Gus Cairns scans the horizon for an HIV cure.
The last time I wrote a review of the progress researchers are making towards a cure for HIV (see Towards a Cure for All, HTU 203 and HTU 204), it was 2011, in the wake of the first documented cure of someone with HIV.
Timothy Ray Brown, also known as the ‘Berlin patient’, had apparently had every trace of active HIV infection removed from his body by means of a bone-marrow transplant containing T-cells resistant to HIV infection. Brown’s case showed that we could cure HIV: but clearly we will need safer, simpler and cheaper ways of achieving the same goal (the transplant, which Brown needed anyway for leukaemia, nearly killed him).
Since then, there has been exciting progress in some areas – including announcements of a few more cases using the same and other strategies. However, it is becoming apparent that there may be no one single cure mechanism for HIV.
We may start achieving cures earlier in two groups of patients. Firstly, for people like Brown, whose condition ethically allows the sort of ‘nothing to lose’ procedure he underwent. Secondly, in people whose HIV is diagnosed very soon after they are infected, where there are signs that using antiretroviral therapy (ART) from the start may suppress HIV reproduction so fast and so deeply that it never really gets going.
Some of these ‘cures’ might better be termed ‘remissions’, because we do not know whether HIV will reappear at some point in the future: there may remain a tiny number of infected cells in the ‘reservoir’ of long-lived, quiescent T-cells, and it is possible they could be reactivated in the future.
The rest of us, though – those with chronic infection, who may be living healthy lives on ART but who would rather not have to take it – may have to wait considerably longer before a cure comes along.
In a few people with cancers, similar cures to Brown’s have been achieved. At a 2013 pre-International AIDS Society (IAS) conference symposium, Towards an HIV cure, researcher Dr Timothy Henrich told delegates that two people with HIV in Boston, who received stem-cell transplants for the treatment of lymphoma, had been maintaining undetectable HIV viral loads without medication for 15 and 7 weeks respectively.1 The decision to take them off ART was only made after the most sensitive available tests failed to detect any HIV genetic material in their cells.
The approach taken here was relatively less toxic than that used for Brown. He had his entire immune system effectively deleted with strong chemo- and radiotherapy before receiving a bone-marrow transplant of new immune cells with the so-called delta-32 CCR5 mutation, which occurs in about 2% of northern Europeans and means they are almost completely resistant to HIV infection.
The Boston patients received an immune-suppressant regimen that consisted only of chemotherapy, and the stem cells were from genetically matched donors without the delta-32 CCR5 mutation.
How did this produce a situation where no more than one in 200,000 immune-system cells in the blood were the patients’ original cells and there was no more than one copy of HIV DNA in 15 million cells?
When we receive a transplant – whether of stem cells or of an organ like a liver – our body recognises it as ‘foreign’ and, unless the parts of the immune system that destroy foreign tissue are suppressed, the transplant can fail. Sometimes, however, the graft’s cells start waging war on the native tissue in what is called graft-versus-host disease (GVHD).
That’s what the researchers believe happened in these cases. “For six to nine months after the transplant, we see a mingling of the donor and host cells, and what happens over time is that the donor cells clear out the host cells.” It had been important to keep the patients on ART during this time, Henrich added, as it protected the donor cells from infection with HIV.
However, he warns: "This is not a practical strategy that we can do for most people with HIV. Stem-cell transplantation is dangerous. There can be up to 20% mortality associated with stem cell transfer in the first year after transplantation".
This danger was underlined earlier this year. Doctors in Minnesota used a similar strategy to the one that cured Brown in a twelve-year-old boy with HIV and leukaemia. They used not bone-marrow cells but stem cells – immune system progenitor cells – found in the blood from the umbilical cords of newborn babies, containing a rich supply of foetal stem cells that can be nudged into turning into a wide variety of cell types. The researchers found enough cord blood with the delta-32 mutation to give these to the boy.
Unfortunately the boy, Eric Blue, lived for less than three months after his transplant.2 The GVHD that in the Boston patients had wiped out most of the remaining HIV-infected cells had, as it can sometimes do in transplant patients, turned lethal: the donor cells mounted a devastating attack on Eric’s body.
In people with chronic HIV, then, we have only achieved cures in a few people where high-risk, complex measures are ethically possible. This year, however, a number of cases of apparent cure or long-term remission from HIV turned up in people subject to a more benign technique – antiretroviral therapy.
In the first report, the talk of the Conference on Retroviruses and Opportunistic Infections (CROI) in Atlanta in March 2013, US researchers identified a case of a functional cure in a baby girl infected with HIV (the ‘Mississippi baby’), who began ART within two days of birth.3 The child has now been off treatment for 18 months, and although HIV DNA (genetic material) has been detected at very low levels in her cells, she has no detectable viral RNA in her blood and her virus is not reproducing – the definition of a functional cure.
