Antibodies against the acetylcholine receptor and myasthenia gravis

Usefullness of detection of antibodies against the acetylcholine receptor i n the diagnosis of myasthenia gravis (MG) MG is an autoimmune disorder incl uding a fatigability of skeletal muscles and the presence of antibodies aga inst the acetylcholine receptor (AChRAbs). These Abs are pathogenic by bloc king the acetylcholine receptor within the neuromuscular junction. A transi ent neonatal MG occurs in 12% of babies born to myasthenic mothers. AChRAbs have been found in 77 to 89% of systemic MG and in 47 to 60% in ocular for m of MG. These Abs are generally directed against the "Main Immunogenic Reg ion" (MIR) within the a subunit of the receptor; others are directed agains t beta, gamma and epsilon subunits. Eleven to 23% of the MG are negative fo r the Abs directed against the a subunit reinforcing the interest to detect the other AChRAbs, specially the anti-epsilon subunit Abs and the Abs dire cted against the muscle-specific receptor tyrosine kinase. Multicenter evaluation in order to compare the results of AChRAbs for a sam e patient from various laboratories, a multicenter study was performed on 2 6 sera with different titers of AChRAbs using radioimmunoassays. Six french laboratories have participated to this study. The antigen was obtained fro m TE671 cells for 5 laboratories (3 used a commercial test) or from human m uscle (1 laboratory). At the end of the study, the qualitative analysis of the results showed a concordance of 92,3%. The interpretation of AChRAbs as says can thus be done by clinicians whatever the test is used. A same quali ty control is nowadays used by the six laboratories in order to estimate qu antitative results according to the assay performed. Usefulness of the other Abs The Abs directed against the striated muscle ar e good markers of thymoma in MG but are not specific for MG. The Abs direct ed against titin which is the major target of the anti-striated muscle Abs, are also good biological markers of thymoma specially in MG adults younger than 60 years. The detection of anti-myoid cells and thymic reticuloepithe lial cells Abs is no more useful due to a lower sensitivity compared to the Abs directed against the striated muscle. (C) 2001, Masson, Paris.