 Study visits: Each participant will be tested at the investigational site, and if qualified, will be treated intravenously through a vein every two weeks for 70 weeks approximately 18 months. The first three infusions must be done at the site, but if the infusions are well tolerated, subsequent infusions may be done by a qualified healthcare provider in the home or other suitable location. Each participant must return to the site every 3 months for evaluation of cognition as well as blood tests and scans of the brain. 