Lentiviruses have been isolated from many species (Table 1), including more than two dozen different African nonhuman primate species. There are two distinct types of human AIDS viruses: HIV-1 and HIV-2. The two types are distinguished on the basis of genome organization and phylogenetic (evolutionary) relationships with other primate lentiviruses. Sequence divergence between HIV-1 and HIV-2 exceeds 50%.
Table1. Representative Members of the Lentivirus Genus
Based on env gene sequences, HIV-1 comprises three dis tinct virus groups (M, N, and O); the predominant M group contains at least 11 subtypes or “clades” (A–K). Recombinant forms of virus are also found in circulation in humans in different geographic regions. Similarly, eight subtypes of HIV-2 (A–H) have been identified. Within each subtype there is extensive variability. The genetic clades do not seem to correspond to neutralization serotype groups, and there is currently no evidence that subtypes differ in biology or pathogenesis.
Numerous lentivirus isolates have been obtained from nonhuman primate species. The primate lentiviruses fall into six major phylogenetic lineages (Table 1). SIV from sooty mangabeys (a type of monkey in West Africa) and HIV-2 are considered to be variants of the same virus, as are chimpanzee isolates and HIV-1. The SIVs from African green monkeys, Sykes monkeys, mandrills, and colobus monkeys represent additional discrete lineages.
The organization of the genomes of primate lentiviruses (human and simian) is very similar. One difference is that HIV-1 and the chimpanzee virus carry a vpu gene, whereas HIV-2 and the SIVsm group have a vpx gene. Other SIV isolates have neither vpu nor vpx genes. The sequences of the gag and pol genes are highly conserved. There is significant divergence among the envelope glycoprotein genes; the sequences of the transmembrane protein portion are more conserved than the external glycoprotein sequences (the protein com ponent exposed on the exterior of the virus particle).
The SIVs appear to be nonpathogenic in their host species of origin (eg, African green monkey or sooty mangabey), species known to be infected in their natural habitats. However, SIVcpz , the precursor of HIV-1, is pathogenic in chimpanzees in the wild, causing AIDS-like pathology and premature death. In contrast, rhesus monkeys are not infected naturally in the wild in Asia but are susceptible to induction of simian AIDS by various SIV isolates. The virus first recovered from captive rhesus monkeys (SIVmac ) is the sooty mangabey/ HIV-2 strain.
The nonprimate lentiviruses establish persistent infections affecting various animal species. These viruses cause chronic debilitating diseases and sometimes immunodeficiency. The prototype agent, visna virus (also called maedi virus), causes neurologic symptoms or pneumonia in sheep in Iceland. Other viruses cause infectious anemia in horses and arthritis and encephalitis in goats. Feline and bovine lentiviruses may cause an immunodeficiency. Nonprimate lentiviruses are not known to infect any primates, including humans.
