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Humanized mouse

A humanized mouse is an immunodeficient engineered to carry functional human cells, tissues, or transgenes, enabling it to mimic aspects of human physiology, particularly the , for preclinical research. These models bridge the gap between traditional mouse studies and by allowing researchers to study human-specific processes in a living organism. Developed primarily in the early , humanized mice emerged from advances in immunodeficient strains like SCID and NOD/SCID mice, which were modified to accept hematopoietic stem cells (HSCs) or other tissues without rejection. Key milestones include the creation of strains such as NSG (NOD-scid IL2Rγ null) mice around 2005, which support robust engraftment of immune components, including T cells, B cells, and myeloid cells. This evolution addressed limitations of earlier xenograft models, providing more reliable platforms for multilineage hematopoiesis and immune reconstitution. Humanized mice have become indispensable tools in biomedical , with applications spanning infectious diseases, , autoimmune disorders, and biology. For instance, they are widely used to model infection by reconstituting the human immune system, allowing evaluation of antiviral therapies and vaccine candidates . In , these models facilitate the study of human tumor growth and responses to checkpoint inhibitors, revealing insights unattainable in standard mice due to species-specific differences. Despite challenges like incomplete immune maturation and risks, ongoing refinements—such as knock-ins and tissue-specific engraftments—continue to enhance their fidelity to human conditions.

Overview

Definition and Characteristics

A humanized mouse is defined as an immunodeficient or engrafted with functional cells, tissues, or expressing human transgenes to recapitulate aspects of physiology, particularly in and disease modeling. These models integrate human hematopoietic cells or immune components into the murine host, enabling the study of -specific responses that are not feasible in standard mice due to differences. Key characteristics include the use of highly immunodeficient strains such as (SCID), NOD-scid IL2Rγ null (NSG), or NOD-shi-scid IL2Rγ null (NOG) mice as hosts, which lack functional T, B, and cells to minimize rejection of human grafts. Engraftment levels vary widely, typically achieving 20-50% human CD45+ cells in peripheral blood in standard NSG models, with advanced strains reaching up to 80%; intrahepatic administration in newborns can yield higher multilineage reconstitution. Levels of humanization range from partial, focusing on the (e.g., reconstitution of human T and B cells in the ), to more comprehensive multi-organ integration via transgenic expression of human cytokines like IL-3, GM-CSF, or thrombopoietin to support human hematopoiesis. Recent strains, such as MISTRG6 (introduced around 2024), incorporate multiple human cytokine knock-ins to support robust myeloid and innate immune cell development. Biological integration involves overcoming species-specific incompatibilities, such as (MHC) mismatches between murine and (HLA) systems, which can impair T-cell functionality; this is addressed in some strains through HLA transgenic modifications to enable HLA-restricted immune responses. cytokine production is often facilitated by knock-in transgenes in the host mouse, promoting differentiation of engrafted hematopoietic stem cells into myeloid, lymphoid, and erythroid lineages. For instance, these models support development of T and B cells, providing a platform for studying immune reconstitution without the limitations of fully syngeneic systems.

Comparison to Other Mouse Models

Humanized mice differ from syngeneic models, which involve transplanting mouse-derived tumors into immunocompetent mice of the same genetic background, by incorporating immune components into immunocompromised hosts. This enables the study of human-specific immune interactions, such as T-cell responses to human tumors, which syngeneic models cannot replicate due to inherent species differences in immune pathways and . In contrast to traditional xenograft models, where human tumor cells are implanted into immunodeficient mice lacking a functional , humanized mice integrate a reconstituted human alongside the tumor engraftment. This allows for the evaluation of immunotherapies, including checkpoint inhibitors, by modeling tumor-immune dynamics that simple xenografts overlook, as the latter primarily assess tumor growth without immune-mediated effects. Unlike transgenic mice, which feature stable insertion of human genes into the mouse genome to study specific molecular functions without human cellular components, humanized models prioritize the dynamic engraftment of human hematopoietic stem cells or tissues for comprehensive reconstitution of physiology. This engraftment approach provides a more holistic representation of human immune and tissue interactions compared to the targeted genetic modifications in transgenics. Humanized mice offer advantages in , including superior prediction of human drug responses in , with studies showing improved concordance rates for efficacy compared to standard models that achieve only around 50% alignment with clinical outcomes. They also facilitate modeling of human-specific pathogens, such as , by supporting viral replication and human immune responses that cannot be adequately studied in non-humanized systems. Despite these benefits, humanized mice present limitations relative to simpler models, including higher costs associated with specialized strains and human tissue sourcing, increased technical complexity in engraftment procedures, and reduced lifespan—often limited to 3-6 months in certain models—due to engraftment-induced stress, , or thymic lymphomas.

