Current combination drug therapies against HIV have proven successful in dramatically lowering levels of the virus in the body, sometimes to undetectable levels. Several recent studies, however, have shown that the virus is not completely eradicated by these treatments, so that patients cannot be considered cured of their infection and must continue on therapy indefinitely. New approaches to treating HIV infection, especially those with the potential to cure, continue to be needed.
Now, researchers at the University of Pennsylvania Medical Center report development of a novel gene-therapy strategy to specifically target cells infected with HIV for attack. The new tactic commandeers the molecular tools that HIV uses to enter and infect cells and employs these against the virus as part of a system to deliver therapeutic genes to only those cells harboring the virus. Additionally, the technique is broadly applicable and could be used against many other viruses that infect using mechanisms similar to that of HIV. The Penn team's findings appear in the November 21 issue of Science.
"Viruses carry molecules on their envelopes that help them bind to receptors on the cell types they infect," explains James A. Hoxie, MD, a professor of medicine and senior author on the study. "We've switched things around, putting the relevant receptors on a retroviral vector that is able then to target the cells infected by HIV."
In the first step of infection by HIV, viral envelope proteins bind to two cell-surface receptors -- to the long-known CD4 receptor and to one of several new receptors discovered only last year, depending on the cell type being attacked. The most important of these so-called coreceptors are CCR5 and CXCR4. CCR5 is used, with CD4, by the strains of HIV that infect macrophages in the earliest stages of infection, also known as the M-tropic viruses. CXCR4 is used -- again with CD4 -- by the T-tropic viruses, those that infect T cells in the later stages of infection and are associated with progression to disease.
After binding and subsequent entry into an immune cell, HIV incorporates its genetic material into the infected cell's genome, which then begins to produce viral proteins along with its own. As is the case with all cells, bits of the complete repertoire of proteins produced inside the cell are displayed on the outside surface of the cell. It is this fact that the Penn scientists were able to turn to their advantage.
The researchers coated a replication-defective retroviral vector with the receptor complexes usually found on the cells that are HIV's prey, hypothesizing that the vector would bind to the viral proteins displayed on infected cells. By incorporating reporter genes into the vector to track its progress, the investigators were able to demonstrate that a vector presenting CD4 and CCR5 receptors successfully located and entered cells infected by M-tropic viruses, and a vector bearing CD4 and CXCR4 did the same for cells infected by T-tropic viruses. Further, neither vector infected cells other than their intended targets.
"People have developed a number of genetic constructs that could be used to block HIV replication," notes Michael J. Endres, PhD, a postdoctoral researcher in Hoxie's laboratory and lead author on the Science study. "But the problem has been that no one has had a way to effectively reach infected cells with this material -- and that's what this is about. One can imagine incorporating one or more of these anti-viral constructs into this new type of vector for therapeutic purposes."
In addition to Hoxie and Endres, the remaining coauthors, all Penn-based, are Salman Jaffer, Beth Haggarty, Julie D. Turner, Benjamin J. Doranz, Peter O'Brien, and Dennis L. Kolson, MD, PhD, an assistant professor of neurology and microbiology.
Research and training grants in support of this work were provided by the National Institutes of Health. Doranz is also the recipient of a predoctoral fellowship from the Howard Hughes Medical Institute.
The University of Pennsylvania Medical Center's sponsored research ranks fifth in the United States, based on grant support from the National Institutes of Health, the primary funder of biomedical research in the nation -- $149 million in federal fiscal year 1996. In addition, for the second consecutive year, the institution posted the highest growth rate in its research activity -- 9.1 percent -- of the top ten U.S. academic medical centers during the same period. News releases from the University of Pennsylvania Medical Center are available to reporters by direct e-mail, fax, or U.S. mail, upon request. They are also posted electronically to the medical center's home page (http://www.med.upenn.edu) and to EurekAlert! (http://www.eurekalert.org), an Internet resource sponsored by the American Association for the Advancement of Science.
Journal
Science