Humoral immune responses in humanized BLT mice immunized with West Nile virus and HIV-1 envelope proteins are largely mediated via human CD5+ B cells

B cell development and antibody responses in human immune system mice: current status and future perspective

Humanized immune system (HIS) mice have been developed and used as a small surrogate model to study human immune function under normal or disease conditions. Although variations are found between models, most HIS mice show robust human T cell responses. However, there has been unsuccessful in constructing HIS mice that produce high-affinity human antibodies, primarily due to defects in terminal B cell differentiation, antibody affinity maturation, and development of primary follicles and germinal centers. In this review, we elaborate on the current knowledge about and previous attempts to improve human B cell development in HIS mice, and propose a potential strategy for constructing HIS mice with improved humoral immunity by transplantation of human follicular dendritic cells (FDCs) to facilitate the development of secondary follicles.

Progress in building clinically relevant patient-derived tumor xenograft models for cancer research

Patient-derived tumor xenograft (PDX) models, a method involving the surgical extraction of tumor tissues from cancer patients and subsequent transplantation into immunodeficient mice, have emerged as a pivotal approach in translational research, particularly in advancing precision medicine. As the first stage of PDX development, the patient-derived orthotopic xenograft (PDOX) models implant tumor tissue in mice in the corresponding anatomical locations of the patient. The PDOX models have several advantages, including high fidelity to the original tumor, heightened drug sensitivity, and an elevated rate of successful transplantation. However, the PDOX models present significant challenges, requiring advanced surgical techniques and resource-intensive imaging technologies, which limit its application. And then, the humanized mouse models, as well as the zebrafish models, were developed. Humanized mouse models contain a human immune environment resembling the tumor and immune system interplay. The humanized mouse models are a hot topic in PDX model research. Regarding zebrafish patient-derived tumor xenografts (zPDX) and patient-derived organoids (PDO) as promising models for studying cancer and drug discovery, zPDX models are used to transplant tumors into zebrafish as novel personalized medical animal models with the advantage of reducing patient waiting time. PDO models provide a cost-effective approach for drug testing that replicates the in vivo environment and preserves important tumor-related information for patients. The present review highlights the functional characteristics of each new phase of PDX and provides insights into the challenges and prospective developments in this rapidly evolving field.

Current Advances in Humanized Mouse Models for Studying NK Cells and HIV Infection

Human immunodeficiency virus (HIV) has infected millions of people worldwide and continues to be a major global health problem. Scientists required a small animal model to study HIV pathogenesis and immune responses. To this end, humanized mice were created by transplanting human cells and/or tissues into immunodeficient mice to reconstitute a human immune system. Thus, humanized mice have become a critical animal model for HIV researchers, but with some limitations. Current conventional humanized mice are prone to death by graft versus host disease induced by the mouse signal regulatory protein α and CD47 signaling pathway. In addition, commonly used humanized mice generate low levels of human cytokines required for robust myeloid and natural killer cell development and function. Here, we describe recent advances in humanization procedures and transgenic and knock-in immunodeficient mice to address these limitations.

The Humanized Mouse Model: What Added Value Does It Offer for HIV Research?

In the early 2000s, novel humanized mouse models based on the transplantation of human hematopoietic stem and progenitor cells (HSPCs) into immunocompromised mice were introduced (hu mice). The human HSPCs gave rise to a lymphoid system of human origin. The HIV research community has greatly benefitted from these hu mice. Since human immunodeficiency virus (HIV) type 1 infection results in a high-titer disseminated HIV infection, hu mice have been of great value for all types of HIV research from pathogenesis to novel therapies. Since the first description of this new generation of hu mice, great efforts have been expended to improve humanization by creating other immunodeficient mouse models or supplementing mice with human transgenes to improve human engraftment. Many labs have their own customized hu mouse models, making comparisons quite difficult. Here, we discuss the different hu mouse models in the context of specific research questions in order to define which characteristics should be considered when determining which hu mouse model is appropriate for the question posed. We strongly believe that researchers must first define their research question and then determine whether a hu mouse model exists, allowing the research question to be studied.

Humanized mouse models for immuno-oncology research

Immunotherapy has emerged as a promising treatment paradigm for many malignancies and is transforming the drug development landscape. Although immunotherapeutic agents have demonstrated clinical efficacy, they are associated with variable clinical responses, and substantial gaps remain in our understanding of their mechanisms of action and specific biomarkers of response. Currently, the number of preclinical models that faithfully recapitulate interactions between the human immune system and tumours and enable evaluation of human-specific immunotherapies in vivo is limited. Humanized mice, a term that refers to immunodeficient mice co-engrafted with human tumours and immune components, provide several advantages for immuno-oncology research. In this Review, we discuss the benefits and challenges of the currently available humanized mice, including specific interactions between engrafted human tumours and immune components, the development and survival of human innate immune populations in these mice, and approaches to study mice engrafted with matched patient tumours and immune cells. We highlight the latest advances in the generation of humanized mouse models, with the aim of providing a guide for their application to immuno-oncology studies with potential for clinical translation.

Teaching a new mouse old tricks: Humanized mice as an infection model for Variola virus

Smallpox, caused by the solely human pathogen Variola virus (VARV), was declared eradicated in 1980. While known VARV stocks are secure, smallpox remains a bioterrorist threat agent. Recent U.S. Food and Drug Administration approval of the first smallpox anti-viral (tecovirimat) therapeutic was a successful step forward in smallpox preparedness; however, orthopoxviruses can become resistant to treatment, suggesting a multi-therapeutic approach is necessary. Animal models are required for testing medical countermeasures (MCMs) and ideally MCMs are tested directly against the pathogen of interest. Since VARV only infects humans, a representative animal model for testing therapeutics directly against VARV remains a challenge. Here we show that three different humanized mice strains are highly susceptible to VARV infection, establishing the first small animal model using VARV. In comparison, the non-humanized, immunosuppressed background mouse was not susceptible to systemic VARV infection. Following an intranasal VARV challenge that mimics the natural route for human smallpox transmission, the virus spread systemically within the humanized mouse before mortality (~ 13 days post infection), similar to the time from exposure to symptom onset for ordinary human smallpox. Our identification of a permissive/representative VARV animal model can facilitate testing of MCMs in a manner consistent with their intended use.

TLR9- and CD40-Targeting Vaccination Promotes Human B Cell Maturation and IgG Induction via pDC-Dependent Mechanisms in Humanized Mice

Mice reconstituted with a human immune system (humanized mice) provide a robust model to study human immunology, vaccinology, and human infectious diseases. However, the development and function of B cells in humanized mice is impaired. B cells from humanized mice are immature and are impaired in IgM to IgG isotype switch in response to infection or vaccination. In the present study we report that Toll-like receptor 9 (TLR9) agonist CpG-B combined with CD40-targeting vaccination triggered human B cell immunoglobin class-switch from IgM⁺ to IgG⁺ B cells in humanized mice. Human B cells from mice vaccinated with CpG-B as adjuvant were more mature in phenotype and produced significant levels of both total IgG and antigen-specific IgG. We found that CpG-B treatment activated human pDCs (plasmacytoid dendritic cells) in vivo to induce interferon-alpha (IFN-α)expression in humanized mice. Pre-depletion of human pDC in vivo abrogated the adjuvant effect of CpG-B. Our results indicate that TLR9 and CD40-targeting vaccination triggers human B cell maturation and immunoglobulin class-switch in a pDC-dependent manner in humanized mice. The findings also shed light on induction of human IgG antibodies in humanized mouse models.

