This CD4+ T-cell subset has a suppressive phenotype and does not produce significant amounts of IFN-γ TNF-α and IL-2 in uninfected subjects, suggesting that these immunostimulatory and/or immunoregulatory CD39 expressing T cell subsets may participate in the pro-inflammatory milieu that could potentially lead to the progression to HAM/TSP. Here, IFNG is linked to tropical spastic paraparesis.