Improving human hemato-lymphoid-system mice by cytokine knock-in gene replacement

Engraftment of adult hematopoietic stem and progenitor cells in a novel model of humanized mice

Pre-clinical use of humanized mice transplanted with CD34+ hematopoietic stem and progenitor cells (HSPCs) is limited by insufficient engraftment with adult non-mobilized HSPCs. Here, we developed a novel immunodeficient mice based on NOD-SCID-Il2γc-/- (NSG) mice to support long-term engraftment with human adult HSPCs. As both Flt3L and IL-6 are critical for many aspects of hematopoiesis, we knock-out mouse Flt3 and knock-in human IL6 gene. The resulting mice showed an increase in the availability of mouse Flt3L to human cells and a dose-dependent production of human IL-6 upon activation. Upon transplantation with low number of human HSPCs from adult bone marrow, these humanized mice demonstrated a significantly higher engraftment with multilineage differentiation of human lymphoid and myeloid cells, and tissue colonization at one year after adult HSPC transplant. Thus, these mice enable studies of human hematopoiesis and tissue colonization over time and may facilitate building autologous models for immuno-oncology studies.

Progress, implications, and challenges in using humanized immune system mice in CAR-T therapy-Application evaluation and improvement

In recent years, humanized immune system (HIS) mice have been gradually used as models for preclinical research in pharmacotherapies and cell therapies with major breakthroughs in tumor and other fields, better mimicking the human immune system and the tumor immune microenvironment, compared to traditional immunodeficient mice. To better promote the application of HIS mice in preclinical research, we selectively summarize the current prevalent and breakthrough research and evaluation of chimeric antigen receptor (CAR) -T cells in various antiviral and antitumor treatments. By exploring its application in preclinical research, we find that it can better reflect the actual clinical patient condition, with the advantages of providing high-efficiency detection indicators, even for progressive research and development. We believe that it has better clinical patient simulation and promotion for the updated design of CAR-T cell therapy than directly transplanted immunodeficient mice. The characteristics of the main models are proposed to improve the use defects of the existing models by reducing the limitation of antihost reaction, combining multiple models, and unifying sources and organoid substitution. Strategy study of relapse and toxicity after CAR-T treatment also provides more possibilities for application and development.

Long-term engraftment of adult hematopoietic progenitors in a novel model of humanized mice

Pre-clinical use of humanized mice transplanted with CD34 + hematopoietic progenitor cells (HPCs) is limited by insufficient engraftment with adult HPCs. Here, we developed a novel immunodeficient mice based in NOD-SCID- Il2γc -/- (NSG) mice to support long-term engraftment with human adult HPCs and tissue colonization with human myeloid cells. As both Flt3L and IL-6 are critical for many aspects of hematopoiesis, we knock-out mouse Flt3 and knock-in human IL6 gene. The resulting mice showed an increase in the availability of mouse Flt3L to human cells, and a dose-dependent production of human IL-6 upon activation. Upon transplantation with low number of human HPCs from adult bone marrow, these humanized mice demonstrated a significantly higher engraftment with multilineage differentiation of human lymphoid and myeloid cells. Furthermore, higher frequencies of human lymphoid and myeloid cells were detected in tissues at one year after adult HPC transplant. Thus, these mice enable studies of human hematopoiesis and tissue colonization over time.

Summary

Pre-clinical use of humanized mice is limited by insufficient engraftment with adult hematopoietic progenitor cells (HPCs). Here, we developed a novel immunodeficient mice which support long-term engraftment with adult bone marrow HPCs and facilitate building autologous models for immuno-oncology studies.

Single-cell RNA sequencing of human lung innate lymphoid cells in the vascular and tissue niche reveals molecular features of tissue adaptation

Innate lymphoid cells (ILCs) are sentinels of healthy organ function, yet it is unknown how ILCs adapt to distinct anatomical niches within tissues. Here, we used a unique humanized mouse model, MISTRG mice transplanted with human hematopoietic stem and progenitor cells (HSPCs), to define the gene signatures of human ILCs in the vascular versus the tissue (extravascular) compartment of the lung. Single-cell RNA sequencing in combination with intravascular cell labeling demonstrated that heterogeneous populations of human ILCs and natural killer (NK) cells occupied the vascular and tissue niches in the lung of HSPC-engrafted MISTRG mice. Moreover, we discovered that niche-specific cues shape the molecular programs of human ILCs in the distinct sub-anatomical compartments of the lung. Specifically, extravasation of ILCs into the lung tissue was associated with the upregulation of genes involved in the acquisition of tissue residency, cell positioning within the lung, sensing of tissue-derived signals, cellular stress responses, nutrient uptake, and interaction with other tissue-resident immune cells. We also defined a core tissue signature shared between human ILCs and NK cells in the extravascular space of the lung, consistent with imprinting by signals from the local microenvironment. The molecular characterization of human ILCs and NK cells in the vascular and tissue niches of the lung provides new knowledge on the mechanisms of ILC tissue adaptation and represents a resource for further studies.

Humanized MISTRG as a preclinical in vivo model to study human neutrophil-mediated immune processes

Introduction

MISTRG mice have been genetically modified to allow development of a human myeloid compartment from engrafted human CD34+ haemopoietic stem cells, making them particularly suited to study the human innate immune system in vivo. Here, we characterized the human neutrophil population in these mice to establish a model that can be used to study the biology and contribution in immune processes of these cells in vivo.

Methods and results

We could isolate human bone marrow neutrophils from humanized MISTRG mice and confirmed that all neutrophil maturation stages from promyelocytes (CD11b–CD16–) to end-stage segmented cells (CD11b+CD16+) were present. We documented that these cells possessed normal functional properties, including degranulation, reactive oxygen species production, adhesion, and antibody-dependent cellular cytotoxicity towards antibody-opsonized tumor cells ex vivo. The acquisition of functional capacities positively correlated with the maturation state of the cell. We found that human neutrophils were retained in the bone marrow of humanized MISTRG mice during steady state. However, the mature segmented CD11b+CD16+ human neutrophils were released from the bone marrow in response to two well-established neutrophil-mobilizing agents (i.e., G-CSF and/or CXCR4 antagonist Plerixafor). Moreover, the neutrophil population in the humanized MISTRG mice actively reacted to thioglycolate-induced peritonitis and could infiltrate implanted human tumors, as shown by flow cytometry and fluorescent microscopy.

Discussion

These results show that functional human neutrophils are generated and can be studied in vivo using the humanized MISTRG mice, providing a model to study the various functions of neutrophils in inflammation and in tumors.

