Across the hall in the department of oncology, her colleagues are trying to inhibit telomerase, whose activity in cancer cells contributes to those cells' unwanted longevity.

But in research published in the Nov. 15, 2004, issue of the Journal of Immunology, Rita Effros' group at the University of California at Los Angeles School of Medicine reports that for killer T cells, the opposite strategy - turning up their telomerase activity - may help them keep HIV in check.

Current HIV therapies are by and large focused on the viral end of things. They interfere with viral replication at several stages of its life cycle, or with viral fusion to host cells. That approach has been instrumental in converting HIV from the inexorable death sentence it once was to more of a chronic disease that can be managed, sometimes for decades. But there is an increasing realization in the medical community that in addition to keeping the viral load low, HIV infection can be kept under control through enhancing the immune system.

The paper, titled "Genetic Manipulation of Telomerase in HIV-Specific CD8+ T Cells: Enhanced Antiviral Functions Accompany the Increased Proliferative Potential and Telomere Length Stabilization," reports on one possible strategy to enhance the immune response to HIV, specifically of cytotoxic "killer" T cells. Other cell types, including helper T cells and B cells, also play a role in the immune response to HIV infection. But killer T cells do most of the heavy lifting, inhibiting viral replication and killing infected cells.

Differentiated cells are able to spawn a limited number of generations; sooner or later, they enter what's known as replicative senescence, losing their ability to divide and undergoing apoptosis. It's usually a good thing, too.

"Evolutionarily, replicative senescence may have arisen as a barrier against tumor formation," Effros, professor of pathology and laboratory medicine and senior author of the paper, told BioWorld Today. "But in chronic infections, you need those cells to keep dividing."

Shortened telomeres are one important feature that signals to a cell that it is getting old, and so Effros and her group decided to investigate whether lengthening telomeres by providing extra telomerase would enable killer T cells to be effective for longer in controlling the virus. In contrast to many other cell types, killer T cells do show a certain degree of endogenous telomerase activity, but such activity is lost with repeated stimulation by immunogens. Asked why the inserted gene does not simply lose its activity, as the T cells' own telomerase does, Effros pointed out that it is inserted in a different chromosomal location than the cells' own telomerase gene, possibly allowing it to escape cellular control mechanisms.

The researchers began by isolating peripheral blood mononuclear cells, a subset of white blood cells, from HIV-positive patients. Stimulating that cell mixture with HIV-derived epitopes enriched those cell mixtures for killer T cells. They then transfected the cultured T cells with the human telomerase gene (hTERT). The hTERT and control constructs were supplied by Geron Corp., of Menlo Park, Calif, which also partly funded the studies reported here through the Industry-University Cooperative Research Program, a University of California matching funds program. Geron itself has programs investigating the use of telomerase inhibitors for the treatment of cancer, as well as a telomerase vaccine currently in Phase I/II trials at Duke University.

Given telomerase's known role in cancer, the scientists checked the cells for possible malignancy-like transformations, but found none. Effros said "this agrees with what we know about tumor cells - they have a lot of mutations."

If 80 percent of success is indeed just showing up, then hTERT cells have an advantage right off the bat: they keep showing up for much longer than their sham-transfected brethren. While the control cells were able to keep dividing for about 25 to 30 generations in culture, hTERT cells were up to generation 66 at the time the manuscript was submitted, and were still dividing. Investigations into the mechanisms of this increase showed that hTERT transfection cells slowed telomere shortening, reduced expression of certain kinases that inhibit cell division, and under certain circumstances, increased expression of CD28, a cellular division signal.

Two Out Of Three Success Rate For Viral Control

In vitro, transfected cells showed some characteristics that suggest they could be more effective at controlling HIV. Transduced T cells inhibited viral replication more strongly than controls. Telomerase also enabled transduced cells to produce cytokines in response to antigenic stimulation, though eventually, the hTERT cells lost that ability as well. Their name notwithstanding, killer T cells eventually lose the ability to kill infected B lymphocytes, and hTERT did not prevent that; transfected T cells were no better than control cells at breaking up infected B cells.

Effros believes that if the approach pans out clinically, gene therapy ultimately will not be the method of choice for treating HIV-infected individuals. Instead, she thinks that small-molecule telomerase enhancers will prove to be a more practical approach. Her group is currently testing such enhancers.

The potential uses of telomerase enhancers are not limited to HIV. "Wherever T cells run out of steam, this approach will be applicable," Effros said. She named age-related diseases as an area of particular interest, saying that replicative senescence of killer T cells is thought to contribute to older people's increased susceptibility to infections.