CD4 and tropical spastic paraparesis: We observed differential expression of PD-1 levels with CD39+CD25+CD4+ T cells (regulatory) having higher expression of PD-1 in HAM/TSP patients, (p = 0.0189) (Fig. S3 A,B), whereas the CD39+CD25−CD4+ T cells (inducer) significantly expressed lower levels of PD-1 in HAM/TSP infected patients compared to asymptomatic controls, (p = 0.0317) (Fig. S3 B).