What is gene therapy and why is it useful for HIV infection?
Gene therapy is a technique used by scientists and doctors to deliver medicinal genes to a patient’s cells. With today’s technological advancements, it is possible to identify the malfunctioning gene responsible for a given disease and then correct it using advanced genetic tools. With gene therapy, scientists can fix the root cause of the malfunction and create cells that manage a patient’s disease instead of relying on constant medication. This modern treatment modality allows patients to live normal lives without the side effects of traditional medicine, and brings previously incurable conditions into striking distance.
The obvious use case for gene therapy is in addressing inherited genetic disorders, but since HIV integrates its DNA with our own DNA when it infects susceptible cells, it can also be argued that HIV causes a non-inherited genetic disorder. Recognizing the opportunity to combat HIV at the genetic level, our team began designing AGT103-T. With careful analysis and creative application, we have developed a gene and cell therapy that we believe can correct HIV infection. Here, we’ll show you how we do it.
What AGT103-T does to fight HIV
For the vast majority of people, exposure to an infectious dose of HIV leads to infection, which if left untreated, inevitably leads to AIDS. HIV infects and kills the very cells that are responsible for generating a long-term immune response to fight the HIV infection. This means that as the virus gains prevalence, the body loses its ability to fight it; an unbreakable cycle in most cases. However, there are people whose immune cells are resistant to the HIV infection, exposing key weaknesses of HIV and providing researchers with actionable examples on how to defeat the virus.
AGT103-T is a gene and cell therapy made up of helper T cells which have been given specialized anti-HIV genes. Helper T cells are central to the adaptive immune system, and the primary target of HIV infection, so protecting these cells allows them to coordinate an anti-HIV response without simply becoming infected. Since T cells can divide, persist and can circulate throughout the body, the AGT103-T cells are able to form a self-sizing army of HIV-resistant T cells which can recognize and respond to HIV wherever it is in the body.
CCR5 is a human protein which spans the cell membrane, allowing cells to detect messenger molecules like CCL3, CCL4, and CCL5 outside of the cell, then cause a change within the cell. While CCR5 is an important protein for immune cells like helper T cells, HIV can use CCR5 as a co-receptor to enter cells. Recognizing that CCR5 is an important part of a cell’s susceptibility to HIV infection, doctors and scientists tested the hypothesis that immune cells without canonical CCR5 could resist HIV infection.
Indeed, in late 2010, Timothy Ray Brown, previously known only as the “Berlin Patient”, made headlines as the first person to be cured of HIV. As a treatment for his leukemia, Brown received a bone marrow transplant from a donor with a natural mutation in CCR5, which in turn, resulted in an immune system with cells that did not produce full-length CCR5. Among these cells were HIV-resistant helper T cells which could coordinate an antiviral immune response and successfully fight the remaining HIV in Brown. Today, the Berlin Patient is joined by the London, New York, City of Hope, and Dusseldorf patients - who all had variations of the CCR5-null stem cell transplant procedure.
Clearly, a population of helper T cells without canonical CCR5 is critical to any attempt to cure HIV with the immune system. For this reason, we engineered AGT103-T cells with a microRNA which knocks down CCR5, rendering the T cell highly resistant to HIV infection. MicroRNAs are precision tools that decrease the expression level of certain genes by “intercepting” mRNA before it can be translated into protein. Since microRNAs bind to RNAs with complementary sequences, a well-designed microRNA does not bind to the mRNAs of other cellular genes, but specifically leads to the degradation of its target mRNA. In this case, the microRNA blocks mRNA that would have otherwise told the cell to make CCR5.
Figure 1: This diagram from the NIH’s National Library of Medicine depicts RNA interference. Short interrupting 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, we can drastically reduce the amount of CCR5 that is produced by the cell, decreasing its susceptibility to HIV infection.
Tat and Vif Targeting
CCR5-null T cells are naturally occurring, but they are quite rare and they are not a perfect cure either, as evidenced by the Essen Patient, so AGT103-T adds additional layers of HIV protection with two more microRNAs. Similar to the microRNA that knocks down CCR5 expression, AGT103-T cells also have the ability to knock down two HIV genes, Tat and Vif, 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 in the assembly of infectious particles, though the precise mechanism are more mysterious.
