Superior human leukocyte reconstitution and susceptibility to vaginal HIV transmission in humanized NOD-scid IL-2Rγ(-/-) (NSG) BLT mice

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.

Generation of improved mouse models for the study of hepatitis C virus

Approximately 3% of the world׳s population suffers from chronic infections with hepatitis C virus (HCV). Although current treatment regimes are capable of effectively eradicating HCV infection from these patients, the cost of these combinations of direct-acting antivirals are prohibitive. Approximately 80% of untreated chronic HCV carriers will be at high risk for developing severe liver disease, including fibrosis, cirrhosis, and hepatocellular carcinoma. A vaccine is urgently needed to lessen this global burden. Besides humans, HCV infection can be experimentally transmitted to chimpanzees, and this is the best model for studies of HCV infection and related innate and adaptive immune responses. Although the chimpanzee model yielded valuable insight, limited availability, high cost and ethical considerations limit their utility. The only small animal models of robust HCV infection are highly immunodeficient mice with human chimeric livers. However, these mice cannot be used to study adaptive immune responses and therefore a more relevant animal model is needed to assist in vaccine development. Novel strains of immunodeficient mice have been developed that allow for the engraftment of human hepatopoietic stem cells, as well as functional human lymphoid cells and tissues, effectively creating human immune systems in otherwise immunodeficient mice. These humanized mice are rapidly emerging as pre-clinical bridges for numerous pathogens that, like HCV, only cause infectious disease in humans. This review highlights the potential these new models have for changing the current landscape for HCV research and vaccine development.

Studies of retroviral infection in humanized mice

Many important aspects of human retroviral infections cannot be fully evaluated using only in vitro systems or unmodified animal models. An alternative approach involves the use of humanized mice, which consist of immunodeficient mice that have been transplanted with human cells and/or tissues. Certain humanized mouse models can support robust infection with human retroviruses including different strains of human immunodeficiency virus (HIV) and human T cell leukemia virus (HTLV). These models have provided wide-ranging insights into retroviral biology, including detailed information on primary infection, in vivo replication and pathogenesis, latent/persistent reservoir formation, and novel therapeutic interventions. Here we describe the humanized mouse models that are most commonly utilized to study retroviral infections, and outline some of the important discoveries that these models have produced during several decades of intensive research.

CD8-positive lymphocytes in graft-versus-host disease of humanized NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ mice

Immunocompromised mice that can support a human immune system are an increasingly important model for the investigation of haemopoietic stem/progenitor cell (HSPC) development and human infectious disease. NOD-SCID IL-2Rγ(-/-) (NSG) mice engrafted with human fetal liver and thymus prior to HSPC engraftment, commonly known as NSG-bone marrow-liver-thymus (NSG-hu-BLT) mice, are one such model and have robust reconstitution of human leucocytes within the peripheral blood and tissues. Four NSG-hu-BLT mice were submitted for diagnostic necropsy examination following the development of alopecia, pruritus and lethargy after HSPC engraftment. Histopathology revealed multifocal to coalescing single keratinocyte cell death in the epidermis and follicles with dermatitis and mild dermal fibrosis. Single-cell hepatocyte cell death was present in three cases, with various degrees of portal fibrosis. In the skin and liver, cell death was associated with lymphocytes that reacted with anti-human CD45, CD3 and CD8 antibodies, consistent with a diagnosis of graft-versus-host disease (GvHD). This study expands on recently reported microscopical features of GvHD in NSG-hu-BLT mice and suggests a role for CD8(+) T lymphocytes in the progression of the disease. NSG-hu-BLT mice represent an excellent model of GvHD, but its prevalence may compromise their use in other fields of biomedical research.

