Mouse Models for Testing Cancer Immunotherapies

Scientifically speaking, the next best thing to testing cancer immunotherapies in humans is using an animal model system designed for the purpose. Animal models are important for learning more about tumorigenesis, metastasis, and therapeutic efficacies. The models used today aim to recreate the tumor microenvironment as faithfully as possible within a non-human animal. Such a task is complex, with some models attempting to simulate both a human tumor and a human immune system. Mouse models are a crucial tool for evaluating the efficacy and safety profiles of immunotherapies prior to their use in human clinical trials. Here’s a look at the types of mouse models currently helping to advance cancer immunotherapies.

Types of mouse models

There are several types of mouse models used in cancer research, and in evaluating the efficacy of potential immunotherapies. One is the syngeneic tumor model, which is widely used because it’s reproducible and relatively easy to make by injecting mouse-derived tumor cell lines into mice. It’s helpful for rapid testing of immunotherapies, but must take into account differences from humans. Genetically engineered mouse models are also widely used. However, both share the disadvantage that they are entirely mouse-based, lacking human tumor or immune components; hence, they are limited when it comes to evaluating efficacy and toxicity of cancer therapeutics. Humanized tumor models in mice were developed to overcome these limitations. In the patient-derived xenograft (PDX) model, patient-derived tumors are implanted into immunocompromised mice. This model can mimic human tumors better, but its lack of human immune system components also limits its utility in evaluating immunotherapies.

In contrast, humanized mouse models—mice modified to include elements of human immune systems—partially overcome this, and their modified immune systems can respond to human implanted tumors. “Humanized mice can be implanted with human tumor cells, allowing for the study of interactions that occur between the human tumor cells, the microenvironment, and the human immune system,” says Bronson. “These models are more complex and can be used to test immunotherapies that target human immune cells.”

The utility of this is critical, because the failure rate for cancer clinical trials is about 97%, notes Jiwon Yang, Senior Scientist at The Jackson Laboratory (JAX). “There are multiple reasons for this high failure rate, and one of them is using the wrong preclinical animal model to test the efficacy and toxicity of drug candidates,” she says. “It is extremely important to test cancer immunotherapy candidates in the model that best represents the human immune system to gauge potential toxicity and efficacy accurately.”

Humanized mouse models

Improved mouse models are crucial for developing better therapies, because despite the high trials failure rate, immunotherapies are one of our best weapons against some types of cancer. Humanized mouse models are constructed using immunodeficient host strains—common ones include NOD, SCID, and NSG mice—which are then engrafted with human immune cells. “The immune system component is vital when examining immunotherapies because these therapeutics target aspects of the immune system leading to an antitumor immune response,” says Steven Bronson, Scientific Product Manager, RMS at Charles River Laboratories. Two common sources of human immune cell grafts are hematopoietic stem cells (HSCs) or peripheral blood mononuclear cells (PBMCs).

Models created using these different methods have distinct strengths and weaknesses. Creating a PBMC model is relatively quick and easy. “The biggest advantage of the PBMC model is that it allows us to create humanized mice that represent each individual, as mice will have the PBMC donor’s immune cells,” says Yang. “However, PBMC models often result in a partial reconstitution of a human immune system and develop acute graft-versus-host disease (GvHD).” JAX is developing new types of NSG mice with enhanced reconstitution of human B, NK, and myeloid cells, along with delayed GvHD. In contrast, the HSC model lacks GvHD and shows excellent human immune cell engraftment, but takes longer to establish (up to 8–12 weeks); in addition, engraftment success varies depending on the HSC donor.

Developing immunotherapies

Mouse models are essential tools for ongoing research on immunotherapies, including monoclonal antibodies, bispecific antibodies, immune system modulators, immune checkpoint inhibitors, and CAR-T cells. JAX has used PBMC humanized mouse models to study safety and efficacy. As a valuable preclinical tool, PBMC mouse models with multiple PBMC donors can better inform pharmaceutical researchers when developing cancer drugs for the general population, such as when finding dosages that maximize efficacy while minimizing toxicity. “The beauty of using the PBMC humanized mouse model is that the model shows donor variability,” says Yang. In the clinic, individual patients react differently to immunotherapies—for example, with different levels of efficacy and major side effects.

PBMC mouse models may also prove a useful tool in screening patients for universal CAR-T (uCAR-T) cell therapy. “Multiple uCAR-T donors can be tested in PBMC humanized mouse models with multiple donors to assess efficacy, toxicity, and expansion of uCAR-T cells,” says Yang. “From there, the scientists would be able to choose the best uCAR-T candidate that shows overall great efficacy with minimum toxicity.”

Humanized mouse models are important for evaluating the efficacy of immune checkpoint inhibitor drugs. Such tools include humanized mouse models expressing human immune checkpoint targets (e.g., PD1), or the human immune system (HIS) reconstructed mouse model “in which severe combined immunodeficient (SCID) mice are reconstituted with functional PBMCs, a class of immune cells that includes lymphocytes (T, B, and NK cells), monocytes, and dendritic cells,” says Shawn Zhou, R&D Director at Cyagen Biomodels.

Innovations in cancer immunotherapy research continue with state-of-the-art immunodeficient mouse models. Newer models such as Cyagen’s C-NKG mice are especially advantageous for immunotherapy due to a range of features, including an absence of mature T, B, and NK immune cells, and dysfunctional macrophages and dendritic cells. Immunodeficient mouse models that express immune system-boosting cytokines are showing promise as tools for evaluating cancer immunotherapies. “For example, human IL2 transgenic C-NKG mice can be used for tumor infiltrating lymphocyte therapy efficacy evaluation, and human IL15 transgenic C-NKG mice can be used for CAR-NK therapy efficacy evaluation,” says Zhou.

Cyagen developed the severe immunodeficiency C-NKG mouse model on the NOC-Scid background strain to knock out the Il2rg gene. C-NKG mice have longer life spans than NOD SCID mice (which have a higher incidence of thymic lymphoma), and are more likely to survive the transplantation of human tissues. “The C-NKG model is compatible with transplanted HSCs, PBMCs, PDXs, CDXs, or adult stem cells and tissues,” says Zhou. Compared to NOD SCID mice, C-NKG mice also allow implantation of a higher proportion of human tissues.

Personalized medicine

Humanized mouse models hold promise for bespoke cancer therapeutics. “Clinically, it is not hard to imagine a future era of personalized medicine where a patient’s tumors are implanted into a mouse humanized with an individual patient’s cells to determine the best therapeutic intervention for that patient,” says Bronson. Indeed, Yang notes that each PBMC mouse model representing a PBMC donor has great potential when it comes to pinpointing the best immunotherapy for a cancer patient. “This mouse model platform can potentially be used as a screening tool to see which individual will benefit from certain immunotherapies and calculate a risk-to-benefit ratio,” says Yang. “If someone is expected to show high toxicity with no efficacy, doctors can look for alternative therapeutics.”