CD4 and progressive multifocal leukoencephalopathy: Our data together with these studies suggest that preserving and augmenting anti-JCPyV CD4 T cells or providing “helper” cytokines in PML-susceptible individuals, and using such interventions to supplement CD8 T cell immunotherapy for PML, could promote differentiation of brain-infiltrating CD8 T cells into CD8 bTRM and improve disease prognosis.