CD4 and tropical spastic paraparesis: Human T-lymphotropic virus type 1-1-infected CD4+ T cells may primarily contribute to development of HAM/TSP, since the number of HTLV-1-infected T cells circulating in the peripheral blood is higher in patients with HAM/TSP than in asymptomatic HTLV-1-infected individuals (Nagai et al., 1998; Yamano et al., 2002); this number is even higher in the CSF of patients with HAM/TSP (Nagai et al., 2001).