PDCD1 and progressive multifocal leukoencephalopathy: The importance of PD-1 in modulating CD8 T cell responses is underscored by clinical data showing that JCPyV-specific CD8 T cells have increased PD-1 expression and blockade of PD-1 on JCPyV-specific CD8 T cells improved T cell responses in patients with PML [75].