Chimp Subspecies Identified as Suspected Source of Human HIV Virus
The HIV virus that causes a large majority of AIDS deaths comes from a particular subspecies of chimpanzees that lives in Southern Cameroon in sub-Saharan Africa, according to a leading AIDS researcher who presented the Merle Sande Memorial Lecture at the J. David Gladstone Institutes building at Mission Bay on Monday.
Beatrice Hahn, MD, professor of medicine and microbiology and co-director of the Center for AIDS Research at the University of Alabama, Birmingham, delivered the lecture, a kick-off for UCSF’s observance of World AIDS Day.
The origin of the HIV virus and the AIDS pandemic it has caused still remains somewhat of a mystery. The source of HIV has been hotly debated ever since the viral cause of the disease was first demonstrated conclusively in 1984.
Some even speculated that government studies of African monkey viruses – part of a biological warfare defense research – were to blame. The HIV virus has been identified in one stored blood sample dating to 1959, and in another from 1960.
Another idea that became popular was that polio vaccination programs conducted in Africa beginning in the late 1950s might have used vaccine prepared using cells from primates, and that these vaccines could have been a source for viral contamination. The World Health Organization later dismissed this idea, concluding that studies of old vaccine preparations revealed no evidence of HIV contamination.
The lecture presented this year by Hahn honors a leader in AIDS research and clinical care. Merle Sande, MD, was the former chief of medicine at San Francisco General Hospital, where he was instrumental in establishing the world’s first unit dedicated to the care of patients with AIDS. Sande, along with other UCSF physicians, played a central role in establishing a model for care and treatment in the early days of the epidemic, before a viral cause had been identified for what then was a nearly inevitably fatal disease.
Sande also was a driving force behind the establishment of the UCSF-affiliated Gladstone Institute of Virology and Immunology, and also for the creation of a major AIDS training, research and treatment center in Kampala, Uganda, called the Infectious Diseases Institute.
The lecture honoring Sande was established by friends and UCSF colleagues when Sande left UCSF to return to his native state and join the University of Washington faculty. The lecture was renamed after Sande died as a result of cancer in 2007.
The event marked the seventh presentation of the Merle Sande Memorial Lecture, and was sponsored by the Gladstone Institute of Virology and Immunology, the UCSF Center for AIDS Research, and San Francisco General Hospital Foundation.
Beatrice Hahn
Primate Roots
In her talk, titled “The Prehistory of HIV-1: Understanding the Primate Roots of Human AIDS,” Hahn presented evidence to pinpoint the chimp origins of human HIV, based on analysis of genetic code from a related family of ape and monkey viruses called SIV. Assisted by former Gladstone postdoctoral fellow Mario Santiago, PhD, Hahn and her lab group worked in African parks across central sub-Saharan Africa, obtaining genetic material from the urine and feces of wild chimpanzees and gorillas, as well as from some chimp groups accustomed to the presence of humans. “There are over 40 species of primates in sub-Saharan Africa that are naturally infected with what is called SIV, or Simian Immunodeficiency Virus,” That’s a misnomer, Hahn said, because in general the virus does not cause immunodeficiency in its primate hosts. Hahn described a surprising, newly discovered exception toward the end of her talk. Hahn and other researchers measure degrees of differences in the genetic codes of the different forms of SIV viruses – or in bits of the virus that have become embedded in the primate’s own genome – to estimate when different forms arose in evolutionary time. For instance, Hahn suggested that there is evidence that SIV may have been present in lemurs many millions of years ago. However, there is no such lineage to mark the evolution of human HIV viruses, Hahn says. “They are the result of cross-species transmission,” Hahn said, something that has been known for more than a decade. But Hahn presented new evidence pointing to a specific population of chimps from which the virus causing the human epidemic could have arisen. The virus first evolved perhaps a century ago, she said. “Chimpanzees acquired their virus, SIVcpz, because they are preying on smaller monkeys, and they have a virus that is a mosaic of viruses.” This mosaic includes large splices of genetic code from SIV viruses common to two different groups of monkeys. Only two subspecies of chimps are naturally infected with this type of virus, she said, and only one subspecies has served as a host for the form of the virus that has been transmitted to gorillas and to humans in the form of HIV1, the most common and deadly form. Roughly six percent of chimps in the southern Cameroon region harbored the virus most closely resembling human HIV1. Evidence that SIV has been present at what may be a fairly steady prevalence for many years in chimps, and that the form that causes most human AIDS arose a century ago, raises the question as to why the human epidemic appeared to take off only in the early 1980s.Transmission to Humans
What caused cross-species transmission to humans? “We don’t know,” Hahn said. “We speculate that the most likely scenario is exposure to infected chimpanzee blood or mucosal secretions in the context of bush meat hunting.” That does not explain the rapid spread of the virus, however. Hahn was willing to speculate, as others have, that changes in social behaviors among humans, such as concentration in urban areas, or changes in sexual health or sexual practices, may have contributed to an environment more conducive to spreading the virus in humans. She also noted that apart from polio vaccine, other vaccines were administered to many millions of Africans in the 1940s and 1950s, and she did not rule out the possibility that these may have contributed to the establishment of the virus in humans. Hahn also reported the striking finding that, contrary to what has long been thought, chimpanzees afflicted with SIV do in fact become sick. Her recent studies of chimps in Gombe National Park in Tanzania indicate that over many years infected chimps are much more likely to die, to fail to give birth, or to bear young that die before age 1. Preliminary research also points to immunological abnormalities similar to what is observed in human AIDS, Hahn said.Merle Sande
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