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Coeliac Disease and Gluten Sensitivity Syndrome

(Download  Coeliac Disease and Gluten Sensitivity Syndrome.pdf)

Introduction

Coeliac disease (CD) is an inflammatory disorder of the small intestine induced by chemicals called "prolamins" (proline-rich amines) which are found in the cereals wheat, barley, and rye. Typically, CD presents with malabsorption characterised by weight loss, abdominal distension, diarrhoea and steatorrhoea. Histology of jejunal mucosa demonstrating villous atrophy establishes the diagnosis and may be used for assessment following dietary intervention. Serological tests have enabled a more selective use of endoscopic procedures in patients with a high probability of disease, and provide an alternative method for monitoring dietary intervention and compliance.

"Gluten Sensitivity Syndrome"

With the introduction of serological screens for CD, disease variants have been recognised which present with features such as fatigue and asymptomatic iron deficiency without the classic malabsorptive symptoms. The breadth of pathology induced by prolamins can extend beyond the gastrointestinal tract, most notably causing the blistering pruritic skin rash of dermatitis herpetiformis and the neurological complication of gluten ataxia. This range of clinical syndromes has prompted some authors to broaden the diagnosis from "gluten-sensitive enteropathy" to "gluten sensitivity syndrome". This broadening of the clinical spectrum of disease may raise the prevalence of such conditions from 1:2000 to as high as 1:100. Clinical conditions associated with gluten sensitivity are listed in Table 1.

The Role of Serological Screening Must Be Re-Evaluated

The diagnostic criteria for CD remain inadequate to encompass the whole range of presentations of gluten sensitivity. The diagnostic "gold standard" requires demonstration of biopsy evidence of small bowel villous atrophy which normalises with dietary restriction and deteriorates upon rechallenge. This will allow most patients with symptomatic gut disease to be diagnosed, but excludes a significant number of people with "latent" coeliac disease, dermatitis herpetiformis, gluten-induced ataxia, and other conditions. In most of the studies of coeliac serology to date, positive antibody results in the absence of demonstrable bowel pathology have been labelled falsely positive, thus reducing the diagnostic specificity of serology. It is quite possible that many of these patients with "normal" small bowel biopsies but high titre antibodies to endomysial, gliadin or transglutaminase antigens could be included in the category of gluten sensitivity syndrome if other clinical features are evident, particularly if these occur in the setting of appropriate HLA haplotypes, family history, IgA deficiency, or other autoimmune conditions such as IDDM or autoimmune thyroiditis (Table 2).

Population Screening?

With the increasing recognition that gluten sensitivity is a more common disorder than had previously been believed, the question arises about the value of population screening. A screening program could be justified by the high sensitivity of current serological tests, the limitations of other diagnostic approaches, concerns about the development of complications such as gut lymphoma, and the ready availability of a suitable dietary intervention. However, this is a controversial area, and many questions about coeliac and related disorders must first be answered before such an approach could be recommended. Issues such as the impact of early intervention on the quality of life and the avoidance of complications of CD must be prospectively studied.

Disease Pathogenesis

Gluten sensitive enteropathy straddles the realms of food hypersensitivity and autoimmunity, and the way in which these two processes are able to combine to produce small bowel mucosal damage has been an area of much controversy. However, with recent advances in the study of CD, especially the identification of the antigenic target transglutaminase, a model of coeliac pathogenesis has become possible (Figure 1).

Figure 1 - Model of Coeliac Pathogenesis

An initial mucosal assault, perhaps due to gut infection or even the toxic effects of gliadin, upregulates tissue transglutaminase (tTG). This enzyme causes cross linking of various proteins, including gliadin. This tTG-gliadin complex represents a "new antigen" (neoantigen) which could then trigger the production of anti-tTG antibodies. Gliadin-specific CD4+ T-helper cells (which exist in the small bowel mucosa of CD patients,) provide help for the production of anti-tTG antibodies by tTG-specific B cells.

This "hapten" model explains a number of other features of CD:

• the gliadin-dependent nature of small bowel damage;
• the localisation of disease to the small bowel, where gliadin is linked with tTG;
• the greater disease specificity of tTG antibodies compared with gliadin antibodies; and
• the high prevalence of the HLA haplotype DQ2 in CD patients, as gliadin-specific T cells in the gut mucosa (but not in peripheral blood) preferentially recognise gliadin peptides presented within the cleft of this cell-surface molecule by antigen-presenting cells.

tTG plays a role in maintaining the integrity of both intestinal crypt micro architecture and the dermo-epidermal junction, so the development of villous atrophy and the lesions of dermatitis herpetiformis would be explained by this final common pathway.

