IFNG and tropical spastic paraparesis: Although there is no significant difference in a functional CD8+ cell assay for the anti-viral efficacy of HTLV-1-specific CD8+ cytotoxic T cells between HAM/TSP patients and HTLV-1 carriers [14], the finding that HTLV-1-specific CD8+ cytotoxic T cells of HAM/TSP patients as the response to HTLV-1 tax produces high levels of proinflammatory cytokines, such as IFN-γ and TNF-α, etc., [15] strongly suggests that HTLV-1-specific CD8+ cytotoxic T cells can function for the induction of bystander mechanism.