Induction of robust cellular and humoral virus-specific adaptive immune responses in human immunodeficiency virus-infected humanized BLT mice

Concurrent administration of IFNα14 and cART in TKO-BLT mice enhances suppression of HIV-1 viremia but does not eliminate the latent reservoir

Combination antiretroviral therapy (cART) prevents HIV-1 replication but does not eliminate the latent reservoir and cure the infection. Type I interferons (IFN) mediate antiviral effects through different mechanisms than cART. We previously showed that IFNα14 is the most potent IFNα subtype against HIV-1 and that it can significantly reduce the HIV-1 proviral reservoir. This study sought to determine whether combining cART with IFNα14 therapy would produce greater reductions in HIV-1 viral and proviral loads than ART alone. Immunodeficient Rag2^-/-γ_c^-/-CD47^-/- C57BL/6 mice were humanized by the BLT method, infected with HIV-1_JR-CSF and the in vivo efficacy of cART was compared with combined cART/IFNα14 therapy. Infection was allowed to establish for 6 weeks prior to 4 weeks of treatment with oral cART either with or without IFNα14. Plasma viral RNA and splenic CD4⁺ T cell viral DNA levels were measured immediately after treatment and after 2 weeks of therapy interruption. Augmentation of cART with IFNα14 resulted in significantly enhanced suppression of HIV-1 plasma viremia. However, no significant reduction in total viral DNA was detectable. Furthermore, virus rebounded after treatment interruption to similar levels in both groups. Thus, augmentation of cART with IFNα14 resulted in a more pronounced reduction of HIV viremia levels over cART alone, but the effect was not potent enough to be detected at the viral DNA level or to prevent virus rebound following therapy interruption in immune system-humanized mice.

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.

A Double Humanized BLT-mice Model Featuring a Stable Human-Like Gut Microbiome and Human Immune System

Humanized mice (hu-mice) that feature a functional human immune system have fundamentally changed the study of human pathogens and disease. They can be used to model diseases that are otherwise difficult or impossible to study in humans or other animal models. The gut microbiome can have a profound impact on human health and disease. However, the murine gut microbiome is very different than the one found in humans.  There is a need for improved pre-clinical hu-mice models that have an engrafted human gut microbiome. Therefore, we created double hu-mice that feature both a human immune system and stable human-like gut microbiome. NOD.Cg-Prkdc^scidIl2rg^tm1Wjl/SzJ (NSG) mice are one of the best animals for humanization due to their high level of immunodeficiency. However, germ-free NSG mice, and various other important germ-free mice models are not currently commercially available. Further, many research settings do not have access to gnotobiotic facilities, and working under gnotobiotic conditions can often be expensive and time consuming. Importantly, germ-free mice have several immune deficiencies that exist even after the engraftment of microbes. Therefore, we developed a protocol that does not require germ-free animals or gnotobiotic facilities. To generate double hu-mice, NSG mice were treated with radiation prior to surgery to create bone-marrow, liver, thymus-humanized (hu-BLT) mice. The mice were then treated with broad spectrum antibiotics to deplete the pre-existing murine gut microbiome. After antibiotic treatment, the mice were given fecal transplants with healthy human donor samples via oral gavage. Double hu-BLT mice had unique 16S rRNA gene profiles based on the individual human donor sample that was transplanted. Importantly, the transplanted human-like microbiome was stable in the double hu-BLT mice for the duration of the study up to 14.5 weeks post-transplant.

Precision mouse models with expanded tropism for human pathogens

A major limitation of current humanized mouse models is that they primarily enable the analysis of human-specific pathogens that infect hematopoietic cells. However, most human pathogens target other cell types, including epithelial, endothelial and mesenchymal cells. Here, we show that implantation of human lung tissue, which contains up to 40 cell types, including nonhematopoietic cells, into immunodeficient mice (lung-only mice) resulted in the development of a highly vascularized lung implant. We demonstrate that emerging and clinically relevant human pathogens such as Middle East respiratory syndrome coronavirus, Zika virus, respiratory syncytial virus and cytomegalovirus replicate in vivo in these lung implants. When incorporated into bone marrow/liver/thymus humanized mice, lung implants are repopulated with autologous human hematopoietic cells. We show robust antigen-specific humoral and T-cell responses following cytomegalovirus infection that control virus replication. Lung-only mice and bone marrow/liver/thymus-lung humanized mice substantially increase the number of human pathogens that can be studied in vivo, facilitating the in vivo testing of therapeutics.

Implantation of lung tissue into humanized mice enables in vivo study of the human immune response to pathogens.

T Lymphocyte Development and Activation in Humanized Mouse Model

Humanized mice, containing engrafted human cells and tissues, are emerging as an important in vivo platform for studying human diseases. Since the development of Nod scid gamma (NSG) mice bearing mutations in the IL-2 receptor gamma chain, many investigators have used NSG mice engrafted with human hematopoietic stem cells (HSCs) to generate functional human immune systems in vivo, results in high efficacy of human cell engraftment. The development of NSG mice has allowed significant advances to be made in studies on several human diseases, including cancer and graft-versus-host-disease (GVHD), and in regenerative medicine. Based on the human HSC transplantation, organ transplantation including thymus and liver in the renal capsule has been performed. Also, immune reconstruction of cells, of the lymphoid as well as myeloid lineages, has been partly accomplished. However, crosstalk between pluripotent stem cell derived therapeutic cells with human leukocyte antigen (HLA) mis/matched types and immune CD3 T cells have not been fully addressed. To overcome this hurdle, human major histocompatibility complex (MHC) molecules, not mouse MHC molecules, are required to generate functional T cells in a humanized mouse model. Here, we briefly summarize characteristics of the humanized mouse model, focusing on development of CD3 T cells with MHC molecules. We also highlight the necessity of the humanized mouse model for the treatment of various human diseases.

Recent Advances in Allergy Research Using Humanized Mice

The prevalence rates of allergic diseases are increasing worldwide, particularly in industrial countries. To date, many mouse models have been generated for allergy research; studies conducted using these models have suggested the importance of cross-talk between immune cells and tissue-resident non-immune cells in the onset of allergic diseases. However, there are several differences between the immune systems of rodents and humans, and human studies are limited. Thus, mice reconstituted with human immune cells are a novel tool for the preclinical evaluation of the efficacy and safety of developing drugs. Genetic technologies for generating humanized mice have improved markedly in recent years. In this review, we will discuss recent progress in allergy research using humanized mice and introduce our recent humanized mouse model of airway inflammation in human immune cells.