Cell-associated transmission of HIV type 1 and other lentiviruses in small-animal models

Small-animal models of lentivirus transmission have repeatedly demonstrated transmission by cell-associated virus via vaginal, rectal, and oral routes. The earliest experiments were in the cat/feline immunodeficiency virus model, followed a decade later by successful vaginal transmission of cell-associated human immunodeficiency virus (HIV) in mice bearing transplanted human immune cells. After early unsuccessful attempts at cell-associated transmission in nonhuman primates, renewed investigation in diverse primate models has now confirmed the findings from the cat and humanized mouse models. Improvements in humanized mouse models have made them the preferred small-animal models to study HIV mucosal transmission. They provide complementary systems to nonhuman primate models to aid in the elucidation of the many remaining questions on the mechanism of and means to prevent both cell-associated and cell-free HIV transmission across mucosal barriers.

Minocycline attenuates HIV-1 infection and suppresses chronic immune activation in humanized NOD/LtsZ-scidIL-2Rγ(null) mice

More than a quarter of a century of research has established chronic immune activation and dysfunctional T cells as central features of chronic HIV infection and subsequent immunodeficiency. Consequently, the search for a new immunomodulatory therapy that could reduce immune activation and improve T-cell function has been increased. However, the lack of small animal models for in vivo HIV study has hampered progress. In the current study, we have investigated a model of cord blood haematopoietic progenitor cells (CB-HPCs) -transplanted humanized NOD/LtsZ-scidIL-2Rγ(null) mice in which progression of HIV infection is associated with widespread chronic immune activation and inflammation. Indeed, HIV infection in humanized NSG mice caused up-regulation of several T-cell immune activation markers such as CD38, HLA-DR, CD69 and co-receptor CCR5. T-cell exhaustion markers PD-1 and CTLA-4 were found to be significantly up-regulated on T cells. Moreover, increased plasmatic levels of lipopolysaccharide, sCD14 and interleukin-10 were also observed in infected mice. Treatment with minocycline resulted in a significant decrease of expression of cellular and plasma immune activation markers, inhibition of HIV replication and improved T-cell counts in HIV-infected humanized NSG mice. The study demonstrates that minocycline could be an effective, low-cost adjunctive treatment to regulate chronic immune activation and replication of HIV.

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.

Humanized mice: novel model for studying mechanisms of human immune-based therapies

The lack of relevant animal models is the major bottleneck for understanding human immunology and immunopathology. In the last few years, a novel model of humanized mouse has been successfully employed to investigate some of the most critical questions in human immunology. We have set up and tested in our laboratory the latest technology for generating mice with a human immune system by reconstituting newborn immunodeficient NOD/SCID-γ(c)(-/-) mice with human fetal liver-derived hematopoietic stem cells. These humanized mice have been deemed most competent as human models in a thorough comparative study with other humanized mouse technologies. Lymphocytes in these mice are of human origin while other hematopoietic cells are chimeric, partly of mouse and partly of human origin. We demonstrate that human CD8 T lymphocytes in humanized mice are fully responsive to our novel cell-based secreted heat shock protein gp96(HIV)-Ig vaccine. We also show that the gp96(HIV)-Ig vaccine induces powerful mucosal immune responses in the rectum and the vagina, which are thought to be required for protection from HIV infection. We posit the hypothesis that vaccine approaches tested in humanized mouse models can generate data rapidly, economically and with great flexibility (genetic manipulations are possible), to be subsequently tested in larger nonhuman primate models and humans.

Vaccination against endogenous retrotransposable element consensus sequences does not protect rhesus macaques from SIVsmE660 infection and replication

