Intravenous immunoglobulin (IVIg) treatment improves muscle strength in Lambert-Eaton myasthenic syndrome (LEMS), but its specific mode of action is unknown. We have delineated its mode of action on neuromuscular blocking properties of LEMS IgG. The effect of sera and purified IgG from six patients with LEMS on evoked quantal release was investigated after direct application to the motor nerve terminal by the perfused macro-patch-clamp electrode in mouse hemidiaphragms. The effect of LEMS IgG was analyzed alone and after coincubation with different concentrations of IVIg or its Fab fragments. All LEMS sera and purified LEMS IgG fractions taken before IVIg treatment inhibited evoked quantal release in a dose-dependent manner. When LEMS IgG was coincubated with a therapeutic IVIg preparation, presynaptic inhibitory activity of LEMS IgG was diminished in a dose-dependent fashion. Monovalent Fab fragments were as effective in neutralizing the activity of LEMS IgG as whole IVIg. These direct neutralizing effects of IVIg may explain its therapeutic efficacy.
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