A Quantitative Analysis of Transglutaminase 2-Mediated Deamidation of Gluten Peptides: Implications for the T-cell Response in Celiac Disease

Abstract

Celiac disease develops in genetically predisposed individuals as the result of an inappropriate intestinal immune response to dietary gluten proteins. T cells present in the intestine of celiac patients recognize gluten peptides in the context of HLA-DQ2 or -DQ8 molecules. Notably, T-cell recognition is increased after these peptides have been deamidated by the enzyme transglutaminase 2. Several T-cell epitopes of gluten exist, and most of these epitopes derive from the alcohol-soluble gliadin fraction. For some of these epitopes, specific T cells can be isolated from intestinal biopsies from nearly all patients, whereas for others, T-cell reactivity could be demonstrated in only a few patients. One reason for this observation could be that the rate of transglutaminase 2 (TG2)-mediated deamidation significantly differs between these peptides, resulting in different amounts of epitopes generated in vivo. In this study, we established a quantitative, mass spectrometry-based approach to measure the kinetics of TG2-mediated deamidation of gliadin-derived, DQ2-restricted epitopes. Our results demonstrate large variations in the degree of deamidation between different peptides and also between individual glutamine residues within each peptide. In general, alpha-gliadin derived epitopes that are frequently recognized by patient T cells showed a significant higher level of deamidation compared to the majority of epitopes from gamma-gliadin that are less frequently recognized. The degree of deamidation of individual residues within a peptide also seems to influence whether some epitopes are better recognized in context of DO2 or DQ8. Thus, the rate of deamidation by TG2 appears to be a factor of importance for the T-cell response to gluten in celiac disease.

Celiac disease T-cell epitopes from gamma-gliadins: immunoreactivity depends on the genome of origin, transcript frequency, and flanking protein variation

“…Variant peptides that displayed a serine (S) at position 5 instead of an phenylalaninine (F) were found to have lost T-cell stimulatory capacity. Moreover, the presence of a positively charged arginine (R) residue at position +1 diminished T-cell proliferation (Additional file 3), most likely because it influenced the deamidation of Q9 as previously observed by Dørum et al [28]. Similarly, a tryptophan (W) at position +2 inhibited the T-cell response (Additional file 3).…”

Tissue Transglutaminase-mediated Glutamine Deamidation of β-Amyloid Peptide Increases Peptide Solubility, Whereas Enzymatic Cross-linking and Peptide Fragmentation May Serve as Molecular Triggers for Rapid Peptide Aggregation

“…TGase-mediated glutamine deamidation is a well known PTM found in dietary gluten protein, which in turn results in an increased T-cell response (44). The high glutamine substrate affinity for TGase cross-linking found in Tau protein is of particular interest here (45), because glutamine deamidation may trigger misfolding of Tau (46), ␣-synuclein, and Huntington protein.…”

T-cell and B-cell immunity in celiac disease

“…Only glutamine residues residing in the sequence QXP, where Q is glutamine, X is any amino acid and P is proline, are targeted by the enzyme [35,36], providing yet an explanation for the high proline content in gluten T-cell epitopes. Interestingly, it was found that those peptides that are most efficiently deamidated are also the ones that are most frequently recognized by T cells in celiac disease patients [37], pointing at a central role for TG2 in shaping the anti-gluten T-cell repertoire. This was signified by the finding that within a complex mixture of gluten peptides, TG2 selectively targeted peptides harboring known T-cell epitopes [38].…”