B cell epitopes of gliadin

A phage displayed dodecapeptide library and synthetic octapeptides spanning the complete sequence of alpha- and gamma-type gliadin and overlapping in six amino acids (pepscan) were screened for binding to human gliadin antibo dies (AGA). Phage display experiments led to four sequences recognized with significantly higher frequency by sera with raised IgA-AGA titres than by control sera. All these peptides contained the core sequence PEQ. Pepscan e xperiments revealed binding of AGA to five prominent regions: (i) QXQPFP (b inding to IgG and IgA, X representing P, Q, and L); (ii) IPEQ (IgG) and WQI PEQ (IgA); (iii) FFQP (IgG) and QGXFQP (IgA, X representing F and S); (iv) PQQLPQ (IgG and IgA), all in alpha-type gliadin; and (v) QPQQPF (IgG and Ig A) in gamma-type gliadin. In two of the sequences (QPQQPF and QQQPFP), subs titution of Q by E resulting in QPEQPF and QEQPFP, respectively, increased significantly binding of AGA from sera of patients with biopsy-proven or su spected coeliac disease (CoD), all positive for endomysium antibodies (EmA) . In contrast, binding of sera with high AGA titre from EmA-negative patien ts (CoD and dermatitis herpetiformis excluded) was not enhanced by this sub stitution. Thus, AGA directed against these modified epitopes can be regard ed as specific for CoD. This is the first study demonstrating that deamidat ion of gliadin improves reactivity of AGA of CoD patients.