Guide: Mice with a human immune system (HIS mice) for the assessment of therapeutics: practical considerations for translational research

主なメッセージ（要約）

• HIS mice are immunodeficient mice engrafted with human cells (such as HSCs and PBMCs) or human tissue (bone marrow, liver, thymus), enabling the in vivo development of many components of the human immune system.
• HIS mice are particularly useful when the research question depends on human immune-cell biology, such as checkpoint blockade, T-cell engagers, cell therapies, cytokine-driven toxicity, or complex tumor-immune interactions.
• A key limitation of conventional HIS models such as NSG-HIS is that myeloid and NK-cell reconstitution is often incomplete, which can reduce translational relevance for mechanisms involving ADCC, phagocytosis, cytokine release, or myeloid immune suppression.
• The BRGSF background was developed to support efficient human engraftment through the introduction of NOD Sirpα polymorphism (Yamauchi et al., 2013) while avoiding some broader NOD-strain abnormalities; BRGSF mice preserve normal C5 complement function, supporting complement-relevant antibody studies better than some NOD-based hosts.
• The genO-BRGSF-HIS model is a human immune system model used for translational immunology and antibody testing. It is especially useful when researchers need to study human immune cell interactions rather than just target binding or pharmacokinetics.
• Model selection matters: choosing the wrong HIS model can lead to false efficacy signals, missed toxicity, poor biomarker interpretation, or limited translation to the clinic.

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はじめに／背景

What is a humanized mouse?

• A humanized mouse is a mouse engineered or engrafted to carry human cells, tissues, genes, or immune components, allowing selected aspects of human biology to be studied in vivo (Fujiwara et al., 2019).

What is a HIS mouse?

• A HIS mouse is a specific type of humanized mouse in which an immunodeficient recipient is transplanted with either human peripheral blood mononuclear cells (PBMCs) or human CD34+ HSCs or related human hematopoietic material. This results in the establishment of populations of human immune cells.

What is the difference between a humanized mouse and a HIS mouse?

• “Humanized mouse” is the broader category, which includes HIS mice and is often used to refer to mice in which a specific target gene has been humanized. “HIS mouse” specifically refers to models with a reconstituted human immune system.

How is a human immune system established in mice?

• Highly immunodeficient neonatal or adult mice are conditioned and transplanted with either human PBMCs or human CD34+ HSCs, commonly from umbilical cord blood, fetal liver, or mobilized peripheral sources depending on the platform and study design. Reconstitution with hPBMCs leads to the presence of circulating blood cell lineages, albeit a low presence of human myeloid cells. Engrafted HSCs migrate to hematopoietic organs and generate multiple human immune lineages over time. Treatment with recombinant Flt3L, a growth factor, in models such as the genO-BRGSF-HIS can then lead to a higher development of lymphoid and myeloid cells.

What human immune cells are present in a HIS mouse?

• Different HIS models support the development of different cell lineages to varying extents. Standard HIS mice support human CD45+ leukocytes, including B cells, T cells, monocytes/macrophage-lineage cells, and in some platforms NK cells to varying degrees.
• The exact proportions depend strongly on the recipient strain, human cell source, age at engraftment, and whether the host expresses human cytokines or other supportive factors.

The genO-BRGSF-HIS model develops both a lymphoid and myeloid compartment, including NK cells, γδ T cells, monocytes and dendritic cells.

What is the difference between CD34+-reconstituted mice and PBMC-reconstituted mice?

• CD34+-reconstituted mice are generated by engrafting human hematopoietic stem cells (HSCs). These stem cells differentiate in vivo and give rise to multiple human immune-cell lineages, including T cells, B cells, and, to varying extents, myeloid and NK cells (Patton et al., 2015; Heuts and Nagy, 2017). They provide a more physiological model of human immune-system development and are generally preferred for long-term translational studies.
• PBMC-reconstituted mice are generated by transferring mature human PBMCs, resulting in rapid engraftment of predominantly mature human T cells. These models are useful for short-term studies requiring immediate human T-cell activity, such as checkpoint inhibitor or T-cell engager screening. However, they rapidly develop xenogeneic graft-versus-host disease (GvHD), have limited study duration, and do not recapitulate physiological immune development (Fujii et al., 2015; Sun et al., 2025).

