A Study of KYV-101, a CD19 CAR T Cell Therapy, in Participants With Treatment Refractory Progressive Multiple Sclerosis

The goal of this study is to test a drug called KYV-101 in people who have progressive multiple sclerosis (MS) and who have not responded to standard therapies to slow disease progression. The main questions it aims to answer are: * What is the highest therapy dose that can be given without causing harm? * Can this therapy enter the central nervous system? Participants will be asked to: * Attend 14 visits plus an 8-day inpatient hospital stay over the course of 58 weeks. * Complete apheresis and chemotherapy treatments in preparation for KVY-101 therapy. * Undergo medical and research testing such as physical and neurological exams, MRI, lumbar puncture, blood draws, questionnaires, and vision assessments.