A triple antibody assay for the quantitation of plasma IgG subclass antibodies to acetylcholine receptors in patients with myasthenia gravis.

Monoclonal anti-human IgG subclass antibodies have been used in an immunoprecipitation assay for the determination of anti-acetylcholine receptor IgG subclasses in plasma from patients with myasthenia gravis. Solubilized acetylcholine receptors labelled with 125I-alpha-bungarotoxin were incubated with patient plasma. Monoclonal mouse antibodies to human IgG subclasses 1-4 were added to the incubation and finally precipitated with anti-mouse IgG antibody. A maximal IgG subclass precipitation of 62-76% was determined with 125I-labelled myeloma IgG subclasses 1-4 added to normal human plasma. The anti-IgG subclass antibodies were added in excess which ensured that the precipitation of IgG2, IgG3 or IgG4 were unchanged, and that of IgG1 was only reduced by 17%, when the plasma IgG concentration was increased by a factor of two. The anti-IgG subclass antibodies were highly specific for their complementary subclasses. Determination of the IgG subclass of the anti-acetylcholine receptor antibodies from 8 patients with myasthenia gravis showed that IgG1 and IgG3 antibodies are predominant. This may support the hypothesis that complement mediated lysis of the neuromuscular end-plate plays a pathogenetic role in myasthenia gravis.