CD4 and tropical spastic paraparesis: Human T-cell leukemia virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a chronic inflammatory disease driven by HTLV-1-infected CD4<sup>+</sup> T cells; however, the phenotypic and functional characteristics of disease-associated T-cell subsets remain incompletely understood.