Inclusion Criteria- Diagnosis of acute ischemic stroke.
- Score of ≥6 points on the National Institute of Health Stroke Scale (NIHSS) at Screening.
- At least 1 acute infarct with largest diameter of more than 2 cm on Baseline brain diffusion-weighted imaging (DWI).
- Participants who have received reperfusion therapy may be eligible to participate but must meet all eligibility criteria and perform the Baseline study magnetic resonance imaging (MRI) after reperfusion therapy has been completed.
- Subjects of childbearing potential must practice effective contraception during the study and be willing and able to continue contraception for at least 3 months after their dose of study treatment.
Exclusion Criteria- Presence of any intracranial hemorrhage (ICH) on head computed tomography (CT) or non-petechial ICH on screening MRI.
- Stroke isolated to the brainstem.
- Presence of coma
- Expected to die OR unable to be evaluated within 5 days.
- Hypotension requiring the use of intravenous (IV) vasopressor support or systolic blood pressure <90 mmHg at the time of randomization.
- Known prior treatment with natalizumab.
- Immunocompromised subjects, as determined by the Investigator.
- History of progressive multifocal leukoencephalopathy (PML).
- Contraindications to MRI, e.g., implanted pacemaker or other contraindicated implanted metal devices, history of or risk for side effects from gadolinium, or claustrophobia that cannot be medically managed.
NOTE: Other protocol-defined inclusion/exclusion criteria may apply.