The enormous sequence diversity of HIV remains a major roadblock to the development of a prophylactic vaccine and new approaches to induce protective immunity are needed. Endogenous retrotransposable elements (ERE) such as endogenous retrovirus K (ERV)-K and long interspersed nuclear element-1 (LINE-1) are activated during HIV-1-infection and could represent stable, surrogate targets to eliminate HIV-1-infected cells. Here, we explored the hypothesis that vaccination against ERE would protect macaques from acquisition and replication of simian immunodeficiency virus (SIV). Following vaccination with antigens derived from LINE-1 and ERV-K consensus sequences, animals mounted immune responses that failed to delay acquisition of SIVsmE660. We observed no differences in acute or set point viral loads between ERE-vaccinated and control animals suggesting that ERE-specific responses were not protective. Indeed, ERE-specific T cells failed to expand anamnestically in vivo following infection with SIVsmE660 and did not recognize SIV-infected targets in vitro, in agreement with no significant induction of targeted ERE mRNA by SIV in macaque CD4+ T cells. Instead, lower infection rates and viral loads correlated significantly to protective TRIM5α alleles. Cumulatively, these data demonstrate that vaccination against the selected ERE consensus sequences in macaques did not lead to immune-mediated recognition and killing of SIV-infected cells, as has been shown for HIV-infected human cells using patient-derived HERV-K-specific T cells. Thus, further research is required to identify the specific nonhuman primate EREs and retroviruses that recapitulate the activity of HIV-1 in human cells. These results also highlight the complexity in translating observations of the interplay between HIV-1 and human EREs to animal models.

DNA cleavage enzymes for treatment of persistent viral infections: recent advances and the pathway forward

Treatment for most persistent viral infections consists of palliative drug options rather than curative approaches. This is often because long-lasting viral DNA in infected cells is not affected by current antivirals, providing a source for viral persistence and reactivation. Targeting latent viral DNA itself could therefore provide a basis for novel curative strategies. DNA cleavage enzymes can be used to induce targeted mutagenesis of specific genes, including those of exogenous viruses. Although initial in vitro and even in vivo studies have been carried out using DNA cleavage enzymes targeting various viruses, many questions still remain concerning the feasibility of these strategies as they transition into preclinical research. Here, we review the most recent findings on DNA cleavage enzymes for human viral infections, consider the most relevant animal models for several human viral infections, and address issues regarding safety and enzyme delivery. Results from well-designed in vivo studies will ideally provide answers to the most urgent remaining questions, and allow continued progress toward clinical application.

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.

Immunodeficient mouse model for human hematopoietic stem cell engraftment and immune system development

Immunodeficient mice engrafted with human immune systems provide an exciting model to study human immunobiology in an in vivo setting without placing patients at risk. The essential parameter for creation of these "humanized models" is engraftment of human hematopoietic stem cells (HSC) that will allow for optimal development of human immune systems. However, there are a number of strategies to generate humanized mice and specific protocols can vary significantly among different laboratories. Here we describe a protocol for the co-implantation of human HSC with autologous fetal liver and thymic tissues into immunodeficient mice to create a humanized model with optimal human T cell development. This model, often referred to as the Thy/Liv or BLT (bone marrow, liver, thymus) mouse, develops a functional human immune system, including HLA-restricted human T cells, B cells, and innate immune cells.

Overcoming current limitations in humanized mouse research

Immunodeficient mice engrafted with human cells and tissues have provided an exciting alternative to in vitro studies with human tissues and nonhuman primates for the study of human immunobiology. A major breakthrough in the early 2000s was the introduction of a targeted mutation in the interleukin 2 (IL-2) receptor common gamma chain (IL2rg(null)) into mice that were already deficient in T and B cells. Among other immune defects, natural killer (NK) cells are disrupted in these mice, permitting efficient engraftment with human hematopoietic cells that generate a functional human immune system. These humanized mouse models are becoming increasingly important for preclinical studies of human immunity, hematopoiesis, tissue regeneration, cancer, and infectious diseases. In particular, humanized mice have enabled studies of the pathogenesis of human-specific pathogens, including human immunodeficiency virus type 1, Epstein Barr virus, and Salmonella typhi. However, there are a number of limitations in the currently available humanized mouse models. Investigators are continuing to identify molecular mechanisms underlying the remaining defects in the engrafted human immune system and are generating "next generation" models to overcome these final deficiencies. This article provides an overview of some of the emerging models of humanized mice, their use in the study of infectious diseases, and some of the remaining limitations that are currently being addressed.

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.

