Of Mice and Men: Humanized Mouse Models Provide Powerful Immune System Insights

What are the challenges with using mouse models for preclinical research and trials?

Mouse models remain the most widely used and genetically accessible mammalian species for in vivo research, advancing our understanding of disease biology and therapeutics. However, despite the similarities between mice and humans, there are differences in biochemical signaling pathways, metabolism, and mechanisms of disease onset and progression. Although there is no single mouse model that predicts outcomes in humans, a carefully designed experiment in an adequate mouse model can provide actionable information for drug development. At TransCure, we create representative mouse models that mimic or contain humanized elements.

WHAT ARE HUMANIZED MOUSE MODELS, AND WHAT ARE THEIR ADVANTAGES OVER OTHER ANIMAL MODELS?

Historically, mouse models were either phenotypically derived or genetically altered to express a specific trait, such as a disease-inducing mutation. However, these models don’t have comprehensive human cell–cell interactions or drug–cell interactions in a physiological context.

Humanized mouse models have a major advantage in better recapitulating human biology because they contain fully functional and sustainable human tissue, commonly the immune system. Human immune stem cells are introduced into genetically engineered mice that are born immunodeficient. Essentially, this reconstitutes a human immune system in the mice and allows investigators to test therapeutic agents in an in vivo system that models human mechanisms of action.

HOW CAN HUMANIZED MOUSE MODELS BE LEVERAGED FOR DRUG DEVELOPMENT?

The availability of a diverse and active human immune system in mice represents significant leverage for preclinical research. Having a fully reconstituted immune system in mice allows researchers to monitor, modulate, and directly target any human immune cell type of interest in vivo.

For cancer research, engrafting these mice with human tumors enables you to test the effectiveness of immune checkpoint inhibitors, CAR T cells, oncolytic viruses, cancer vaccines, and other cancer-targeting agents.

For immunology, you can model specific diseases, such as Crohn's disease and inflammatory bowel disease, and test protective and anti-inflammatory agents. For example, for infectious diseases like HIV, a humanized mouse model provides unprecedented access and assets to test prophylactic antiviral agents in vivo.

How do you generate CD34+ mice to end up with functional human immune cells?

The method for generating these mice varies between labs and institutions, but there are four essential steps:

1. Use severely immunodeficient transgenic mice as hosts for the repopulated human immune system. This reduces interference with the mechanism being investigated.
2. Remove residual hematopoietic stem cells from the bone marrow of mice to promote the development of a complete and long-lasting human immune system. While irradiation can be used, we find chemoablation to be more effective.
3. Engraft CD34+ hematopoietic stem cells. We prefer to engraft mice with CD34+ cells from human umbilical cord blood, as this results in a more diverse and active human immune system compared to engrafting peripheral blood mononuclear cells, which only lead to T cells persisting and the development of graft-versus-host disease.
4. (Optional) Boost certain myeloid or lymphoid populations. We do this through hydrodynamic plasmid-based cytokine gene delivery, which leads to appropriate and time-limited expression of cytokines to boost certain immune cell populations.