Data from: Enhanced B-cell receptor recognition of the autoantigen transglutaminase 2 by efficient catalytic self-multimerization

A hallmark of the gluten-driven enteropathy celiac disease is autoantibody
production towards the enzyme transglutaminase 2 (TG2) that catalyzes the
formation of covalent protein-protein cross-links. Activation of
TG2-specific B cells likely involves gluten-specific CD4 T cells as
production of the antibodies is dependent on disease-associated HLA-DQ
allotypes and dietary intake of gluten. IgA plasma cells producing TG2
antibodies with few mutations are abundant in the celiac gut lesion. These
plasma cells and serum antibodies to TG2 drop rapidly after initiation of
a gluten-free diet, suggestive of extrafollicular responses or germinal
center reactions of short duration. High antigen avidity is known to
promote such responses, and is also important for breakage of
self-tolerance. We here inquired whether TG2 avidity could be a feature
relevant to celiac disease. Using recombinant enzyme we show by dynamic
light scattering and gel electrophoresis that TG2 efficiently utilizes
itself as a substrate due to conformation-dependent homotypic association,
which involves the C-terminal domains of the enzyme. This leads to the
formation of covalently linked TG2 multimers. The presence of exogenous
substrate such as gluten peptide does not inhibit TG2 self-cross-linking,
but rather results in formation of TG2-TG2-gluten complexes. The celiac
disease autoantibody epitopes, clustered in the N-terminal part of TG2,
are conserved in the TG2-multimers as determined by mass spectrometry and
immunoprecipitation analysis. TG2 multimers are superior to TG2 monomer in
activating A20 B cells transduced with TG2-specific B-cell receptor, and
uptake of TG2-TG2-gluten multimers leads to efficient activation of
gluten-specific T cells. Efficient catalytic self-multimerization of TG2
and generation of multivalent TG2 antigen decorated with gluten peptides
suggest a mechanism by which self-reactive B cells are activated to give
abundant numbers of plasma cells in celiac disease. Importantly, high
avidity of the antigen could explain why TG2-specific plasma cells show
signs of an extrafollicular generation pathway.