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The influence of age on the induction of experimental autoimmune myasthenia gravis (EAMG) was investigated. Immunization with acetylcholine receptor (AChR) or injection of varying amounts of anti-AChR mAb 35 into young adult (10-12 wk) BN rats induced severe signs of EAMG including weight loss and decrement of muscle action potential, whereas aged BN rats (120-130 wk) did not show any clinical signs of EAMG. Serum anti-AChr mAb titers were not significantly different in young and aged rats up to 24 h after administration of mAb. No significant AChR loss was demonstrated in aged rats, whereas similarly treated young rats showed extensive AChR loss. In contrast to young rats, no degradation of the postsynaptic membrane could be demonstrated by electron microscopy in aged rats. C component C3 and C5b-9 membrane attack complex could be demonstrated at the neuromuscular junction in both young and aged mAb-treated rats. However, infiltrating macrophages and necrotic muscle fibers were seen only in young rats. These results suggest that the postsynaptic membrane in aged rats is resistant to autoantibody attack. AChR degradation by antigenic modulation may be less efficient in aged rats as a result of altered AChR density and distribution or rigidity of the postsynaptic membrane. Age-related resistance in the EAMG model can provide more information about the factors that determine the severity of myasthenia gravis. Manipulation of AChR density or lipid composition of the postsynaptic membrane may be of therapeutic interest in myasthenia gravis.

Susceptibility to experimental autoimmune myasthenia gravis (EAMG) was found to decrease with aging in both Lewis and Brown Norway (BN) rats. In this study, the difference in susceptibility between young and aged Lewis and BN rats was used to analyze factors determining the clinical severity of EAMG. The incidence and severity of muscular weakness did not correlate with acetylcholine receptor (AChR) loss nor with the ability of antibodies to interfere with AChR function. Aged rats showed significantly lower anti-rat AChR antibody titers than young rats and developed less severe or no clinical signs of disease. In individual young or aged rats, however, no significant correlation was found between the clinical signs of disease and anti-rat AChR titer. Neuromuscular transmission was found to change with aging as measured by single-fiber electromyography (SFEMG). In aged BN rats, increased jitter and blockings were found even before EAMG induction. Despite this disturbed neuromuscular transmission, these aged BN rats were clinically resistant against induction of EAMG. The results of this study indicate that the age-related susceptibility to EAMG is influenced by factors determined by the immune attack as well as mechanisms at the level of the neuromuscular junction.

Autoimmune diseases against cell surface receptors are the result of a mainly antibody-mediated attack on membrane receptors. This results in a hypofunction of the target organ; occasionally antibodies can exert an agonist effect, e.g. in Graves' disease. Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction associated with a plethora of other diseases, mainly autoimmune diseases. Antibodies against the acetylcholine receptor (AChR) reduce the number of receptors necessary for efficient neuromuscular transmission. The effector mechanisms of MG can be studied elegantly in an experimental animal model in rodents immunized with AChR or injected with antibodies against AChR. The thymus is thought to play a central role in the induction of MG. Microscopic analysis of these thymuses revealed a follicular hyperplasia of the medulla or a lympho-epithelial thymoma. Thymectomy results in clinical improvement along with a decline in anti-AChR antibody titres. Additional therapeutic measures include anticholinesterase drugs, immunosuppression and plasmapheresis.

We have studied the isoelectric focusing pattern of antibodies expressed in rats with experimental autoimmune myasthenia gravis (EAMG) induced by immunization with acetylcholine receptors (AChR) purified from Torpedo californica. Sera or tissue eluates were obtained at intervals in the course of disease and subjected to isoelectric focusing. Subsequently, the focused antibodies were detected by autoradiography of gels labelled with 125I-alpha-bungarotoxin conjugated AChR. Reverse electrofocusing was used to separate complexes of antibody and AChR formed in vivo, thereby allowing detection of the full spectrotype (banding pattern). As little as 1.1 X 10(-12) moles of monoclonal antibodies (MoAbs) to AChR yielded distinct bands of radiolabelled antigen binding by this technique. The anti-AChR MoAbs studied showed a multitude of bands localized in neutral to alkaline position. The clonotypes expressed in late post-immunization sera were compared to early sera. The spectrotypes of immunized Lewis and Brown Norway rats were not identical. In early sera most of the isoelectric focusing bands were specific for T. californica AChR, whereas in late sera further expansion of the repertoire produced bands that reacted with rat muscle AChR as well. The focused bands that bound rat AChR also bound T. californica AChR. The anti-AChR antibodies eluted from muscles of rats with EAMG showed similar binding patterns to anti-receptor antibodies in rats' sera. These results indicate that the antibody specificities detected in serum are the same specificities which are effective in binding to muscle AChR in vivo. Minor specificities of serum anti-receptor antibodies are not disproportionally represented in the antibodies actually bound at the neuromuscular junction in EAMG.