The Effects of Tysabri Treatment on Vaccination Response and Lymphocyte Subsets in Subjects With Relapsing Forms of Multiple Sclerosis

Inclusion Criteria: * able to give written informed consent * diagnosis of a relapsing form of MS and must fall within the therapeutic indication stated in the approved label for Tysabri * aged 18-60 years, inclusive at the time of consent * free of signs and symptoms suggestive of any serious opportunistic infection, based on medical history, physical examination, or laboratory testing * must have a known history of tetanus toxoid immunization Major Exclusion Criteria: * tetanus toxoid vaccination less than 2 years prior to Screening * known hypersensitivity to tetanus-diphtheria vaccine or KLH or any other administered vaccinations or their components (such as thimerosal) * known allergy to shellfish * history of active tuberculosis or undergoing treatment for tuberculosis * previous exposure to KLH or vaccines containing KLH components (e.g., cancer vaccines) * known history of human immunodeficiency virus (HIV), hepatitis C, or hepatitis B infection * history of, or available abnormal laboratory results indicative of any significant disease * history of malignancy * history of organ transplantation (including anti-rejection therapy) * history of severe allergic or anaphylactic reactions or known drug hypersensitivity * a clinically significant infectious illness within 30 days prior to the Screening visit * prior exposure to Tysabri, rituximab, any murine protein, or any therapeutic monoclonal antibody at any time * receipt of intravenous (IV) or intramuscular (IM) immunoglobulin within 6 months of screening * live virus, bacterial vaccines, or any other vaccines within 3 months of screening * treatment with immunosuppressant medications within 6 months prior to screening * treatment with cyclophosphamide within 1 year prior to screening * treatment with immunomodulatory medications (interferon beta and glatiramer acetate) within 2 weeks prior to screening * treatment with systemic corticosteroids within 4 weeks prior to screening * treatment with any investigational product or approved therapy or vaccination for investigational use within 6 months prior to Screening * women who are breastfeeding, pregnant, or planning to become pregnant during the study * female subjects who are not postmenopausal for at least 1 year, surgically sterile (does not include tubal ligation), or willing to practice effective contraception during the study