Evidence for an antigen-driven selection process in human autoantibodies against acetylcholine receptor.

Autoantibodies to the nicotinic acetylcholine receptor (AChR) play a central role in the neurological symptoms associated with myasthenia gravis (MG). A better knowledge of the structural organization and of the mechanisms leading to the production of these antibodies may help in understanding the pathogenesis of the disease. To achieve this, four IgG anti-AChR monoclonal autoantibodies obtained in a previous work were derived from lymphoid cells of MG patients. Two of them (MH1 and MH6) were capable of modulating in vitro the expression of AChR at the surface of TE-671 cells. We report here the complete nucleotide sequence of the heavy and light chains of these four antibodies. Although it is difficult to address the issue of VH gene usage in anti-AChR autoantibodies because of the limited number of clones studied, our results associated with others which have appeared in the literature point to non-stochastic usage by anti-AChR antibody of some defined VH genes belonging to VH2 and VH5 minifamilies overexpressed in the fetal repertoire. The second and major aim of this work was to assess the role of an antigen-driven selection process in the production of anti-AChR autoantibodies. When comparing the expressed sequences to their closest germline counterparts, it appeared that all four studied clones displayed numerous mutations in VH regions. In particular, MH1 and MH6, characterized by their AChR modulating capacity, displayed a higher than expected number of mutations and replacements occurring in CDR regions. These data point to an antigen-driven selection process. On the contrary, the mutational process observed in the MH% clone was borderline and that of MH7 was compatible with a random process. Interestingly, when comparing mutations in heavy and light chains, a significantly lower number of mutations were expressed in light chains for the four clones.