Humanized Mice for the Evaluation of Novel HIV-1 Therapies

With the discovery of antiretroviral therapy, HIV-1 infection has transitioned into a manageable but chronic illness, which requires lifelong treatment. Nevertheless, complete eradication of the virus has still eluded us. This is partly due to the virus’s ability to remain in a dormant state in tissue reservoirs, ‘hidden’ from the host’s immune system. Also, the high mutation rate of HIV-1 results in escape mutations in response to many therapeutics. Regardless, the development of novel cures for HIV-1 continues to move forward with a range of approaches from immunotherapy to gene editing. However, to evaluate in vivo pathogenesis and the efficacy and safety of therapeutic approaches, a suitable animal model is necessary. To this end, the humanized mouse was developed by McCune in 1988 and has continued to be improved on over the past 30 years. Here, we review the variety of humanized mouse models that have been utilized through the years and describe their specific contribution in translating HIV-1 cure strategies to the clinic.

Advances in Human Immune System Mouse Models for Studying Human Hematopoiesis and Cancer Immunotherapy

Immunotherapy has established itself as a promising tool for cancer treatment. There are many challenges that remain including lack of targets and some patients across various cancers who have not shown robust clinical response. One of the major problems that have hindered the progress in the field is the dearth of appropriate mouse models that can reliably recapitulate the complexity of human immune-microenvironment as well as the malignancy itself. Immunodeficient mice reconstituted with human immune cells offer a unique opportunity to comprehensively evaluate immunotherapeutic strategies. These immunosuppressed and genetically modified mice, with some overexpressing human growth factors, have improved human hematopoietic engraftment as well as created more functional immune cell development in primary and secondary lymphoid tissues in these mice. In addition, several new approaches to modify or to add human niche elements to further humanize these immunodeficient mice have allowed a more precise characterization of human hematopoiesis. These important refinements have opened the possibility to evaluate not only human immune responses to different tumor cells but also to investigate how malignant cells interact with their niche and most importantly to test immunotherapies in a more preclinically relevant setting, which can ultimately lead to better success of these drugs in clinical trials.

Rejection of xenogeneic porcine islets in humanized mice is characterized by graft-infiltrating Th17 cells and activated B cells

Xenogeneic porcine islet transplantation is a promising potential therapy for type 1 diabetes (T1D). Understanding human immune responses against porcine islets is crucial for the design of optimal immunomodulatory regimens for effective control of xenogeneic rejection of porcine islets in humans. Humanized mice are a valuable tool for studying human immune responses and therefore present an attractive alternative to human subject research. Here, by using a pig-to-humanized mouse model of xenogeneic islet transplantation, we described the human immune response to transplanted porcine islets, a process characterized by dense islet xenograft infiltration of human CD45⁺ cells comprising activated human B cells, CD4⁺ CD44⁺ IL-17⁺ Th17 cells, and CD68⁺ macrophages. In addition, we tested an experimental immunomodulatory regimen in promoting long-term islet xenograft survival, a triple therapy consisting of donor splenocytes treated with ethylcarbodiimide (ECDI-SP), and peri-transplant rituximab and rapamycin. We observed that the triple therapy effectively inhibited graft infiltration of T and B cells as well as macrophages, promoted transitional B cells both in the periphery and in the islet xenografts, and provided a superior islet xenograft protection. Our study therefore indicates an advantage of donor ECDI-SP treatment in controlling human immune cells in promoting long-term islet xenograft survival.

Moving beyond the mousetrap: current and emerging humanized mouse and rat models for investigating prevention and cure strategies against HIV infection and associated pathologies

The development of safe and effective combination antiretroviral therapies for human immunodeficiency virus (HIV) infection over the past several decades has significantly reduced HIV-associated morbidity and mortality. Additionally, antiretroviral drugs have provided an effective means of protection against HIV transmission. Despite these advances, significant limitations exist; namely, the inability to eliminate HIV reservoirs, the inability to reverse lymphoid tissues damage, and the lack of an effective vaccine for preventing HIV transmission. Evaluation of the safety and efficacy of therapeutics and vaccines for eliminating HIV reservoirs and preventing HIV transmission requires robust in vivo models. Since HIV is a human-specific pathogen, that targets hematopoietic lineage cells and lymphoid tissues, in vivo animal models for HIV-host interactions require incorporation of human hematopoietic lineage cells and lymphoid tissues. In this review, we will discuss the construction of mouse models with human lymphoid tissues and/or hematopoietic lineage cells, termed, human immune system (HIS)-humanized mice. These HIS-humanized mouse models can support the development of functional human innate and adaptive immune cells, along with primary (thymus) and secondary (spleen) lymphoid tissues. We will discuss applications of HIS-humanized mouse models in evaluating the safety and efficacy of therapeutics against HIV reservoirs and associated immunopathology, and delineate the human immune response elicited by candidate HIV vaccines. In addition to focusing on how these HIS-humanized mouse models have already furthered our understanding of HIV and contributed to HIV therapeutics development, we discuss how emerging HIS-humanized rat models could address the limitations of HIS-mouse models.

The Utility of Human Immune System Mice for High-Containment Viral Hemorrhagic Fever Research

Human immune system (HIS) mice are a subset of humanized mice that are generated by xenoengraftment of human immune cells or tissues and/or their progenitors into immunodeficient mice. Viral hemorrhagic fevers (VHFs) cause severe disease in humans, typically with high case fatality rates. HIS mouse studies have been performed to investigate the pathogenesis and immune responses to VHFs that must be handled in high-containment laboratory facilities. Here, we summarize studies on filoviruses, nairoviruses, phenuiviruses, and hantaviruses, and discuss the knowledge gained from using various HIS mouse models. Furthermore, we discuss the complexities of designing and interpreting studies utilizing HIS mice while highlighting additional questions about VHFs that can still be addressed using HIS mouse models.

Use of Humanized Mouse Models for Studying HIV-1 Infection, Pathogenesis and Persistence

Despite decades of intensive basic and clinical research efforts, there is still no successful vaccine candidate against human immunodeficiency virus (HIV-1). Standard combined antiretroviral therapy (cART) has been successfully developed and has given remarkable results suppressing HIV-1 infection and transmission. However, cART cannot fully clear the virus from the infected patients. A cure for HIV-1 is highly desirable to stop both the spread of the virus in humans and disease progression in HIV-1 patients. A safe and effective cure strategy for HIV-1 infection will require appropriate animal models that properly mimic HIV-1 infection and advance HIV-1 cure research. Animal models have been a crucial tool in the drug discovery process for investigation of HIV-1 disease mainly in preclinical evaluations of antiretroviral drugs and vaccines. An ideal animal model should recapitulate the main aspects of human-specific HIV-1 infection and pathogenesis with their associated immune responses, while permitting invasive antiretroviral studies. The best humanized mouse models would allow a thorough evaluation of antiretroviral strategies that are aimed towards reducing the establishment and size of the HIV-1 reservoirs. In this review, we evaluate multiple humanized mouse models while presenting their strengths and limitations for HIV-1 research. These humanized mouse models have been tailored in recent decades and heavily employed to address specific quintessential and remaining questions of HIV-1 persistence, pathogenesis and ultimately, eradication.

