Study visits will occur at Days 0, 1, 3, 7, 10, 14, 18, 21, 25, 28, 42, 56, 84, 112, 168, and 175. Visits will include a physical examination, medical history review, and blood collection. Neurocognitive testing will take place on Day 168. Some participants may take part in optional study procedures at various time points during the study including mucosal secretion collection, rectosigmoid biopsy, lymph node biopsy, leukapheresis, and lumbar puncture.