CD4 and tropical spastic paraparesis: Although other risk factors have been proposed, few differ systematically between ACs and patients with HAM/TSP after normalisation for proviral load: the frequency of certain lymphocyte subsets (HTLV-1-specific CD4+ T cells [7]; FoxP3+CD4+ T cells [8]; natural killer (NK) cells [9]); the level of expression of HTLV-1 genes [10] and the pattern of integration of the HTLV-1 provirus in the host cell genome [11].