Humanized mice as a model to study human hematopoietic stem cell transplantation

Hematopoietic stem cell (HSC) transplantation has the potential to treat a variety of human diseases, including genetic deficiencies, immune disorders, and to restore immunity following cancer treatment. However, there are several obstacles that prevent effective HSC transplantation in humans. These include finding a matched donor, having a sufficient number of cells for the transplant, and the potency of the cells in the transplant. Ethical issues prevent effective research in humans that could provide insight into ways to overcome these obstacles. Highly immunodeficient mice can be transplanted with human HSCs and this process is accompanied by HSC homing to the murine bone marrow. This is followed by stem cell expansion, multilineage hematopoiesis, long-term engraftment, and functional human antibody and cellular immune responses. As such, humanized mice serve as a model for human HSC transplantation. A variety of conditions have been analyzed for their impact on HSC transplantation to produce humanized mice, including the type and source of cells used in the transplant, the number of cells transplanted, the expansion of cells with various protocols, and the route of introduction of cells into the mouse. In this review, we summarize what has been learned about HSC transplantation using humanized mice as a recipient model and we comment on how these models may be useful to future preclinical research to determine more effective ways to expand HSCs and to determine their repopulating potential in vivo.

Humanized mice for the study of infectious diseases

Many of the pathogens that cause human infectious diseases do not infect rodents or other mammalian species. Small animal models that allow studies of the pathogenesis of these agents and evaluation of drug efficacy are critical for identifying ways to prevent and treat human infectious diseases. Immunodeficient mice engrafted with functional human cells and tissues, termed 'humanized' mice, represent a critical pre-clinical bridge for in vivo studies of human pathogens. Recent advances in the development of humanized mice have allowed in vivo studies of multiple human infectious agents providing novel insights into their pathogenesis that was otherwise not possible.

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.

Assessment of benzene-induced hematotoxicity using a human-like hematopoietic lineage in NOD/Shi-scid/IL-2Rγnull mice

Despite recent advancements, it is still difficult to evaluate in vivo responses to toxicants in humans. Development of a system that can mimic the in vivo responses of human cells will enable more accurate health risk assessments. A surrogate human hematopoietic lineage can be established in NOD/Shi-scid/IL-2Rγ^null (NOG) mice by transplanting human hematopoietic stem/progenitor cells (Hu-NOG mice). Here, we first evaluated the toxic response of human-like hematopoietic lineage in NOG mice to a representative toxic agent, benzene. Flow cytometric analysis showed that benzene caused a significant decrease in the number of human hematopoietic stem/progenitor cells in the bone marrow and the number of human leukocytes in the peripheral blood and hematopoietic organs. Next, we established chimeric mice by transplanting C57BL/6 mouse-derived bone marrow cells into NOG mice (Mo-NOG mice). A comparison of the degree of benzene-induced hematotoxicity in donor-derived hematopoietic lineage cells within Mo-NOG mice indicated that the toxic response of Hu-NOG mice reflected interspecies differences in susceptibilities to benzene. Responses to the toxic effects of benzene were greater in lymphoid cells than in myeloid cells in Mo-NOG and Hu-NOG mice. These findings suggested that Hu-NOG mice may be a powerful in vivo tool for assessing hematotoxicity in humans, while accounting for interspecies differences.

Rapid evolution of HIV-1 to functional CD8⁺ T cell responses in humanized BLT mice

The development of mouse/human chimeras through the engraftment of human immune cells and tissues into immunodeficient mice, including the recently described humanized BLT (bone marrow, liver, thymus) mouse model, holds great promise to facilitate the in vivo study of human immune responses. However, little data exist regarding the extent to which cellular immune responses in humanized mice accurately reflect those seen in humans. We infected humanized BLT mice with HIV-1 as a model pathogen and characterized HIV-1-specific immune responses and viral evolution during the acute phase of infection. HIV-1-specific CD8(+) T cell responses in these mice were found to closely resemble those in humans in terms of their specificity, kinetics, and immunodominance. Viral sequence evolution also revealed rapid and highly reproducible escape from these responses, mirroring the adaptations to host immune pressures observed during natural HIV-1 infection. Moreover, mice expressing the protective HLA-B*57 allele exhibited enhanced control of viral replication and restricted the same CD8(+) T cell responses to conserved regions of HIV-1 Gag that are critical to its control of HIV-1 in humans. These data reveal that the humanized BLT mouse model appears to accurately recapitulate human pathogen-specific cellular immunity and the fundamental immunological mechanisms required to control a model human pathogen, aspects critical to the use of a small-animal model for human pathogens.

