HTLV-1 has been linked to a chronic, slowly progressive neurologic condition termed HTLV-1 associated spastic paraparesis or tropical spastic paraparesis (HAM/TSP) which affects about 1% of the infected individuals. The disease is thought to be due to a T cell viral induced immunopathological process. A high frequency of HTLV-1 specific CD8 T cells are found in patients with HAM/TSP. The immune system of patients infected with HTLV-1 appears to be dysregulated, and increased spontaneous T cell proliferation can be demonstrated in vitro. This is in part due to continuous antigenic stimulation and due to transactivation by the HTLV-1 encoded Tax protein of host immunomodulatory genes such as CD80, CD86, IL-2 and its receptor. In addition, an increased viral load has been demonstrated in symptomatic individuals compared to asymptomatic HTLV-1 carriers. It is thought that the local immune response to the virus within the central nervous system plays a role in the pathogenesis of progressive spastic encephalomyeloneuropathy of HAM/TSP. Therefore, reduction of spontaneous T cell proliferation and viral replication, as well as decrease of the compromise of the blood brain barrier may ameliorate the immune-mediated component of the process which leads to inflammatory destruction of nervous tissue in HAM/TSP.