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Human T-Cell Lymphotropic Virus

Human T-cell lymphotropic virus (HTLV) are retroviruses which can cause an uncommon type of T-cell lymphocytic leukemia and non-Hodgkin lymphoma in adults called adult T-cell leukemia/lymphoma (ATLL). There are four known types of HTLV (HTLV-1, HTLV-2, HTLV-3, and HTLV-4); HTLV-1 is the most pathogenic for humans while HTLV-2 usually produces mild neurological disease. Both are prevalent worldwide. HTLV-3 and HTLV-4 have only been identified in Central Africa and usually affect non-human hominids. Despite infecting at least 10 million people worldwide, HTLV is still considered a neglected disease.

Human T-Cell Lymphotropic Virus

The human T-cell lymphotropic virus, human T-lymphotropic virus, or human T-cell leukemia-lymphoma virus (HTLV) family of viruses are human retroviruses that cause a type of cancer called adult T-cell leukemia/lymphoma and a demyelinating disease called HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). They were discovered by Robert Gallo and colleagues in 1980 and belong to a larger group of primate T-lymphotropic viruses (PTLVs). Members of this family that infect humans are called HTLVs, and those that infect Old World monkeys are called Simian T-lymphotropic viruses (STLVs). Four types of HTLVs (HTLV-1, HTLV-2, HTLV-3, and HTLV-4) have been identified. The original name for HIV, the virus that causes AIDS, was HTLV-3. A closely related virus is bovine leukemia virus BLV (Martinez et al., 2019).
HTLV-1 is a retrovirus belonging to the family retroviridae and the genus deltaretrovirus which predominately infects CD4+ T cells. It can be transmitted from mother to child, through sexual contact, and through contaminated blood products. The virus affects up to 10 million people worldwide with endemic regions of infection in Southwest Japan, sub-Saharan Africa, South America, the Caribbean, and regions of the Middle East and Australo-Melanesia. The major geographic subtypes are Cosmopolitan subtype A, Central African subtype B, Australo-Melanesian subtype C, and Central African/Pygmies subtype D. Cosmopolitan subtype A is the most widespread subtype (endemic subgroups in Japan, Central and South America, the Caribbean, North and West Africa, and regions of the Middle East). It is directly associated to one of the most aggressive T cell malignancies; Adult T Cell Leukemia-Lymphoma (ATLL) and a progressive neurological disorder, Tropical Spastic Paraparesis/ HTLV-1 Associated Myelopathy (TSP/HAM). Between 1 in 20 and 1 in 25 infected people are thought to develop cancer as a result of the virus (Verdonck et al., 2007).
HTLV-2 closely related to HTLV-1 and may be linked to cutaneous T-cell lymphoma (CTCL). HTLV-2 is far less prevalent than HTLV-1 with an estimated 800,000 infected individuals worldwide. Most documented HTLV-2 infected individuals are found in the United States highly concentrated in the Native American and intravenous drug user populations. A similar epidemiologic pattern is found in the second most HTLV-2 infected region, Brazil. HTLV-2 is divided into four molecular subtypes; a, b, c, and d. HTLV-2a and HTLV-2b are commonly found in the Americas and Europe whereas HTLV-2c and HTLV-2d are found predominantly in Brazil and Central Africa. HTLV-2 is associated with milder neurologic disorders and chronic pulmonary infections. No specific illnesses have yet been associated with HTLV-3 and HTLV-4 (Eusebio-Ponce et al., 2019).

References

  • Eusebio-Ponce E, Anguita E, Paulino-Ramirez R, Candel FJ. HTLV-1 infection: An emerging risk. Pathogenesis, epidemiology, diagnosis and associated diseases. Rev Esp Quimioter. 2019 Dec;32(6):485-496.
  • Martinez MP, Al-Saleem J, Green PL. Comparative virology of HTLV-1 and HTLV-2. Retrovirology. 2019 Aug 7;16(1):21.
  • Verdonck K, González E, Van Dooren S, Vandamme AM, Vanham G, Gotuzzo E. Human T-lymphotropic virus 1: recent knowledge about an ancient infection. Lancet Infect Dis. 2007 Apr;7(4):266-81.

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