CD4 and progressive multifocal leukoencephalopathy: High rates of HIV-positive PML cases are thought to result from a number of factors, such as the presence of CNS HIV, which may either directly or indirectly contribute to PML neuropathogenesis, CD4+ T-cell loss with compromised CNS immune surveillance, and the activation of CD8+ Cytotoxic T Lymphocyte (CTL) responses with the destruction of infected oligodendrocytes [32].