IFNG and tropical spastic paraparesis: Peripheral blood mononuclear cells (PBMC) naturally infected by HTLV-1, such as in HAM/TSP disease, precociously express intracellular Tax protein and interferon-γ (IFN-γ) in mitogen- stimulated short term culture, while the expression of IL-2 and TNF-α requires longer culture time [9, 10].