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The Malabsorption Syndrome versus Celiac Disease: A Diagnostic Reappraisal

Andrej Beseda, Marián Bencat, Lubica Korinkova, Janka Papanová, Julius Rajcani*

Pathology, Diagnostic center, Alpha medical Ltd, Martin, Slovakia

Abstract: The most frequent cause of malabsorption syndrome (MAS) in developed countries is the celiac disease (CD). There are two possible approaches for exact diagnostic of this disorder: in case of full blown clinical symptoms the golden standard is duodenal biopsy. When it shows classical mucosa lesions (Marsh type 3 mucosa atrophy of various grades), the diagnosis of CD is confirmed. However, if intraepithelial lymphocyte infiltration (IEL) and/or hyperplastic crypts can be seen in the absence of mucosa atrophy (Marsh type 1 and/or 2 lesions) only, it is reasonable to examine the serologic markers. The most frequent enzyme linked immunosorbent assay (ELISA) tests are: detection of tissue transglutaminase 2 antibody (anti-tTGase 2, especially of IgA class) and the deamidated gliadin peptide antibody (anti-DGP) assay. A simpler serological test, useful for screening, is the detection of anti-endomysium antibody (AEmA) by indirect fluorescent antibody (IFA) in monkey intestine or esophagus sections. This is reasonable to combine with the microdot test for anti-gliadin antibody (AGA), especially by using the endomysium/gliadin mosaic kit (Euroimmun). Taken together, at least three serological markers (anti-tTGase 2, anti-DGP and AGA) are highly specific for CD; their detection first (i.e. prior to duodenal biopsy) is recommended in children as well as in adults showing atypical, silent and or latent forms of CD. In both of them, serology represents a reliable non-invasive approach, which allows precise diagnosis with a probability of nearly 90 %. Another advantage of the non-invasive serological technic is that it can be easily repeated when following the outcome of gluten-free diet (GFD). Nevertheless, duodenal biopsy should be performed in any case, if the serology does not correlate with clinical symptoms. In such patients as well as in those revealing positive serology but negative and/or non-specific histology, immunogenetic typing should be performed in order to assess the expression of DQ2 [DQA*0501-DQB*0201] or DQ8 [DQA*0301-DQB1*0302] haplotypes of the human leukocyte antigen (HLA) class II region critical for the development of CD.

Keywords: malabsorption syndrome, celiac disease, gluten sensitivity, diagnostic, duodenal biopsy, serological markers

Citation: Andrej Beseda, Marián Bencat, Lubica Korinkova, Janka Papanová, Julius Rajcani, The Malabsorption Syndrome versus Celiac Disease: A Diagnostic Reappraisal. International Journal of Celiac Disease, 3 (4). 118-131. January 2015. doi:10.12691/ijcd-3-4-5

1. Introduction

Malabsortion syndrome (MAS) is a clinical entity [1] with variable signs and symptoms associated with disturbed resorption of nutrients. As shown in Table 1, in MAS the impaired resorption of lipids (especially triglycerids) prevails, followed by disturbed ingestion of amino acids, saccharides (intolerance for galactose and glucose in children), minerals and certain vitamins (cobolamin/vitamin B12 and folate). There are two basic forms of MAS (Table 2): primary form (which develops due to primary damage of the epithelium cells lining the intestinal villi and crypts) and the secondary form (when clinical symptoms related to mucosal damage occur due to external influences such as infection, surgery and/or in response to drugs and chemicals). In the past, MAS occurring in tropical countries (such as India) has been referred to as tropical sprue, but recently the term tropical malabsorption (TM) is preferred. TM is mainly of infectious origin, most probably associated with giardiasis, strongiloidiasis and/or intestinal tuberculosis [2,3], whereas the MAS occurring in developed countries is frequently related to CD. Thus, the etiology of TM in tropical areas usually differs from that of non-infective CD occurring in countries with temperate climate. This notion is supported by the existence of the epidemic form TM reported in Pakistan [4], Thailand [5] and/or in other South-East Asia regions including Malaysia [6]. The emergence of TM in the subtropical Caribbean countries such as Puerto Rico, was probably related to poor sanitary conditions, i.e. just indirectly associated with the climate itself [7,8].

In recent years, the frequency of infective TM in developing countries has declined, supporting the secondary role of tropical climate and stressing the importance of sanitary conditions and/or of the increasing use of antibiotics and related drugs. Nevertheless, no clear evidence exists in favor of this statement. It should be kept in mind that some overlap may exist between the genuine (primary) intestinal lesions associated with CD and the mild of forms of infective TM. Such overlap condition is referred to as small intestinal bacterial overgrowth (SIBO). The SIBO syndrome has been associated with the irritable bowel syndrome (IBS), a common condition in countries with temperate climate [9,10]. Symptoms of SIBO include abdominal pain, bloating, flatulence, and diarrhea, similar to those observed in patients with IBS. The clinical diagnosis of IBS is based on a combination of similar symptoms. It is possible that some patients may be misdiagnosed as IBS, particularly if no specific diagnostic tests are performed. The SIBO represents an increased and/or abnormal number of bacteria in the small intestine. The bacterial nature of SIBO can be proven by the Hydrogen breath testing (H₂BT) either with glucose or lactulose. An abnormally high bacterial count (≥ 10⁵ colony-forming units/ml) in the proximal small intestine may approximate the amounts of bacteria normally found in the colon. More precisely, the stomach and proximal small bowel (including duodenum) in adults normally contain relatively small numbers of bacteria. The bacterial concentration in the gut increases from 10–4 colony forming units per ml (cfu/ml) in the duodenum and the jejunum up to 10–5 cfu/ml in the proximal ileum; however, it easily reaches 10–8 cfu/ml in the terminal ileum and 1010–12 cfu/ml in the caecum [11,12]. The flora in the upper small bowel consists mainly of Gram-positive rods; the numbers of Gram negatives are low and the anaerobes, so much frequent in colon, are very rare [13]. In the past, SIBO had been interpreted acting in accord with other predisposing conditions, such as impaired intestinal motility or failure of the gastric-acid barrier [14,15]. Taking advantage of the glucose hydrogen breath test (GHBT) and/or the ¹⁴C-xylose breath tests (both considered for fairly reliable), there was shown that in IRB only mildly increased bacterial counts occur within the small-bowel. Therefore, the potent motility alterations in IBS cannot be predicted by increased small-bowel bacterial flora, rather the opposite may be the case [16]. Thus, the association between SIBO and IBS remains very uncertain [17], even when SIBO related IBS patients would respond to antibiotic treatment by temporary improvement. The SIBO patients, therefore, should be examined for serologic markers, especially when the clinical signs are indicative for CD in order to exclude the latter [18,19].

