肌肉特异性酪氨酸激酶在重症肌无力中的作用(综述)

约有20％重症肌无力(MG)患者血清中检测不到乙酰胆碱受体抗体(AChRAb)，称之为“血清反应阴性MG”(seronegative MG，SNMG)。有报道71％的SNMG患者血清中存在肌肉特异性酪氨酸激酶(MUSK)抗体(MuSKAb)，MuSKAb阳性SNMG患者的流行病学特征、If缶床特点和治疗效果与由AChRAb介导的MG(seropositive MG，SPMG)有所不同，存在较大争议。     

肌肉特异性酪氨酸激酶抗体、血清阴性能否解释重症肌无力

20％～40％的重症肌无力（MG）病人在他们的血清中没有发现乙酰胆碱受体抗体（AchRab），被称为血清阴性重症肌无力的病人（Seronegative MG，SNMG）。Hoch等最近在血清阴性重症肌无力体内发现了一个新特殊抗原——肌肉特异性酪氨酸激酶（MuSK）。Lyanagi等发现38％～70％的血清阴性重症肌无力病人体内有肌肉特异性酪氨酸激酶抗体（MusKab）。随后，Scudefi等报道66％AchR阴性重症肌无力病人可用免疫沉淀法检查出MusK。但Jon Lindstrom提出了这样一个问题：抗MusK自身抗体能够解释血清阴性的重症肌无力吗？     

血清阴性重症肌无力研究进展

乙酰胆碱受体(acetylcholine receptor,AChR)抗体阴性的重症肌无力(myasthenia gravis,MG)称为血清阴性重症肌无力(SNMG)。SNMG在发病机制、临床表现和治疗上都与AChR抗体阳性MG不同。近年来研究者在SNMG患者血清中发现多种MG相关抗体,如肌肉特异性酪氨酸激酶(muscle-specific receptor tyro-sine kinase,MuSK)抗体、低亲和力AChR抗体、低密度脂蛋白受体相关蛋白4(low density lipoprotein receptor-related protein 4,LRP4)抗体等。本文就SNMG发病机制与临床表现的研究进展做一简要综述。     

肌肉特异性酪氨酸激酶抗体与重症肌无力

重症肌无力（MG）是一种主要累及神经-肌肉接头突触后膜乙酰胆碱受体（acetylcholine receptor，AChR）的自身免疫性疾病。近来研究发现肌肉特异性酪氨酸激酶（muscle-specific kinase，MUSK）抗体（MuSKAb）与MG发病机制有关。MuSKAb阳性MG临床表现和治疗与血清AChR（＋）MG存在明显不同。MuSKAb阳性MG病情重，容易引起漏诊和误诊。     

AChRAb阳性和阴性重症肌无力患者IL-4和IFN-γ分泌细胞的检测

将行胸腺切除术的重症肌无力（MG）患者分成乙酰胆碱受体抗体（AChRAb）阳性和阴性2组，采用免疫酶点法检测其外周血、骨髓和胸腺白细胞介素-4（IL－4）分泌细胞和干扰素-γ（IFN－γ）分泌细胞的数量。结果表明AChRAb阳性组其外周血和骨髓中IL－4和IFN－γ分泌细胞数量均显著高于AChRAb阴性组（P＜0．05），而胸腺组织两组间差异无显著性意义（P＞0．05）。提示IL－4和IFN－γ在MG自身抗体的产生中均有重要作用，TH细胞的不同亚类都参与MG的免疫调节。     

乙酰胆碱受体抗体及肌肉特异性受体酪氨酸激酶抗体均阳性的重症肌无力1例报告

<正>重症肌无力(MG)是一种累及神经-肌肉接头处乙酰胆碱受体(AchR)的自身免疫性疾病。约80%的MG患者血清AchR-Ab(+),但约20%的患者为阴性,称为血清阴性MG(SNMG)[1]。目前国内将肌肉特异性受体酪氨酸激酶抗体(Mu SK-Ab)阳性的MG归类于SNMG,乙酰胆碱受体抗体(AchR-Ab)及Mu SK-Ab同时阳性的重症肌无力尚无报道。     

黄芪复方对重症肌无力患者血清乙酰胆碱受体抗体水平的影响

目的采用中药黄芪复方调节重症肌无力(myastheniagravis,MG)患者血清乙酰胆碱受体抗体(acetylcholinereceptorantibody,AChRAb)水平,以探讨其治疗MG的可能作用机制。方法应用酶联免疫吸附法(ELISA)检测74例MG患者治疗前后血清AChRAb水平,并采用许(氏)临床绝对和相对记分法观察病情严重程度和判定中药复方疗效。结果MG患者治疗前血清AChRAb水平(108±007)显著高于健康对照组(016±011)(P<001),治疗后MG患者血清AChRAb水平(072±006)明显下降(P<001)。结论具有补益脾肾作用的中药黄芪复方能下调MG患者血清AChRAb水平,从而纠正MG患者免疫失衡状态,临床绝对记分和相对记分对其疗效的判定有一定意义。     