HIV-1 immunopathogenesis in humanized mouse models

In recent years, the technology of constructing chimeric mice with humanized immune systems has markedly improved. Multiple lineages of human immune cells develop in immunodeficient mice that have been transplanted with human hematopoietic stem cells. More importantly, these mice mount functional humoral and cellular immune responses upon immunization or microbial infection. Human immunodeficiency virus type I (HIV-1) can establish an infection in humanized mice, resulting in CD4(+) T-cell depletion and an accompanying nonspecific immune activation, which mimics the immunopathology in HIV-1-infected human patients. This makes humanized mice an optimal model for studying the mechanisms of HIV-1 immunopathogenesis and for developing novel immune-based therapies.

Mucosal HIV-1 transmission and prevention strategies in BLT humanized mice

Clinical trials testing microbicides and related biomedical interventions to block HIV transmissions have produced contradictory results and to date it is unclear why. Further elucidation of the molecular basis of mucosal HIV transmission and extensive pharmacokinetic and pharmacodynamic analyses are essential to implementing effective prevention strategies. Animal models are of critical importance to this effort and bone marrow-liver-thymus (BLT) humanized mice have recently emerged as a powerful small animal research platform for in vivo efficacy evaluation of mucosal and parenteral HIV-1 prevention interventions. The availability of this validated system for the pre-clinical evaluation of HIV-1 prevention approaches will accelerate the implementation of the best candidate interventions into clinical trials.

In vivo suppression of HIV by antigen specific T cells derived from engineered hematopoietic stem cells

The HIV-specific cytotoxic T lymphocyte (CTL) response is a critical component in controlling viral replication in vivo, but ultimately fails in its ability to eradicate the virus. Our intent in these studies is to develop ways to enhance and restore the HIV-specific CTL response to allow long-term viral suppression or viral clearance. In our approach, we sought to genetically manipulate human hematopoietic stem cells (HSCs) such that they differentiate into mature CTL that will kill HIV infected cells. To perform this, we molecularly cloned an HIV-specific T cell receptor (TCR) from CD8+ T cells that specifically targets an epitope of the HIV-1 Gag protein. This TCR was then used to genetically transduce HSCs. These HSCs were then introduced into a humanized mouse containing human fetal liver, fetal thymus, and hematopoietic progenitor cells, and were allowed to differentiate into mature human CD8+ CTL. We found human, HIV-specific CTL in multiple tissues in the mouse. Thus, genetic modification of human HSCs with a cloned TCR allows proper differentiation of the cells to occur in vivo, and these cells migrate to multiple anatomic sites, mimicking what is seen in humans. To determine if the presence of the transgenic, HIV-specific TCR has an effect on suppressing HIV replication, we infected with HIV-1 mice expressing the transgenic HIV-specific TCR and, separately, mice expressing a non-specific control TCR. We observed significant suppression of HIV replication in multiple organs in the mice expressing the HIV-specific TCR as compared to control, indicating that the presence of genetically modified HIV-specific CTL can form a functional antiviral response in vivo. These results strongly suggest that stem cell based gene therapy may be a feasible approach in the treatment of chronic viral infections and provide a foundation towards the development of this type of strategy.

There is a desperate need for the development of new therapeutic strategies to eradicate HIV infection. HIV actively subverts the potent natural immune responses against it, particularly cellular cytotoxic T lymphocyte (CTL) responses. The development of a therapy that allows long-lived immune self-containment of HIV and restoration of these CTL responses by the host would be ideal. Through genetic manipulation of human blood-forming stem cells, we introduced a molecule– an HIV-targeting T cell receptor (TCR)–that allowed the generation of functional HIV-specific CTLs following differentiation within human tissues in a humanized mouse model. To assess if these newly developed, HIV-specific CTLs can allow active suppression of HIV replication, we infected these mice with HIV. We found that the development of genetically modified, HIV-specific CTLs in these mice results in the presence of a functional antiviral CTL response in vivo that significantly lowers viral replication following HIV infection. These results have strong implications for the use of this technology to engineer the human immune response to combat viral infections and suggest that genetic engineering via HSCs may allow tailoring of the immune response to target and eradicate HIV.

