The development of models that accurately reflect human biology is a persistent challenge in biomedical research, given the inherent differences between human physiology and that of traditional laboratory animals. To bridge this gap, scientists have engineered the humanized mouse, which incorporates human genetic, cellular, or tissue components into a small, living system. This modification allows researchers to study human-specific diseases and test therapies in an in vivo environment that is far more relevant than standard animal models. This technology provides a reliable platform for investigating complex biological processes, particularly those involving the human immune system and cancer progression.
What is a Humanized Mouse
The term “humanized mouse” covers two distinct approaches to incorporating human elements into the murine model. One category includes genetically humanized mice, which are engineered with human genes inserted into their genome, often replacing the corresponding mouse gene. These models are used to study the function of specific human proteins or to create a mouse susceptible to a human-specific pathogen.
The second, more common category consists of chimeric humanized mice, created by transplanting human cells, tissues, or organs into an immunodeficient mouse host. These models contain a mixture of mouse and human cells. Within this group, two types are frequently utilized: Human Immune System (HIS) mice and Patient-Derived Xenograft (PDX) models.
HIS mice are generated for studying the human immune response, making them useful for infectious disease and immunology research. PDX models involve the direct implantation of a patient’s tumor tissue into the mouse, allowing the cancer to grow while retaining the complexity of the original human tumor. Creating both types of chimeric models requires a severely immunocompromised mouse strain that will not reject the foreign human cells and tissues.
Methods for Creating Humanized Models
Genetic modification and cellular transplantation are the two main strategies for creating humanized mice. For genetically humanized models, targeted gene editing technologies are foundational. Scientists frequently employ the CRISPR/Cas9 system, which precisely cuts and modifies the mouse genome.
CRISPR/Cas9 allows for the targeted insertion of human genes. This can involve replacing the native mouse gene with its human counterpart, known as a “knock-in,” or inserting the human gene into a highly active, non-disruptive location like the ROSA26 locus. Small-scale changes, such as a single nucleotide polymorphism (SNP) modification or an exon swap, can also be made to ensure the mouse gene expresses a humanized isoform of a protein. This approach ensures the human protein is expressed in the correct mouse tissues and at relevant levels.
For chimeric models, the use of highly immunodeficient host mice, such as the NOD-scid-gamma null (NSG) strain, is essential. These mice lack T, B, and natural killer (NK) cells, preventing the rejection of transplanted human material. To create a Human Immune System (HIS) mouse, researchers transplant human hematopoietic stem cells (HSCs), typically isolated from umbilical cord blood, into the host mouse. These stem cells migrate to the mouse’s bone marrow and thymus, where they differentiate and mature into various types of functional human immune cells, establishing a human immune system within the mouse.
Research Uses for Humanized Mice
Humanized mice are an indispensable tool across several fields of biomedical research because they permit the study of human-specific processes. One primary use is in the study of human infectious diseases. Many human viruses, such as the Human Immunodeficiency Virus (HIV) and Hepatitis C Virus (HCV), are highly species-specific and cannot successfully infect or replicate in a standard mouse host.
HIS mice, with their functional human immune cells, allow researchers to model the complete viral life cycle and the complex human immune response to these pathogens. This provides a platform for testing antiviral drugs and vaccine candidates against the actual human virus. The ability to model the human immune system also extends to the development of immunotherapies, where humanized mice are used to study how new drugs, such as checkpoint inhibitors, interact with human T-cells and other immune components.
In oncology, Patient-Derived Xenograft (PDX) models have transformed cancer therapy development. PDX models are created by surgically implanting a fragment of a patient’s tumor directly into an immunodeficient mouse. This allows the tumor to grow while maintaining the original characteristics of the patient’s cancer, including its genomic mutations, cellular diversity, and architecture. This fidelity makes PDX models significantly more predictive than older models using immortalized cell lines. PDX models are now used to guide personalized medicine decisions by testing a patient’s tumor against a panel of drugs to determine the most effective treatment.
The Ethics of Humanized Animal Models
The creation and use of humanized animal models introduce unique ethical considerations beyond the general welfare of laboratory animals. A central concern is the potential for introducing human cognitive or behavioral traits into a non-human species, raising debates about consciousness and moral status. Although the goal is to humanize specific systems, the possibility that human neural stem cells could engraft in the mouse brain raises questions about whether the animal’s capacity for pain or sentience might be altered.
These models are governed by stringent regulatory oversight, primarily through Institutional Animal Care and Use Committees (IACUCs) in the United States and similar international bodies. The IACUC reviews and approves all research protocols involving vertebrate animals, ensuring that scientific benefits justify the animal model’s use. Their review is guided by the fundamental principles of the “Three Rs”: Replacement of animal models with non-animal alternatives whenever possible, Reduction of the number of animals used to the minimum required for statistical validity, and Refinement of procedures to minimize animal pain and distress. Researchers must provide specific justification for the genetic or cellular modifications and detail measures taken to mitigate any potential increase in suffering due to the human elements.