History

Early Immunodeficient Models

The severe combined immunodeficient (SCID) mouse model emerged as a foundational tool in the late 20th century, discovered in 1983 as a spontaneous autosomal recessive mutation in the Prkdc gene on the C.B-17 strain background. This mutation impairs DNA-dependent protein kinase catalytic subunit (DNA-PKcs), essential for V(D)J recombination during lymphocyte development, resulting in a profound lack of functional T and B cells while retaining innate immunity components like natural killer (NK) cells. The SCID phenotype provided the first reliable murine host for xenotransplantation, enabling initial experiments in human cell engraftment. In 1988, researchers demonstrated successful reconstitution by implanting human fetal thymus and liver fragments into SCID mice, creating the SCID-hu model that supported human hematolymphoid differentiation for several months. Building on this, the /SCID strain was developed in 1995 by introducing the Prkdc^scid mutation onto the non-obese diabetic () background, which inherently features multiple innate immune defects including reduced cell activity, absent hemolytic complement, and impaired function. These enhancements minimized rejection of human grafts, yielding 5- to 10-fold higher engraftment levels compared to standard SCID mice, often reaching 10-30% human cell chimerism in the . Concurrently, in the early 1990s, targeted knockouts of recombination-activating genes and Rag2 were introduced, providing cleaner models of adaptive immunity deficiency by completely blocking B- and T-cell maturation without the partial "leakiness" observed in SCID mice. Rag-deficient strains offered broader and became alternatives for human cell studies. Early applications of these models in research highlighted their utility; for instance, peripheral blood leukocyte-engrafted SCID mice (hu-PBL-SCID) supported productive HIV-1 infection, replicating viral depletion of human CD4+ T cells . A key milestone came in 2005 with the generation of /SCID mice carrying a null mutation in the interleukin-2 receptor gamma chain gene (Il2rg), termed NSG mice (a similar strain, NOG, was developed in 2002), which eliminated residual cell activity and signaling through common gamma chain receptors. This strain achieved unprecedented multi-lineage human hematopoiesis, including robust engraftment of myeloid, lymphoid, and erythroid lineages from human hematopoietic stem cells, with chimerism levels exceeding 50% in some tissues. These early immunodeficient models laid the groundwork for subsequent humanization strategies by establishing viable hosts tolerant to immune cells.

Advancements in Human Cell Engraftment

In the early 2000s, advancements in humanized mouse models built upon foundational immunodeficient strains like SCID and to enhance human hematopoietic cell engraftment through genetic modifications supporting human signaling. A key innovation involved the introduction of human transgenes, such as IL-3, GM-CSF, and , into NSG mice, creating strains like NSG-SGM3 that promoted the survival and differentiation of human myeloid and other immune cells. These knock-in models significantly improved multilineage reconstitution, with engraftment levels in peripheral blood chimerism reaching up to 50-70% in some cases, compared to lower efficiencies in unmodified hosts. A pivotal development in 2006 was the bone marrow-liver-thymus (BLT) engraftment protocol, which involved the surgical co-implantation of fetal liver and fragments under the of immunodeficient mice, followed by intravenous injection of autologous CD34+ hematopoietic stem cells from the same fetal tissue. This approach enabled development of a functional immune system, including T cell education in a thymic microenvironment, leading to robust multilineage engraftment and lymphoid formation with chimerism levels often exceeding 50% in blood and tissues. The BLT model represented a major leap in , allowing for more physiological immune responses than prior peripheral blood leukocyte transfers. During the 2010s, protocols shifted toward using adult-derived hematopoietic stem cells to address ethical and availability concerns with fetal tissues, with intravenous injection of mobilized peripheral blood CD34+ cells into preconditioned NSG or similar hosts achieving multi-lineage reconstitution of B, T, and NK cells. These methods, often involving low-dose irradiation or busulfan conditioning, yielded peripheral blood chimerism of 20-60% and sustained engraftment for months, facilitating studies of adult human hematopoiesis without fetal dependencies. Engraftment efficiency broadly improved from around 10% in early models to up to 80% in optimized strains by the late 2010s, driven by refined cell dosing and host preconditioning. Recent innovations from 2020 to 2025 have further refined engraftment through and niche optimization, including / modifications to host genomes for enhanced support of cell integration. For instance, -edited immunodeficient mice have been developed to express -specific factors improving vascularization and tissue integration of engrafted cells, boosting long-term multilineage chimerism. A 2024 study introduced mice, engineered with thymic epithelial cells via optimized niches, enabling mature class-switching, hypermutation, and neutralizing responses—achieving near-complete immune reconstitution with diverse B and T cell repertoires. These advances have elevated overall engraftment efficiencies, with some models now sustaining 70-80% chimerism in peripheral blood for over a year.