A single-cell map of vascular and tissue lymphocytes identifies proliferative TCF-1+ human innate lymphoid cells

Innate lymphoid cells (ILCs) play important roles in tissue homeostasis and host defense, but the proliferative properties and migratory behavior of especially human ILCs remain poorly understood. Here we mapped at single-cell resolution the spatial distribution of quiescent and proliferative human ILCs within the vascular versus tissue compartment. For this purpose, we employed MISTRG humanized mice as an in-vivo model to study human ILCs. We uncovered subset-specific differences in the proliferative status between vascular and tissue ILCs within lymphoid and non-lymphoid organs. We also identified CD117^-CRTH2^-CD45RA⁺ ILCs in the spleen that were highly proliferative and expressed the transcription factor TCF-1. These proliferative ILCs were present during the neonatal period in human blood and emerged early during population of the human ILC compartment in MISTRG mice transplanted with human hematopoietic stem and progenitor cells (HSPCs). Single-cell RNA-sequencing combined with intravascular cell labeling suggested that proliferative ILCs actively migrated from the local vasculature into the spleen tissue. Collectively, our comprehensive map reveals the proliferative topography of human ILCs, linking cell migration and spatial compartmentalization with cell division.

CD116+ fetal precursors migrate to the perinatal lung and give rise to human alveolar macrophages

Despite their importance in lung health and disease, it remains unknown how human alveolar macrophages develop early in life. Here we define the ontogeny of human alveolar macrophages from embryonic progenitors in vivo, using a humanized mouse model expressing human cytokines (MISTRG mice). We identified alveolar macrophage progenitors in human fetal liver that expressed the GM-CSF receptor CD116 and the transcription factor MYB. Transplantation experiments in MISTRG mice established a precursor-product relationship between CD34-CD116+ fetal liver cells and human alveolar macrophages in vivo. Moreover, we discovered circulating CD116+CD64-CD115+ macrophage precursors that migrated from the liver to the lung. Similar precursors were present in human fetal lung and expressed the chemokine receptor CX3CR1. Fetal CD116+CD64- macrophage precursors had a proliferative gene signature, outcompeted adult precursors in occupying the perinatal alveolar niche, and developed into functional alveolar macrophages. The discovery of the fetal alveolar macrophage progenitor advances our understanding of human macrophage origin and ontogeny.

CD5 Surface Expression Marks Intravascular Human Innate Lymphoid Cells That Have a Distinct Ontogeny and Migrate to the Lung

Innate lymphoid cells (ILCs) contribute to immune defense, yet it is poorly understood how ILCs develop and are strategically positioned in the lung. This applies especially to human ILCs due to the difficulty of studying them in vivo. Here we investigated the ontogeny and migration of human ILCs in vivo with a humanized mouse model (“MISTRG”) expressing human cytokines. In addition to known tissue-resident ILC subsets, we discovered CD5-expressing ILCs that predominantly resided within the lung vasculature and in the circulation. CD5⁺ ILCs contained IFNγ-producing mature ILC1s as well as immature ILCs that produced ILC effector cytokines under polarizing conditions in vitro. CD5⁺ ILCs had a distinct ontogeny compared to conventional CD5^- ILCs because they first appeared in the thymus, spleen and liver rather than in the bone marrow after transplantation of MISTRG mice with human CD34⁺ hematopoietic stem and progenitor cells. Due to their strategic location, human CD5⁺ ILCs could serve as blood-borne sentinels, ready to be recruited into the lung to respond to environmental challenges. This work emphasizes the uniqueness of human CD5⁺ ILCs in terms of their anatomical localization and developmental origin compared to well-studied CD5^- ILCs.

A STAT5B-CD9 axis determines self-renewal in hematopoietic and leukemic stem cells

The transcription factors STAT5A and STAT5B are critical in hematopoiesis and leukemia. They are widely believed to have redundant functions but we describe a unique role for STAT5B in driving the self-renewal of hematopoietic and leukemic stem cells (HSCs/LSCs). We find STAT5B to be specifically activated in HSCs and LSCs, where it induces many genes associated with quiescence and self-renewal, including the surface marker CD9. Levels of CD9 represent a prognostic marker for patients with STAT5-driven leukemia and our findings suggest that anti-CD9 antibodies may be useful in their treatment to target and eliminate LSCs. We show that it is vital to consider STAT5A and STAT5B as distinct entities in normal and malignant hematopoiesis.

IL-6 blockade reverses bone marrow failure induced by human acute myeloid leukemia

Most patients with acute myeloid leukemia (AML) die from complications arising from cytopenias resulting from bone marrow (BM) failure. The common presumption among physicians is that AML-induced BM failure is primarily due to overcrowding, yet BM failure is observed even with low burden of disease. Here, we use large clinical datasets to show the lack of correlation between BM blast burden and degree of cytopenias at the time of diagnosis. We develop a splenectomized xenograft model to demonstrate that transplantation of human primary AML into immunocompromised mice recapitulates the human disease course by induction of BM failure via depletion of mouse hematopoietic stem and progenitor populations. Using unbiased approaches, we show that AML-elaborated IL-6 acts to block erythroid differentiation at the proerythroblast stage and that blocking antibodies against human IL-6 can improve AML-induced anemia and prolong overall survival, suggesting a potential therapeutic approach.

Innate Immune Reconstitution in Humanized Bone Marrow-Liver-Thymus (HuBLT) Mice Governs Adaptive Cellular Immune Function and Responses to HIV-1 Infection

Humanized bone marrow-liver-thymus (HuBLT) mice are a revolutionary small-animal model that has facilitated the study of human immune function and human-restricted pathogens, including human immunodeficiency virus type 1 (HIV-1). These mice recapitulate many aspects of acute and chronic HIV-1 infection, but exhibit weak and variable T-cell responses when challenged with HIV-1, hindering our ability to confidently detect HIV-1-specific responses or vaccine effects. To identify the cause of this, we comprehensively analyzed T-cell development, diversity, and function in HuBLT mice. We found that virtually all HuBLT were well-reconstituted with T cells and had intact TCRβ sequence diversity, thymic development, and differentiation to memory and effector cells. However, there was poor CD4+ and CD8+ T-cell responsiveness to physiologic stimuli and decreased TH1 polarization that correlated with deficient reconstitution of innate immune cells, in particular monocytes. HIV-1 infection of HuBLT mice showed that mice with higher monocyte reconstitution exhibited greater CD8+ T cells responses and HIV-1 viral evolution within predicted HLA-restricted epitopes. Thus, T-cell responses to immune challenges are blunted in HuBLT mice due to a deficiency of innate immune cells, and future efforts to improve the model for HIV-1 immune response and vaccine studies need to be aimed at restoring innate immune reconstitution.

