Turning HIV’s power against itself may help target hidden virus – another step towards a cure

Researchers used selected molecules to make human cells less tolerant of damage, so that reactivating hidden HIV becomes a clear trigger for cell death. While making cells more vulnerable to dying may sound counterintuitive, the strategy ensures that cells harbouring HIV are eliminated, removing the hidden virus they contain. This in turn means there potentially will be no viable virus left to re-do the spreading all over again in the absence of treatment.

Although the approach is still at a very early pre-clinical stage and represents a proof of concept – in other words, a preliminary test to show the idea can work – it may be the missing step in the ‘shock-and-kill’ cure strategies. Findings were published by Dr Min Li of the Houston Methodist Research Institute and colleagues in The Journal of Infectious Diseases.

Despite effective treatment a portion of the virus remains in hiding in various parts of the body that are known as reservoirs. Many cure strategies try to eliminate these reservoirs which would otherwise become the source of new rounds of infection once treatment is paused.

The virus uses its own proteins to put these reservoir cells into a state of deep sleep where the cells can survive for decades. Not only that, but the virus also modifies these cells so that they are more resistant to programmed cell death (apoptosis).

Apoptosis is a key component of healthy cell aging and turnover in our bodies. It gets activated once the cell senses it is too old or damaged to continue living. Usually, having a virus would be the ideal signal for the cell to pull the suicide switch, ensuring it clears itself from the body without harming any other cell.

Often cells use another approach called autophagy to ‘digest’ damaged components and reuse their building blocks to build new, healthier components. When cellular damage is minor, recycling damaged parts via autophagy preserves the cell rather than destroying it entirely through apoptosis. But when HIV is present, the recycling process makes these cells more resilient, allowing them to survive longer while carrying intact virus that can spread again to healthy cells.

The study

What the Texan researchers did was to block autophagy and also block cellular pathways that make the cells less likely to undergo apoptosis. In theory, this would make all cells containing intact virus undergo apoptosis upon HIV’s activation in them, as even the small damage caused by the virus’ proteins would be enough to trigger programmed cell death. These cells undergoing apoptosis would mean that the last bits of intact HIV left in hiding would also be flushed out, leaving no viable virus to ‘restart’ the infection.

The experiment was done on humanised mice (mice whose immune system is engineered to mimic that of humans) and then repeated in a lab dish on human immune cells that had been harvested from the blood of people with HIV. In both cases, the special treatment led to a lack of viral activity upon stopping antiretroviral therapy (ART), and in the case of mice there was no viral rebound.

The approach close-up

The experiment used four different drugs in order to address each required step of the ‘shock and kill’ approach. A compound called ABT-263 was used to dampen the cellular signals that stop or delay apoptosis, thus making the cell more prone to dying upon damage. Another molecule called SAR405 was used to block autophagy, further lowering the cell’s ability to counter apoptosis. Two other molecules known as latency reversing agents were added to the cocktail. Their role is to wake up the dormant HIV, speeding up the process as otherwise the virus may stay in hiding for years; it is essential for the virus to reactivate and cause some cell damage in order for this strategy to work.

By design this would work best in cells containing only intact virus capable of reactivating and producing new rounds of infection; this is about three percent of all the virus in cells as the rest are defective. Cells with defective virus wouldn’t be targeted by this approach (unless the defective virus can still express cell-damaging genes) as defective viruses often lack critical genes required for reactivating, spreading and targeting new cells. Since they wouldn’t cause cell damage, the cells harbouring them wouldn’t undergo apoptosis.

Both the mice and the cells were infected with HIV to establish the reservoirs. Then they were put on ART which lowered their viral loads to undetectable levels. In the experimental group, the four drugs described above were added alongside ART as part of the selective elimination of cells with intact HIV. Two ART drugs (raltegravir and fostemsavir) were kept in the mix to stop new infections while the hidden virus was being reactivated. After the treatment period, all drugs were stopped. In the humanised mice, there was an eight-week observational period where the researchers looked for viral rebound. For the human immune cells cultured in the lab dish, just one measurement was taken right after the treatment ended.

For eight weeks after stopping the shock-and-kill treatment and ART, 69% of mice showed no signs of viral rebound while all mice that were on ART alone experienced viral rebound. Those mice that experienced rebound despite the treatment were found to have intact virus sequences detectable in their cells, while none of the mice without rebound had detectable intact virus in their immune cells from their spleen or brain tissues – two known reservoir sites. On the other hand, the proportion of cells harbouring defective virus was comparable in all mice – those treated with ART alone and those treated with the experimental cocktail regardless of experiencing viral rebound or not.

Outcome of the experimental treatment in immune cells from people with HIV

Similarly, upon stopping ART, HIV wasn’t detected in the experimentally treated immune cells isolated from the blood of people and cultured in a lab dish, while all samples on ART alone still had HIV particles. Further testing showed the absence of intact HIV sequences in the samples treated with the shock-and-kill approach, while defective HIV sequences were still present – confirming that the treatment only targets reservoir cells with intact HIV capable of restarting the infectious cycle.

Final thoughts

Earlier studies concluded that most of the virus leftover in cells after starting treatment is defective, incapable of spreading to other cells; in other words – inert. Less than three percent of the virus present in cells seems viable and capable of spreading. Therefore, a cure strategy has to focus on this three percent.

It's also plausible that if the cure strategy targeted all cells containing HIV, including those with defective virus, there would be a chance for massive cell death potentially triggering the immune system and leading to serious side-effects, some of them life-threatening. Thus, specific targeting of only the portion of the reservoir that matters would bypass potential immune triggering too. However, leaving defective virus sequences might also sustain a heightened inflammatory state in the body even after a potential cure.

This study builds on previous steps and approaches used in the shock-and-kill context, but it adds a rather smart and elegant step towards the ‘kill’ part of the strategy. It is elegant as it puts the virus in a self-destructive loop; if the virus causes damage, it dies with the cell, if it doesn’t cause damage, the cell lives along with the ‘harmless’ virus.

It is important to remember that this is only a basic science study at a very early pre-clinical stage. There could be several problems with it; for example, humanised mice models cannot exactly mimic the human immune behaviour and those apoptosis-stimulating molecules can come with unpredicted and severe off-target effects as apoptosis is a core mechanism of our cellular health and even small interventions on that level may lead to significant consequences. However, the study is significant in that it acts as a proof of concept and broadens the horizon on possible cure approaches.