Humanized Mice as an Effective Evaluation System for Peptide Vaccines and Immune Checkpoint Inhibitors

Peptide vaccination was developed for the prevention and therapy of acute and chronic infectious diseases and cancer. However, vaccine development is challenging, because the patient immune system requires the appropriate human leukocyte antigen (HLA) recognition with the peptide. Moreover, antigens sometimes induce a low response, even if the peptide is presented by antigen-presenting cells and T cells recognize it. This is because the patient immunity is dampened or restricted by environmental factors. Even if the immune system responds appropriately, newly-developed immune checkpoint inhibitors (ICIs), which are used to increase the immune response against cancer, make the immune environment more complex. The ICIs may activate T cells, although the ratio of responsive patients is not high. However, the vaccine may induce some immune adverse effects in the presence of ICIs. Therefore, a system is needed to predict such risks. Humanized mouse systems possessing human immune cells have been developed to examine human immunity in vivo. One of the systems which uses transplanted human peripheral blood mononuclear cells (PBMCs) may become a new diagnosis strategy. Various humanized mouse systems are being developed and will become good tools for the prediction of antibody response and immune adverse effects.

Alcohol consumption increases susceptibility to pneumococcal pneumonia in a humanized murine HIV model mediated by intestinal dysbiosis

Alcohol use in persons living with HIV (PLWH) worsens the severity of bacterial pneumonia. However, the exact mechanism(s) by which this occurs remain ill-defined. We hypothesized that alcohol in the setting of HIV infection decreases Streptococcus pneumoniae clearance from the lung through mechanisms mediated by the gut microbiota. Humanized BLT (bone marrow, liver, thymus) mice were infected with 1 × 104 TCID50 of HIV (BAL and JRCSF strains) via intraperitoneal (i.p.) injection. One week post-HIV infection, animals were switched to a Lieber-DeCarli 5% ethanol diet or an isocaloric control diet for 10 days. Alcohol-fed animals were also given two binges of 2 g/kg ethanol on days 5 and 10. Feces were also collected, banked, and the community structures were analyzed. Mice were then infected with 1 × 105 CFU (colony-forming units) of S. pneumoniae and were sacrificed 48 h later. HIV-infected mice had viral loads of ∼2 × 104 copies/mL of blood 1 week post-infection, and exhibited an ∼57% decrease in the number of circulating CD4+ T cells at the time of sacrifice. Fecal microbial community structure was significantly different in each of the feeding groups, as well as with HIV infection. Alcohol-fed mice had a significantly higher burden of S. pneumoniae 48 h post-infection, regardless of HIV status. In follow-up experiments, female C57BL/6 mice were treated with a cocktail of antibiotics daily for 2 weeks and recolonized by gavage with intestinal microbiota from HIV+ ethanol-fed, HIV+ pair-fed, HIV- ethanol-fed, or HIV- pair-fed mice. Recolonized mice were then infected with S. pneumoniae and were sacrificed 48 h later. The intestinal microbiota from alcohol-fed mice (regardless of HIV status) significantly impaired clearance of S. pneumoniae. Collectively, these data indicate that alcohol feeding, as well as alcohol-associated intestinal dysbiosis, compromise pulmonary host defenses against pneumococcal pneumonia. Determining whether HIV infection acts synergistically with alcohol use in impairing pulmonary host defenses will require additional study.

Immunization of BLT Humanized Mice Redirects T Cell Responses to Gag and Reduces Acute HIV-1 Viremia

BLT (bone marrow-liver-thymus) humanized mice, which reconstitute a functional human immune system, develop prototypic human virus-specific CD8⁺ T cell responses following infection with human immunodeficiency virus type 1 (HIV-1). We explored the utility of the BLT model for HIV-1 vaccine development by immunizing BLT mice against the conserved viral Gag protein, utilizing a rapid prime-boost protocol of poly(lactic-co-glycolic) acid microparticles and a replication-defective herpes simplex virus (HSV) recombinant vector. After HIV-1 challenge, the mice developed broad, proteome-wide gamma interferon-positive (IFN-γ⁺) T cell responses against HIV-1 that reached magnitudes equivalent to what is observed in HIV-1-infected individuals. The functionality of these responses was underscored by the consistent emergence of escape mutations in multiple CD8⁺ T cell epitopes during the course of infection. Although prechallenge vaccine-induced responses were largely undetectable, the Gag immunization increased both the magnitude and the kinetics of anamnestic Gag-specific T cell responses following HIV-1 infection, and the magnitude of these postchallenge Gag-specific responses was inversely correlated with acute HIV-1 viremia. Indeed, Gag immunization was associated with a modest but significant 0.5-log reduction in HIV-1 viral load when analyzed across four experimental groups of BLT mice. Notably, the HSV vector induced elevated plasma concentrations of polarizing cytokines and chemotactic factors, including interleukin-12p70 (IL-12p70) and MIP-1α, which were positively correlated with the magnitude of Gag-specific responses. Overall, these results support the ability of BLT mice to recapitulate human pathogen-specific T cell responses and to respond to immunization; however, additional improvements to the model are required to develop a robust system for testing HIV-1 vaccine efficacy.IMPORTANCE Advances in the development of humanized mice have raised the possibility of a small-animal model for preclinical testing of an HIV-1 vaccine. Here, we describe the capacity of BLT humanized mice to mount broadly directed HIV-1-specific human T cell responses that are functionally active, as indicated by the rapid emergence of viral escape mutations. Although immunization of BLT mice with the conserved viral Gag protein did not result in detectable prechallenge responses, it did increase the magnitude and kinetics of postchallenge Gag-specific T cell responses, which was associated with a modest but significant reduction in acute HIV-1 viremia. Additionally, the BLT model revealed immunization-associated increases in the plasma concentrations of immunomodulatory cytokines and chemokines that correlated with more robust T cell responses. These data support the potential utility of the BLT humanized mouse for HIV-1 vaccine development but suggest that additional improvements to the model are warranted.

Accelerated thymopoiesis and improved T-cell responses in HLA-A2/-DR2 transgenic BRGS-based human immune system mice

Human immune system (HIS) mouse models provide a robust in vivo platform to study human immunity. Nevertheless, the signals that guide human lymphocyte differentiation in HIS mice remain poorly understood. Here, we have developed a novel Balb/c Rag2^-/- Il2rg^-/- Sirpa^NOD (BRGS) HIS mouse model expressing human HLA-A2 and -DR2 transgenes (BRGSA2DR2). When comparing BRGS and BRGSA2DR2 HIS mice engrafted with human CD34⁺ stem cells, a more rapid emergence of T cells in the circulation of hosts bearing human HLA was shown, which may reflect a more efficient human T-cell development in the mouse thymus. Development of CD4⁺ and CD8⁺ T cells was accelerated in BRGSA2DR2 HIS mice and generated more balanced B and T-cell compartments in peripheral lymphoid organs. Both B- and T-cell function appeared enhanced in the presence of human HLA transgenes with higher levels of class switched Ig, increased percentages of polyfunctional T cells and clear evidence for antigen-specific T-cell responses following immunization. Taken together, the presence of human HLA class I and II molecules can improve multiple aspects of human B- and T-cell homeostasis and function in the BRGS-based HIS mouse model.

