The utility of the new generation of humanized mice to study HIV-1 infection: transmission, prevention, pathogenesis, and treatment

HIV Reservoir: How to Measure It?

PURPOSEOF REVIEW

In the current quest for a complete cure for HIV/AIDS, the persistence of a long-lived reservoir of cells carrying replication-competent proviruses is the major challenge. Here, we describe the main elements and characteristics of several widely used assays of HIV latent reservoir detection.

RECENT FINDINGS

To date, researchers have developed several different HIV latent reservoir detection assays. Among them, the in vitro quantitative viral outgrowth assay (QVOA) has been the gold standard for assessing latent HIV-1 viral load. The intact proviral DNA assay (IPDA) based on PCR also demonstrated the predominance of defective viruses. However, these assays all have some drawbacks and may still be inadequate in detecting the presence of ultralow levels of latent virus in many patients who were initially thought to have been cured, but eventually showed viral rebound. An accurate and precise measurement of the HIV reservoir is therefore needed to evaluate curative strategies, aimed to functional cure or sterilizing cure.

Use of Hu-PBL Mice to Study Pathogenesis of Human-Restricted Viruses

Different humanized mouse models have been developed to study human diseases such as autoimmune illnesses, cancer and viral infections. These models are based on the use of immunodeficient mouse strains that are transplanted with human tissues or human immune cells. Among the latter, mice transplanted with hematopoietic stem cells have been widely used to study human infectious diseases. However, mouse models built upon the transplantation of donor-specific mature immune cells are still under development, especially in the field of viral infections. These models can retain the unique immune memory of the donor, making them suitable for the study of correlates of protection upon natural infection or vaccination. Here, we will review some of these models and how they have been applied to virology research. Moreover, the future applications and the potential of these models to design therapies against human viral infections are discussed.

Generation and Characterization of a Humanized Lung Xenograft Mouse Model for Studying Henipavirus Pathogenesis

The development of humanized mouse models has recently opened new avenues in the field of infectious diseases. These models allow research on many human viruses that were once difficult to study, because finding suitable animal models of infection can be challenging, cost prohibitive, and often do not entirely recapitulate all parameters of the disease. Here, we describe the procedure of human immune system reconstitution (humanization) of NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice by the bone marrow, liver, and thymus (BLT) reconstitution method as well as the process of human lung engraftment. We then describe how to infect these human lung grafts with the paramyxovirus Nipah virus (NiV) that can cause lethal respiratory disease in humans, and for which there is only limited understanding of pathogenesis to acute lung injury.

The development and improvement of immunodeficient mice and humanized immune system mouse models

Animal models play an indispensable role in the study of human diseases. However, animal models of different diseases do not fully mimic the complex internal environment of humans. Immunodeficient mice are deficient in certain genes and do not express these or show reduced expression in some of their cells, facilitating the establishment of humanized mice and simulation of the human environment in vivo. Here, we summarize the developments in immunodeficient mice, from the initial nude mice lacking T lymphocytes to NOD/SCID rg^null mice lacking T, B, and NK cell populations. We describe existing humanized immune system mouse models based on immunodeficient mice in which human cells or tissues have been transplanted to establish a human immune system, including humanized-peripheral blood mononuclear cells (Hu-PBMCs), humanized hematopoietic stem cells (Hu-HSCs), and humanized bone marrow, liver, thymus (Hu-BLT) mouse models. The different methods for their development involve varying levels of complexity and humanization. Humanized mice are widely used in the study of various diseases to provide a transitional stage for clinical research. However, several challenges persist, including improving the efficiency of reconstructing the human B cell immune response, extending lifespan, improving the survival rate of mice to extend the observation period, and improving the development of standardized commercialized models and as well as their use. Overall, there are many opportunities and challenges in the development of humanized immune system mouse models which can provide novel strategies for understanding the mechanisms and treatments of human disease.

