HUMAN ANTI-NICOTINIC ACETYLCHOLINE-RECEPTOR RECOMBINANT FAB FRAGMENTSISOLATED FROM THYMUS-DERIVED PHAGE DISPLAY LIBRARIES FROM MYASTHENIA-GRAVIS PATIENTS REFLECT PREDOMINANT SPECIFICITIES IN SERUM AND BLOCK THE ACTION OF PATHOGENIC SERUM ANTIBODIES

Myasthenia gravis (MG) is a prototype Ab-mediated autoimmune disease i n which Abs against nicotinic acetylcholine receptors (AChR) induce lo ss of functional receptors at the neuromuscular junction. Germinal cen ters present in MG hyperplastic thymus contain activated B cells spont aneously producing anti-human AChR (anti-huAChR) Ab in vitro. To acces s the anti-huAChR repertoire, phage display Fab libraries of thymic ly mphocytes were constructed from two MG patients. Four Fabs highly spec ific for huAChR were isolated that bind to determinants in or near the main immunogenic region. These anti-huAChR Fabs showed evidence of si gnificant somatic mutations, supporting the idea that the anti-huAChR Ab response in MG patients is driven by Ag. Two Fabs were able to inhi bit up to 90% of donor serum anti-huAChR Abs. Competition with serum a nti-huAChR Ab was also observed in unrelated MG patients and indicate that anti-huAChR Fabs bind to epitopes on huAChR recognized by the maj ority of MG patients. In vitro antigenic modulation studies demonstrat ed that anti-huAChR Fabs were able to induce AChR loss when cross-link ed by an anti-Fab Ab but not as monovalent Fab. Moreover, anti-huAChR Fabs were able to protect against AChR loss by antigenic modulation in duced by MG serum Abs, suggesting a potential therapeutic role for the se recombinant Fabs in patients with a myasthenic crisis.