Animal Models of Hepatitis B Virus Infection-Success, Challenges, and Future Directions

Chronic hepatitis B virus (HBV) infection affects more than 250 million people worldwide, which greatly increases the risk for terminal liver diseases, such as liver cirrhosis and hepatocellular carcinoma (HCC). Even though current approved antiviral therapies, including pegylated type I interferon (IFN) and nucleos(t)ide analogs, can effectively suppress viremia, HBV infection is rarely cured. Since HBV exhibits a narrow species tropism and robustly infects only humans and higher primates, progress in HBV research and preclinical testing of antiviral drugs has been hampered by the scarcity of suitable animal models. Fortunately, a series of surrogate animal models have been developed for the study of HBV. An increased understanding of the barriers towards interspecies transmission has aided in the development of human chimeric mice and has greatly paved the way for HBV research in vivo, and for evaluating potential therapies of chronic hepatitis B. In this review, we summarize the currently available animal models for research of HBV and HBV-related hepadnaviruses, and we discuss challenges and future directions for improvement.

Humanized Mouse Models to Evaluate Cancer Immunotherapeutics

Immunotherapy is at the forefront of cancer treatment. The advent of numerous novel approaches to cancer immunotherapy, including immune checkpoint antibodies, adoptive transfer of CAR (chimeric antigen receptor) T cells and TCR (T cell receptor) T cells, NK (natural killer) cells, T cell engagers, oncolytic viruses, and vaccines, is revolutionizing the treatment for different tumor types. Some are already in the clinic, and many others are underway. However, not all patients respond, resistance develops, and as available therapies multiply there is a need to further understand how they work, how to prioritize their clinical evaluation, and how to combine them. For this, animal models have been highly instrumental, and humanized mice models (i.e., immunodeficient mice engrafted with human immune and cancer cells) represent a step forward, although they have several limitations. Here, we review the different humanized models available today, the approaches to overcome their flaws, their use for the evaluation of cancer immunotherapies, and their anticipated evolution as tools to help personalized clinical decision-making.

Building the Next Generation of Humanized Hemato-Lymphoid System Mice

Since the late 1980s, mice have been repopulated with human hematopoietic cells to study the fundamental biology of human hematopoiesis and immunity, as well as a broad range of human diseases in vivo. Multiple mouse recipient strains have been developed and protocols optimized to efficiently generate these “humanized” mice. Here, we review three guiding principles that have been applied to the development of the currently available models: (1) establishing tolerance of the mouse host for the human graft; (2) opening hematopoietic niches so that they can be occupied by human cells; and (3) providing necessary support for human hematopoiesis. We then discuss four remaining challenges: (1) human hematopoietic lineages that poorly develop in mice; (2) limited antigen-specific adaptive immunity; (3) absent tolerance of the human immune system for its mouse host; and (4) sub-functional interactions between human immune effectors and target mouse tissues. While major advances are still needed, the current models can already be used to answer specific, clinically-relevant questions and hopefully inform the development of new, life-saving therapies.

Continuous human uterine NK cell differentiation in response to endometrial regeneration and pregnancy

Immune cell differentiation is critical for adequate tissue-specific immune responses to occur. Here, we studied differentiation of human uterine natural killer cells (uNK cells). These cells reside in a tissue undergoing constant regeneration and represent the major leukocyte population at the maternal-fetal interface. However, their physiological response during the menstrual cycle and in pregnancy remains elusive. By surface proteome and transcriptome analysis as well as using humanized mice, we identify a differentiation pathway of uNK cells in vitro and in vivo with sequential acquisition of killer cell immunoglobulin-like receptors and CD39. uNK cell differentiation occurred continuously in response to the endometrial regeneration and was driven by interleukin-15. Differentiated uNK cells displayed reduced proliferative capacity and immunomodulatory function including enhanced angiogenic capacity. By studying human uterus transplantation and monozygotic twins, we found that the uNK cell niche could be replenished from circulation and that it was under genetic control. Together, our study uncovers a continuous differentiation pathway of human NK cells in the uterus that is coupled to profound functional changes in response to local tissue regeneration and pregnancy.

Distinct developmental pathways from blood monocytes generate human lung macrophage diversity

The study of human macrophages and their ontogeny is an important unresolved issue. Here, we use a humanized mouse model expressing human cytokines to dissect the development of lung macrophages from human hematopoiesis in vivo. Human CD34⁺ hematopoietic stem and progenitor cells (HSPCs) generated three macrophage populations, occupying separate anatomical niches in the lung. Intravascular cell labeling, cell transplantation, and fate-mapping studies established that classical CD14⁺ blood monocytes derived from HSPCs migrated into lung tissue and gave rise to human interstitial and alveolar macrophages. In contrast, non-classical CD16⁺ blood monocytes preferentially generated macrophages resident in the lung vasculature (pulmonary intravascular macrophages). Finally, single-cell RNA sequencing defined intermediate differentiation stages in human lung macrophage development from blood monocytes. This study identifies distinct developmental pathways from circulating monocytes to lung macrophages and reveals how cellular origin contributes to human macrophage identity, diversity, and localization in vivo.

Tumor-macrophage crosstalk: how to listen

The tumor microenvironment contains many cellular components influencing tumor behaviors, such as metastasis, angiogenesis and chemo-resistance. Tumor-associated macrophages (TAMs) are one of such components that can also manipulate the overall prognosis and patient survival. Analysis of tumor-macrophage crosstalk is crucial as tumor cells can polarize circulatory monocytes into TAMs. Such trans-polarization of macrophages support tumor mediated evasion and suppression of immune response. Additionally, such TAMs significantly influence tumor growth and proliferation, making them a potential candidate for precision therapeutics. However, the failure of macrophage-dependent therapies at clinical trials emphasizes the fault in current perception and research modality. This review discussed this field's progress regarding emerging model systems with a focused view on the in vitro platforms. The inadequacy of currently available models and their implications on existing studies also analyzed. The need for a conceptual and experimental leap toward a human-relevant in vitro custom-built platform for studying tumor-macrophage crosstalk is acknowledged.

Benefits and limitations of humanized mice in HIV persistence studies

Significant advances in the treatment of HIV infection have been made in the last three decades. Antiretroviral therapy (ART) is now potent enough to prevent virus replication and stop disease progression. However, ART alone does not cure the infection, primarily because HIV can persist in stable long-term reservoir cells including latently-infected CD4 + T cells. A central goal of the HIV research field is to devise strategies to eliminate these reservoirs and thereby develop a cure for HIV. This requires robust in vivo model systems to facilitate both the further characterization of persistent HIV reservoirs and evaluation of methods for eliminating latent virus. Humanized mice have proven to be versatile experimental models for studying many basic aspects of HIV biology. These models consist of immunodeficient mice transplanted with human cells or tissues, which allows development of a human immune system that supports robust infection with HIV. There are many potential applications for new generations of humanized mouse models in investigating HIV reservoirs and latency, but these models also involve caveats that are important to consider in experimental design and interpretation. This review briefly discusses some of the key strengths and limitations of humanized mouse models in HIV persistence studies.