Table 1. Clinical symptoms in MAS and recommended laboratory tests

Table 2. The etiology of malabsorption syndrome (MAS)

As described below, the genuine CD is characterized by typical small-intestinal mucosa lesions mainly occurring in genetically susceptible individuals (HLA II haplotypes DQ2 and DQ8), who respond by MAS to the dietary ingestion of wheat gluten (and/or similar prolamins) present in barley and rye [reviewed by Kagnoff, [20]]. In addition to villous atrophy of intestinal mucosa, higher platelet counts were found as a relevant diagnostic sign [21]. On other hand, TM is an infectious disease, which may start acutely by an episode of gastrointestinal infection. It may be partially treated with antibiotics before reaching a regional healthcare center. Since TM is a relatively serious disease, which cannot be treated by gluten-free diet (GFD), it must not be mistaken for CD. The sudden emergence of TM in developed countries with a high standard of hygiene is possible, especially among tourists who have returned from tropical areas such as South Asia [22]. In presented review, we briefly reconsider the classical signs of CD as well as the lesions seen in duodenal biopsy, along with the diagnostic significance of recently described serologic markers, which, under careful precautions, can be regarded for highly specific. The serologic markers are of special importance in children, in which they have been regarded for a superior diagnostic tool [23,24]. On other hand, duodenal biopsy still remains the golden diagnostic standard in any case (even in children), when clinical signs do not correlate with positive serologic tests [25].

2. Histological Findings in CD and Related Diseases

From the intestine lumen, nutrients are absorbed by the tall columnar epithelium cells (at least 30 μm high), also called enterocytes, lining the finger-like extensions of mucosa. The absorption surface at apical end of each enterocyte is substantially increased by the brush border (as described by light microscopy), which is represented by approximately 1 μm long microvilli clearly seen by electron microscopy. The villi are composed of loose connective tissue, which contains capillary and lymphatic vessel loops and a few smooth muscle cells. The connective belonging to lamina propria on one hand extends to above mentioned villi and on other hand, it surrounds the simple tubular intestinal glands, the so called crypts of Lieberkuehn, which are inverted structures increasing the inner space of intestine. In addition to absorptive enterocytes, the villi are covered by goblet cells, by antigen presenting microfold cells (M cells), by regenerative stem cells, by exocrine Paneth cells and, at last but not least, by the so called enteroendocrine cells, producing paracrine hormones such as somatostatin, serotonin, gastrin and others. The above mentioned exocrine and entero-endocrine cells as well as the majority of stem cells are mainly located in the wall of tubular crypts [26]. The lateral membranes of cylindric epithelium cells are interconnected by intercellular desomosomes and gap junctions equipped with adherent as well as occludent zonules. These intercellular structures contain special junctional proteins such as occludin, claudin 3, E-cadherin and the zona occludens protein. The expression of junction proteins, when compared to non-celiac controls, was found decreased already at the early stage of CD. At early infiltrative phase of CD (referred to as Marsh type 1/stage I/grade A lesion), the expression of junction proteins correlates negatively with the increased number of IEL. The expression of claudin 3 shows a negative correlation with the diarrhea score. These findings indicate that the integrity of intestinal epithelium might be disrupted already before progression to the full blown classical Marsh type 3 CD lesion [27].

Table 3. Overview of histological lesions of duodenal mucosa in CD (modified from Marsh classification)¹

