In this study, 60 subjects will be randomized to receive either PRO 140 or placebo in a 1:1 ratio (i.e. 30 subjects per arm). PRO 140 or placebo will be administered as a 350 mg subcutaneous injection on Day -3 or Day -2 prior to stem cell infusion, on the day of stem cell infusion (Day 0), and then weekly for 30 days (at Week 1, Week 2, Week 3 and Week 4) after which it will be administered every two weeks for up to 100±7 days (at Week 6, Week 8, Week 10, Week 12 and Week 14). Subjects will return to clinic for two Follow-up visits at 2 weeks after the last treatment visit, and one year after the first treatment visit.