Aug. 27, 2003 Three Johns Hopkins researchers propose, for the first time, that HIV and other retroviruses can use a Trojan horse style of infection, taking advantage of a cloak of human proteins to sneak into cells.
The hypothesis explains 20 years of perplexing observations and suggests new ways to reduce HIV transmission and treat HIV infection, but it also implies that existing approaches to developing vaccines against HIV won't work. A description of the hypothesis and its supporting evidence appear in the Proceedings of the National Academy of Sciences, scheduled for publication online this week.
"Most researchers have focused on viral proteins when trying to understand HIV's mechanisms or develop vaccines," says James Hildreth, M.D., Ph.D., professor of pharmacology and molecular sciences in Hopkins' Institute for Basic Biomedical Sciences. "But so many aspects of retroviral biology have not been reconciled, including HIV, that we have to take a broader view. If our hypothesis is true and retroviruses can rely on human proteins, vaccines based solely on a few key viral proteins will never be able to completely prevent infection. There needs to be serious attention to this hypothesis."
Even if a vaccine against the viral proteins physically blocks a retrovirus's primary way of infecting cells, the retrovirus's ability to enter new cells by way of its cover of human proteins -- the Trojan horse -- provides previously unrecognized ways to escape the vaccine's effects, says Stephen Gould, Ph.D., professor of biological chemistry in the Institute for Basic Biomedical Sciences.
To go from cell to cell, all retroviruses are packaged in "envelopes" made from viral proteins and proteins from human cell membranes. The prevailing view is that the viral proteins do all the work to enter new cells, and the human proteins are just along for the ride. But the Hopkins team suggests that sometimes the viral proteins take the back seat, and the retrovirus relies instead on the cells' own mechanism for shuttling molecules from one cell to another.
"New hypotheses are frequently huge jumps from current thinking, that then occasionally turn out to be true. This is not one of those times," says Gould. "This hypothesis links what is known about how molecules are transported within and between cells and a great deal of what is known about HIV and other retroviruses. When the pieces are put together, it's such an obvious connection. The biggest surprise is that the idea hasn't been widely discussed before."
Researchers elsewhere, for example, have shown that a version of HIV completely missing its key envelope protein can still infect cells in the laboratory, strong support for the Trojan horse effect.
"Despite that observation being 'impossible' under the prevailing view of how HIV gets into cells, people have said that this little bit of infection can't be important," says Gould, an expert on cellular transport vehicles. "But just because something isn't big, doesn't mean it's not important -- this little bit of infection offers the retrovirus a chance to survive, mutate and thrive in infected people. In general, our hypothesis makes HIV appear nastier than we think it is, and we already think it's a pretty nasty virus."
But all is not lost, the Hopkins team says. The new hypothesis, and some quirky observations from the past, highlight the potential of targeting immune responses against the human proteins in the virus's envelope, instead of the viral proteins, as a way to prevent infection.
Each person's immune system innately "knows" to attack and destroy tissue from other people, a characteristic reflected in the need to have appropriate blood and organ "matches" in transfusions and transplants. The human proteins that elicit these immune responses are among those found in the viral envelope.
The researchers suggest that heightening this immune response by vaccinating people with small amounts of these human proteins (called "alloimmunization") could be a very cost-effective way to reduce the rate of new HIV infections, especially in developing countries. The immune system would immediately attack the viral envelope, and the virus would be degraded before the person's own cell's could become infected.
"Unlike current vaccine approaches, which target particular viral proteins, this new vaccination strategy has the decided advantage of working against all strains of HIV as well as against other retroviruses," says Gould.
"Harnessing this immune response may be the only near-term prospect we have to reduce the rate of new HIV infections," says Hildreth. "There's lots of evidence supporting the idea of alloimmunization to fight HIV transmission. We need to push for it and push hard, especially since this could be done in developing countries today."
Already, some clinical reports indicate that people whose tissue and blood types don't match are less likely to infect one another with HIV or other retroviruses. However, a person who is a "good" tissue match for his or her infected partner is more likely to become infected. Unfortunately, the proposed vaccine strategy might not help protect people from their rare "good" matches.
The proposed Trojan horse mode of infection stemmed from trying to explain a number of observations in their respective labs that couldn't be explained by existing models of HIV biology. However, these as-yet unpublished findings can easily be explained if HIV can act like an exosome, tiny pockets made from cell membranes that are used to send molecules between cells.
Gould, an expert in the basic biology of the cell, Hildreth, an expert on HIV's biology, and graduate student Amy Booth also reviewed decades of scientific reports about the viral envelope or "particle" -- the way retroviruses package themselves -- and more recent discoveries about exosomes.
The retroviral particles and exosomes contain the same human proteins, are built by the same machinery and have the same job -- transferring payloads from one cell to another. With these and other striking similarities, the researchers suggest retroviral particles are treated like regular exosomes. In this way, the retroviral particle is protected from immune attack and able to enter cells throughout the body.
"This 'Trojan exosome hypothesis' explains why retroviruses carry the human proteins they do, why they are able to survive even in people with healthy immune systems, and why traditional approaches have failed to generate an effective HIV vaccine," says Gould. "The evolutionary implications are similarly revealing, both for how retroviruses evolved and why animals possess intense tissue rejection responses."
Adds Hildreth: "Reexamining previous experiments and analyzing new results in the context of this hypothesis have the potential to revolutionize what we know about retroviruses and what we can do to fight the spread of HIV and other retroviruses."
The researchers note that the immunization method suggested by their hypothesis and others' observations is already performed safely to alleviate some instances of infertility, reducing the hurdles to testing its applicability in HIV prevention.
The work was supported by the National Institutes of Health and the Johns Hopkins Fund for Medical Discovery.
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