The early lesions at the infiltrative phase of CD appear first on surface epithelium (Figure 1A), where the increased number of IEL dominates; the infiltrate consists mainly of CD3posit T- lymphocytes [28]. Marsh and Crowe [29] elaborated the histological classification of duodenal mucosa pattern in CD, introducing the recent three-degree grading of mucosa lesions, still used worldwide with small modifications. In type 1 lesions, the IEL count is increased, but the villous atrophy is absent. Precise counts of the IEL/enterocyte ratio as performed by several authors were based on the CD3 marker stain [30,31,32]. The borderline for pathological IEL has been determined as minimum at 25 CD3⁺ T-lymhocytes per 100 enterocytes, while a range from 25 to 4 CD3⁺ lymphocytes per 100 enterocytes may be accepted for basic value (Table 3). Even though confirming the improved precision of CD3posit T-lymphocyte counts in duodenal biopsy sections, some authors stress that the borderline IEL criterion alone may not be satisfactory for the diagnosis of the early stage of gluten-sensitive enteropathy [33]. The finding of increased IEL counts in the absence of other mucosa lesions classified as Marsh type 1, is not specific for CD. Increased IELs counts in the small bowel mucosa have been found also in TM, in food protein intolerance (enteropathy associated with substances other than gluten), in Helicobacter pylori associated gastritis and/or in peptic duodenitis, not excluding some parasitic and/or viral infections (Table 2); similar accumulation of CD3posit T cells appears during the development of intestinal lymphoma [34]. In the meantime, additional diagnostic approaches beyond the conventional histology of early Marsh type 1 lesion, have been introduced to elucidate the diagnosis of CD [35,36]. The recent ESPGHAN guidelines for the diagnosis of early CD lesions encompasses both, a high CD3posit T lymphocyte (IEL) count as well as the presence of deposits in the lamina propria consisting of IgA class antibody and tTGase 2 antigen. The latter finding increases the likelihood of CD development [37] along with the positive serum antibody markers. Asymptomatic patients are typically diagnosed through serologic screening, which can be done first if the individual has atypical symptoms and/or if he is a first-degree relative to a patient with CD [38]. However, the diagnosis of CD requires duodenal biopsy, when the adult patient is on GFD due to long lasting positive serology.

Figure 1A. Schematic representation of morphological changes of enterocytes in CD. Enterocytes become lower, they show basophilic cytoplasm and irregular nuclei. The brush border is thin and later on disappears. In subtotal atrophy the enterocytes are non-differentiated cuboidal cells with round shaped small nuclei and basophilic cytoplasm

A considerable of population of CD3posit T cells has a different T cell receptor (TCR), namely that composed of γδsubunits, instead of the usual αβsubunits. In progressed CD, characterized by mucosal atrophy, the increase of CD3plus/TCRγδ lymphocytes and concomitant decrease of CD3minus IEL counts was described by flow cytometry of duodenal biopsy eluates [39]. For such investigations, the duodenal biopsy sample was immediately processed with a suitable buffer (HBSS). To prepare a relatively pure IEL suspension, the total villous epithelium fraction must have been removed along with at least a part of crypt epithelium cells. Immunohistochemical analysis was done using directly labeled monoclonal antibodies to define the different IEL subsets. The following sera were used by above mentioned authors: 1. anti-CD45-APC /clone 2D1; 2. anti CD3-PerCP/clone SK7, 3. anti CD103-FITC/clone Ber-ACT8 and 3. anti-TCRγδ-PE/clone 11F2. Such thorough analysis cannot be accomplished at routine diagnostic work. We still believe that consequent use of the CD3 marker staining at histological examination of duodenal tissue sections, considerably improves the evaluation of IEL counts. This statement is evident when looking at given examples of duodenal biopsy samples (Figure 2 and Figure 3). Fully convincing histological diagnostic signs are inflammatory infiltration, increased IEL counts (> 25/100), villous atrophy of at least mild grade with an uneven brush border (Figure 1A, Marsh type 3a lesion). In addition, the critical leukocyte antigen (HLA) haplotypes DQ2 and/or DQ8 should be found by immunogenotyping. The importance of serologic markers in the case of uncertain histological findings as well as in the monitoring of therapy (especially when following the effect of GFD), comes from the high specificity and non-invasive nature of blood sampling. The key role of serology is also related to the fact that persistent IEL may still remain in a proportion of CD patients, who have undergone a long-term strict GFD [40,41]. In this connection, the so called refractory form of CD has been described. The seronegative form of CD (SNCD) is also known, being defined by negative anti-tTGase 2 antibodies in the presence of a positive duodenal biopsy (histological lesions at least of Marsh type 3a). SNCD is characterized by mucosal deposits of above mentioned immunocomplexes containing tTGase 2 antigen and anti-tTGase 2 class IgA antibody. The deposits in question may counteract the passage of anti-tTGase antibody into the bloodstream explaining the continuing seronegativity [42]. Another reason for seronegativity may be the incomplete maturation of plasma cells with consequent failure of antibody production. This condition often characterizes immunoglobulin deficiencies, and indeed, SNCD is common in subjects with this defect (see below).

Further histological criterion for early CD lesions is the elongation of crypts. In the hyperplasia phase (Marsh type 2), the crypt/villum length ratio is of 1:2 or 1:3 (Figure 1B), while the conventional (normal) ratio uses to be 1:5 (Table 4). The elongation of crypts is related to the proliferation of regenerative stem cells, which prevail within the crypts. Their activated division can be assessed using the Ki-67 proliferation marker staining better than looking for mitoses in hematoxylin and erythrosine sections. The hyperplasia of crypts along with a given stage of villous atrophy (usually graded partial or subtotal) and enterocyte alterations (Figure 1A) are the dominant features of Marsh type 3 lesions. As presented on Figure 4, the differences between lesion subtypes when compared to unchanged (and/or regular sized) villous structures can be easily recognized. These changes comprise a dynamically interrelated series of events, culminating in a final and severe flat-destructive lesion. In contrast to about 25 % gluten-sensitive enteropathies, no case of complete villous blunting (Marsh type 3c) has been reported in TM [22]. As shown on Figure 1B and mentioned in Table 4, in TM the subtotal atrophy of villi is not associated with hyperplasia of crypts. The latter rather show a hypoplastic pattern, so that the VH : CH ratio remains unchanged. TM is also characterized by incomplete duodenal villous blunting which never shows a flat mucosa. Furthermore, there is some involvement of terminal ileum along with a marked inflammation of the lamina propria in that region. The anti-tTGase 2 antibody, so typical for CD (with the exception of rare SNCD cases) is not found in TM.