CD4dim CD8bright T Cells Home to the Brain and Mediate HIV Neuroinvasion

CD4^dim CD8^bright T cells are a mature population of CD8⁺ T cells that upon activation upregulate CD4 dimly on their surface. Expression of CD4 on these cells suggests that they can be an additional source of HIV neuroinvasion and persistence in the brain. We used HIV-infected NOD/SCID/IL-2rcγ^-/- (NSG) humanized mice to track CD4^dim CD8^bright T cell homing to the brain and define their role in HIV dissemination into the brain. We report here that CD4^dim CD8^bright T cells are found in the brain at a median frequency of 2.6% and in the spleen at median frequency of 7.6% of CD3⁺ T cells. In the brain, 10 to 20% of CD4^dim CD8^bright T cells contain integrated provirus, which is infectious as demonstrated by viral outgrowth assay. CD4^dim CD8^bright T cells in the brain exhibited significantly higher expression of the brain homing receptors CX3CR1 and CXCR3 in comparison to their single-positive CD8⁺ T cell counterpart. Blocking lymphocyte trafficking into the brain of humanized mice via anti-VLA4 and anti-LFA1 antibodies reduced CD4^dim CD8^bright T cell trafficking into the brain by 60% and diminished brain HIV proviral DNA by 72%. Collectively, our findings demonstrate that CD4^dim CD8^bright T cells can home to the brain and support productive HIV replication. These studies also reveal for the first time that CD4^dim CD8^bright T cells are capable of HIV neuroinvasion and are a reservoir for HIV. IMPORTANCE We report here a seminal finding of a novel population of T cells, termed CD4^dim CD8^bright T cells, that plays a role in HIV neuroinvasion and is a reservoir for HIV in the brain.

Engineered Zinc Finger Protein Targeting 2LTR Inhibits HIV Integration in Hematopoietic Stem and Progenitor Cell-Derived Macrophages: In Vitro Study

Human hematopoietic stem/progenitor cell (HSPC)-based gene therapy is a promising direction for curing HIV-1-infected individuals. The zinc finger protein (2LTRZFP) designed to target the 2-LTR-circle junction of HIV-1 cDNA was previously reported as an intracellular antiviral molecular scaffold that prevents HIV integration. Here, we elucidate the efficacy and safety of using 2LTRZFP in human CD34⁺ HSPCs. We transduced 2LTRZFP which has the mCherry tag (2LTRZFPmCherry) into human CD34⁺ HSPCs using a lentiviral vector. The 2LTRZFPmCherry-transduced HSPCs were subsequently differentiated into macrophages. The expression levels of pro-apoptotic proteins of the 2LTRZFPmCherry-transduced HSPCs showed no significant difference from those of the non-transduced control. Furthermore, the 2LTRZFPmCherry-transduced HSPCs were successfully differentiated into mature macrophages, which had normal phagocytic function. The cytokine secretion assay demonstrated that 2LTRZFPmCherry-transduced CD34⁺ derived macrophages promoted the polarization towards classically activated (M1) subtypes. More importantly, the 2LTRZFPmCherry transduced cells significantly exhibited resistance to HIV-1 integration in vitro. Our findings demonstrate that the 2LTRZFPmCherry-transduced macrophages were found to be functionally and phenotypically normal, with no adverse effects of the anti-HIV-1 scaffold. Our data suggest that the anti-HIV-1 integrase scaffold is a promising antiviral molecule that could be applied to human CD34⁺ HSPC-based gene therapy for AIDS patients.

Perspectives on Non-BLT Humanized Mouse Models for Studying HIV Pathogenesis and Therapy

A variety of humanized mice, which are reconstituted only with human hematopoietic stem cells (HSC) or with fetal thymus and HSCs, have been developed and widely utilized as in vivo animal models of HIV-1 infection. The models represent some aspects of HIV-mediated pathogenesis in humans and are useful for the evaluation of therapeutic regimens. However, there are several limitations in these models, including their incomplete immune responses and poor distribution of human cells to the secondary lymphoid tissues. These limitations are common in many humanized mouse models and are critical issues that need to be addressed. As distinct defects exist in each model, we need to be cautious about the experimental design and interpretation of the outcomes obtained using humanized mice. Considering this point, we mainly characterize the current conventional humanized mouse reconstituted only with HSCs and describe past achievements in this area, as well as the potential contributions of the humanized mouse models for the study of HIV pathogenesis and therapy. We also discuss the use of various technologies to solve the current problems. Humanized mice will contribute not only to the pre-clinical evaluation of anti-HIV regimens, but also to a deeper understanding of basic aspects of HIV biology.

