Activation of small intestinal
gluten-reactive CD4+ T cells is a critical event in
celiac disease. Such cells predominantly recognise
gluten peptides in which specific glutamines are deamidated. Deamidation may be catalysed by intestinal
tissue transglutaminase (TG2), a
protein which is also the main
autoantigen in
celiac disease. Our aim was to study how the two main catalytic activities of
transglutaminase--deamidation and transamidation (cross-linking) of an immunodominant
gliadin epitope--are influenced by the presence of acceptor
amines in the intestinal mucosa, and thereby contribute to further elucidation of the pathogenetic mechanisms in
celiac disease. We prepared
monoclonal antibodies, reacting specifically with the non-deamidated
epitope QPFPQPQLPYPQPQ-
amide and/or the deamidated
epitope QPFPQPELPYPQPQ-
amide. A solid phase immunoassay combined with gel filtration chromatography was used to analyse deamidation and cross-linking of these
peptides to
proteins. Our results show that QPFPQPQLPYPQPQ-
amide was deamidated when incubated with purified TG2, with fresh mucosal sheets and with mucosal homogenates. Of other
transglutaminases tested, only Streptoverticillium
transglutaminase was able to generate the deamidated
epitope. A fraction of the non-deamidated
epitope was cross-linked to
proteins, including TG2. The results suggest that intestinal TG2 is responsible for generation of the active deamidated
epitope. As the
epitope often occurs in a repeat structure, the result may be cross-linking of a deamidated, i.e., activated cell
epitope. Alternatively, the deamidation may occur by reversal of the cross-linking reaction. The results provide a basis for the suggestion that binding of a
peptide to a
protein, in connection to its modification to a
T cell epitope, might be a general explanation for the role of TG2 in
celiac disease and a possible mechanism for the generation of
autoantigens.