Specificities of antibodies to acetylcholine receptors

Specificities of antibodies to acetylcholine receptors in sera from myasthenia gravis patients measured by monoclonal antibodies

The pattern of antibody specificities in sera from patients with myasthenia gravis (MG) was determined by the ability of monoclonal antibodies against defined determinants on the acetylcholine receptor molecule to inhibit binding of the serum antibodies to receptor from human muscle. We found that MG patients produce fundamentally the same pattern of specificities as that produced by animals immunized with receptor purified from fish electric organs or mammalian muscle. Most of the antibodies are directed at the "main immunogenic region' which is located on the extracellular surface of the alpha subunit and is distinct from the acetylcholine binding site. Regions on the beta and gamma subunits near the main immunogenic region are also significantly immunogenic. In one patient the proportions of antibodies to various regions are constant over time despite changes in total antibody amount and clinical state. Between patients there is no obvious correlation between antibody specificities and clinical state. These data suggest that the autoimmune response in MG is stimulated by human receptor rather than a crossreacting (e.g., viral) antigen and that in both MG and experimental autoimmune MG the pattern of specificities produced is determined by the inherently immunogenic structural features of the receptor molecule. They also suggest that the wide differences in clinical state sometimes observed between patients with the same total concentration of antireceptor antibody are due primarily to differences in endogenous factors which affect the safety factor for neuromuscular transmission rather than to the presence of especially pathogenic antireceptor specificities.

Purification of anti-acetylcholine receptor antibody from patients with myasthenia gravis

The purification of specific anti-acetylcholine receptor antibodies from the plasma of 3 patients with myasthenia gravis was achieved by affinity chromatography using an immunoadsorbent of alpha-bungarotoxin-Sepharose 4B covalently linked to acetylcholine receptor extracted from human leg muscle. The specific antibody was eluted with potassium thiocyanate. The antibody characteristics were monitored throughout the purification procedure. The high avidity of the antibodies for the acetylcholine receptor was preserved but there was some loss of lambda light chain specific antibody.

Anti-acetylcholine receptor antibody: use of polyethylene glycol as an aid to precipitation of antibody-receptor complexes in determination of light chain and subclass

Polyethylene glycol 6000 (PEG) was used as an aid to precipitation of antibody-acetylcholine receptor (AChR) complexes. In the absence of anti-human IgG, 8% PEG can be used to precipitate antibody-AChR complexes. In the presence of low titre specific antiserum, 3% PEG selectively precipitates anti-IgG-IgG-AChR complexes and allows analysis of light chain and subclass contribution to the anti-AChR. The specificity of the various antisera is presented and results in 16 patients described.

Humoral immunity in myasthenia gravis: clinical correlations of anti-receptor antibody avidity and titer

Antibody to human acetylcholine receptor (AChR Ab) in myasthenia gravis (MG) correlates with clinical (Osserman) classification. Patients in remission R) or with ocular only (I) symptoms differed significantly from those with generalized disease (IIA, IIB, III, IV) (p less than 0.01, p less than 0.05 respectively). Patients with mild generalized disease (IIA) differed significantly from those with acute severe (III) or chronic severe (IV) disease (p less than 0.01). However, within each clinical class titers ranged over two or three orders of magnitude. This variation in AChR Ab titer for patients with similar diseases severity was not explained by differences in immunoglobulin class. All patients produced IgG AChR Ab and occasional patients produced IgM or IgA at less than 10% of their IgG titer. No IgM to IgG switch was identified. In MG patients negative for AChR Ab by immunoprecipitation assay, blockade of toxin binding to extracted human AChR could still be identified indicating antibody specificity to the toxin binding site. The avidity of AChR Ab for receptor assayed in six myasthenic patients with differing severities of disease, varied widely with T1/2 (time to half-maximal binding) ranges from 25 to 81 minutes. However, differences in AChR Ab avidity did not explain differences in severity of disease in patients with similar titers. AChR Ab was fractionated in six patients into IgG kappa and IgG lambda; in four patients AChR Ab activity could be demonstrated in both fractions. Thus, differences among MG patients as a group are due to production of several AChR Ab idiotypes, with individual patients being oligoclonal or polyclonal as well. Differences in IgG subclass (complement fixation) and site of attachment of AChR Ab to receptor subunits may resolve these differences.