Types

Peripheral Blood Leukocyte-Engrafted Models

Peripheral blood leukocyte-engrafted models, commonly known as Hu-PBL models, involve the direct injection of mature peripheral blood leukocytes (PBLs) into immunodeficient mice to achieve rapid reconstitution of a human immune component, primarily for short-term studies of T-cell responses. The seminal Hu-PBL-SCID model, developed in 1988, utilizes intravenous or of activated human PBLs into severe combined immunodeficient (SCID) mice, which lack functional T and B s, allowing for efficient engraftment without rejection. This approach results in substantial human immune reconstitution, with 20-50% human CD45+ leukocytes observed in the and of recipient mice. The mechanism of engraftment in these models favors the expansion of mature T cells, including both + helper and + cytotoxic subsets, which dominate the human immune compartment due to their and proliferation in response to xenogeneic stimuli. However, B cells and natural killer () cells show limited persistence and function, as the murine lacks the necessary supportive niches, such as human-specific and stromal elements, for their long-term and activity. This T-cell-centric reconstitution makes Hu-PBL models particularly valuable for investigating T-cell-mediated immune responses, such as , production, and antigen-specific , in a . Variants of the Hu-PBL model, such as those using /SCID mice, enhance engraftment efficiency and tolerance by incorporating additional defects in innate immunity, including reduced NK cell activity and complement function, which minimize early rejection of human cells. These models have been applied in studies of allograft rejection, where human T cells mounted against transplanted human tissues mimic clinical immune responses. Engraftment typically peaks at 4-6 weeks post-injection, after which human leukocyte levels decline due to the onset of xenogeneic (GVHD), limiting the model's utility to short-term experiments. During peak engraftment, human IgG serum levels can reach up to 1 mg/ml, reflecting polyclonal B-cell-derived production, though without significant class switching or affinity maturation.

Hematopoietic Stem Cell-Engrafted Models

(HSC)-engrafted models utilize human + HSCs, typically isolated from blood or , transplanted into immunodeficient mice to achieve sustained, multi-lineage human hematopoiesis. The foundational Hu-SRC-SCID model, established in , demonstrated that intravenous injection of human cells into sublethally irradiated SCID mice could lead to multilineage reconstitution, including myeloid and lymphoid lineages, following stimulation. Subsequent adaptations incorporated purified + cells from blood, administered via intravenous or intra-bone injection, to enhance engraftment efficiency and reproducibility. Standard protocols involve preconditioning recipient SCID mice with total body irradiation at 200-300 cGy to deplete endogenous hematopoiesis and facilitate niche availability in the . Following transplantation of 1-5 × 10^5 CD34+ cells, human cell engraftment in the can reach up to 40% of total hematopoietic cells, supporting the production of human red blood cells and platelets alongside other lineages. These models enable long-term engraftment lasting 6-12 months, far exceeding the short-term, T-cell-dominant reconstitution seen in peripheral blood leukocyte-engrafted models. Key advancements include the development of /SCID/IL2Rγ-null (NSG) mice in the early 2000s, which exhibit reduced innate immunity and support higher levels of HSC engraftment and lymphoid development compared to standard SCID strains. Further enhancements in the 2010s involved engineering NSG variants with cytokine transgenes, such as (SCF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3), to promote robust B-cell maturation and production that were limited in earlier models. Additionally, advances in mobilizing adult HSCs using (G-CSF) enabled the use of peripheral blood-derived + cells as an alternative to sources, improving accessibility for engraftment studies. In these systems, T-cell maturation proceeds independently of a thymus, relying instead on peripheral expansion and mouse thymic support for positive selection.

