A Multicenter Study to Assess the Effect of Plasma Exchange in Accelerating the Clearance of Natalizumab in Subjects With Multiple Sclerosis (MS)

Inclusion Criteria: * considered by the Investigator to be free of signs and symptoms suggestive of any serious opportunistic infection * willing to discontinue and remain free from concomitant immunosuppressive or immunomodulatory treatment (including interferon beta and glatiramer acetate) for the duration of the study * willing and able to comply with the site's plasma exchange protocol which may require hospitalization or daily visits Exclusion Criteria: * considered by the Investigator to be immunocompromised * history of, or available abnormal laboratory results indicative of any major disease that would preclude the administration of a recombinant humanized antibody immunomodulating agent for the duration of the study. * condition(s) considered to be contraindication(s) for plasma exchange, including but not limited to bleeding diathesis, hypotension, or vascular access limitations