Humoral immunity in myasthenia gravis: biochemical characterization of acquired antireceptor antibodies and clinical correlations

The titer and characteristics of antiacetylcholine receptor antibody (AChR-Ab) were investigated in 184 patients with myasthenia gravis. Mean AChR-Ab titers of each clinical grade increased with the severity of the disease. AChR-Ab was always of an IgG class. IgM (5 of 92) and IgA (2 of 48) class AChR-Ab were detected, but only concurrently with IgG and in low concentrations. IgG subclass 3 was not prominent. In 3 patients with AChR-Ab titers in the normal range, blockade of bungarotoxin binding to receptor could still be demonstrated. AChR-Ab from 6 patients was heterogeneous in affinity for receptor, reactivity from human ocular and gastrocnemius muscle, and blockade ot toxin binding. AChR-Ab was oligoclonal in 4 of 6 patients, as shown by concurrent production of AChR-Ab IgG of both kappa and lambda types. Amniotic fluid and fetal cord serum did not interfere with antibody-receptor interaction. Variation in the pattern of weakness among patients was a function of both the heterogeneity of AChR antibodies and the antigenic uniqueness of receptor complexes from different human muscles.

A rapid immunoadsorbent radioimmunoassay for anti-acetylcholine receptor antibody

Antibody to the human nicotinic acetylcholine receptor has been demonstrated in the disorder myasthenia gravis and is the pathologic factor producing the characteristic symptoms of the disorder. A rapid, quantitative and sensitive radioimmunoassay using human acetylcholine receptor, affinity labeled with iodinated alpha-bungarotoxin, and utilizing the immunoadsorbent protein A-bearing Staphylococcal aureus is described. The assay has proven as effective and more efficient than double antibody immunoprecipitation assays in diagnostic screening, evaluation of various forms of therapy, determining kinetics of receptor and antibody interaction, and the production of antireceptor antibody by peripheral blood lymphocytes in culture.

Cross-reactivity of anti-acetylcholine receptor autoantibodies

The heterogeneity of the specificities of anti-acetylcholine receptor (anti-AChR) antibodies of myasthenia gravis (MG) patients has been demonstrated by comparing reactions against a panel of xenogeneic AChR. For each patient there was a more or less unique cross-reactivity profile. Such heterogeneity emphasizes the need to use human AChR for the routine detection of anti-AChR. In vitro cross-reactivity was important in predicting the effect of anti-AChR after passive transfer to rats. Specificity may influence the outcome in human neonates receiving maternal anti-AChR via the placenta. In contrast to the extreme heterogeneity seen in spontaneous MG, the antibodies associated with D-penicillamine-induced MG were more homogeneous.

Myasthenia gravis. Anti-acetylcholine receptor antibodies

Human acetylcholine receptor and the Lefvert method (13) were used to determine the serum levels of anti-acetylcholine receptor antibodies in 27 patients with Myasthenia Gravis. Antibodies were detected in 22 patients (81%). Negative results were generally obtained in patients having milder forms of the disease. To find out whether serum levels of antibodies correlate with the severity of the disease, we compared the median antibody levels in patients with Type 1 and 2A Myasthenia Gravis with those of patients with type 2B and 3. Significantly lower titers were observed in the first group. Nevertheless, there were many overlapping values in the range of each class of the disease. A more definite relationship was observed in the follow-up of single patients. As a rule, clinical deterioration was accompanied by an increase of antibody levels and viceversa.

