To identify the mode of viral transmission in the HTLV-1 VS we examined the ultrastructure of the VS using electron tomography of cell-cell conjugates spontaneously formed either between autologous CD4+ T cells isolated from the PBMCs of HTLV-1-infected HAM/TSP individuals with high proviral load or between MS9 cells (a chronically HTLV-1-infected cell line), as a donor T-cell, and the Jurkat cell line, as a target T-cell. Here, CD4 is linked to tropical spastic paraparesis.