We further analyzed if CD4+OX40+ T cells in HAM/TSP patients were capable of producing the inflammatory and neurotoxic cytokines, IFN-γ and TNF-α, which, according to the bystander damage hypothesis, could cause central nervous system (CNS) inflammation and demyelination seen in HAM/TSP patients [33,34]. Here, CD4 is linked to tropical spastic paraparesis.