FOXP3 and tropical spastic paraparesis: Furthermore, during HTLV-1 infection, the frequency of HTLV-1–negative Foxp3 + CD4+ T cells is positively correlated with the HTLV-1 proviral load (PVL) [18, 19], the most important risk factor for developing HAM/TSP, and CTL activity is negatively correlated with the frequency of HTLV-1–negative Foxp3 + CD4+ cells [19].