Humanized Mouse Models for the Study of Infection and Pathogenesis of Human Viruses

The evolution of infectious pathogens in humans proved to be a global health problem. Technological advancements over the last 50 years have allowed better means of identifying novel therapeutics to either prevent or combat these infectious diseases. The development of humanized mouse models offers a preclinical in vivo platform for further characterization of human viral infections and human immune responses triggered by these virus particles. Multiple strains of immunocompromised mice reconstituted with a human immune system and/or human hepatocytes are susceptible to infectious pathogens as evidenced by establishment of full viral life cycles in hope of investigating viral–host interactions observed in patients and discovering potential immunotherapies. This review highlights recent progress in utilizing humanized mice to decipher human specific immune responses against viral tropism.

Potential Pitfalls of the Humanized Mice in Modeling Sepsis

Humanized mice are a state-of-the-art tool used to study several diseases, helping to close the gap between mice and human immunology. This review focuses on the potential obstacles in the analysis of immune system performance between humans and humanized mice in the context of severe acute inflammation as seen in sepsis or other critical care illnesses. The extent to which the reconstituted human immune system in mice adequately compares to the performance of the human immune system in human hosts is still an evolving question. Although certain viral and protozoan infections can be replicated in humanized mice, whether a highly complex and dynamic systemic inflammation like sepsis can be accurately represented by current humanized mouse models in a clinically translatable manner is unclear. Humanized mice are xenotransplant animals in the most general terms. Several organs (e.g., bone marrow mesenchymal cells, endothelium) cannot interact with the grafted human leukocytes effectively due to species specificity. Also the interaction between mice gut flora and the human immune system may be paradoxical. Often, grafting is performed utilizing an identical batch of stem cells in highly inbred animals which fails to account for human heterogeneity. Limiting factors include the substantial cost and restricting supply of animals. Finally, humanized mice offer an opportunity to gain knowledge of human-like conditions, requiring careful data interpretation just as in nonhumanized animals.

Analysis of ROR1 Protein Expression in Mice with Reconstituted Human Immune System Components

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is an oncofetal antigen expressed on multiple tumors and has no significant expression on normal human tissues. ROR1 is highly upregulated in chronic lymphocytic leukemia (CLL) B cells. NOD-scid IL2rg^−/− (NSG) mice engrafted with human CD34⁺ hematopoietic progenitor cells (huNSG) achieved multilineage human immune cell reconstitution including B cells, T cells, NK cells, and DCs. Like the CLL patients, huNSG mice have abnormally high percentage of CD5-expressing B cells in the periphery. In light of this, we aim to determine whether ROR1 is expressed on huNSG B cells. Using flow cytometry analysis, we found that ROR1 was highly expressed in a proportion of bone marrow, spleen, and blood B cells, which were mostly immature B cells. Transplantation of the oncogene TCL-1-transduced CD34⁺ cells in neonatal NSG mice did not increase the frequency of ROR1-expressing B cells, but the mouse with the highest engraftment of transduced cells developed a tumor-like lump consisting of a high percentage of ROR1-expressing B cells. This study highlights the potential use of huNSG mice to study B cell malignant diseases and to evaluate immunotherapeutics targeting ROR1.

Humanized Mice Engrafted With Human HSC Only or HSC and Thymus Support Comparable HIV-1 Replication, Immunopathology, and Responses to ART and Immune Therapy

Immunodeficient mice reconstituted with human immune tissues and cells (humanized mice) are relevant and robust models for the study of HIV-1 infection, immunopathogenesis, and therapy. In this study, we performed a comprehensive comparison of human immune reconstitution and HIV-1 infection, immunopathogenesis and therapy between immunodeficient NOD/Rag2-/-/γc-/- (NRG) mice transplanted with human HSCs (NRG-hu HSC) and mice transplanted with HSCs and thymus fragments (NRG-hu Thy/HSC) from the same donors. We found that similar human lymphoid and myeloid lineages were reconstituted in NRG-hu HSC and NRG-hu Thy/HSC mice, with human T cells more predominantly reconstituted in NRG-hu Thy/HSC mice, while NRG-hu HSC mice supported more human B cells and myeloid cells reconstitution. HIV-1 replicated similarly and induced similar T cell depletion, immune activation, and dysfunction in NRG-hu HSC and NRG-hu Thy/HSC mice. Moreover, combined antiretroviral therapy (cART) inhibited HIV-1 replication efficiently with similar persistent HIV-1 reservoirs in both models. Finally, we found that blocking type-I interferon signaling under cART treatment transiently activated HIV-1 reservoirs, enhanced T cell recovery and reduced HIV-1 reservoirs in both HIV-1 infected NRG-hu HSC and NRG-hu Thy/HSC mice. In summary, we report that NRG-hu Thy/HSC and NRG-hu HSC mice support similar HIV-1 infection and similar HIV-1 immunopathology; and HIV-1 replication responds similarly to cART and IFNAR blockade therapies. The NRG-hu HSC mouse model reconstituted with human HSC only is sufficient for the study of HIV-1 infection, pathogenesis, and therapy.

Evaluating Human Immune Responses for Vaccine Development in a Novel Human Spleen Cell-Engrafted NOD-SCID-IL2rγNull Mouse Model

The lack of preclinical models able to faithfully predict the immune responses which are later obtained in the clinic is a major hurdle for vaccines development as it increases markedly the delays and the costs required to perform clinical studies. We developed and evaluated the relevance to human immune responses of a novel humanized mouse model, humanized-spleen cells-NOD-SCID-gamma null (Hu-SPL-NSG), in which we grafted human spleen cells in immunodeficient NOD-SCID-IL-2rγnull (NSG) mice. We selected the malaria vaccine candidate, Liver Stage Antigen 3-Full Length, because we had previously observed a major discrepancy between preclinical and clinical results, and compared its immunogenicity with that of a shorter form of the molecule, LSA3-729. NSG mice engrafted with human spleen lymphocytes were immunized with either LSA3-FL or LSA3-729, both adjuvanted with montanide ISA720. We found that the shorter LSA3-729 triggered the production of human antibodies and a T-helper-type 1 cellular immune response associated with protection whereas LSA3-FL did not. Results were consistent in five groups receiving lymphocytes from five distinct human donors. We identified antigenic regions in the full-length molecule, but not in the shorter version, which induced T-regulatory type of cellular responses. These regions had failed to be predicted by previous preclinical experiments in a wide range of animal models, including primates. Results were reproducible using spleen cells from all five human donors. The findings in the Hu-SPL-NSG model were similar to the results obtained using LSA3-FL in the clinic and hence could have been used to predict them. The model does not present graft versus host reaction, low survival of engrafted B lymphocytes and difficulty to raise primary immune responses, all limitations previously reported in humanized immune-compromised mice. Results also point to the shorter construct, LSA3-729 as a more efficient vaccine candidate. In summary, our findings indicate that the Hu-SPL-NSG model could be a relevant and cost-saving choice for early selection of vaccine candidates before clinical development, and deserves being further evaluated.

