About five years ago, George Church and his new postdoc, Ying Kai Chan, sat hunched over a laptop in the office of the genetics pioneer at Harvard, staring in amazement at an old newspaper.
The article documented early trials for Glybera, the first and then only gene therapy approved anywhere on the planet. Less than three dozen patients have ever received it, but in the years before Luxturna and Zolgensma, it gave researchers an example to which they can refer if gene therapy works.
However, Chan and Church were shocked to see that researchers doing clinical trials gave high-dose immunosuppressive drugs to patients who were often reserved for organ transplants, such as mycophenolic acid and cyclosporine. And when they biopsyed the muscles of patients, they were filled with T cells, indicating an immune response in action.
The results were particularly surprising because Glybera used the viral vector AAV, a delivery system that led to a boom in gene therapy, precisely because it was widely believed to evade the immune response that doomed the field in the 1990s.
Ying Kai Chan‘There are so many people who are now engaged in gene therapy and even when you say to them,’ Did you know that cyclosporine is used? Do you know that all these things are used? “People are like ‘Huh, what?’ Chan tells Endpoints News. “Glybera was the poster child, but people don’t seem to appreciate how much immunosuppression is needed.”
Chan trained as a viral immunologist and he entered the church laboratory with an intuition that viral vectors, which were hollowed out and dished out no taxis, are still viruses and are still treated by the body as such. Gradually, the field came to his mind. Several monkey studies have shown that high doses of AAV can be toxic to certain neurons, results that companies reluctantly accept. And last year, three deaths in a high-dose trial raised AAV safety issues, even though it has not yet been necessarily linked to an immunological response.
Meanwhile, Chan has been working on new methods to disguise AAV to make it safer and reduce the need for immunosuppressants. This week, he, Church, and a larger team at the Wyss Institute published the work in Scientific Translational Medicine, showing how the weaving of specific strands of human DNA into the vector can neutralize one of the body’s most important defenses against foreign invaders.
“It’s very much inspired by nature,” Chan said.
One of the first ways that Jim Wilson, a gene therapy pioneer, showed that the body could respond to AAV was through a set of sentinels called toll-like receptors. These guard posts are one of the first defensive layers of the immune system, which raises alarm when they detect something that appears strange. However, this means that normal cells need a way to tell the receptors that they are safe – a coding key that only human cells know.
That encryption key is encoded in a few DNA strands at the tips of telomeres, the shift-shaped strands at the end of chromosomes that are sometimes involved in aging. Chan incorporated these strands into the DNA of an AAV2 vector, the serotype used in Luxturna. When the vector is injected, the strands must bind to the toll-like receptors in the body and tell the receptors not to sound the alarm.
When the team injected it into the muscles, liver and eye of pig and mouse models, it caused a significantly reduced immune response as a traditional vector, Chan reported in STM.
The results contribute to a range of new technologies emerging from laboratories across the country to combat AAV immunogenicity. Wilson’s laboratory offered a way to use microRNAs – short strands that reduce the expression of a specific gene in a particular cell – to reduce the neural effects. And Dyno Therapeutics, an offshoot of the church lab, uses engineering and machine learning to invent completely new vectors, hoping to find some that can prevent the immune system.
Chan has now helped start a new company, along with ARCH and a few other VCs to form Ally Therapeutics, a biotechnology still being abandoned that seeks to reduce the immunogenicity of viral vectors.
Yet he admits he was hoping for more results than he eventually had. Although its technology successfully suppressed the immune response in pigs and mice, the results were less profound in monkeys.
Chan’s team injected the vector into the eyes of non-human primates, a part of the body where much of the immune system cannot penetrate, and consequently toll-like receptors are of utmost importance. They saw improved safety when administered under the retina, but it injected directly into the vitreal jelly in the center of the eye, still causing significant inflammation. Intravitreal injection is important for tackling various conditions and for delivering eye gene therapy safer and easier, as only ophthalmic surgeons can administer sub-retinally.
However, the new article is just version 1.0 of the approach, Chan said, and they have since come up with significant improvements.
More broadly, the field has a long way to go. Animal models, for example, are still poor predictors of the immune response in humans, making translation difficult and putting big holes in safety tests. A vector that is immune silent in monkeys can still cause reactions in humans and vice versa. Although their role in animals is well documented, it is still not clear how large a role toll-like receptors play in the human response to AAV.
Yet, says Chan, they achieved what they wanted to do: they improved the vector and in the process helped the field wake up with a problem that had been overlooked for years.
“There are still challenges,” Chan said. “What we really wanted to achieve was to raise awareness and come up with a promising solution. I would say that we have made progress on both fronts. ”