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Two studies published last month described promising new approaches which could stop HIV infecting cells. They could either eliminate or permanently suppress the activity of the so-called ‘reservoir’ cells which are the source of new virus in people taken off antiretroviral therapy (ART), and are the reason HIV infection is lifelong.

The first class of drugs starve cells of the 'fuel' they need to produce HIV, while the second permamently suppresses the spark that ignites that fuel, so to speak.

A team from the Institut Pasteur in Paris found that the vulnerability of T-cells to infection was critically dependent on how fast they are metabolising – how high their energy demand is and how fast they are turning fuels like glucose into energy. It found that cells with a high energy consumption are far more vulnerable to infection, and that although their speed of metabolism did correlate to what kind of T-cell they were, it was an independent kind of vulnerability to HIV that could be targeted by a new class of drugs.

By slowing down cells’ metabolism by feeding them with a ‘dummy’ inactive form of glucose called 2-DG, they were able to selectively kill HIV-infected cells in the lab dish, even ones not actively producing HIV. One advantage of 2-DG is that it can be produced cheaply and is already under investigation in cancer, as a way of ‘starving’ tumour cells of energy.

In the other study a team from the University of Pittsburgh screened a number of compounds that interfere with the genetic process that cause cells to become active – which means, in the case of HIV reservoir cells, that they start producing virus again. If cell activation, even off ART, could be prevented, then this would provide long-term remission from HIV infection – a so-called functional cure.

This potential cure strategy has been called ‘block and lock’ by researchers, as it depends on suppressing HIV reservoir cells, as opposed to ‘kick and kill’, which is based on activating them. The first compound capable of doing this, called dCA, was found in experiments on mice in 2015. The Pittsburgh team screened a number of other compounds for similar activity and found one, PF-3758309, which was active at very low doses. The next part of the research will look at whether its suppressive effects in reservoir cells persist. PF-3758309 is also under investigation as a cancer therapy.

It has been known for many years that T-cells that are more activated, meaning that they display markers on their surface like CD38 that show how reactive they are to infections, and more differentiated, meaning that they display markers such as CD45RA indicating that they have developed specialised cell-killing ability, are much more likely to be infected with HIV than cells which are quiescent and undifferentiated.

Dr Asier Sáez-Cirión from the Institut Pasteur found that cells’ vulnerability to HIV infection is also dependent on cells’ metabolic (energy) requirements; in particular, how fast they burn glucose. His team did a series of experiments that showed that, by and large, cells that were more activated and differentiated also burn glucose faster, which is to be expected, as they are doing more work.

They took T-cells and managed to infect 12% of them in the lab dish. The most differentiated cells, T-effector-memory cells, were the most liable to infection, with 20% infected.

But, importantly, 0.9% of the least active and differentiated cells, the T-naïve cells, were also infected. The one thing that distinguished these low-activation but infected cells was that they had unusually high energy demands for cells of their type.

One interesting aspect of the role of glucose metabolism in HIV infection is that it seems to sustain ongoing replication, rather than first infection. For HIV infection to happen at all, cells need to express on their surfaces the receptor molecules CD4 and CCR5, which are present in greater density on more differentiated cells. But for ongoing, productive infection to be sustained the cell also needs to keep burning glucose at a high rate, and indeed viral production peaked in these cells some three to five days after initial infection rather than immediately. So drugs that interfered with glucose metabolism may act at a different stage of the viral timeline than entry inhibitors.

The scientists therefore cultivated infected cells with several drugs that inhibited glucose metabolism. Such drugs could pose a significant threat of toxicity, as they interfere with one of the most basic and universal biological processes.

However, they found that an altered version of glucose, 2-deoxyglucose or 2-DG, “decreased HIV infection of CD4 T-cells with minimal cell toxicity.” This molecule looks like glucose to cellular receptors but can’t be ‘cracked’ to release energy in the way regular glucose can. Its specific effect was to reduce the ability of the cellular machinery to produce viral components. Interestingly, 2-DG worked retrospectively – it shut down viral replication in cells even if they had been infected eight hours previously, and did so so efficiently that it effectively reversed potential new infections.

If HIV does manage to infect cells with low activity and differentiation, they are more likely to turn into quiescent ‘reservoir’ cells that are potential sources of future waves of virus rather than cells currently producing lots of viral particles. Importantly, 2-DG reduced the number of both reservoir cells and actively productive ones.

2-DG was considerably less toxic than other metabolic suppressors tried by the researchers. This seemed to be because although it caused a ‘glucose famine’ that killed T-cells, it was more likely to kill HIV-infected cells than non-infected cells, probably because their energy requirements are greater.

Dr Sáez-Cirión’s team did not just experiment on cells infected in the lab dish; they also took T-cells from six people with HIV on ART, added a cellular activating chemical, and then added 2-DG. The glucose analogue potently stopped the T-cells from reactivating and producing new virus.

These experiments are early-stage preclinical investigations, and many steps will be needed to find out if using metabolic inhibitors is safe. But they have found a new vulnerability of HIV to a class of molecules that has already been investigated in cancer therapy and are simple, easy-to-make. The fact that they work on cellular rather than viral machinery suggests that viral resistance might not be a problem, and they may hold the potential to be used as agents that could preferentially kill HIV-infected reservoir cells.

The idea of permanently suppressing reservoir cell activity as a way of functionally curing HIV has been called ‘block and lock’, to distinguish it from the opposite strategy of ‘kick and kill’, which theorised that if reservoir cells could be induced to come out of hiding and start producing virus, the immune system or cell-killing drugs could preferentially kill them and reduce the reservoir size. ‘Kick and kill’ has produced rather disappointing results so far; while some drugs have certainly reversed latency (i.e. ‘woken up’ reservoir cells), the reservoir has not shrunk in most experiments.

Permanent maintenance of latency (‘block and lock’) was initially seen as a less viable cure strategy as it was thought latency suppressors would have to be taken life-long; they would just be another class of ART.

There was considerable interest therefore in a report in 2015 that a drug called didehydro-cortistatin A (dCA) not only stopped reservoir cells reactivating, but seemed to exert a continued influence, maintaining cellular latency, despite the presence of stimulant chemicals, for months after it was withdrawn. It was theorised that dCA caused a permanent change to the shape of chromatin, the packaged DNA that occupies the nucleus of every cell and contains the genes, thus locking reservoir cells into stasis.

Molecules that stop genes being expressed are known as kinase inhibitors. Dr Benni Vargas from the University of Pittsburgh investigated 418 other, diverse kinase inhibitors to find out if they too could force cells to maintain latency even in the presence of reactivating drugs. These chemicals included prostratin and panobinostat, both of which have been used in ‘kick and kill’ experiments.

They found four substances that blocked the activity of four different types of reactivating drug. The most potent of these was a molecule called PF-3758309, an inhibitor of a protein called PAK-1 that performs a variety of roles in shaping the internal components and operation of cells. It completely blocked the latency-reversing activity of drugs like panobinostat at a concentration 3300 times lower than a toxic dose.

Dr Vargas’s team is now further investigating PF-3758309 and similar molecules to see “whether any of these compounds will drive HIV-1 into deep latency, similar to what is observed with d-CA, this proving a unique approach to identifying a functional cure for HIV.”

Valle-Casuso JC et al. Cellular metabolism is a major determinant of HIV-1 reservoir seeding in CD4⁺ T cells and offers an opportunity to tackle infection. Cell Metabolism 29, 1-16. 2019.

Vargas B et al. Inhibitors of signalling pathways that block reversal of HIV-1 latency. Antimicrobial Agents and Chemotherapy, early online publication, 2018. DOI: 10.1128/AAC.01744-18