Modeling HIV-associated neurocognitive disorders in mice: new approaches in the changing face of HIV neuropathogenesis

It is well established that infection with the human immunodeficiency virus (HIV) leads to immune suppression. Less well known is the fact that long-term, progressive HIV disease is associated with the development of cognitive deficits. Since the introduction of combined antiretroviral therapy (cART), the clinical presentation of HIV infection has evolved into a chronic illness with very low levels of viral replication and chronic immune activation, with compliant affected individuals surviving for decades with a high quality of life. Despite these advances, many HIV-infected individuals develop some degree of neurodegeneration and cognitive impairment. The underlying pathophysiological mechanisms are not well understood, and there are no effective treatments. Thus, there is an unmet need for animal models that enable the study of HIV-associated neurocognitive disorders (HAND) and the testing of new therapeutic approaches to combat them. Here, we review the pros and cons of existing mouse models of HIV infection for addressing these aims and propose a detailed strategy for developing a new mouse model of HIV infection.

Alpha interferon and HIV infection cause activation of human T cells in NSG-BLT mice

The development of small animal models for the study of HIV transmission is important for evaluation of HIV prophylaxis and disease pathogenesis. In humanized bone marrow-liver-thymus (BLT) mice, hematopoiesis is reconstituted by implantation of human fetal liver and thymus tissue (Thy/Liv) plus intravenous injection of autologous liver-derived hematopoietic stem progenitor cells (HSPC). This results in reconstitution of human leukocytes in the mouse peripheral blood, lymphoid organs, and mucosal sites. NOD-scid interleukin-2 receptor-negative (IL-2Rγ(-/-)) (NSG)-BLT mice were inoculated intravaginally with HIV and were monitored for plasma viremia by a branched DNA assay 4 weeks later. T-cell activation was determined by expression of CD38 and HLA-DR on human CD4(+) and CD8(+) T cells in mouse peripheral blood at the time of inoculation and 4 weeks later. Additional BLT mice were treated with human alpha interferon 2b (IFN-α2b) (intron A) and assessed for T-cell activation. Productive HIV infection in BLT mice was associated with T-cell activation (increases in CD38 mean fluorescence intensity and both the frequency and absolute number of CD38(+) HLA-DR(+) T cells) that correlated strongly with plasma viral load and was most pronounced in the CD8(+) T-cell compartment. This T-cell activation phenotype was recapitulated in NSG-BLT mice treated with intron A. HIV susceptibility correlated with the number of HSPC injected, yet a number of mice receiving the Thy/Liv implant alone, with no HSPC injection, were also susceptible to intravaginal HIV. These results are consistent with studies linking T-cell activation to progressive disease in humans and lend support for the use of NSG-BLT mice in studies of HIV pathogenesis.

