Diversity and Evolution of Primate Lentiviruses

The human immunodeficiency viruses (HIV-1 and -2) together with the simian immunodeficiency viruses (SIV) comprise the primate lentivirus family. Since the isolation of the first SIV in 1985 [15], our knowledge of the diversity of these primate lentiviruses has continuously developed by the isolation and characterization of new SIV strains from additional species of African simians. It is now clear that the SIVs are a large group of viruses that can be found naturally in feral and domesticated African primates, such as guenons, mangabeys, mandrills, and chimpanzees; to indicate the species from which each SIV was isolated they are given a short suffix, such as SIVagm for the virus derived from African green monkeys. Most of these African primates are natural hosts for these viruses, but some infections are the result of recent cross-species transmissions. In those species that are natural hosts, the proportion of animals that are seropositive in the wild can be quite high [5, 33, 55, 60], and infected primates do not seem to develop any clinical symptoms [22, 51, 66]. The reasons for this lack of pathogenicity are still not well understood. However the lack of pathogenicity does not seem to be based on inherent properties of the virus, since for example Asian macaques inoculated with SIV from sooty mangabeys or African green monkeys can develop AIDS-like symptoms similar to those of humans after HIV infection [35, 59]. Because SIV is the most closely related lentivirus to HIV, SIV infection of monkeys has become the best animal model for studying the pathogenesis of, and efficacy of vaccines against, HIV infection in humans. In addition, the evaluation of new SIV strains is important to better understand the origins of HIV-1 and -2 and to assess the potential for additional lentiviruses to infect the human population.