重症肌无力患者血清Ryanodine受体抗体检测及意义

目的探讨Ryanodine受体（RyR）抗体在重症肌无力（myasthenia gravis，MG）诊断中的临床意义。方法采用ELISA法检测89例MG患者、66例其他神经系统疾病患者和66名正常对照者血清RyR抗体水平。结果MG组血清RyR抗体阳性率显著高于其他神经系统疾病组和正常对照组（P〈0．05），其敏感性和特异性分别为55．0％和91．7％。晚发型MG患者血清RyR抗体阳性率（74．4％）明显高于早发型MG（37．0％，P〈0．05）。MG合并胸腺瘤（MGT）和未合并胸腺瘤（nMGT）患者血清RyR抗体阳性率差异无统计学意义（P〉0．05）。将MG患者根据Osserman评分进行分型，各型血清RyR抗体阳性率及其吸光度值大小与病情严重程度无相关性（P〉0．05）。结论RyR抗体多见于晚发型MG，对诊断MG具有较高的敏感性和特异性。     

重症肌无力血清中碳酸酐酶Ⅲ其抗体的检测

目的：检测重症肌无力（MG）患者血清中碳酸酐酶Ⅲ（CAⅢ）及其抗体水平，探讨MG骨骼肌减少的CAⅢ是否参与MG的免疫发病。方法：用Western blot检测18例MG患者、16例其他神经肌肉疾病（ONMD）患者和15名健康人血清中CAⅢ水平；用Western blot与酶联免疫吸附测定（ELISA）检测上述血清中抗CAⅢ的抗体水平。结果：Western blot显示，正常人、MG和ONMD患者血清中都存在CAⅢ，但该蛋白的谱带密度在3组人的血清中差异无统计学意义（P〉0．05）；Western blot与ELISA表明，上述血清中不存在抗CAⅢ的抗体。结论：正常人、MG和ONMD患者血清中均存在CAⅢ蛋白，但不存在抗该蛋白的抗体，说明MG骨骼肌减少的CAⅢ蛋白可能没有直接参与MG的免疫发病。     

重症肌无力患者血清连接素抗体与临床的关系

目的 探讨重症肌无力（MG）患者血清连接素抗体（Titin—Ab）水平与病情及胸腺异常的关系,以及其对诊断和预后判断的价值。方法 应用酶联免疫吸附法（ELISA）检测MG患者（MG组）52例、非MG（其他神经系统疾病）患者（NMG组）18例、健康对照者（NC组）50名血清中Titin—Ab水平,并对其中10例进行胸腺手术的MG患者随访。结果 MG组Titin—Ab阳性率为36．5％,NMG组和NC组均为阴性,MG组显著高于NC组和NMG组（均P〈0．01）;以MG伴胸腺瘤（MGT）组阳性率最高（83．3％）,MG伴胸腺萎缩（MGA）次之（37．5％）,胸腺正常的MG组为23．0％,MG伴胸腺增生组为阴性;Titin．Ab诊断MGT特异性高于胸腺CT（P〈0．05）。Titin—Ab阳性MG患者抗体水平与肌无力程度呈显著正相关（r=0．562,P〈0．01）;对10例不同胸腺病理类型的MG患者手术前后Titin—Ab测定显示差异具有显著性（P〈0．05）。结论 Titin—Ab阳性多见于MGT和MGA者,诊断MGT的特异性高于CT;Titin—Ab对MG的诊断、病情及预后判断均有帮助。     

MuSK antibody clearance during serial sessions of plasmapheresis for myasthenia gravis

To evaluate the relationship of antibody clearance and clinical improvement for myasthenia gravis (MG) patients with MuSK antibody (MuSKAb) receiving one course of double-filtration plasmapheresis (DFP) treatment, we prospectively recorded their MG scores and measured MuSKAb concentrations before and after each DFP treatment session in two MuSKAb-positive patients. The clinical improvement trends roughly paralleled the reductions of MG score, except for the late rebound of MuSKAb in the 6th DFP treatment session, without concomitant clinical worsening in one patient. Longitudinal analysis on the serological changes during the serial sessions revealed that the levels of MuSKAb fell by 44%, 59%, 79%, 82%, 92%, and 71% in patient 1 and 52%, 70%, 78%, 87%, and 90% in patient 2. The titers of MuSKAb decreased constantly during the first 5 DFP treatment sessions. MuSKAb clearance per session for the 11 sessions ranged from 19% to 56%, with a mean of 45%. In conclusion, DFP was effective for MuSKAb removal and amelioration of clinical weakness for our two patients with MuSKAb. A 5-session protocol cleared 90% of serum MuSKAb.     

