ANTIACETYLCHOLINE RECEPTOR FAB FRAGMENTS ISOLATED FROM THYMUS-DERIVEDPHAGE DISPLAY LIBRARIES FROM MYASTHENIA-GRAVIS PATIENTS REFLECT PREDOMINANT SPECIFICITIES IN SERUM AND BLOCK THE ACTION OF PATHOGENIC SERUMANTIBODIES

Myasthenia gravis (MG) is a prototype antibody-mediated autoimmune dis ease in which antibodies against nicotinic acetylcholine receptors (AC hR) induce loss of functional receptors at the neuromuscular junction. Germinal centers present in MG hyperplastic thymus contain activated B-cells spontaneously producing anti-human AChR (huAChR) Ab in vitro. In order to access the anti-huAChR repertoire phage display Fab librar ies of thymic lymphocytes were constructed from two MG patients. A tot al of four Fabs highly specific for huAChR were isolated that bind to determinants in or near the main immunogenic region (MIR). These anti- huAChR Fabs showed evidence of significant somatic mutations supportin g the notion that the anti-huAChR Ab response in MG patients is driven by antigen. A total of two Fabs were able to inhibit up to 90% of don or serum anti-huAChR antibodies. Competition with serum anti-huAChR Ab was also observed in unrelated MG patients and indicate that anti-huA ChR Fabs bind to epitopes on huAChR recognized by the majority of MG p atients. In vitro antigenic modulation studies demonstrated that anti- huAChR Fabs were able to induce AChR loss when cross-linked by an anti -Fab antibody but. not as monovalent Fab. Moreover, anti-huAChR Fabs w ere able to protect against AChR loss by antigenic modulation induced by MG serum antibodies suggesting a potential therapeutic role for the se recombinant Fabs in patients with a myasthenic crisis. (C) 1997 Els evier Science B.V.