Human hematopoietic stem cell maintenance and myeloid cell development in next-generation humanized mouse models

Next-generation humanized mice differentially support human HSPC maintenance and myelopoiesis. MISTRG mice support long-term human HSPC maintenance demonstrated by quaternary transplantation and development of human tissue macrophages.

Humanized mice as preclinical models for myeloid malignancies

Humanized mice have proven to be invaluable for human hematological translational research since they offer essential tools to dissect disease biology and to bridge the gap between pre-clinical testing of novel therapeutics and their clinical applications. Many efforts have been placed to advance and optimize humanized mice to support the engraftment, differentiation, and maintenance of hematopoietic stem cells (HSCs) and the human hematological system in order to broaden the scope of applications of such models. This review covers the background of humanized mice, how they are used as platforms to model myeloid malignancies, and the various current and potential approaches to further enhance their utilization in biomedical research.

Origin and ontogeny of lung macrophages: from mice to humans

Macrophages are tissue-resident myeloid cells with essential roles in host defense, tissue repair, and organ homeostasis. The lung harbors a large number of macrophages that reside in alveoli. As a result of their strategic location, alveolar macrophages are critical sentinels of healthy lung function and barrier immunity. They phagocytose inhaled material and initiate protective immune responses to pathogens, while preventing excessive inflammatory responses and tissue damage. Apart from alveolar macrophages, other macrophage populations are found in the lung and recent single-cell RNA-sequencing studies indicate that lung macrophage heterogeneity is greater than previously appreciated. The cellular origin and development of mouse lung macrophages has been extensively studied, but little is known about the ontogeny of their human counterparts, despite the importance of macrophages for lung health. In this context, humanized mice (mice with a human immune system) can give new insights into the biology of human lung macrophages by allowing in vivo studies that are not possible in humans. In particular, we have created humanized mouse models that support the development of human lung macrophages in vivo. In this review, we will discuss the heterogeneity, development, and homeostasis of lung macrophages. Moreover, we will highlight the impact of age, the microbiota, and pathogen exposure on lung macrophage function. Altered macrophage function has been implicated in respiratory infections as well as in common allergic and inflammatory lung diseases. Therefore, understanding the functional heterogeneity and ontogeny of lung macrophages should help to develop future macrophage-based therapies for important lung diseases in humans.

Human macrophages and innate lymphoid cells: Tissue-resident innate immunity in humanized mice

Macrophages and innate lymphoid cells (ILCs) are tissue-resident cells that play important roles in organ homeostasis and tissue immunity. Their intricate relationship with the organs they reside in allows them to quickly respond to perturbations of organ homeostasis and environmental challenges, such as infection and tissue injury. Macrophages and ILCs have been extensively studied in mice, yet important species-specific differences exist regarding innate immunity between humans and mice. Complementary to ex-vivo studies with human cells, humanized mice (i.e. mice with a human immune system) offer the opportunity to study human macrophages and ILCs in vivo within their surrounding tissue microenvironments. In this review, we will discuss how humanized mice have helped gain new knowledge about the basic biology of these cells, as well as their function in infectious and malignant conditions. Furthermore, we will highlight active areas of investigation related to human macrophages and ILCs, such as their cellular heterogeneity, ontogeny, tissue residency, and plasticity. In the near future, we expect more fundamental discoveries in these areas through the combined use of improved humanized mouse models together with state-of-the-art technologies, such as single-cell RNA-sequencing and CRISPR/Cas9 genome editing.

HTLV-1 viral oncogene HBZ drives bone destruction in adult T cell leukemia

Osteolytic bone lesions and hypercalcemia are common, serious complications in adult T cell leukemia/lymphoma (ATL), an aggressive T cell malignancy associated with human T cell leukemia virus type 1 (HTLV-1) infection. The HTLV-1 viral oncogene HBZ has been implicated in ATL tumorigenesis and bone loss. In this study, we evaluated the role of HBZ on ATL-associated bone destruction using HTLV-1 infection and disease progression mouse models. Humanized mice infected with HTLV-1 developed lymphoproliferative disease and continuous, progressive osteolytic bone lesions. HTLV-1 lacking HBZ displayed only modest delays to lymphoproliferative disease but significantly decreased disease-associated bone loss compared with HTLV-1-infected mice. Gene expression array of acute ATL patient samples demonstrated increased expression of RANKL, a critical regulator of osteoclasts. We found that HBZ regulated RANKL in a c-Fos-dependent manner. Treatment of HTLV-1-infected humanized mice with denosumab, a monoclonal antibody against human RANKL, alleviated bone loss. Using patient-derived xenografts from primary human ATL cells to induce lymphoproliferative disease, we also observed profound tumor-induced bone destruction and increased c-Fos and RANKL gene expression. Together, these data show the critical role of HBZ in driving ATL-associated bone loss through RANKL and identify denosumab as a potential treatment to prevent bone complications in ATL patients.

Epigenetic aging of human hematopoietic cells is not accelerated upon transplantation into mice

Background

Transplantation of human hematopoietic stem cells into immunodeficient mice provides a powerful in vivo model system to gain functional insights into hematopoietic differentiation. So far, it remains unclear if epigenetic changes of normal human hematopoiesis are recapitulated upon engraftment into such “humanized mice.” Mice have a much shorter life expectancy than men, and therefore, we hypothesized that the xenogeneic environment might greatly accelerate the epigenetic clock.

Results

We demonstrate that genome-wide DNA methylation patterns of normal human hematopoietic development are indeed recapitulated upon engraftment in mice—particularly those of normal early B cell progenitor cells. Furthermore, we tested three epigenetic aging signatures, and none of them indicated that the murine environment accelerated age-associated DNA methylation changes.

Conclusions

Epigenetic changes of human hematopoietic development are recapitulated in the murine transplantation model, whereas epigenetic aging is not accelerated by the faster aging environment and seems to occur in the cell intrinsically.

