TAT and progressive multifocal leukoencephalopathy: In addition to the synergistic effect of Tat and JCV in oligodendrocytes, cytotoxic CD8+ T cells aggregate at demyelinated lesion sites in the brain to engage JCV-infected oligodendrocytes, which tend to control JCV dissemination, but at the cost of oligodendrocyte death and further demyelination in PML [40].