by Kathleen Scalise
Besides preventing HIV from killing T-cells, anti-retroviral treatments may increase the body's ability to generate new cells in some patients, according to recent data from a new diagnostic technique developed by Berkeley and UCSF researchers. The technique allows human cells, including T-cells, to be directly traced from the time they are created until they die.
T-cell death versus production is an ongoing debate in the AIDS community. Strategies for preventing or reversing immune deficiency depend on knowing what makes T-cell count fall.
Previously many researchers believed the underlying cause of AIDS, which is immune deficiency, could best be explained by abnormally high rates of cell death that forced the immune system to battle back with high rates of new cell generation. But the new technique has revealed an important role for the T-cell production systems as well. It found T-cell production rates were higher in patients on potent protease inhibitor treatments.
"Our research shows that T-cell counts in AIDS patients on protease inhibitor treatments are closely related to the number of cells their bodies can produce each day," said Marc Hellerstein, associate professor of nutritional sciences at Berkeley and associate professor of medicine at UCSF. "T-cells do have a shorter life-span in AIDS than in healthy people, but the production of new cells is also an important factor in determining T-cell counts."
Hellerstein presented the data Feb. 3 at the fifth Conference on Retroviruses and Opportunistic Infections in Chicago. A description of the new technique was published in the Jan. 20 issue of the Proceedings of the National Academy of Sciences. Joseph M. McCune of the Gladstone Institute of Virology and Immunology at UCSF collaborated in the studies, conducted with AIDS patients at San Francisco General Hospital.
AIDS is caused by the retrovirus HIV. Like many other viruses, HIV enters human cells in order to reproduce, since viruses cannot replicate on their own. It attacks and gains entry to cells that have CD4 receptors on their surface. The most significant of these are helper T lymphocytes, or T-cells, which respond to foreign substances and perform critical functions in warding off infections.
From the time the virus is first detectable, it begins to replicate in T-cells. This replication eventually destroys the cell. However, in early stages of the infection T-cell levels generally remain near normal. This may be because the immune system is still able to churn out new cells to replace those that are destroyed. This period of the disease is the so-called dormant stage since few symptoms are apparent and it can last years.
Eventually, however, T-cells are destroyed more rapidly than the immune system can produce them, causing a drastic reduction in their number. The result is a serious impairment of the immune system and increased susceptibility to opportunistic infections, a hallmark of full-blown AIDS.
The body's T-cell stores in AIDS have been compared to a sink with the drain wide open and the faucet on full force.
"The big question has been, is there a reason to turn up the faucet, if the drain is still wide open?" said Hellerstein. "Well, now we have reasons to believe that the faucet is involved, as well."
Hellerstein and McCune came to this conclusion by tagging T-cells using deuterium, or "heavy" hydrogen, to label DNA in T-cells as they are produced. Deuterium is a non-radioactive, and thus safe, hydrogen isotope double the weight of regular hydrogen. It can only be detected in newly generated DNA by mass spectrometry and affords a very reliable count of the number of new cells generated.
In the past, cell generation could not be directly measured because techniques involved radioactive or toxic chemicals unsuitable for human consumption. The availability of a safe, reliable method for measuring the rate of cell generation in humans is expected to be useful in a number of diseases.
Use of this technique in AIDS patients revealed tremendous individual variability, said Hellerstein. Some patients have an impaired T cell regeneration system, while other patients make extremely high numbers of T cells. Also, some patients responded to protease inhibitor therapy with high T-cell production rates and improvement of T-cell counts, he said, while other patients did not get a boost in their T-cell counts, despite suppressing the virus. These latter individuals were unable to increase T-cell production rates. Still other patients developed resistance of the virus to protease inhibitors, but maintained high T-cell counts. They were found to be producing tremendous numbers of T-cells each day. These different response patterns might represent clinically useful sub-groups of patients, says Hellerstein.
"Identification of patients who have good virologic control but cannot generate more T-cells may suggest the need for immunostimulatory therapies in addition to antiviral drugs, for example," he said. Such therapies might include transplanting immune stem cells or thymic tissue, or administering agents such as interleukin-2.
Berkeley has applied for a patent on the new technique and SpectruMedix Corporation of State College, Pennsylvania, has acquired rights to the technique. Spectru-
Medix is a U.S. company producing medical and scientific technologies and associated instrumentation.
The technique is expected to be useful for treatment of a wide variety of diseases, including AIDS, cancer, heart problems and osteoporosis. It allows physicians to safely and rapidly measure the efficacy of particular drug therapies in individual patients and gives physicians the ability to tailor treatment based on individual response to the drug. The test is not yet commercially available to physicians.