Immunoglobulin-E-binding epitopes of wheat allergens in patients with food allergy to wheat and in mice experimentally sensitized to wheat proteins (1)
Differences in the recognition of gliadin epitopes between patients according to the severity of their symptoms. The characterization of wheat protein epitopes to propose a diagnostic tool for exercise-induced anaphylaxis (EIA) due to wheat that combined epitopes from w5-gliadins and high-molecular-weight (HMW) glutenin subunits. High-resolution, three-dimensional structures are valuable for determining the structures of possible discontinuous epitopes, but are available for only a few wheat allergens, one being lipid transfer protein (LTP), which has been shown to be a highly-ordered compact protein. The w-gliadins usually have no cysteines and hence no disulphide bonds. The a-gliadins, g-gliadins, and the low-molecular-weight (LMW)-glutenin subunits are composed of two major domains: (1) an N-terminal, repetitive domain that is similar to the single domain of the w-gliadins, being rich in glutamine and proline residues and probably unstructured and flexible in solution; and (2) a non-repetitive, C-terminal domain that includes either three or four intramolecular disulphide bonds, which link specific cysteine residues.
Five of the patient sera exhibited IgE binding to the LTP membrane. The peptide 37QARSQSDRQS46 was bound by IgE from four patients (25, 33, 60, and 82) with a positive, but weak, intensity. This peptide corre- sponded to the loop linking the helix a2 to the first part of a3. Pooled sera from the LTP1 sensitized mice detected two peptides found in the LTP1 sequence: P37-46 and a mouse-specific epitope, 53GIARGIHNLN62, localized in the loop between the a3 and a4 helices.
Reduction and alkylation of the wheat LTP1 strongly modified the protein secondary structure contents with a drastic reduction of helix content (15% vs. 47%) and an increase of b strands (25% vs. 6%) and unordered structures (43% vs. 32%). The reduced and alkylated LTP1 was very poorly recognized by patient IgE and mouse IgE.
w5-gliadin, 15 of them (75%), mostly from adults, displayed positive responses against consensus IgE-binding motif of o5-gliadin being QQX1PX2QQ (X1 being I, L, F, S, or Y and X2 Q, and E). W2-gliadin, eight of the 10 sera (80%) displayed positive responses for w2-gliadin peptides which contain PQQPFP motif. A-gliadin, in the repetitive domain, the sequence 27QQQFPGQQQQ36 recognized by three sera was very close to the w5-gliadin consensus epitope. Four epitopes in the non-repetitive domain contained a cysteine or were in the vicinity of a cysteine. Low-molecular-weight-glutenin, Only one of the epitopes, recognized by six sera (39QQPIQQQPQQ48) was immunodominant.
For the w5-gliadin sequence, three of the human epitopes were bound by mouse IgE (QQFPQQQ, QQLPQQQ, and QQSPQQQ) and the peptide QQLPQQ was recognized with the highest intensity. Mouse IgE specifically detected other peptides (QQEFPQQQ, QQQFPQQEFP). The responses of mouse IgE to w2-gliadin peptides were of low intensity. Two epitopic zones were common to human and mouse IgE (43–60 and 105–116); these zones contained notably the peptides FPTPQQQFPE and QQSFPLQPQQ. Four IgE-binding epitopes in a-gliadin were common to mice and wheat allergic patients: three were in the repetitive domain (23PLVQQQ28, 27QQQFPGQQQQ36, 55YLQLQP60) and one localized in the C-terminal fragment (224SFQQPQQQYP233).
The repeated epitopic sequences of w5-gliadin may promote very efficient mast cell degranulation and contribute to the development of severe symptoms in these patients. The epitope 27QQQFPGQQQQ36 of a-gliadin might certainly lead to cross-reactions with w5-gliadins. Similarly, epitopes 39QQPIQQQPQQ48 and 47QQFPQQQPCS56 might be responsible for cross-reactions between some LMW-glutenin subunits and w5-gliadins. Cross-reactions might also occur for some patients between w2 and w5-gliadins via the sequences QQPFPQ, QQQFPEQ, or QQFPQ.