Figure 1B. Schematic representation of the villi and crypts of intestinal mucosa in CD. For explanation see Table 4 (VH: villous height, CH: crypt height)

Table 4. Explanations to the legend of the Figure 1B

Figure 2. Classical CD lesions seen in duodenal biopsy by HE stain (in B, C and D) as compared to normal duodenal mucosa (A)

Figure 3. Classical CD lesions in duodenal biopsy: CD3 staining of T lymphocytes (from C to F) as compared to normal duodenal mucosa (at A and B)

Taken together, in CD four possible diagnostic patterns are recognized: (1) The typical most commonly diagnosed form, characterized by full blown gastrointestinal signs (intestinal malabsorption), developed villous atrophy along with other classical histological lesions (histology plus). (2) Atypical or extra intestinal form, where the gastrointestinal symptoms are minimal or even absent, but if examined by duodenal biopsy, a fully developed villous atrophy can be seen along with the serological manifestations (serology plus, histology plus, clinical signs minimal or atypical). Since the onset of their disease, these patients may not be considered for having CD, at least not from gastrointestinal point of view. A large number of them goes undiagnosed, or becomes examined for other reasons such as iron deficiency, osteoporosis, short stature or infertility. (3) Silent form of CD, which represents a group of patients who have no clinical symptoms, but villous atrophy can be discovered during endoscopy undertaken for another reason. If tested, the CD related autoantibodies can be detected by serology. (4) In latent from, the individuals possessing genetic susceptibility (DQ2 or DQ8 HLA II haplotypes), for example the relatives of CD patients, will subsequently develop CD despite of the currently normal intestinal mucosa, especially when remaining on a standard (gluten-containing) diet. Another latent CD patients show positive autoimmune serology, but still reveal normal mucosa by histology (representing the serology positive, but histology negative variant) or show the Marsh type 1 infiltrative lesion, which is not significant for CD [43]. These latter patients are very difficult to identify and even more questionable is their classification among those really having CD. Some of these patients may reveal a history of previous CD positive histology, who responded to GFD by retaining a normal mucosal histology, while other just reacted by keeping an increased IEL manifestation, which itself is not specific so that any diagnostic decision becomes difficult. The latter patients fulfill the category of the refractory form of CD, represented by patients with positive serology along with the minimal Marsh type 1 histology, who do not respond to GFD by improved mucosa lesions. Serum antibodies have poor sensitivity after staying on GFD longer than 1 year or even more. The RCD is defined just clinically: either as a recurrence of CD symptoms after a former period of response to GFD, or some kind of duodenal deterioration despite of the strict GFD in cases which had been verified clinically and confirmed either serologically or by previous biopsies. Some of latter patients with more severe (Marsh type 3) but GFD non-responding histological lesions may develop complications such as ulcerative jejuno-ileitis or enteropathy type I - associated T-cell lymphoma (EATL). The lymphocytes of the intestinal infiltrate when gaining an aberrant phenotype and genotype undergo clonal expansion then developing EATL. They might represent a gradual stepwise transformation starting from increased accumulation of IEL, probably developing the refractory celiac disease and subsequent lymphoma in situ. These transformational stages of the progressing celiac disease show increasing risk of type 1 EATL development, which decreases the survival of such patients [44]. In EATL patients as result of deterioration of the intestinal mucosa may or may not show ulcerations [45].

Figure 4. Various type CD lesions in duodenal biopsy by as seen by CD3 lymphocyte staining: the early infiltrative lesion (A) compared with the classical lesion (B, partial atrophy) as well as with progressed lesions (C and D) showing subtotal atrophy

It should noticed that biopsy sampling in the bulb and/or distal duodenum alone would yield sufficient histological results, i.e. may neither confirm nor exclude the diagnosis of CD [46], especially not in the absence of positive serologic results (see below). The histological lesions in CD may be patchy rather than diffuse and/or they may be confined to the bulb. Data, although scanty, suggests that pathologic lesions involve only 30-50% of the entire small bowel mucosa [47]. Interestingly enough, in a small proportion of cases the ileum biopsy may display more severe lesions than the duodenal sites. A mild IEL infiltration accompanied with eosinophilic leukocytes can be seen in the colon mucosa as well. Current guidelines recommend obtaining multiple biopsies from the bulb as well as distal duodenum when evaluating for CD. Adherence to these recommendations among gastroenterologists treating adults is low, and unfortunately, there are no overall accepted recommendations at all available for pediatric gastroenterologists [48].