A Hitchhiker's guide to humanized mice: new pathways to studying viral infections

Humanized mice are increasingly appreciated as an incredibly powerful platform for infectious disease research. The often very narrow species tropism of many viral infections, coupled with the sometimes misleading results from preclinical studies in animal models further emphasize the need for more predictive model systems based on human cells rather than surrogates. Humanized mice represent such a model and have been greatly enhanced with regards to their immune system reconstitution as well as immune functionality in the past years, resulting in their recommendation as a preclinical model by the US Food and Drug Administration. This review aims to give a detailed summary of the generation of human peripheral blood lymphocyte-, CD34⁺ haematopoietic stem cell- and bone marrow/liver/thymus-reconstituted mice and available improved models (e.g. myeloid- or T-cell-only mice, MISTRG, NSG-SGM3). Additionally, we summarize human-tropic viral infections, for which humanized mice offer a novel approach for the study of disease pathogenesis as well as future perspectives for their use in biomedical, drug and vaccine research.

Improved B cell development in humanized NOD-scid IL2Rγnull mice transgenically expressing human stem cell factor, granulocyte-macrophage colony-stimulating factor and interleukin-3

Introduction

Immunodeficient mice engrafted with human immune systems support studies of human hematopoiesis and the immune response to human‐specific pathogens. A significant limitation of these humanized mouse models is, however, a severely restricted ability of human B cells to undergo class switching and produce antigen‐specific IgG after infection or immunization.

Methods

In this study, we have characterized the development and function of human B cells in NOD‐scid IL2Rγ^null (NSG) mice transgenically expressing human stem cell factor (SCF), granulocyte macrophage colony‐stimulating factor (GM‐CSF), and IL‐3 (NSG‐SGM3) following engraftment with human hematopoietic stem cells, autologous fetal liver, and thymic tissues (bone marrow, liver, thymus or BLT model). The NSG‐SGM3 BLT mice engraft rapidly with human immune cells and develop T cells, B cells, and myeloid cells.

Results

A higher proportion of human B cells developing in NSG‐SGM3 BLT mice had a mature/naive phenotype with a corresponding decrease in immature/transitional human B cells as compared to NSG BLT mice. In addition, NSG‐SGM3 BLT mice have higher basal levels of human IgM and IgG as compared with NSG BLT mice. Moreover, dengue virus infection of NSG‐SGM3 BLT mice generated higher levels of antigen‐specific IgM and IgG, a result not observed in NSG BLT mice.

Conclusions

Our studies suggest that NSG‐SGM3 BLT mice show improved human B cell development and permit the generation of antigen‐specific antibody responses to viral infection.

Concise review: humanized models of tumor immunology in the 21st century: convergence of cancer research and tissue engineering

Despite positive testing in animal studies, more than 80% of novel drug candidates fail to proof their efficacy when tested in humans. This is primarily due to the use of preclinical models that are not able to recapitulate the physiological or pathological processes in humans. Hence, one of the key challenges in the field of translational medicine is to "make the model organism mouse more human." To get answers to questions that would be prognostic of outcomes in human medicine, the mouse's genome can be altered in order to create a more permissive host that allows the engraftment of human cell systems. It has been shown in the past that these strategies can improve our understanding of tumor immunology. However, the translational benefits of these platforms have still to be proven. In the 21st century, several research groups and consortia around the world take up the challenge to improve our understanding of how to humanize the animal's genetic code, its cells and, based on tissue engineering principles, its extracellular microenvironment, its tissues, or entire organs with the ultimate goal to foster the translation of new therapeutic strategies from bench to bedside. This article provides an overview of the state of the art of humanized models of tumor immunology and highlights future developments in the field such as the application of tissue engineering and regenerative medicine strategies to further enhance humanized murine model systems.

Humanized mouse G6 anti-idiotypic monoclonal antibody has therapeutic potential against IGHV1-69 germline gene-based B-CLL

In 10-20% of the cases of chronic lymphocytic leukemia of B-cell phenotype (B-CLL), the IGHV1-69 germline is utilized as VH gene of the B cell receptor (BCR). Mouse G6 (MuG6) is an anti-idiotypic monoclonal antibody discovered in a screen against rheumatoid factors (RFs) that binds with high affinity to an idiotope expressed on the 51p1 alleles of IGHV1-69 germline gene encoded antibodies (G6-id(+)). The finding that unmutated IGHV1-69 encoded BCRs are frequently expressed on B-CLL cells provides an opportunity for anti-idiotype monoclonal antibody immunotherapy. In this study, we first showed that MuG6 can deplete B cells encoding IGHV1-69 BCRs using a novel humanized GTL mouse model. Next, we humanized MuG6 and demonstrated that the humanized antibodies (HuG6s), especially HuG6.3, displayed ∼2-fold higher binding affinity for G6-id(+) antibody compared to the parental MuG6. Additional studies showed that HuG6.3 was able to kill G6-id(+) BCR expressing cells and patient B-CLL cells through antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). Finally, both MuG6 and HuG6.3 mediate in vivo depletion of B-CLL cells in NSG mice. These data suggest that HuG6.3 may provide a new precision medicine to selectively kill IGHV1-69-encoding G6-id(+) B-CLL cells.

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.

Effects of lymphocyte profile on development of EBV-induced lymphoma subtypes in humanized mice

Epstein-Barr virus (EBV) infection causes both Hodgkin's lymphoma (HL) and non-Hodgkin's lymphoma (NHL). The present study reveals that EBV-induced HL and NHL are intriguingly associated with a repopulated immune cell profile in humanized mice. Newborn immunodeficient NSG mice were engrafted with human cord blood CD34(+) hematopoietic stem cells (HSCs) for a 8- or 15-wk reconstitution period (denoted (8w)hN and (15w)hN, respectively), resulting in human B-cell and T-cell predominance in peripheral blood cells, respectively. Further, novel humanized mice were established via engraftment of hCD34(+) HSCs together with nonautologous fetal liver-derived mesenchymal stem cells (MSCs) or MSCs expressing an active notch ligand DLK1, resulting in mice skewed with human B or T cells, respectively. After EBV infection, whereas NHL developed more frequently in B-cell-predominant humanized mice, HL was seen in T-cell-predominant mice (P = 0.0013). Whereas human splenocytes from NHL-bearing mice were positive for EBV-associated NHL markers (hBCL2(+), hCD20(+), hKi67(+), hCD20(+)/EBNA1(+), and EBER(+)) but negative for HL markers (LMP1(-), EBNA2(-), and hCD30(-)), most HL-like tumors were characterized by the presence of malignant Hodgkin's Reed-Sternberg (HRS)-like cells, lacunar RS (hCD30(+), hCD15(+), IgJ(-), EBER(+)/hCD30(+), EBNA1(+)/hCD30(+), LMP(+)/EBNA2(-), hCD68(+), hBCL2(-), hCD20(-/weak,) Phospho STAT6(+)), and mummified RS cells. This study reveals that immune cell composition plays an important role in the development of EBV-induced B-cell lymphoma.

Animal Models of Gastrointestinal and Liver Diseases. The difficulty of animal modeling of pancreatic cancer for preclinical evaluation of therapeutics

Pancreatic ductal adenocarcinoma (PDAC) is relatively rare but extremely lethal. Standard cytotoxic therapeutics provide little benefit. To date, newer targeted therapeutics have also not been highly successful. Often novel therapeutics that have appeared to perform well in preclinical models have failed in the clinic. Many factors contribute to these failures, but the one most often attributed is the shortcomings of the preclinical models. A plethora of animal models now exist for PDAC, including cell line xenografts, patient-derived xenografts, a wide variety of genetic mouse models, and syngeneic xenografts. These models have generated a tremendous amount of information useful for the understanding of PDAC. Yet none seems to well predict clinical outcomes of new treatments. This review will discuss how genetic instability and cellular heterogeneity make this disease so difficult to model accurately. We will also discuss the strengths and weaknesses of many of the popular models. Ultimately we will argue that there is no perfect model and that the best approach to understanding clinical performance is the use of multiple preclinical models with an understanding of their salient features.

