Human T-cell leukemia virus type 1: replication, proliferation and propagation by Tax and HTLV-1 bZIP factor

•HTLV-1 primarily transmits by cell-to-cell contact.•HTLV-1 increases the number of infected cells by clonal proliferation to facilitate its transmission.•HTLV-1 bZIP factor (HBZ) is responsible for immunophenotype of HTLV-1 infected cells and adult T-cell leukemia (ATL) cells.•HBZ and Tax have opposing functions important for replication and proliferation.

Human T-cell leukemia virus type 1 (HTLV-1) spreads primarily by cell-to-cell transmission. Therefore, HTLV-1 promotes the proliferation of infected cells to facilitate transmission. In HTLV-1 infected individuals, the provirus is present mainly in effector/memory T cells and Foxp3+ T cells. Recent study suggests that this immunophenotype is acquired by infected cells through the function of HTLV-1 bZIP factor (HBZ). Tax, which is encoded by the plus strand, is crucial for viral replication and de novo infection, while HBZ, encoded by the minus strand, is important for proliferation of infected cells. Importantly, HBZ and Tax have opposing functions in most transcription pathways. HBZ and Tax cooperate in elaborate ways to permit viral replication, proliferation of infected cells and propagation of the virus.