Modeling Human Antitumor Responses In Vivo Using Umbilical Cord Blood-Engrafted Mice

Mice engrafted with human immune cells offer powerful in vivo model systems to investigate molecular and cellular processes of tumorigenesis, as well as to test therapeutic approaches to treat the resulting cancer. The use of umbilical cord blood mononuclear cells as a source of human immune cells for engraftment is technically straightforward, and provides T lymphocytes and autologous antigen-presenting cells (including B cells, monocytes, and DCs) that bear cognate antigen presenting molecules. By using a human-specific oncogenic virus, such as Epstein-Barr virus, de novo neoplastic transformation of the human B cells can be induced in vivo in a manner that models progressive stages of tumorigenesis from nascent neoplasia to the establishment of vascularized tumor masses with an immunosuppressive environment. Moreover, since tumorigenesis occurs in the presence of autologous T cells, this type of system can be used to investigate how T cells become suppressed during tumorigenesis, and how immunotherapies counteract immunosuppression. This minireview will provide a brief overview of the use of human umbilical cord blood transplanted into immunodeficient murine hosts to model antitumor responses.

Expression of Exogenous Cytokine in Patient-derived Xenografts via Injection with a Cytokine-transduced Stromal Cell Line

Patient-derived xenograft (PDX) mice are produced by transplanting human cells into immune deficient mice. These models are an important tool for studying the mechanisms of normal and malignant hematopoiesis and are the gold standard for identifying effective chemotherapies for many malignancies. PDX models are possible because many of the mouse cytokines also act on human cells. However, this is not the case for all cytokines, including many that are critical for studying normal and malignant hematopoiesis in human cells. Techniques that engineer mice to produce human cytokines (transgenic and knock-in models) require significant expense before the usefulness of the model has been demonstrated. Other techniques are labor intensive (injection of recombinant cytokine or lentivirus) and in some cases require high levels of technical expertise (hydrodynamic injection of DNA). This report describes a simple method for generating PDX mice that have exogenous human cytokine (TSLP, thymic stromal lymphopoietin) via weekly intraperitoneal injection of stroma that have been transduced to overexpress this cytokine. Use of this method provides an in vivo source of continuous cytokine production that achieves physiological levels of circulating human cytokine in the mouse. Plasma levels of human cytokine can be varied based on the number of stromal cells injected, and cytokine production can be initiated at any point in the experiment. This method also includes cytokine-negative control mice that are similarly produced, but through intraperitoneal injection of stroma transduced with a control vector. We have previously demonstrated that leukemia cells harvested from TSLP-expressing PDX, as compared to control PDX, exhibit a gene expression pattern more like the original patient sample. Together the cytokine-producing and cytokine-negative PDX mice produced by this method provide a model system that we have used successfully to study the role of TSLP in normal and malignant hematopoiesis.

Genetically engineered mesenchymal stromal cells produce IL-3 and TPO to further improve human scaffold-based xenograft models

Recently, NOD-SCID IL2Rγ^-/- (NSG) mice were implanted with human mesenchymal stromal cells (MSCs) in the presence of ceramic scaffolds or Matrigel to mimic the human bone marrow (BM) microenvironment. This approach allowed the engraftment of leukemic samples that failed to engraft in NSG mice without humanized niches and resulted in a better preservation of leukemic stem cell self-renewal properties. To further improve our humanized niche scaffold model, we genetically engineered human MSCs to secrete human interleukin-3 (IL-3) and thrombopoietin (TPO). In vitro, these IL-3- and TPO-producing MSCs were superior in expanding human cord blood (CB) CD34⁺ hematopoietic stem/progenitor cells. MLL-AF9-transduced CB CD34⁺ cells could be transformed efficiently along myeloid or lymphoid lineages on IL-3- and TPO-producing MSCs. In vivo, these genetically engineered MSCs maintained their ability to differentiate into bone, adipocytes, and other stromal components. Upon transplantation of MLL-AF9-transduced CB CD34⁺ cells, acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) developed in engineered scaffolds, in which a significantly higher percentage of myeloid clones was observed in the mouse compartments compared with previous models. Engraftment of primary AML, B-cell ALL, and biphenotypic acute leukemia (BAL) patient samples was also evaluated, and all patient samples could engraft efficiently; the myeloid compartment of the BAL samples was better preserved in the human cytokine scaffold model. In conclusion, we show that we can genetically engineer the ectopic human BM microenvironment in a humanized scaffold xenograft model. This approach will be useful for functional study of the importance of niche factors in normal and malignant human hematopoiesis.

Modeling Natural Killer Cell Targeted Immunotherapies

Animal models have extensively contributed to our understanding of human immunobiology and to uncover the underlying pathological mechanisms occurring in the development of diseases. However, mouse models do not reproduce the genetic and molecular complexity inherent in human disease conditions. Human immune system (HIS) mouse models that are susceptible to human pathogens and can recapitulate human hematopoiesis and tumor immunobiology provide one means to bridge the interspecies gap. Natural killer cells are the founding member of the innate lymphoid cell family. They exert a rapid and strong immune response against tumor and pathogen-infected cells. Their antitumor features have long been exploited for therapeutic purposes in the context of cancer. In this review, we detail the development of highly immunodeficient mouse strains and the models currently used in cancer research. We summarize the latest improvements in adoptive natural killer (NK) cell therapies and the development of novel NK cell sources. Finally, we discuss the advantages of HIS mice to study the interactions between human NK cells and human cancers and to develop new therapeutic strategies.

Experimental animal modeling for immuno-oncology

Immuno-oncology (I/O) research has intensified significantly in recent years due to the breakthrough development and the regulatory approval of several immune checkpoint inhibitors, leading to the rapid expansion of the new discovery of novel I/O therapies, new checkpoint inhibitors and beyond. However, many I/O questions remain unanswered, including why only certain subsets of patients respond to these treatments, who the responders would be, and how to expand patient response (the conversion of non-responders or maximizing response in partial responders). All of these require relevant I/O experimental systems, particularly relevant preclinical animal models. Compared to other oncology drug discovery, e.g. cytotoxic and targeted drugs, a lack of relevant animal models is a major obstacle in I/O drug discovery, and an urgent and unmet need. Despite the obvious importance, and the fact that much I/O research has been performed using many different animal models, there are few comprehensive and introductory reviews on this topic. This article attempts to review the efforts in development of a variety of such models, as well as their applications and limitations for readers new to the field, particularly those in the pharmaceutical industry.

Establishing human leukemia xenograft mouse models by implanting human bone marrow-like scaffold-based niches

To begin to understand the mechanisms that regulate self-renewal, differentiation, and transformation of human hematopoietic stem cells or to evaluate the efficacy of novel treatment modalities, stem cells need to be studied in their own species-specific microenvironment. By implanting ceramic scaffolds coated with human mesenchymal stromal cells into immune-deficient mice, we were able to mimic the human bone marrow niche. Thus, we have established a human leukemia xenograft mouse model in which a large cohort of patient samples successfully engrafted, which covered all of the important genetic and risk subgroups. We found that by providing a humanized environment, stem cell self-renewal properties were better maintained as determined by serial transplantation assays and genome-wide transcriptome studies, and less clonal drift was observed as determined by exome sequencing. The human leukemia xenograft mouse models that we have established here will serve as an excellent resource for future studies aimed at exploring novel therapeutic approaches.

