Celiac disease is a small intestine inflammatory disorder with multiple organ involvement, sustained by an inappropriate immune response to dietary gluten. Anti-transglutaminase antibodies are a typical serological marker in patients with active disease, and may disappear during a gluten-free diet treatment. Involvement of infectious agents and innate immunity has been suggested but never proven. Molecular mimicry is one of the mechanisms that links infection and autoimmunity.
Methods and Findings
In our attempt to clarify the pathogenesis of celiac disease, we screened a random peptide library with pooled sera of patients affected by active disease after a pre-screening with the sera of the same patients on a gluten-free diet. We identified a peptide recognized by serum immunoglobulins of patients with active disease, but not by those of patients on a gluten-free diet. This peptide shares homology with the rotavirus major neutralizing protein VP-7 and with the self-antigens tissue transglutaminase, human heat shock protein 60, desmoglein 1, and Toll-like receptor 4. We show that antibodies against the peptide affinity-purified from the sera of patients with active disease recognize the viral product and self-antigens in ELISA and Western blot. These antibodies were able to induce increased epithelial cell permeability evaluated by transepithelial flux of [3H] mannitol in the T84 human intestinal epithelial cell line. Finally, the purified antibodies induced monocyte activation upon binding Toll-like receptor 4, evaluated both by surface expression of activation markers and by production of pro-inflammatory cytokines.
Our findings show that in active celiac disease, a subset of anti-transglutaminase IgA antibodies recognize the viral protein VP-7, suggesting a possible involvement of rotavirus infection in the pathogenesis of the disease, through a mechanism of molecular mimicry. Moreover, such antibodies recognize self-antigens and are functionally active, able to increase intestinal permeability and induce monocyte activation. We therefore provide evidence for the involvement of innate immunity in the pathogenesis of celiac disease through a previously unknown mechanism of engagement of Toll-like receptor 4.
A subset of anti-transglutaminase IgA antibodies recognize the viral protein VP-7, suggesting a possible involvement of rotavirus infection in the pathogenesis of celiac disease through a mechanism of molecular mimicry.
Celiac disease is an autoimmune, digestive disorder in which the small intestine (the part of the gut that absorbs nutrients from food) is damaged. In autoimmune diseases, the immune system, which normally provides protection against foreign invaders, attacks a person's own tissues. In celiac disease, this attack is triggered by eating food containing gluten, a mixture of proteins found in wheat, barley, and rye. To avoid malnutrition, people with celiac disease—about one in 100 people of north European descent—must follow a strict, lifelong gluten-free diet, one that avoids baked products, wheat, pasta, and many other foods. If they fail to do this, their immune system may attack not only their gut but also their brain, skin, joints, and other tissues, in part through the production of antibodies (autoantibodies) that recognize a protein (self-antigen) called tissue transglutaminase. Celiac disease is diagnosed also by looking for these autoantibodies in patients' blood when they are on a gluten-containing diet; they rapidly disappear when a gluten-free diet is adopted.
Why Was This Study Done?
A gluten-free diet keeps celiac disease in check but does not cure it and is very difficult to follow. Even the minute amounts of gluten found in medicines, for example, can trigger the production of autoantibodies and active disease. But developing a cure is impossible without a better understanding of how celiac disease develops. Why, for example, do celiac disease patients make anti-transglutaminase antibodies? Were they made initially to ward off an infectious agent but unfortunately also recognized transglutaminase? In this study, the researchers asked whether “molecular mimicry”—cross-reactivity between self-molecules and foreign molecules on bacteria or viruses (pathogens)—might initiate celiac disease. They also asked whether innate immunity (the part of the immune system that responds quickly to general features on pathogens) as well as adaptive immunity (the production of antibodies and immune cells that recognize specific features on pathogens) is involved in the development of celiac disease.
What Did the Researchers Do and Find?
The researchers purified antibodies from blood provided by patients with celiac disease when they were eating food containing gluten and when they were on a gluten-free diet. They used these to identify celiac peptide, a synthetic protein fragment that was recognized only by the antibodies made by patients with active disease. By searching a database of pathogen proteins, the researchers discovered that rotavirus protein VP-7 contains a very similar peptide; a search of a database of human proteins indicated that celiac peptide also resembles peptides found in tissue transglutaminase, Toll-like receptor 4 (TLR4; a protein involved in the innate immune response), and several other self-antigens. Patient antibodies purified through their ability to bind to celiac peptide also bound to VP-7 and to these self-antigens, and only patients with active disease made these antibodies. The researchers also investigated whether these anti-celiac peptide antibodies might affect the gut or the innate immune system. The antibodies increased the permeability of a layer of gut cells growing in a laboratory dish by interacting with the self-antigen desmoglein 1. This protein helps to make impermeable seals between the cells that line the gut so that food antigens in the gut cannot seep out into the tissues where the immune system might detect them. In addition, by binding to TLR4, the anti-celiac peptide antibodies activated monocytes—cells that function in both the innate and adaptive immune response.
What Do These Findings Mean?
The finding that some anti-transglutaminase antibodies recognize the viral protein VP-7 could mean that rotavirus infection, which causes gastroenteritis, helps to initiate celiac disease in susceptible individuals through molecular mimicry. Furthermore, the identification of other self-antigens that contain peptides recognized by the antibodies made during active disease starts to explain why damage occurs outside the gut in people with celiac disease. The ability of these antibodies to recognize all these peptides could be coincidental, but the observation that the antibodies have relevant functional effects—the ability to increase intestinal permeability and to activate monocytes—makes this less likely. More research is needed to reveal exactly how infections and the innate immune response affect the development of celiac disease, but every piece of new information brings the possibility of a cure a little closer.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0030358.
US National Institute of Diabetes and Digestive and Kidney Diseases, information for patients on celiac disease
MedlinePlus encyclopedia entries on celiac disease and on autoimmunity
Wikipedia pages on celiac disease and on autoimmunity (note that Wikipedia is a free online encyclopedia that anyone can edit)