Humanized mice are specialized laboratory models engineered to contain functional human components, such as cells, tissues, or genes. They create a more accurate representation of human biology within a small animal host. This allows researchers to study human-specific diseases and test therapies, including drugs and vaccines, in an environment that better mimics the human body than a standard laboratory mouse. These models bridge the translational gap between basic animal research and clinical trials in humans.
The Essential Host: Immunodeficient Mice
Standard laboratory mice possess an active immune system that would immediately recognize and reject any foreign human cells or tissues introduced into their bodies. This rejection, known as xenograft rejection, is mounted by the host’s T cells, B cells, and Natural Killer (NK) cells. To overcome this biological barrier, the first step in creating a humanized mouse is to use a genetically modified, immunodeficient strain as the host.
Early models, such as the SCID (Severe Combined Immunodeficiency) mouse, lacked T and B cells, but still retained functional NK cells and a degree of immune activity that limited human cell engraftment. Modern humanized mouse models rely on highly immunodeficient strains like the NSG (NOD Scid Gamma) mouse. This strain carries two significant mutations: the scid mutation, which eliminates T and B cells, and a knockout of the Interleukin-2 receptor common gamma chain (\(IL2r\gamma^{null}\)), which eliminates functional NK cells and impairs cytokine signaling.
This profound lack of a functional immune system, coupled with genetic modifications that enhance the bone marrow’s ability to support human cells, creates a permissive environment. The NSG mouse essentially provides an empty biological “niche” where human cells can be transplanted and allowed to grow without being destroyed by the mouse’s own defenses. This biological prerequisite allows the humanization process to succeed and sustain the human components long-term.
Technical Methods for Humanization
The creation of a humanized mouse is achieved through two main strategies: the direct transplantation of human biological material or the genetic engineering of the mouse genome. Transplantation is the most common method for modeling complex human systems like the immune system. This process often begins with the isolation of specific human components, such as hematopoietic stem cells (HSCs) from umbilical cord blood or fetal liver tissue.
These isolated human cells are then introduced into the pre-conditioned immunodeficient host, typically via intravenous injection or sometimes intrahepatic injection in neonatal mice. The recipient mice often undergo sublethal irradiation beforehand to create space in the bone marrow and further suppress any remaining mouse immune activity. Once injected, the human HSCs migrate to the mouse’s bone marrow, where they can engraft and begin to differentiate, generating a multi-lineage human immune system within the mouse.
Another transplantation approach involves using mature peripheral blood mononuclear cells (PBMCs), which are injected into the host to rapidly reconstitute a human T-cell population. While this method is faster, the resulting humanized mice have a short lifespan due to the onset of xenogeneic graft-versus-host disease (GVHD), which occurs when the transplanted human immune cells attack the mouse tissues. For models focused on non-immune organs, such as the liver, human hepatocytes or liver tissue fragments may be transplanted after the mouse’s liver has been chemically or genetically damaged to create a regenerative niche.
The second primary method is genetic engineering, which focuses on modifying the mouse’s own DNA to express specific human proteins or receptors. Using advanced gene-editing tools like CRISPR/Cas9, researchers can replace a mouse gene with its human equivalent, a process known as “knock-in.” This technique is frequently used when a study requires a mouse to express a single, specific human molecule, such as a drug target or a viral receptor like human ACE2 for COVID-19 research. This method results in a mouse that is genetically humanized, rather than cell-humanized, and is useful for studying human-specific pathways or drug interactions without needing a full human immune system replacement.
Specific Research Models and Utility
Humanized Immune System Models (Hu-mice), created by transplanting human hematopoietic stem cells (HSCs), are invaluable for studying the human immune response in a living system. These models are extensively used in infectious disease research, allowing for the study of human-tropic pathogens like HIV and COVID-19, which cannot infect standard mice. They are also crucial for testing new immunotherapies, such as checkpoint inhibitors or CAR T-cell therapies, in a setting that contains functional human immune cells.
Humanized Liver Models are a specific type of chimerism, often created using strains like the FRG mouse, modified to allow human hepatocyte engraftment. These models are utilized for toxicology studies and to investigate drug metabolism, as the enzymatic pathways in mouse livers often differ significantly from those in humans. They are also employed in research on human liver diseases, such as Hepatitis B and C, offering a unique platform to test antiviral therapies and understand disease progression.
A third major application involves Humanized Tumor Models, often using Patient-Derived Xenografts (PDX). In this technique, actual tumor tissue from a human patient is implanted into an immunodeficient mouse. When combined with a humanized immune system, these PDX models allow researchers to test personalized cancer treatments in a system where the human tumor interacts with a human immune environment. This provides a relevant tool for predicting a patient’s response to chemotherapy, targeted therapy, or immunotherapy before beginning clinical treatment.