Humanized hemato-lymphoid system mice

Over the last decades, incrementally improved xenograft mouse models, supporting the engraftment and development of a human hemato-lymphoid system, have been developed and now represent an important research tool in the field. The most significant contributions made by means of humanized mice are the identification of normal and leukemic hematopoietic stem cells, the characterization of the human hematopoietic hierarchy, and their use as preclinical therapy models for malignant hematopoietic disorders. Successful xenotransplantation depends on three major factors: tolerance by the mouse host, correct spatial location, and appropriately cross-reactive support and interaction factors such as cytokines and major histocompatibility complex molecules. Each of these can be modified. Experimental approaches include the genetic modification of mice to faithfully express human support factors as non-cross-reactive cytokines, to create free niche space, the co-transplantation of human mesenchymal stem cells, the implantation of humanized ossicles or other stroma, and the implantation of human thymic tissue. Besides the source of hematopoietic cells, the conditioning regimen and the route of transplantation also significantly affect human hematopoietic development in vivo. We review here the achievements, most recent developments, and the remaining challenges in the generation of pre-clinically-predictive systems for human hematology and immunology, closely resembling the human situation in a xenogeneic mouse environment.

The Zebrafish Xenograft Platform: Evolution of a Novel Cancer Model and Preclinical Screening Tool

Animal xenografts of human cancers represent a key preclinical tool in the field of cancer research. While mouse xenografts have long been the gold standard, investigators have begun to use zebrafish (Danio rerio) xenotransplantation as a relatively rapid, robust and cost-effective in vivo model of human cancers. There are several important methodological considerations in the design of an informative and efficient zebrafish xenotransplantation experiment. Various transgenic fish strains have been created that facilitate microscopic observation, ranging from the completely transparent casper fish to the Tg(fli1:eGFP) fish that expresses fluorescent GFP protein in its vascular tissue. While human cancer cell lines have been used extensively in zebrafish xenotransplantation studies, several reports have also used primary patient samples as the donor material. The zebrafish is ideally suited for transplanting primary patient material by virtue of the relatively low number of cells required for each embryo (between 50 and 300 cells), the absence of an adaptive immune system in the early zebrafish embryo, and the short experimental timeframe (5-7 days). Following xenotransplantation into the fish, cells can be tracked using in vivo or ex vivo measures of cell proliferation and migration, facilitated by fluorescence or human-specific protein expression. Importantly, assays have been developed that allow for the reliable detection of in vivo human cancer cell growth or inhibition following administration of drugs of interest. The zebrafish xenotransplantation model is a unique and effective tool for the study of cancer cell biology.

Peripheral blood CD34+ cells efficiently engraft human cytokine knock-in mice

Human CD34⁺ hematopoietic stem and progenitor cells (HSPCs) can reconstitute a human hemato-lymphoid system when transplanted into immunocompromised mice. Although fetal liver-derived and cord blood-derived CD34⁺ cells lead to high engraftment levels, engraftment of mobilized, adult donor-derived CD34⁺ cells has remained poor. We generated so-called MSTRG and MISTRG humanized mice on a Rag2^-/-Il2rg^-/- background carrying a transgene for human signal regulatory protein α (SIRPα) and human homologs of the cytokine macrophage colony-stimulating factor, thrombopoietin, with or without interleukin-3 and granulocyte-macrophage colony-stimulating factor under murine promoters. Here we transplanted mobilized peripheral blood (PB) CD34⁺ cells in sublethally irradiated newborn and adult recipients. Human hematopoietic engraftment levels were significantly higher in bone marrow (BM), spleen, and PB in newborn transplanted MSTRG/MISTRG as compared with nonobese diabetic/severe combined immunodeficient Il2rg^-/- or human SIRPα-transgenic Rag2^-/-Il2rg^-/- recipients. Furthermore, newborn transplanted MSTRG/MISTRG mice supported higher engraftment levels of human phenotypically defined HSPCs in BM, T cells in the thymus, and myeloid cells in nonhematopoietic organs such as liver, lung, colon, and skin, approximating the levels in the human system. Similar results were obtained in adult recipient mice. Thus, human cytokine knock-in mice might open new avenues for personalized studies of human pathophysiology of the hematopoietic and immune system in vivo.

A humanized bone marrow ossicle xenotransplantation model enables improved engraftment of healthy and leukemic human hematopoietic cells

Xenotransplantation models represent powerful tools for the investigation of healthy and malignant human hematopoiesis. However, current models do not fully mimic the components of the human bone marrow (BM) microenvironment, and they enable only limited engraftment of samples from some human malignancies. Here we show that a xenotransplantation model bearing subcutaneous humanized ossicles with an accessible BM microenvironment, formed by in situ differentiation of human BM-derived mesenchymal stromal cells, enables the robust engraftment of healthy human hematopoietic stem and progenitor cells, as well as primary acute myeloid leukemia (AML) samples, at levels much greater than those in unmanipulated mice. Direct intraossicle transplantation accelerated engraftment and resulted in the detection of substantially higher leukemia-initiating cell (LIC) frequencies. We also observed robust engraftment of acute promyelocytic leukemia (APL) and myelofibrosis (MF) samples, and identified LICs in these malignancies. This humanized ossicle xenotransplantation approach provides a system for modeling a wide variety of human hematological diseases.

Modeling BCR-ABL and MLL-AF9 leukemia in a human bone marrow-like scaffold-based xenograft model

Although NOD-SCID IL2Rγ^-/- (NSG) xenograft mice are currently the most frequently used model to study human leukemia in vivo, the absence of a human niche severely hampers faithful recapitulation of the disease. We used NSG mice in which ceramic scaffolds seeded with human mesenchymal stromal cells were implanted to generate a human bone marrow (huBM-sc)-like niche. We observed that, in contrast to the murine bone marrow (mBM) niche, the expression of BCR-ABL or MLL-AF9 was sufficient to induce both primary acute myeloid leukemia (AML) and acute lymphocytic leukemia (ALL). Stemness was preserved within the human niches as demonstrated by serial transplantation assays. Efficient engraftment of AML MLL-AF9 and blast-crisis chronic myeloid leukemia patient cells was also observed, whereby the immature blast-like phenotype was maintained in the huBM-sc niche but to a much lesser extent in mBM niches. We compared transcriptomes of leukemias derived from mBM niches versus leukemias from huBM-like scaffold-based niches, which revealed striking differences in the expression of genes associated with hypoxia, mitochondria and metabolism. Finally, we utilized the huBM-sc MLL-AF9 B-ALL model to evaluate the efficacy of the I-BET151 inhibitor in vivo. In conclusion, we have established human niche models in which the myeloid and lymphoid features of BCR-ABL⁺ and MLL-AF9⁺ leukemias can be studied in detail.

