It has been observed that patients with PML display higher percentages of PD-1 + CD4 + and CD8 + T cells in the CSF and blood (Cortese et al. 2019) and increased expression of PD-1 on JCV-specific CD8 + cytotoxic T-lymphocytes (Yang et al. 2023). Here, CD4 is linked to progressive multifocal leukoencephalopathy.