January 1998 Volume 1, Number 3-4, Section 1
Newly Uncovered Viral Reservoir
In a recently published article in Science, Dr Robert Siliciano of Johns Hopkins University discussed his findings of a viral reservoir of post integrated proviral DNA persisting in resting CD4 memory t-cells. These cells are in a reversible state of non-productive infection but can be capable of producing infectious virus if stimulated by antigen. He said that the frequency of these cells and their decay rate are important to evaluating the effect on the potential for eradication.
Using a sensitive assay he found the frequency of these cells to be extremely low. The replication competent integrated proviral DNA was found in less than .01% of resting CD4+ t-cells. He took a lymph node biopsy of 14 individuals and found that the frequency of these cells are not different in the blood and the lymph node. This is important because it may allow researchers to make implications about lymph tissue proviral DNA just by taking a peripheral blood sample. Lymph tissue biopsies can be difficult to perform. He ran an independent assay, a virus culture, because there was thinking that all the DNA was not competent to reproduce virus. His findings were confirmed that some DNA is competent to reproduce virus.
In order to determine the decay rate of these cells, Siliciano entered into a collaboration with clinical researchers to see if this reservoir persists in individuals who have been on long term highly active antiretroviral therapy (HAART). For this study, he selected patients: who were likely to maintain a high level of compliance to the regimens; who were on three or four drugs at least one of which was a protease inhibitor; who had a rapid response to therapy (who became undetectable within 2-3 months); and, who remained undetectable (200 copies/ml) with multiple measurements for the period of study including the times for which they took blood to look for latently-infected cells.
Replication competent proviral DNA was found in resting CD4+ T-cells in essentially all the individuals (18/18), including those who were on therapy for up to 30 months, and it was found that it can persist for a while. In four patients they were unable to get enough resting CD4 T-cells to conduct proper tests. Unfortunately, he concluded that the frequency of these cells in the study individuals was not reduced by being on therapy for a longer period of time. The frequency for those who were on therapy the longest was not much different for those individuals who were on therapy for a short period of time. But, he only took one blood sample from each person. So, he was unable to observe a change in the amount of HIV DNA for an individual. You have to take two blood samples over time to observe a change. He believes that we will have to conduct studies following individuals over time to try and determine the decay rates of these cells.
For comparison sake, it has been generally accepted that free virus only persists for minutes or hours. Cells that produce most of the virus last only a day or two. Cells with unintegrated DNA retain the ability to produce virus for only a few days and extracellular virus particles bound to follicular dendretic cells (in the lymph tissue) last for about two weeks. Although we have less information on chronically-infected macrophages, their halflife appears to be about two weeks. The halflife of resting CD4+ T-cells with integrated proviral DNA could be 5 to 7 months. This is consistent with the fact that these are memory T-cells and their biological function is to persist. Some researchers have said that memory cells can last for a number of years.
However, after being on HAART for a period of time, an individual may experience a partial restoration of the immune system. If the CD4 cells increase appreciably in conjunction with some immune restoration it is possible that the immune system could control virus that might be produced by the activation of these resting cells. It is also possible that the therapy itself might control a burst of viral replication that could occur from activation of this latent virus. This can only be determined by following individuals over time.
David Ho and the ACTG are planning to conduct studies of strategies designed to target these CD4 memory cells harboring latent virus. Stay tuned.
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