Abstract
Enzymatic transamidation of gliadins by microbial transglutaminase (mTG) inhibits interferon-γ (IFN-γ) secretion by intestinal T cell lines in patients with celiac disease (CD). To gain insight into the cellular mechanisms underlying the down-regulatory effects of transamidation, we tested a single recombinant α-gliadin (r-gliadin) harbouring two immunodominant peptides, p13 (aa. 120–139) and p23 (aa. 220–239), in HLA-DQ8 transgenic mice, a model of gluten sensitivity. Mice were intranasally immunised with r-gliadin or r-gliadin transamidated by mTG (K-r-gliadin) along with cholera toxin, and the response of mesenteric lymph node cells was analysed by cytokine multiplex assay. An in vitro challenge with r-gliadin was characterised by secretion of specific cytokines featuring both innate immunity and the Th1/Th2/Th17 pattern of the adaptive response. Notably, transamidation specifically down-regulated the Th1 response. Structural studies performed on K-r-gliadin confirmed that specific glutamine residues in p13 and p23, previously found to be deamidated by tissue transglutaminase, were also transamidated by mTG. In silico analysis, simulating p13 and p23 peptide binding to HLA-DQ8 showed that these glutamines, in the form of glutamate, could interact by means of salt bridges with peculiar amino acids of the alpha chain of HLA-DQ8, suggesting that their transamidation may influence the HLA-restricted recognition of these peptides. Thus, the structural findings provided a rationale to explain the down-regulation of the r-gliadin-specific Th1 response following transamidation.
Highlights
Celiac disease (CD) is a common food-sensitive enteropathy in humans caused by absence of oral tolerance to wheat gluten
A preliminary deamidation step catalysed by tissue transglutaminase appears crucial [3], as the negative charges introduced in the gliadin molecules by this reaction significantly increase the binding affinity of peptides to DQ2 [4,5,6] and DQ8 transgenic (DQ8) [7] heterodimers
We previously showed that enzymatic transamidation of gliadin by microbial transglutaminase was effective at blocking IFN-γ production by intestinal T cells in patients with CD [10]. mTG is a transamidase of the endo-γglutamine:ε-lysine transferase type [11]
Summary
Celiac disease (CD) is a common food-sensitive enteropathy in humans caused by absence of oral tolerance to wheat gluten. Almost all patients with CD express the MHC class II molecules HLA-DQ2.5, HLA-DQ8, or HLA-DQ2.2, with HLA-DQ2.5 being the major genetic risk factor [1]. This suggests the dominant role of adaptive immunity in the pathogenesis of CD. A preliminary deamidation step catalysed by tissue transglutaminase (tTG) appears crucial [3], as the negative charges introduced in the gliadin molecules by this reaction significantly increase the binding affinity of peptides to DQ2 [4,5,6] and DQ8 [7] heterodimers. Unlike tTG, mTG is a calcium-independent, low molecular weight protein, which has several advantages in food industrial applications [12]
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