Cortese I, et al. - Given a possible contribution of programmed cell death protein 1 (PD-1), a negative regulator of immune response, in impaired viral clearance, researchers investigated patients with progressive multifocal leukoencephalopathy (PML; an opportunistic brain infection caused by the JC virus) to see if anti–JC virus immune activity in patients with PML could be strengthened by PD-1 blockade with pembrolizumab. Eight adults with PML, each with a different underlying predisposing condition, were administered a 2 mg per kilogram of body weight dose of pembrolizumab every 4 to 6 weeks. At least one but no more than three doses were administered to the patient. In line with the hypothesis, this study suggests that pembrolizumab reduces JC viral load and increases CD4+ and CD8+ activity against the JC virus in some patients with PML. Five of the eight patients who received pembrolizumab showed clinical improvement or stabilization.