Vaccine targets against Moraxella catarrhalis

INTRODUCTION

Moraxella catarrhalis is a prominent pathogen that causes acute otitis media in children and lower respiratory tract infections in adults, resulting in a significant socioeconomic burden on healthcare systems globally. No vaccine is currently available for M. catarrhalis. Promising M. catarrhalis target antigens have been characterized in animal models and should soon enter human clinical trials.

AREAS COVERED

This review discusses the detailed features and research status of current candidate target antigens for an M. catarrhalis vaccine. The approaches for assessing M. catarrhalis vaccine efficacy are also discussed.

EXPERT OPINION

Targeting the key molecules contributing to serum resistance may be a viable strategy to identify effective vaccine targets among M. catarrhalis antigens. Elucidating the role and mechanisms of the serum and mucosal immune responses to M. catarrhalis is significant for vaccine target selection, testing and evaluation. Developing animal models closely simulating M. catarrhalis-caused human respiratory diseases is of great benefit in better understanding pathogenesis and evaluating vaccine efficacy. Carrying out clinical trials will be a landmark in the progress of M. catarrhalis vaccine research. Combined multicomponent vaccines will be a focus of future M. catarrhalis vaccine studies.

Application of Humanized Mice in Immunological Research

During the past decade, the development of humanized mouse models and their general applications in biomedical research greatly accelerated the translation of outcomes obtained from basic research into potential diagnostic and therapeutic strategies in clinic. In this chapter, we firstly present an overview on the history and current progress of diverse humanized mouse models and then focus on those equipped with reconstituted human immune system. The update advancement in the establishment of humanized immune system mice and their applications in the studies of the development of human immune system and the pathogenesis of multiple human immune-related diseases are intensively reviewed here, while the shortcoming and perspective of these potent tools are discussed as well. As a valuable bridge across the gap between bench work and clinical trial, progressive humanized mouse models will undoubtedly continue to play an indispensable role in the wide area of biomedical research.

Dengue virus models based on mice as experimental hosts

Dengue virus (DENV) causes dengue fever, a widely distributed endemic disease transmitted by mosquitoes. The complex interaction of DENV with the human immune system has complicated the development of an effective vaccine. This may be attributed, at least in part, to the lack of a suitable animal model capable to reproduce symptoms observed in humans. Mouse models are simple but usually rely on host-adapted virus strains or immunodeficient mouse lineages. Recent evidences indicated that some natural DENV strains are capable to infect immunocompetent mice. In addition, humanized mouse lineages can more faithfully reproduce some of the symptoms observed in humans. Such experimental models are valuable tools for the study of DENV biology.

Use of Humanized Mice to Study the Pathogenesis of Autoimmune and Inflammatory Diseases

Animal models of disease have been used extensively by the research community for the past several decades to better understand the pathogenesis of different diseases and assess the efficacy and toxicity of different therapeutic agents. Retrospective analyses of numerous preclinical intervention studies using mouse models of acute and chronic inflammatory diseases reveal a generalized failure to translate promising interventions or therapeutics into clinically effective treatments in patients. Although several possible reasons have been suggested to account for this generalized failure to translate therapeutic efficacy from the laboratory bench to the patient's bedside, it is becoming increasingly apparent that the mouse immune system is substantially different from the human. Indeed, it is well known that >80 major differences exist between mouse and human immunology; all of which contribute to significant differences in immune system development, activation, and responses to challenges in innate and adaptive immunity. This inconvenient reality has prompted investigators to attempt to humanize the mouse immune system to address important human-specific questions that are impossible to study in patients. The successful long-term engraftment of human hematolymphoid cells in mice would provide investigators with a relatively inexpensive small animal model to study clinically relevant mechanisms and facilitate the evaluation of human-specific therapies in vivo. The discovery that targeted mutation of the IL-2 receptor common gamma chain in lymphopenic mice allows for the long-term engraftment of functional human immune cells has advanced greatly our ability to humanize the mouse immune system. The objective of this review is to present a brief overview of the recent advances that have been made in the development and use of humanized mice with special emphasis on autoimmune and chronic inflammatory diseases. In addition, we discuss the use of these unique mouse models to define the human-specific immunopathological mechanisms responsible for the induction and perpetuation of chronic gut inflammation.

BLT humanized mice as a small animal model of HIV infection

Humanized mice are valuable models for the research and development of vaccine strategies and therapeutic interventions to control or eradicate HIV. The BLT humanized mouse model is particularly promising because the combination of transplantation of human fetal pluripotent hematopoietic stem cells with surgical engraftment of human fetal thymic tissue results in improved T cell reconstitution, maturation, and selection. To date, the BLT humanized mouse model has been used to study many aspects of HIV infection including prevention, mucosal transmission, HIV-specific innate and adaptive immunity, viral latency, and novel antiretroviral and immune-based therapies for suppression and reservoir eradication. Here we describe recent advances and applications of the BLT humanized mouse model of HIV infection and discuss opportunities to further improve this valuable small animal model.

Recovery of a human natural antibody against the noncollagenous-1 domain of type IV collagen using humanized models

BACKGROUND

Anti-glomerular basement membrane nephritis and Goodpasture syndrome result from autoantibody (Ab)-mediated destruction of kidney and lung. Ab target the noncollagenous 1 (NC1) domain of alpha3(IV) collagen, but little is known about Ab origins or structure. This ignorance is due in part to the inability to recover monoclonal Ab by transformation of patients' blood cells. The aim of this study was to assess the suitability of two humanized models for this purpose.

METHODS

NOD-scid-gamma immunodeficient mice were engrafted either with human CD34+ hematopoietic stem cells (HSC) (Hu-HSC mice) and immunized with alpha3(IV)NC1 collagen containing the Goodpasture epitopes or with nephritis patients' peripheral blood leukocytes (PBL) (Hu-PBL mice). After in vivo immune cell development and/or expansion, recovered human B cells were Epstein Barr virus (EBV)-transformed, screened for antigen (Ag) binding, electrofused with a mouse-human heterohybridoma, subcloned, and human Ab RNA sequenced by PCR after reverse transcription to cDNA. Flow cytometry was used to assess human B cell markers and differentiation in Hu-PBL mice.

RESULTS

Sequence analysis of a human Ab derived from an immunized Hu-HSC mouse and reactive with alpha3(IV)NC1 collagen reveals that it is encoded by unmutated heavy and light chain genes. The heavy chain complementarity determining region 3, a major determinant of Ag binding, contains uncommon motifs, including an N-region somatically-introduced highly hydrophobic tetrapeptide and dual cysteines encoded by a uniquely human IGHD2-2 Ab gene segment that lacks a murine counterpart. Comparison of human and mouse autoantibodies suggests that structurally similar murine Ab may arise by convergent selection. In contrast to the Hu-HSC model, transformed human B cells are rarely recovered from Hu-PBL mice, in which human B cells terminally differentiate and lose expression of EBV receptor CD21, thus precluding their transformation and recovery.