Bone Marrow, Liver, and Thymus Models

The , liver, and (BLT) humanized mouse model involves the surgical implantation of human fetal and liver fragments under the of immunodeficient mice, such as /SCID strains, followed by intravenous injection of autologous + hematopoietic stem cells derived from the same fetal liver tissue. This approach, first described in 2006, facilitates the development of a structured by providing a thymic scaffold for T-cell . In the BLT model, de novo human thymopoiesis occurs within the implanted thymic tissue, resulting in the production of HLA-restricted T cells that recognize human molecules. These mice achieve high levels of human engraftment, typically 70-90% human hematopoietic cells in peripheral blood, including functional B cells capable of producing IgM and IgG antibodies. The model supports multilineage immune reconstitution, including T cells, B cells, natural killer cells, monocytes, and dendritic cells, conferring the highest degree of immune functionality among human cell engraftment models. Variants of the BLT model include adaptations using adult tissues, such as mobilized peripheral blood CD34+ cells instead of fetal sources, to reduce ethical concerns associated with fetal tissue procurement while maintaining robust engraftment. In the 2020s, refinements incorporating NOD-scid IL2Rγnull (NSG) strains, known as NSG-BLT mice, have enhanced utility for studying HIV-1 latency and persistence, enabling prolonged observation of viral reservoirs post-antiretroviral therapy interruption. Despite its strengths, the model carries a risk of (GVHD) onset after approximately 8-12 weeks in some configurations, limiting long-term studies, though strain-specific modifications can extend viability beyond 20 weeks without GVHD. These mice have been instrumental in vaccine testing, demonstrating human-like adaptive immune responses to pathogens such as and Epstein-Barr virus.

Organoid and Tissue Transplant Models

Organoid and tissue transplant models involve the implantation of human-derived organoids or tissues into immunodeficient mice to recapitulate specific organ functions, such as drug metabolism and tissue-specific physiology, independent of immune reconstitution. These models leverage the three-dimensional architecture of organoids to better mimic human organ microenvironments compared to two-dimensional cultures. Immunodeficient hosts, such as NSG mice, facilitate tolerance and engraftment of human tissues. Liver humanization through or tissue transplantation emerged prominently in the , with key advancements including the implantation of human s or fetal liver tissues into strains like /SCID or FRG mice to enable human-specific studies. For instance, in 2011, human -transplanted mice demonstrated repopulation of up to 90% of the liver with human cells, replicating enzymatic profiles for pharmacokinetic evaluation. These models have been instrumental in assessing drug toxicity and metabolism, showing human-like responses to compounds metabolized by () enzymes. Beyond the liver, brain organoids derived from induced pluripotent cells (iPSCs) have been transplanted into mice to model neurodegeneration, with 2023 studies demonstrating functional integration of organoids into striatal circuits, restoring motor function in models without tumor formation. Similarly, intestinal organoids transplanted under the capsule of humanized mice support research by enabling microbial exposure that induces epithelial immune responses and IgA production, mimicking gut-microbe interactions. Transplantation techniques include subcutaneous implantation for accessibility and orthotopic placement, such as under the , where a small incision allows gentle deposition of organoids into a created using a needle. Vascularization occurs via host endothelium invasion or co-engraftment with endothelial cells, promoting nutrient delivery and long-term survival; for example, transplanted liver organoids integrate with vasculature to form perfused structures. Recent 2025 advances include multi-zonal liver organoids that self-assemble from iPSC-derived progenitors to exhibit zone-specific functions like urea cycle activity, enhancing their utility for transplantation in injury models. Engraftment survival reaches up to several months, with iPSC-derived liver organoids maintaining human albumin secretion and CYP450-mediated for at least five weeks post-transplantation in mice. These features underscore the models' potential for studying organ-specific diseases and drug responses. In 2025, further improvements in vascularized multi-organoid systems have enabled better integration and functionality in mouse hosts.

