![]() ![]() Several lentiviruses have been identified in various species of non-human primates as well as in other mammalian species. (a) Phylogenetic relationship of HIV-1 and HIV-2 to other retroviruses Most non-human and human primate lentivirus, oncovirus and spumavirus isolates so far analysed are complex retroviruses ( Wilkenson et al., 1994). The defining feature of complex retroviruses is that in addition to gag, pol and env structural genes, they encode genes which regulate expression of structural genes (see Section 1.1.7). In addition to their morphological classification, retroviruses have been described as ‘simple’ or ‘complex’ according to their genome organization ( Cullen, 1993 Figure 2). The lentiviruses and spumaviruses each constitute a genus the oncoviruses have been subdivided into five genera. More recently, the International Committee on the Taxonomy of Viruses has divided the Retroviridae family into seven genera on the basis of genetic structure. The third subfamily, the spumaviruses (Latin, spuma = foam), so called because of the characteristic ‘foamy’ appearance induced in infected cells in vitro, have not been conclusively linked to any disease ( Schweizer et al., 1994 Ali et al., 1996). The second group, the lentiviruses (Latin, lentus = slow), cause a variety of diseases including immunodeficiency and wasting syndromes, usually after a long period of clinical latency. The oncoviruses (Greek, onkos = mass, swelling) consist of four morphological subtypes which are associated with tumours in naturally or experimentally infected animals, and non-oncogenic related viruses. On this basis three subfamilies were defined. Traditionally, retroviruses (family Retroviridae) have been classified according to a combination of criteria including disease association, morphology and cytopathic effects in vitro ( Table 1 Weiss et al., 1985). Embedded within the lipid bilayer are the viral envelope glycoproteins: the external surface glycoprotein (SU gp120) and the transmembrane glycoprotein (TM gp41), which are non-covalently associated on the virion surface ( Gelderblom, 1991 Barker et al., 1995). ![]() The myristoylated matrix protein (MA p17) lies just below the lipid bilayer which surrounds the virion. The viral core is a complex made up of RT (p55/66), endonuclease or integrase (IN p32), protease (PR p10, p12 or p15 1), and nucleocapsid proteins (NC p6 and p7) and two copies of positive strand viral RNA, all of which is surrounded by an icosahedral capsid protein (CA p24). The inter-relationship of the genomic RNA, core proteins and surrounding viral envelope is schematically represented in Figure 1. Each HIV-1 virion measures approximately 120 nm in diameter and has a condensed cylindrical core surrounded by a lipid membrane. Lentiviruses contain a diploid, single-stranded RNA genome within a protein core. HIV-1 and HIV-2 are the only known human lentiviruses.Īll retroviruses share a similar overall morphology, but there is variation in detail ( Table 1). It is this ‘backward’ transfer of genetic information from RNA to DNA which classifies these viruses as retroviruses. All of these retroviruses belong to the lentivirus subfamily, have an RNA genome and replicate via a DNA intermediate (a ‘provirus’) by means of a viral RNA-directed DNA polymerase, more commonly called reverse transcriptase (RT). Between 19, several non-human primates were shown to harbour related retroviruses. ![]() Initially the virus was referred to as lymphadenopathy-associated virus (LAV) or human T-cell lymphotropic virus type III (HTLV-III) the name human immunodeficiency virus was established in 1986. Later, a second virus was discovered in West Africa (HIV-2) that was sufficiently different from HIV-1 in its serological and molecular characteristics to be considered a separate, but related, virus ( Clavel et al., 1986). The human immunodeficiency virus type 1 (HIV-1) was discovered in 1983 ( Barré-Sinoussi et al., 1983) and firmly associated with the acquired immunodeficiency syndrome (AIDS) in 1984 ( Gallo et al., 1984). ![]()
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