The new Japanese regulation on human/non-human chimeras: should we worry?

In March 2019 Japan modified its norms regarding research with human/non-human chimeras. The amended rules allow the creation of chimeras with human brain cells, and the subsequent transfer of the resulting creature to an uterus, where it can develop for more than 14 days, eventually until term. At this moment, the real consequences of this new regulation in actual research are still uncertain. However, many concerning issues have already been identified. This paper will start by addressing traditional topics involving this practice: the use of non-human animals in research, the use of human stem cells in scientific experimentation and the creation of human/non-human chimeras. Subsequently, it will analyze the new concerning issues brought on by the 2019 amendment: the use of human brain cells, the transfer of the chimera to an uterus and its development for more than 14 days, and the possibility of using animals which present close similarities with humans. In the end, the paper will conclude that in spite of the legal and ethical hazards that this new regulation might carry, it should be allowed under strict scrutiny.

Substantial induction of non-apoptotic CD4 T-cell death during the early phase of HIV-1 infection in a humanized mouse model

Several mechanisms underline induction of CD4 T-cell death by human immunodeficiency virus (HIV) infection. For a long time, apoptosis was considered central to cell death involved in the depletion of CD4 T cells during HIV infection. However, which types of cell death are induced during the early phase of HIV infection in vivo remains unclear. In this study, CD4 T-cell death induced in early HIV infection was characterized using humanized mice challenged with CCR5-tropic (R5) or CXCR4-tropic (X4) HIV-1. Results showed that CD4 T-cell death was induced in the spleen 3 days post-challenge with both R5 and X4 HIV-1. Although cell death without caspase-1 and caspase-3/7 activation was preferentially observed, caspase-1⁺ pyroptosis was also significantly induced within the memory subpopulation by R5 or X4 HIV-1 and the naïve subpopulation by X4 HIV-1. In contrast, caspase-3/7⁺ apoptosis was not enhanced by either R5 or X4 HIV-1. Furthermore, phosphorylated mixed lineage kinase domain-like protein⁺ necroptosis was induced by only X4 HIV-1. These findings indicate that various types of non-apoptotic CD4 T-cell death, such as pyroptosis and necroptosis, are induced during the early phase of HIV infection in vivo.

Humanized mice are precious tools for evaluation of hematopoietic gene therapies and preclinical modeling to move towards a clinical trial

Over the last decade, incrementally improved xenograft mouse models, which support the engraftment and development of a human hemato-lymphoid system, have been developed and represent an important fundamental and preclinical research tool. Immunodeficient mice can be transplanted with human hematopoietic stem cells (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. The most significant contributions made by these humanized mice are the identification of normal and leukemic hematopoietic stem cells, the characterization of the human hematopoietic hierarchy, screening of anti-cancer therapies and their use as preclinical models for gene therapy applications. This review article focuses on several gene therapy applications that have benefited from evaluation in humanized mice such as chimeric antigen receptor (CAR) T cell therapies for cancer, anti-viral therapies and gene therapies for multiple monogenetic diseases. Humanized mouse models have been and still are of great value for the gene therapy field since they provide a more reliable understanding of sometimes complicated therapeutic approaches such as recently developed therapeutic gene editing strategies, which seek to correct a gene at its endogenous genomic locus. Additionally, humanized mouse models, which are of great importance with regard to testing new vector technologies in vivo for assessing safety and efficacy prior toclinical trials, help to expedite the critical translation from basic findings to clinical applications. In this review, innovative gene therapies and preclinical studies to evaluate T- and B-cell and HSC-based therapies in humanized mice are discussed and illustrated by multiple examples.

Development of Onchocerca volvulus in humanized NSG mice and detection of parasite biomarkers in urine and serum

BACKGROUND

The study of Onchocerca volvulus has been limited by its host range, with only humans and non-human primates shown to be susceptible to the full life cycle infection. Small animal models that support the development of adult parasites have not been identified.