If HIV genes are already inside the cell's genome, or find a way around the CCR5 knockdown, the anti-Tat and anti-Vif microRNAs can selectively intercept mRNAs coming from the nucleus if they resemble Tat or Vif. Thee extra layers of protection take AGT103-T cells beyond normal CCR5-null cells and allows them to block the HIV life cycle in multiple places, similar to a HAART regimen. HAART regimens are currently the standard of care for HIV, and they typically consist of 3 antiretroviral medications which block the HIV life cycle in at least 2 places. In order to truly advance HIV treatment and revolutionize the standard of care, AGT103-T was designed around proven strategies to block HIV replication and blends knowledge from traditional medicine with lessons from previous HIV cure cases.
Gag-specific T cells
Not all T cells are created equal, in fact, conformity would be terrible for your immune system. Instead, each T cell bears a unique T cell receptor, allowing the whole population of T cells to recognize a wide diversity of molecular patterns. To elicit an effective anti-HIV immune response, we’re interested in amplifying and protecting the helper T cells which recognize HIV-associated molecular patterns. To create a batch of AGT103-T cells with this specific focus, we can take advantage of a pre-existing function of immune cells: clonal selection and clonal expansion, pictured below.
Figure 1: When the body is exposed to an antigen, a molecular pattern associated with a pathogen, the T cells which react to that specific antigen divide and differentiate. After the primary immune response, some of those antigen-reactive T cells are retained as a long-lived memory cell population. Photo credit to Bergstrom and Antia 2006.
Your immune system is powerful, so it needs to be precisely targeted in order to avoid collateral damage. Clonal expansion is when the specific subset of T cells that recognize and respond to an antigen divide and differentiate. Through this mechanism, the immune system can clone the specific T cells it needs to vigorously fight a particular pathogen without causing too much collateral damage. By understanding how to induce clonal expansion, we can amplify the population of T cells which recognize HIV’s signature molecular patterns before we give them the protective gene therapy.
Using the right cell culturing techniques, we can take normal T cells from the body, selectively expand the subset of T cells which recognize high-value HIV antigens, and then use AGT103 on that enriched T cell population. The result is a population of HIV-resistant AGT103-T cells which are enriched in their capability to recognize viral patterns. These cells can then become memory T cells and enhance many other aspects of the anti-HIV immune response. Since AGT103-T cells are engineered from patients own cells, and no new proteins are generated upon engineering, these cells can be returned to the patient without fear of being rejected.
How does AGT103-T behave inside of a person living with HIV?
To match theory to reality, AGT103-T was tested extensively during the preclinical phase. Since AGT103-T is made using free-floating T cells in the blood, a simple blood draw makes it possible to create AGT103-T and test it outside the body. After observing the behavior of AGT103-T cells in a laboratory setting, the next step was to create AGT103-T and infuse it into human trial participants. When the FDA approved the phase 1 study for AGT103-T, there was an electric excitement in the air.
Data from the phase 1 trial of AGT103-T was published in the journal Frontiers in Medicine in late 2022, showing safety metrics from AGT103-T recipients. According to the data, no serious adverse events were recorded during the trial, AGT103-T cells were detected for extended periods of time in the body, and HIV responsiveness was measured to assess functionality of those cells. Since this was the first time AGT103-T was ever infused into a human trial participant, risk was minimized by maintaining antiretroviral medication during the trial. To observe the behavior of AGT103-T cells in trial participants whose HIV was not being suppressed by antiretroviral medication, another study was opened whereby trial participants were withdrawn from antiretroviral medication.
As each clinical study is completed, news from the clinic will be analyzed, explained, and published. If you are interested in supporting our mission or following our progress, we invite you to share this article with your friends, follow our social media pages, and sign up for our newsletter. Today, HIV has only been cured under specific circumstances, but tomorrow, there will be a safe, scalable therapeutic available at scale. As we progress toward that future, we hope you’ll join us in our contribution to the gene and cell therapy revolution.