Grains in the Gut: Recipe for Disaster?

Diet and IBD; ten years ago these two topics would invite controversy. Many specialists were divided in their opinions about diet. Some thought it might matter and others not. To complicate issues, there were whole groups of independently minded patients successfully reducing their symptoms following Elaine Gottschall’s diet, outlined in her book, Breaking the Vicious Cycle. This diet called for the elimination of all starches and sugars; a diet that the average IBD patient, myself included, might find difficult. Now, there is evidence that certain grain enzymes can function like adjuvants in the intestines leading to inflammation.

Poor mucus allows the interaction between cereal enzymes and the immune cells below.

Main points:

  • For some IBD patients, following a cereal-grain free diet helps their symptoms.
  • Cereal grains contain an enzyme that can initiate the innate immune system via a receptor that normally recognizes bacterially associated molecules.
  • Celiac patients, who are reactive to cereal gluten, develop increased amounts of Th17 cells.
  • Ulcerative colitis patients with active disease and colitis models have poor mucus allowing the penetration of bacteria to the epithelial surface.

Take home message: Poor mucus in IBD combined with exposure to cereal enzymes may combine to aggravate inflammation.

In my opinion, things are changing and many more specialists are looking at diet with a more open mind, especially, as it is now clear that intestinal microbiota play a role in health. To highlight this change, I decided to look at an article that focuses on Celiac disease published at the end of 2012 in the Journal of Experimental Medicine. Although it is not about IBD, the concepts discussed could very well be applied to other inflammatory diseases of the intestinal tract.

Celiac disease is different than IBD. For one, it is localized in the small intestines and not the colon and it is caused by immune reactions to a specific antigen, cereal grain-derived gluten. Moreover, it only happens in individuals that can present the gluten peptides in their MHC class II molecules (HLA-DQ8 and HLA-DQ2). For this reason, it is accepted that T cells are involved in the etiology of Celiac disease. Extensive studies have been done to determine the precise peptides involved, and a recent article in PloS ONE, has indicated that these T cells are likely to be Th17.

However, as the immunologists among us know, antigen specific T cells do not appear out of thin air. They need instructions from cells of the innate immune system. Thus, it was a mystery where that trigger came from. This is where it gets exciting. It appears that cereal grains, besides providing peptide antigens, also stimulate Toll-like Receptor 4 (TLR4) through α-amylase/trypsin inhibitors (ATIs). Ironically, this almost suggests that in the right predisposed individuals, cereal grains are like vaccinations containing both antigens and adjuvants.

Naturally, with all articles that look at triggering via TLR4, lipopolysaccharide (LPS) contamination is an issue. To control for this, the author’s used a clever trick of digesting away the ATIs in their preparations and then demonstrating that the activity was lost. LPS, being a sugar, is not susceptible to enzymatic degradation. If it were responsible for the observed effects than enzymatic digestion would have had no effect.

To make their point, the authors stimulated human cell lines, TLR4 and MyD88 deficient mice (in vitro and in vivo) and Celiac patient intestinal biopsies with ATIs. It all turned out, as one would expect; ATI stimulation led to the same effects as LPS. They observed copious production of IL-8, TNFα, IL-12 and MCP-1, the baddies of the pro-inflammatory response. They also nicely observed the maturation of dendritic cells. It would have been interesting to see precisely what kinds of T cells differentiate from these dendritic cells. They suggest Th1 would be induced due to the IL-12, but they did not look at other cytokines like IL-6, IL-23 or IL-1β, which are associated with Th17 responses.

During their discussion, they wondered how it is possible that these kinds of responses are controlled in healthy individuals (remember that reaction to ATIs is not dependent on a specific HLA). They suggest that the difference the reactions found in Celiac patients and healthy ones could be a result of either a lowered immune threshold or a lack of inhibitory mechanisms at the level of TLR4 signaling in Celiac patients. I would suggest that they are missing another factor: mucus.

Mucus covers the intestinal walls and protects the epithelial barrier from luminal contents. I do not suspect that cereal-grain derived ATIs are mucus privileged. This would mean that in healthy patients contact with TLR4 probably doesn’t happen on a regular basis. Yet, it appears that for colitis susceptible individuals the mucus layer is different. An article, appearing in Gut, looked at the mucus layer of a multitude of spontaneous colitis models (Muc2-/-, C1galt1−/−, Tlr5-/-, Il10-/- and Slc9a3-/-) and found that all had defective mucus. The authors than examined Ulcerative colitis patients and found that all relapsing patients and some patients in remission had mucus that was extremely penetrable. Could it be that it was also more penetrable for ATIs?

Not all genetically susceptible individuals (with the right HLA) develop Celiac disease. Perhaps, the difference lies in their mucus quality. This could also be of importance to IBD patients. Are those that respond best to restrictive diets, the ones that are most aggravated by ingested ATIs? Looking at the results of these two articles, it seems that the combination of weak mucus and ATIs could lead to even more inflammation. Truly, a recipe for disaster.

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment.

Sources

Gottschall, E. G. (1994) Breaking the Vicious Cycle: Intestinal Health Through DietThe Kirkton Press

Johansson, M. E. V., Gustafsson, J. K., Holmen-Larsson, J., Jabbar, K. S., Xia, L., Xu, H., et al. (2013). Bacteria penetrate the normally impenetrable inner colon mucus layer in both murine colitis models and patients with ulcerative colitis. Gut. doi:10.1136/gutjnl-2012-303207

Junker, Y., Zeissig, S., Kim, S. J., Barisani, D., Wieser, H., Leffler, D. A., et al. (2012). Wheat amylase trypsin inhibitors drive intestinal inflammation via activation of toll-like receptor 4. The Journal of Experimental Medicine, 209(13), 2395–2408. doi:10.1084/jem.20102660

Sjöberg, V., Sandström, O., Hedberg, M., Hammarström, S., Hernell, O., & Hammarström, M.-L. (2013). Intestinal T-cell Responses in Celiac Disease – Impact of Celiac Disease Associated Bacteria. (K. Sestak, Ed.)PloS One, 8(1), e53414. doi:10.1371/journal.pone.0053414.t003

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