Serological Screens for Coeliac Disease

Historically, anti-reticulin antibodies (ARA) were used as a serological screen, but this test has been replaced by the more reliable anti-gliadin antibodies (AGA) and anti-endomysial antibodies (EMA). While AGA are useful for monitoring dietary compliance, testing for IgA EMA by indirect immunofluorescence on monkey oesophagus or human umbilical cord substrates has until recently been regarded as the most reliable serological screen for CD. The identification of the antigenic target of these antibodies, the enzyme tissue transglutaminase (tTG), has allowed the development of ELISAs for detecting antibodies to tTG as a diagnostic tool.

Two studies have been performed by Hunter Immunology Unit comparing an in-house ELISA for detecting antibodies to tTG (TGA) with a commercial TGA kit against traditional serodiagnostic tests.

Pertinent findings included the following:

• The sensitivity of IgA EMA was equal to that of IgA TGA (70%). This sensitivity was inferior to the expected sensitivity of about 90% for this assay in a population of patients with a 10% prevalence of IgA deficiency
• The specificity of a positive IgA TGA antibody was 100%
• The additional measurement of IgG AGA raised the diagnostic sensitivity of the serological approach to 93%, with the specificity falling to 60%

A number of caveats for the use of TGA screening were raised by the HIU study:

• In keeping with the known problem with false-positive AGA serology in Down syndrome patients, we found that 13% of the Down syndrome patients studied had falsely positive IgA TGA results. Therefore, as is the case with AGA results, TGA antibody positivity should be regarded with caution in patients with Down syndrome.
• In patients with systemic autoimmune disorders such as systemic lupus erythematosus, 13% were falsely positive for IgA TGA and IgA AGA. Therefore, caution should be exercised in interpreting TGA results in patients with systemic autoimmune disease, particularly if they are SSA positive.

Recommended Strategy for Screening Patients with Suspected Gluten Sensitivity (including CD)

Four important factors have been considered in making our recommendations for coeliac screening (Figure 2):

• As this is a screening rather than a diagnostic strategy, any loss of specificity incurred by measuring IgG AGA is acceptable to ensure that the possibility of gluten sensitivity or CD is not prematurely discounted on the basis of a negative serological test.
• Endomysial antibodies use monkey oesophagus, a technology which poses ethical problems in a species under threat. In addition, the high cost and semiquantitative nature of the assay makes it desirable to replace EMA with a test offering similar diagnostic performance. Transglutaminase antibody measurement by ELISA fulfils this objective, and should replace EMA as part of the screening strategy.
• Given the high incidence of IgA deficiency in CD populations (up to 10%), it is important to know the IgA status of a patient by measuring the total serum IgA level.
• Serodiagnostic screens should be interpreted with caution in patients with Down syndrome and systemic autoimmune disease.

Figure 2 - Coeliac Screening

Normal Ranges

In normal individuals, transglutaminase antibodies are negative (<20 ELISA Units) and endomysial antibodies are negative. The reference ranges for gliadin antibodies are age-dependent (Table 3).

Specimen Collection

A 5 mL clotted blood sample is preferred. Alternatively, a 5 mL EDTA (plasma) tube is acceptable. Samples may be transported at 4° C for up to 48 hours.

References

1. Maki M, Collin P. Coeliac disease. Lancet 1997; 349: 1755-1759
2. Walker-Smith JA, Guandalini S, Schmitz J et al. Revised criteria for diagnosis of coeliac disease. Arch Dis Child 1990; 65: 909-11
3. Unsworth DJ, Brown DL. Serological screening suggests that adult coeliac disease is underdiagnosed in the UK and increases the incidence by up to 12%. Gut 1994; 35: 61-64
4. Bruce SE, Bjarnason I, Peters TJ. Human jejunal transglutaminase: demonstration of activity, enzyme kinetics and substrate specificity with special relation to gliadin and coeliac disease. Clin Sci 1985;68:573-579
5. Lundin KEA, Scott H, Hansen T, Paulsen G, Halstensen TS, Fausa O, et al. Gliadin-specific, HLA-DQ(1*0501,1*0201) restricted T cells isolated from the small intestinal mucosa of celiac disease patients. J Exp Med 1993;178:187-196
6. Molberg O, Kett K, Scott H, Thorsby E, Sollid LM, Lundin KEA. Gliadin specific, HLA-DQ2-restricted T cells are commonly found in small intestinal biopsies from coeliac disease patients, but not from controls. Scand J Immunol 1997;46:103-109.
7. Petterson A, Sjoberg K, Lernmark …, Eriksson S. HLA genotypes in coeliac disease and in healthy individuals carrying gliadin antibodies. Eur J Gastroenterol Hepatol 1993;5:445-450.
8. Ferreira M, Davies SL, Butler M et al. Endomysial antibody: is it the best screening test for coeliac disease? Gut 1992; 33: 1633-37
9. Dieterich W, Ethnis T, Bauer M et al. Identification of tissue transglutaminase as the autoantigen of coeliac disease. Nature Medicine 1993; 7: 797-801

Written by:                     Dr Glenn Reeves and Karla Lemmert, Immunology, HAPS
Written:                          June 2000
Reviewed:                      August 2001