This trial is testing a new cell therapy called MuSK-CAART for patients with a severe muscle disease known as MuSK myasthenia gravis. These patients have harmful antibodies that attack their muscles. The therapy uses special cells to find and stop these bad antibodies, potentially leading to long-term relief from the disease. Rituximab has been used to treat MuSK myasthenia gravis, showing some success in patients who are refractory to standard treatments.