Patient description: The patient is a 16-year-old girl recently diagnosed with myasthenia gravis, class IIa. She complains of diplopia and weakness affecting in her upper extremities. She had a positive anti-AChR antibody test, and her single fiber electromyography (SFEMG) was positive. She is on acetylcholinesterase inhibitor treatment combined with immunosuppressants. But she still has some symptoms. She does not smoke or use illicit drugs. She is not sexually active, and her menses are regular. Her physical exam and lab studies are not remarkable for any other abnormalities. BP: 110/75 Hgb: 11 g/dl WBC: 8000 /mm3 Plt: 300000 /ml Creatinine: 0.5 mg/dl BUN: 10 mg/dl Beta hcg: negative for pregnancy

Clinical trial eligibility criteria: eligible ages (years): 18.0-80.0,  Myositis Myasthenia Gravis For myositis III. Idiopathic myositis 1. Myositis as defined by the 119th ENMC: 1. Proximal myopathy with weakness 2. Subacute or insidious onset over 18 years 3. Myogenic syndrome on EMG (optional) 4. Muscle fibre necrosis and regeneration and/or inflammatory cell infiltrate on muscular biopsy 2. Specific AAbs : anti-synthetases (anti-JO1, anti-PL7, or anti-PL12), or anti-SRP. IV. Refractory to the conventional treatments Resistance to conventional treatments is defined as an inadequate response to, or intolerable side effects with conventional treatments, such as corticosteroids, azathioprine, methotrexate, cyclophosphamide, cyclosporine, IgIV and/or plasma exchange. At least one or more of these drugs or therapeutical approaches (used alone or as a combination) must have been unsuccessfully tested before inclusion. Inadequate response is defined as the lack of improvement and/or the degradation of evaluation parameters (defined bellow) despite these conventional therapies, that led to a modification or a reintroduction of treatment For myasthenia III. Generalised MG Generalised seropositive MG as defined by the Texas Clinical Classification System: 1. Extraocular muscle weakness quantified with MG muscle score (MMS), whose inter and inter observer reproducibility has been demonstrated [44]. 2. Specific AAbs : anti-AchR IV. Refractory to the conventional treatments Resistance to conventional treatments is defined as an inadequate response to, or intolerable side effects with conventional treatments, such as corticosteroids, azathioprine, methotrexate, cyclophosphamide, cyclosporine, IgIV and/or plasma exchange. At least one or more of these drugs or therapeutical approaches (used alone or as a combination) must have been unsuccessfully tested before inclusion. Inadequate response is defined as the lack of improvement and/or the degradation of evaluation parameters (defined bellow) despite these conventional therapies, that led to a modification or a reintroduction of treatment  Other muscular diseases, such as: 1. body myositis 2. Macrophagic myofasciitis 3. Inherited myopathies Secondary IM to one other connective tissue disorders 1. Systemic scleroderma (ARA and/or "LEROY AND criteria) 2. Sjögren's syndrome (European criteria) 3. Systemic lupus erythematosus (ACR criteria) 4. Rheumatoid arthritis (ACR criteria) 5. Mixed connective tissue disease (ACR criteria) Other myasthenic syndrome, such as: 1. Non generalised, ocular MG 2. Lambert Eaton syndrome 3. MG associated with malignant thymoma 4. Inherited myasthenic syndrome Cancer (or cancer-associated myositis) Age < 18 years Pregnancy HIV seropositivity Evolutive infection (B, C hepatitis, tuberculosis) Lack of approved consent