Genetic and Chimeric Models

Genetic humanized mouse models involve the insertion of genes into the mouse genome to confer specific human-like physiological or immunological traits, enabling stable, heritable expression across generations. These transgenic approaches typically target immune-related loci, such as (HLA) genes, to facilitate MHC-matched immune responses. For instance, Hu-HLA transgenic mice express HLA class I or II molecules, allowing for the development of HLA-restricted T-cell responses that mimic and improve the evaluation of efficacy or tumor immunity. Similarly, transgenic expression of cytokines like GM-CSF and IL-3 supports the differentiation and function of human myeloid cells, enhancing the model's utility in studying hematopoiesis without requiring exogenous cell engraftment. Advancements in , particularly CRISPR-Cas9 in the 2020s, have enabled precise knock-in and knock-out strategies to replace mouse genes with human orthologs, further refining humanization. A prominent example is the humanization of the SIRPα gene, where the mouse Sirpa locus is replaced with human SIRPA, alleviating the incompatibility between mouse SIRPα and human that otherwise leads to macrophage-mediated rejection of human cells. This modification significantly enhances the engraftment and survival of human hematopoietic cells, particularly macrophages, in the mouse host, providing a more permissive environment for studying human immune dynamics. These genetic alterations are stably transmitted through the , ensuring consistent phenotypes in progeny and reducing variability in experimental outcomes. Chimeric humanized models, developed from 2017 onward, utilize blastocyst complementation techniques to fuse early-stage embryos with pluripotent stem cells, achieving multi-tissue humanization without direct genetic integration into the . In this method, induced pluripotent stem cells (iPSCs) are injected into immunodeficient blastocysts, allowing cells to contribute to various lineages during development, with chimerism levels reaching up to 10% cells in some tissues like the or vasculature. These models support the formation of humanized structures, such as neural or vascular networks, and offer insights into interspecies developmental compatibility, though human contribution remains limited compared to intraspecies chimeras. Unlike engraftment-based alternatives, these chimeras provide inherent, non-transient humanization for studying . In 2025, genetic humanized mouse models dominate the market with a 55% share, driven by their reproducibility and applicability in , particularly for antibody humanization. Seminal platforms, such as those with megabase-scale humanization of immunoglobulin loci, enable mice to produce fully human antibodies in response to immunization, accelerating the development of therapeutic monoclonal with reduced risks. These models have been instrumental in generating high-affinity antibodies for and infectious diseases, bypassing the need for humanization techniques.

Applications

Infectious Disease Modeling

Humanized mice provide a critical platform for studying human-specific infectious , enabling the examination of pathogen-host immune interactions that cannot be adequately replicated in standard models due to species-specific barriers in entry, replication, and immune . These models, particularly those with engrafted human hematopoietic cells or tissues, support the full lifecycle of human pathogens and allow of immune responses, persistence, and therapeutic interventions in a controlled setting. By recapitulating aspects of immunity, such as T-cell and antibody-mediated control, humanized mice facilitate insights into disease and development for viruses that do not naturally infect mice. In HIV/AIDS research, bone marrow-liver-thymus (BLT) and hematopoietic stem cell (HSC)-engrafted models effectively mimic viral reservoirs and latency, harboring latent HIV-1 in human T cells, macrophages, and other long-lived cells similar to human infection. These models demonstrate sustained viremia, CD4+ T-cell depletion, and the establishment of tissue reservoirs upon infection, providing a system to study persistence despite antiretroviral therapy (ART). Recent 2023 investigations using these models have evaluated long-acting antiretrovirals, achieving up to 90% viral suppression in treated animals while revealing residual latent reservoirs in lymphoid tissues, informing strategies for HIV cure. For instance, in HSC-engrafted non-obese diabetic severe combined immunodeficiency gamma chain knockout (NSG) mice, ART reduces plasma viremia to undetectable levels but fails to eliminate integrated proviral DNA, highlighting the need for latency-reversing agents. Liver-humanized mouse models, generated by engrafting human hepatocytes into immunodeficient strains like Fah-/- Rag2-/- Il2rg-/- (FRG), are essential for hepatitis B virus (HBV) and hepatitis C virus (HCV) studies, supporting chronic infections that persist for months and recapitulating the full viral lifecycle absent in unmodified mice. Human hepatocytes in these models enable HBV cccDNA formation and HCV RNA replication, allowing assessment of viral spread, immune-mediated liver damage, and therapeutic efficacy; for example, chronic HBV infection in such mice lasts at least 169 days with stable human albumin levels, mimicking human disease progression. These platforms have been used to test nucleoside analogs and entry inhibitors, demonstrating reduced viral loads and histological changes akin to human chronic hepatitis. For emerging pathogens, ACE2-transgenic humanized mice have been instrumental in COVID-19 modeling since 2020, with strains like K18-hACE2 expressing human ACE2 receptors to permit SARS-CoV-2 entry and replication in respiratory and systemic tissues, leading to severe disease outcomes including neuroinfection and weight loss. These models, often combined with human immune reconstitution, evaluate vaccine candidates and monoclonal antibodies, showing dose-dependent protection and reduced viral titers post-immunization through 2025 studies. Similarly, immune-reconstituted humanized mice assess influenza and Zika virus vaccine efficacy; for influenza, BLT models support human-like antibody responses and viral clearance, while for Zika, HSC-engrafted mice generate neutralizing human antibodies and T-cell immunity, with DNA vaccines conferring robust protection against lethal challenge by eliciting prM/E-specific responses. Specific examples underscore the utility of these models in dissecting immune-pathogen dynamics. In peripheral blood leukocyte (PBL)-engrafted humanized mice, Epstein-Barr virus (EBV) infection elicits robust human T-cell responses, including HLA-A2-restricted cytotoxic activity against infected B cells, leading to control of lymphoproliferation and persistent infection without full clearance. For , standard mouse cells resist replication due to type I pathway differences, such as murine STAT2-mediated restriction, but human cell engraftment in NSG mice overcomes this barrier, enabling high , production, and T-cell activation that mirror human severe disease.

