Localizing Pathogenically Relevant Transglutaminase 2 in Celiac Disease

Celiac disease (CeD) is a gluten-induced, HLA-DQ2 or -DQ8 dependent inflammatory disorder of the small intestine for which no non-dietary therapy is available. Transglutaminase 2 (TG2) is the target of CeD-specific autoantibodies and is also involved in disease pathogenesis. In a CeD patient TG2 catalyzes the formation of deamidated gluten peptides that bind to HLA-DQ2/8 with high affinity and are recognized as epitopes by disease- specific CD4+ T cells. However, the location where TG2 exerts its pathogenic action is unknown. The overarching hypothesis of our proposal is that TG2 derived from enterocytes shed into the intestinal lumen is the source of pathogenically relevant enzyme in CeD. This luminal TG2 reacts with gluten peptides to form covalent complexes recognized by TG2-specific B cells in Peyer’s patches. In turn, these B cells present gluten antigens to disease-specific T cells, while also deriving help from these T cells. This hypothesis can explain how TG2 autoantibodies are formed in CeD. Three Aims involving in vitro and in vivo studies are designed to test our hypothesis, while taking advantage of the complementary capabilities of the three collaborating laboratories. Specific aim 1: Two key features of the above hypothesis will be tested at a biochemical level. First, the ability of antigenic gluten peptides to form metastable covalent intermediates at the TG2 active site will be probed. Second, a novel isotope labeling assay will be developed to verify that luminal TG2 in the mouse intestine can recognize and deamidate dietary gluten. Under this Aim we will also engineer a gut-impermeable TG2 inhibitor, which will be used in Aim 3 to validate the pathogenic role of luminal TG2. Specific aim 2: Using shed enterocytes collected from the human jejunal lumen as well as human organoid cultures, we will demonstrate that intestinal epithelial cells harbor abundant catalytically competent TG2 at the time of shedding. The ability of human enterocyte-derived TG2 to form covalent adducts with antigenic gluten peptides will also be verified. Our organoid cultures will also be used to identify disease-relevant environmental factors that are most effective at increasing steady-state TG2 activity in the small intestinal lumen. Specific aim 3: This Aim will test the pathogenic role of enterocyte-derived luminal TG2 in two mouse models of CeD. In one model of established CeD, we will test whether TG2-gluten peptide covalent complexes in the gut lumen can stimulate collaboration between TG2-specific B cells and gluten-specific T cells. In another model where CeD can be induced by feeding gluten, the requirement for TG2 expression in enterocytes or alternately in the myeloid compartment will be tested. First-generation TG2 inhibitors are already undergoing human clinical trials. If the overarching hypothesis of our proposal is correct, it will establish that TG2 inhibition in the intestinal lumen is sufficient to protect against gluten- induced villous atrophy in CeD. Not only will this motivate the design of a safer and more efficacious therapies, but our gut-impermeable inhibitors may also serve as next-generation drug prototypes for this lifelong disorder.