Humanized Mice for Studying Human Immune Responses and Generating Human Monoclonal Antibodies

The new-generation humanized (Hu) mouse models permit multilineage human hematopoiesis and generate T cells, B cells, macrophages, and dendritic cells required for a coordinated human immune response. Therefore, any desired antigen or human-specific pathogens that can infect humanized mice can be used to generate human antibody responses. Two leading humanized mouse models are currently being used. The Hu-HSC model uses the transplantation of human hematopoietic stem cells (HSCs), whereas the BLT mouse model is created by transplantation of human fetal liver, thymus, and HSC. A number of human pathogens such as HIV-1, dengue, Epstein-Barr virus, and hepatitis C virus have been studied in these systems. Responder antigen-specific B cells from these animals can be collected and used to generate human monoclonals by B-cell immortalization or by single-cell PCR methods to "rescue" antibody-producing genes for ectopic expression. Both models generate cellular and humoral immune responses. However, the antibodies generated are primarily of the IgM type because of the inefficient immunoglobulin class switch resulting in the suboptimal production of antigen-specific affinity-matured IgG. The current Hu mouse models thus far have permitted the analysis of human "antibodyome," and recent reports demonstrated their utility in generating human monoclonal antibodies. Ongoing efforts at further refinements are expected to make these systems more efficient in the near future.

Humanized Mice as Preclinical Models in Transplantation

Animal models have been instrumental in our understanding of the mechanisms of rejection and the testing of novel treatment options in the context of transplantation. We have now entered an exciting era with research on humanized mice driving advances in translational studies and in our understanding of the function of human cells in response to pathogens and cancer as well as the recognition of human allogeneic tissues in vivo. In this chapter we provide a historical overview of humanized mouse models of transplantation to date, outlining the distinct strains and share our experiences in the study of human transplantation immunology.

Phenotypic, Morphological and Adhesive Differences of Human Hematopoietic Progenitor Cells Cultured on Murine versus Human Mesenchymal Stromal Cells

Xenogenic transplantation models have been developed to study human hematopoiesis in immunocompromised murine recipients. They still have limitations and therefore it is important to delineate all players within the bone marrow that could account for species-specific differences. Here, we evaluated the proliferative capacity, morphological and physical characteristics of human CD34⁺ hematopoietic stem and progenitor cells (HSPCs) after co-culture on murine or human bone marrow-derived mesenchymal stromal cells (MSCs). After seven days, human CD34⁺CD133^– HSPCs expanded to similar extents on both feeder layers while cellular subsets comprising primitive CD34⁺CD133⁺ and CD133⁺CD34^– phenotypes are reduced fivefold on murine MSCs. The number of migrating HSPCs was also reduced on murine cells suggesting that MSC adhesion influences cellular polarization of HSPC. We used atomic force microscopy-based single-cell force spectroscopy to quantify their adhesive interactions. We found threefold higher detachment forces of human HSPCs from murine MSCs compared to human ones. This difference is related to the N-cadherin expression level on murine MSCs since its knockdown abolished their differential adhesion properties with human HSPCs. Our observations highlight phenotypic, morphological and adhesive differences of human HSPCs when cultured on murine or human MSCs, which raise some caution in data interpretation when xenogenic transplantation models are used.

A chimeric mouse model to study immunopathogenesis of HCV infection

Several human hepatotropic pathogens including chronic hepatitis C virus (HCV) have narrow species restriction, thus hindering research and therapeutics development against these pathogens. Developing a rodent model that accurately recapitulates hepatotropic pathogens infection, human immune response, chronic hepatitis, and associated immunopathogenesis is essential for research and therapeutics development. Here, we describe the recently developed AFC8 humanized liver- and immune system-mouse model for studying chronic hepatitis C virus and associated human immune response, chronic hepatitis, and liver fibrosis.

Impacts of humanized mouse models on the investigation of HIV-1 infection: illuminating the roles of viral accessory proteins in vivo

Human immunodeficiency virus type 1 (HIV-1) encodes four accessory genes: vif, vpu, vpr, and nef. Recent investigations using in vitro cell culture systems have shed light on the roles of these HIV-1 accessory proteins, Vif, Vpr, Vpu, and Nef, in counteracting, modulating, and evading various cellular factors that are responsible for anti-HIV-1 intrinsic immunity. However, since humans are the exclusive target for HIV-1 infection, conventional animal models are incapable of mimicking the dynamics of HIV-1 infection in vivo. Moreover, the effects of HIV-1 accessory proteins on viral infection in vivo remain unclear. To elucidate the roles of HIV-1 accessory proteins in the dynamics of viral infection in vivo, humanized mouse models, in which the mice are xenotransplanted with human hematopoietic stem cells, has been utilized. This review describes the current knowledge of the roles of HIV-1 accessory proteins in viral infection, replication, and pathogenicity in vivo, which are revealed by the studies using humanized mouse models.

Evaluation of the efficiency of human immune system reconstitution in NSG mice and NSG mice containing a human HLA.A2 transgene using hematopoietic stem cells purified from different sources

Severely immunodeficient mice such as the NOD/SCID/IL2rγ(null) (NSG) strain can be engrafted with human hematopoietic stem cells (HSCs), resulting in chimeric mice containing many components of the human immune system (Human Immune System mice or HIS mice). HIS mice can both support the replication of and recapitulate much of the immunological response to a variety of pathogens, including ones with strict human tropism, such as HIV-1. In an effort to develop a better mouse model for human infectious pathogen infection and possible immune resolution, we compared the human immune system reconstitution of NSG mice following injection with human CD34(+) HSCs purified from either fetal liver (FL) or umbilical cord blood (UCB). We analyzed reconstitution in standard NSG mice as well as a derivative of these mice containing an HLA.A2 encoding transgene (NSG.A2). HSCs from both sources effectively reconstituted hematopoietic lineages when injected into NSG mice. In marked contrast, total CD45(+) human hematopoietic cells in NSG.A2 mice were well reconstituted by HSCs from UCB but very poorly by HSCs purified from FL. Moreover, the reconstitution of T cell lineages in NSG.A2 mice by HSCs from UCB was inferior to that obtained using NSG mice. We also found that FL CD34(+) HSCs contain a much higher percentage of cells with a phenotype consistent with primitive progenitors than UCB HSCs. We discuss possible explanations for the influence of the HLA.A2 transgene on hematopoietic reconstitution using the two sources of HSCs.

