CD4 and tropical spastic paraparesis: Although HTLV-I-specific CTL responses have been demonstrated in ACs and patients with HAM/TSP [23],[24], high expression of IFN-γ in CD8+ T cells specifically in HAM/TSP patients compared to ACs have been reported to be induced by interaction with virus-infected CD4+ T cells and CD8+ T cells [8],[9],[25].