Celiac Disease

Clinical significance: The Anti-Gliadin (GAF-3X) serves for the determination of antibodies against gliadin in the serological diagnosis of gluten-sensitive enteropathy (celiac disease) and Duhring’s dermatitis herpetiformis. Celiac disease is an autoimmune disease which occurs in predisposed individuals as a reaction to gluten sensitivity. We know today that after absorption in the lamina propria of the intestinal mucosa gliadin is deamidated by tissue transglutaminase (tTG). In this process particular glutamine residues are replaced by glutamic acid residues. In individuals with a genetic predisposition pieces (peptides) of gliadin modified (reconstructed) in this way bind to e.g. HLADQ2/8 molecules of the antigen presenting cells and are presented to helper T cells. An extensive immune reaction is triggered, causing pathological tissue changes, particularly damage to the small intestine. Components of this immune reaction are antibodies against the deamidated gliadin generated by tTG and also antibodies against tTG itself. The frequency of diagnosed celiac disease amounts to around 1:300 in western Ireland, Italy, the USA, the Middle East, India and Cuba, around 1:1,000 in Germany and Austria and around 1:2,000 to 1:4,500 in other European countries.

Gluten is found in various cereals (e.g. wheat, barley, rye). It consists of a mixture of proteins, which can be divided into two groups: prolamins and glutelins, with gliadin being the most frequent prolamin. If patients with celiac disease consume food containing gluten, this will finally lead to damage to the mucous membranes of the small intestine, which shows a flat, mosaic-like surface without any villi structure but with deep visible crypts. This, in turn, results in functional disorders. In the Marsh classification the intestinal histopathological grade of the disease is divided into three levels:

• Marsh type I: increase in intraepithelial lymphocytes (> 40 IEL/100 epithelial cells) with normal mucous membrane architecture

• Marsh type II: additional crypt hyperplasia with normal villi

• Marsh type III: IEL increase, crypt hyperplasia, degeneration of epithelial cells and villous atrophy.Type III is further divided into Marsh IIIA (partial villous atrophy), Marsh IIIB (subtotal villous atrophy) and Marsh IIIC (complete villous atrophy).

The clinical symptoms comprise fatigue (78%), borborygmus (72%), abdominal pain (64%), diarrhoea (56%), effects of malabsorption (44%) with weight loss, anaemia and growth retardation in children, vomiting (16%), constipation (12%) and bone pains (12%). Some patients with gluten- sensitive enteropathy also suffer from Duhring's dermatitis herpetiformis (10%), a chronic skin disease accompanied by the formation of blisters. In a prolonged course, mainly in adults, the risk of melanoma, particularly intestinal T-cell lymphoma, is about 10%.

If other autoimmune diseases are present in addition to celiac disease, the risk of developing splenic hypofunction amounts to approximately 60%, independent of the duration of gluten-free diet. In severe cases of celiac disease, the prevalence increases to 80%. A deficiency of IgM memory B lymphocytes, which play a role in the protection from bacteria, particularly encapsulated bacteria (e.g. Streptococcus pneumoniae), is held responsible for the hypofunction of the spleen accompanying celiac disease. In unproblematic cases of celiac disease or without the presence of further autoimmune diseases the probability of developing a splenic hypofunction is about 20%.

HLA antigens play a decisive role in celiac disease and Duhring's dermatitis herpetiformis, whereby there are regional differences in frequency and gene combinations. The HLA-DQ2 and HLA-DQ8 coding genes account for around 40% of the genetic influence.

An important contribution to the diagnosis of gluten-sensitive enteropathy and Duhring’s dermatitis herpetiformis is provided by the (non-invasive) determination of antibodies (IgA and IgG) against tissue transglutaminase and deamidated gliadin peptides (e.g. GAF-3X).

The clinical diagnosis is rounded off by the serological investigation. The determination of antibodies against gliadin is also suitable for control of a gluten-free diet or a gluten loading test. The prevalence of specific antibodies in children with celiac disease under gluten-free diet depends on the patient’s compliance with the diet and amounts to around 15-30% according to literature. If a relapse takes place under gluten loading, the level of IgA and IgG antibodies against gliadin rises within a few days.

During the acute phase of a gluten-sensitive enteropathy, antibodies of class IgA against endomysium/ tissue transglutaminase and/or gliadin are almost always detected. IgA-negative celiac disease patients show a higher incidence of latent forms (around 13%), recurrent infections (around 30%) and atopic diseases (around 13%) than IgA-positive patients. Furthermore, it has been observed that a selective IgA antibody deficiency against gliadin is associated with a very high titer of class IgG anti-gliadin antibodies in almost 100% of cases. IgM antibodies against gliadin are detected occasionally, but generally only when antibodies of classes IgA and IgG against gliadin are also present. Anti-gliadin IgM plays therefore almost no diagnostic role.

The serological determination of IgG and IgA antibodies should be examined in first-degree relatives of celiac disease patients, as well.

With the development of a state-of-the-art “designer antigen”, from which the immunological reactive surface is created, the Anti-Gliadin (GAF-3X) is a recombinant “gliadin-analogue fusion peptide”, which produces a positive reaction almost exclusively in patients with celiac disease and Duhring’s dermatitis herpetiformis, but not in healthy persons or patients with other gastrointestinal diseases. The fusion peptide consists of two components: a deamidated epitope of gliadin consisting of nine amino acids, and an artificial gliadin-homologue octapeptide. The deamidated epitope is probably pathophysio- logically relevant for celiac disease and makes up no more than 2% of the total size of gliadin. The remaining 98% of the gliadin molecule is not used in these tests – immunological ballast, which serves predominantly as a target for unspecific reactions. This explains the increase in specificity for the Anti-Gliadin-(GAF-3X). Moreover, the construct is expressed in trimer form in order to also increase the responsiveness of the test system.

For a reliable, comprehensive diagnosis, including identification of IgA-deficiency patients, the deter- mination of both antibody classes anti-gliadin (GAF-3X) IgG and anti-gliadin (GAF-3X) IgA is recommended.