The purpose of this study is to determine if CP-675,206, a monoclonal antibody to CTLA4, is safe and well-tolerated, reduces viral load, and improves immune function in patients infected with HIV.

Inclusion Criteria:

• HIV infected man or woman at least 18 years of age who is available for a follow up period of at least 3 months
• Has been on a stable antiretroviral regimen for ≥3 months and willing to remain on current antiretroviral regimen for an additional 3 months or has been off of antiretroviral therapy for at least 8 weeks and willing to remain off of antiretroviral therapy for another 3 months
• Plasma HIV 1 RNA ≥5,000 copies/mL measured by Roche Amplicor HIV 1 Monitor at the screening visit
• CD4 T-cells ≥200 cells/mm3

Exclusion Criteria:

• History of, or significant evidence of risk for, chronic inflammatory or autoimmunedisease (eg, Addison's disease, asthma, celiac disease, multiple sclerosis, Graves disease, Hashimoto's thyroiditis, inflammatory bowel disease, psoriasis, rheumatoid arthritis, systemic lupus erythematosus, etc)
• History of inflammatory bowel disease (eg, Crohn's disease or ulcerative colitis), celiac disease, or other chronic gastrointestinal conditions associated with diarrhea, or current acute colitis of any origin, and any history of diverticulitis (even a single episode) or evidence of diverticulitis at baseline, including evidence limited to CT-scan only
• Exposure within the previous 3 months to a drug known to have immunomodulatory effects (eg, IL 2)