METHODOLOGY/PRINCIPAL FINDINGS

We hypothesized that highly immunodeficient NSG mice would support the survival and maturation of O. volvulus and alteration of the host microenvironment through the addition of various human cells and tissues would further enhance the level of parasite maturation. NSG mice were humanized with: (1) umbilical cord derived CD34+ stem cells, (2) fetal derived liver, thymus and CD34+ stem cells or (3) primary human skeletal muscle cells. NSG and humanized NSG mice were infected with 100 O. volvulus infective larvae (L3) for 4 to 12 weeks. When necropsies of infected animals were performed, it was observed that parasites survived and developed throughout the infection time course. In each of the different humanized mouse models, worms matured from L3 to advanced fourth stage larvae, with both male and female organ development. In addition, worms increased in length by up to 4-fold. Serum and urine, collected from humanized mice for identification of potential biomarkers of infection, allowed for the identification of 10 O. volvulus-derived proteins found specifically in either the urine or the serum of the humanized O. volvulus-infected NSG mice.

CONCLUSIONS/SIGNIFICANCE

The newly identified mouse models for onchocerciasis will enable the development of O. volvulus specific biomarkers, screening for new therapeutic approaches and potentially studying the human immune response to infection with O. volvulus.

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.

Ultra-Sensitive HIV-1 Latency Viral Outgrowth Assays Using Humanized Mice

In the current quest for a complete cure for HIV/AIDS, highly sensitive HIV-1 latency detection methods are critical to verify full viral eradication. Until now, the in vitro quantitative viral outgrowth assays (qVOA) have been the gold standard for assessing latent HIV-1 viral burden. However, these assays have been inadequate in detecting the presence of ultralow levels of latent virus in a number of patients who were initially thought to have been cured, but eventually showed viral rebound. In this context, new approaches utilizing in vivo mouse-based VOAs are promising. In the murine VOA (mVOA), large numbers of CD4⁺ T cells or PBMC from aviremic subjects are xenografted into immunodeficient NSG mice, whereas in the humanized mouse-based VOA (hmVOA) patient CD4⁺ T cell samples are injected into BLT or hu-hematopoetic stem cells (hu-HSC) humanized mice. While latent virus could be recovered in both of these systems, the hmVOA provides higher sensitivity than the mVOA using a fewer number of input cells. In contrast to the mVOA, the hmVOA provides a broader spectrum of highly susceptible HIV-1 target cells and enables newly engrafted cells to home into preformed human lymphoid organs where they can infect cells in situ after viral activation. Hu-mice also allow for both xenograft- and allograft-driven cell expansions with less severe GvH providing a longer time frame for potential viral outgrowth from cells with a delayed latent viral activation. Based on these advantages, the hmVOA has great potential in playing an important role in HIV-1 latency and cure research.

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.

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.

Contribution of Human Lung Parenchyma and Leukocyte Influx to Oxidative Stress and Immune System-Mediated Pathology following Nipah Virus Infection

Nipah virus (NiV) is a zoonotic emerging paramyxovirus that can cause fatal respiratory illness or encephalitis in humans. Despite many efforts, the molecular mechanisms of NiV-induced acute lung injury (ALI) remain unclear. We previously showed that NiV replicates to high titers in human lung grafts in NOD-SCID/γ mice, resulting in a robust inflammatory response. Interestingly, these mice can undergo human immune system reconstitution by the bone marrow, liver, and thymus (BLT) reconstitution method, in addition to lung tissue engraftment, giving altogether a realistic model to study human respiratory viral infections. Here, we characterized NiV Bangladesh strain (NiV-B) infection of human lung grafts from human immune system-reconstituted mice in order to identify the overall effect of immune cells on NiV pathogenesis of the lung. We show that NiV-B replicated to high titers in human lung grafts and caused similar cytopathic effects irrespective of the presence of human leukocytes in mice. However, the human immune system interfered with virus spread across lung grafts, responded to infection by leukocyte migration to small airways and alveoli of the lung grafts, and accelerated oxidative stress in lung grafts. In addition, the presence of human leukocytes increased the expression of cytokines and chemokines that regulate inflammatory influx to sites of infection and tissue damage. These results advance our understanding of how the immune system limits NiV dissemination and contributes to ALI and inform efforts to identify therapeutic targets.IMPORTANCE Nipah virus (NiV) is an emerging paramyxovirus that can cause a lethal respiratory and neurological disease in humans. Only limited data are available on NiV pathogenesis in the human lung, and the relative contribution of the innate immune response and NiV to acute lung injury (ALI) is still unknown. Using human lung grafts in a human immune system-reconstituted mouse model, we showed that the NiV Bangladesh strain induced cytopathic lesions in lung grafts similar to those described in patients irrespective of the donor origin or the presence of leukocytes. However, the human immune system interfered with virus spread, responded to infection by leukocyte infiltration in the small airways and alveolar area, induced oxidative stress, and triggered the production of cytokines and chemokines that regulate inflammatory influx by leukocytes in response to infection. Understanding how leukocytes interact with NiV and cause ALI in human lung xenografts is crucial for identifying therapeutic targets.