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研究に適したモデルを選ぶ方法

まずは、次の質問を自分に問いかけてみてください：

• Does my antibody rely on human immune effector cells rather than only target neutralization?
• Do I need human T cells, human myeloid cells, human NK cells, or all three?
• Is the main question efficacy, mechanism, toxicity, PK/PD, or biomarker discovery?
• Is a short, reductionist screen sufficient, or do I need decision-grade translational data?
• Which humanized mouse model provides the best immune reconstitution?

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Comparison of mice with a human immune system

Takeaway: Models that develop both a human lymphoid and myeloid compartment without associated side effects provide the highest translational value.

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Suggested comparison criteria

Immune reconstitution

• Classical NSG-HIS and related models support broad human engraftment, but immune completeness remains partial.
• BRGSF-based hosts were specifically developed to enable efficient human hematopoietic engraftment through NOD Sirpα-mediated tolerance of human grafts, enabling a higher yield of human myeloid cells.

骨髄系コンパートメント

• Conventional HIS models often underperform for human myeloid-cell abundance and function, which is why cytokine-enhanced strains were developed. However, the latter often develop health issues associated with an overexpression of certain cytokines.
• For studies where macrophages, monocytes, dendritic cells, or myeloid suppressor biology are central, researchers should verify myeloid readouts prospectively before committing to efficacy work. The genO-BRGSF-HIS mouse model has been shown to develop a robust myeloid cell compartment (Martin et al., 2024, Martin et al., 2025).

NK cells

• Human NK-cell development is frequently weak in standard NSG-HIS settings because the murine cytokine environment is suboptimal for human NK maturation.
• If the therapeutic mechanism depends on ADCC or NK-cell activation, NK-cell presence and function should be measured before dosing.
• The genO-BRGSF-HIS mouse model has been shown to develop a population of NK cells, which can be further developed following Flt3L stimulation (Martin et al., 2024, Martin et al., 2025).

Study duration

• HIS mouse studies generally require a substantial engraftment period before treatment.
• Study duration can be limited by xenogeneic inflammation, health deterioration, or protocol-specific GVHD-like pathology depending on the platform and human graft.
• The genO-BRGSF-HIS mouse model however, has been shown to enable studies of long duration (over 52 weeks).

GvHD

• GvHD-like pathology remains a major practical concern in humanized mouse models, especially in mature immune-cell transfer settings and in longer studies.
• HSC-based HIS models typically offer better duration and lower immediate xenoreactivity than PBMC-based humanization, but they do not eliminate longer-term immune pathology. Nonetheless, up to date, no development of GvHD has been reported in the genO-BRGSF-HIS mice.

Translational relevance

• HIS models outperform non-humanized hosts or syngeneic models when the question depends on human immune-cell biology, but they still do not replicate the full complexity of patients.
• For therapeutic antibodies, the best translational value comes from matching the model to the specific MoA: immune reconstitution, Fc relevance, and tumor context all matter.
• The genO-BRGSF-HIS model has been shown to have a better reconstitution of the myeloid compartment without the requirement of exogenous cytokine administration or over-expression of cytokines which frequently result in health problems. This, combined with the lack of GvHD development, make this model one of the most translatable tools.

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Human Immune Reconstitution

Which human immune cell populations develop in genO-BRGSF-HIS mice?

• Based on the broader HIS literature and BRGSF-derived host biology, genO-BRGSF-HIS mice support multilineage human hematopoiesis, including lymphoid and myeloid compartments, including γδ T cells, NK cells, dendritic cells, etc. As with all HIS systems, the depth of myeloid and NK reconstitution must be confirmed experimentally rather than assumed.

Can genO-BRGSF-HIS support long-term immune reconstitution?

• The genO-BRGSF-HIS model is routinely used in medium- to longer-term studies (over 30 weeks) after immune establishment (Martin et al., 2025, Labarthe et al., 2020). The practical upper limit depends on study design and disease burden.

Does the genO-BRGSF-HIS model support human myeloid cells?

• Yes, the genO-BRGSF-HIS model has been described to develop myeloid cells such as monocytes/macrophages, dendritic cells and neutrophils (Martin et al., 2024, Martin et al., 2025).

