Inner strength: gene therapy aims to build cells that thwart HIV replication

Science News, March 14, 1998 by John Travis

Like an attacking army that has bridged a castle's moat and scaled its walls, the AIDS virus is free to enjoy the spoils of war once it has invaded an immune cell. Infected cells may attempt suicide or try to alert other cells to HIV's presence, but the virus seems to stymie these ploys. Secure in its new home, HIV hijacks the cell's internal machinery and begins to make copies of itself.

While recent combinations of antiviral medications have shown a dramatic ability to curtail HIV replication, they must be taken several times a day, are expensive, can produce undesirable side effects, and don't work for everyone.

"There are now increasing numbers of HIV-infected patients that are failing combination therapies," says Wayne A. Marasco of the Dana Farber Cancer Institute in Boston. One recent study suggested that more than half of all AIDS patients may not benefit from the drug treatments, he notes.

Consequently, investigators like Marasco continue to explore the possibility of building a better immune system, one whose cells safely tolerate the normally deadly intrusion of HIV. In particular, they hope to equip immune cells with various genes that thwart the virus' ability to reproduce.

A decade ago, Nobel prize-winning virologist David Baltimore bestowed the name "intracellular immunization" on this idea of using gene therapy to create HIV-resistant immune cells (SN: 10/1/88, p. 213). He and other investigators imagined periodically infusing people with the hardier cells or even genetically engineering immune cell precursors, so-called stem cells, to permanently reconstitute a person's immune system with cells that check HIV.

"The advantage of intracellular immunization, if it works right, is that it wouldn't require continually taking pills," says Baltimore, now president of the California Institute of Technology in Pasadena and head of the national effort to develop an AIDS vaccine.

Today, with physicians employing the strategy in a number of safety trials with HIV-infected people, intracellular immunization is moving beyond the test-tube proving ground. Furthermore, investigators have been buoyed up by news that one version of intracellular immunization has apparently protected a few macaque monkeys from the ravages of SIV, a monkey virus that closely resembles HIV.

"Intracellular immunization works in the most relevant animal model we have for HIV infection," says study leader Richard A. Morgan of the National Human Genome Research Institute in Bethesda, Md.

The origins of intracellular immunization date to 1988, when investigators first blocked the replication of a virus by providing cells with genes that encode a mutant version of a protein naturally made by the virus. In that study--of a herpesvirus, not HIV--the mutant proteins interfered with the actions of the normal viral protein, apparently by competing for the same cellular targets.

With that result in hand, scientists immediately began speculating about trying the same strategy against HIV. Two obvious targets were Tat and Rev, proteins that are used early in the infection process and are essential to the replication of the AIDS virus. "You shut those two down, you shut down the virus," says Larry A. Couture of Ribozyme Pharmaceuticals in Boulder, Colo.

Several research groups have made progress with a mutant version of Rev that disrupts HIV's attempts at replication without seeming to harm the infected cell. Mutant versions of Tat are also under study.

Other intracellular immunization strategies have also drawn a bead on Tat. Several years ago, inspired by antibodies' talent for binding proteins of bacteria or viruses floating in the bloodstream, Marasco and his colleagues wondered if they could engineer antibodies to perform similar duties inside cells. These so-called intrabodies might home in on bacterial or viral proteins or even on mutant cellular proteins that drive the uncontrolled growth of cancer cells, they proposed.

Normally, immune cells secrete antibodies or display these proteins on their surface. Recently, scientists have grown increasingly adept at shaping antibodies to their needs by mixing and matching the many genes that encode the proteins. Indeed, Marasco and his colleagues found that they could design antibodies which would remain in a cell and even move to specific cellular regions.

Initially, the group created intrabodies that fuse to gp160, a precursor of the protein that makes up HIV's outer envelope. By binding gp160, the intrabodies deprived any new viruses of a protein that plays a crucial role in HIV's ability to attach to cells and infect them. Marasco's group found that the normal AIDS virus is 1,000 times more infectious than the viruses manufactured in cells with the added intrabody gene.

Because those first intrabodies did not actually stem the production of HIV, the scientists shifted their attention to Tat. They have since developed intrabodies that zero in on that crucial protein and virtually arrest HIV replication. Later this year, Marasco plans to start testing this Tat intrabody on HIV-infected people for whom current drug therapies have failed.