Engineered Bispecific Antibodies with Exquisite HIV-1-Neutralizing Activity

While the search for an efficacious HIV-1 vaccine remains elusive, emergence of a new generation of virus-neutralizing monoclonal antibodies (mAbs) has re-ignited the field of passive immunization for HIV-1 prevention. However, the plasticity of HIV-1 demands additional improvements to these mAbs to better ensure their clinical utility. Here, we report engineered bispecific antibodies that are the most potent and broad HIV-neutralizing antibodies to date. One bispecific antibody, 10E8V2.0/iMab, neutralized 118 HIV-1 pseudotyped viruses tested with a mean 50% inhibitory concentration (IC50) of 0.002 μg/mL. 10E8V2.0/iMab also potently neutralized 99% of viruses in a second panel of 200 HIV-1 isolates belonging to clade C, the dominant subtype accounting for ∼50% of new infections worldwide. Importantly, 10E8V2.0/iMab reduced virus load substantially in HIV-1-infected humanized mice and also provided complete protection when administered prior to virus challenge. These bispecific antibodies hold promise as novel prophylactic and/or therapeutic agents in the fight against HIV-1.

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.

Neuroteratogenic Viruses and Lessons for Zika Virus Models

The Centers for Disease Control and Prevention has confirmed that Zika virus (ZIKV) causes congenital microcephaly. ZIKV now joins five other neuroteratogenic (NT) viruses in humans and ZIKV research is in its infancy. In addition, there is only one other NT human arbovirus (Venezuelan equine encephalitis virus), which is also poorly understood. But further insight into ZIKV can be found by evaluating arboviruses in domestic animals, of which there are at least seven NT viruses, three of which have been well studied. Here we review two key anatomical structures involved in modeling transplacental NT virus transmission: the placenta and the fetal blood-brain barrier. We then survey major research findings regarding transmission of NT viruses for guidance in establishing a mouse model of Zika disease that is crucial for a better understanding of ZIKV transmission and pathogenesis.

Human-animal chimeras: ethical issues about farming chimeric animals bearing human organs

Recent advances in stem cells and gene engineering have paved the way for the generation of interspecies chimeras, such as animals bearing an organ from another species. The production of a rat pancreas by a mouse has demonstrated the feasibility of this approach. The next step will be the generation of larger chimeric animals, such as pigs bearing human organs. Because of the dramatic organ shortage for transplantation, the medical needs for such a transgressive practice are indisputable. However, there are serious technical barriers and complex ethical issues that must be discussed and solved before producing human organs in animals. The main ethical issues are the risks of consciousness and of human features in the chimeric animal due to a too high contribution of human cells to the brain, in the first case, or for instance to limbs, in the second. Another critical point concerns the production of human gametes by such chimeric animals. These worst-case scenarios are obviously unacceptable and must be strictly monitored by careful risk assessment, and, if necessary, technically prevented. The public must be associated with this ethical debate. Scientists and physicians have a critical role in explaining the medical needs, the advantages and limits of this potential medical procedure, and the ethical boundaries that must not be trespassed. If these prerequisites are met, acceptance of such a new, borderline medical procedure may prevail, as happened before for in-vitro fertilization or preimplantation genetic diagnosis.

