ABSTRACT
Human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus that was identified as a causative agent of human diseases. HTLV-1 belongs to the delta-type retroviruses, which also include bovine leukemia virus, human T-cell leukemia virus type 2, and simian T-cell leukemia virus. Indeed, HTLV-1 replication is not active in vivo; this fact indicates that the mechanism of HTLV-1 transmission is quite different from that of another human retrovirus, human immunodeficiency virus 1, which vigorously replicates in vivo to promote efficient transmission through cell- free virions. Replication of HTLV-1 is known to be generally suppressed in vivo. Most HTLV-1-infected individuals are asymptomatic and do not develop any HTLV-1-mediated diseases during their lifetime. HTLV-1 induces malignancy and several inflammatory diseases in some of the infected carriers. HTLV-1 can increase its copy number in vivo by clonal proliferation of infected cells and by de novo infection from infected cells to uninfected cells.