Humanized Mouse Model of HIV-1 Latency with Enrichment of Latent Virus in PD-1+ and TIGIT+ CD4 T Cells

Combination anti-retroviral drug therapy (ART) potently suppresses HIV-1 replication but does not result in virus eradication or a cure. A major contributing factor is the long-term persistence of a reservoir of latently infected cells. To study this reservoir, we established a humanized mouse model of HIV-1 infection and ART suppression based on an oral ART regimen. Similar to humans, HIV-1 levels in the blood of ART-treated animals were frequently suppressed below the limits of detection. However, the limited timeframe of the mouse model and the small volume of available samples makes it a challenging model with which to achieve full viral suppression and to investigate the latent reservoir. We therefore used an ex vivo latency reactivation assay that allows a semiquantitative measure of the latent reservoir that establishes in individual animals, regardless of whether they are treated with ART. Using this assay, we found that latently infected human CD4 T cells can be readily detected in mouse lymphoid tissues and that latent HIV-1 was enriched in populations expressing markers of T cell exhaustion, PD-1 and TIGIT. In addition, we were able to use the ex vivo latency reactivation assay to demonstrate that HIV-specific TALENs can reduce the fraction of reactivatable virus in the latently infected cell population that establishes in vivo, supporting the use of targeted nuclease-based approaches for an HIV-1 cure.IMPORTANCE HIV-1 can establish latent infections that are not cleared by current antiretroviral drugs or the body's immune responses and therefore represent a major barrier to curing HIV-infected individuals. However, the lack of expression of viral antigens on latently infected cells makes them difficult to identify or study. Here, we describe a humanized mouse model that can be used to detect latent but reactivatable HIV-1 in both untreated mice and those on ART and therefore provides a simple system with which to study the latent HIV-1 reservoir and the impact of interventions aimed at reducing it.

Blocking HIV-1 Infection by Chromosomal Integrative Expression of Human CD4 on the Surface of Lactobacillus acidophilus ATCC 4356

Lactobacillus bacteria are potential delivery vehicles for biopharmaceutical molecules because they are well-recognized as safe microorganisms that naturally inhabit the human body. The goal of this study was to employ these lactobacilli to combat human immunodeficiency virus type 1 (HIV-1) infection and transmission. By using a chromosomal integration method, we engineered Lactobacillus acidophilus ATCC 4356 to display human CD4, the HIV-1 receptor, on the cell surface. Since human CD4 can bind to any infectious HIV-1 particles, the engineered lactobacilli can potentially capture HIV-1 of different subtypes and prevent infection. Our data demonstrate that the CD4-carrying bacteria are able to adsorb HIV-1 particles and reduce infection significantly in vitro and also block intrarectal HIV-1 infection in a humanized mouse model in preliminary tests in vivo Our results support the potential of this approach to decrease the efficiency of HIV-1 sexual transmission.IMPORTANCE In the absence of an effective vaccine, alternative approaches to block HIV-1 infection and transmission with commensal bacteria expressing antiviral proteins are being considered. This report provides a proof-of-concept by using Lactobacillus bacteria stably expressing the HIV-1 receptor CD4 to capture and neutralize HIV-1 in vitro and in a humanized mouse model. The stable expression of antiviral proteins, such as CD4, following genomic integration of the corresponding genes into this Lactobacillus strain may contribute to the prevention of HIV-1 sexual transmission.

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.

Cellular HIV Reservoirs and Viral Rebound from the Lymphoid Compartments of 4'-Ethynyl-2-Fluoro-2'-Deoxyadenosine (EFdA)-Suppressed Humanized Mice

Although antiretroviral therapy (ART) greatly suppresses HIV replication, lymphoid tissues remain a sanctuary site where the virus may replicate. Tracking the earliest steps of HIV spread from these cellular reservoirs after drug cessation is pivotal for elucidating how infection can be prevented. In this study, we developed an in vivo model of HIV persistence in which viral replication in the lymphoid compartments of humanized mice was inhibited by the HIV reverse transcriptase inhibitor 4′-ethynyl-2-fluoro-2′-deoxyadenosine (EFdA) to very low levels, which recapitulated ART-suppression in HIV-infected individuals. Using a combination of RNAscope in situ hybridization (ISH) and immunohistochemistry (IHC), we quantitatively investigated the distribution of HIV in the lymphoid tissues of humanized mice during active infection, EFdA suppression, and after drug cessation. The lymphoid compartments of EFdA-suppressed humanized mice harbored very rare transcription/translation-competent HIV reservoirs that enable viral rebound. Our data provided the visualization and direct measurement of the early steps of HIV reservoir expansion within anatomically intact lymphoid tissues soon after EFdA cessation and suggest a strategy to enhance therapeutic approaches aimed at eliminating the HIV reservoir.

An injectable bone marrow-like scaffold enhances T cell immunity after hematopoietic stem cell transplantation

The use of allogeneic hematopoietic stem cell transplantation (HSCT) to cure multiple disorders is limited by deficiency and dysregulation of T-cells. Here we report a biomaterial-based scaffold that mimics features of T-cell lymphopoiesis in the bone marrow. The bone marrow cryogel (BMC) releases bone morphogenetic protein-2 to recruit stromal cells, and presents the Notch ligand Delta-like ligand-4 to facilitate T-cell lineage specification of mouse and human hematopoietic progenitor cells. BMCs subcutaneously injected in mice at the time of HSCT enhanced T-cell progenitor seeding of the thymus, T-cell neogenesis and diversification of the T-cell receptor repertoire. Peripheral T-cell reconstitution increased ~6-fold in mouse HSCT and ~2-fold in human xenogeneic HSCT. Furthermore, BMCs promoted donor CD4⁺ regulatory T-cell generation and improved survival after allogeneic HSCT. Compared with adoptive transfer of T-cell progenitors, BMCs increased donor chimerism, T-cell generation and antigen-specific T-cell responses to vaccination. BMCs may provide an off-the-shelf approach for enhancing T-cell regeneration and mitigating graft-versus-host disease in HSCT.

