Immunodeficient Mouse Models

What is the primary difference between immunocompetent and immunodeficient mice?
The difference lies in the functional status of their immune systems:

Immunocompetent mice: These mice have a functional immune system, including T, B, and NK cells. Common strains include C57BL/6 and BALB/c. They support studies of normal immune responses, syngeneic tumors, vaccines, and autoimmune diseases. They also support immune checkpoint inhibitor testing using agents that recognize mouse targets or appropriately humanized target models.

Immunodeficient mice: These mice carry genetic mutations that impair specific immune components. Strains such as BALB/c nude (deficient in mature T cells) and NKG (lacking mature T cells, B cells, and functional NK cells) support the engraftment of human cells and tissues, including tumors, by reducing immune-mediated rejection.

Nude mice (1960s). The story began with the discovery of the athymic nude mouse in 1962. A Foxn1 mutation leaves these mice without a thymus and mature T cells, and in 1969 they hosted the first human tumor xenograft. But they keep functional B cells and highly active NK cells, which limits the human tissues they can accept.

SCID mice (1983). A mutation in the Prkdc gene blocks both T and B cell development. However, many SCID mice become “leaky” as they age, producing some functional T and B cells, and their NK activity stays high.

NOD-SCID mice (1990s). Moving the scid mutation onto the NOD background added the NOD strain’s innate immune defects, including reduced NK cell, macrophage and complement function, and greatly reduced leakiness. Residual NK activity and a short lifespan caused by thymic lymphomas remained limitations.

Il2rg-null mice (2000s). The major breakthrough came from disrupting signaling through the IL-2 receptor common gamma chain gene (Il2rg) on the NOD-scid background, as reported for NOG (2002) and NSG (2005) mice. This removed functional NK cells. Cyagen's NKG mouse (NOD.Cg-Prkdc^scid Il2rg^em1Cya/Cya) belongs to this generation: it lacks mature T cells, B cells and functional NK cells and is one of the most permissive hosts for human cell and tissue engraftment.

Next-generation strains. Newer strains build on this platform. Some express human cytokines, some lack mouse MHC molecules, and some carry Kit mutations, so they better support human immune and hematopoietic cells.

Establishing a human tumor xenograft follows a standardized workflow:


1. Select the host. Choose a strain based on the tumor type and study goal. Options range from T cell-deficient BALB/c nude mice, often enough for CDX, to severely immunodeficient NKG mice (lacking T, B and functional NK cells), which suit difficult-to-engraft PDX, hematologic tumors and humanized models.

2. Prepare the material. For CDX, expand human cancer cells in culture, harvest them in log-phase growth, and resuspend them in serum-free medium, often mixed with a basement membrane matrix such as Matrigel. For PDX, keep fresh patient tumor tissue in cold preservation medium, cut it into small fragments (about 2–3 mm³), and implant it as soon as possible.

3. Inoculate or implant. Inject cells or implant tissue fragments subcutaneously (usually into the flank) for easy monitoring, or orthotopically (into the organ of origin) to better recreate the native tumor microenvironment and metastatic behavior. Hematologic tumors are usually injected intravenously.

4. Monitor and passage. Track tumor growth with calipers or non-invasive imaging such as bioluminescence. Once tumors reach a defined size, passage PDX tumors into new mice and cryopreserve them to build a stable model bank.

5. Validate. Confirm that the xenograft keeps the features of the original tumor with histopathology (H&E and immunohistochemistry) and STR or genomic profiling. Rule out mouse-cell overgrowth and EBV-associated human lymphoma before using the model in efficacy studies.

Immunodeficient mice are needed because of immune rejection. A mouse with an intact immune system recognizes human cells as foreign, and its T cells, supported by B cells, antibodies, NK cells, macrophages and complement, reject the human graft within days to weeks. Strains such as BALB/c nude, NOD-SCID and NKG mice lack some or all of these immune barriers, so human tumor cells can survive, become vascularized by the host, and grow as solid tumors. Researchers can then study the behavior of real human cancers in a living system and test human-specific anti-cancer therapies in vivo before clinical development.

Loss of functional NK cells. Knocking out Il2rg (γc/CD132) blocks IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 signaling, which prevents NK cell development. This is the main reason human cell engraftment is so efficient.

Reduced innate rejection. The NOD background carries a SIRPα variant that binds human CD47 and lowers macrophage phagocytosis of human cells, along with a complement C5 deficiency.

Stability. Leakiness is minimal, so mice rarely develop functional mouse T or B cells as they age.

Longevity. Unlike NOD-SCID mice, Il2rg-null strains rarely develop thymic lymphomas and live much longer, which supports long-term studies such as humanized immune system models.

Support for humanization. Derivative strains engineered to express human cytokines (e.g., IL-3, GM-CSF, SCF, IL-15) or to lack mouse MHC molecules better support specific human immune cell lineages. Strains lacking mouse MHC molecules can reduce xenogeneic GvHD, particularly in PBMC-humanized models.

SCID (Severe Combined Immunodeficient) mice carry a mutation in the Prkdc gene, which results in a near-complete deficiency of functional T and B lymphocytes.

Key Applications:
1. Tumor Xenografts (CDX): Widely used as hosts for the transplantation of human tumor cell lines to evaluate preclinical drug efficacy.
2. Humanization: Serving as a platform for reconstituting the human immune system.
3. Immunology Research: Studying the basic functions of the innate immune system (e.g., NK cells) in the absence of adaptive immunity.