In the present study, we showed for the first time that in vivo targeting of TG2 in coeliac disease occurs in the form of in situ endomysial, reticulin, and jejunal subepithelial autoantibody binding. Patient IgA found deposited on extracellular TG2 in the liver, lymph nodes, and muscles indicates that the coeliac disease autoantigen is widely accessible to the intestinally produced21
circulating coeliac autoantibodies throughout the body. Based on clinical evidence of the occurrence of extraintestinal manifestations, coeliac disease should be regarded as a systemic disease and not solely involving the intestinal tract. The present primary observations may constitute one important denominator for gluten induced extraintestinal disease manifestations.
This study also showed that coeliac autoantibodies are deposited in the morphologically normal small intestinal mucosa before they can be detected in the circulation, and furthermore that the TG2 specific IgA deposition precedes the formation of the gluten induced flat jejunal lesion. This raises the possibility that autoantibodies against TG2, apart from T lymphocytes, contribute to the pathogenesis of mucosal atrophy in both coeliac disease and dermatitis herpetiformis. Also, in all other organs studied, TG2 was targeted by autoantibodies, as shown by the TG2 specificity of the IgA co-eluted together with TG2 from coeliac tissues. At the same time, conventional elution agents (for example, urea) capable of removing serum endomysial antibodies coated on tissue sections,32
did not extract any specific IgA from the same tissues (data not shown), which would strongly argue against the possibility that the eluted IgA represents circulating (blood) antibodies.
However, in the gluten dependent skin disease of dermatitis herpetiformis, IgA further targets type 3 transglutaminase in the skin.33
The coeliac disease autoantigen, TG2, is a ubiquitous cellular protein which also functions on the cell membrane associated with integrins.34
In the extracellular space, TG2 is involved in the cell-extracellular matrix interactions and also in the remodelling and stabilisation of the extracellular matrix.35
As coeliac antibodies have been shown to influence the protein cross linking enzymatic activity of TG2,22
their presence may interfere with the normal performance of the enzyme in the turnover of the extracellular matrix and cytokines. It may also be hypothesised that binding of autoantibodies to TG2 also disturbs TG2 mediated cellular adhesion. It has been shown that subepithelial blistering leading to loss of surface epithelial cells occurs in the coeliac mucosa,36
coincident with findings indicating that the most pronounced IgA deposition is characteristically seen subepithelially.24,26
None the less, ulceration is not a feature of coeliac disease. Our study also showed that TG2 stands in intimate relation with mucosal blood vessels where deposition of coeliac IgA (fig 1I) might influence their nutritive and architectural function. There is remodelling of vessels in the coeliac flat lesion, and angiogenesis is very likely important for mucosal regeneration on diet when a normal villous structure without scarring is achieved.1
Coeliac disease may also induce architectural changes in extraintestinal tissues—for example, dental enamel defects in permanent teeth or loss of Purkinje cells in the brain.8,37
Our patients with extraintestinal manifestations and IgA autoantibody deposition in their organs evinced no specific abnormalities on conventional histology (table 2). However, some inflammation was present and it is possible that, as in the case of the jejunum,13
more time is required to develop advanced lesions. The most general pathology, scarring, seems to be related to the bioactivity of TG235
and may be inhibited by the presence of antibodies. The observed clinical symptoms and signs (for example, size of the lymph nodes, muscular weakness, and proteinuria) were clearly responsive to gluten withdrawal, which suggests an ongoing pathology in the respective organs. In situ endomysial antibody deposition would also be a reasonable case for the gastrointestinal motility disorder observed in other clinical studies.38
Direct evaluation of the effect and dynamics of extraintestinal TG2 antibody deposition is especially difficult as repeated biopsies were not possible in our cases. In addition, our liver patients also had independent hepatic diseases (case 4: cystic fibrosis, parvovirus B19 infection39
; case 5: hepatitis B infection). TG2 specific autoantibodies found in the morphologically normal appendix again indicate that a clinically silent deposition of TG2 specific antibodies in various organs can occur in any coeliac patient. It remains to be established which factors determine organ manifestations or whether there are contributing factors directing the antibodies to particular organs.
The ability of coeliac autoantibodies to inhibit the bioactivity of TG222
and the resulting decrease in the availability of active TGF-β23
could also be part of the mechanisms which elicit the diverse extraintestinal disease entities. In fact, TGF-β is known as an elementary regulatory growth factor in organogenesis and tissue homeostasis maintenance, in addition to its known function as a proinflammatory and immunosuppressive factor. Furthermore, as the present results show the self-antigen to be targeted by immunoglobulins early on in disease development, it is intriguing to hypothesise that an initially ongoing Th2-type mucosal inflammation passes on to the classical Th1-type inflammatory process only later, as the mucosa deteriorates.
As the histological changes in coeliac organ manifestations are non-specific, examination of TG2 specific immunoglobulin deposition by direct immunofluorescence might be a useful diagnostic tool in the differential diagnosis of organ diseases of unknown aetiology. For example, lymph node enlargement, hitherto not acknowledged as a presenting symptom in coeliac disease, was a prominent feature in two of our patients.
TG2 related IgA deposits in the morphologically normal jejunum were predictive of forthcoming overt coeliac disease with villous atrophy. They may thus be regarded as a further marker of developing coeliac disease and coeliac disease latency, and a promising diagnostic tool in cases with low grade enteropathy. In agreement with previous studies,24,26
our results support the notion that subepithelial IgA deposits are specific for gluten induced enteropathy. However, further studies are needed to confirm this in larger patient groups and to explore the kinetics of IgA deposition in relation to gluten intake and intraepithelial lymphocyte counts. Moreover, it should be established whether humoral targeting of TG2 occurs in gluten induced peripheral or central nervous system diseases, or heart, bone, or reproductive system involvement.
In conclusion, the widespread in situ endomysial, reticulin, and jejunal binding of TG2 specific IgA antibodies found in the tissues of our coeliac patients with developing disease or extraintestinal organ manifestations strongly supports the concept that humoral immunity is involved in pathogenesis of coeliac disease.