CD4 and tropical spastic paraparesis: Few factors have been identified that differ systematically between asymptomatic carriers and patients with HAM at a given proviral load: the frequency of certain lymphocyte subsets (HTLV-1-specific CD4+ T cells [3,4]; FoxP3+ CD4+ T cells [5]; natural killer (NK) cells [6] and NKT cells [7]); the level of expression of HTLV-1 genes in fresh PBMCs [5,8-10]; and the pattern of integration of the HTLV-1 provirus in the host cell genome [11].