Alpha-bungarotoxin blocking antibodies in neonatal myasthenia gravis: frequency and selectivity

Direct proof of the in vivo pathogenic role of the AChR autoantibodies from myasthenia gravis patients

Several studies have suggested that the autoantibodies (autoAbs) against muscle acetylcholine receptor (AChR) of myasthenia gravis (MG) patients are the main pathogenic factor in MG; however, this belief has not yet been confirmed with direct observations. Although animals immunized with AChR or injected with anti-AChR monoclonal Abs, or with crude human MG Ig fractions exhibit MG symptoms, the pathogenic role of isolated anti-AChR autoAbs, and, more importantly, the absence of pathogenic factor(s) in the autoAb-depleted MG sera has not yet been shown by in vivo studies. Using recombinant extracellular domains of the human AChR α and β subunits, we have isolated autoAbs from the sera of four MG patients. The ability of these isolated anti-subunit Abs and of the Ab-depleted sera to passively transfer experimental autoimmune MG in Lewis rats was investigated. We found that the isolated anti-subunit Abs were at least as efficient as the corresponding whole sera or whole Ig in causing experimental MG. Abs to both α- and β-subunit were pathogenic although the anti-α-subunit were much more efficient than the anti-β-subunit ones. Interestingly, the autoAb-depleted sera were free of pathogenic activity. The later suggests that the myasthenogenic potency of the studied anti-AChR MG sera is totally due to their anti-AChR autoAbs, and therefore selective elimination of the anti-AChR autoAbs from MG patients may be an efficient therapy for MG.

Failure of intravenously administered immunoglobulin in the treatment of neonatal myasthenia gravis

Neonatal myasthenia gravis is uncommon and life threatening. We describe the use of intravenously administered immunoglobulin, in addition to conventional modalities, in a neonate with severe neonatal myasthenia gravis. However, despite this aggressive management, the child had a prolonged period of weakness requiring intensive care.

Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Association of neonatal myasthenia gravis with antibodies against the fetal acetylcholine receptor

The specificities of autoantibodies directed against the acetylcholine receptor (AChR) for embryonic and adult muscle AChR were studied in 22 mothers with myasthenia gravis (MG) and in their newborns using human fetus and normal adult muscle AChR preparations. 12 mothers had transmitted MG to their neonates with, in three cases, antenatal injury. A clear correlation was found between occurrence of neonatal MG (NMG) and the high overall level of anti-AChR antibodies (embryonic or adult muscle AChR). However, a strong correlation was also found between occurrence of NMG and the ratio of anti-embryonic AChR to anti-adult muscle (Te/Ta) AChR antibodies (P < 0.0002). Taken together, these data suggest that autoantibodies directed against the embryonic form of the AChR could play a predominant role in the pathogenesis of NMG. Paradoxically, the three cases with antenatal injury presumably the most severe form of NMG, were not associated with high Te/Ta. At the clinical level, these observations could prove helpful in the prediction of transmission of NMG.

Myasthenia gravis with polyhydramnios in the fetus of an asymptomatic mother

Transient myasthenia gravis was observed in an infant whose mother had no signs of neuro-muscular disease. The case presented with severe polyhdramnios at 31 weeks. Sonographic and invasive work-up showed only an absence of fetal swallowing. At birth, the infant had severe muscle weakness and respiratory distress. He had high titres of anti-acetylcholine receptor (anti-AChR) antibodies of maternal origin. Anti-AChR antibodies were also found in stored samples of fetal serum and amniotic fluid. To our knowledge, this is the first case of neonatal myasthenia gravis for which neither past nor present evidence of disease could be obtained in the mother. Anti-AChR antibody testing should be considered in cases of unexplained polyhydramnios.

Follow-up study of muscle function in children of mothers with myasthenia gravis during pregnancy

Most infants whose mothers have myasthenia gravis are healthy at birth, but 10% to 15% have a transient neonatal form of myasthenia gravis. In this study, the muscular function and neuromuscular transmission were examined in 31 children, aged 3 months to 31 years (median, 10 years), of 15 myasthenic mothers. Eleven of these children had had the neonatal form of myasthenia gravis. The children were examined clinically and with neurophysiologic methods. Blood samples were taken for HLA typing, creatine kinase levels, and myoglobin and acetylcholine receptor antibody studies. Twenty-nine of the 31 children had no signs of neuromuscular disease. Two children (who had had neonatal myasthenia gravis) had a moderate stationary myopathy, probably unrelated to the myasthenia gravis of their mother. Creatine kinase levels were normal for all subjects. Acetylcholine receptor antibody levels were similar to those of a control population. The HLA type B8 antigen was not significantly more prevalent in the children who had had neonatal myasthenia gravis than in the healthy children. Neonatal myasthenia gravis in a previous sibling was the only factor in the material that predicted the occurrence of myasthenic symptoms in the neonatal period.

Transient neonatal myasthenia gravis

Transient neonatal myasthenia gravis is a postsynaptic neuromuscular transmission defect occurring in 21% of infants born to women with active (and, less commonly, in remission) acquired myasthenia gravis. Although passive-transfer acetylcholine receptor (AChR) antibodies are found in the majority of these newborns, their pathogenic role is questionable because only some infants are symptomatic. Pathogenesis in infants without AChR antibodies is unknown. There is still no biologic marker for prenatal identification of this subpopulation of newborns, although HLA typing may be a promising tool. Sucking, swallowing, and respiratory difficulties are the most common presenting signs in the first day of life. Final diagnosis is done when administration of acetylcholinesterase agents transiently corrects the neuromuscular transmission defect. Serum AChR antibody titers follow the same pattern as their mothers. Supportive management and anticholinesterase agents prior to feedings are necessary in about 80% of patients. In the majority of infants the condition resolves spontaneously.

Anti-acetylcholine receptor antibodies in neonatal myasthenia gravis: heterogeneity and pathogenic significance

Precipitating, blocking and modulating anti-AChR antibodies and their capacity to recognize embryo or adult muscle were investigated in parallel in maternal and neonatal sera of fifty-two newborns and their myasthenic mothers. Twenty-four babies presented neonatal myasthenia gravis (NMG) with a common expression in twenty cases and foetal involvement in four cases. Occurrence of NMG was clearly related to levels of maternal precipitating, decamethonium blocking (DC blocking) and modulating antibodies (respectively P less than 0.001, P less than 0.02, P less than 0.01, Mann-Whitney test), these three parameters being interrelated. The only significant difference between mothers with severely affected babies and others with mild NMG newborns concerned DC blocking antibodies. Transfer fraction of DC blocking antibodies was significantly higher in NMG than in asymptomatic babies. Our data suggest that (1) the amount of anti-AChR antibodies, whatever the tested category, is involved in the pathogeny of NMG in a more direct manner than in adult MG where the correlation with their levels is poor, and (2) among antibodies, those with blocking effects could be preponderant for triggering NMG and be involved in the severity of the disease in the child.