Biomedical research often requires animal models that can accept human cells or tissues without rejection, especially for studying the human immune system. This need led to the development of immunodeficient mice, which lack a functioning immune system. These mice allow for the investigation of human-specific biological processes, from cancer progression to infectious diseases.
Among these, the NRG mouse is a notable model. Its severe immunodeficiency makes it an excellent recipient for human cells and tissues, enabling researchers to create accurate models of human health. Studying these processes in a living organism provides insights not possible through cell cultures or computer simulations alone.
Genetic Foundation of NRG Mice
The name “NRG” is an acronym for the three genetic features of this mouse strain: NOD, Rag1, and IL2rg. Each component contributes a specific defect to the mouse’s immune system to create a state of severe immunodeficiency. The foundation is the Non-Obese Diabetic (NOD) genetic background, which confers several immune alterations, including functional defects in innate immune cells like macrophages. A feature of the NOD background is a variation in the Sirpa gene, which helps the mouse’s macrophages tolerate transplanted human cells and reduces the chance of rejection.
Building on the NOD background, NRG mice have a “knockout” of the Rag1 gene, meaning the gene has been intentionally inactivated. The Rag1 gene is responsible for V(D)J recombination, a process where developing immune cells shuffle gene segments to create a vast diversity of receptors. Without a functional Rag1 gene, both B cells and T cells, the primary drivers of adaptive immunity, cannot mature. This modification eliminates the branch of the immune system responsible for recognizing specific pathogens.
The final modification is a knockout of the gene for the interleukin-2 receptor common gamma chain (IL2rg or γc). This protein is a shared component of receptors for several cytokines, which are signaling molecules necessary for lymphocyte development. The absence of the IL2rg chain has the most significant impact on Natural Killer (NK) cells, as they require signaling through this receptor to develop, resulting in their complete absence.
The Resulting Immunodeficient State
The combination of the NOD background with the Rag1 and IL2rg knockouts produces a mouse with a profound immunodeficiency. The most direct consequence is the complete absence of the three major lymphocyte populations: T cells, B cells, and NK cells. This lack of cells means the mouse is incapable of mounting an adaptive immune response, the highly specific defense mechanism that targets pathogens and generates immunological memory. Without B cells, the mouse cannot produce antibodies, and without T cells, it cannot coordinate a targeted cellular attack or eliminate infected cells.
This absence of adaptive immunity is compounded by defects in the innate immune system. The NOD background leads to poorly functioning macrophages and dendritic cells. The elimination of NK cells due to the IL2rg mutation removes another line of innate defense specialized in destroying stressed or cancerous cells. The result is an animal that is an ideal recipient for foreign cells for research.
The inability of NRG mice to recognize and attack foreign material means they can accept grafts of human cells and tissues without triggering immune rejection. This feature allows researchers to study human components in a living system. Furthermore, the Rag1 mutation makes these mice more resistant to radiation compared to some other immunodeficient strains, an advantage when pre-conditioning is needed for experiments.
Core Applications in Biomedical Research
The ability of NRG mice to host human cells makes them useful in several research fields.
- Oncology: Researchers create patient-derived xenograft (PDX) models by implanting a patient’s tumor into an NRG mouse. Because the mouse’s immune system is suppressed, the human tumor can grow, allowing for testing the efficacy of different cancer drugs on a specific patient’s cancer in a personalized way.
- Infectious Disease: These mice can be engrafted with human immune cells to study pathogens that only infect humans. This approach allows for investigating how a pathogen interacts with the human immune system, how it causes disease, and how potential vaccines or antiviral therapies might work.
- Stem Cell Biology: Scientists can transplant human hematopoietic stem cells into NRG mice and observe their development into various types of blood and immune cells. This is used to understand the fundamental processes of human blood formation and to test stem cell transplantation therapies.
- Immunology: Despite lacking their own immune system, NRG mice serve as living platforms for human immune cells. This enables the study of human immune function and responses to new therapies in a controlled in vivo environment.
Advancing Human Disease Modeling through Humanization
A primary application of NRG mice is the creation of “humanized” models, which involves engrafting them with components of an entire human system. The most common approach is to build a human immune system within the mouse. This creates a more complex and accurate model for studying human biological processes and predicting how therapies might behave in people.
One method involves injecting human hematopoietic stem cells (HSCs), often from umbilical cord blood, into NRG mice. These stem cells travel to the mouse’s bone marrow, where they differentiate and produce a multi-lineage human immune system, including T cells and B cells. This process allows for long-term studies of the development and function of a human immune system within a living organism.
Another technique uses peripheral blood mononuclear cells (PBMCs), which are mature immune cells from an adult human donor. Engrafting PBMCs into an NRG mouse provides a rapid way to establish a model with adult human T cells. By observing how a human immune system responds to a new drug or a pathogen inside the mouse, researchers can make more informed predictions about how that therapy might behave in people.