PD-1 Inhibition to Determine CNS Reservoir of HIV-Infection

Background: HIV affects millions of people. The disease may "hide" in the brain, even in people with well-controlled HIV without cancer. Then it may "wake up" and continue. The drug pembrolizumab uses the body's immune system to fight cells like cancer cells. It is approved to treat some cancers but not HIV. Researchers want to see if it is safe for HIV-positive people without cancer. This study is not for HIV treatment; only one dose of the drug will be used. Objective: To learn if the drug pembrolizumab, used to treat certain cancers, is safe for HIV-positive people. Eligibility: Adults ages 18 and older with HIV who are in another NIH protocol Design: Participants will be screened with: * Medical history * Physical and neurological exams * Blood tests * Lumbar puncture. The lower back will be numbed. A needle will remove fluid from between back bones. * FDG-PET/CT. A radioactive sugar will be injected in a thin plastic tube (catheter) inserted in an arm vein. Participants will rest for an hour, urinate, then lie in the scanner. A mask will hold the head still. * Leukapheresis. An optional procedure at baseline. White blood cells are removed from you using a serum cell separator machine Women who can become pregnant cannot take pembrolizumab. Men who take it must use 2 kinds of contraception. Participants will have up to 7 more visits, which repeat some screening tests. At 1 visit, participants will get one dose of pembrolizumab by catheter for 30 minutes. They will get allergy and pain medicines. At 2 visits, participants will have a brain MRI. They will get a contrast agent by catheter. They will lie in a metal cylinder that takes pictures for 1-2 hours. They will get earplugs for loud sounds.