Human Anti-HIV-1 gp120 Monoclonal Antibodies with Neutralizing Activity Cloned from Humanized Mice Infected with HIV-1

Broadly neutralizing, anti-HIV-1 gp120 mAbs have been isolated from infected individuals, and there is considerable interest in developing these reagents for Ab-based immunoprophylaxis and treatment. As a means to identify potentially new anti-HIV Abs, we exploited humanized NOD-scid IL2rγnull mice systemically infected with HIV-1 to generate a wide variety of Ag-specific human mAbs. The Abs were encoded by a diverse range of variable gene families and Ig classes, including IgA, and several showed significant levels of somatic mutation. Moreover, the isolated Abs not only bound target Ags with similar affinity as broadly neutralizing Abs, they also demonstrated neutralizing ability against multiple HIV-1 clades. The use of humanized mice will allow us to use our knowledge of HIV-1 gp120 structure and function, and the immune response targeting this protein, to generate native human prophylactic Abs to reduce the infection and spread of HIV-1.

Selective expansion of myeloid and NK cells in humanized mice yields human-like vaccine responses

Mice engrafted with components of a human immune system have become widely-used models for studying aspects of human immunity and disease. However, a defined methodology to objectively measure and compare the quality of the human immune response in different models is lacking. Here, by taking advantage of the highly immunogenic live-attenuated yellow fever virus vaccine YFV-17D, we provide an in-depth comparison of immune responses in human vaccinees, conventional humanized mice, and second generation humanized mice. We demonstrate that selective expansion of human myeloid and natural killer cells promotes transcriptomic responses akin to those of human vaccinees. These enhanced transcriptomic profiles correlate with the development of an antigen-specific cellular and humoral response to YFV-17D. Altogether, our approach provides a robust scoring of the quality of the human immune response in humanized mice and highlights a rational path towards developing better pre-clinical models for studying the human immune response and disease.

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.

Leukocyte Tracking Database, a collection of immune cell tracks from intravital 2-photon microscopy videos

Recent advances in intravital video microscopy have allowed the visualization of leukocyte behavior in vivo, revealing unprecedented spatiotemporal dynamics of immune cell interaction. However, state-of-the-art software and methods for automatically measuring cell migration exhibit limitations in tracking the position of leukocytes over time. Challenges arise both from the complex migration patterns of these cells and from the experimental artifacts introduced during image acquisition. Additionally, the development of novel tracking tools is hampered by the lack of a sound ground truth for algorithm validation and benchmarking. Therefore, the objective of this work was to create a database, namely LTDB, with a significant number of manually tracked leukocytes. Broad experimental conditions, sites of imaging, types of immune cells and challenging case studies were included to foster the development of robust computer vision techniques for imaging-based immunological research. Lastly, LTDB represents a step towards the unravelling of biological mechanisms by video data mining in systems biology.

A Small-Molecule CD4-Mimetic Compound Protects Bone Marrow-Liver-Thymus Humanized Mice From HIV-1 Infection

Background

Small-molecule CD4-mimetic compounds (CD4mc) inhibit human immunodeficiency virus (HIV-1) entry by blocking binding to the CD4 receptor and by premature triggering of the viral envelope glycoprotein (Env) spike.

Methods

The efficacy of a CD4mc in protecting bone marrow-liver-thymus (BLT) humanized mice from vaginal HIV-1 challenge was evaluated.

Results

Intravaginal application of the CD4mc JP-III-48, either before or simultaneously with virus challenge, protected BLT humanized mice from HIV-1JR-CSF infection in a dose- dependent manner.

Conclusion

The direct antiviral effects of a CD4mc prevent HIV-1 infection in a murine model of sexual transmission.

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.

Noninvasive PET Imaging of T cells

The rapidly evolving field of cancer immunotherapy recently saw the approval of several new therapeutic antibodies. Several cell therapies, for example, chimeric antigen receptor-expressing T cells (CAR-T), are currently in clinical trials for a variety of cancers and other diseases. However, approaches to monitor changes in the immune status of tumors or to predict therapeutic responses are limited. Monitoring lymphocytes from whole blood or biopsies does not provide dynamic and spatial information about T cells in heterogeneous tumors. Positron emission tomography (PET) imaging using probes specific for T cells can noninvasively monitor systemic and intratumoral immune alterations during experimental therapies and may have an important and expanding value in the clinic.

Human Immune System Mice for the Study of Human Immunodeficiency Virus-Type 1 Infection of the Central Nervous System

Immunodeficient mice transplanted with human cell populations or tissues, also known as human immune system (HIS) mice, have emerged as an important and versatile tool for the in vivo study of human immunodeficiency virus-type 1 (HIV-1) pathogenesis, treatment, and persistence in various biological compartments. Recent work in HIS mice has demonstrated their ability to recapitulate critical aspects of human immune responses to HIV-1 infection, and such studies have informed our knowledge of HIV-1 persistence and latency in the context of combination antiretroviral therapy. The central nervous system (CNS) is a unique, immunologically privileged compartment susceptible to HIV-1 infection, replication, and immune-mediated damage. The unique, neural, and glia-rich cellular composition of this compartment, as well as the important role of infiltrating cells of the myeloid lineage in HIV-1 seeding and replication makes its study of paramount importance, particularly in the context of HIV-1 cure research. Current work on the replication and persistence of HIV-1 in the CNS, as well as cells of the myeloid lineage thought to be important in HIV-1 infection of this compartment, has been aided by the expanded use of these HIS mouse models. In this review, we describe the major HIS mouse models currently in use for the study of HIV-1 neuropathogenesis, recent insights from the field, limitations of the available models, and promising advances in HIS mouse model development.

SIVcpz closely related to the ancestral HIV-1 is less or non-pathogenic to humans in a hu-BLT mouse model

The HIV-1 pandemic is a consequence of the cross-species transmission of simian immunodeficiency virus in wild chimpanzees (SIVcpz) to humans. Our previous study demonstrated SIVcpz strains that are closely related to the ancestral viruses of HIV-1 groups M (SIVcpzMB897) and N (SIVcpzEK505) and two SIVcpz lineages that are not associated with any known HIV-1 infections in humans (SIVcpzMT145 and SIVcpzBF1167), all can readily infect and robustly replicate in the humanized-BLT mouse model of humans. However, the comparative pathogenicity of different SIVcpz strains remains unknown. Herein, we compared the pathogenicity of the above four SIVcpz strains with HIV-1 using humanized-BLT mice. Unexpectedly, we found that all four SIVcpz strains were significantly less pathogenic or non-pathogenic compared to HIV-1, manifesting lower degrees of CD4+ T-cell depletion and immune activation. Transcriptome analyses of CD4+ T cells from hu-BLT mice infected with SIVcpz versus HIV-1 revealed enhanced expression of genes related to cell survival and reduced inflammation/immune activation in SIVcpz-infected mice. Together, our study results demonstrate for the first time that SIVcpz is significantly less or non-pathogenic to human immune cells compared to HIV-1. Our findings lay the groundwork for a possible new understanding of the evolutionary origins of HIV-1, where the initial SIVcpz cross-species transmission virus may be initially less pathogenic to humans.

