Humanized Mouse Models for Cell and Gene Therapy Development

Cell and gene therapies (CGTs) are a rapidly emerging area of pharmaceutical development and manufacturing. An estimated 1,200 treatments are currently in clinical trials globally, with the market for manufacturing CGTs predicted to grow by 29% per year.¹

One driver for the growth of CGTs is the development of immunotherapies, and other cancer treatments, to tackle a growing burden of cancer.¹ And, according to Brian Soper, Ph.D., Senior Scientific Engagement Manager at The Jackson Laboratory (JAX), this has led to the growing importance of humanized mouse models. “There are a number of immunotherapies that have gone almost exclusively through…humanized platforms to get them to clinical trials and later approved,” he says.

The advantages of humanized mice, according to Simon Authier, Ph.D., a Principal Director at Charles River Laboratories, are that they combine the flexibility of an animal model adapted to multiple diseases with the ability to maintain viable human tissue.

What is a humanized mouse model?

Researchers often approach JAX after searching the literature for similar studies targeting their desired cell population. There isn’t a single definition of a humanized mouse model, Dr. Soper says, meaning that customers need to understand what they plan to do with the mice.

“Some mice only have a human gene knocked into [their DNA] and this would be to serve as a target for drug discovery,” Dr. Soper adds. In this type of model, for example, a company developing a monoclonal antibody (mAb)—a common class of biologic that isn’t a CGT—might target a human protein expressed on the surface of a mouse cell. However, as Dr. Soper explains, the immune response would be from the mouse’s immune system. According to Dr. Authier, humanized models where genes regulating mouse immunity have been knocked out, mutated, or replaced are most suitable for evaluating single-cell therapies.

An alternative, according to Dr. Soper, is to use an immunodeficient mouse engrafted with human immune cells or tissues, which serve as targets for a therapy. There’s also a “next-level” mouse model, where an immunodeficient mouse with engrafted tissues is further engineered with the genes to express human proteins or growth factors. “It’s kind of ‘doubly humanized’,” he says. “Not only has it got the human genes, but it’s also got human cells supported by the expression of those genes.”

Humanized mice models with human gene therapy targets, pathways, and systems, Dr. Authier adds, are most suited to studies that require the mice to display a human phenotype and give a similar response to a human patient.

Choosing a humanized mouse model

According to Dr. Soper, a key aspect to choosing a humanized mouse model is understanding the scientific questions you’re trying to answer. The primary questions, Dr. Authier elaborates, include whether the model can sufficiently mimic the human condition and whether the model exhibits a relevant therapeutic effect once the CGT has been administered.

Probably the most critical question is which cell population you are targeting, explains Dr. Soper. Some humanized mouse models demonstrate a lot of T-cell involvement, whereas—for more complex cancer therapies—you may need a mouse model with a wide range of human immune cells present and where the human cancer can be growing at the same time.

This leads to a second question, Dr. Soper notes, over whether previous studies have been carried out on the tumor. “If it’s never been grown before in a humanized mouse model, [we’d recommend] doing a quick pilot study to get the tumor growth kinetics in the context of these human immune cells—then you know how to set up both the immune cells and the tumor.”

Early studies can also include a baseline analysis of the mouse model, to ensure whether their immune cells are behaving as expected, explains Monika Buczek, Ph.D., Director of Humanized Immune Model Core at Taconic Biosciences. “If they aren’t, there’s the opportunity to go back to your mouse vendor to see if anything can be adjusted in your experimental conditions or vivarium.”

Another early assessment involves testing the maximum dosage of therapy that the mouse model can withstand. “[Administering] a maximum tolerated dose before following the animals to see what they do has prevented some large studies going downhill very quickly,” she says.

Other important considerations include the timescale of the study and type of humanized mouse model to be used. For short-term, acute studies lasting a month or less, with lots of T-cell involvement, Dr. Buczek recommends peripheral blood mononuclear cell (PBMC) humanized mouse models. These models, she explains, have a T-cell response within a couple of weeks of engraftment, but the mice swiftly succumb to Graft versus Host Disease.

For more complex longer studies, she recommends CD34⁺ hematopoietic stem cell engrafted, humanized mice models. These can take 5 to 10 weeks after engraftment to generate a fully humanized immune response with B, T, and myeloid cell populations, and 15–20 weeks for decent numbers of suitable absolute T-cell counts. She believes that understanding these criteria is important for getting the most out of humanized mouse models because, as they progress to later timepoints post engraftment, there’s a trade-off between reconstitution of the engrafted immune system, and the health and longevity of the mouse.

Regardless of which experiments you’re performing, both Drs. Buczek and Soper recommend anyone new to humanized mouse models reach out to field application scientists at their preferred mouse supplier. “There’s no one webpage or article that can answer every question,” says Dr. Soper. “So, it’s a good idea to talk to an expert as soon as you can.”

References

1.  Global Cell and Gene Therapy Manufacturing Market Outlook (2023 to 2033), Jan 2023