A recent report demonstrated that, when the frequency of PD-1+ T cells were compared among healthy controls, patients with HAM/TSP and Progressive Multifocal Leukoencephalopathy (PML) which is a rare, often fatal, demyelinating disease caused by reactivation of the ubiquitous JC virus, HAM/TSP patients as well as PML patients showed a significant higher percentage of PD-1 expression on CD4+ and CD8+ T cells in CSF than healthy controls [97]. Here, CD4 is linked to tropical spastic paraparesis.