3. Pathogenesis of Celiac Disease

As already mentioned, CD is an autoimmune disorder occurring in genetically susceptible individuals, who reveal certain human leukocyte antigen (HLA) class II haplotypes, when expressing either the DQ2 [DQA*0501-DQB*0201] or the DQ8 [DQA*0301-DQB1*0302] regions. The HLA II molecules function as co-receptors of antigen presenting cells (APC) involved in the processing of exogenous antigens. The APCs are dendritic cells, macrophages and/or other phagocytes engulfing foreign protein antigens. These are first cleaved and then their immunogenic peptides are presented to antigen recognizing receptors of reactive T cells, such as the CD3plus/CD4plus helper T-lymphocytes [49]. Structural polymorphisms of major histocompatibility complex subloci identify the codon switches within the second exon that control the third hyper variable region in the outer domain of HLA II molecule. The shape of different grooves formed by the subunits of HLA II molecule subtypes, confers a kind of affinity for capturing a certain antigenic peptide. When presented at the APC surface, this mechanism may explain the restricted genetic susceptibility for the development of CD, associated with a pathologic immune response to gluten and related prolamins [50]. Several prolamins (of wheat, rye, and barley) contain recurring tetrapeptide motif sequences that are predicted to have a beta-reverse-turn secondary structure, appearing active at least by in vitro assays [47]. The immune-mediated responses in jejunal tissue at gluten sensitivity arise in the lamina propria, in association with macrophages and the in abundance activated CD3plus/CD4plus T-lymphocytes (helper T-cells). Many other inflammatory consequences flow from these interactions, involving activation of mast cells, eosinophil and neutrophil granulocytes, induction of cytokines (such as IL-15) and other products of inflammation, along with increased hyper permeability of the microvasculature and up regulation of leukocyte adhesion molecules. The result is a doubling of the volume of lamina propria connective tissue accompanied with a less or more severe flattened villi lined by low differentiated enterocytes indicating local inflammation and proliferation [30,47,51]. The gliadin peptide (a cleavage product of gluten), in addition evokes the dose-responsive infiltration of the surface enterocyte layer with CD3⁺/CD8⁺ T-lymphocytes (forming the IEL described above). The T cell receptors (TCR) of the IEL cells consist of gamma-delta subunits, i.e. of different protein chains. The γδTCR CD3⁺/CD8⁺ T- lymphocytes then function as local intraepithelial cytotoxic T cells. Unlike to αβCD3⁺ T-cells, the γδCD3⁺ T-cells had not undergone antigen selection rendering them tolerant to the self (own body) antigens.

In ingested food, the gluten molecule (which precipitates the pathological immune response) is cleaved by luminal and/or brush-border enzymes of enterocytes into amino acids and α-gliadin peptides that are resistant to further degradation. It has been hypothesized that the increased intestinal permeability (as manifested by above mentioned decrease in expression of junction proteins) is an early but crucial event supporting the transport of α -gliadin through the epithelial barrier by a not well defined paracellular mechanism and/or by apical-to-basal transcytosis [52]. After crossing the epithelial cell barrier and entering lamina propria, theαgliadin peptides are cross-linked, and their charged glutamic acid residues are deamidated by tissue transglutaminase 2 (tTGase 2), to produce deamidated glutamine peptides (DGP). The DGP molecules are bound with high affinity either to DQ2 or to DQ8 HLA II receptor molecules expressed on the surface of APCs. In such way, when inducing the CD3plus/CD4plus T-cell-specific responses, their help provided to B cells elicits the gluten specific antibody response (this antibody is not an autoantibody). Gliadin is also thought to stimulate the innate immune system among intestinal epithelial cells directly or through the up regulation of IL-15. The IL-15 cytokine has been recognized to upregulate the MIC-A, a stress molecule on enterocytes and the NKG2D receptor of NK (natural killer) cells, promoting an initial enterocyte damage. IL-15, in turn, activates altered signaling through NK/G2D and other CD94/NKG2 receptors, and on the CD45 (transmembrane tyrosine kinase) molecules of IELs, whose cognate ligands on intestinal epithelial cells are also upregulated by IL-15 and IFN-γ. Taken together, the complex mechanism of CD lesions is mediated: 1. By the innate immunity provided through NK cells and 2. By IELs, both of which are involved in early as well as in late phases of CD pathogenesis, 3. By the role of cytokines such as IL-15 and IFNγ, thought to underlay the increased release of tissue TGase 2 that leads to deamidation of gluten peptides, and 4. By the sequence of, and/or for the relationship between the helper CD4⁺ T cell responses and intestinal B cells, stimulating local IgA production in the patients who develop CD. The inflammatory cytokines, the apoptotic proteins (granzyme B and perforin), and activated cytotoxic proteins (metallo-proteinases) may also be responsible for the early damage to intestinal tissue as seen in biopsy specimens coming from CD patients [20]. Furthermore, autoantibodies are formed against the own tissue enzymes such as tTGase 2 (expressed in increased amounts) along with the anti-DGP autoantibodies.

