CD4 and tropical spastic paraparesis: The main hypothesis to explain HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) neuropathogenesis is the so-called “Bystander damage.” It suggests that the presence of IFN-γ-secreting HTLV-1-infected CD4+T cells and their recognition by virally specific cytotoxic CD8+T cells in the CNS, induce microglia to secrete cytokines, such as TNF-α, which may be toxic for the myelin of neurons.