotypic frequencies for the biallelic polymorphisms of TGF-β1
, albeit on a smaller number of patients, did not reveal any significant differences from healthy controls. The IL-10 gene does have a number of different polymorphisms but the site selected in this study (−1082) is known to influence IL-10 production in lymphocytes, and differences in IL-10 levels have been measured in serum and from activated peripheral blood cells.15, 20, 24
Lymphocytes have been found in excessive numbers in the intestinal mucosa in cases of post infective irritable bowel syndrome (PI-IBS)13
but the question of whether in vivo IL-10 production within the intestine is similarly influenced by genotype remains to be addressed.
IBS is almost certainly a multifactorial condition with different combinations of factors operating in any one individual. Thus it is likely that a certain facet of the condition under genetic control will only account for a relatively small proportion of patients. Furthermore, if that facet has no biological marker, identifying appropriate subgroups for more indepth evaluation presents difficulties. At first sight, PI-IBS might seem a relatively straightforward group to study but this will inevitably contain patients with pre-existing forme fruste IBS as well as de novo cases, either of which might have the genetic trait under scrutiny. Similar pitfalls could accompany another potentially fruitful line of enquiry—that is, those patients with a family history. It is highly likely that this group could contain patients in whom learning from parents is just as important as inheritance from the same.
This study was not powered to assess an association between our findings and any particular characteristic of the condition, such as diarrhoea, constipation, bloating, pain, or even apparent precipitation by dysentery. However, the fact that in such an unselected group of patients a significant trend emerged strongly suggests that it might contain a subgroup in whom genes controlling inflammation might be important and this warrants further investigation.
Future studies on PI-IBS will have to be very carefully designed to ensure homogeneity of the groups under study as far as possible. This should include documenting the infecting organism, not forgetting viruses, considering measurement of markers of inflammation, both serological and mucosal, and possibly assessing gastrointestinal physiology or mucosal permeability. In addition, the role of dietary antigens may also have to be taken into account.
In conclusion, the association found between IL-10 genotype frequencies in an unselected group of IBS patients suggests there may be a subgroup of patients in whom this gene is critically important to the development of their disease. In addition, these results provide some support for the concept that genetic factors may also contribute to the pathogenesis of this condition.