Inhibitory effect of α-fetoprotein on the binding of myasthenia gravis antibody to acetylcholine receptor

The binding of myasthenia gravis antibody to acetylcholine receptor (AcChoR) as measured in vitro by radioimmunoassay with ¹²⁵I-labeled α-bungarotoxin (α-BuTx), can be blocked by amniotic fluid, maternal serum, and umbilical cord serum. This inhibitory effect is due to α-fetoprotein present in high concentrations in amniotic fluid and serum, as shown by: (i) selective removal of several components from amniotic fluid and serum; (ii) selective addition of different components present in amniotic fluid and serum, including α-fetoprotein, to the radioimmunoassay; (iii) correlation between the inhibitory effect of both amniotic fluid and serum and between the amounts of α-fetoprotein they contain; (iv) blocking of the α-fetoprotein effect by pretreatment of the amniotic fluid with anti-α-fetoprotein antibody. The inhibitory effect of α-fetoprotein in vitro suggests a similar effect in vivo in pregnant women with myasthenia gravis. This effect may explain in part the variability in the development of neonatal myasthenia gravis in the babies, due to transplacental transfer of maternal anti-AcChoR antibody, only after delivery and only in the minority of the cases. It also may explain the appearance of remissions in females with myasthenia gravis during the second and third trimesters of pregnancy. Similar phenomena observed during pregnancy in other autoimmune and immunopathogenic diseases also might be attributed to activity of α-fetoprotein.