NSG mice are a specialized tool in modern biomedical research, offering a platform for studying human biology in a living system. The acronym “NSG” stands for NOD scid gamma, referring to genetic modifications that make these mice profoundly immunodeficient. These alterations create a “blank slate” where the mouse’s immune system is largely absent. This immunodeficiency allows for the introduction and study of foreign cells and tissues, especially human ones, without rejection. Their development has advanced our capacity to understand human diseases and test therapeutic strategies.
The Genetic Makeup of NSG Mice
NSG mice have three genetic alterations that contribute to their immunodeficient state. First, NOD refers to the Non-obese diabetic background strain. This background reduces innate immune system aspects, including a non-functional hemolytic complement system and diminished macrophage and dendritic cell activity. It also reduces natural killer (NK) cell activity, aiding foreign cell acceptance.
Second, scid stands for severe combined immunodeficiency. This mutation affects the Prkdc gene, involved in V(D)J recombination. This process is fundamental for T and B cell development, primary components of the adaptive immune system. Mice with this mutation lack functional T and B lymphocytes, eliminating adaptive immune responses.
The final modification is IL2RĪ³null, a null mutation in the gene encoding the interleukin-2 receptor gamma chain. This gamma chain is a shared component of receptors for several interleukins (IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21). Its absence severely impairs natural killer (NK) cell development and function. This triple deficiency creates one of the most immunodeficient mouse models, highly permissive for human cell engraftment.
Creating Humanized Mice
NSG mice’s profound immunodeficiency makes them suitable for accepting foreign cells and tissues, a process called engraftment. Engraftment is the successful integration and function of transplanted cells or tissues within a recipient. When tissue is transplanted between different species, like human to mouse, it’s a xenograft. NSG mice are adept recipients for xenografts because their compromised immune system prevents rejection.
Scientists introduce human hematopoietic stem cells (HSCs) into newborn NSG mouse pups, often via intrahepatic injection. These HSCs, identified by the CD34+ marker and often from umbilical cord blood, differentiate into various human immune cell types. Over time, these stem cells develop into a diverse human immune system within the mouse, including T, B, and myeloid cells, creating a “humanized” mouse. This allows study of human immune system development and function in a living model.
Another application involves implanting patient tumor tissue directly into NSG mice, forming Patient-Derived Xenograft (PDX) models. These PDX models allow human tumors to grow while largely retaining the genetic and biological characteristics of the original patient tumor. This approach provides a living model of a patient’s cancer, enabling researchers to study tumor progression and test personalized therapies.
Key Research Applications
Humanized NSG mice have impacted biomedical research, particularly in understanding and treating human diseases. These models bridge the gap between in vitro studies and human clinical trials.
In oncology, Patient-Derived Xenograft (PDX) models are a major application, advancing preclinical testing. These models involve implanting a patient’s tumor tissue directly into NSG mice, allowing the tumor to grow while largely preserving its original genetic, architectural, and microenvironmental characteristics. This approach provides a living avatar of a patient’s cancer, enabling researchers to test personalized cancer therapies and study tumor progression.
For instance, these models evaluate immunotherapy efficacy, such as immune checkpoint inhibitors like pembrolizumab and nivolumab, which activate the immune system against cancer by blocking specific pathways. Researchers observe how different tumors respond to these treatments and investigate resistance mechanisms, which is difficult in cell cultures or standard mouse models.
For infectious diseases, NSG mice are instrumental, especially for studying human-specific pathogens like HIV. Since HIV cannot naturally infect standard mice, humanized NSG models, which develop human immune systems, are invaluable. These mice recapitulate key aspects of HIV pathogenesis, including viral replication, human CD4+ T cell depletion, and latent viral reservoir establishment within lymphoid tissues. This capability allows testing of new antiviral drugs, combination antiretroviral therapies (cART), and vaccines against HIV in a living system, providing insights into viral lifecycle and treatment efficacy.
Beyond specific diseases, humanized NSG mice are powerful tools in immunology and stem cell research. They enable scientists to observe the differentiation and development of human hematopoietic stem cells into various immune cell lineages (T, B, and myeloid cells) within a controlled in vivo environment. This provides a direct window into human immune system development and function, allowing studies on human hematopoiesis and tissue colonization. Researchers investigate specific human immune responses, such as inflammatory reactions and cytokine production, and explore gene therapies for blood-based genetic disorders by modifying human stem cells before engraftment.
Model Advancements and Refinements
While standard NSG mice offer a powerful platform, scientists refine these models to better mimic the human immune system’s complexities. A limitation of the basic NSG model is its incomplete support for the full development and function of certain human immune cell types, especially myeloid cells and some T cell subsets. Additionally, xenogeneic graft-versus-host disease (xeno-GVHD) in some humanized models can shorten the experimental window.
To overcome these challenges, advanced NSG strains are developed. The NSG-SGM3 mouse, for instance, expresses human cytokines like Stem Cell Factor (SCF), Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), and Interleukin-3 (IL-3). These human cytokines more effectively support engraftment and differentiation of human myeloid cells (monocytes, dendritic cells, and granulocytes), leading to a more robust human immune system. However, these models can experience progressive anemia and myeloid cell hyperactivation syndrome, which may limit long-term study duration.
Another advancement is the NSGW41 mouse, which carries a mutation in the Kit gene. This mutation creates a more receptive environment within the mouse bone marrow, allowing efficient engraftment of human hematopoietic stem cells without prior irradiation. NSGW41 mice also demonstrate improved support for human erythropoiesis, leading to better formation of human red blood cells and platelets. These refinements enhance NSG models’ utility, enabling more comprehensive and accurate studies of human biology.