CD4 and tropical spastic paraparesis: HAM is a chronic progressive myelopathy, with a theorised pathophysiology involving expanding populations of HTLV-1-infected CD4 + and HTLV-1-specific CD8 + T lymphocytes more capably migrating into the spinal cord, at which point, their interaction with one another generates neurotoxic cytokines such as IFN-γ and TNF-α, inadvertently initiating chronic myelitis (Höllsberg 1997).