Promise and Potential of Immune System Engrafted Mouse Models

In a recently published article in Genetic Engineering & Biotechnology News, Taconic Bioscience's Dr. Leon Hall, Senior Director, Global Scientific Development and Translational Discovery Services, discusses the promise and potential of human immune system engrafted mouse models.

The mouse remains one of the most commonly used models for biomedical research, primarily due to the fact that 99% of mouse genes have a human equivalent, with functional similarities in assessing normal human biology and disease. In addition, mice are easier to house, and have short generation and breeding periods. Yet fundamental differences between the species have limited the mouse's utility for research, evidenced by the approximately 94% failure rate for drugs that make it through preclinical testing into clinical trials. Demand is growing for animal models that can better predict how drug therapies will act in humans, spurring aggressive developments in improved animal models of human immune system function, as well as other models of human physiology in mice.

Thirty years after the first mouse model was developed to carry a human transgene, technology now supports more sophisticated strategies. Today, improving predictability of human-cell specific drugs can be accomplished in several ways: through genetic modifications that replace mouse genes with the human versions of a gene or genes, modified to carry human transgenes, or by engrafting the mouse with human cells or tissues. Among the most promising developments is the ability to directly replace the mouse ortholog with the human sequence, using highly efficient genome editing technologies.

In the past, to insert human genes into the corresponding mouse locus via homologous recombination required using embryonic stem (ES) cells. Current genome-editing technologies and tools - including the cluster regularly interspaced short palindromic repeats (CRISPR/Cas9 system) - provide significant advantages over ES cells. Specifically, they enable direct microinjection of components of the gene-editing machinery into fertilized eggs, allowing mice to be generated much faster. The CRISPR/Cas9 system has also been used to develop mice carrying multiple genetic alterations targeting different loci, accelerating the time it takes to engineer compound mutants.

Modeling the Immune System

The use of mice with defective immune systems as host to human cells and tissues is an increasingly common approach to model human immuno-biology in rodents. For instance, the athymic nude mouse model, established in the 1960s, possesses an intrinsic defect in T cell development. It has given way to a much wider range of immunodeficient models. The scid mutation, a spontaneous mutation in a DNA repair enyzme that is required for proper B and T cell differentiation, is capable of accepting low-levels of engraftments of mature human peripheral blood mononuclear cells (PBMCs) and hematopoietic stem cells (HSCs), while the non-obese diabetic (NOD) scid mice support slightly improved, yet still limited immune cell engraftment. This has prompted development of new models like the CIEA NOG mouse®, a NOD/scid mouse with disruption of the common gamma chain of the IL2 cytokine receptor. The NOG mouse is capable of extremely efficient HSC engraftment and supports development of a more functional human immune system, yielding even greater predictability of study results.

In the study of hematopoiesis and immune system development, four methods are primarily used for engrafting a functional human immune system on a mouse model:

• The Hu-HSC model involves transplanting HSCs that give rise to most blood cell lineages.
• The Hu-PBMC model is reconstituted with PBMCs.
• The SCID-Hu model has liver and thymus tissue fragments engrafted under the kidney capsule.
• The BLT (bone marrow-liver-thymus) model is a modification of the SCID-Hu mouse, but with HSCs derived from the matching liver tissue transplanted and used to repopulate the bone marrow.

In addition, immunodeficient strains are being combined with transgenic expression of human genes to overcome some of the limitations of immunodeficient models. For instance, cytokines required for proper hematopoiesis and immune system development in humans are not entirely conserved in mice. Both transgenic expression and knock-in of human cytokines have resulted in improved reconstitution of specific functional hematopoietic cell lineages.

Immune System Engraftment at Work

Today's advanced immune system engrafted mouse strains are seeing a great deal of utility, particularly in overcoming inherent differences between mice and humans. For example, some human pathogens - including HIV-1 and hepatitis C - can't infect other species because those species lack certain host factors. By reconstituting mice with a human immune system or the transgenic expression of human specific factors, those models can be infected with the desired pathogen.

The study of how drugs metabolize is another area where mouse models of human physiology prove valuable. Mice and humans show profound differences in liver-mediated metabolism of drug therapies. Genetically engineered mouse models can express different components of human drug metabolism pathways, helping predict human drug-drug interactions, toxicity and efficacy more accurately. Already, human liver engrafted mice have been generated and used in drug metabolism research, along with viral hepatitis studies.

Given the challenges of studies using large animal models like non-human primates, genetically engineered and human cell engrafted models of human physiology will play an increasingly vital role in biomedical research. These models already are used for drug candidate testing, analysis of proteins involved in drug metabolism, and the study of the immune system, infectious disease, regenerative medicine and cancer biology, among other applications. Incorporating these models into biomedical research and carefully choosing the right model best suited to each application can greatly advance the understanding of disease and the development of more effective therapies.