The HTLV-1-infected CD4+ T lymphocytes of HAM/TSP patients exhibit spontaneous proliferation, in addition to an increased production of proinflammatory cytokines such as IFN-γ, TNF-α, IL-1, and IL-6; neurotoxic cytokines such as IFN-γ and TNF-α, in particular, are found at high concentrations in the spinal fluid of HAM/TSP patients [9,16,50,51]. Here, CD4 is linked to tropical spastic paraparesis.