Fig.1 Genomic organization of SIV. (Sauter & Kirchhoff, 2014)
Simian immunodeficiency viruses (SIVs) are primate lentiviruses that infect 45 nonhuman primate species in Sub-Saharan Africa. SIV infection is not pathogenic in natural host species such as sooty mangabeys (SMs) and African green monkeys (AGMs). SIV infection in chimps, on the other hand, can cause a disease similar to acquired immunodeficiency syndrome (AIDS) in humans, which is caused by HIV. SIVs are morphologically and biologically related to HIV-1 and HIV-2. SIV and HIV share molecular biology features such as target cells, cell infection mechanisms, and key features of virus persistence. HIV is thought to have originated in apes as a SIV and to have crossed the species barrier from apes to humans sometime in the first half of the twentieth century. SIV is a perfect model for HIV pathogenesis.
SIV is a member of the genus Lentivirus and the family Retroviridae. Long terminal repeats are found at each end of the lentivirus genome, as well as three genes (env, gag, and pol) that encode three virion structural proteins (envelope, core and enzymatic proteins respectively). Furthermore, the genome encodes a large number of non-structural proteins with complex regulatory functions, including Rev, Tat, Nef, Vif, Vpr, Vpu (HIV-1 only), and Vpx (HIV-2 and SIV only).
Fig.2 HIV structure (van Heuvel, et al., 2022); and SIVcpz genome (Sharp, et al., 2005).
SIV has been around for at least 32,000 years. SIV genetic diversity and evolution are complicated. Many SIV strains appear to be recombinant, and the parental strains are frequently difficult to identify. Primate lentiviruses were classified into at least seven distinct lineages based on comparisons of their full-length genome sequences and the functional similarity of their genes, such as SIVcpz in chimps, SIVsm in sooty mangabeys, and SIVcol in guereza colobus. The majority of NHP species are infected with a species-specific SIV lineage. Surprisingly, different animals from the same primate species can be infected with multiple SIVs.
HIV-1 could be the result of multiple cross-species transmission events. Cross-transmission of SIV to humans is thought to have occurred numerous times over the last 32,000 years. According to the phylogenetic tree analysis, HIV-1 Groups M and N are descended from SIVcpzPtt and SIVcpzPts, respectively, while HIV-1 Group O is descended from SIVgor. SIVcpz was also created through lentiviral host switching and recombination. HIV-2 is the result of an unrelated transmission event in which the virus was passed from sooty mangabeys to humans.
Fig.3 SIV and HIV lineages. (Klatt, et al., 2012)
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