By Jeff Boyle, Ph.D., CSO at Addimmune
As we commemorated World AIDS Day on December 1st, an annual event that serves as a reminder of the global struggle to end HIV-related stigma, honor those we have lost, and commit to a day when HIV is no longer a public health threat – the progress over the past 35 years has been nothing short of transformational. We have witnessed the evolution of AIDS from a death sentence to now a chronic illness, manageable for most with antiretroviral treatment (ART). This is a result of collaboration across industry, government, and community stakeholders to increase disease awareness, access to critical testing and care, an investment in novel areas of research that have broadened understanding of the virus and provide insight into how we can improve outcomes and quality of life for people living with HIV.
As a first line therapy for HIV, ARTs have continued to improve since the early 1990s. Their evolution from a multi-pill combination therapy to modern long-acting injectables has continued to improve our ability to manage HIV. As a result, many people with HIV are living for far longer than when the epidemic began as most patient segments have seen a significant drop in morbidity and mortality.
Despite this progress, the fight against the rapidly mutating virus is far from over. People living with HIV are still facing treatment challenges, such as poor adherence and clinical failure of first-line ART, that contribute to an ongoing need for innovation on behalf of patients. HIV is particularly difficult to eliminate because it is integrated into the DNA of host immune cells, within these viral reservoirs the potential is always there for the virus to mutate rapidly to escape pressure from the host cell and/or ARTs, particularly when adherence is poor.
Today, we find ourselves confronted with a sobering reality and growing public health risk: in the U.S approximately 45,000 people living with HIV are clinically failing first-line ART treatment, and an estimated 91,000 patients are non-adherent for hard-to-address reasons such as housing instability, mental health disorders, and substance abuse; and the consequences of poor adherence and clinical outcomes extend beyond individual patients. Poor viral control leads to ongoing transmission risk within U.S. communities that result in 35,000 to 40,000 new infections each year.
To address these critical challenges, one of the ultimate long-term goals in HIV research today is to move beyond existing ARTs and deliver the first potential cure to patients. In the pursuit of an HIV cure, researchers are exploring two possible strategies to address the virus. The first is in the form of a sterilizing cure – the removal of all replication-competent virus. The second would be a possible functional cure – durable viral control without the need for antiretroviral therapy.
Why Gene and Cell Therapy?
As a modality, gene and cell therapy has continued to show promising results in areas such as cancer and rare inherited disorders. To date, more than 30 gene and cell therapy products have been approved in the U.S., with more anticipated in the coming years. As our understanding of HIV molecular biology and pathogenesis has broadened, it has opened opportunities to explore gene therapy as a potential solution for delivering long-term viral suppression for patients. We’ve seen natural resistance to HIV in rare individuals with mutations in CCR5, a protein on the surface of immune cells, which is a major coreceptor crucial for the virus’ entry into human cells. Using these individuals as bone marrow donors in HIV patients we have also seen proof of concept that you can functionally cure HIV. The first documented patient to be cured of HIV –known as “the Berlin patient” – was given an ablative bone marrow transplant from such a donor with a rare mutation to CCR5 (missing part of the CCR5 protein), essentially blocking the virus from entry. The transplant was done as part of his treatment for acute myeloid leukemia, Since the Berlin patient, a handful of bone marrow transplants from donors with similar CCR5 mutations, as part of their cancer treatments, have resulted in similar functional cures.
Through the potential of gene and cell therapy, researchers are now seeking to replicate these results in patients without the need for rare and potentially risky transplant procedures. Across the industry, teams are applying the technology to HIV to prevent de novo infection of CD4 T cells by silencing genes enabling entry/infection by the virus and by silencing genes needed for the virus to replicate and propagate infection.
Closing the Door for Viral Entry by Inhibiting CCR5 Expression
Across the industry researchers are leveraging multiple strategies to target CCR5 and limit HIV’s ability to bind and infect healthy cells.
Silencing or knocking down CCR5 is one of the approaches researchers are exploring that has been shown to hold promise in preclinical and clinical studies.
Silencing RNAs (siRNAs) and microRNAs (miRNAs) are important mechanisms used by human cells to regulate what proteins the cell can make, and in what amounts. By taking advantage of this pre-existing mechanism and simply directing it against CCR5, this could significantly reduce the amount of CCR5 that is produced by cells, thereby decreasing its susceptibility to HIV infection.
Ribozymes are catalytic RNA molecules that are increasingly being used for sequence-specific inhibition of gene expressions. They can be designed to inhibit CCR5 expression offering an approach to anti-HIV gene therapy and have been shown to have the potential for interfering with different stages of the viral cycle. Researchers are exploring combinatorial uses of ribozymes allowing for multiple HIV-1 sequences to be attacked simultaneously. This approach could circumvent viral resistance caused by mutation.