CONCLUSIONS

Hu-HSC mice reveal that potentially pathogenic B cells bearing unmutated Ig receptors reactive with the NC1 domain on alpha3(IV) collagen can be generated in, and not purged from, the human preimmune repertoire. Uniquely human gene elements are recruited to generate the antigen binding site in at least a subset of these autoantibodies, indicating that humanized models may provide insights inaccessible using conventional mouse models.

Development of human B cells and antibodies following human hematopoietic stem cell transplantation to Rag2(-/-)γc(-/-) mice

Humanized mice represent a valuable model system to study the development and functionality of the human immune system. In the RAG-hu mouse model highly immunodeficient Rag2(-/-)γc(-/-) mice are transplanted with human CD34(+) hematopoietic stem cells, resulting in human hematopoiesis and a predominant production of B and T lymphocytes. Human adaptive immune responses have been detected towards a variety of antigens in humanized mice but both cellular and humoral immune responses tend to be weak and sporadically detected. The underlying mechanisms for inconsistent responses are poorly understood. Here, we analyzed the kinetics of human B cell development and antibody production in RAG-hu mice to better understand the lack of effective antibody responses. We found that T cell levels in blood did not significantly change from 8 to 28 weeks post-engraftment, while B cells reached a peak at 14 weeks. Concentrations of 3 antibody classes (IgM, IgG, IgA) were found to be at levels about 0.1% or less of normal human levels, but human antibodies were still detected up to 32 weeks after engraftment. Human IgM was detected in 92.5% of animals while IgG and IgA were detected in about half of animals. We performed flow cytometric analysis of human B cells in bone marrow, spleen, and blood to examine the presence of precursor B cells, immature B cells, naïve B cells, and plasma B cells. We detected high levels of surface IgM(+) B cells (immature and naïve B cells) and low levels of plasma B cells in these organs, suggesting that B cells do not mature properly in this model. Low levels of human T cells in the spleen were observed, and we suggest that the lack of T cell help may explain poor B cell development and antibody responses. We conclude that human B cells that develop in humanized mice do not receive the signals necessary to undergo class-switching or to secrete antibody effectively, and we discuss strategies to potentially overcome these barriers.

Fatal autoimmunity in mice reconstituted with human hematopoietic stem cells encoding defective FOXP3

Mice reconstituted with a human immune system provide a tractable in vivo model to assess human immune cell function. To date, reconstitution of murine strains with human hematopoietic stem cells (HSCs) from patients with monogenic immune disorders have not been reported. One obstacle precluding the development of immune-disease specific "humanized" mice is that optimal adaptive immune responses in current strains have required implantation of autologous human thymic tissue. To address this issue, we developed a mouse strain that lacks murine major histocompatibility complex class II (MHC II) and instead expresses human leukocyte antigen DR1 (HLA-DR1). These mice displayed improved adaptive immune responses when reconstituted with human HSCs including enhanced T-cell reconstitution, delayed-type hypersensitivity responses, and class-switch recombination. Following immune reconstitution of this novel strain with HSCs from a patient with immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome, associated with aberrant FOXP3 function, mice developed a lethal inflammatory disorder with multiorgan involvement and autoantibody production mimicking the pathology seen in affected humans. This humanized mouse model permits in vivo evaluation of immune responses associated with genetically altered HSCs, including primary immunodeficiencies, and should facilitate the study of human immune pathobiology and the development of targeted therapeutics.

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.

Hypogammaglobulinemia in BLT humanized mice--an animal model of primary antibody deficiency

Primary antibody deficiencies present clinically as reduced or absent plasma antibodies without another identified disorder that could explain the low immunoglobulin levels. Bone marrow-liver-thymus (BLT) humanized mice also exhibit primary antibody deficiency or hypogammaglobulinemia. Comprehensive characterization of B cell development and differentiation in BLT mice revealed other key parallels with primary immunodeficiency patients. We found that B cell ontogeny was normal in the bone marrow of BLT mice but observed an absence of switched memory B cells in the periphery. PC-KLH immunizations led to the presence of switched memory B cells in immunized BLT mice although plasma cells producing PC- or KLH- specific IgG were not detected in tissues. Overall, we have identified the following parallels between the humoral immune systems of primary antibody deficiency patients and those in BLT mice that make this in vivo model a robust and translational experimental platform for gaining a greater understanding of this heterogeneous array of humoral immunodeficiency disorders in humans: (i) hypogammaglobulinemia; (ii) normal B cell ontogeny in bone marrow; and (iii) poor antigen-specific IgG response to immunization. Furthermore, the development of strategies to overcome these humoral immune aberrations in BLT mice may in turn provide insights into the pathogenesis of some primary antibody deficiency patients which could lead to novel clinical interventions for improved humoral immune function.

Dengue virus infection induces broadly cross-reactive human IgM antibodies that recognize intact virions in humanized BLT-NSG mice

The development of small animal models that elicit human immune responses to dengue virus (DENV) is important since prior immunity is a major risk factor for developing severe dengue disease. This study evaluated anti-DENV human antibody (hAb) responses generated from immortalized B cells after DENV-2 infection in NOD-scid IL2rγ(null) mice that were co-transplanted with human fetal thymus and liver tissues (BLT-NSG mice). DENV-specific human antibodies predominantly of the IgM isotype were isolated during acute infection and in convalescence. We found that while a few hAbs recognized the envelope protein produced as a soluble recombinant, a number of hAbs only recognized epitopes on intact virions. The majority of the hAbs isolated during acute infection and in immune mice were serotype-cross-reactive and poorly neutralizing. Viral titers in immune BLT-NSG mice were significantly decreased after challenge with a clinical strain of dengue. DENV-specific hAbs generated in BLT-NSG mice share some of the characteristics of Abs isolated in humans with natural infection. Humanized BLT-NSG mice provide an attractive preclinical platform to assess the immunogenicity of candidate dengue vaccines.

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.

Generation of improved humanized mouse models for human infectious diseases

The study of human-specific infectious agents has been hindered by the lack of optimal small animal models. More recently development of novel strains of immunodeficient mice has begun to provide the opportunity to utilize small animal models for the study of many human-specific infectious agents. The introduction of a targeted mutation in the IL2 receptor common gamma chain gene (IL2rg^null) in mice already deficient in T and B cells led to a breakthrough in the ability to engraft hematopoietic stem cells, as well as functional human lymphoid cells and tissues, effectively creating human immune systems in immunodeficient mice. These humanized mice are becoming increasingly important as pre-clinical models for the study of human immunodeficiency virus-1 (HIV-1) and other human-specific infectious agents. However, there remain a number of opportunities to further improve humanized mouse models for the study of human-specific infectious agents. This is being done by the implementation of innovative technologies, which collectively will accelerate the development of new models of genetically modified mice, including; i) modifications of the host to reduce innate immunity, which impedes human cell engraftment; ii) genetic modification to provide human-specific growth factors and cytokines required for optimal human cell growth and function; iii) and new cell and tissue engraftment protocols. The development of “next generation” humanized mouse models continues to provide exciting opportunities for the establishment of robust small animal models to study the pathogenesis of human-specific infectious agents, as well as for testing the efficacy of therapeutic agents and experimental vaccines.

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Humanized mice support pre-clinical analyses of human-specific infectious agents.

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Novel technologies are generating new humanized mouse models.

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Innovations to improve human immune responses in humanized mice are becoming available.

