This clinical trial is focused on studying a disease called Progressive Multifocal Leukoencephalopathy (PML), which affects the brain and is found in patients with weakened immune systems. The study is specifically for those who are immunocompromised but do not have HIV and are not taking certain medications for chronic inflammatory diseases. The treatment being tested in this trial is a medication called Pembrolizumab, which is given as an injection into a vein. Pembrolizumab is a type of protein that helps the immune system fight diseases.

The purpose of this study is to see if Pembrolizumab can help reduce the amount of a virus called JCV in the fluid around the brain and spinal cord, known as cerebrospinal fluid (CSF). The study will monitor patients over a period of time to check if the virus levels decrease. Participants will receive regular check-ups and tests to track their progress and the effects of the treatment. These tests include measuring the virus levels in the CSF and assessing neurological health using scales like the NIH Stroke Scale and the Modified Rankin Scale, which help evaluate the impact of the disease on daily activities and overall neurological function.

Throughout the study, the health and safety of participants will be closely monitored. The trial will also look at any side effects or adverse events that may occur during the treatment. The study aims to provide valuable information on whether Pembrolizumab can be an effective treatment for PML in patients with compromised immune systems, potentially offering a new option for managing this challenging condition.