The Staff of Life?
Understanding and Diagnosing Celiac Disease

For many people the world over, bread is a dietary staple. But for those with celiac disease, bread is anything but sustaining.

What is celiac disease?
Celiac disease (CD), also known as gluten-sensitive enteropathy, is caused by a lifelong intolerance to gliadin—a component of gluten found in wheat, rye, barley and oats. Ingestion of these grains or any food augmented with gluten evokes a destructive autoimmune response targeting the intestinal villi. Villous atrophy (a flattening out of the intestinal villi) accompanied by crypt hyperplasia (Figure 1) progresses as long as gluten is consumed, resulting in malabsorption and malnutrition despite a well-balanced diet. Other disorders, such as anemia, may arise as further consequences of malnourishment. Untreated CD, as a potential progenitor of lymphoma and other small-bowel cancers, can be fatal, although the risk to celiac patients of developing such cancers is only slightly greater than that of the general population.¹

Figure 1. Appearance of villi in normal vs. celiac-diseased intestine.

To understand CD and other gluten intolerances, one must delve into the intricacies of human genetics, molecular biology, immunology and autoimmunity.

The autoimmune jigsaw puzzle
When a food containing gliadin is ingested, the gliadin stimulates the production of tissue transglutaminase (tTG), a fairly ubiquitous enzyme responsible for converting glutamine to glutamic acid by deamidation. While gliadin appears to be a preferred substrate for tTG, only specific glutamines in the molecule are deamidated.² This deamidation generates charge changes that enable gliadin, possibly complexed with tTG, to form novel epitopes that are presented to molecules made by the major histocompatibiltiy complex (MHC—see sidebar.) In normal individuals, the deamidated gliadin is unrecognized by the major histocompatibility molecules (MHM) encoded by the MHC, traversing safely through the intestinal lumen as digestion progresses.

In celiac patients, however, gliadin is erroneously recognized as an invader. This ignites a series of events that result in intestinal destruction (Figure 2). These individuals possess variant alleles of the human leukocyte antigen (HLA) component of the MHC—either HLA-DQ2 or HLA-DQ8⁴—which encode MHMs on dendritic and T-helper cells that present gliadin as a pathogen.⁵

Figure 2. Steps in the intestinal autoimmune response invoked by gliadin (G).3

1. Digestion of gluten (A) generates G fragments. 2. G infiltrates submucosa through predisposed leaky cell junctions. 3. tTG deamidates G at select glutamines, yielding deamidated gliadin (DG) and also activates tissue growth factor TGF b1. 4. Dendritic cells (D) phagocytose DG and DG-tTG complexes. 5. DG and DG-tTG are presented by DQ2 MHM to T helper cells (TH). 6. TH phagocytose DG and DG-tTG complexes, produce inflammatory cytokines (C) and tumor necrosis factor a (TNFa). 7. TNFa attracts fibroblasts (F) that secrete metalloproteinases (MPs). MPs destroy the connective tissue of lamina propria, making villi leakier. 8. TH stimulates amplification of tTG IgA–producing B cells (B). 9. tTG-IgA inhibits activation of TGF b1 by tTG, inhibiting growth and organization of villous cells. 10. MICA is expressed on the villous endothelial cell surface, targeting cells for attack by cytotoxic T cells (CTLs).

Somewhat unexpectedly, not all people possessing one of these variants are afflicted with celiac disease. It is conjectured that a weakening or injury to the mucosa precedes the onset of the disease, allowing gluten to leak inappropriately from the intestinal lumen into the lamina propria (the underlying vascular connective tissue supporting the intestinal epithelium) and invoking the first stage of the autoimmune response⁶ (Figure 2). This permeability can be congenital or develop at any time as a result of illness, injury or other physiologic stress, such as pregnancy.

Once within the lamina propria, dendritic cells bearing the HLA-DQ2 or HLA-DQ8 MHMs activate gliadin-specific CD4+ (helper) T lymphocytes, inducing the release of cytokines that inflame the lamina propria, resulting in damage to the intestinal mucosa.⁷ Simultaneously, the large-scale production of anti-tTG IgA antibodies by B cells is triggered, inhibiting tTG activation of tissue growth factor b1 (TGF b1), preventing the growth and organization of villous cells, thus further damaging the tTG-producing endomysial connective tissue underlying the villi.⁶

Additionally, gliadin-specific CD8+ (cytotoxic) T cells (CTLs) proliferate in the intestinal epithelia comprising the villi and crypts, even in asymptomatic individuals.⁷ Interaction of gliadin with the major histocompatibility molecule encoded by at least one MHC class I A (MICA) variant of the HLA-A2 locus has been shown to activate these killer T cells, directing them to destroy MICA-expressing villi.⁸

Elusive diagnosis
The diagnosis of classic celiac disease is usually straightforward. Whether presenting in infancy, childhood or adulthood, the classic celiac patient appears malnourished with a bloated abdomen and wasted limbs. Patients frequently also complain of a wide variety of other symptoms, including osteopenia (bone pain), fatigue, rash, steatorrhea (foul smelling, floating stool) and abdominal pain. Children may be growth delayed and/or developmentally delayed and of very short stature.

