HTLV-1–Infected CD4 ⁺ T Cells Drive Spontaneous Lymphoproliferation and Virus-Specific CD8 ⁺ Cytotoxic T Lymphocyte Expansion in HAM/TSP

BACKGROUND AND OBJECTIVES: Spontaneous lymphoproliferation (SP) is an ex vivo phenomenon where lymphocytes proliferate without exogenous stimulation in peripheral blood mononuclear cells (PBMCs) from patients with human T-cell leukemia virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP). SP is thought to reflect spinal cord pathology in HAM/TSP, but its cellular mechanisms remain unclear. METHODS: PBMCs from 30 patients with HAM/TSP, 12 asymptomatic HTLV-1 carriers, and 8 healthy controls (HCs) were labeled with a proliferation tracer and cultured for 6 days without exogenous stimulation. Flow cytometry was used to identify T-cell subsets, HTLV-1-infected cells, and HTLV-1-specific cytotoxic T lymphocytes (CTLs). Additional analyses included CTL antigen specificity, proviral load (PVL), phenotypic profiling (differentiation, activation, exhaustion markers), and cytokine/chemokine quantification in culture supernatants. RESULTS: T-cell proliferation. Proliferating infected cells exhibited a Th1 phenotype, and most proliferating T cells coexpressed activation and exhaustion markers with memory phenotypes. IL-6 and IFN-γ levels were significantly elevated in culture supernatants from patients with HAM/TSP compared with HCs. IFN-γ levels in SP supernatants correlated with CSF PVL. SP responses in HTLV-1 carriers were attenuated, with reduced proliferation compared with patients with HAM/TSP. DISCUSSION: CTL expansion. This phenomenon appears to recapitulate certain immunologic features of neuroinflammation in HAM/TSP. This ex vivo model may help evaluate therapies targeting HTLV-1-infected cells and immune-mediated pathology.