Shutting the door to HIV? (part 2 of 2)

This is post two of a blog on HIV gene therapy; I’d highly recommend post one to help with understanding of what is to come, that being the discussion of a study in which CCR5 is deleted from cells of HIV infected individuals.

Over recent years ‘gene editing’ has started to take off, with new technologies being developed and made safe. One such approach uses artificial proteins known as zinc finger nucleases (ZFNs). These proteins are a combination of two naturally occurring proteins, a zinc finger protein and a nuclease (known as Fok1). Zinc fingers bind to specific stretches of DNA, while a nuclease is an enzyme capable of cutting through double stranded DNA. Combining the two produces a protein that can cut through DNA at specific locations.

A diagram demonstrating the principle of
ZFN cleavage of DNA

When breaks are made in both strands of DNA, a cell attempts to repair the damage – with one of three outcomes. The cell dies, the DNA is stitched back together and the original sequence is preserved, or the DNA is repaired with the random insertion or deletion of parts of the original base sequence. The third of these outcomes leads to the deletion of a gene. Using ZFNs to damage DNA at specific sites therefore allows for deletion of a specific gene – such as CCR5.

This leads me to the aforementioned study. In the work, published in the New England Journal of Medicine, HIV infected patients had blood taken, CD4 T-cells collected and then treated with a ZFN to specifically delete CCR5. Following the procedure, these cells were then infused back into the patient.

Twelve individuals received the ZFN treatment, in a study designed primarily to determine if this was safe. To that end they mostly achieved their aim. The majority of the patients in the study developed only mild reactions with 130 reported side effects such as, fever, chills, headaches etc. Out of these, only 32 of the side effects were linked directly to the modified cells, with the remaining number being attributed to the infusion process. Sadly one of the twelve did develop a severe adverse reaction and needed hospitalisation, but this was a response to the infusion, rather than the modified cells themselves. It would be nice to have none of these issues, but, as with any medical procedure, there is a likelihood of side effects, and the ones caused by infusion of the CCR5 deleted cells are largely acceptable.

So what impact did these cells have? As you may expect, the infusion of 10 billion or so cells caused an increase in the number of CD4 T-cells (the loss of CD4 T-cells is a hallmark feature of HIV infection). The mutant cells were detected at a reasonably high level, making up 13.9% of the circulating CD4 T-cells at any given time. Furthermore, the mutant cells were estimated to stay in an individual for close to 96 weeks. At this stage it’s clear that the infusion of CCR5 deleted cells is largely safe, and that these cells can survive in the blood for a fairly long period of time.

The main aim of this study was to assess the safety of CCR5 ZFN treatment, but there was also some investigation into the impact these CCR5 deleted cells had on HIV levels. All of the patients were on antiretroviral therapy at the start of the trial, and, as a result, had undetectable levels of HIV. However, six of the twelve patients were taken off their medication for twelve weeks, four weeks after the infusion of CCR5 deleted cells. Initially these patients showed virologic rebound, the phenomenon where HIV levels start to rise as soon as therapy is stopped. A peak in this was seen at six to eight weeks following cessation of therapy. However, at that point the levels began to decline – perhaps, optimistically, suggesting a protective effect from the CCR5 deleted cells. Moreover, as CD4 T-cell death occurred from the increased level of HIV, it was noted that the CCR5 deleted cells died at a slower rate than normal CD4 T-cells, further suggesting these cells may be protected from HIV infection.

At this stage no real conclusions can be drawn about the efficacy of infusing CCR5 deleted cells for treating HIV. This study achieved its aim to demonstrate safety of the procedure, but little can be learned about any protective effect at this stage. However, that is not to say this isn’t a promising avenue for HIV therapy in the near future.

An artistic representation of HIV showing the outer layer being
peeled away to show the inner core

A few aspects still need to be ironed out. For instance, of the approximately 10 billion cells infused to the patient, only 20% of these had the CCR5 deletion, meaning there are still a large number of cells for the virus to infect. Furthermore, the patients will continue to make their own immune cells from their own stem cells, which won’t have the CCR5 deletion, and will therefore, effectively, dilute the impact of the CCR5 deleted cells. Continual infusions may therefore be necessary for any protective effect. Alternatively, it may be possible to use a similar technique to engineer a CCR5 deletion into stem cells, allowing the patients to develop modified cells for the rest of their life.

Another issue may arise from the fact that HIV is a rapidly evolving virus, capable of becoming resistant to any treatment given in isolation. I would speculate that if a patient, infected with HIV were to be taken off all medication and given CCR5 deleted cells, then HIV viruses that bind CXCR4 would become more prevalent, or perhaps some other work around would evolve. What impact this may have is unknown at this stage.

No doubt a large scale, phase 2 clinical trial is, soon to be, or already underway to assess the protective effects of the CCR5 deletion against HIV over a longer time period of time, with a larger number of patients. It will be interesting to see how much clinical impact this procedure can truly have. And what ways the virus may find to escape the intervention.