Humanized-BLT mouse model of Kaposi's sarcoma-associated herpesvirus infection

Lack of an effective small-animal model to study the Kaposi's sarcoma-associated herpesvirus (KSHV) infection in vivo has hampered studies on the pathogenesis and transmission of KSHV. The objective of our study was to determine whether the humanized BLT (bone marrow, liver, and thymus) mouse (hu-BLT) model generated from NOD/SCID/IL2rγ mice can be a useful model for studying KSHV infection. We have tested KSHV infection of hu-BLT mice via various routes of infection, including oral and intravaginal routes, to mimic natural routes of transmission, with recombinant KSHV over a 1- or 3-mo period. Infection was determined by measuring viral DNA, latent and lytic viral transcripts and antigens in various tissues by PCR, in situ hybridization, and immunohistochemical staining. KSHV DNA, as well as both latent and lytic viral transcripts and proteins, were detected in various tissues, via various routes of infection. Using double-labeled immune-fluorescence confocal microscopy, we found that KSHV can establish infection in human B cells and macrophages. Our results demonstrate that KSHV can establish a robust infection in the hu-BLT mice, via different routes of infection, including the oral mucosa which is the most common natural route of infection. This hu-BLT mouse not only will be a useful model for studying the pathogenesis of KSHV in vivo but can potentially be used to study the routes and spread of viral infection in the infected host.

Human immune system development and survival of non-obese diabetic (NOD)-scid IL2rγ(null) (NSG) mice engrafted with human thymus and autologous haematopoietic stem cells

Immunodeficient mice bearing targeted mutations in the IL2rg gene and engrafted with human immune systems are effective tools for the study of human haematopoiesis, immunity, infectious disease and transplantation biology. The most robust human immune model is generated by implantation of human fetal thymic and liver tissues in irradiated recipients followed by intravenous injection of autologous fetal liver haematopoietic stem cells [often referred to as the BLT (bone marrow, liver, thymus) model]. To evaluate the non-obese diabetic (NOD)-scid IL2rγ(null) (NSG)-BLT model, we have assessed various engraftment parameters and how these parameters influence the longevity of NSG-BLT mice. We observed that irradiation and subrenal capsule implantation of thymus/liver fragments was optimal for generating human immune systems. However, after 4 months, a high number of NSG-BLT mice develop a fatal graft-versus-host disease (GVHD)-like syndrome, which correlates with the activation of human T cells and increased levels of human immunoglobulin (Ig). Onset of GVHD was not delayed in NSG mice lacking murine major histocompatibility complex (MHC) classes I or II and was not associated with a loss of human regulatory T cells or absence of intrathymic cells of mouse origin (mouse CD45(+) ). Our findings demonstrate that NSG-BLT mice develop robust human immune systems, but that the experimental window for these mice may be limited by the development of GVHD-like pathological changes.

Humoral immunity in humanized mice: a work in progress

Humanized mice historically have not been good models of human humoral immunity induced by either infection or immunization. However, newer versions of humanized mice generated in severely immunodeficient mice with a targeted disruption of the IL2Rγc gene have recently been reported to produce antigen-specific class-switched human antibodies, with some demonstrating neutralizing activities. Here we review the growing ability of humanized mice to support the study of human humoral immune responses, discussing the current and future potential of these models as well as their current limitations.

HIV-specific CD8⁺ T-cell immunity in humanized bone marrow-liver-thymus mice

CD8(+) T-cell responses play a critical role in the control of human immunodeficiency virus (HIV) infection, and recent vaccine studies in nonhuman primates now demonstrate the ability of T cells to prevent the early dissemination of simian immunodeficiency virus and perhaps clear residual infection. Recent advances in humanized mouse models, in particular the humanized bone marrow-liver-thymus (BLT) mouse model, show promise in their ability not only to support sustained infection with HIV, but also to recapitulate human HIV-specific immunity. The availability of a small-animal model with which to study human-specific immune responses to HIV would greatly facilitate the elucidation of mechanisms of immune control, as well as accelerate the iterative testing of promising vaccine candidates. Here we discuss data from our recent study detailing the composition and efficacy of HIV-specific CD8(+) T-cell responses in humanized BLT mice that was recently presented at a Harvard Center for AIDS Research symposium on humanized mouse models for HIV vaccine design.

Human immune responses and potential for vaccine assessment in humanized mice

The new humanized mouse models with a transplanted human immune system have a capacity for de novo multilineage human hematopoiesis and generate T cells, B cells, macrophages, dendritic cells and NK cells. Of the two current leading humanized mouse models, the hu-HSC model is created by human hematopoietic stem cell (HSC) engraftment whereas the BLT mouse model is prepared by co-transplantation of human fetal liver, thymus and HSC. Humoral and cellular immune responses are seen in both models after immunization with antigens or infection with hematotropic pathogens such as EBV, HIV-1 and dengue viruses. While consistent antigen specific IgM production is seen, IgG responses were found to be generally feeble which is attributed to inefficient immunoglobulin class switching. BLT mice permit human HLA restricted T cell responses due to the autologous human thymus contributing to T cell maturation. Use of HLA Class I and II transgenic hu-HSC mice recently demonstrated that the HLA restriction deficiency could be overcome in this model. However, the overall vigor of the immune responses needs further improvement in both the models to approach that of the human. Towards this goal, supplementation with human cytokines and growth factors by transgenesis to improve human cell reconstitution and their homeostatic maintenance are beginning to yield improved mouse strains to create more robust human immune competent mice for immunoprophylaxis studies.

New generation humanized mice for virus research: comparative aspects and future prospects

Work with human specific viruses will greatly benefit from the use of an in vivo system that provides human target cells and tissues in a physiological setting. In this regard humanized mice (hu-Mice) have played an important role in our understanding of viral pathogenesis and testing of therapeutic strategies. Limitations with earlier versions of hu-Mice that lacked a functioning human immune system are currently being overcome. The new generation hu-Mouse models are capable of multilineage human hematopoiesis and generate T cells, B cells, macrophages and dendritic cells required for an adaptive human immune response. Now any human specific pathogen that can infect humanized mice can be studied in the context of ongoing infection and immune responses. Two leading humanized mouse models are currently employed: the hu-HSC model is created by transplantation of human hematopoietic stem cells (HSC), whereas the BLT mouse model is prepared by transplantation of human fetal liver, thymus and HSC. A number of human specific viruses such as HIV-1, dengue, EBV and HCV are being studied intensively in these systems. Both models permit infection by mucosal routes with viruses such as HIV-1 thus allowing transmission prevention studies. Cellular and humoral immune responses are seen in both the models. While there is efficient antigen specific IgM production, IgG responses are suboptimal due to inefficient immunoglobulin class switching. With the maturation of T cells occurring in the autologous human thymus, BLT mice permit human HLA restricted T cell responses in contrast to hu-HSC mice. However, the strength of the immune responses needs further improvement in both models to reach the levels seen in humans. The scope of hu-Mice use is further broadened by transplantation of additional tissues like human liver thus permitting immunopathogenesis studies on hepatotropic viruses such as HCV. Numerous studies that encompass antivirals, gene therapy, viral evolution, and the generation of human monoclonal antibodies have been conducted with promising results in these mice. For further improvement of the new hu-Mouse models, ongoing work is focused on generating new strains of immunodeficient mice transgenic for human HLA molecules to strengthen immune responses and human cytokines and growth factors to improve human cell reconstitution and their homeostatic maintenance.