Investigating an mRNA CAR T-cell Therapy, Known as Descartes-08, as a Potential Approach to Treat Myasthenia Gravis

Inclusion Criteria: * Patient must be at least 18 years of age. * Patient must have generalized myasthenia gravis (gMG), Myasthenia Gravis Foundation of America (MGFA) clinical classification grades 2-4 at the time of Sscreening. * MG-Activities of Daily Living (MG ADL) total score ≥ 6. * Concomitant immunosuppressive drugs must be deemed necessary by the investigator. The dose must be stable for a minimum of 8 weeks prior to Baseline visit. * If a patient is using corticosteroids, the daily dose should not exceed 40 mg/day of prednisone equivalent. The dose must have been stable for a minimum of 8 weeks prior to Baseline visit. * Acetylcholine receptor autoantibody (anti-nAChR) titer or anti-AChR cluster antibody must be above the reference laboratory upper normal limit (UNL) and documented within the past 10 years of screening. * Patient must be willing to return for all study visits. * Patient must be able to give written informed consent. * Women of childbearing potential must agree to use highly effective birth control from Screening until 14 days post last dose of Descartes-08, Exclusion Criteria: * Major chronic illness that is not well managed at the time of study entry and in the opinion of the investigator may increase the risk to the patient. * Diagnosis of gMG within 12 months of screening. * No history of systemic treatment for gMG other than acetylcholine esterase inhibitors. * Diagnosis of a neuromuscular disease other than gMG. * Patient is pregnant or lactating. * Treatment with intravenous immunoglobulin (IVIG) or plasma exchange within 4 weeks prior to the Baseline visit. * Treatment with rituximab or ocrelizumab within 12 months prior to Baseline visit; treatment with calcineurin inhibitors (e.g. tacrolimus, cyclosporine, cyclophosphamide), Neonatal Fc receptor antagonists, and/or other biologics within 3 weeks prior to planned leukapheresis and within 8 weeks prior to Baseline visit. * The patient has started treatment with a complement 5a (C5a) inhibitor, such as eculizumab, within 8 weeks of Baseline visit. (NOTE: patients who have been receiving a C5a inhibitor for more than 8 weeks and meet other criteria for enrollment are eligible for treatment). * Prior treatment with B-cell maturation antigen (BCMA)-directed therapy (e.g. monoclonal antibody, T-cell engager, or chimeric antigen receptor T-cell \[CAR-T\]). * Abnormal prothrombin (PT)/international normalized ratio (INR) or partial thromboplastin time (PTT) increased \> 1.5-fold above the normal range at Screening or patient is on anticoagulation therapy (except in cases of elevated PTT with documented lupus anticoagulant; or in patients who have been on stable doses of anticoagulation therapy for more than 6 months of venous thromboembolism (VTE) diagnosis; or in patients on stable doses of anticoagulation therapy for at least 8 weeks of atrial fibrillation diagnosis; these conditions will not be exclusionary unless, in the investigator's opinion, they make participation in the study unsafe). * Absolute neutrophil count (ANC) \< 1000 cells/microliter. * Hemoglobin \< 8.0 g/dL. * Platelets \< 50,000/mm3. * Alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) \> 3x above normal. * Creatine clearance less than 30 mL/min. * History of primary immunodeficiency, organ, or allogeneic bone marrow transplant. * Patients must be seronegative for hepatitis B surface antigen. * Patients must be seronegative for hepatitis C antibody. If hepatitis C antibody test is positive, then patients must be tested for the presence of viremia by reverse transcriptase polymerase chain reaction (RT-PCR) and must be hepatitis C virus (HCV) ribonucleic acid (RNA) negative. * History of positive human immunodeficiency virus (HIV) or positive HIV at screening. * Active tuberculosis or positive QuantiFERON test at screening. * Any other clinical or laboratory abnormality that, in the opinion of the investigator, may jeopardize the subject's ability to participate in the study or could affect study outcome. * Any active significant cardiac or pulmonary disease that, in the opinion of the Principal Investigator, is significant and/or uncontrolled. Note: Patients with asthma and chronic obstructive pulmonary disease (COPD) controlled with inhaled medications are allowed. * History of malignancy that required treatment in the past 3 years, except for squamous cell carcinoma, basal cell carcinoma of the skin, or breast or early-stage colon cancer that is surgically removed and did not require adjuvant chemotherapy or radiotherapy. * Treatment with any investigational agent 4 weeks prior to screening or 5 half-lives of the investigational drug (whichever is longer). * Receipt of a live vaccination within 4 weeks prior to Baseline visit or intent to receive live vaccination during the study (Note: messenger RNA \[mRNA\]-based vaccines such as those against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are not considered live; likewise, the Janssen Covid-19 vaccine is not live). * History of significant recurrent infections or any active infection that in the opinion of the Investigator may interfere with the patient's participation in the opinion of the investigator. * Any known psychiatric illness that in the opinion of the Investigator, may interfere with the patient's participation in the study in the opinion of the investigator.