Radiation, Chimpanzees, and the Origin of AIDS

RADIATION, CHIMPANZEES, AND THE ORIGIN OF AIDS BRANDON P. REINES* The sudden appearance of two distinct pathogenic human immunodeficiency viruses (HIVs) on the African continent in the 1980s constitutes an extraordinary anomaly [I]. Several investigators have offered hypotheses that attempt to explain it. Two such hypotheses now dominate discussion: (1) a cross-species infection hypothesis, which posits that increased capture and transport of African primates in the 1960s and 1970s led to two separate interspecific transmission events of HIVs from primates to humans, most likely through a bite wound [2, 3]; and (2) an altered selection pressure hypothesis, which holds that benign HIVs have long existed exogenously in human beings but evolved to virulence because of drastic changes in the behavior of their human hosts. In particular, Ewald postulates that increasing sexual promiscuity in Africa had effectively eliminated selection pressure against virulence. While HIVs that had killed their human hosts outright would have been selected against under usual conditions, with rare opportunities for inter-host transfer, the rapid transmission of HIVs from host to host in semen would have virtually assured their survival regardless of lethality. In fact, more infectious virulent strains might tend to outcompete less virulent strains. Ewald's theory is attractive because it explains how a relatively-benign HIV, HIV-2, might have evolved in the socially more stable West African region. At the same time, however, if construed as an epidemiologic theory of the explosive appearance of the two HIVs in specific geographical locations in Africa, Ewald's and other evolutionary hypotheses come up short. It is for that reason, as well as the well-known pathogenicity of viruses that "jump the species barrier," that both Doolittle and Myers hypothesize that two independent cross-species transmission events *The Center for Health Science Policy, 1673 Columbia Road, NW, 200, Washington, DC 20009.© 1995 by The University of Chicago. All rights reserved. 003 1-5982/95/390 1-0944$0 1 .00 Perspectives in Biology andMedicine, 39, 2 ¦ Winter 1996 | 187 from primates must have occurred on the African continent, one in West Africa and the other in Central Africa [2, 3]. Doolittle approaches an epidemiologic account of the origin of HIVs when he writes that: There is another aspect that is often ignored in discussing these issues: it seems impossible for a retrovirus to remain exogenous and infectious for millions of years and yet retain a recognizable sequence. The rate of sequence change while the virus is under the replicative influence of its reverse transcriptase is overwhelming, and it is this rate that investigators are observing among HIV isolates. What must happen is that infectious retroviruses eventually invade the host germ line and become "endogenized" .... [2, p.389] In addition, Doolittle points out that chimpanzees and human beings share an endogenous retrovirus which occurs at the same multiple points in the genomes of the two species [5], "indicating that the genomic diaspora occurred before the divergence of these species" [2, p.389]. His next hypothesis, which had been inspired by the now well-known pathogenicity of the SIV of sootey mangabeys themselves [6], is particularly bold. He suggests an underlying mechanism to account for the observed relative or complete benignity of exogenous simian immunodeficiency viruses (SIVs) in their primate host, as opposed to their pathogenic potential in nonhost primate species: such "xenotropism" is due to the fact that host species must already carry their species-specific retrovirus in their germline. Doolittle proffers that HIV-2 most likely arose by cross-species transmission of a progenitor virus from sootey mangabeys 20 to 40 years ago and implies that HIV-I arose similarly from either a mandrill or chimpanzee. Critics of the chimpanzee hypothesis assert correctly that HIV- 1—like viruses have been isolated only from two captive, wild-born chimpanzees in Gabon and Zaire [7, 8]. Luc Montagnier explains away such findings as flukes due to the common, if poorly grounded, practice among primate dealers of injecting chimpanzees with human serum to passively protect them from human bacteria and viruses [9]. The two chimpanzees had nonetheless been caged for several years prior to isolation of the HIV-I -like virus and had had no documented prior exposure to...