Therapeutic Efficacy of Vectored PGT121 Gene Delivery in HIV-1-Infected Humanized Mice

Broadly neutralizing antibodies (bNAbs) are being explored for HIV-1 prevention and cure strategies. However, administration of purified bNAbs poses challenges in resource-poor settings, where the HIV-1 disease burden is greatest. In vivo vector-based production of bNAbs represents an alternative strategy. We investigated adenovirus serotype 5 (Ad5) and adeno-associated virus serotype 1 (AAV1) vectors to deliver the HIV-1-specific bNAb PGT121 in wild-type and immunocompromised C57BL/6 mice as well as in HIV-1-infected bone marrow-liver-thymus (BLT) humanized mice. Ad5.PGT121 and AAV1.PGT121 produced functional antibody in vivo. Ad5.PGT121 produced PGT121 rapidly within 6 h, whereas AAV1.PGT121 produced detectable PGT121 in serum by 72 h. Serum PGT121 levels were rapidly reduced by the generation of anti-PGT121 antibodies in immunocompetent mice but were durably maintained in immunocompromised mice. In HIV-1-infected BLT humanized mice, Ad5.PGT121 resulted in a greater reduction of viral loads than did AAV1.PGT121. Ad5.PGT121 also led to more-sustained virologic control than purified PGT121 IgG. Ad5.PGT121 afforded more rapid, robust, and durable antiviral efficacy than AAV1.PGT121 and purified PGT121 IgG in HIV-1-infected humanized mice. Further evaluation of vector delivery of HIV-1 bNAbs is warranted, although approaches to prevent the generation of antiantibody responses may also be required.

IMPORTANCE Broadly neutralizing antibodies (bNAbs) are being explored for HIV-1 prevention and cure strategies, but delivery of purified antibodies may prove challenging. We investigated adenovirus serotype 5 (Ad5) and adeno-associated virus serotype 1 (AAV1) vectors to deliver the HIV-1-specific bNAb PGT121. Ad5.PGT121 afforded more rapid, robust, and durable antiviral efficacy than AAV1.PGT121 and purified PGT121 IgG in HIV-1-infected humanized mice.

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.

Bone degradation machinery of osteoclasts: An HIV-1 target that contributes to bone loss

Bone deficits are frequent in HIV-1-infected patients. We report here that osteoclasts, the cells specialized in bone resorption, are infected by HIV-1 in vivo in humanized mice and ex vivo in human joint biopsies. In vitro, infection of human osteoclasts occurs at different stages of osteoclastogenesis via cell-free viruses and, more efficiently, by transfer from infected T cells. HIV-1 infection markedly enhances adhesion and osteolytic activity of human osteoclasts by modifying the structure and function of the sealing zone, the osteoclast-specific bone degradation machinery. Indeed, the sealing zone is broader due to F-actin enrichment of its basal units (i.e., the podosomes). The viral protein Nef is involved in all HIV-1-induced effects partly through the activation of Src, a regulator of podosomes and of their assembly as a sealing zone. Supporting these results, Nef-transgenic mice exhibit an increased osteoclast density and bone defects, and osteoclasts derived from these animals display high osteolytic activity. Altogether, our study evidences osteoclasts as host cells for HIV-1 and their pathological contribution to bone disorders induced by this virus, in part via Nef.

Humanized Mice as Unique Tools for Human-Specific Studies

With an increasing human population, medical research is pushed to progress into an era of precision therapy. Humanized mice are at the very heart of this new forefront where it is acutely required to decipher human-specific disease pathogenesis and test an array of novel therapeutics. In this review, “humanized” mice are defined as immunodeficient mouse engrafted with functional human biological systems. Over the past decade, researchers have been conscientiously making improvements on the development of humanized mice as a model to closely recapitulate disease pathogenesis and drug mechanisms in humans. Currently, literature is rife with descriptions of novel and innovative humanized mouse models that hold a significant promise to become a panacea for drug innovations to treat and control conditions such as infectious disease and cancer. This review will focus on the background of humanized mice, diseases, and human-specific therapeutics tested on this platform as well as solutions to improve humanized mice for future clinical use.

An advanced BLT-humanized mouse model for extended HIV-1 cure studies

OBJECTIVE

Although bone marrow, liver, thymus (BLT)-humanized mice provide a robust model for HIV-1 infection and enable evaluation of cure strategies dependent on endogenous immune responses, most mice develop graft versus host disease (GVHD), limiting their utility for extended HIV cure studies. This study aimed to: evaluate the GVHD-resistant C57 black 6 (C57BL/6) recombination activating gene 2 (Rag2)γcCD47 triple knockout (TKO)-BLT mouse as a model to establish HIV-1 latency. Determine whether TKO-BLT mice could be maintained on antiretroviral therapy (ART) for extended periods of time. Assess the rapidity of viral rebound following therapy interruption.

DESIGN

TKO-BLT mice were HIV-1 infected, treated with various ART regimens over extended periods of time and assayed for viral rebound following therapy interruption.

METHODS

Daily subcutaneous injection and oral ART-mediated suppression of HIV-1 infection was tested at various doses in TKO-BLT mice. Mice were monitored for suppression of viremia and cellular HIV-1 RNA and DNA prior to and following therapy interruption.

RESULTS

Mice remained healthy for 45 weeks posthumanization and could be treated with ART for up to 18 weeks. Viremia was suppressed to less than 200 copies/ml in the majority of mice with significant reductions in cellular HIV-1 RNA and DNA. Treatment interruption resulted in rapid viral recrudescence.

CONCLUSION

HIV-1 latency can be maintained in TKO-BLT mice over extended periods on ART and rapid viral rebound occurs following therapy removal. The additional 15-18 weeks of healthy longevity compared with other BLT models provides sufficient time to examine the decay kinetics of the latent reservoir as well as observe delays in recrudescence in HIV-1 cure studies.