Intrabody Targeting CCR5
Intrabody is an intracellular single chain variable fragment antibody that can bind to a protein of interest and potentially render it dysfunctional. Researchers developed single chain antibodies to CCR5 targeting the protein and used an HIV-1 vector delivery to the primary CD4 T cells. This disrupted CCR5 cell surface expression blocking the cell surface transport of the envelop proteins of HIV-1 and decreased viral loads in CD4 T cells and macrophages.
CC-chemokines bind to their receptor CCR5 with high affinity. Intrakines are modified intracellular chemokines and have been shown to bind newly synthesized CCR5 and prevent their transport to the cell surface. However, one of the challenges with this approach was reported to be incomplete inhibition of CCR5.
Clustered Regularly Interspaced Short Palindromic Repeats (“CRISPR”) gene editing to cut apart the HIV viral genome. Two guide RNAs are used to excise the integrated copies of the HIV genome. The approach has demonstrated suppression of viral RNA in rodent models and is now the subject of a Phase I clinical trial. When HIV infects a cell, it integrates a permanent copy of the HIV genome into the cell’s DNA. Upon the cell’s division, it will copy the HIV genome as if it were its own genetic material, which is one of the reasons why HIV-infected T cells are so difficult to clear.
Added Protection – Silencing the HIV Genome by Targeting Highly-Conserved Sites
Even though CCR5-null T cells are naturally occurring, they are quite rare, and they are not a perfect cure either, as evidenced by the Essen Patient. The rapidly mutating HIV genes can still find a way around the CCR5 knock down. While HIV usually uses CCR5 (R5-tropic) during the early stages of infection, the virus may later switch to use CXCR4 (X4-tropic), another coreceptor for entry into a host cell. This has prompted researchers to explore ways to add extra layers of protection blocking the HIV life cycle in multiple places, similar to the HAART regimen which blocks it in at least two places.
Tat and Vif have been identified as two sites on the HIV genome for targeting using microRNAs. Tat is a multifunctional protein whose primary role is to enhance transcription of the HIV genome, meaning that if the cell makes Tat, it will then read the HIV genome more frequently, creating a positive feedback loop. Vif is also multifunctional, and it plays a significant role through the inhibition of natural resistance mechanisms to viral infection.
By silencing these highly conserved parts of the HIV genome critical for viral replication, this could provide broader protection against new infection with the X4-tropic virus or any existing virus infecting the cell.
Combining Gene and Cell Therapy to Support Sustained Immunity
To support sustained immunity against HIV, researchers are also exploring the use of a combined gene and cell therapy approach, utilizing resistant HIV-specific CD4 T cells. These cells, crucially involved in aiding antibody and CD8 T cells, are particularly susceptible to infection, limiting the immune system’s ability to respond and naturally suppress the virus. Despite their pivotal role, well-treated aviremic individuals exhibit low levels of these cells. The potential strategy involves blocking CCR5 in these cells to preserve it for other infections. The potential lies in expanding and genetically fortifying these cells against infection, coupled with in vitro activation. This comprehensive gene and cell therapy approach holds promise in creating a robust defense against HIV, marking a significant stride in the ongoing battle against the virus.
The Promise of Gene therapy
While we may have come a long way in treating HIV since the 1980s, our work is far from over as the devastation caused by HIV continues to endure with approximately 39 million people across the globe living with HIV. There is an urgent need for innovation to provide options, especially for those who are failing on current treatments and having compromised adherence.
Imagine a world where one could take a one-and-done single-dose cell and gene therapy for HIV and never have to think of it again.
Gene and cell therapy has the potential to make that world a reality in addition to complementing or augmenting current antiretroviral therapies. However, there is a need for more clinical research to address some of the challenges in this field such as targeting specificity of the anti-HIV gene and viral escape.
I remain optimistic about the potential of gene and cell therapy to enable people living with HIV to fight the virus, ultimately providing a functional cure that could liberate them from a lifetime of treatment and the burdens associated with it.
About the Author
Jeff Boyle, Ph.D. is the CSO of Addimmune. Jeff has over 25 years of experience in immunology research, assay development and commercialization and has held leadership positions at biopharmaceutical and diagnostic companies. Most recently Jeff served as President of Ellume USA LLC building the organization from the ground up, constructing and commissioning an automated manufacturing facility for COVID-19 home tests in partnership with the Department of Defense. He also served as a board member on the Maryland Technology Council and BioHealth Innovation. He has maintained a long research career with over 30 publications and has received multiple high level academic awards. He is an inventor listed on several patents in fields including vaccines, immuno-oncology and medical devices that have been launched as revenue-generating products globally. Previously Jeff held multiple executive roles at QIAGEN including the Global Head of its Immune Monitoring Diagnostic Franchise and R&D leadership positions at CSL Ltd in its clinical stage vaccine and biologic development programs.