IFNG and tropical spastic paraparesis: In HTLV-1 infection, although HTLV-1-specific CTL responses were detected in both asymptomatic carriers and HAM/TSP patients, high expression of IFN-γ in CD8+ T cells specifically in HAM/TSP patients compared to asymptomatic carriers have been reported to be induced by interaction with virus-infected CD4+ T cells and CD8+ T cells [89–91].