In vivo activation of latent HIV with a synthetic bryostatin analog effects both latent cell "kick" and "kill" in strategy for virus eradication

The ability of HIV to establish a long-lived latent infection within resting CD4+ T cells leads to persistence and episodic resupply of the virus in patients treated with antiretroviral therapy (ART), thereby preventing eradication of the disease. Protein kinase C (PKC) modulators such as bryostatin 1 can activate these latently infected cells, potentially leading to their elimination by virus-mediated cytopathic effects, the host's immune response and/or therapeutic strategies targeting cells actively expressing virus. While research in this area has focused heavily on naturally-occurring PKC modulators, their study has been hampered by their limited and variable availability, and equally significantly by sub-optimal activity and in vivo tolerability. Here we show that a designed, synthetically-accessible analog of bryostatin 1 is better-tolerated in vivo when compared with the naturally-occurring product and potently induces HIV expression from latency in humanized BLT mice, a proven and important model for studying HIV persistence and pathogenesis in vivo. Importantly, this induction of virus expression causes some of the newly HIV-expressing cells to die. Thus, designed, synthetically-accessible, tunable, and efficacious bryostatin analogs can mediate both a "kick" and "kill" response in latently-infected cells and exhibit improved tolerability, therefore showing unique promise as clinical adjuvants for HIV eradication.

Host and Viral Factors in HIV-Mediated Bystander Apoptosis

Human immunodeficiency virus (HIV) infections lead to a progressive loss of CD4 T cells primarily via the process of apoptosis. With a limited number of infected cells and vastly disproportionate apoptosis in HIV infected patients, it is believed that apoptosis of uninfected bystander cells plays a significant role in this process. Disease progression in HIV infected individuals is highly variable suggesting that both host and viral factors may influence HIV mediated apoptosis. Amongst the viral factors, the role of Envelope (Env) glycoprotein in bystander apoptosis is well documented. Recent evidence on the variability in apoptosis induction by primary patient derived Envs underscores the role of Env glycoprotein in HIV disease. Amongst the host factors, the role of C-C Chemokine Receptor type 5 (CCR5), a coreceptor for HIV Env, is also becoming increasingly evident. Polymorphisms in the CCR5 gene and promoter affect CCR5 cell surface expression and correlate with both apoptosis and CD4 loss. Finally, chronic immune activation in HIV infections induces multiple defects in the immune system and has recently been shown to accelerate HIV Env mediated CD4 apoptosis. Consequently, those factors that affect CCR5 expression and/or immune activation in turn indirectly regulate HIV mediated apoptosis making this phenomenon both complex and multifactorial. This review explores the complex role of various host and viral factors in determining HIV mediated bystander apoptosis.

Humanized mouse models of latent HIV infection

Antiretroviral therapy can efficiently control HIV viral replication, resulting in low viral loads and sustained CD4⁺ T cell counts in HIV-infected persons. However, fast viral rebound occurs in most infected persons when therapy is interrupted. The principal component of persistent infection is a latent but replication-competent HIV reservoir. The long half-life of this reservoir is a major barrier to cure, and its elimination is the target of important research efforts. Animal models that can recapitulate this aspect of human infection are needed to examine the HIV reservoir in tissues in vivo, and to test eradication strategies. In this review, we will summarize recent studies using humanized mouse models to examine different aspects of the viral reservoir.

Human immune system mouse models of Ebola virus infection

Human immune system (HIS) mice, immunodeficient mice engrafted with human cells (with or without donor-matched tissue), offer a unique opportunity to study pathogens that cause disease predominantly or exclusively in humans. Several HIS mouse models have recently been used to study Ebola virus (EBOV) infection and disease. The results of these studies are encouraging and support further development and use of these models in Ebola research. HIS mice provide a small animal model to study EBOV isolates, investigate early viral interactions with human immune cells, screen vaccines and therapeutics that modulate the immune system, and investigate sequelae in survivors. Here we review existing models, discuss their use in pathogenesis studies and therapeutic screening, and highlight considerations for study design and analysis. Finally, we point out caveats to current models, and recommend future efforts for modeling EBOV infection in HIS mice.

Humanized mouse models: Application to human diseases

Humanized mice are superior to rodents for preclinical evaluation of the efficacy and safety of drug candidates using human cells or tissues. During the past decade, humanized mouse technology has been greatly advanced by the establishment of novel platforms of genetically modified immunodeficient mice. Several human diseases can be recapitulated using humanized mice due to the improved engraftment and differentiation capacity of human cells or tissues. In this review, we discuss current advanced humanized mouse models that recapitulate human diseases including cancer, allergy, and graft-versus-host disease.

Chimeric antigen receptor engineered stem cells: a novel HIV therapy

Despite the success of combination antiretroviral therapy (cART) for suppressing HIV and improving patients' quality of life, HIV persists in cART-treated patients and remains an incurable disease. Financial burdens and health consequences of lifelong cART treatment call for novel HIV therapies that result in a permanent cure. Cellular immunity is central in controlling HIV replication. However, HIV adopts numerous strategies to evade immune surveillance. Engineered immunity via genetic manipulation could offer a functional cure by generating cells that have enhanced antiviral activity and are resistant to HIV infection. Recently, encouraging reports from several human clinical trials using an anti-CD19 chimeric antigen receptor (CAR) modified T-cell therapy for treating B-cell malignancies have provided valuable insights and generated remarkable enthusiasm in engineered T-cell therapy. In this review, we discuss the development of HIV-specific chimeric antigen receptors and the use of stem cell based therapies to generate lifelong anti-HIV immunity.

Human Cytomegalovirus Induces Cellular and Humoral Virus-specific Immune Responses in Humanized BLT Mice

The strict species specificity of Human Cytomegalovirus (HCMV) has impeded our understanding of antiviral adaptive immune responses in the context of a human immune system. We have previously shown that HCMV infection of human hematopoietic progenitor cells engrafted in immune deficient mice (huNSG) results in viral latency that can be reactivated following G-CSF treatment. In this study, we characterized the functional human adaptive immune responses in HCMV latently-infected huBLT (humanized Bone marrow-Liver-Thymus) mice. Following infection, huBLT mice generate human effector and central memory CD4+ and CD8+ T-cell responses reactive to peptides corresponding to both IE and pp65 proteins. Additionally, both HCMV specific IgM and IgG B-cell responses with the ability to neutralize virus were detected. These results indicate that the HCMV huBLT mouse model may provide a valuable tool to study viral latency and reactivation as well as evaluate HCMV vaccines and immune responses in the context of a functional human immune system.