The apparent suppression of the girl’s HIV to levels below which it could start replicating – either because there was no fully functional HIV DNA left, or because her own immune system was controlling any remnants – was initially greeted with some scepticism. Was the HIV truly an infection, or residual maternal virus? Would HIV eventually reappear? There was nervousness about the claim of researcher Deborah Persaud that “This is our Timothy Brown moment”. Even if this was not an isolated case and could be repeated, few people get the chance to start ART within days of infection.
However, a study published soon after CROI confirmed not only that long-term off-treatment remission of HIV was possible, it might even be quite common – and that ART did not necessarily have to be started within the first day or two. French researchers4 found 14 (now 26) adult patients from a group called the VISCONTI cohort who also started ART soon after infection, subsequently stopped it, and had not had to re-start because they had largely – and in eight cases completely – maintained undetectable viral loads for four to ten years after stopping therapy.
Furthermore, the researchers suggested that the only reason such cases are not more common is simply because, once having started ART, few people stop. They claimed that 15% (later revised downwards to 5 to 10%) of people with HIV, if ART was started within six months of HIV infection and maintained for at least a year, could subsequently become so-called ’post-treatment controllers’.
Their estimate is a stark contrast to findings from studies conducted between 1996 and 2000, soon after the introduction of highly active ART, which found no evidence that people who began treatment in primary infection soon after acquiring HIV could control HIV after stopping treatment. The key difference is that earlier studies looked at HIV control in people who had only received treatment for 12 to 18 months. The French patients had been on treatment for an average of three years before stopping, and all started treatment within ten weeks of infection, compared to within six months in previous studies.
A further study, presented at the IAS conference, came up with another patient – a 67-year-old German man - who had started ART within —three months of infection and stayed on it for five years, but who stopped his HIV therapy in 2004 and, apart from a small initial viral ‘rebound’, has not had a detectable viral load result since.5
This patient’s CD4 and CD8 cells had strong anti-HIV responses, meaning his immune system was actively preventing viral replication. This is also characteristic of ‘elite controllers’, people who maintain undetectable viral loads without therapy and, essentially, generate their own long-term remission from active HIV.
Being an elite controller might not be good for you. Another presentation at the IAS cure symposium found that HIV responses in elite controllers were characterised by increased activation of virus-fighting proteins such as interferon alpha which, as anyone who has taken it as hepatitis C treatment knows, creates symptoms of its own.6 Another found high levels of a second immunomodulator with known harmful effects called galectin 9.7 Elite controllers don’t just experience physical malaise: it had already been shown that they have higher levels of cardiovascular disease than average, similar to other people with HIV not on ART.8
The VISCONTI researchers found that the viral control seen in their patients (and the Mississippi baby) had almost an opposite explanation from that seen in the majority of elite controllers, whose immune cells tended to be rather unresponsive to HIV. Infection spread slowly through the body, which allowed the immune system time to recognise HIV and mount a vigilant response to it.
In contrast, the VISCONTI patients’ immune cells were unusually sensitive to HIV infection and their acute HIV infection period was characterised by high viral load. HIV invaded so fast, it gave their immune systems no time to react. Normally, this would result in the body being ‘seeded’ with a large reservoir of HIV-infected CD4 cells that would start pumping out virus as soon as viral suppression with ART was removed.
The theory goes that, if ART is started quickly enough and maintained for long enough, the viral reservoir remains small. In normal HIV infection, even in people on ART, residual HIV - burning away like a pilot light - remains; as soon as ART is removed it ‘lights the fire’ again, stimulating cells into producing a new burst of HIV.
In the VISCONTI patients, on the other hand, there was so little HIV around that, when ART was taken away, the immune system – which, remember, had never been given time to ‘recognise’ HIV – simply acted as if it was not there at all.
At the IAS cure symposium, Persaud mentioned unpublished data showing that a number of the other children she has studied might be in the same position as the Mississippi baby – but after a decade on ART, rather than 18 months. In some cases, HIV DNA that was still detectable when the children were six or seven years old could no longer be found at twice that age.
This presents an ethical dilemma. The Mississippi baby case was only identified because the girl’s mother stopped coming for appointments when her daughter was 18 months old, reappearing six months later. Similarly, the VISCONTI patients had taken themselves off ART voluntarily, often in structured treatment-interruption studies. Other people who have come off therapy in the past, however, have experienced disastrous crashes in their CD4 counts; at the very least, a viral ‘rebound’ upon stopping treatment will replenish the reservoir of persistently infected cells.
So how do we decide who to take off therapy? What is the threshold for judging that a person is a potential viral controller? Are HIV DNA counts the best guide? Or is the disappearance of HIV antibodies, as happened for Timothy Brown and the Mississippi baby but not for all the VISCONTI patients, a better guide?
Most people who start ART later, however, are unlikely to achieve control of their HIV without more help because their reservoir of persistently infected cells is bigger, having had longer to be ‘seeded’. Several approaches are being tried to halt ongoing infection, but the one that has received the most attention has been the so-called 'kick and kill' approach.
