As scientists study immunomodulation—activating or inhibiting parts of the immune system—to find new therapies for cancer, they continue their struggle to find translationally relevant models that recapitulate the human immune system. The Jackson Laboratory (JAX) has developed a comprehensive catalog of mouse models, many applicable for cancer immunomodulation research. These models include mice with genetically defined tumors and active components of the human immune system. Researchers are using these mouse models and JAX preclinical in vivo services to examine how the immune system or tumors respond to potential therapeutics.

Researchers need models that best replicate the human condition as closely as possible. That’s rarely easy to accomplish, but mouse models keep improving, especially those that include elements of the human immune system. “These models are a scientific bridge between the mouse immune system and the human immune system,” says Aaron Rose, Ph.D., opportunity development manager at JAX.

Before the creation of mouse models with human immune systems, cancer researchers were limited to studying treatments in mice bearing human tumors with no human immune cells. “Now, we modulate the immune system with a treatment, such as a bispecific or monoclonal antibody, and study its effect on a tumor,” Rose explains. “That’s where these models come in: Seeing how a drug affects the human immune system’s response to a human tumor before we put it in a human.”

Why mice?Many features make mice a good choice for cancer immunomod-

ulation research. “The mouse and human genomes are over 85% identical,” says Guangwen “Gary” Ren, Ph.D., assistant professor at JAX. “So, there’s not much variability, and there are many powerful genetic tools, such as knockouts, to define the precise mechanism of how a drug works.”

JAX starts with a mouse, such as the NSG™ (NOD scid gamma) mouse, that lacks an immune system. Then, particular features of a human immune system are engrafted, such as hematopoietic stem cells (HSCs) or peripheral blood mononuclear cells (PBMCs). In these humanized mice, a specific tumor—even a portion of a human patient’s—can be grown in the mouse. Then, as Ren points out, “scientists can work with a model that provides an intact immune microenvironment in a tumor, as well as organ tissue.”

JAX continues to develop new models to represent the human immune system and to allow more ways to investigate mechanism of action. Rose says, “Humanized NSG™ opens investigations of the newest antibodies, like bispecific antibodies that target two things: the antigen on a cancer cell’s surface and the T-cell receptor, thus drastically increasing the numbers of T-cell able to target tumors.”

A new example of the humanized NSG™ platform is the NSG™-IL15, which expresses human interleukin-15 in order to stimulate natural killer cells (NK cells). With previous mouse models, so few NK cells engrafted that the mice could not be used to study NK cell–based mechanisms. NSG™-IL15, Rose says, “allows investigations of antibodies whose mechanism of action is known to be mediated through antibody-dependent cellular cytotoxicity (ADCC).”

Modeling the needsThe features required in a mouse model depend entirely on how

it will be used. As Rose begins working with a new customer, he asks: “What scientific questions do you need to answer?”

In considering the best mouse model, scientists often want to know what immune cells are present. “That’s a good question,” Rose says. ”Each model has a unique human immune cell repertoire and therefore a unique case.”

Scientists at Merck Research Laboratories (MRL), in Palo Alto, California, use JAX mouse models, mostly in immune-related studies

Generating new models at the speed of oncology research

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of colorectal cancer. When asked what she looks for in such mouse models, Barbara Joyce-Shaikh, Ph.D., associate principal scientist at MRL, says, “It depends on each project, and we look at our drug targets as a lump of clay.” For example, Merck scientists turn genes on and off to see how that changes the impact of a compound on a target.

“To do that, we need mouse models with many different types of mutations,” Joyce-Shaikh says, “and JAX has so many from which to choose.” She adds, “JAX has a great database that they can use to suggest the best mouse model to use in a new project.”

Sometimes, only a humanized mouse model created by the insertion of a human gene meets the needs of researchers. “In one project, we had a gene that had no mouse or other rodent orthologs—not even one in monkeys—only human,” Joyce-Shaikh says. “For preclinical work we needed a humanized system that included that human gene, and JAX developed one.” With that model, Joyce-Shaikh and her colleagues provided data for two new drugs that are now being tested in humans. “It used to be rare to use genes that are only in humans,” Joyce-Shaikh says, “but we’re now seeing it more and more.”

Switching the immune sourceThe source of the cells that are used to generate human immune

cells in a mouse model impact the results. JAX NSG™ mice engrafted with CD34+ HSCs, for example, contain human immune cells that developed from stem cells in the context of mouse tissues, and so their development was different from that of adult human immune cells. As an alternative, JAX scientists are exploring engrafting NSG™ with adult human PBMCs, which contain lymphocytes, B cells, T cells, monocytes, granular cells, and dendritic cells as the immune system source.

The factors that support the development of the immune system are also of critical importance. When engrafting immunodeficient mice with CD34+ cells, the mouse system drives the differentiation of the immune system, which requires added growth factors and cytokines to support development of some cell types. PBMCs, on the other hand, come with a mature immune system that was developed in the human donor.

Nonetheless, PBMCs come with their own problems. One downside is that with the exception of T cells, most PBMC-derived immune cells fail to persist. Plus, the human T cells eventually trigger graft-versus-host disease (GVHD) in response to the mouse tissue, which is absent in human immune cells developed in a mouse from CD34+ stem cells.

Balancing the benefitsAt Bicycle Therapeutics (BT), scientists use JAX mouse models

developed from CD34+ cells and PBMCs. In particular, BT’s director of in vivo pharmacology, Johanna Lahdenranta, Ph.D., and her colleagues, worked with JAX to develop a humanized mouse that incorporates human CD137, a tumor necrosis factor receptor. Activating CD137 on immune cells, such as NK cells and others, could turn them into cancer killers.

So, BT and JAX scientists collaborated to create a PBMC-humanized mouse that includes human CD137, plus an engrafted human tumor. JAX even created mouse models from several PBMC donors to find the one that worked best for the intended experiments. “They looked at the

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level of immune-cell infiltration in the tumor and characterized the CD137 expression in the immune-cell populations until they identified the best possible tumor models for us,” Lahdenranta says.

Using this mouse model, BT scientists will test bicyclic peptides—known at BT as Bicycles—that include two stabilizing loops in their structure, which Lahdenranta describes as “a unique drug modality,” to know when experiments are showing a drug-driven immune response, not GVHD. To do that, Lahdenranta says, “Your vendor needs to be knowledgeable and able to guide you in the proper humanization protocol.” She adds, “JAX can tell me when to switch from PBMC-humanization to CD34+.”

More than modelsIn preclinical research, biopharmaceutical companies need reliable

methods to decide when to move ahead with a potential medicine. In immunoncology research, that confidence requires effective engraftment of the human tumor in the best mouse model to collect a series of data, from validating a target and analyzing a potential drug’s efficacy to simulating clinical trials.

JAX characterizes the phenotype of each model and provides various methods—formalin-fixed, paraffin-embedded slides of a patient-derived tumor, a range of mouse models, advanced data analysis, and more—to study specific features of a drug. Plus, JAX continually improves its tools. “At JAX, we’re trying to develop a different generation of humanized mice,” Ren says. “We keep seeing more functional similarity with human physiology, but there’s always room to better optimize the system.”

Getting the best results depends on a solid relationship between experts at JAX and the global research community. Commenting on her work with JAX, Lahdenranta says, “The depth of experience in their scientists is very important to me.”

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