Chemoattractant-mediated leukocyte trafficking enables HIV dissemination from the genital mucosa

HIV vaginal transmission accounts for the majority of newly acquired heterosexual infections. However, the mechanism by which HIV spreads from the initial site of viral entry at the mucosal surface of the female genital tract to establish a systemic infection of lymphoid and peripheral tissues is not known. Once the virus exits the mucosa it rapidly spreads to all tissues, leading to CD4⁺ T cell depletion and the establishment of a viral reservoir that cannot be eliminated with current treatments. Understanding the molecular and cellular requirements for viral dissemination from the genital tract is therefore of great importance, as it could reveal new strategies to lengthen the window of opportunity to target the virus at its entry site in the mucosa where it is the most vulnerable and thus prevent systemic infection. Using HIV vaginal infection of humanized mice as a model of heterosexual transmission, we demonstrate that blocking the ability of leukocytes to respond to chemoattractants prevented HIV from leaving the female genital tract. Furthermore, blocking lymphocyte egress from lymph nodes prevented viremia and infection of the gut. Leukocyte trafficking therefore plays a major role in viral dissemination, and targeting the chemoattractant molecules involved can prevent the establishment of a systemic infection.

In Vivo Models of Human Immunodeficiency Virus Persistence and Cure Strategies

Current HIV therapy is not curative regardless of how soon after infection it is initiated or how long it is administered, and therapy interruption almost invariably results in robust viral rebound. Human immunodeficiency virus persistence is therefore the major obstacle to a cure for AIDS. The testing and implementation of novel yet unproven approaches to HIV eradication that could compromise the health status of HIV-infected individuals might not be ethically warranted. Therefore, adequate in vitro and in vivo evidence of efficacy is needed to facilitate the clinical implementation of promising strategies for an HIV cure. Animal models of HIV infection have a strong and well-documented history of bridging the gap between laboratory discoveries and eventual clinical implementation. More recently, animal models have been developed and implemented for the in vivo evaluation of novel HIV cure strategies. In this article, we review the recent progress in this rapidly moving area of research, focusing on the two most promising model systems: humanized mice and nonhuman primates.

Noninvasive Imaging of Human Immune Responses in a Human Xenograft Model of Graft-Versus-Host Disease

The immune system plays a crucial role in many diseases. Activation or suppression of immunity is often related to clinical outcome. Methods to explore the dynamics of immune responses are important to elucidate their role in conditions characterized by inflammation, such as infectious disease, cancer, or autoimmunity. Immuno-PET is a noninvasive method by which disease and immune cell infiltration can be explored simultaneously. Using radiolabeled antibodies or fragments derived from them, it is possible to image disease-specific antigens and immune cell subsets. Methods: We developed a method to noninvasively image human immune responses in a relevant humanized mouse model. We generated a camelid-derived single-domain antibody specific for human class II major histocompatibility complex products and used it to noninvasively image human immune cell reconstitution in nonobese diabetic severe combined immune deficiency γ-/- mice reconstituted with human fetal thymus, liver, and liver-derived hematopoietic stem cells (BLT mice). Results: We showed imaging of infiltrating immunocytes in BLT mice that spontaneously developed a graft-versus-host-like condition, characterized by alopecia and blepharitis. In diseased animals, we showed an increased PET signal in the liver, attributable to infiltration of activated class II major histocompatibility complex⁺ T cells. Conclusion: Noninvasive imaging of immune infiltration and activation could thus be of importance for diagnosis and evaluation of treatment of graft-versus-host disease and holds promise for other diseases characterized by inflammation.

Longitudinal imaging of HIV-1 spread in humanized mice with parallel 3D immunofluorescence and electron tomography

Dissemination of HIV-1 throughout lymphoid tissues leads to systemic virus spread following infection. We combined tissue clearing, 3D-immunofluorescence, and electron tomography (ET) to longitudinally assess early HIV-1 spread in lymphoid tissues in humanized mice. Immunofluorescence revealed peak infection density in gut at 10–12 days post-infection when blood viral loads were low. Human CD4+ T-cells and HIV-1–infected cells localized predominantly to crypts and the lower third of intestinal villi. Free virions and infected cells were not readily detectable by ET at 5-days post-infection, whereas HIV-1–infected cells surrounded by pools of free virions were present in ~10% of intestinal crypts by 10–12 days. ET of spleen revealed thousands of virions released by individual cells and discreet cytoplasmic densities near sites of prolific virus production. These studies highlight the importance of multiscale imaging of HIV-1–infected tissues and are adaptable to other animal models and human patient samples.

DOI:http://dx.doi.org/10.7554/eLife.23282.001

A novel humanized mouse model with significant improvement of class-switched, antigen-specific antibody production

Humanized mice are a powerful tool for the study of human hematopoiesis and immune function in vivo. However, the existing models cannot support robust adaptive immune responses, especially the generation of class-switched, antigen-specific antibody responses. Here we describe a new mouse strain, in which human interleukin 6 (IL-6) gene encoding the cytokine that is important for B- and T-cell differentiation was knocked into its respective mouse locus. The provision of human IL-6 not only enhanced thymopoiesis and periphery T-cell engraftment, but also significantly increased class switched memory B cells and serum immunoglobulin G (IgG). In addition, immunization with ovalbumin (OVA) induced OVA-specific B cells only in human IL-6 knock-in mice. These OVA-specific antibodies displayed the highest frequency of somatic mutation, further suggesting that human IL-6 is important for efficient B-cell activation and selection. We conclude that human IL-6 knock-in mice represent a novel and improved model for human adaptive immunity without relying on complex surgery to transplant human fetal thymus and liver. These mice can therefore be used to exploit or evaluate immunization regimes that would be unethical or untenable in humans.

Targeting type I interferon-mediated activation restores immune function in chronic HIV infection

Chronic immune activation, immunosuppression, and T cell exhaustion are hallmarks of HIV infection, yet the mechanisms driving these processes are unclear. Chronic activation can be a driving force in immune exhaustion, and type I interferons (IFN-I) are emerging as critical components underlying ongoing activation in HIV infection. Here, we have tested the effect of blocking IFN-I signaling on T cell responses and virus replication in a murine model of chronic HIV infection. Using HIV-infected humanized mice, we demonstrated that in vivo blockade of IFN-I signaling during chronic HIV infection diminished HIV-driven immune activation, decreased T cell exhaustion marker expression, restored HIV-specific CD8 T cell function, and led to decreased viral replication. Antiretroviral therapy (ART) in combination with IFN-I blockade accelerated viral suppression, further decreased viral loads, and reduced the persistently infected HIV reservoir compared with ART treatment alone. Our data suggest that blocking IFN-I signaling in conjunction with ART treatment can restore immune function and may reduce viral reservoirs during chronic HIV infection, providing validation for IFN-I blockade as a potential therapy for HIV infection.