TALEN gene editing takes aim on HIV

Transcription activator-like effector nucleases (TALENs) are one of several types of programmable, engineered nucleases that bind and cleave specific DNA sequences. Cellular machinery repairs the cleaved DNA by introducing indels. In this review, we emphasize the potential, explore progress, and identify challenges in using TALENs as a therapeutic tool to treat HIV infection. TALENs have less off-target editing and can be more effective at tolerating HIV escape mutations than CRISPR/Cas-9. Scientists have explored TALEN-mediated editing of host genes such as viral entry receptors (CCR5 and CXCR4) and a protein involved in proviral integration (LEDGF/p75). Viral targets include the proviral DNA, particularly focused on the long terminal repeats. Major challenges with translating gene therapy from bench to bedside are improving cleavage efficiency and delivery, while minimizing off-target editing, cytotoxicity, and immunogenicity. However, rapid improvements in TALEN technology are enhancing cleavage efficiency and specificity. Therapeutic testing in animal models of HIV infection will help determine whether TALENs are a viable HIV treatment therapy. TALENs or other engineered nucleases could shift the therapeutic paradigm from life-long antiretroviral therapy toward eradication of HIV infection.

Experimental systems for studying Plasmodium/HIV coinfection

Coinfections with Human Immunodeficiency Virus (HIV) and Plasmodium, the causative agents of AIDS and malaria, respectively, are frequent and their comorbidity especially in sub-Saharan Africa is high. While clinical studies suggest an influence of the two pathogens on the outcome of the respective infections, experimental studies on the molecular and immunological impact of coinfections are rare. This reflects the limited availability of suitable model systems that reproduce key properties of both pathologies. Here, we discuss key aspects of coinfection with a focus on currently established experimental systems, their limitations for coinfection studies and potential strategies for their improvement.

Pulmonary Tuberculosis in Humanized Mice Infected with HIV-1

Co-infection with HIV increases the morbidity and mortality associated with tuberculosis due to multiple factors including a poorly understood microbial synergy. We developed a novel small animal model of co-infection in the humanized mouse to investigate how HIV infection disrupts pulmonary containment of Mtb. Following dual infection, HIV-infected cells were localized to sites of Mtb-driven inflammation and mycobacterial replication in the lung. Consistent with disease in human subjects, we observed increased mycobacterial burden, loss of granuloma structure, and increased progression of TB disease, due to HIV co-infection. Importantly, we observed an HIV-dependent pro-inflammatory cytokine signature (IL-1β, IL-6, TNFα, and IL-8), neutrophil accumulation, and greater lung pathology in the Mtb-co-infected lung. These results suggest that in the early stages of acute co-infection in the humanized mouse, infection with HIV exacerbates the pro-inflammatory response to pulmonary Mtb, leading to poorly formed granulomas, more severe lung pathology, and increased mycobacterial burden and dissemination.

Using animal models to overcome temporal, spatial and combinatorial challenges in HIV persistence research

Research challenges associated with understanding HIV persistence during antiretroviral therapy can be categorized as temporal, spatial and combinatorial. Temporal research challenges relate to the timing of events during establishment and maintenance of HIV persistence. Spatial research challenges regard the anatomical locations and cell subsets that harbor persistent HIV. Combinatorial research challenges pertain to the order of administration, timing of administration and specific combinations of compounds to be administered during HIV eradication therapy. Overcoming these challenges will improve our understanding of HIV persistence and move the field closer to achieving eradication of persistent HIV. Given that humanized mice and non-human primate HIV models permit rigorous control of experimental conditions, these models have been used extensively as in vivo research platforms for directly addressing these research challenges. The aim of this manuscript is to provide a comprehensive review of these recent translational advances made in animal models of HIV persistence.

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

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

Characterization of neuropathology in the HIV-1 transgenic rat at different ages

The transgenic HIV-1 rat (Tg) is a commonly used neuroHIV model with documented neurologic/behavioral deficits. Using immunofluorescent staining of the Tg brain, we found astrocytic dysfunction/damage, as well as dopaminergic neuronal loss/dysfunction, both of which worsening significantly in the striatum with age. We saw mild microglial activation in young Tg brains, but this decreased with age. There were no differences in neurogenesis potential suggesting a neurodegenerative rather than a neurodevelopmental process. Gp120 CSF levels exceeded serum gp120 levels in some animals, suggesting local viral protein production in the brain. Further probing of the pathophysiology underlying astrocytic injury in this model is warranted.

