Clinical symptoms and basis of diagnostics

Coeliac disease (CD) is a chronic inflammatory intestinal disease with known environmental trigger and a strong genetic component.  The external trigger is ingested wheat gluten or related proteins from rye, barley and possibly oats.  CD is a multigenic disorder with a strong HLA association.  HLA-DQ2 and DQ8 confer susceptibility together with, as yet unidentified, gene loci.  Expression of HLA-DQ2 and DQ8 molecules that present gliadin-derived peptides to CD4 T cells is necessary but not sufficient to develop CD.  The non-HLA locus that shows evidence for association with CD is the cytotoxic T lymphocyte-associated (CTLA-4/CD28) gene region.  Gene products of this region have a regulatory role on T cell function and in maintaining tolerance to self-antigens.

The mechanism of intestinal immunopathogenesis involves HLA-restricted CD4 T cells in the lamina propria as well as an immune reaction by CD8 T cells in the intestinal epithelium.  Loss of tolerance to gluten-derived gliadin peptides provides the external trigger for CD.  The self antigen involved in CD immunopathogenesis is an enzyme tissue transglutaminase type 2 (tTG) which is found apart from many other tissues in the intestine under the epithelium and in the brush border.  Gliadin is an excellent substrate for tTG that changes positively charged glutamines to negatively charged glutamic acid residues by deamidation in gliadin peptides.  These tTG-transformed peptides are the immunodominant parts of the 33mer gliadin peptide that is resistant to digestion by gastric and pancreatic enzymes and is presented by HLA-DQ2 and DQ8 to specific CD4 T cells in the lamina propria.  Autoantibody formation against tTG is a hallmark of CD serology.

The disease is also known as gluten sensitive enteropathy.  It causes considerable morbidity, often presenting with diarrhoea, malnutrition and failure to thrive in infants, or diarrhoea and osteoporosis in later life.  Infertility, autoimmune diseases and lymphomas have also been associated with CD.  The disease typically has a wide range of clinical manifestations resulting in patients having to seek help from many different specialists.

Coeliac disease has remained under-diagnosed.  Recently its prevalence has increased dramatically, and it is considered to affect 1 in 100 to 200 of the European population.  Several reports indicate a widespread occurrence of CD in many countries around the world.  Intensified serologic screening has contributed to the increased prevalence figures.  Serologic screening of the population, especially high-risk groups, is important to identify CD, for example, among diabetes mellitus, other autoimmune disease and lymphocytic colitis patients.

The diagnosis of CD is based on characteristic changes in intestinal biopsy samples and improvement in clinical symptoms and histological tests on a gluten-free diet.  Typical histology shows villous atrophy with crypt hyperplasia and intraepithelial lymphocytosis.

Diagnostics is not easy since many patients have very few symptoms (silent or latent disease) or have varying atypical symptoms.

Serological tests have an important role in the diagnostics and follow-up of patients with CD.  Several serological screening tests are available.  They include anti-gliadin antibodies (both IgA and IgG class), anti-endomysium antibodies and anti-tTG antibodies (both IgA and IgG class).  Antibodies against the external trigger antigen, gliadin, are less specific for the diagnostics of CD than the autoantibodies against tTG which are also detected in assays using endomysium as the target antigen.  Titres of tTG and endomysial antibodies correlate with the degree of villous atrophy and severity of the symptoms.  However, some patients with partial villous atrophy may not have antibodies against tTG or endomysium but they have antibodies against gliadin.  Thus anti-gliadin antibodies have their role in the diagnosis of CD and in addition they may prove important in analysis of other gluten-dependent disorders.

Antibodies to tTG and endomysium appear in almost 100% of the patients with active CD and in 80% of those with dermatitis herpetiformis.  Endomysium antibodies are detected by indirect immunofluorescence using either monkey esophagus or human umbilical cord as substrate.  The autoantigen for endomysial antibodies is the enzyme tTG which has led to the development of ELISA assays with guinea pig or human tTG.  ELISA with recombinant or native human tTG has better sensitivity than ELISA using guinea pig tTG.

The IgA antibody titres decrease or become negative in patients on a gluten-free diet but antibodies reappear when gluten is ingested and thus reflect compliance with the diet.  Intestinal biopsy remains the gold standard of CD diagnostics but, especially in adults, follow-up of anti-gliading or autoantibody levels against tTG has in many cases reduced the need for multiple biopsies.

Selective IgA deficiency is found in a 10 fold higher frequency among CD patients than in the normal population.  This presents a challenge to serological diagnostics, since IgA antibodies against gliadin and tTG are lacking.  These patients can be diagnosed with IgG class antibody tests.

In patients with total IgA deficiency high values of IgG antibodies to gliadin or endomysium or tTG are found.  It has been observed that IgA class antigliadin antibodies are more disease specific and IgG antibodies can be found in normal controls as well as other disease controls such as Crohn's disease, liver diseases and other gastrointestinal disorders.

It has become evident that gluten sensitivity is no longer restricted just to intestinal problems.  A large spectrum of both clinical and histological features induced by gluten is observed in genetically susceptible DQ2-positive individuals.  The clinical spectrum may vary from active disease to a silent CD and histological findings may vary from flat mucosa at one end, to a normal villous and crypt architecture, but with an abnormally high density or count of intraepithelial lymphocytes at the other end.  Especially in family studies, genetically susceptible family members may have normal villous architecture but immunological signs of sensitization to gluten (gliadin antibodies) or endomysium and tTG.  In follow-up studies some of those subjects have developed coeliac lesions in their gut mucosa.  This finding has evoked the term latent CD.

25% of patients with CD also have immune response-mediated lesions in the skin and thus a cutaneous manifestation of CD, dermatitis herpetiformis (DH), and 50-70% of patients with DH have lesions in the gut and consequently also coeliac-related antibodies.  Consequently these serological tests can also be used in the diagnostics of DH.  DH is a bullous skin disease affecting mainly the extensor surfaces of the major joints.  In DH granular IgA deposits are found in upper dermis and IgA-containing immune complexes have a possible role in the pathogenesis of DH.  Antibodies from the sera of patients react with tTG type 2 but it has been shown that the major autoantigen in DH is a related enzyme epidermal tissue transglutaminase or tTG type 3.

Effective treatment of CD is a strict life-long gluten-free diet.  A favorable clinical response is accompanied by a decline in IgA class antibodies to gliadin or endomysium/tTG usually in six months, and IgG antibodies take more than six months to decline.  The measurement of IgA class antibodies is especially suitable for the monitoring of the dietary compliance which is mandatory for the healing of the disease.  Since anti-gliadin antibodies are formed against the provoking agent, gliadin, they disappear rapidly when the gluten free diet starts.  The fall in the titre of antibodies to endomysium/tTG when on gluten-free diet reflects healing of the intestinal mucosa.  Therefore it is advisable to follow both anti-gliadin and endomysium/tTG antibodies to control compliance to GFD in CD patients.

Written by

The Biofile Scientific Team

2.9.2005

See Biofons coeliac disease panel.

Reference list coming in the near future.