Propagating Humanized BLT Mice for the Study of Human Immunology and Immunotherapy

The humanized bone marrow-liver-thymus (BLT) mouse model harbors a nearly complete human immune system, therefore providing a powerful tool to study human immunology and immunotherapy. However, its application is greatly limited by the restricted supply of human CD34⁺ hematopoietic stem cells and fetal thymus tissues that are needed to generate these mice. The restriction is especially significant for the study of human immune systems with special genetic traits, such as certain human leukocyte antigen (HLA) haplotypes or monogene deficiencies. To circumvent this critical limitation, we have developed a method to quickly propagate established BLT mice. Through secondary transfer of bone marrow cells and human thymus implants from BLT mice into NSG (NOD/SCID/IL-2Rγ^-/-) recipient mice, we were able to expand one primary BLT mouse into a colony of 4-5 proBLT (propagated BLT) mice in 6-8 weeks. These proBLT mice reconstituted human immune cells, including T cells, at levels comparable to those of their primary BLT donor mouse. They also faithfully inherited the human immune cell genetic traits from their donor BLT mouse, such as the HLA-A2 haplotype that is of special interest for studying HLA-A2-restricted human T cell immunotherapies. Moreover, an EGFP reporter gene engineered into the human immune system was stably passed from BLT to proBLT mice, making proBLT mice suitable for studying human immune cell gene therapy. This method provides an opportunity to overcome a critical hurdle to utilizing the BLT humanized mouse model and enables its more widespread use as a valuable preclinical research tool.

Visualizing the Behavior of HIV-Infected T Cells In Vivo Using Multiphoton Intravital Microscopy

The introduction of multiphoton microscopy has dramatically broadened the scope of intravital imaging studies and has allowed researchers to validate and refine basic mechanistic concepts in many areas of biology within the context of physiologically relevant tissue microenvironments. This has also led to new insights into the behavior of immune cells at steady state, and how their behaviors are altered during an immune response. At the same time, advances in the humanized mouse model have allowed for in vivo studies of strictly human pathogens, such as HIV-1. Here, we describe in detail an intravital microscopy approach to visualize the dynamic behavior of HIV-infected T cells within the lymph nodes of live, anesthetized humanized mice.

Humanized mouse as an appropriate model for accelerated global HIV research and vaccine development: current trend

Humanized mouse models currently have seen improved development and have received wide applications. Its usefulness is observed in cell and tissue transplant involving basic and applied human disease research. In this article, the development of a new generation of humanized mice was discussed as well as their relevant application in HIV disease. Furthermore, current techniques employed to overcome the initial limitations of mouse model were reviewed. Highly immunodeficient mice which support cell and tissue differentiation and do not reject xenografts are indispensable for generating additional appropriate models useful in disease study, this phenomenom deserves emphases, scientific highlight and a definitive research focus. Since the early 2000s, a series of immunodeficient mice appropriate for generating humanized mice has been successively developed by introducing the IL-2Rγ^null gene (e.g. NOD/SCID/γc^null and Rag2^nullγc^null mice) through various genomic approaches. These mice were generated by genetically introducing human cytokine genes into NOD/SCID/γc^null and Rag2^nullγc^null mouse backgrounds. The application of these techniques serves as a quick and appropriate mechanistic model for basic and therapeutic investigations of known and emerging infections.

T-Regulatory Cells and Vaccination "Pay Attention and Do Not Neglect Them": Lessons from HIV and Cancer Vaccine Trials

Efficient vaccines are characterized by the establishment of long-lived memory T cells, including T-helper (effectors and follicular) and T-regulatory cells (Tregs). While the former induces cytotoxic or antibody responses, the latter regulates immune responses by maintaining homeostasis. The role of Tregs in inflammatory conditions is ambiguous and their systematic monitoring in vaccination along with effector T-cells is not instinctive. Recent studies from the cancer field clearly showed that Tregs suppress vaccine-induced immune responses and correlate with poor clinical benefit. In HIV infection, Tregs are needed during acute infection to preserve tissue integrity from an overwhelmed activation, but are not beneficial in chronic infection as they suppress anti-HIV responses. Current assays used to evaluate vaccine-induced specific responses are limited as they do not take into account antigen-specific Tregs. However, new assays, such as the OX40 assay, which allow for the simultaneous detection of a full range of Th-responses including antigen-specific Tregs responses, can overcome these issues. In this review article we will revise the role of Tregs in vaccination and review the recent work performed in the field, including the available tools to monitor them, from novel assays to humanized mouse models.

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.

Humanized mice in infectious diseases

The pathogenesis of infectious agents with human tropism can only be properly studied in an in vivo model featuring human cells or tissue. Humanized mice represent a small animal model featuring human cells or tissue that can be infected by human-specific viruses, bacteria, and parasites and also providing a functional human immune system. This makes the analysis of a human immune response to infection possible and allows for preclinical testing of new vaccines and therapeutic agents. Results of various studies using humanized mice to investigate pathogens with human tropism are presented in this review. In addition, the limitations of humanized mice and methods to improve this valuable animal model are discussed.

Efficient Inhibition of HIV Replication in the Gastrointestinal and Female Reproductive Tracts of Humanized BLT Mice by EFdA

Background

The nucleoside reverse transcriptase inhibitor (NRTI) 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) in preclinical development exhibits improved safety and antiviral activity profiles with minimal drug resistance compared to approved NRTIs. However, the systemic antiviral efficacy of EFdA has not been fully evaluated. In this study, we utilized bone marrow/liver/thymus (BLT) humanized mice to investigate the systemic effect of EFdA treatment on HIV replication and CD4+ T cell depletion in the peripheral blood (PB) and tissues. In particular, we performed a comprehensive analysis of the female reproductive tract (FRT) and gastrointestinal (GI) tract, major sites of transmission, viral replication, and CD4+ T cell depletion and where some current antiretroviral drugs have a sub-optimal effect.