Cancer Research

Humanized mice serve as critical platforms for studying tumor engraftment and responses in , enabling the recapitulation of human tumor-immune interactions. Patient-derived xenografts (PDX) implanted into NOD-scid IL2rgamma null (NSG) mice engrafted with human peripheral blood mononuclear cells (PBMCs) or hematopoietic stem cells (HSCs) facilitate testing of chimeric receptor T-cell (CAR-T) therapies. For example, HER2-targeted CAR-T cells have demonstrated significant tumor reduction in humanized models of and other solid tumors, mirroring clinical scenarios where autologous T cells target patient-specific cancers. Similarly, recent immuno-oncology studies using 2024 models have validated PD-1 blockade efficacy, with anti-PD-1 antibodies enhancing T-cell infiltration and reducing tumor growth in NSG mice bearing non-small cell PDXs. In and research, HSC-engrafted humanized mice provide robust models for (AML) and myelodysplastic syndromes (MDS), allowing evaluation of targeted therapies within a functional human hematopoietic system. These models support multilineage engraftment and disease progression, as seen in NSG-SGM3 mice where patient-derived AML cells retain mutational profiles like TP53 and SF3B1. A 2025 review highlights their utility in assessing , a inhibitor, combined with hypomethylating agents, showing synergistic effects in overcoming resistance in MDS PDX models without excessive toxicity to normal hematopoiesis. For solid tumors, organoid co-engraftment in humanized mice advances modeling of breast and prostate cancers by integrating tumor organoids with human immune components to study the tumor microenvironment. Patient-derived breast cancer organoids paired with PDXs in immunodeficient strains engrafted with human HSCs enable testing of therapies that target tumor-stroma interactions. Additionally, human natural killer (NK) cells in these models enhance antibody-dependent cellular cytotoxicity (ADCC), as demonstrated in IL-15-supported NSG variants where NK cells potentiate anti-tumor responses. Humanized mice offer improved prediction of clinical responses compared to non-humanized xenografts, with studies indicating higher concordance rates for outcomes due to authentic human immune-tumor dynamics. In Hu-PBL (human PBMC-engrafted) models, bispecific antibodies targeting CD3 and tumor antigens, such as CD123 in AML, have shown potent T-cell redirection and tumor clearance, informing designs for bispecific T-cell engagers.