Anti-acetylcholine receptor antibodies

Early suggestions that a humoral factor might be implicated in the disorder of neuromuscular transmission in myasthenia gravis have been confirmed by the detection of anti-AChR antibody in 85-90% of the patients with generalised disease and in 75% of cases with restricted ocular myasthenia. Plasma exchange reveals that serum anti-AChR usually has an inverse relationship to muscle strength and present evidence indicates that patients responding to thymectomy and immunosuppressive drug treatment usually show a consistent decline in serum anti-AChR titres. The antibody is heterogeneous and can lead to a loss of muscle AChR by several mechanisms. Anti-AChR is produced in the thymus in relatively small amounts. Anti-AChR antibody synthesis by thymic lymphocytes and pokeweed stimulated peripheral lymphocytes in culture provides a means of studying the effect of different lymphocyte populations in vitro. Analysis of clinical, immunological and HLA antigen characteristics in MG suggest that more than one mechanism may underlie the breakdown in tolerance to AChR, leading to the production of anti-AChR antibodies.

Experimental autoimmune myasthenia gravis

Injection of animals with purified acetylcholine receptor in complete Freund's adjuvant causes development of antibodies which crossreact with receptors in muscle. The crossreacting antibodies impair neuromuscular transmission. Animals with experimental autoimmune myasthenia gravis (EAMG) are excellent models for studying the complex mechanisms by which the autoimmune response to receptor in myasthenia gravis causes muscle weakness. This review first briefly describes the discovery of EAMG. Then, to provide the necessary perspective, receptor structure and function and properties of anti-receptor antibodies are discussed, followed by a brief review of the pathological mechanisms in EAMG.

Monoclonal antibodies used to probe acetylcholine receptor structure: localization of the main immunogenic region and detection of similarities between subunits

Seventeen cell lines producing monoclonal antibodies against Torpedo californica (torpedo) acetylcholine receptor (AcChoR) and its subunits were established. By using these antibodies as probes, we identified: (i) a similar antigenic determinant on alpha and beta torpedo subunits, (ii) a similar antigenic determinant on gamma and delta subunits, (iii) antigenic determinants unique for alpha or beta torpedo AcChoR subunits, (iv) a small region on the alpha subunit that dominates the immunogenicity of native torpedo AcChoR in rats (a monoclonal antibody directed at this region could bind to rat AcChoR in vivo and cause passive experimental autoimmune myasthenia gravis), and (v) antigenic determinants on torpedo subunits recognized in AcChoR from other species. The unexpected similarities between alpha and beta and between gamma and delta subunits raise the possibility that the complex four-subunit structure of AcChoR was derived from a simpler precursor and suggests that these antigenic similarities might reflect some structural and functional homologies.

Immunochemical similarities between subunits of acetylcholine receptors from Torpedo, Electrophorus, and mammalian muscle

Polypeptide chains composing acetylcholine receptors from the electric organs of Torpedo californica and Electrophorus electricus were purified and labeled with 125I. Immunochemical studies with these labeled chains showed that receptor from Electrophorus is composed of three chains corresponding to the alpha, beta, and gamma chains of receptor from Torpedo but lacks a chain corresponding to the delta chain of Torpedo. Experiments suggest that receptor from mammalian muscle contains four groups of antigenic determinants corresponding to all four of the Torpedo chains. Binding of 125I-labeled chains was measured by quantitative immune precipitation and electrophoresis. Antisera to the following immunogens were used: denatured alpha, beta, gamma, and delta chains of Torpedo receptor, native receptor from Torpedo and Electrophorus electric organs and from rat and fetal calf muscle, and human muscle receptor (from autoantisera of patients with myasthenia gravis). The four chains of Torpedo receptor were immunologically distinct from one another and from higher molecular weight chains found in electric organ membranes. Antibodies to these chains reacted very efficiently with native Torpedo receptor, but the reverse was not true. Antibodies to native receptor from Torpedo and Electrophorus reacted slightly with each of the chains of the corresponding receptor. However, cross-reaction between chains and antibodies to any native receptor was most obviuos with the alpha chain of Torpedo or the corresponding alpha' chain of Electrophorus. Antiserum to alpha chains exhibited higher titer aginst receptor from denervated rat muscle. Antibodies from myasthenia gravis patients did not cross-react detectably with 125I-labeled chains from electric organ receptors. Most interspecies cross-reaction occurred at conformationally dependent determinants whose subunit localization could not be determined by reaction with the denatured chains.