But not all patients have classic symptoms and CD is not always easy to diagnose. Manifestations can be elusive and counterintuitive (e.g., obesity has been noted in some celiacs) and frequently diagnosed as other diseases that are thought to occur with greater prevalence (Table 1). Many of these occurrences are secondary to the vitamin and mineral imbalances caused by malabsorption.

Table 1. Examples of CD misdiagnosed as other disorders.

* Occurs in approximately 5% of all celiac patients, especially those with the HLA-DQ8 haplotype. † Occurs in up to 50% of gravid CD patients not following a gluten-free diet.

As a result of CD's chameleon-like nature, in countries where the awareness of CD symptoms is low, correct diagnosis takes 11 years on average,¹⁷ with some individuals remaining undiagnosed for over 28 years!⁹ Misdiagnosis based on less common symptoms is further confounded by the mistaken belief that CD is rare. However, the prevalence of CD in populations of European descent (including the United States and Australia) ranges from 1:100 to 1:200 (0.5% to 1%), and is even greater in high-risk populations (Table 2). The prevalence in Middle-Eastern populations is similar to that in the European population.¹⁸ While it is rarely seen in Africans and Asians, the prevalence among African-, Hispanic- and Asian-Americans is 1:236.¹⁹

Greater awareness aids in the early diagnosis of CD. This is vital. Not only can early diagnosis significantly improve quality of life, but the longer a CD sufferer is exposed to gluten, the greater that person's risk of developing other autoimmune deficiencies (10.5% to 34%; Table 2).²⁰ This may result from a similar dysfunctional presentation of other proteins to the MHC²⁵ as the HLA-DQ2 genotype is linked to other autoimmune diseases.^26,27 In fact, one proposed mechanism for the development of type I diabetes in celiacs suggests that prolonged exposure to gluten increases the presence of, and intestinal permeability to, interleukin-4, resulting in the circulation of normally gut-localized Th2 lymphocytes to the pancreas, where they selectively destroy insulin-producing islet cells.²⁶

Table 2. Prevalence of celiac disease in high-risk populations.

Serological approaches to diagnosis
The baffling diversity of celiac disease symptoms underscores the necessity of serological testing. Traditionally, antigliadin, antireticulin and antiendomysial (EMA) antibodies have been used as first-stage serologic diagnostic aids. However, the US National Institutes of Health (NIH) now recommend assaying for anti-human tissue transglutaminase IgA (anti-tTG IgA) as the test of first choice.4 This assay displays very high sensitivity and specificity in both children and adults,²⁸ but must be administered while the patient is consuming a gluten-containing diet because these antibodies diminish with adherence to a gluten-free diet.²⁹ Confirmation of CD and the determination of the extent of intestinal damage are then made on the basis of duodenal biopsy.

Since the prevalence of selective IgA deficiency (a severe or total lack of all IgA class antibodies) in celiacs is 10 to 15 percent greater than in the general population, some sufferers appear as false negatives when tested for anti-tTG IgA.4 In cases where a selective IgA deficiency is suspected, an anti-tTg IgG or antigliadin IgG assay can be used as a second screen.³⁰ Antigliadin IgG and antigliadin IgA assays are also useful for diagnosing non–gut-related (especially neurological) gluten sensitivities since approximately 66 percent of these patients have no intestinal involvement and may not produce antibodies to tTG.¹⁰ Assays for anti-tTG IgA, antigliadin IgG and antigliadin IgA are under development on the IMMULITE^® family of systems. All three assays have demonstrated sensitivity, specificity and precision similar to those of comparable assays from other sources.

Treating gluten intolerance–related diseases
CD symptoms and physiological abnormalities abate within a few weeks of adopting a gluten-free diet, which is currently the only available treatment. The greatest impediment to a successful outcome lies in patient compliance as the elimination of wheat, rye, barley and oats^‡ is very restrictive; great care must be taken especially with packaged foods, in which gluten-based additives are common. Periodic serologic monitoring of celiac patients is a valuable tool for assessing treatment compliance and success.

Alternative therapy may be available within the next decade. One pharmacological approach focuses on the use of a bacterial enzyme, prolyl endopeptidase (PEP), to degrade the normally proteolysis-resistant gliadin. In vitro digestion by PEP effectively destroys g-gliadin epitopes, although a-gliadin epitopes are less completely digested. In vivo trials in rats have been promising, suggesting that dietary supplementation of PEP will degrade gliadin, preventing the T cell response.^31,32 Alternatively, the development of DQ2-blocking agents may hold promise not only for the treatment of celiac disease, but of other DQ2-mediated autoimmune disorders.³³

Should mass screening for CD be adopted?
Because of the prevalence of CD in populations of Middle Eastern and European extraction and the potentially serious impact on the health of affected individuals, mass screening is being considered. Many practical, economic and ethical questions must be addressed:³⁴

Perhaps one of the greatest concerns involves the degree of dietary compliance that can be expected from individuals who are serologically positive but asymptomatic. Is it worthwhile conducting mass screening if patient compliance with "treatment" is unlikely? Hopefully, the development of alternative pharmacological therapies will remove this impediment, and by combining palatable therapy with simple screening practices, the staff of life could some day be safely consumed by all.

^‡ Many celiacs can consume oats.