HTLV-1-Infected CD4⁺ T Cells Drive Spontaneous Lymphoproliferation and Virus-Specific CD8⁺ Cytotoxic T Lymphocyte...

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.

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.

CD8⁺ T cells exhibited greater proliferation than CD4⁺ T cells during SP. Among proliferating CD4⁺ T cells, 38.15% were CADM1+, 61.84% were Tax+, and PVL analysis indicated that nearly all proliferating CD4⁺ T cells were HTLV-1 infected. The frequency of HTLV-1 Tax 301-309-specific CTLs among CD8⁺ T cells increased from 3.81% to 17.67% during SP. After a subsequent resting phase, IFN-γ responses were detected in 37.38% of CD8⁺ T cells to a Tax peptide mixture and in 33.42% to an HTLV-1 bZIP factor peptide mixture, together accounting for over 70% of proliferating CD8⁺ T cells. Sorted CD4⁺ T cells proliferated independently, whereas sorted CD8⁺ T cells did not; however, coculture with CD4⁺ T cells restored CD8⁺ 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.

SP is driven by HTLV-1-infected CD4⁺ T cells, which are essential for sustaining HTLV-1-specific CD8⁺ 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.