simian immunodeficiency virus

simian immunodeficiency virus
Name	Simian immunodeficiency virus
Caption	Diagram of SIV virion structure
Taxon	Lentivirus
Synonyms	Simian T-lymphotropic virus type III

simian immunodeficiency virus is a retrovirus that infects various non-human primate species across Africa. It belongs to the same Lentivirus genus as human immunodeficiency virus and shares significant genetic and structural similarities. The study of these viruses in their natural hosts, such as sooty mangabeys and African green monkeys, has been fundamental to understanding the origins of the AIDS pandemic. Research conducted at institutions like the National Institutes of Health and the University of Alabama at Birmingham has revealed critical insights into viral evolution and pathogenesis.

Classification and evolution

SIVs are classified within the Orthoretrovirinae subfamily and the Lentivirus genus. Phylogenetic analyses, pioneered by scientists like Beatrice Hahn, show that SIVs have co-evolved with their primate hosts over millennia, resulting in a vast diversity of strains. Distinct viral lineages are named for their host species, such as SIVcpz from chimpanzees and SIVsmm from sooty mangabeys. The evolutionary history of these viruses, studied through techniques like molecular clock analysis, indicates multiple cross-species transmission events. Major research on this topic has been conducted at the University of Nottingham and the Institut Pasteur.

Virology and structure

The virion structure is characteristic of lentiviruses, featuring a lipid bilayer envelope studded with glycoprotein spikes. Its genome consists of two identical strands of positive-sense RNA and encodes key proteins like Gag, Pol, and Env. The viral reverse transcriptase enzyme, a target for antiretroviral drugs, facilitates the conversion of viral RNA into DNA. Structural biology studies using cryo-electron microscopy at facilities like the Scripps Research Institute have elucidated the atomic structure of the capsid and other components. The virus also encodes accessory proteins such as Nef and Vif that modulate the host immune system.

Transmission and host range

Natural transmission occurs primarily through mucosal contact during mating, blood-to-blood contact from bites, and vertically from mother to offspring. The host range is extensive, encompassing over 40 African primate species, including mandrills, guenons, and De Brazza's monkey. In their natural hosts, such as the sooty mangabey, infection typically does not cause severe disease, a phenomenon studied extensively at the Tulane National Primate Research Center. However, transmission to non-natural hosts, like Asian macaques in research colonies, can result in an AIDS-like syndrome.

Pathogenesis and disease

Pathogenesis varies dramatically between natural and non-natural hosts. In natural hosts like the African green monkey, infection is generally non-pathogenic despite high viral loads, a state of disease tolerance studied at the Yerkes National Primate Research Center. In contrast, infection of rhesus macaques, an Asian species, leads to progressive depletion of CD4+ T-cells, opportunistic infections, and simian AIDS, mirroring human disease. Key pathogenic mechanisms involve chronic immune activation and dysfunction of the gastrointestinal tract mucosa, as researched at the Harvard Medical School.

Relationship to HIV

SIVs are the direct precursors to human immunodeficiency virus types 1 and 2. HIV-1, the pandemic strain, originated from cross-species transmission of SIVcpz from chimpanzees in central Africa, a discovery made by teams led by Beatrice Hahn and Paul Sharp. HIV-2 stems from SIVsmm in sooty mangabeys in West Africa. The HIV/AIDS pandemic is believed to have begun following multiple zoonotic events, possibly linked to activities like bushmeat hunting. This origin story was clarified through work at the University of Montpellier and the Robert Koch Institute.

Research and significance

SIV infection in rhesus macaques is the premier animal model for HIV/AIDS research, enabling studies of vaccine candidates, antiretroviral therapy, and cure strategies. Landmark trials like the RV144 vaccine regimen were informed by SIV studies. Research at the Oregon National Primate Research Center and the Wisconsin National Primate Research Center has advanced understanding of latent reservoirs and immune correlates of protection. The study of these viruses remains critical for global public health efforts led by the World Health Organization and continues to inform the search for an effective HIV vaccine.

Category:Retroviruses Category:Primates Category:Virology