Central Nervous System Uptake of Anti-CD8+ T Cell Minibodies in Multiple Sclerosis and Progressive Multifocal Leukoencephalopathy

INCLUSION CRITERIA: In order to be eligible to participate in this study, an individual must meet all of the following criteria: Multiple Sclerosis Inclusion Criteria Enrolled in the NINDS Natural History Study for MS (protocol 89-N-0045) Able to understand, and willing to sign, a written, informed consent document. Willing to comply with all study procedures and available for the duration of the study. Male or female, aged >=18. Diagnosis of MS according to the 2017 revision of the McDonald diagnostic criteria48 (in the presence or absence of a clinical relapse). For females of reproductive potential: agrees to use highly effective contraception for at least one month prior to screening and during study participation. PML Inclusion Criteria Enrolled in the NINDS Natural History Study for PML (protocol 13-N-0017) Able to understand and willing to sign a written, informed consent document Willing to comply with all study procedures and available for the duration of the study. Male or female, aged >=18. Diagnosis of definite PML according to 2013 AAN Consensus Criteria49 or PML-IRIS based on clinical, radiological and laboratory evidence. For females of reproductive potential: agrees to use highly effective contraception for at least one month prior to screening and during study participation. EXCLUSION CRITERIA: An individual who meets any of the following criteria will be excluded from participation in this study: Pregnant or lactating. Contraindications for MRI gadolinium contrast administration or 3T MRI. History of, or current diagnosis with, concomitant medical or clinical conditions that would adversely affect participation in this study. Weighs > 350 lb (158 kg; weight limit for the scanner table) or is unable to fit within the MRI or PET imaging gantry. Severe claustrophobia unresponsive to oral anxiolytics.