Circulating IgA class anti-tTG autoantibodies and EMA are highly prevalent and characteristic in active coeliac disease.15,20,21
Our results show that in as many as 98.7% of IgA deficient coeliac patients, disease specific IgG anti-tTG antibodies and EMA can be detected. Therefore, the humoral response targeting tTG as an autoantigen should be considered as a regular and diagnostically useful feature in all coeliac patients, both IgA competent and IgA deficient.
The human recombinant tTG based ELISA used in this study clearly out performs the guinea pig substrate based IgG class anti-tTG antibody test, earlier shown to be highly unspecific.15
Immunofluorescent investigation of serum IgG EMA has severe technical drawbacks because secondary antibodies against human IgG often bind to the tissue connective fibres in a similar non-specific manner.28
While the positive EMA reaction is easy to recognise, up to 50% of negative samples may show endomysial staining when monkey oesophagus substrate is used.26
For more specific results, human IgG1 specific or monkey immunoglobulin preadsorbed secondary antibodies,18,29,30
or substrates with low background (human fetal tissues, jejunum, or appendix) are needed.14,19
Given these technical requirements, only few laboratories offer IgG class coeliac antibody testing on a routine basis. In contrast, although the performance of IgG AGA tests is not optimal, they are widely accessible because they can be performed with simple ELISA methods. Introduction of an efficient ELISA for IgG anti-tTG antibody testing could clearly enhance the feasibility of detection of IgG coeliac autoantibodies for laboratories that have not mastered the IgG EMA method.
An advantage of the measurement of IgG anti-tTG antibodies with ELISA is the more precise quantification of serum antibody concentrations which can make monitoring a gluten free diet effective and more convenient. In the present study, a clear decrease was detected, even in patients where IgG-EMA was still considerably positive.
Due to impaired responses to mucosal immunostimuli, IgA deficient subjects frequently present with enteral complaints1
but coeliac disease combined with IgA deficiency accounts in general for only a few of cases in each gastroenterology centre.9
The prospective use of IgG EMA enabled us to identify a high number of IgA deficient coeliac patients, and also those with moderate and mild symptoms, and therefore we included all IgA deficient patients evaluated by biopsy during the same period to avoid significant bias. The ELISA used here identified coeliac patients effectively whereas none of the non-coeliac subjects had IgG anti-tTG antibody levels in the range of untreated coeliac patients (fig 1).
Furthermore, the decrease in IgG autoantibodies seems to be very slow in IgA deficient coeliac patients and most of our patients were still positive after more than two or three years on a gluten free diet. In contrast, IgA competent coeliac patients became negative for both IgA and IgG anti-tTG antibodies by one year on a gluten free diet.31
Dietary interviews in our centre did not reveal more lapses in coeliacs with IgA deficiency than in those without (data not shown), and mucosal healing was observed at low levels of IgG autoantibody positivity. In fact, all of these patients had several fold reductions in IgG anti-tTG antibody concentrations on a gluten free diet, clearly showing that IgG coeliac autoantibodies are indeed gluten dependent. Specific antibody production is dependent on T helper lymphocyte function, and T cell priming, cytokine profiles, and B cell responsiveness show alterations in IgA deficiency.32,33
Therefore, the slow disappearance kinetics of IgG coeliac antibodies may be part of the immunoregulatory defect seen in IgA deficiency. However, there are a number of reports showing that individuals with HLA B8 DR3 haplotypes have alterations in their immune response, regardless of their IgA status, and hence it is not clear whether these features are directly due to IgA deficiency as such or associated HLA genes.34
We also found that 9.8% of apparently healthy IgA deficient adult blood donors were positive for both IgG EMA and anti-tTG antibodies and may have undetected coeliac disease. Biopsies were not performed in these patients but they were genetically similar to the clinically diagnosed IgA deficient coeliac patients. Given the high specificity of the EMA and anti-tTG antibody tests in our clinical cases, the results further support the fact that known IgA deficient subjects have at least a 10-fold increased risk of coeliac disease6
and should be an important target group for case finding.
However, IgA deficiency is often not known at the time of coeliac antibody testing and the firstline use of IgG EMA instead of serum IgA measurements was successful in population screening.14
The IgG anti-tTG antibody ELISA seems to be an easy tool to evaluate samples with negative IgA EMA or anti-tTG antibodies and unknown serum IgA levels. However, the cost effectiveness of this approach remains to be established.
Humoral IgA deficiency is usually defined as a total serum IgA of <0.05 g/l but levels may fluctuate as a response to mucosal antigens and cytokines.35
Four of our IgA deficient patients resumed normal serum IgA levels on a gluten free diet although IgA in IgA competent coeliac patients usually decreases on a gluten free diet.36
One prominent regulator of the IgA class switch is transforming growth factor β (TGF-β), and mice lacking the TGF-β receptor type II develop serum IgA deficiency.37
Although TGF-β1 is increased in the coeliac mucosa,38
its action to promote epithelial cell differentiation on the crypt-villus axis seems to be impaired.39
The action of TGF-βs on the IgA switch also may be altered in genetically susceptible individuals who may also benefit from early recognition and treatment of coeliac disease in terms of IgA production and related pathology.
In conclusion, IgG anti-tTG antibodies measured with human tTG are highly reliable serum markers of coeliac disease in IgA deficient subjects. To ensure the detection of these patients, IgG anti-tTG measurements should be integrated into diagnostic and screening strategies and coeliac disease should be considered in all IgA deficient individuals.