4. Celiac Disease versus Inflammatory Bowel Disease: Serological Markers and Differential Diagnostic

AS already mentioned, the diagnostic signs of CD (which complement duodenal biopsy) are the presence of critical HLA-DQ2 and/or HLA-DQ8 haplotypes in susceptible patients and their still healthy relatives, the diverse but occasionally typical clinical manifestations (summarized in Table 1) and the production of special autoantibodies and anti-gliadin antibodies (summarized in Table 6). Out of the latter, the traditional anti-endomysium antibodies (AEmA) can be detected by indirect fluorescent antibody (IFA) test [53]. Sections from intestinal mucosa show positive AEmA staining in their lamina propria, while the fine reticular (honey comb-like) fluorescence within the subepithelial connective tissue of villi indicates the reaction of the antireticulin antibody (ARA). The detection of ARA, as described by Calvani et al. [54], even though useful in basic screening, is much less specific than that of AEmA. Nevertheless, the presence of both, AEmA and ARA together, predicts the diagnosis of CD with a high probability [55,56]. When using the endomysium/gliadin diagnostic mosaic kit (Euroimmun) consisting of sections of primate liver, rat oesophagus and/or small intestine along with the gliadin coated microdots (taking advantage of a pure and specific gliadin antigen), the parallel detection of antibodies to gliadin along with AEmA and ARA is of great diagnostic value [57]. Later on, the AEmA has turned out to react with the tTGase 2 antigen being overexpressed in lamina propria along with DGP produced from the digested gluten at same site. The physiological activity of tTGase 2 as well as its amount expressed in the gastric and/or intestinal mucosa are enhanced in CD patients, since the production of this enzyme becomes activated in order to cleave the gluten in the ingested food into gliadin. Both autoantibodies are typical for CD and, if present together, they are considered for CD specific. Therefore, it is recommended to confirm the results of positive AEmA as found by IFA screening, by the detection of antibodies to tTGase 2 and to DGP by appropriate ELISA tests [58,59]. It is up most useful to determine the tTGase 2 antibody reactivity also in the IgA immunoglobulin class. Most recently, microdot slides and/or the ELISA microplates are available coated with a highly specific gliadin peptide, consisting of two but three times repeated gliadin-specific epitopes (GAF-3x). In either modification, the anti-gliadin antibody (AGA) test shows high specificity and significance. Nevertheless, the widely accepted serological tests available for CD still call for standardization, in order to compare the results from different laboratories [60]. According to novel recommendations, if the IgA anti-tTGase 2 antibody level shows a value greater than the 10-fold cut-off, such result can be regarded for quite satisfactory for decision making without an additional need of duodenal biopsy [61]. However, this notion was seriously challenged by the findings of Makovický et al. [25], who documented that certain pediatric patients may be seronegative while the disease is already active, so that the incidence of CD in the pediatric population is higher than expected from serological results. This is an important finding, which demonstrates the lack of a strict association between autoantibodies and lesions. It justifies the use of biopsies for conformation diagnosis in any case when negative serology does not correlate with clinical symptoms. Therefore, some diagnostic algorhitms, based on comparing the clinical, endoscopic and serological context, should be reconsidered (see legend to Figure 5).

Figure 5. The presented diagnostic algorhitm takes into account that extremely high anti-tTGase 2 IgA levels (reaching tenfold of the borderline value defined by Vermeersch et al. [61]) may confirm the diagnosis of CD without biopsy. In the case of medium and/or lower anti-tTGase 2 levels, duodenal biopsy should be taken to confirm the results of serology. The serology is a prior (first) choice in children and in adults, whose clinical symptoms are neither typical nor relevant. Even if the serological markers cannot be regarded in CD diagnostic for absolutely superseding, their advantage is in simplicity, high specificity and in noninvasive way of sampling. In cases showing typical symptoms, vice versa, the serology is at second line of choice, especially important for confirming the diagnosis in cases of uncertain biopsy results. Finally, for the therapeutic strategy when serology is positive, but histology negative, an assessment of inherited risk by HLA II DQ genotyping seems inevitable.

Table 5. Statistical analysis of the serology versus biopsy in both possible orders¹

The clinicians, on one hand, take an advantage of serological marker testing due to their simplicity and lower costs. According to our general experience, the IFA test combined with microdot and immunoblot assays can be regarded for representative and may be preferentially used by the pathologist [62]. For this reason, were perform the AEmA, ARA and AGA testing together, i.e. in the same serum sample using the endomysium/gliadin diagnostic mosaic kit. It comes from our statistics that during the last 3 years (from 2012 to 2014) out of 975 serum samples examined by AGA, the diagnosis of CD was positive in 6.1 %. This frequency is in compliance with the positive ratios observed at our Diagnostic center by other autoantibody screening tests. Alternatively, however, from the 415 serum samples submitted from the patients under the diagnosis of MAS, nearly the same proportion (5 %) was AGA positive (Table 5). Taken together, the serological screening for CD in suspicious patients revealing atypical or unclear clinical signs, revealed an average positive rate of 5.8 %, represented by 80 AGA positive patients. From these, over the half has been checked by duodenal biopsy in order to confirm the diagnosis. In those cases, when biopsy was performed at first choice, clear-cut and progressed CD lesions were seen in 20.2 % cases confirming the clinical diagnosis without the need of any serology. Additional 24 % of duodenal biopsies showed mild lesions, which could not been always regarded for CD-specific. In these, but also in MAS patients showing no relevant CD lesions, serological tests were recommended by the pathologist in order to clarify the diagnosis. Thus, in above mentioned cases serology is needed either to confirm or exclude the diagnosis of CD. Serology namiesto it is further useful for repeated follow up of suspicious cases with negative histology, especially when the patients express the high risk DQ2 [DQA*0501-DQB*0201] or DQ8 [DQA*0301-DQB1*0302] HLA II regions (Figure 5).

Table 6. Serological markers in CD and inflammatory bowel disease

In contrast to CD, the intestinal bowel disease (IBD) is a designation for two practically different clinical entities, namely ulcerative colitis (UC) and Crohn´s disease. While the former is characterized by inflammatory lesions located at the surface mucosa of large intestine (especially in colon sigmoides and upper rectum areas), in Crohn´s disease slowly developing deep transmural granulomatous inflammation involves mainly the terminal ileum. In UC the intestinal mucosa goblet cell antibody (GAB) predominates along with the ANCA-like antibodies, which react with the H1 histone and the mycobacterial histone 1 homologue called Hup B. In general, UC is characterized by the presence of blood granulocyte reactive antibodies (pANCA and xANCA), which are relatively frequent. Namely, pANCA occurs with the probability ranging from 75 to 100 %, while xANCA appears by the probability of 50 %, while the frequency of cANCA is rare, just at about 10% [63]. In contrast, in the pANCA antibody appears rarely in Crohn´s disease (less than 20% probability), but more frequently, by at least 40 % of patients, autoantibodies to exocrine pancreas (acinus cells) can be found (Table 6, see legend for explaining the details).

