CD4 and progressive multifocal leukoencephalopathy: More recently, pembrolizumab, a humanized monoclonal antibody (MAB) that binds to the programmed cell death protein 1 (PD-1) receptor on lymphocytes, has been shown in 2 studies published in 2019, to reduce JCV viral load, increase CD4+ and CD8+ T cell activity against JCV, and induce clinical stabilization or improvement in up to 62.5% of patients with PML [42–44].