A widely publicized theory that human AIDS viruses originated in recent years from related viruses harbored by African green monkeys has been refuted by new data showing distinct differences in the molecular structure of the human and monkey viruses, University of Tokyo researchers reported today.
The new findings lend support to other explanations for the origins of human AIDS viruses. These include their beginnings in common ancestors of humans and primates, their presence in isolated human populations for hundreds or thousands of years, or the existence of a yet-to-be identified prototype AIDS-like virus that first infected humans in modern times.
"Interspecies transmission is unlikely," said the report in the British journal Nature by Masashi Fukasawa, Masanori Hayami and other scientists at the University of Tokyo's Institute of Medical Studies and Japan's National Institute of Health.
"Although interspecies transmission may have occurred in ancient times," the report stated, there appears to have been no transmission between primates "for a long time."
The report presents the first complete gene sequence of an authentic AIDS-like virus isolated from a naturally infected African monkey from Kenya. Parts of the monkey virus sequence differed from typical human AIDS virus sequences by more than 50%.
No Evidence of Ill Effects
Although it is estimated that many African green monkeys are infected with such viruses, there is no evidence that they suffer any ill effects as a result.
According to the report, this "lack of pathogenicity" may be related to the genetic characteristics of the green monkey virus. The researchers determined that it lacks a gene that is found in human AIDS viruses. But because the function of the missing "R" gene is not known, the researchers cannot be certain its absence is why the infected monkeys show no illness.
The report also confirms a Nature article in February showing that earlier "isolates" of AIDS-like viruses from African monkeys in fact resulted from contamination of blood samples in a Harvard School of Public Health laboratory.
Claims made by Myron Essex and Phyllis Kanki, the Harvard researchers, on the basis of their erroneous data played a key role in popularizing the theory of recent cross-species transmission as well as a related theory that a variant human AIDS virus from West Africa did not appear to cause AIDS. The Harvard researchers have acknowledged their mistaken findings.
"We don't know what the origins of the human AIDS viruses are," said Carel Mulder of the University of Massachusetts Medical School, who wrote a commentary on the new data for Nature.
"It is not the African green monkey," he said in a telephone interview. "We don't know of any other monkey virus that is similar enough to the human viruses that it could be their predecessor."
Despite the continued uncertainty about the origins of the human immunodeficiency virus, most researchers state unequivocally that it occurred naturally and spread throughout the world in a silent epidemic in the 1970s before the first AIDS cases were reported in 1981.
Infection in Africa
In addition, most researchers agree that large numbers of Africans became infected with the AIDS virus before it spread as significantly in other parts of the world.
So far, researchers have obtained complete gene sequence data for two major human AIDS viruses. HIV-1, the original AIDS virus, is believed to be responsible for almost all of the tens of thousands of acquired immune deficiency syndrome cases that have occurred worldwide.
HIV-2, a variant AIDS virus first detected in West Africa, has a gene sequence about 40% different from HIV-1. HIV-2 also appears to cause diseases similar to those caused by the original AIDS virus, but the relative pathogenicity of the two human viruses is a subject of scientific dispute.
Researchers also have gene sequence data confirming the existence of two AIDS-like viruses from monkeys, which are known as simian immunodeficiency viruses.
One is the African green monkey virus, which is described in the current issue of Nature. The other was isolated in 1984 from Asian macaques kept at the New England Regional Primate Research Center in Southborough, Mass. This virus apparently contaminated green monkey and human blood samples at the Harvard School of Public Health laboratory where macaque specimens were also being studied.
The macaque virus has many similarities to HIV-2. But its origins remain a mystery and it may be "an artifact of captivity acquired from another species," according to Dr. Ronald C. Desrosiers, the leader of the primate center research team who sequenced the virus. This is because the virus has only been found in Asian macaques born in primate centers in the United States and has never been detected in Asian monkeys caught in the wild.
In the months ahead, Desrosiers and others expect that the existence of additional AIDS-like viruses from primates will be confirmed.
For example, Hayami's research group in Tokyo has sequence data on another simian immunodeficiency virus from a West African baboon known as a mandrill that has been submitted for publication. And a yet-to-be-identified research team is attempting to isolate an AIDS-like virus from chimpanzees from Gabon, according to Dr. Luc Montagnier of the Pasteur Institute in Paris.