Autoimmune and Immunological Studies

Humanized mouse models have proven instrumental in elucidating the pathogenesis of autoimmune diseases, particularly (T1D), by incorporating human genetic elements and tissues into immunodeficient backgrounds. In NOD-humanized models engrafted with human , T1D progression has been observed through mechanisms involving expression, where human T cells recognize islet autoantigens, leading to insulitis and beta-cell destruction. These 2025 models highlight polygenic control of T cell-mediated , mirroring human T1D more closely than traditional by demonstrating human-specific responses and immune infiltration patterns. For rheumatoid arthritis (RA), HLA-transgenic mice engrafted with human synovial tissues serve as platforms for studying autoantibody production and joint pathology. In HLA-DR4 transgenic models, citrullinated proteins like fibrinogen trigger arthritis via T cell activation and autoantibody formation, recapitulating RA synovial inflammation and pannus formation observed in patients. These systems reveal how human HLA alleles present arthritogenic peptides, driving B cell responses and autoantibodies such as anti-citrullinated protein antibodies (ACPAs), which are absent in non-transgenic controls. In basic , advanced humanized models from 2024 enable the study of class-switching and critical for transplantation. The mouse, engineered for estrogen-supported immune maturation, supports mature class-switched, hypermutated responses, including IgG and IgA production by B cells, addressing limitations in prior models lacking full human lymphoid architecture. These models also facilitate , as demonstrated by IL-2 muteins expanding human regulatory T cells (Tregs) to prolong allograft survival without broad . Multi-lineage reconstitution in and HSC-engrafted mice underpins these findings by providing diverse human immune compartments. Humanized systems further uncover species differences in Treg function, such as reduced suppressive capacity of human Tregs compared to murine counterparts in suppressing effector T cells, influencing autoimmune regulation. Additionally, humanized mice model (GVHD), a key immunological complication post-transplantation, by injecting human peripheral mononuclear cells into immunodeficient hosts like NSG strains. This xenogeneic GVHD mimics human acute and chronic forms, with T cell activation causing multi-organ damage, including skin, liver, and gut involvement, allowing evaluation of therapeutic interventions targeting donor T cells. These models emphasize -specific cytokine profiles and tissue tropism, distinguishing them from syngeneic murine GVHD.

Regenerative Medicine and Gene Therapy

Humanized mice have emerged as valuable platforms for advancing through the engraftment of (iPSC)-derived cells into organoid models, particularly for cardiac and . In these models, iPSC-derived organoids are transplanted into immunodeficient mice, enabling long-term engraftment and functional integration of tissues. For instance, vascularized liver organoids generated from iPSCs demonstrate scalable production and sustained albumin secretion post-transplantation in mice, recapitulating complex cellular interactions essential for hepatic repair. Similarly, 2023 studies highlighted the vascular integration of iPSC-derived cardiac organoids in humanized hosts, where endothelial cells facilitate and maturation, mimicking native vascularization critical for regenerative therapies. These approaches allow researchers to study -specific tissue repair mechanisms that are challenging to replicate or in non-human models. In applications, humanized mouse models with chimeric livers provide an ideal system for evaluating (AAV) delivery, such as in treatments for hemophilia. Liver-chimeric mice engrafted with human hepatocytes, like the TIRFA or FRG strains, exhibit enhanced efficiency with clinically relevant AAV serotypes, enabling targeted expression of clotting factors in human cells. Recent advances in editing of human hematopoietic stem cells (HSCs) , reported in 2024 and 2025, demonstrate efficient on-target modifications in engrafted HSCs within humanized mice, achieving up to 29% editing levels via lipid nanoparticle delivery without significant . However, these models reveal human-specific off-target effects in editing, where genetic variations alter cleavage sites compared to murine genomes, underscoring the need for variant-aware assessments to improve therapeutic precision. Beyond organ-specific regeneration, humanized mice support broader applications in and neural repair. Human skin grafts transplanted onto humanized mice models enable the study of full-thickness wound closure, with robust re-epithelialization and immune-mediated healing observed post-injury, providing insights into human dermal regeneration. For neural repair, brain-chimeric mice generated by grafting iPSC-derived human neurons into models facilitate circuit restoration, as evidenced by 2025 studies showing integration and functional recovery of motor deficits in hosts. The growing relevance of these models is reflected in the humanized mouse market for , projected to reach USD 113.6 million in 2025 with a (CAGR) of 5.8%, driven by demand for translational regenerative research.

