Celiac disease: how immune experiments connected genes, grain and tissue
Celiac disease acquired an increasingly specific immune explanation through experiments on inherited genes, intestinal cells and antibodies. Its classification is autoimmune: the immune system reacts against the body's own tissues. Gluten supplies an external trigger, but the resulting process involves more than difficulty digesting a food. MedlinePlus Genetics describes inflammation that shortens and flattens the small intestine's absorptive villi. The research behind that classification followed several distinct trails, from a shared genetic combination to immune recognition of particular proteins.
One trail became unusually clear in 1989. Ludvig Sollid and colleagues examined DNA from 94 unrelated children with celiac disease, using probes that distinguished versions of the HLA-DQA1 and HLA-DQB1 genes. All but one child could share a particular combination of those genes. That amounted to 98.9% of the group. The finding narrowed the genetic association to a pair of components of an HLA-DQ molecule, rather than leaving it as a general observation that the condition sometimes ran in families.
The arrangement was more revealing than a simple shared stretch of DNA. The relevant genes could occur together on one inherited chromosome, or their combination could be assembled from versions carried on opposite members of a chromosome pair. The researchers described these arrangements as cis and trans. Their interpretation was that the children could consequently share the same two-part HLA-DQ molecule. The result concerned a selected group already affected by celiac disease; it did not establish that everyone with that genetic combination would develop intestinal inflammation.
In 1993, Knut Lundin and colleagues investigated the immune cells in the affected organ itself. They exposed small-intestinal biopsy material from adults with celiac disease to gluten in laboratory conditions, then isolated activated T cells. The team established cell lines and clones that recognized gliadin, a component of wheat gluten. Antibodies that blocked HLA molecules helped identify which molecules presented that material to the cells. The predominant route involved HLA-DQ, including the particular DQ2 combination associated with the condition. The responding clones belonged to the CD4 group and carried diverse T-cell receptors. This connected the genetic association with a measured cellular response inside intestinal tissue. The experiment used removed biopsy material and cultured cells, so its findings described those preparations rather than the entire course of illness in every participant.
The grain side of celiac disease presented a separate technical obstacle: gliadin was a mixture rather than one uniform antigen. In January 2000, E. Arentz-Hansen and colleagues published an investigation using a commercial Nordic wheat cultivar. They sequenced 16 alpha-gliadin genes with complete coding sequences. Those genes specified 11 distinct proteins, only one of which was already represented in the Swiss-Prot database. A single wheat strain therefore supplied considerable molecular variety before any immune response was measured.
The team produced individual gliadin proteins and tested their recognition by intestinal T cells. The published figures compared crude gliadin mixtures with particular recombinant proteins, including digested preparations modified with tissue transglutaminase. Cell activation was measured through release of interleukin-2. The contribution to celiac disease research was a defined set of antigens: immune responses could be compared against known proteins instead of an incompletely characterized flour-derived mixture. That made the complexity of the trigger experimentally visible without assigning identical effects to every gliadin sequence.
A 2002 investigation approached immune recognition from the body's side. Miklós Sárdy and colleagues compared celiac disease with dermatitis herpetiformis, a related blistering skin disorder. Their serum collection included 104 people with the intestinal condition and 59 with the skin condition, alongside several control groups. They investigated antibodies reacting with tissue transglutaminase and with the related epidermal enzyme, transglutaminase 3. These were human proteins, making the target of the antibody response different in kind from an incoming grain protein.
Both patient groups had antibodies reacting with the two enzymes, but the skin group showed stronger binding to epidermal transglutaminase and a population of antibodies specific to it. Examination of the skin deposits identified that epidermal enzyme together with IgA. The investigators distinguished it from tissue transglutaminase as the dominant autoantigen in dermatitis herpetiformis. Their comparison gave celiac disease research a more precise boundary: related gluten-associated conditions could share immune features while differing in the principal human protein targeted in a particular tissue.
The genetic background also has limits. MedlinePlus Genetics explains that HLA-DQA1 and HLA-DQB1 help the immune system distinguish the body's proteins from foreign proteins. The associated variants occur in about 30% of the general population, yet only about 3% of people with those variants develop the condition. Environmental factors and other genes appear to contribute. Parents, siblings and children of affected people have an estimated 4%–15% chance of developing it. The pattern of inheritance remains unknown, despite the documented tendency for the condition to cluster in families.
- MedlinePlus Genetics, Celiac disease: autoimmune classification and genetic susceptibility — medlineplus.gov
- Sollid and colleagues, Evidence for a primary association of celiac disease to a particular HLA-DQ alpha/beta heterodimer, 1989 — pmc.ncbi.nlm.nih.gov
- Lundin and colleagues, Gliadin-specific HLA-DQ-restricted T cells isolated from intestinal mucosa, 1993 — pmc.ncbi.nlm.nih.gov
- Arentz-Hansen and colleagues, Production of a panel of recombinant gliadins for the characterisation of T cell reactivity, 2000 — pmc.ncbi.nlm.nih.gov
- Sárdy and colleagues, Epidermal Transglutaminase (TGase 3) Is the Autoantigen of Dermatitis Herpetiformis, 2002 — pmc.ncbi.nlm.nih.gov