Two years ago, the UK became the first country to approve Casgevy, a Crispr-based therapy for the blood disorders sickle cell disease and β-thalassaemia. This year saw the world’s first personalised Crispr therapy for a baby boy with a rare genetic condition. Such gene therapy breakthroughs have paved the way for over 250 clinical trials, some with the potential to treat once-incurable diseases, including HIV/Aids.

Recent studies investigating Crispr’s potential to cure HIV have been encouraging. ‘The long-term goal is for a one-time or limited treatment that either excises or permanently inactivates HIV DNA,’ says molecular biologist and biochemist Elena Herrera Carrillo, at the Institute of Parasitology and Biomedicine López-Neyra-CSIC in Granada, Spain. However, Herrera Carrillo cautions that we are still far from the clinic. ‘The data are promising but remain at a very premature stage.’

A promising tool

In 1981, the Centers for Disease Control and Prevention (CDC) reported the first cases of a rare fungal pneumonia – typically affecting severely immunosuppressed people – in five previously healthy young men in Los Angeles, two of whom had already died. What was causing people’s immune systems to suddenly break down, turning harmless microorganisms into killers? At the time, very little was known about this condition that was so new it didn’t have a name. There were no tests or recognised treatments, and by the time symptoms appeared, patients had only months to live.

As the emerging disease began to affect a wider population, the CDC named the condition acquired immune deficiency syndrome (Aids) a year later. Around the same time, the first human retrovirus was discovered in leukaemia patients – an RNA virus, called human T-cell lymphotropic virus type 1, that hides in the DNA of a host cell. Like leukaemia, Aids is associated with abnormal T-cell function. And in 1983, virologist Luc Montagnier and his team at the Pasteur Institute in Paris were the first to isolate what would later be called the human immunodeficiency virus (HIV ) from the T cells of a patient in the early stages of infection.

But it would take about a decade to understand how HIV behaves. HIV enters CD4+ T helper cells by binding to the CD4 receptor and to either CCR5 or CXCR4, acting as co-receptors. Once inside, HIV uses its reverse transcriptase enzyme to convert its RNA into double-stranded DNA. The virus then hijacks the host’s cellular machinery to make new viral RNA and proteins. HIV then uses a protease to cut up these proteins into functional pieces that assemble into new, mature viruses that bud off to infect other cells.

Infected T cells eventually die, and with fewer T helper cells the immune system loses its ability to properly activate other immune cells. This weakened immunity, if left untreated, can progress to Aids.

Today, about 40 million people around the world live with HIV, and more than 600,000 die from related illnesses each year. Since the start of the epidemic, huge advances have been made in significantly reducing both new HIV infections and deaths. This includes antiretroviral therapy (ART), the current gold-standard treatment that involves a daily, monthly or bi-monthly combination of drugs that target different stages of the HIV life cycle to suppress viral replication to undetectable – and therefore untransmittable – levels.

But only three-quarters of people living with HIV are on treatment and, with the recently proposed HIV funding cuts by major donor countries to foreign aid, including the US and UK, there could be up to 11 million extra HIV infections by 2030, setting progress on HIV targets back decades.

Despite its massive success, ART isn’t a cure and has to be taken for life – sometimes causing serious side effects. And even with a long-term treatment regimen, the virus can develop resistance. Another major obstacle is latent reservoirs. As part of its life cycle, HIV permanently integrates its DNA into the host cell’s genome, becoming a provirus. But sometimes, this provirus can go ‘silent’ for a long time, becoming undetectable by both the immune system and drug therapy. If ART stops, these latent reservoirs can reactivate at any time, producing more viruses.

Enter Crispr

‘Crispr-based approaches offer a potentially efficient and durable way to tackle HIV infection and persistence because they can act on the provirus integrated into host genomes,’ says Herrera Carrillo.

The only cure for HIV ever achieved was in three patients receiving stem cell transplants to treat severe blood cancers – all three also had HIV. To treat two diseases at once, doctors chose donors with a rare mutation that deletes both copies of the CCR5 gene, which encodes the CCR5 co-receptor used by most HIV strains, especially during early infection.

And it worked. After the transplant, none of the patients had detectable levels of HIV, even after stopping ART. But this transplant isn’t a practical or safe way to treat most people with HIV. Instead, researchers want to use Crispr to cut out CCR5 genes in patients’ stem cells.

In a paper published in early 2025, Christian Boutwell at the Ragon Institute of Mass General Brigham, Massachusetts Institute of Technology and Harvard University, and his team used Crispr to edit the CCR5 gene in over 90% of human blood stem cells – with minimal or no detectable off-target effects.

These edited stem cells were then transplanted into humanised mice. When exposed to a high dose of HIV strains that prefer to bind to CCR5 instead of CXCR4, the CCR5-edited mice remained uninfected and maintained healthy CD4+ T cell levels – even after a second HIV exposure at five times the original dose. Testing different proportions of edited stem cells found that an over 90% edit rate provided the strongest protection against HIV.

But reducing CCR5 expression only helps prevent future infections. ‘The search for an HIV cure goes hand-in-hand with strategies aimed at eliminating, reducing or controlling latent reservoirs,’ says Herrera Carrillo.