Does the genO-BRGSF-HIS model support human NK cells?

• Yes, the genO-BRGSF-HIS model has been described to develop NK cells (Martin et al., 2024, Martin et al., 2025). For NK-reliant antibodies, include baseline NK frequency, phenotype, activation markers, and functional readouts in study qualification.

Which immune functions are faithfully reproduced?

• genO-BRGSF-HIS mice are valuable for human T-cell responses, tumor-immune interactions, cytokine biology, and selected safety signals that are impossible to analyze in standard murine hosts.

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用途

Immuno-oncology

• The genO-BRGSF-HIS can be positioned for immuno-oncology studies, particularly where human immune context is more informative than a simple xenograft response.

Immune checkpoint inhibitor studies

• genO-BRGS-HIS mice have been used in a tumor-bearing study of nivolumab, where treatment led to a significant reduction in tumor growth, supporting the model’s value for human immune-mediated toxicity and mechanism studies (Capasso et al., 2019).

Can the genO-BRGSF-HIS reproduce human tumor microenvironments?

• Yes, the genO-BRGSF-HIS model can reproduce human tumor microenvironments (TME), and the TME in this model has been shown to be dependent on tumor burden and tumor type (Martin et al., 2025)

T細胞エンゲージャー

• The genO-BRGSF-HIS mice are well suited to T-cell engager strategies when the core mechanism depends on human T-cell activation, trafficking, and anti-tumor function (Martin et al., 2025, Majocchi et al., 2025)

Cell therapies

• genO-BRGSF-HIS mice are relevant for CAR-T, CAR-NK, and other adoptive human cell therapies, provided the immune compartment is sufficiently established and the study question is matched to the model’s strengths.

Safety assessment

• genO-BRGSF-HIS mice have been described as a valuable tool for the safety assessment of therapeutics, by analyzing the development of cytokine release syndrome (CRS) (Martin et al., 2024, Thisted et al., 2024).

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Experimental design

What level of immune-cell engraftment is required before study initiation?

• There is no universal threshold; studies should define a pre-specified minimum human CD45+ engraftment and then verify the specific effector populations relevant to the MoA.
• For checkpoint inhibitors or T-cell engagers, prioritize human T-cell levels and phenotype.
• For ADCC / innate mechanisms, prioritize NK and myeloid qualification, not only total hCD45.

Which human cell source should be used?

• Cord blood CD34+ cells are widely used and support robust HIS generation in multiple platforms.
• The cell source influences lineage output and kinetics, so source selection should be aligned with the biological question.
• It is important to include several human cell donors, given the variability of responses according to donor.

How does GvHD affect HIS mouse studies?

• GvHD-like pathology can shorten the usable study window, confound efficacy readouts, and alter cytokine and tissue pathology results.
• The genO-BRGSF-HIS mouse model, however, is not affected by GvHD.

What are the limitations of humanized mouse models?

• Major limitations include inter-animal variability, incomplete immune maturation, species-mismatched cytokine environments, xenogeneic artefacts, and incomplete tumor microenvironment fidelity.

How predictive are humanized mouse models for clinical outcomes?

• They are best viewed as translational bridge models rather than direct clinical replicas. However, the genO-BRGSF-HIS model has helped achieve IND approvals, such as for the SNS-101 compound developed by Sensei Bio (Thisted et al., 2024).

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よくある質問

What is a humanized mouse?

A mouse carrying human cells, genes, or tissues to model selected aspects of human biology in vivo.

What is a HIS mouse?

An immunodeficient mouse reconstituted with human peripheral blood mononucleated cells (PBMCs) or with human hematopoietic cells so that a human immune system develops.

What is the genO-BRGSF-HIS mouse?

A BRGSF-derived HIS model intended for studies requiring a reconstituted human immune system; peer-reviewed BRGSF literature supports the host background’s efficient human engraftment and preserved complement function. This model has been demonstrated to develop functional human lymphoid and myeloid compartments.

Which humanized mouse model should I use?

The genO-BRGSF-HIS model has been shown to be a powerful tool for studying different therapies and mechanisms: T-cell biology, myeloid biology, NK biology, Fc-mediated function, or safety.

What are the differences between NSG, NOG and genO-BRGSF-HIS mice?

