Wheat gliadins modified by deamidation are more efficient than native gliadins in inducing a Th2 response in Balb/c mice experimentally sensitized to wheat allergens (1)
Gliadins is monomeric and soluble in an alcohol/ water mixture. Many types of gliadins are found in each group: w-gliadins in the sulphur-poor group and a-b-, g-gliadins in the sulphur-rich group, each of them being highly polymorphic. Two pathologies display distinct immunological mechanisms and effectors: mostly IgE-mediated for food allergy versus T-cell mediated in celiac disease. Murine models constitute good tools to improve our understanding of allergic sensitization mechanisms and symptom elicitation. IgG1 production in mice is induced by IL-4- and IL-5-secreting Th2 cells and is followed by IL-4-induced IgE production. The Th2 response is downregulated by Th1 cell activation that causes INF-g secretion, inducing IgG2a production. A Balb/c model of sensitization to native gliadins by intra-peritoneal (IP) injections.
Freeze-dried native (NG) and deamidated (DG) gliadins were rendered soluble in 70% ethanol at 5 mg/mL and then slowly diluted at 0.1 mg/mL in sterile phosphate-buffered saline (PBS). Three groups of mice were established: the first (control) group was sensitised with aluminium hydroxide (Alum) diluted in PBS, the second group was sensitized with 10 mg NG adsorbed on Alum and the third group was sensitized with 10 mg DG adsorbed on Alum. Intraperitoneal (IP) sensitizations were performed at days 0, 10, 20 and 30.
Purified gliadin fractions chemically deamidated under acidic conditions displayed deamidation levels as follows : 52% for a/b-gliadins, 32% for g-gliadins, 36% for w1.2-gliadins and 51% for w5-gliadins. The deamidation level of the total gliadin extract, which included a/b-, g- and o-gliadins, reached 53% which was within the range obtained for separate gliadin types. The migration of native and deamidated total gliadins in an acid-PAGE gel revealed two main differences.
These patient sera displayed high concentrations of IgE that were reactive against two particular gliadins: native and deamidated g and w1.2 gliadins.
Both NG-sensitized and DG-sensitized mice secreted significantly higher levels of gliadin-specific IgG1. Both NG-sensitized and DG-sensitized mice secreted significantly higher levels of total and gliadin-specific IgE. Spleen cells from both NG-sensitized and DG-sensitized mice secreted significantly higher levels of IL-4 than control mice.
Both NG- and DG-sensitized mice secreted gliadin-specific IgG2a when compared to the control group. Activation with native gliadins resulted in a significant secretion of INFg by spleen cells in both NG and DG-sensitized mice.
Severe symptoms were observed in both NG-sensitized and DG-sensitized mice, with equivalent average scores. Both NG-sensitized and DG-sensitized mice secreted significantly high levels of histamine. NG-sensitized mice displayed elevated concentrations of IgE specific to the different classes of native gliadins (i.e. a/b-, g-, w1.2- and w5-gliadins) and which were roughly related to the proportion of each gliadin class in wheat flour. Mice sensitized with DG displayed a totally different profile: they displayed higher concentrations of IgE specific to g- and w1.2-gliadins.
This allergenic effect must be associated with some of the physicochemical properties of DG. DG are more soluble in water, so when they were injected via the IP route they may spread more easily in the blood and thus come into contact with the immune system. In humans, a dominant epitope (‘‘QPQQPFP’’) has been identified in the repetitive region of g and w1.2 gliadins, and it is not contained in the amino-acid sequences of either a or w5 gliadins. Gliadins are particularly rich in Gln residues. DG is more soluble at pH 7 than at an acidic pH. DG may aggregate in the stomach at an acidic pH and thus be more resistant than NG to digestion by stomach enzymes (pepsin).