Fucoidan therapy decreases the proviral load in patients with human T-lymphotropic virus type-1-associated neurological disease

Background: Human T-lymphotropic virus type-1 (HTLV-1) is a human retrovirus that causes HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and adult T-cell leukaemia (ATL). A higher viral load in individuals with HTLV-1 infection increases their risk of developing HAM/TSP and ATL. Moreover, the high proviral load is associated with the clinical progression of HAM/TSP. Reduction of the number of HTLV-1-infected cells is therefore crucial for preventing and treating HTLV-1-associated diseases. Recently, fucoidan, a complex sulphated polysaccharide derived from marine seaweed, has been demonstrated to exert inhibitory effects on HTLV-1 infection in vitro. In this study, we examined the in vivo effects of fucoidan on HTLV-1 infection. Methods: In this single-centre open-label trial, 13 patients with HAM/TSP were treated with 6 g fucoidan daily for 6-13 months. The HTLV-1 proviral DNA load and frequencies of HTLV-1-specific CD8⁺ T-cells, natural killer cells, invariant natural killer T-cells and dendritic cells in the peripheral blood were analysed. Furthermore, the in vitro inhibitory effect of fucoidan on cell-to-cell HTLV-1 infection was examined by using luciferase reporter cell assays. Results: Fucoidan inhibited the cell-to-cell transmission of HTLV-1 in vitro. Furthermore, fucoidan therapy resulted in a 42.4% decrease in the HTLV-1 proviral load without affecting the host immune cells. During the treatment, no exacerbation was observed. Four patients with HAM/TSP developed diarrhoea, which improved immediately after stopping fucoidan administration. Conclusions: Fucoidan is a new potential therapeutic agent for the prevention and treatment of HTLV-1-associated diseases.