HIV Replication Is Not Controlled by CD8+ T Cells during the Acute Phase of the Infection in Humanized Mice

HIV replication follows a well-defined pattern during the acute phase of the infection in humans. After reaching a peak during the first few weeks after infection, viral replication resolves to a set-point thereafter. There are still uncertainties regarding the contribution of CD8⁺ T cells in establishing this set-point. An alternative explanation, supported by in silico modeling, would imply that viral replication is limited by the number of available targets for infection, i.e. CD4⁺CCR5⁺ T cells. Here, we used NOD.SCID.gc^-/- mice bearing human CD4⁺CCR5⁺ and CD8⁺ T cells derived from CD34⁺ progenitors to investigate the relative contribution of both in viral control after the peak. Using low dose of a CCR5-tropic HIV virus, we observed an increase in viral replication followed by “spontaneous” resolution of the peak, similar to humans. To rule out any possible role for CD8⁺ T cells in viral control, we infected mice in which CD8⁺ T cells had been removed by a depleting antibody. Globally, viral replication was not affected by the absence of CD8⁺ T cells. Strikingly, resolution of the viral peak was equally observed in mice with or without CD8⁺ T cells, showing that CD8⁺ T cells were not involved in viral control in the early phase of the infection. In contrast, a marked and specific loss of CCR5-expressing CD4⁺ T cells was observed in the spleen and in the bone marrow, but not in the blood, of infected animals. Our results strongly suggest that viral replication during the acute phase of the infection in humanized mice is mainly constrained by the number of available targets in lymphoid tissues rather than by CD8⁺ T cells.

Of mice and monkeys: can animal models be utilized to study neurological consequences of pediatric HIV-1 infection?

Pediatric human immunodeficiency virus (HIV-1) infection remains a global health crisis. Children are much more susceptible to HIV-1 neurological impairments than adults, which can be exacerbated by coinfections. Neurological characteristics of pediatric HIV-1 infection suggest dysfunction in the frontal cortex as well as the hippocampus; limited MRI data indicate global cerebral atrophy, and pathological data suggest accelerated neuronal apoptosis in the cortex. An obstacle to pediatric HIV-1 research is a human representative model system. Host-species specificity of HIV-1 limits the ability to model neurological consequences of pediatric HIV-1 infection in animals. Several models have been proposed including neonatal intracranial injections of HIV-1 viral proteins in rats and perinatal simian immunodeficiency virus (SIV) infection of infant macaques. Nonhuman primate models recapitulate the complexity of pediatric HIV-1 neuropathogenesis while rodent models are able to elucidate the role specific viral proteins exert on neurodevelopment. Nonhuman primate models show similar behavioral and neuropathological characteristics to pediatric HIV-1 infection and offer a stage to investigate early viral mechanisms, latency reservoirs, and therapeutic interventions. Here we review the relative strengths and limitations of pediatric HIV-1 model systems.

Limitations of Current in Vivo Mouse Models for the Study of Chikungunya Virus Pathogenesis

Chikungunya virus (CHIKV) is an arthropod-borne alphavirus that causes febrile chikungunya fever (CHIKF) in humans. This disease is debilitating and characterized by acute fever onset and chronic incapacitating polyarthralgia. CHIKF pathogenesis remains poorly defined with no approved vaccines and therapies. Recent outbreaks in the Caribbean islands have elevated concerns over the possibility of a global pandemic. Tremendous efforts have been made to develop relevant mouse models to enable the study of infection and immunity against this viral disease. Among them, the more common C57BL/6 mouse model demonstrated the ability to recapitulate the symptoms shown in infected humans, including self-limiting arthritis, myositis, and tenosynovitis. This has facilitated the unraveling of some key factors involved in disease pathogenesis of CHIKF. However, the stark differences in immune response between humans and mouse models necessitate the development of an animal model with an immune system that is more genetically similar to the human system for a better representation. In this paper, we aim to uncover the limitations of the C57BL/6 model and discuss alternative mouse models for CHIKV research.

Application of Humanized Mice in Immunological Research

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

Dengue virus models based on mice as experimental hosts

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

Long lasting control of viral rebound with a new drug ABX464 targeting Rev - mediated viral RNA biogenesis

BACKGROUND

Current therapies have succeeded in controlling AIDS pandemic. However, there is a continuing need for new drugs, in particular those acting through new and as yet unexplored mechanisms of action to achieve HIV infection cure. We took advantage of the unique feature of proviral genome to require both activation and inhibition of splicing of viral transcripts to develop molecules capable of achieving long lasting effect on viral replication in humanized mouse models through inhibition of Rev-mediated viral RNA biogenesis.

