HIV-I and HIV-2 are believed to be the result of cross .species transmission from simian immunodeficiency virus (SIV)-infected chimpanzees and sooty mangabeys, respectively, which represent 2 (SIVcpz and SIVsm) of the 6 major lentiviral phylogenetic lineages (1,2).
HIV-2 type-specific protease primers allow the amplification of HIV-2 at least subtypes A and B (13), and HIV-2/SIV integrase primers amplify HIV-2 and at least 5 major SIV lineages including SIVcpz, SIVsm, SIVagm (African green monkey), SIVmndBK12 (mandrill), and SIVsyk (Sykes monkey) (14).
The evidence indicates that two simian immunodeficiency viruses (SIV), one from chimpanzees (SIVcpz) and the other from sooty mangabeys (SIVsm
), crossed the species barrier to humans, generating HIV-1 and HIV-2, respectively.
The closest simian relatives of HIV-1 and HIV-2 have been found in the common chimpanzee (Pan troglodytes) and the sooty mangabey (Cercocebus atys), respectively (6-8), and phylogenetic evidence indicates that lentiviruses from these species (SIVcpz and SIVsm
, respectively) have been transmitted to humans on at least eight occasions (5,9).