NSG-HIS and NOG-HIS are the classical benchmark HIS models, while BRGSF-derived hosts were designed to maintain strong human engraftment through NOD Sirpα without the full set of NOD-related abnormalities; genO-BRGSF also retains normal C5 complement, which may be advantageous in complement-relevant antibody settings. Additionally, the genO-BRGSF-HIS model does not require exogenous cytokine treatment for the development of myeloid or NK cell populations. Furthermore, it is known to have a long therapeutic window, without the development of GvHD. The key difference between NSG and NOG mice is that the NSG mouse is a knock-out strain for the IL2rg gene, while the NOD strain has a truncated IL2rg gene.

Can genO-BRGSF-HIS mice be used for immuno-oncology?

Yes, particularly when efficacy or safety depends on human immune-cell interactions (Martin et al., 2024, Martin et al., 2025).

Does the genO-BRGSF-HIS model support NK cells?

Yes, the genO-BRGSF-HIS model has been described to develop NK cells (Martin et al., 2024, Martin et al., 2025).

Does the genO-BRGSF-HIS model develop myeloid cells?

Yes, the genO-BRGSF-HIS model has been described to develop myeloid cells such as monocytes/macrophages, dendritic cells and neutrophils (Martin et al., 2024, Martin et al., 2025).

What are the limitations of HIS mouse models?

This depends on the HIS model used (see comparative table above). However, limitations may include variability, incomplete immune maturation, limited innate-cell fidelity, xenogeneic artefacts, imperfect clinical predictivity and cost.

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参考文献

Capasso A., et al. (2019).  Characterization of immune responses to anti-PD-1 mono and combination immunotherapy in hematopoietic humanized mice implanted with tumor xenografts. J Immunother Cancer. 7:37. doi: 10.1186/s40425-019-0518-z

Fujiwara, S. (2019). Humanised mice: A brief overview on their diverse applications in biomedical research. Journal of Cellular Physiology, 233(4), 2889–2901.  DOI: 10.1002/jcp.26022

Fujii H., et al. (2015). Humanised chronic graft-versus-host disease in NOD-SCID IL2rγ−/− (NSG) mice with G-CSF-mobilised peripheral blood mononuclear cells following cyclophosphamide and total body irradiation. PLoS One, 10(7), e0133216.

Heuts, F. and Nagy, N. (2017). Mice with reconstituted human immune system components as a tool to study immune cell interactions. Methods in Molecular Biology, 1532, 229–240.

Labarthe L., et al. (2020). Frontline Science: Exhaustion and senescence marker profiles on human T cells in BRGSF-A2 humanized mice resemble those in human samples. J Leukoc Biol. 107:27–42. doi: 10.1002/JLB.5HI1018-410RR

Martin G., et al. (2024). Myeloid and dendritic cells enhance therapeutics-induced cytokine release syndrome features in humanized BRGSF-HIS preclinical model. Front Immunol. 15:1357716. doi: 10.3389/fimmu.2024.1357716

Martin G., et al. (2025). Tumor-dependent myeloid and lymphoid cell recruitment in genO-BRGSF-HIS mice: a novel tool for evaluating immunotherapies. Front. Immunol. 16:1624724. doi: 10.3389/fimmu.2025.1624724

Patton, J., et al. (2015). Evaluation of the efficiency of human immune system reconstitution in NSG mice and NSG mice containing a human HLA.A2 transgene using haematopoietic stem cells purified from different sources. Journal of Immunological Methods, 422, 13–21. DOI: 10.1016/j.jim.2015.02.007

Sun, Z., et al. (2025). Application of humanised mice in the safety experiments of antibody drugs. Animal Models and Experimental Medicine, 8(6), 1023–1032. DOI: 10.1002/ame2.12562

Thisted T., et al. (2024). VISTA checkpoint inhibition by pH-selective antibody SNS-101 with optimized safety and pharmacokinetic profiles enhances PD-1 response. Nat Commun. 15:2917. doi: 10.1038/s41467-024-47256-x

Yamauchi, T., et al. (2013). Polymorphic Sirpa is the genetic determinant for NOD-based mouse lines to achieve efficient human cell engraftment. Blood, 121(8), 1316–1325. DOI: 10.1182/blood-2012-06-440354

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