Challenges and Future Directions

Technical and Biological Limitations

Humanized mouse models exhibit significant engraftment variability, primarily due to differences in the quality and source of human + hematopoietic stem cells (HSCs), which can lead to inconsistent reconstitution levels across experiments. For instance, HSCs derived from or fetal liver often achieve higher engraftment rates compared to those from adult , but donor-specific factors such as genetic polymorphisms and cell viability further contribute to this variability. Additionally, the onset of (GVHD) typically limits the duration of studies to 3-6 months in HSC-engrafted models, as human T cells attack murine tissues, causing progressive immune dysfunction and requiring early endpoint termination to maintain . Incomplete humanization remains a core biological limitation, as these models fail to fully recapitulate the immune architecture. Notably, there is an absence of human lymph nodes and (GALT), which impairs the development of organized secondary lymphoid structures essential for adaptive immune responses. Myeloid cell function is also suboptimal, with poor and of human monocytes, macrophages, and dendritic cells, largely attributable to a mismatch between human and murine SIRPα, which disrupts and innate immune surveillance. In bone marrow-liver-thymus () models, GVHD risks are somewhat mitigated but still highlight these gaps in lymphoid organization. Technical challenges further constrain the utility of humanized mice, including high production costs exceeding $10,000 per model when accounting for specialized immunodeficient strains, human cell sourcing, surgical engraftment, and longitudinal monitoring. Preconditioning regimens, such as sublethal , introduce additional variability by inducing inconsistent myeloablation and potential off-target effects on murine tissues, complicating across cohorts. These factors demand rigorous protocols to minimize batch-to-batch differences. Biologically, humanized mice cannot support human pregnancy or a fully humanized (CNS), limiting their applicability to reproductive or neuroinflammatory studies. Species-specific metabolic differences, particularly in hepatic profiles, result in altered (PK), with reports of up to 30% discordance in clearance rates between humanized liver models and human data, which can mislead predictions of and . These gaps underscore the models' partial fidelity to human physiology despite advances in engraftment techniques.

Emerging Improvements and Ethical Considerations

Recent advancements in CRISPR/Cas9 technology have enhanced the compatibility of humanized mouse models by enabling precise editing of host immune genes, such as HLA class I and II loci, to reduce rejection of human cells and improve engraftment efficiency. For instance, in 2025, CRISPR-edited mice demonstrated successful integration of allogeneic human regulatory T cells, facilitating better functional reconstitution of the human immune system without severe . Similarly, Cas9-mediated editing of human + hematopoietic stem cells in immunodeficient mice has allowed for the recapitulation of specific human immune gene losses, advancing studies . These modifications prioritize targeted gene knockouts to mimic human physiological responses more accurately. Efforts to optimize engraftment protocols have focused on refining regimens and selections, leading to more consistent human cell reconstitution in models like NSG and NCG mice. While AI-driven approaches remain emerging, standardized protocols for intravenous injection of human + cells into newborn or adult hosts have achieved high-level, multi-lineage engraftment, supporting long-term development up to several months. In 2025, these optimizations have been pivotal for immuno-oncology research, where humanized mice provide clinically relevant insights into therapies like CAR-T cells, though variability in engraftment rates persists as a challenge targeted by ongoing refinements. Multi-organ humanization has progressed through interspecies chimerism techniques, with 2025 studies on complementation primarily demonstrating enhanced integration in rat-mouse and human-pig models, including contributions to organs like liver and , though human-mouse chimeras face ongoing efficiency challenges due to developmental mismatches. Such advances build on 2024 embryo model research, where human cells populated multiple embryonic structures in interspecies contexts, offering a platform for studies without full transmission. Ethical considerations surrounding humanized mice, particularly neural chimeras, center on the potential for human-like arising from significant human neural cell integration, prompting debates on moral status and animal . A 2019 analysis highlighted risks of enhanced cognitive capacities in chimeric brains, advocating for oversight to prevent unintended humanization. The 2023 Third Summit on Editing reinforced global consensus against heritable modifications, deeming them unacceptable for clinical use due to safety and equity concerns, with guidelines extending to research to prohibit reproductive applications. In 2025, developed regulations for cell-based models, guided by ethical considerations to balance research benefits and risks. Animal welfare issues in long-term models include monitoring for chronic stress from human cell burdens, as emphasized in Society for Research (ISSCR) guidelines, which call for refined endpoints and humane husbandry to minimize suffering in extended engraftment studies. Future directions emphasize hybrid systems integrating humanized mice with (OoC) platforms to combine systemic effects with microfluidic precision for disease modeling. iPSC-derived OoCs, projected for widespread use by 2025, enable by recapitulating patient-specific responses to drugs, potentially reducing reliance on animal models while accelerating translation. These integrations could address limitations like by validating OoC predictions in chimeric contexts, fostering ethical, efficient preclinical pipelines.

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