IFNG and tropical spastic paraparesis: HTLV-1-infected CD4+ cells of HAM/TSP patients exhibit spontaneous proliferation with an increased production of proinflammatory cytokines such as interferon (IFN)-γ, TNF-α, IL-1 and IL-16 and neurotoxic cytokines IFN-γ and TNF-α, which are found in high concentrations in the spinal fluid of HAM/TSP patients [20,56,57].