Preclinical studies using Crispr to cut out HIV from the genome of infected cells have shown promise in reducing viral reservoirs. A 2019 study in humanised mice showed that combining long-acting slow-effective release antiretroviral therapy (Laser ART) – a formulation that prolongs drug activity – with Crispr, excised 60–80% of integrated HIV DNA across various tissues, including the spleen, lymph nodes and brain, in some animals. Similarly, a 2023 study developed a dual Crispr approach targeting both the CCR5 gene and an HIV DNA region essential for replication. Combined with Laser ART, this treatment eliminated HIV in over half of infected humanised mice. Neither study had off-target effects or viral rebound for the most part.

Clinical trials

The tricky part is translating these Crispr-based strategies from preclinical models to human clinical trials. After its preclinical success, the first-in-human clinical trial of the Crispr-based HIV therapy, EBT-101, began in 2022.

Developed by California-based biotech company Excision Bio Therapeutics, EBT-101 was administered to HIV-positive adults on ART using the adeno-associated virus (AAV) delivery system. Directed by two guide RNAs, a Crispr-Cas9 system was meant to remove a large portion of HIV DNA. After 12 weeks, participants who maintained viral suppression stopped taking ART to see if the therapy had prevented viral rebound – but it failed.

In Herrera Carrillo’s view, the main limitation why the only Crispr trial for HIV did not prevent HIV rebound is because a single non-targeted AAV-Crispr administration cannot target all reservoir cells. ‘Safe and effective delivery to latent reservoirs remains a huge challenge, as no delivery system currently reaches all infected cells,’ she adds.

One reason is that the large size of the Crispr-Cas9 system makes it difficult to deliver to reservoirs hidden deep within some tissues, including the lymphatic system and other hard-to-access sites such as the central nervous system. Also, its size limits the choice of delivery vectors, excluding some that might otherwise be safer or more efficient. To address this, Herrera Carrillo and her team at her former institute, Amsterdam UMC, delivered smaller Crispr systems from different bacteria – saCas9 and the even smaller cjCas9 – to T cells using lentiviral vectors. In two unpublished proof-of-concept studies, both saCas9 and cjCas9 suppressed HIV with only a single guide RNA. Moreover, saCas9, combined with two guide RNAs, was able to excise part of the viral DNA, which further enhanced the antiviral effect and prevented viral escape in the experimental setting.

But while better delivery systems are crucial, the most effective therapy for curing HIV will likely require combination strategies. ‘Gene editing could be combined with “shock and kill” approaches, broadly neutralising antibodies (bNAbs), immune-based therapies, or other strategies designed to target latent HIV reservoirs and enhance the immune response,’ Herrera Carrillo says.

In one study published this year, researchers at the Peter Doherty Institute for Infection and Immunity in Melbourne developed a novel lipid nanoparticle formulation called LNP X that delivers mRNA to T cells to reverse latency. In latent HIV-infected cells, LNP X reactivated HIV transcription without inducing T cell activation or toxicity in vitro. LNP X also delivered a Crispr activation system to T cells to better enable gene-specific transcription.

Sharon Lewin, an infectious disease physician, virologist and one of the lead authors of this study, says that delivering mRNA very efficiently to a resting CD4+ T cell using this novel lipid nanoparticle has never been possible before. Lewin says that, although Crispr was delivered to resting cells using LNP X, they think they can do better on the delivery front. ‘The challenge here is that Crispr activation requires a very large piece of mRNA,’ says Lewin. ‘Once solved, we would likely combine Crispr activation with a drug that also can kill infected cells.’

‘Immune-based therapies, such as enhancing T cell responses or using bNAbs, are being explored to help the body clear infected cells more efficiently,’ says Herrera Carrillo. ‘Working together, these strategies could attack HIV from multiple fronts: Crispr to precisely target the viral genome, “shock and kill” to expose the hidden virus and immune stimulation or bNAbs to eliminate what remains,’ she adds.

Prasanta Dash, a pharmacologist and experimental neuroscientist at the University of Nebraska Medical Center, strongly believes Crispr-based therapies will be an integral component for any cure for HIV in the future. ‘Of course, it will be a multimodal component,’ he says. ‘Any single therapy is unlikely to be successful in eliminating all the integrated HIV from an infected individual’s latent reservoirs.’

Herrera Carrillo believes that large-scale manufacturing and distribution of Crispr-based HIV treatments – along with fair intellectual property and pricing strategies – are essential for ensuring access in low-resource settings. ‘If these hurdles are overcome, Crispr-based cures could greatly reduce inequities in HIV outcomes worldwide.’

While there are already several obstacles to overcome before a Crispr-based cure can enter the clinic, the long-term effects on tissues of long-acting drugs and/or bNAbs are the most critical side effects that would need to be assessed before moving into humans, Dash says. ‘Future steps include developing a Crispr delivery system that specifically targets the HIV reservoirs, uncovering the best combination strategies to achieve complete viral elimination success and limiting any adverse effects of the developed techniques.’