High frequencies of Th1-type CD4+ T cells specific to HTLV-1 Env and Tax proteins in patients with HTLV-1–associated myelopathy/tropical spastic paraparesis

CD4+ T cells are critical for inducing and maintaining efficient humoral and cellular immune responses to pathogens. The CD4+ T-cell response in human T-lymphotropic virus 1 (HTLV-1) infection has not been studied in detail. However, CD4+ T cells have been shown to predominate in early lesions in HTLV-1–associated myelopathy/tropical spastic paraparesis (HAM/TSP). We present direct estimates of HTLV-1 Env- and Tax-specific CD4+ T-cell frequencies in patients infected with HTLV-1. We first showed that there was a strong bias toward the Th1 phenotype in these HTLV-1–specific CD4+ T cells in patients with HAM/TSP. We then demonstrated significantly higher frequencies of HTLV-1–specific Th1-type CD4+ T cells in HAM/TSP patients than in asymptomatic HTLV-1 carriers. The majority of these HTLV-1–specific CD4+ T cells did not express HTLV-1 Tax and were therefore unlikely to be infected by HTLV-1. High frequencies of activated HTLV-1–specific CD4+ T cells of the Th1 phenotype might contribute to the initiation or pathogenesis of HAM/TSP and other HTLV-1–associated inflammatory diseases.