HIV/Host Interaction Elucidated

SETTING THE STAGE

Two reports in 1998 by Malim's group⁵ and David Kabat's group at the Oregon Health and Science University in Portland⁶ were the first to describe an anti-HIV cellular phenotype, explains Malim. "We and Kabat's group had shown [that] the reason Vif was essential was most likely because it inhibited a natural antiviral mechanism that was in human cells," he says.

A puzzling feature of HIV infection lay in the susceptibility of some cells, but not others, to HIV propagation. Without Vif, the HIV virus is rendered noninfectious in non-permissive human T lymphocytes (the main target for HIV-1). Permissive cell types, on the other hand, support production of HIV virus even without Vif. This observation, says Malim, led to a model proposing that nonpermissive cells possess an antiviral mechanism normally suppressed by Vif.

Ann Sheehy, a postdoctoral fellow, set out to find that mechanism. "We devised a strategy to identify what those genes might be, based on differential expression and screening," says Malim. They selected two closely related cell lines, whose primary difference was that Vif was required for HIV infection in one line but not the other. They sifted through candidates until they came upon a protein that, when expressed, could inhibit the replication of Vif-deficient HIV-I. "The clincher experiment was to take different cell lines and infect with δvif," says Jim Hoxie, a virologist at the University of Pennsylvania. Permissive cells could make the virus just fine. "But if you give [the cell] this gene, now you could shut the door and prevent that cell from making virus," he explains. They called the gene CEM15 after the human T-cell line, CEM, and the fifteenth clone screened.

Not only did they identify the CEM15 protein as the human cell target, but they also were the first to reveal that CEM15 bore more than a passing resemblance to mRNA-editing enzymes called cytosine deaminases, which convert cytidines to uridines and are known as APOBECs. The CEM15 protein would subsequently become known as APOBEC3G.

"Vif is one example of how, in humans, a virus got past our molecular defense mechanisms," explains Pomerantz. As it turns out, the permissive cells have lost their expression of APOBEC3G, accounting for their ability to support the production of infectious virus, regardless of whether Vif is present.

A year later, in 2003, the three groups described how APOBEC3G destroys HIV replication through hypermutation of the viral genome. Robert Doms, a virologist at the University of Pennsylvania, says the effects are extensive. "The virus is structurally wild type. It looks and smells like normal HIV, but it peters out over time because it has been hobbled by all of these mutations ... when added to cells it would not establish a productive infection."

The impact of these papers, says Trono, was this realization that cellular mechanisms are in place that exist to gum up the works of invading viruses, effectively protecting us against infection via this previously unidentified mechanism.

Wes Sundquist at the University of Utah writes via E-mail: "The series of papers from the Malim, Zhang/Pomerantz, and Trono labs explained these observations beautifully by demonstrating that cells possess an innate antiviral defense mechanism that can actually act to 'recode' retroviral genomes as they are being reverse-transcribed."

HIV FIGHTS BACK

Courtesy of Michael Malim

Vif (green) binds to APOBEC3G or APOBEC3F (orange) and bridges an interaction with a cullin5 SCF-like E3 ubiquitin ligase complex. This results in polyubiquitination of APOBEC proteins and subsequent proteolytic degradation by the proteasome.

Subsequent studies now reveal how Vif circumvents this host defense mechanism. The mechanism involves Vif binding to APOBEC3G, sending the latter to the protein scrap yard, the proteasome.⁷⁸ "HIV can replicate in humans because its countermeasure, [Vif], gets APOBEC3G out of the way. Vif interacts with APOBEC3G and promotes its degradation," says Stevenson.

According to Doms, key issues remain unsolved: elucidating the normal function of APOBEC3G and its role in the innate immune system, and whether other viruses might be subject to this genetic regulation. Zhang says much remains to be done. "In vitro, you can't get Vif and APOBEC3G to bind." Though he suggests that RNA "may be the middleman," Zhang says his group's major goal is to learn how Vif binds to APOBEC3G and triggers degradation.

Trono and colleagues recently showed that APOBEC3G is the reason why 95% of individuals infected with hepatitis B virus manage to escape chronic infection with the virus.⁹ Furthermore, Joe Sodroski and colleagues at Harvard University recently described another cellular defense factor, called TRIM5α, which acts as a negative cofactor for simian and human immunodeficiency viruses.¹⁰ "There are likelyothers," says Stevenson.

Some are investigating the therapeutic potential of such interactions. "We're trying to build a vaccine, but cells have their own factors that are more potent than anything the immune system can throw at viruses," says Stevenson. Such an approach could tip the balance in favor of the cellular defense mechanisms by blocking mechanisms such as Vif, thereby allowing cellular cofactors tomanifest their activity and block the activity of the virus, he explains.

Unlike existing anti-HIV therapies that are aimed at envelope proteins and enzymes, no drug is yet available that targets a regulatory gene, says Pomerantz. But he adds, Vif could be an attractive target: "When you knock out Vif you have a dead virus."

Identifying such a crucial virus-host interaction raises the possibility of new therapeutic strategies, not only for HIV but also against other viruses as well. "Once you know how something works, now you can start talking about intervention, and that means drugs,"says Hoxie. "It's as elegant as it gets in science, to see something like this."