Dual infections occur more often than realized
Current blood testing procedures are inadequate
Can individuals become infected with more than one HIV-1 strain? For many years, researchers have said dual infections rarely occurred. Even with sophisticated polymerase chain reaction (PCR) cloning, they had difficulty detecting anything but the most closely related viruses in any infected individual.
"Rarely did people find a second strain," says Patricia Fultz, PhD, professor of microbiology at the University of Alabama in Birmingham. "Therefore, it was generally considered that those who were infected with one HIV strain could not be infected by a second. That was curious to a lot of people. To many others, it gave hope that a vaccine might prevent secondary infection."
Counting herself among the curious, Fultz and a team of Alabama researchers have been studying HIV in chimpanzees for a decade, exploring the possibility of a vaccine as prevention from HIV infection.
Their studies have found evidence suggesting that the number of humans infected with two or more HIV-1 strains is probably larger than previously thought.1 The research also supports the theory that, because a large number of recombinant HIV-1 strains exist around the world, multiple dual infections have occurred. At least 10% of all HIV strains isolated are recombinant.
However, the research team said current blood-testing procedures are unable to identify those strains, further complicating the search for a vaccine.
The research involves HIV-infected chimpanzees, many of whom have been infected between one and 10 years, with the majority showing no evidence of disease. The chimpanzees were exposed either intravenously or by a mucosal route to an unrelated HIV-1 strain and then were monitored for increases in antibody titers and plasma viral burdens.
To distinguish the two HIV strains, both universal- and strain-specific primers were used in PCR testing of cellular DNA from blood and lymph nodes. Researchers found it possible to identify the second strain, but only within the first six weeks after exposure to the second strain.
"That indicated that the prior infection was downregulating and bringing under control the second strain that the animal was exposed to, which is what you would like in a vaccine," says Fultz.
In some animals, universal primers — which should pick up any of the subtypes throughout the world — showed no evidence of the second strain. The research group then designed PCR primers that would recognize only the strain that had infected the animal. They found that with few exceptions, every animal was infected with the second virus.
"It means that primers preferentially pick up one strain," says Fultz. "Therefore, previous testing may have shown that people had only one virus, but they probably were infected with two viruses." Because the nucleotide sequences of the HIV-1 strains with which individuals are infected are rarely known, Fultz is not optimistic about detecting secondary strains using available methods.
As for the chimpanzees, researchers are monitoring them to test the dynamics between the two viral strains to see if one strain will predominate over the other for the entire infection; whether the secondary strain will at some point predominate; whether recombinants will be generated; whether a more pathogenic virus is generated; and whether that strain will predominate.
"Some strains are known to replicate extremely well in chimps, and they establish viral loads in the animal that are easily measurable," says Fultz. "Other strains replicate much less efficiently, and often the viral burden in the animal — the amount of virus in their blood — is too low to routinely detect."
The questions they pose: If an animal is infected with a virus that replicates poorly, as with an attenuated virus vaccine, what will happen when it is exposed to a virus that replicates much more efficiently? Will the new virus’ ability to replicate be downregulated? Or will the more efficient virus predominate over time?
"We have noticed that the second virus initially replicates normally, and that the second virus is held in control, which is good. It’s what we want to see in a vaccine. However, we just have to wait. We’ll only know with time whether that second, more efficient virus is going to be held in check for years," says Fultz.
1. Fultz P, Wei Q, Yue L. Are infections of two (or more) HIV-1 strains really rare events? Presented at the 38th Interscience Conference on Antimicrobial Agents and Chemotherapy. Abstract No. I-33. San Diego; September 1998.