Regarding the role of the virus, the in vitro increase in CD80+ T cells (but not CD86+ T or CD80+/CD86+ B cells) was significantly correlated to the proviral load in both HTLV-1 infected individuals (p = 0.013, Spearman’s r = 0.66, n = 13) and HAM/TSP patients (p = 0.0046, Spearman’s r = 0.91, n = 8). This evidence concerns the gene CD80 and tropical spastic paraparesis.