The primary objective of the study is to assess the clinical effects of natalizumab versus placebo in acute ischemic stroke on clinical measures of functional independence and activities of daily living. The secondary objective of the study is to explore dose and exposure response and the clinical treatment effects of natalizumab versus placebo in acute ischemic stroke on the following: measures of independence, activities of daily living, neurologic function, quality of life, cognition, and safety and tolerability