Results

EFdA treatment resulted in reduction of HIV-RNA in PB to undetectable levels in the majority of treated mice by 3 weeks post-treatment. HIV-RNA levels in cervicovaginal lavage of EFdA-treated BLT mice also declined to undetectable levels demonstrating strong penetration of EFdA into the FRT. Our results also demonstrate a strong systemic suppression of HIV replication in all tissues analyzed. In particular, we observed more than a 2-log difference in HIV-RNA levels in the GI tract and FRT of EFdA-treated BLT mice compared to untreated HIV-infected control mice. In addition, HIV-RNA was also significantly lower in the lymph nodes, liver, lung, spleen of EFdA-treated BLT mice compared to untreated HIV-infected control mice. Furthermore, EFdA treatment prevented the depletion of CD4+ T cells in the PB, mucosal tissues and lymphoid tissues.

Conclusion

Our findings indicate that EFdA is highly effective in controlling viral replication and preserving CD4+ T cells in particular with high efficiency in the GI and FRT tract. Thus, EFdA represents a strong potential candidate for further development as a part of antiretroviral therapy regimens.

Humanized mice for HIV and AIDS research

HIV has a very limited species tropism that prevents the use of most conventional small animal models for AIDS research. The in vivo analysis of HIV/AIDS has benefited extensively from novel chimeric animal models that accurately recapitulate key aspects of the human condition. Specifically, immunodeficient mice that are systemically repopulated with human hematolymphoid cells offer a viable alternative for the study of a multitude of highly relevant aspects of HIV replication, pathogenesis, therapy, transmission, prevention, and eradication. This article summarizes some of the multiple contributions that humanized mouse models of HIV infection have made to the field of AIDS research. These models have proven to be highly informative and hold great potential for accelerating multiple aspects of HIV research in the future.

Protection of Humanized Mice From Repeated Intravaginal HIV Challenge by Passive Immunization: A Model for Studying the Efficacy of Neutralizing Antibodies In Vivo

Humanized mice reconstituted with a human immune system can be mucosally infected with human immunodeficiency virus (HIV), opening up the possibility of studying HIV transmission in a small-animal model. Here we report that passive immunization with the broadly neutralizing antibody b12 protected humanized mice against repetitive intravaginal infection in a dose-dependent manner. In addition, treatment with the antibody PGT126, which is more potent in vitro, was more efficacious in vivo and provided sterilizing protection. Our results demonstrate that humanized mice can be used as a small-animal model to study the efficacy and mechanism of broadly neutralizing antibody protection against HIV acquisition.

Smoothened Agonist Reduces Human Immunodeficiency Virus Type-1-Induced Blood-Brain Barrier Breakdown in Humanized Mice

Human Immunodeficiency Virus type-1 (HIV)-associated neurocognitive disorder is characterized by recruitment of activated/infected leukocytes into the CNS via disrupted Blood Brain Barrier (BBB) that contributes to persistent neuro-inflammation. In this report, humanized NOD/scid-IL2Rγ_c^null mice were used to establish that impaired Sonic hedgehog (Shh) signaling is associated with loss of BBB function and neurological damage, and that modulating Shh signaling can rescue these detrimental effects. Plasma viral load, p24 levels and CD4⁺ T cells were measured as markers of productive HIV infection. These mice also showed impaired exclusion of Evans blue dye from the brain, increased plasma levels of S100B, an astrocytic protein, and down-regulation of tight junction proteins Occludin and Claudin5, collectively indicating BBB dysfunction. Further, brain tissue from HIV⁺ mice indicated reduced synaptic density, neuronal atrophy, microglial activation, and astrocytosis. Importantly, reduced expression of Shh and Gli1 was also observed in these mice, demonstrating diminished Shh signaling. Administration of Shh mimetic, smoothened agonist (SAG) restored BBB integrity and also abated the neuropathology in infected mice. Together, our results suggest a neuroprotective role for Shh signaling in the context of HIV infection, underscoring the therapeutic potential of SAG in controlling HAND pathogenesis.

In vivo analysis of the effect of panobinostat on cell-associated HIV RNA and DNA levels and latent HIV infection

BACKGROUND

The latent reservoir in resting CD4(+) T cells presents a major barrier to HIV cure. Latency-reversing agents are therefore being developed with the ultimate goal of disrupting the latent state, resulting in induction of HIV expression and clearance of infected cells. Histone deacetylase inhibitors (HDACi) have received a significant amount of attention for their potential as latency-reversing agents.

RESULTS

Here, we have investigated the in vitro and systemic in vivo effect of panobinostat, a clinically relevant HDACi, on HIV latency. We showed that panobinostat induces histone acetylation in human PBMCs. Further, we showed that panobinostat induced HIV RNA expression and allowed the outgrowth of replication-competent virus ex vivo from resting CD4(+) T cells of HIV-infected patients on suppressive antiretroviral therapy (ART). Next, we demonstrated that panobinostat induced systemic histone acetylation in vivo in the tissues of BLT humanized mice. Finally, in HIV-infected, ART-suppressed BLT mice, we evaluated the effect of panobinostat on systemic cell-associated HIV RNA and DNA levels and the total frequency of latently infected resting CD4(+) T cells. Our data indicate that panobinostat treatment resulted in systemic increases in cellular levels of histone acetylation, a key biomarker for in vivo activity. However, panobinostat did not affect the levels of cell-associated HIV RNA, HIV DNA, or latently infected resting CD4(+) T cells.

CONCLUSION

We have demonstrated robust levels of systemic histone acetylation after panobinostat treatment of BLT humanized mice; and we did not observe a detectable change in the levels of cell-associated HIV RNA, HIV DNA, or latently infected resting CD4(+) T cells in HIV-infected, ART-suppressed BLT mice. These results are consistent with the modest effects noted in vitro and suggest that combination therapies may be necessary to reverse latency and enable clearance. Animal models will contribute to the progress towards an HIV cure.

TREX1 Knockdown Induces an Interferon Response to HIV that Delays Viral Infection in Humanized Mice

Despite their antiviral effect, the in vivo effect of interferons on HIV transmission is difficult to predict, because interferons also activate and recruit HIV-susceptible cells to sites of infection. HIV does not normally induce type I interferons in infected cells, but does if TREX1 is knocked down. Here, we investigated the effect of topical TREX1 knockdown and local interferon production on HIV transmission in human cervicovaginal explants and humanized mice. In explants in which TREX1 was knocked down, HIV induced interferons, which blocked infection. In humanized mice, even though TREX1 knockdown increased infiltrating immune cells, it delayed viral replication for 3-4 weeks. Similarly intravaginal application of type I interferons the day before HIV infection induced interferon responsive genes, reduced inflammation, and decreased viral replication. However, intravenous interferon enhanced inflammation and infection. Thus, in models of human sexual transmission, a localized interferon response inhibits HIV transmission but systemic interferons do not.