The antiviral T-helper (Th)1 phenotype is also dominant among HTLV-1-specific CD4+ T cells in both ACs and patients with HAM/TSP [93], and there is a higher frequency of IFN-γ, TNF-α, and IL-2 production by CD4+ T cells in patients with HAM/TSP compared to AC of a similar PVL [93, 94]. Here, CD4 is linked to tropical spastic paraparesis.