Predominant development of mature and functional human NK cells in a novel human IL-2-producing transgenic NOG mouse

We generated a severe immunodeficient NOD/Shi-scid-IL-2Rγ(null) (NOG) mouse substrain expressing the transgenic human IL-2 gene (NOG-IL-2 Tg). Upon transfer of human cord blood-derived hematopoietic stem cells (HSCs), CD3(-)CD56(high)CD16(+/-) cells developed unexpectedly, predominantly in the NOG-IL-2 Tg (hu-HSC NOG-IL-2 Tg). These cells expressed various NK receptors, including NKp30, NKp44, NKp46, NKG2D, and CD94, as well as a diverse set of killer cell Ig-like receptor molecules at levels comparable to normal human NK cells from the peripheral blood, which is evidence of their maturity. They produced levels of granzyme A as high as in human peripheral blood-derived NK cells, and a considerable amount of perforin protein was detected in the plasma. Human NK cells in hu-HSC NOG-IL-2 Tg produced IFN-γ upon stimulation, and IL-2, IL-15, or IL-12 treatment augmented the in vitro cytotoxicity. Inoculation of K562 leukemia cells into hu-HSC NOG-IL-2 Tg caused complete rejection of the tumor cells, whereas inoculation into hu-HSC NOG fully reconstituted with human B, T, and some NK cells did not. Moreover, when a CCR4(+) Hodgkin's lymphoma cell line was inoculated s.c. into hu-HSC NOG-IL-2 Tg, the tumor growth was significantly suppressed by treatment with a therapeutic humanized anti-CCR4 Ab (mogamulizumab), suggesting that the human NK cells in the mice exerted active Ab-dependent cellular cytotoxicity in vivo. Taken together, these data suggest that the new NOG-IL-2 Tg strain is a unique model that can be used to investigate the biological and pathological functions of human NK cells in vivo.

Insufficient interleukin-12 signalling favours differentiation of human CD4(+) and CD8(+) T cells into GATA-3(+) and GATA-3(+) T-bet(+) subsets in humanized mice

Differentiation of CD4(+) T cells into type 1 or type 2 subsets is mediated by the expression of the opposing lineage defining transcription factors T-bet and GATA-3. However, the existence of GATA-3(+) T-bet(+) CD4(+) T cells in mice suggests functional plasticity of these subsets. Little is known about type 1 and type 2 plasticity of human T-cell subsets in vivo. Here, we show that in the xenogeneic environment of humanized mice, which lacks a functional immune-regulatory network, human CD4(+) and, notably, CD8(+) T cells preferentially differentiate into interleukin (IL)-4(+) GATA-3(+) and IL-4(+) interferon-γ(+) GATA-3(+) T-bet(+) subsets. Treatment with recombinant human IL-12 or expansion of IL-12-producing human dendritic cells in vivo reverted this phenotype and led to the down-regulation of GATA-3 expression. These changes also correlated with improved antiviral immune responses in humanized mice. In conclusion, our study shows the capacity of human CD4(+) and CD8(+) T cells for stable co-expression of GATA-3 and T-bet in humanized mice and reveals a critical role for IL-12 in regulating this phenotype.

Treg vaccination in autoimmune type 1 diabetes

Foxp3⁺ regulatory T (Treg) cells are critical contributors to the establishment and maintenance of immunological self-tolerance. Autoimmune type 1 diabetes (T1D) is characterized by the loss of self-tolerance to the insulin-producing β cells in the pancreas and the destruction of β cells, resulting in the development of chronic hyperglycemia at diagnosis. The application of strong agonistic T-cell receptor ligands provided under subimmunogenic conditions functions as a critical means for the efficient de novo conversion of naive CD4⁺ T cells into Foxp3⁺ Treg cells. The specific induction of Treg cells upon supply of strong-agonistic variants of certain self-antigens could therefore function as a critical instrument in order to achieve safe and specific prevention of autoimmunity such as T1D via the restoration of self-tolerance. Such immunotherapeutic strategies are being developed, and in the case of T1D aim to restrict autoimmunity and β-cell destruction. In this review, we discuss the requirements and opportunities for Treg-based tolerance approaches with the goal of interfering with autoimmune T1D.

Animal models of Epstein Barr virus infection

Epstein Barr virus (EBV) was the first human tumor virus to be identified. Despite 50years of research on this oncogenic virus, no therapeutic or prophylactic vaccine is available against this pathogen. In part, the development of such a vaccine is hampered by the lack of in vivo models for EBV infection and immune control. However, with the advent of mice with reconstituted human immune system components (HIS mice), certain aspects of EBV associated diseases and immune responses can be modeled in vivo. In this review, we will discuss the insights that can be gained from these experiments, and how immune system components can be manipulated to interrogate their function during EBV infection. Finally, we will compare EBV immunobiology in HIS mice to infection by EBV-related viruses in monkeys, and we will outline the strengths and weaknesses of these two in vivo models of EBV infection. Both of these models show great promise as a platform for preclinical EBV vaccine testing.

Kit regulates HSC engraftment across the human-mouse species barrier

In-depth analysis of the cellular and molecular mechanisms regulating human HSC function will require a surrogate host that supports robust maintenance of transplanted human HSCs in vivo, but the currently available options are problematic. Previously we showed that mutations in the Kit receptor enhance engraftment of transplanted HSCs in the mouse. To generate an improved model for human HSC transplantation and analysis, we developed immune-deficient mouse strains containing Kit mutations. We found that mutation of the Kit receptor enables robust, uniform, sustained, and serially transplantable engraftment of human HSCs in adult mice without a requirement for irradiation conditioning. Using this model, we also showed that differential KIT expression identifies two functionally distinct subpopulations of human HSCs. Thus, we have found that the capacity of this Kit mutation to open up stem cell niches across species barriers has significant potential and broad applicability in human HSC research.

Critical assessment of human antibody generation in humanized mouse models

Immunodeficient mice reconstituted with human hematopoietic stem cells provide a small-animal model for the study of development and function of human hematopoietic cells in vivo. However, in the current models, the immune response, and especially the humoral response by the human immune cells is far from optimal. The B cells found in these mice exhibit an immature and abnormal phenotype correlating with a reduced capacity to produce antigen-specific affinity matured antibodies upon infection or immunization. Herein, we review the current state of knowledge of development, function and antibody production of human B cells and discuss the obstacles for the improvement of these models.