The effectiveness of thymectomy on seronegative generalized myasthenia gravis: comparing with seropositive cases

OBJECTIVES: To investigate the efficacy of thymectomy between patients with seronegative myasthenia gravis (SNMG) and seropositive myasthenia gravis (SPMG). METHODS: We present here the first Taiwanese retrospective paired cohort study comparing the effectiveness of thymectomy among 16 seronegative and 32 seropositive MG patients after matching for age-of-onset and time-to-thymectomy, and following up over a mean of 35 +/- 20 (7-86) months. Clinical characteristics and complete stable remission (CSR) rates were compared and analyzed between the groups. RESULTS: There were no major clinical differences between the two groups except for our finding of a lower percentage of SNMG receiving preoperative plasmapheresis or human immunoglobulin than SPMG (31% for SNMG vs 72% for SPMG, P = 0.007). CSR rates calculated using the Kaplan-Meier method were similar in the two groups (38% for SNMG vs 50% for SPMG, P = 0.709). The median time for CSR was 47.4 months for SNMG and 48.2 months for SPMG. Thymic hyperplasia were the most common pathology (69% for SNMG vs 88% for SPMG, P = 0.24). During the follow-up period, we found no group difference on prednisolone or pyridostigmine dosages. Significant postoperative dosage reductions on pyridostigmine, but not on prednisolone, were found in both groups. CONCLUSIONS: Thymectomy has a comparable response among SNMG and SPMG in our study. Thymic hyperplasia is prevalent in our SNMG patients and thymectomy may also be a therapeutic option to increase the probability of remission or improvement in SNMG. More prospective controlled trial will be helpful in the future.     

Detection and characterization of MuSK antibodies in seronegative myasthenia gravis

Antibodies to rat muscle specific kinase, MuSK, have recently been identified in some generalized "seronegative" myasthenia gravis (SNMG) patients, who are often females with marked bulbar symptoms. Using immunoprecipitation of (125)I-labelled-human MuSK, 27 of 66 (41%) seronegative patients were positive, but 18 ocular SNMG patients, 105 AChR antibody positive MG patients, and 108 controls were negative. The antibodies are of high affinity (Kds around 100 pM) with titers between 1 and 200 nM. They bind to the extracellular Ig-like domains of soluble or native MuSK. Surprisingly they are predominantly in the IgG4 subclass. MuSK-antibody associated MG may be different in etiological and pathological mechanisms.     

IgG from "seronegative" myasthenia gravis patients binds to a muscle cell line, TE671, but not to human acetylcholine receptor

Antibodies to acetylcholine receptor (AChR) are found in 85% of patients with myasthenia gravis (seropositive MG [SPMG]) and are thought to be pathogenic; but in 15% of MG patients, the standard immunoprecipitation test for anti-AChR is negative (seronegative MG [SNMG]). Here, we used a novel approach, fluorescence-activated cell sorting analysis, to measure binding of SPMG and SNMG IgG antibodies to rhabdomyosarcoma cell lines that express human adult (TE671-epsilon) or fetal (TE671-gamma) AChR, and to human embryonic kidney (HEK) fibroblasts that express adult human AChR (HEK-AChR). We found that whereas most SPMG antibodies bind to all three cell lines, IgG from 8 of 15 SNMG sera/plasmas bind to the surface of both TE671 cell lines but not to HEK-AChR cells. These results indicate that SNMG antibodies bind to a muscle surface antigen that is not the AChR, which strongly supports previous studies that suggest that SNMG should be considered a distinct subtype of MG.     

Effect of sera from seronegative myasthenia gravis patients on neuromuscular junctions

About 85 % of patients with generalized myasthenia gravis (MG) have anti-nicotinic acetylcholine receptor (nAChR) antibodies in their sera (seropositive MG; SPMG). The other 15 % (seronegative MG; SNMG) are also considered to have antibody-mediated disease, but the nature of the antibodies in SNMG is not fully understood. We investigated the effect of sera from patients with MG on spontaneous muscle action potentials and acetylcholine (ACh)-induced potentials, and we examined the localization of epitopes recognized by SPMG sera or SNMG sera. SPMG sera and SNMG sera inhibited spontaneous muscle action potentials and ACh-induced potentials in the spinal-muscle co-culture system. However, spontaneous muscle action potentials and ACh-induced potentials in the neuromuscular junctions were strongly blocked by SPMG serum, whereas they were weakly blocked by SNMG serum. Both types of sera reacted strongly with the neuromuscular junctions in normal rat muscles, as shown by double immunostaining with serum and α-bungarotoxin. The SPMG epitope remained in the neuromuscular junctions, whereas that of SNMG disappeared after denervation of the sciatic nerve. Therefore, we suggest that the skeletal muscle weakness in SNMG may be due to an interaction with presynaptic neuromuscular transmission and nAChR.     

