 Participants in this study will be randomly assigned to receive a single dose of one of several possible doses of PRO 140 or placebo. Participants will remain in the clinic for observation and evaluation for 24 hours after the single-dose administration. Follow-up visits will occur at 2, 3, 5, 7, 10, 14, 28, 42, and 60 days post-treatment. Physical exams, electrocardiograms ECGs, vital signs measurement, adverse event reporting, and blood and urine collection will occur at most visits. 