Transferrin Receptor Marks a Foxp3-Low Treg-like Inflammatory T Cell Subset Associated with Disease Severity in HAM/TSP.

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+ T cells; however, the phenotypic and functional characteristics of disease-associated T-cell subsets remain incompletely understood. We analyzed samples using flow cytometry (n = 3-5 per group) and RNA-seq (n = 13), focusing on CADM1highCD4+ T cells enriched for HTLV-1-infected cells to evaluate a transferrin receptor (TfR)-expressing subset. TfR+CADM1highCD4+ T cells were detected in both asymptomatic carriers and patients with HAM, but their frequency among CD4+ T cells was higher in HAM patients. These cells exhibited a Treg-like phenotype with higher Foxp3 and CTLA-4 expression than TfR- cells and showed increased Ki-67 positivity, consistent with proliferation. Despite this phenotype, they produced interferon-γ, indicating inflammatory potential, while Foxp3 expression was lower in HAM patients than in asymptomatic carriers, suggesting a more inflammatory phenotype. Furthermore, TfR transcript levels (RNA-seq TPM) correlated with clinical indicators of disease activity, including neopterin and CXCL10 protein levels, and the Osame motor disability score. Collectively, these findings suggest that TfR identifies a proliferative, Foxp3-low, Treg-like inflammatory CD4+ T-cell subset that is associated with disease activity in HAM.