A presumed etiological mechanism of IBD is the dysregulated immunopathologic response to certain antigens of the own bacterial flora [64]. Namely, in Crohn´s disease heterologous antibodies have been found reacting with epitopes of Bacteroides caccae (Ton-B linked outer membrane protein) and Pseudomonas fluorescens associated bacterial protein I-2. More recently, the chance of correct IFA diagnostic in Crohn´s disease has been improved by introducing the so called CIBD mosaic (Euroimmun), in which the pancreas sections used to detect the pancreas antibodies (PAB), were replaced with recombinant pancreas acinus cell antigens (rPAB/Ag1 and/or rPAB/Ag2). Namely, the mosaic contains two transfected cell lines: HEK-293-CUZD1 expressing the zona pellucida (CUB) domain protein (CUZD1) corresponding to the exocrine acinus zymogen granule membrane protein and the transfected HEK293-GP2 cell line expressing the pancreas glycoprotein GP2 [65]. Furthermore, the CIBD mosaic in question contains the human adenocarcinoma cell line HT29-18N2 producing a goblet cell-like antigen in vitro, which is identical with that present in primate intestinal goblet cell sections as confirmed with a series of various lectins [66]. Summing up, in UC pANCA and GAB predominate, while in Crohn´s disease the PAB prevails along with heterologous antibodies to the yeast (Saccharomyces cerevisae) surface antigen (ASCA), which occurs with a frequency of about 90%. The detection of ASCA by IFA is quite reliable, while additional heterologous antibodies reacting with several bacterial glycans (ALCA, ACCA, AMCA, anti-L anti-C) and/or above mentioned Pseudomonas fluorescence antigens can be identified by ELISA. Nevertheless, the latter antibodies are not quite specific for Crohn´s disease, so that in the absence of ASCA their significance seems questionable [67].

5. Disorders Closely Related to CD and Their Diagnostic

Strong associations between CD and various autoimmune diseases, including autoimmune thyroid disease, type 1 diabetes mellitus, primary biliary cirrhosis, inflammatory bowel diseases and autoimmune adrenal insufficiency, have been demonstrated. It is believed that 2-5% of patients with autoimmune thyroid disorders have celiac disease, due to common genetic characteristics [68,69]. Hyperplasia of crypts, as mentioned in Table 3 and Table 4, occurs in several other MAS related conditions such as TM; relatively rarely it can be found in diabetes and in dermatitis herpetiformis (DH). The DH is a chronic blistering skin lesion, which is neither related to nor caused by herpes virus: the name just means that its appearance looks similar to herpes. When first described by L. A. Duhring in 1884, it was characterized as a skin disease of unknown etiology. The connection with celiac disease was recognized in 1967, but confirmed just recently by the presence of CD-related serologic markers [70] along with the association to HLA-DQ2 haplotype and gluten sensitivity [71,72]. Despite of the belief that DH is a quite specific manifestation of CD, the exact causal mechanism of skin lesion formation is still obscure. The skin affected by DH presents edema of epidermis, infiltration by neutrophil granulocytes and IgA deposits. The latter have an increased prevalence in the dermis, namely its closest areas to the epidermis. The direct IFA staining (anti-HuIgA/FITC) of uninvolved skin shows IgA deposits in dermal papillae and patchy granular IgA accumulation along the epidermal basement membrane. The biopsy of jejunal mucosa shows a lower degree of partial villous atrophy, and in addition, many hyperplastic crypts. In contrast, hypoplasia of crypts can be seen in severe cases of TM, in some cases of progressed (late) stage CD at the and in cases which are unresponsive to GFD (compare the upper paragraph). In adults with steatorrhea, deposition of collagen in the lamina propria is not an unfrequent change seen in duodenal biopsy samples obtained by the end-stage of CD.

