Why are the natural hosts of SIV resistant to AIDS?

An increasing number of African primate species have been shown to be infec ted in the wild with their own distinct variants of simian immunodeficiency virus. The most striking feature of these natural host systems is the lack of AIDS-like disease despite long-term infection. In the African green mon key (AGM)/SIVagm system there is no evidence that a vigorous antiviral immu ne response, a lack of variability or a low virus load accounts for this la ck of pathogenicity. New-born AGMs appear to be even more resistant to the virus than adults, despite their immature immune system and higher pool of target cells. The fact that AGMs, unlike HIV-infected humans, lack a humora l immune response to non-denatured Gag protein and do not show trapping of virus in the lymph nodes suggested that tolerance to Gag might prevent the formation of immune complexes which would normally be filtered out by the l ymphoid tissues with detrimental results. This apparent tolerance to Gag is a common feature of many, if not all, of the natural host systems and migh t explain why the lymph nodes and immune system in general remain intact in these primates in the face of continuous, high level virus replication. (C ) 1999 Published by Elsevier Science B.V. All rights reserved.