The goal of this project is to understand the mechanism of how highly disease specific autoantibodies are generated in response to the exposure to a foreign antigen. IgA autoantibodies reactive with the enzyme transglutaminase 2 (TG2) are typical of coeliac disease (CD). These antibodies are only present in subjects who are HLA-DQ2 or -DQ8, and their production is dependent on dietary gluten exposure. This suggests that CD4+ gluten reactive T cells, which are found in CD patients and which recognise gluten peptides deamidated by TG2 in context of DQ2 or DQ8, are implicated in the generation of these autoantibodies. Many small intestinal IgA+ plasma cells express membrane Ig hence allowing isolation of antigen specific cells. Whereas control subjects lack anti-TG2 IgA+ plasma cells, on average 10% of the plasma cells of CD patients are specific for TG2. We have sorted single TG2 reactive IgA+ plasma cells, cloned their VH and VL genes and expressed recombinant mAbs. So far we have expressed 26 TG2 specific mAbs. There is a strong bias for VH5-51 usage, and surprisingly the antibodies are modestly mutated. TG2 acts on specific glutamine residues and can either crosslink these to other proteins (transamidation) or hydrolyse the glutamine to a glutamate (deamidation). None of the 18 mAbs tested affected either transamidation or deamidation leading us to hypothesise that retained crosslinking ability of TG2 when bound to membrane Ig of B cells is an integral part of the anti-TG2 response. Four models of how activation of TG2 specific B cells is facilitated by TG2 crosslinking and the help of gluten reactive CD4 T cells are proposed. These four models will be extensively tested including doing in vivo assays with a newly generated transgenic anti-TG2 immunoglobulin knock-in mouse model.
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