Autoantibody detection by a live cell-based assay in conventionally antibody-tested triple seronegative Myasthenia gravis

Autoantibody testing is the mainstay in confirming the diagnosis of autoimmune myasthenia gravis (MG). However, in approximately 15% of patients, antibody testing in clinical routine remains negative (seronegative MG). This study aimed at assessing the prevalence of “clustered” AChR- and MuSK- and LRP4- autoantibodies using a live cell-based assay in a large German cohort of seronegative myasthenia gravis (SNMG) patients. A total of 67 SNMG patients were included. Clustered AChR-ab were identified in 4.5% (n = 3) of patients. Two out of the three patients showed binding to the adult AchR as well as the fetal AchR. None of the patients was positive for MuSK- or LRP4-autoantibodies. There were no differences in clinical characteristics between the patients with and without clustered AChR-ab detection. Comparison of clinical data of our cohort with clinical data from the nationwide Myasthenia gravis registry showed broad similarities between seronegative MG patients of both cohorts.     

Modulation of acetylcholine receptor expression in seronegative myasthenia gravis

In a previous study, we demonstrated a compensatory mechanism for regulating acetylcholine receptor (AChR) gene expression in muscle biopsies from seropositive and seronegative (SN) myasthenia gravis (MG) patients. To further characterize the AChR regulation mechanisms involved in SNMG disease, we investigated the effects of MG sera on nicotinic AChR expression (at the protein and messenger RNA [mRNA] levels) in cultured human muscle cells. Sera from SNMG patients induced an in vitro increase in the level of nicotinic AChR beta-subunit mRNA but did not cause a decrease in AChR protein level. This apparent discrepancy was not due to a higher level of AChR synthesis as demonstrated by analysis of AChR turnover. In SN patients, the increase in beta-subunit mRNA level was followed after 48 hours by a slight increase in the amount of AChR surface protein. This regulation of nicotinic receptor expression was due to the purified IgG-containing fraction. Thus, sera from SNMG patients contain an immunoglobulin that induces an increase in AChR mRNA without causing a decrease in AChR protein level, suggesting an indirect regulatory mechanism involving another surface molecule. This model is therefore useful for defining the targets involved in the pathogenesis of SNMG disease.     

Widely varying TNF-α levels in patients with myasthenia gravis

Animal studies have indicated an important role of tumor necrosis factor-alpha (TNF-α) in the pathogenesis of myasthenia gravis (MG), and trials of monoclonal antibodies that block TNF-α have shown clinical improvement. However, before a TNF-α blocking agent is proposed for treatment of MG, whether serum TNF-α level correlates with the patient’s condition should be confirmed. Therefore, we evaluated the relationship between the serum TNF-α level and clinical factors, including the quantitative MG score and the anti-acetylcholine receptor antibody level, in 33 MG patients. TNF-α levels ranged from 0.44 to 3.63 pg/mL and did not correlate with clinical factors. Overall, we found that serum TNF-α levels varied greatly among MG patients.     

血清乙酰胆碱受体抗体、突触前膜抗体和胸腺瘤相关抗体与重症肌无力发病关系的研究

本研究用ＥＬＩＳＡ法同步、动态地检测了２８５例ＭＧ患者血清乙酰胆碱受体抗体（ＡｃｈＲａｂ）、突触前膜抗体（ＰｓＭａｂ）和胸腺瘤相关抗体（ＣＡＥａｂ）的变化。实验结果表明，ＭＧ患者体内增高的ＡｃｈＲａｂ是ＭＧ发病的主要体液因素。部分血清ＡｃｈＲａｂ阴性ＭＧ患者（１７．５％）的发病，可能与血清ＰｓＭａｂ异常有关。另一部分血清ＡｃｈＲａｂ和ＰｓＭａｂ均异常的ＭＧ患者（１２．６％），则可能同时存在骨骼肌神经肌接头突触前和突触后的损害。血清ＣＡＥａｂ测定在辅助诊断胸腺瘤方面具有高度的灵敏度和特异性，可作为临床早期诊断胸腺瘤的过筛检查。