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Double-seronegative myasthenia gravis (dSN-MG, without detectable AChR and MuSK antibodies) presents a serious gap in MG diagnosis and understanding. Recently, autoantibodies against the low-density lipoprotein receptor-related protein 4 (LRP4) have been identified in several dSN-MG sera, but with dramatic frequency variation (∼2-50%). We have developed a cell based assay (CBA) based on human LRP4 expressing HEK293 cells, for the reliable and efficient detection of LRP4 antibodies. We have screened about 800 MG patient sera from 10 countries for LRP4 antibodies. The overall frequency of LRP4-MG in the dSN-MG group (635 patients) was 18.7% but with variations among different populations (range 7-32.7%). Interestingly, we also identified double positive sera: 8/107 anti-AChR positive and 10/67 anti-MuSK positive sera also had detectable LRP4 antibodies, predominantly originating from only two of the participating groups. No LRP4 antibodies were identified in sera from 56 healthy controls tested, while 4/110 from patients with other neuroimmune diseases were positive. The clinical data, when available, for the LRP4-MG patients were then studied. At disease onset symptoms were mild (81% had MGFA grade I or II), with some identified thymic changes (32% hyperplasia, none with thymoma). On the other hand, double positive patients (AChR/LRP4-MG and MuSK/LRP4-MG) had more severe symptoms at onset compared with any single positive MG subgroup. Contrary to MuSK-MG, 27% of ocular dSN-MG patients were LRP4 antibody positive. Similarly, contrary to MuSK antibodies, which are predominantly of the IgG4 subtype, LRP4 antibodies were predominantly of the IgG1 and IgG2 subtypes. The prevalence was higher in women than in men (female/male ratio 2.5/1), with an average disease onset at ages 33.4 for females and 41.9 for males. Overall, the response of LRP4-MG patients to treatment was similar to published responses of AChR-MG rather than to MuSK-MG patients.

Myasthenia gravis (MG) is an autoimmune disorder caused by autoantibodies that target the neuromuscular junction, leading to muscle weakness and fatigability. Currently available treatments for the disease include symptomatic pharmacological treatment, immunomodulatory drugs, plasma exchange, thymectomy and supportive therapies. Different autoantibody patterns and clinical manifestations characterize different subgroups of the disease: early-onset MG, late-onset MG, thymoma MG, muscle-specific kinase MG, low-density lipoprotein receptor-related protein 4 MG, seronegative MG, and ocular MG. These subtypes differ in terms of clinical characteristics, disease pathogenesis, prognosis and response to therapies. Patients would, therefore, benefit from treatment that is tailored to their disease subgroup, as well as other possible disease biomarkers, such as antibodies against cytoplasmic muscle proteins. Here, we discuss the different MG subtypes, the sensitivity and specificity of the various antibodies involved in MG for distinguishing between these subtypes, and the value of antibody assays in guiding optimal therapy. An understanding of these elements should be useful in determining how to adapt existing therapies to the requirements of each patient.

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (a1-10, b1-4, c, d, and e). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal ⁄ nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

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 a and ft torpedo subunits, (il) a similar antigenic determinant on 7 and 6 subunits, (iii) antigenic determinants

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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 a subunit and is distinct from the acetylcholine binding site. Regions on the P and y 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 ofthe 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.

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.