Can Gene Therapy Provide a Cure for HIV?

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Can Gene Therapy Provide a Cure for HIV?

Can Gene Therapy Provide a Cure for HIV?

Early research on gene therapy held immense promise for curing genetic diseases.

However, due to safety concerns, clinical trials slowed down, demanding a more cautious approach.

With advances in genetic engineering, the first gene therapy product targeting genetic mutations received approval in 2017. Gene therapy (GT) can be conducted either in vivo or ex vivo, with ex vivo methods offering several advantages.

Ex vivo therapy allows the characterization and selection of genetically modified cells before administering them to the patient, minimizing immune rejection by using the patient’s own cells.

This review focuses on ex vivo gene therapy, highlighting the latest progress to enhance its efficiency and safety, and providing a comprehensive overview of ongoing HIV gene therapy research.

Can Gene Therapy Provide a Cure for HIV?

1. Introduction

The sequencing of the human genome has expanded our understanding of genetic diseases and the pathology underlying chronic and infectious conditions. Today, over 5,000 gene therapy products are in clinical trials for a wide range of diseases (as of October 30, 2023). A key aspect of developing gene therapies involves selecting an effective delivery method to ensure precise expression of the desired gene at the target site, while minimizing adverse effects. The development of gene therapy products (GTPs) is stringent, given potential risks such as tumor formation or germline transmission.

Gene delivery can be achieved using in vivo or ex vivo methods. In vivo therapy involves delivering genetic material directly into the patient’s body, whereas ex vivo therapy modifies cells outside the body, which are later returned to the patient. The latter offers safer alternatives as cells are genetically manipulated and tested before reintroduction. Ex vivo gene therapy has shown promise in several areas, such as treating blood disorders with hematopoietic stem cells (HSCs) or neurological conditions.

2. Gene Therapy Strategies for HIV Cure

Finding a cure for HIV is challenging due to the presence of latent reservoirs—viruses that remain dormant but can replicate if antiretroviral therapy (ART) is stopped. These reservoirs exist in difficult-to-access tissues like the gut-associated lymphoid tissue (GALT) and central nervous system (CNS), contributing to viral persistence even during ART. The immune system’s limited ability to combat HIV also complicates efforts, as neutralizing antibodies and effective T-cell responses are insufficient.

Efforts to cure HIV aim to:

Eliminate latent reservoirs or keep them inactive, and
Rebuild the immune system to resist reinfection.
Emerging approaches include genetic engineering of T-cells and B-cells to enhance immune responses, such as chimeric antigen receptor T-cell (CAR-T) therapy. The International AIDS Society has established minimum clinical efficacy criteria for a potential cure, including maintaining viral load below 200 copies/mL for over two years with minimal adverse effects.

Four Strategies for HIV Gene Therapy

Stem Cell Transplants:
So far, six patients have been cured through hematopoietic stem cell transplantation—most notably the Berlin, London, and New York patients. These transplants involved donors with a homozygous CCR5Δ32 mutation, which prevents HIV from entering immune cells. However, due to the risks, cost, and limited availability of CCR5Δ32 donors, this approach is not scalable for widespread use.
Shock and Kill Strategy:
This approach activates latent viruses with latency-reversing agents, followed by ART to prevent new infections and immune responses to eliminate infected cells. Though promising, this method has not yet incorporated gene therapy but is part of broader efforts to cure HIV.
Gene Editing:
Technologies like CRISPR, TALEN, and zinc-finger nucleases (ZFNs) are used to target the CCR5 gene or excise viral DNA from infected cells. Tools such as short interfering RNA (siRNA) or shRNA can also silence CCR5 expression. Additionally, engineering recombinases like TRE to target HIV-specific sequences offers another route to remove integrated viral DNA.
Block and Lock Strategy:
This strategy aims to achieve functional cures by keeping HIV dormant without eliminating it. Through epigenetic modifications, latent viruses are permanently silenced, preventing viral replication even without ART.

3. Ex Vivo Gene Therapy

A critical factor in ex vivo gene therapy is determining the cell source for modification. Autologous cells (derived from the patient) are preferred to avoid immune rejection. However, in conditions like HIV, using the patient’s cells may not be suitable due to the risk of reinfection. Allogeneic cells (from donors) are gaining interest, despite challenges like rejection or graft-versus-host disease (GvHD). Sources include hematopoietic stem cells (HSCs) from bone marrow, umbilical cord blood, peripheral blood, and placenta.

3.1 Sources of Hematopoietic Stem Cells

Bone Marrow:
Bone marrow-derived HSCs are frequently used in gene therapy. Osteoblasts in the bone marrow microenvironment help maintain HSC viability by secreting growth factors. Harvesting involves aspirating marrow from the iliac crest under anesthesia, typically collecting around 1.5 liters.
Peripheral Blood:
HSCs from peripheral blood require mobilization using granulocyte-colony stimulating factor (G-CSF) or drugs like plerixafor (AMD3100) to release HSCs into circulation. Collection is done via leukapheresis, although the process can cause side effects such as bone pain and fatigue.
Umbilical Cord Blood (UCB):
Cord blood is rich in HSCs but limited in quantity, making it challenging for large-scale applications. Nevertheless, it is an ethical and readily available source.
Placenta:
Recently, the placenta has emerged as an attractive HSC source due to its low immunogenicity and pluripotency. Placental cells express low levels of HLA antigens, reducing the likelihood of immune rejection. Additionally, plerixafor infusion can increase the yield of HSCs from placental tissue.

3.2 Isolation and Purification of HSCs

To ensure effective transplantation, it is essential to purify and enrich HSC populations. Magnetic-activated cell sorting (MACS) is the most common method, utilizing CD34 antibodies to isolate CD34+ HSCs. Advanced sorting techniques such as fluorescence-activated cell sorting (FACS) are also employed for higher purity.

Conclusion

HIV gene therapy has advanced significantly, with ex vivo methods offering safer and more controlled approaches. Strategies like stem cell transplantation, shock-and-kill, gene editing, and block-and-lock show varying degrees of promise in achieving a functional or sterilizing cure. Ex vivo gene therapy remains at the forefront of these efforts, with continuous improvements in cell engineering, isolation, and transplantation methods. Future research will focus on refining these approaches, optimizing immune responses, and combining strategies to develop a scalable and effective HIV cure.