Initially, gene-stimulating drugs would be taken that 'kick' the normally quiescent central memory reservoir cells into becoming activated and producing some HIV. As long as this remains suppressed at controllable levels with ART, the hope is that by becoming activated, the cells turn into 'effector' cells with short lives; they die, and the reservoir is drained.
It would need to be drained very well: one study where HIV reappeared in someone with fewer than two-in-a-billion HIV-infected reservoir cells9 shows that spontaneous control is unlikely to be perfect in all cases and that there may need to be further stages where drugs are taken that seek out and kill off the activated reservoir cells, driving their number down still further. Then something like a therapeutic vaccine might be given that magnifies the body's natural immune response to HIV and contains the activation of the tiny number of remaining HIV-infected cells.
The class of drugs furthest along in investigations into reservoir-cell activators are called HDAC inhibitors. Some are already in use as anti-cancer drugs. Professor Martin Tolstrup of Aarhus University in Denmark summarised his team's recent research at the IAS cure symposium.10
This research was the subject of an article in the UK's Sunday Telegraph which implied in a misleading headline that a cure for HIV might be achieved “within months”. The team issued a correction and the article was subsequently modified – but it is evidence of huge public interest that these ‘hope for a cure’ articles keep appearing.
The researchers gave 15 men with HIV twelve doses of an HDAC inhibitor called panobinostat over eight weeks. They found that, after the first dose, 60% of participants expressed low but detectable levels of HIV RNA in their blood, despite being on ART, compared with only 28% before panobinostat.
The team will publish data on HIV RNA and DNA detected within cells soon, and will test groups of reservoir cells to find out how many remain with hidden HIV infections and how many can produce replication-competent HIV. The hope is that, if the panobinostat can drain the reservoir sufficiently, it might be safe to take people off ART for a monitored treatment interruption.
As mentioned above, we may need additional therapies that actively seek out and destroy the cells activated by immune stimulants such as HDAC inhibitors.
Dr Victor Garcia of the University of North Carolina introduced such a cell-killing missile at the IAS conference.11 In this molecule, a broadly neutralising antibody (3B3) that attaches itself exclusively to HIV surface proteins, is joined on to a toxin, (PE38, derived from the Pseudomonas bacterium). The antibody attaches the molecule to activated cells from which HIV is budding and the toxin enters the cells and kills them.
This molecule was injected into mice that had been infected with human HIV. Three weeks later they were started on ART. Four and five weeks after that they were given two doses of 3B3-PE38.
Even though ART dramatically suppresses production of HIV within immune cells, some still remains. The ART, as expected, produced a hundred-fold drop in HIV RNA inside cells, but the bacterial toxin produced a further 6.5-fold drop (0.8 logs) on top. More importantly, the absolute number of cells expressing HIV RNA decreased from between 1100 and 20,000 per gram of tissue to between 600 and 3000 per gram, an approximately six-fold drop in the presumed size of the reservoir.
Genetic cell manipulation, immune therapies and therapeutic vaccines that contain any onward infection of HIV from activated cells to other cells may also be a crucial part of the 'kick and kill' strategy, both to encourage the body to kill or contain the tiny fraction of HIV-infected cells left after reservoir 'draining' and to prevent onward infection of HIV into new cells during the 'kick' phase.
These therapies are potentially safer than the hazardous cell-transplant approach, because many are based on what is called autologous transplant. Cells are taken out of the patient’s own body and then genetically modified in some way – to make them immune to HIV or to stimulate an immune response to HIV – and are re-injected. This introduces a new population of cells into the body but not one that risks rejection or graft-versus-host disease.
In HTU 206,writer Matt Sharp recounted his experience of receiving a transplant of his own cells that had been genetically modified to be immune to HIV: the ‘graft’ became a large proportion of the immune system in most participants in the study and, in Matt’s case, it also improved a persistently low CD4 count.
Autologous transplants can also be used as a form of therapeutic vaccine. In one study presented at the IAS conference by the Institut Pasteur in France, the immune-system cells called natural killer (NK) cells, which represent the body's first line of defence against viruses, were used.12 When these sensitised NK cells were mixed with CD4 cells, ‘virgin’ dendritic (immune system) cells and live HIV, their presence reduced the proportion of dendritic cells infected with HIV from 45 to 25% and of infected CD4 cells from 35 to 20%. These reductions might not sound big but they may be enough to contain residual viral replication in a person whose reservoir of hidden HIV-infected cells has been depleted by the previous methods.
More impressive was the performance of a vaccine which used HIV components wrapped inside a cytomegalovirus (CMV) shell. When given to monkeys before infection with the simian form of HIV, this vaccine produced a stunted infection which eventually dwindled to nothing (see The monkeys’ tale).13 The researchers speculate that if this vaccine is safe and effective in humans – and that’s a big if – it might form part of a cure too.
These experiments are in very early clinical or pre-clinical stages, and are just a sample of a whole number of experimental approaches designed to drive ongoing HIV infection down to the absolute minimum. In the long run, they may make it possible even for people with chronic infection to come off ART for long periods – and maybe even a lifetime – without HIV reappearing. Then we really would have a cure.
