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CD4+ class II–restricted T cells specific for self antigens are thought to be involved in the pathogenesis of most human autoimmune diseases and molecular mimicry between foreign and self ligands has been implicated as a possible mechanism for their activation. In this report we introduce combinatorial peptide libraries as a powerful tool to identify cross-reactive ligands for these T cells. The antigen recognition of a CD4+ T cell clone (TCC) specific for myelin basic protein peptide (MBP) (86-96) was dissected by the response to a set of 220 11-mer peptide sublibraries. Based on the results obtained with the libraries for each position of the antigen, artificial peptides were found that induced proliferative responses at much lower concentrations than MBP(86-96). In addition stimulatory ligands derived from protein sequences of self and microbial proteins were identified, some of them even more potent agonists than MBP(86-96). These results indicate that: (a) for at least some autoreactive CD4+ T cells antigen recognition is highly degenerate; (b) the autoantigen used to establish the TCC represents only a suboptimal ligand for the TCC; (c) a completely random and unbiased approach such as combinatorial peptide libraries can decrypt the spectrum of stimulatory ligands for a T cell receptor (TCR).

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Transglutaminase 2 (TG2) in the extracellular matrix is largely inactive but is transiently activated upon certain types of inflammation and cell injury. The enzymatic activity of extracellular TG2 thus appears to be tightly regulated. As TG2 is known to be sensitive to changes in the redox environment, inactivation through oxidation presents a plausible mechanism. Using mass spectrometry, we have identified a redox-sensitive cysteine triad consisting of Cys 230 Human transglutaminase 2 (TG2) 2 modifies protein-or peptide-bound glutamine residues by either cross-linking their reactive carboxamide side chains to primary amines or by deamidation, converting glutamine residues to glutamate (1, 2). Ca 2ϩ is required for this catalytic activity and induces conformational changes in the enzyme, which arrange the active-site residues for catalysis, including Cys 277 (3-6). As TG2 is abundantly expressed in both the intracellular and extracellular environments of many tissues, its catalytic activity must be tightly regulated to avoid excess modification of cellular and tissue components. GTP and GDP act as allosteric inhibitors by inducing a closed conformation in which the active site is buried (6 -8). Low Ca 2ϩ concentration and high GTP/GDP concentration in the cytosol typically prevent TG2 activation within cells. Despite conditions in the extracellular milieu that favor activation, extracellular TG2 also appears to be predominantly inactive under normal conditions but can be activated by certain types of inflammation and cell injury (9).The catalytic activity of TG2 is implicated in the pathogenesis of several human diseases, including celiac disease (10). Celiac disease is caused by an aberrant immune response to proline-and glutamine-rich peptides from dietary gluten in the small intestine of genetically predisposed individuals (11). In the celiac immune response, the enzymatic activity of TG2 is crucial, as TG2-mediated deamidation of gluten peptides increases their T-cell antigenicity (12, 13).In contrast to our knowledge of the mechanistic basis for TG2 inactivity in the intracellular environment, the mechanisms underlying regulation of TG2 activity in the extracellular compartment remain unclear. TG2 harbors no disulfide bonds in its native state, which is unusual for enzymes in the extracellular environment (14). Previous studies have shown that TG2 is susceptible to oxidation, resulting in inactivation (19 -22). Thus, modulation of enzymatic activity through oxidation presents a plausible mechanism for regulation of extracellular TG2 activity. We have investigated the events underlying oxidative inactivation of TG2 and report the identification of a redox-sensitive cysteine triad consisting of Cys 230 , Cys 370 , and Cys 371 . Within this triad, Cys 230 appears to set the threshold for intramolecular disulfide bond formation and thereby inactivation. Oxidation was influenced by the presence of Ca 2ϩ and substrate, suggesting that the local environment can modulate and fine-tune oxidative inactivation of T...

BackgroundCeliac disease is a small intestinal inflammatory disorder characterized by malabsorption, nutrient deficiency, and a range of clinical manifestations. It is caused by an inappropriate immune response to dietary gluten and is treated with a gluten-free diet. Recent feeding studies have indicated oats to be safe for celiac disease patients, and oats are now often included in the celiac disease diet. This study aimed to investigate whether oat intolerance exists in celiac disease and to characterize the cells and processes underlying this intolerance.Methods and FindingsWe selected for study nine adults with celiac disease who had a history of oats exposure. Four of the patients had clinical symptoms on an oats-containing diet, and three of these four patients had intestinal inflammation typical of celiac disease at the time of oats exposure. We established oats-avenin-specific and -reactive intestinal T-cell lines from these three patients, as well as from two other patients who appeared to tolerate oats. The avenin-reactive T-cell lines recognized avenin peptides in the context of HLA-DQ2. These peptides have sequences rich in proline and glutamine residues closely resembling wheat gluten epitopes. Deamidation (glutamine→glutamic acid conversion) by tissue transglutaminase was involved in the avenin epitope formation.ConclusionsWe conclude that some celiac disease patients have avenin-reactive mucosal T-cells that can cause mucosal inflammation. Oat intolerance may be a reason for villous atrophy and inflammation in patients with celiac disease who are eating oats but otherwise are adhering to a strict gluten-free diet. Clinical follow-up of celiac disease patients eating oats is advisable.