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 antibodies (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 experiments revealed binding of AGA to five prominent regions: (i) QXQPFP (binding to IgG and IgA, X representing P, Q, and L); (ii) IPEQ (IgG) and WQIPEQ (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 IgA) in gamma-type gliadin. In two of the sequences (QPQQPF and QQQPFP), substitution of Q by E resulting in QPEQPF and QEQPFP, respectively, increased significantly binding of AGA from sera of patients with biopsy-proven or suspected coeliac disease (CoD), all positive for endomysium antibodies (EmA). In contrast, binding of sera with high AGA titre from EmA-negative patients (CoD and dermatitis herpetiformis excluded) was not enhanced by this substitution. Thus, AGA directed against these modified epitopes can be regarded as specific for CoD. This is the first study demonstrating that deamidation of gliadin improves reactivity of AGA of CoD patients.