IFNG and tropical spastic paraparesis: As we have demonstrated high HTLV-1 tax expression in HAM/TSP CD4+CD25+CCR4+ T cells (Foxp3−) and low HTLV-1 tax expression in ATL CD4+CD25+CCR4+ T cells (Foxp3+) (Figure 6), it was suggested that HTLV-1 expression intracellularly may act as a “switch” that directs T cell differentiation of CD4+CD25+CCR4+ T cells from Foxp3+ to IFN-γ+Foxp3− T cells.