TNFRSF4 and infection: The number of HHV-6B-infected HSB2-CD134 cells increased rapidly for 3 days and then these cells died via the lytic cycle 4 days post-infection (d.p.i.), whereas 100% of the MT-4 cells were infected by multi-step growth at 4 d.p.i. MT-4 cells are human T-cell lymphotropic virus type 1 (HTLV-1) which transform and express HTLV-1 Tax, which is a transcriptional activator of the viral long terminal repeat [49,50].