In allergic enteropathy which develops in response to allergens (antigens) other than gluten, the mucosal connective tissue is infiltrated by eosinophilic granulocytes, but neither villous atrophy, nor altered crypts and/or no increased IEL can be seen. In progressive lymphoid hyperplasia, the jejunal mucosa reveals large secondary follicles, while the germinal centers regularly present in ileum mucosa become overshadowed by the massive and diffuse lymphocyte infiltration. Hypogammaglobulinemia is a form of primary immunodeficiency, as an uncommon condition in which gastrointestinal symptoms may be the only presentation [73]. In duodenal mucosa biopsies of such patients, the morphologic picture of massive infiltration in lamina propria by plasma cells, occasional villous atrophy and nodular lymphoid hyperplasia can be found. Serum globulins are low in all the patients. As result of severe immunodeficiency, the concomitant opportunistic infections may include intestinal parasites (giardia, lamblia, Cryptosporidium parvum, Isospora belli) and/or cytomegalovirus. If hypogammaglobulinemia accompanies IBS (may result into MAS), it may be combined with recurrent upper and lower respiratory tract infections, defective antibody responses, a granulomatous disease mimicking sarcoidosis and/or lymphoma (usually of B-cell origin). This rare disorder also called common variable immunodeficiency (CVIS) is difficult to manage [74]. The so called Waldenström macroglobulinemia (WM) has been initially thought to be associated with a variety of lymphoid neoplasms that can secondarily cause hyperviscosity syndrome. Subsequently, the WM combined with lymphoplasmacytic lymphoma (LPL) and had been designated lymphoplasmacytic lymphoma/Waldenström macroglobulinemia [75]. In the latter disease, jejunal mucosa shows extracellular deposits of PAS positive material in the connective tissue of lamina propria, which surrounds the lymphatic vessels and/or distends their internal lumen. The results of flow cytometry show that the immunophenotype of LPL/WM is variable, but overlaps mainly with that of other B-cell lymphoproliferative disorders [76]. In contrast, in Whipple disease the connective tissue of villi is infiltrated with large mononuclear phagocyes, which cytoplasm is filled in with PAS positive material showing rod shaped or granular appearance. Jejunal biopsy plays important role in the detection of lymphangiectasaia which is associated with severe protein loss and closely related to CD, to TM, to giardiases but occasionally also to Crohn’s disease. The mucosal lymphatics within the tips of villi are extremely widened in the absence of any villous atrophy. The lamina propria may contain scattered macrophages filled with PAS positive granules and/or fat droplets. Waldman et al. [77] reported the first cases of such intestinal lymphangiectasia, associated with edema and hypoproteinemia (along with hypoalbuminemia and hypogammaglobulinemia). Biopsies of the small bowel [78] showed prominent dilated lymphatics in the lamina propria region, often with extension into submucosa, but this lesions may be quite focal. A rare association of the intestinal lymphangiectasia with CD has been noted [79]; however, in most with primary intestinal lymphangiectasia, the intestinal villi appear to be otherwise normal on routine microscopic evaluation apart from the dilated lacteals. In addition, iron deficiency may develop [80] but it is not clear if there is any impairment of iron absorption in primary intestinal localized in the duodenum. The disease, as a rule, occurs in small children, often in neonates, but is rare in adults. A critical diagnostic element in adults with lymphangiectasia is the exclusion of malignancies including lymphoma, Whipple’s disease or chronic inflammatory disorders such as Crohn’s disease, all of which may cause MAS or at least profound changes in intestinal permeability and/or protein-losing enteropathy [81]. Whipple disease is a rare, multi-organ disease with prominent intestinal manifestations. Clinical signs preceding the diagnosis are articular, digestive (MAS) and/or neurologic. At progressed stage of the disease, patients present diarrhea, weight loss and some other symptoms of MAS, though a minority does not show any gastrointestinal symptomatology. In Whipple disease, histopathology is the key for differential diagnosis, but may this may require multiple and occasionally even repeated sampling. AS a rule, the villous pattern looks distorted by conspicuous infiltrate consisting of large mononuclear cells PAS positive material. The macrophages are located in loose connective tissue of widened villi containing neutral fat. The central role of PCR has been confirmed for the detection of the associated Gram-positive bacillus named Tropheryma whippelii, which otherwise is very difficult to culture [82].

6. Conclusions

The main structural features of CD, as seen in duodenal biopsy, include villous atrophy at the small intestine mucosa along with crypt hyperplasia and of CD3plus T-lymphocyte infiltration of the intraepithelial compartments (IEL). Later on, the inflammatory T-lymphocyte accumulation in the lamina propria becomes abundant. In the past, duodenal biopsy was regarded for the golden diagnostic standard which should be always performed in order to confirm the diagnosis of CD. However, in patients who show atypical and/or incomplete clinical symptoms, the reasonable strategy is to examine the well-defined and highly significant serological markers first. Serological tests include screening of AEmA by IFA, detection of tTGase 2 antibodies (especially in the IgA class) by ELISA and/or detection AGA using the so called microdots as antigen. The serologic markers are of great importance; when positive, they are indicative for the running CD. Thus, relatively simple and non-invasive serological tests can ensure that either asymptomatic individuals or those with mild (atypical) clinical forms are identified. Patients who are serologically positive for CD, but still show normal histology, represent its latent form. In these, duodenal biopsy may be performed to distinguish between the silent and/or latent cases. Another recently introduced test for solving the latter situation is HLA II genotyping. Those individuals who reveal classical CD symptoms as well as positive serology (at least two or all three markers positive by two different tests) and, in addition, show the high risk HLA II haplotypes (HLA-DQ2 and/or HLA-DQ8) can be regarded for CD positive even in the absence of convincing duodenal biopsy lesions (Figure 5 shows the alternative diagnostic algorithms). In all cases mentioned above, starting GFD is justified. Repeated duodenal biopsy can be performed in order to follow the patient´s response to GFD and to other therapeutic measures previously positive cases, but mainly in adults. On other hand, at repeated testing, the non-invasive serologic follow up is highly recommended especially for children in whom obtaining the biopsy samples might be difficult and/or in those adults who have shown negative or not convincing biopsy.

Abbreviations

CD – celiac disease (patient disorder), MAS – malabsorption syndrome, IBS –irritable bowel disease, TM – tropical malabsorption, SIBO- small intestine bacterial overgrowth, GFD – gluten free diet, IEL - intraepithelial lymphocyte, AEmA - anti-endomysium antibody, anti-tTGase 2 - tissue transglutaminase 2 antibody, anti-DGP – deamidated gliadin peptide antibody, AGA - anti-gliadin antibody, SNCD – seronegative celiac disease, TCR – T cell receptor, HLA – human leukocyte antigen, CD – cluster of differentiation (immunocyte marker), APC – antigen presenting cell, PAB – pancreas antibody, UC – ulcerative colitis, ANCA – anti-netrophil cytoplasmic antibody, IL – interleukin, IFN – interferon, IFA – indirect fluorescent antibody (assay), ELISA – enzyme linked immunosorbent assay.

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