CD4 and tropical spastic paraparesis: Several reports have demonstrated the mechanism in HAM/TSP patients including dysregulation in CD4+ regulatory T cells (44), degenerate specificity and exhaustion in HTLV-1-specific CD8+ T cells (45–50), and increased T cell proliferation due to high expression of the common γ chain family of cytokines and their receptors, such as IL-2 and IL-15, associated with HTLV-1 gene transactivation (51, 52).