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

BLT mice, constructed by surgical implantation of human fetal thymus-liver tissues and intravenous delivery of autologous CD34+ haematopoietic stem cells into adult non-obese diabetic/severe combined immunodeficiency mice, were evaluated for vaccine-induced humoral immune responses. Following engraftment, these mice developed a human lymphoid system; however, the majority of the peripheral human B lymphocytes displayed an immature phenotype as evidenced by surface CD10 expression. Over 50% of the human B cells in the periphery but not in the bone marrow also expressed the CD5 antigen, which is found only infrequently on mature follicular B cells in humans. A single intramuscular immunization with recombinant viral envelope antigens, e.g., HIVgp140 and West Nile Virus envelope proteins, together with the immune stimulatory KLK/ODN1a composition) [corrected] adjuvant resulted in seroconversion characterized by antigen-specific human antibodies predominantly of the IgM isotype. However, repeated booster immunizations did not induce secondary immune responses as evidenced by the lack of class switching and specific IgM levels remaining relatively unchanged. Interestingly, the peripheral CD19+  CD5+ but not the CD19+  CD5- human B lymphocytes displayed a late developing CD27+  IgM+ memory phenotype, suggesting that the CD5+ B-cell subset, previously implicated in 'natural antibody' production, may play a role in the vaccine-induced antibody response. Furthermore, human T lymphocytes from these mice demonstrated suboptimal proliferative responses and loss of co-stimulatory surface proteins ex vivo that could be partially reversed with human interleukin-2 and interleukin-7. Therefore, vaccine-induced immune responses in BLT mice resemble a T-cell-independent pathway that can potentially be modulated in vivo by the exogenous delivery of human cytokines/growth factors.

HIV latency in the humanized BLT mouse

Even after extended treatment with powerful antiretroviral drugs, HIV is not completely eliminated from infected individuals. Latently infected CD4(+) T cells constitute one reservoir of replication-competent HIV that needs to be eliminated to completely purge virus from antiretroviral drug-treated patients. However, a major limitation in the development of therapies to eliminate this latent reservoir is the lack of relevant in vivo models that can be used to test purging strategies. Here, we show that the humanized BLT (bone marrow-liver-thymus) mouse can be used as both an abundant source of primary latently infected cells for ex vivo latency analysis and also as an in vivo system for the study of latency. We demonstrate that over 2% of human cells recovered from the spleens of HIV-infected BLT mice can be latently infected and that this virus is integrated, activation inducible, and replication competent. The non-tumor-inducing phorbol esters prostratin and 12-deoxyphorbol-13-phenylacetate can each induce HIV ex vivo from these latently infected cells, indicating that this model can be used as a source of primary cells for testing latency activators. Finally, we show activation-inducible virus is still present following suppression of plasma viral loads to undetectable levels by using the antiretroviral drugs zidovudine, indinavir sulfate, and didanosine, demonstrating that this model can also be used to assess the in vivo efficacy of latency-purging strategies. Therefore, the HIV-infected BLT mouse should provide a useful model for assessment of HIV latency activators and approaches to eliminate persistent in vivo HIV reservoirs.

Current humanized mouse models for studying human immunology and HIV-1 immuno-pathogenesis

A robust animal model for "hypothesis-testing/mechanistic" research in human immunology and immuno-pathology should meet the following criteria. First, it has well-studied hemato-lymphoid organs and target cells similar to those of humans. Second, the human pathogens establish infection and lead to relevant diseases. Third, it is genetically inbred and can be manipulated via genetic, immunological and pharmacological means. Many human-tropic pathogens such as HIV-1 fail to infect murine cells due to the blocks at multiple steps of their life cycle. The mouse with a reconstituted human immune system and other human target organs is a good candidate. A number of human-mouse chimeric models with human immune cells have been developed in the past 20 years, but most with only limited success due to the selective engraftment of xeno-reactive human T cells in hu-PBL-SCID mice or the lack of significant human immune responses in the SCID-hu Thy/Liv mouse. This review summarizes the current understanding of HIV-1 immuno-pathogenesis in human patients and in SIV-infected primate models. It also reviews the recent progress in the development of humanized mouse models with a functional human immune system, especially the recent progress in the immunodeficient mice that carry a defective gammaC gene. NOD/SCID/gammaC(-/-) (NOG or NSG) or the Rag2(-/-)gammaC(-/-) double knockout (DKO) mice, which lack NK as well as T and B cells (NTB-null mice), have been used to reconstitute a functional human immune system in central and peripheral lymphoid organs with human CD34(+) HSC. These NTB-hu HSC humanized models have been used to investigate HIV-1 infection, immuno-pathogenesis and therapeutic interventions. Such models, with further improvements, will contribute to study human immunology, human-tropic pathogens as well as human stem cell biology in the tissue development and function in vivo.