`Shock and kill` approach awakens latent HIV in test tube

Keith Alcorn
Published: 05 June 2009

Cells harbouring latent HIV can be shocked into expressing virus in the test tube using a class of drugs previously thought to be too toxic for large-scale use, suggesting the possibility of more rapid progress towards human studies of HIV eradication, according to Italian researchers in this month’s edition of Retrovirology.

Dr. Enrico Garaci, president of the Istituto Superiore di Sanità (the Italian Institute of Health) and Dr. Andrea Savarino, a retrovirologist working at the institution, worked with a team of researchers to study the so-called "barrier of latency" which has been the main obstacle to HIV eradication from the body.

HIV’s genome is incorporated into cells but a small proportion of infected cells remain unactivated, ensuring that the genome remains latent. When the cell is activated new virions are produced, triggering a new round of HIV infection of cells.

The persistence of latently infected cells is widely assumed to be the major barrier to curing HIV infection, although scientists still need to identify all the cell types and body tissues in which HIV might remain latent.

Methods of purging the reservoir of latent cells are still being debated, and there is concern that most approaches could be toxic and therefore risky to study in people otherwise doing well on HIV treatment.

One mechanism by which latency is known to be maintained is the presence of histone deacetylases (HDACs) within cells. These enzymes can be inhibited by a class of compounds called HDAC inhibitors, some of which are used in cancer treatment. However HDACs are found widely within the human body, regulating cell growth and death, so their inhibition can lead to a range of toxicities.

The researchers looked at 32 types of HDAC inhibitors belonging to a class that acts on a small group of enzymes (class I HDACs). They found that at non-toxic quantities, class I HDAC inhibitors could cause some of the latently infected cells to awaken.

They repeated the experiment and added a drug that reduces levels of cellular glutathione, called 'buthionine sulfoximine' (BSO). This drug enabled the class I HDAC inhibitors to act on more (but not all) of the latently infected cells. The 'awakened' infected cells then died out while the non-infected cells remained intact, despite being subjected to the combination of BSO and HDAC inhibitors.

Previous research using valproic acid, another HDAC inhibitor used in the treatment of mood disorders, showed that it did not have a great effect on the latent reservoir in humans.

The Italian researchers believe that buthionine sulfoxamine may be a useful additive to an HDAC inhibitor because it lowers levels of glutathione, creating oxidative stress within the cells that contributes to viral transcription.

Furthermore, as the drug and HIV replication deplete cellular glutathione, it moves the cell towards a state in which it is more likely to self-destruct, thus removing the virus-producing cell from the reservoir.

"I really hope this study may open new avenues to the development of weapons able to eliminate the HIV-infected cells from the body", said Dr. Andrea Savarino.

"Such weapons, in combination with antiretroviral therapies, could hopefully allow people living with HIV/AIDS to get rid of the virus and return to a normal life. Of note, there are testable drug combinations composed of molecules that have passed phase I clinical trials for safety in humans".

This type of approach has been dubbed 'shock and kill'. "Although this type of approach is largely accepted by the scientific community", adds Dr. Savarino, "to be honest, we have to take into consideration that some scientists are sceptical about this approach, and others even think that a cure for HIV/AIDS will never be found. Experiments using animal models will shed a new light on this difficult problem."

While the results are promising, the researchers point out that the possible efficacy of the shock-and-kill approach remains a matter of debate. For example, they cite recent findings suggesting that "there are different cellular reservoirs for HIV-1 latency and that each reservoir may require a specific activation strategy," and the fact that "viral factors, along with cellular factors, may contribute to HIV-1 quiescence, and these factors may not be controlled by strategies using HDAC [inhibitors]."

Reference

Savarino A et al. “Shock and kill” effects of class-1-selective histone deacetylase inhibitors in combination with the glutathione synthesis inhibitor buthionine sulfoxamine in cell line models for HIV-1 quiescence. Retrovirology 6: 52, 2009.

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