Human T-lymphotropic virus 1

Human T-cell lymphotropic virus type 1 or human T-lymphotropic virus type 1 (HTLV-I), also called the adult T-cell lymphoma virus type 1, is a retrovirus of the human T-lymphotropic virus (HTLV) family that has been implicated in several kinds of diseases including very aggressive adult T-cell lymphoma (ATL), HTLV-I-associated myelopathy, uveitis, Strongyloides stercoralis hyper-infection and some other diseases. It is thought that about 1–5% of infected persons develop cancer as a result of the infection with HTLV-I over their lifetimes. Adult T-cell lymphoma (ATL) was discovered in 1977 in Japan. The symptoms of ATL were different from other lymphomas known at the time. It was suggested that ATL is caused by the infection of a retrovirus called ATLV. Strikingly, ATLV had the transforming activity in vitro. These studies established that the retrovirus infection is the cause of ATL. The retrovirus is now generally called HTLV-I because later studies proved that ATLV is the same as the firstly identified human retrovirus called HTLV discovered by Bernard Poiesz and Francis Ruscetti and their co-workers in the laboratory of Robert C. Gallo at the National Cancer Institute. Infection with HTLV-I, like infection with other retroviruses, probably occurs for life. A patient infected with HTLV can be diagnosed when antibodies against HTLV-1 are detected in the serum. HTLV-1 is a retrovirus belonging to the family retroviridae and the genus deltaretrovirus. It has a positive-sense RNA genome that is reverse transcribed into DNA and then integrated into the cellular DNA. Once integrated, HTLV-1 continues to exist only as a provirus which can spread from cell to cell through a viral synapse. Few, if any, free virions are produced and there is usually no detectable virus in the blood plasma though the virus is present in genital secretions. Like HIV, HTLV-1 predominately infects CD4+ T cells. The viral RNA is packed into the icosahedral capsid which is contained inside the protein inner envelope. The lipid outer envelope is of host cell origin but contains viral transmembrane and surface proteins. The virion is spherical in shape with a diameter of about 100 nm. Seven HTLV-1 genotypes are recognised—HTLV-1a through HTLV-1g. It is estimated that from 10 to 20 million people worldwide are infected; 3–8 million of them are in Africa. The most widespread genotype is type A. Types B, D, E, F and G have only been isolated from Central Africa. Type C is only present in Asia. Simian HTLV-1 genotypes are interspersed in between the human genotypes indicating frequent animal-human and human-animal transmission. The only human genotype that does not have a simian relative is A. It is thought that genotypes B, D, E, F and G originated in Africa from closely related STLV about 30,000 years ago, while the Asian genotype C is thought to have originated independently in Indonesia from the simians present there. Two subtypes are found in Japan: a transcontinental subgroup and a Japanese subgroup.