RESULTS

Current HIV therapies reduce viral load during treatment but titers rebound after treatment is discontinued. We devised a new drug that has a long lasting effect after viral load reduction. We demonstrate here that ABX464 compromises HIV replication of clinical isolates of different subtypes without selecting for drug resistance in PBMCs or macrophages. ABX464 alone, also efficiently compromised viral proliferation in two humanized mouse models infected with HIV that require a combination of 3TC, Raltegravir and Tenofovir (HAART) to achieve viral inhibition in current protocols. Crucially, while viral load increased dramatically just one week after stopping HAART treatment, only slight rebound was observed following treatment cessation with ABX464 and the magnitude of the rebound was maintained below to that of HAART for two months after stopping the treatment. Using a system to visualize single HIV RNA molecules in living cells, we show that ABX464 inhibits viral replication by preventing Rev-mediated export of unspliced HIV-1 transcripts to the cytoplasm and by interacting with the Cap Binding Complex (CBC). Deep sequencing of viral RNA from treated cells established that retained viral RNA is massively spliced but importantly, normal cellular splicing is unaffected by the drug. Consistently ABX464 is non-toxic in humans and therefore represents a promising complement to current HIV therapies.

CONCLUSIONS

ABX464 represents a novel class of anti-HIV molecules with unique properties. ABX464 has a long lasting effect in humanized mice and neutralizes the expression of HIV-1 proviral genome of infected immune cells including reservoirs and it is therefore a promising drug toward a functional cure of HIV.

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

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

Humanized mice dually challenged with R5 and X4 HIV-1 show preferential R5 viremia and restricted X4 infection of CCR5(+)CD4(+) T cells

CCR5-tropic (R5) immunodeficiency virus type 1 (HIV-1) strains are highly transmissible during the early stage of infection in humans, whereas CXCR4-tropic (X4) strains are less transmissible. This study aimed to explore the basis for early phase R5 and X4 HIV-1 infection in vivo by using humanized mice dually challenged with R5 HIV-1NLAD8-D harboring DsRed and X4 HIV-1(NL-E) harboring EGFP. Whereas R5 HIV-1 replicated well, X4 HIV-1 caused only transient viremia with variable kinetics; however, this was distinct from the low level but persistent viremia observed in mice challenged with X4 HIV-1 alone. Flow cytometric analysis of HIV-1-infected cells revealed that X4 HIV-1 infection of CCR5(+)CD4(+) T cells was significantly suppressed in the presence of R5 HIV-1. X4 HIV-1 was more cytopathic than R5 HIV-1; however, this was not the cause of restricted X4 HIV-1 infection because there were no significant differences in the mortality rates of CCR5(+) and CCR5(-) cells within the X4 HIV-1-infected cell populations. Taken together, these results suggest that restricted infection of CCR5(+)CD4(+) T cells by X4 HIV-1 (occurring via a still-to-be-identified mechanism) might contribute to the preferential transmission of R5 HIV-1 during the early phase of infection.

Pharmacodynamics of long-acting folic acid-receptor targeted ritonavir-boosted atazanavir nanoformulations

Long-acting nanoformulated antiretroviral therapy (nanoART) that targets monocyte-macrophages could improve the drug's half-life and protein-binding capacities while facilitating cell and tissue depots. To this end, ART nanoparticles that target the folic acid (FA) receptor and permit cell-based drug depots were examined using pharmacokinetic and pharmacodynamic (PD) tests. FA receptor-targeted poloxamer 407 nanocrystals, containing ritonavir-boosted atazanavir (ATV/r), significantly increased drug bioavailability and PD by five and 100 times, respectively. Drug particles administered to human peripheral blood lymphocyte reconstituted NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ mice and infected with HIV-1ADA led to ATV/r drug concentrations that paralleled FA receptor beta staining in both the macrophage-rich parafollicular areas of spleen and lymph nodes. Drug levels were higher in these tissues than what could be achieved by either native drug or untargeted nanoART particles. The data also mirrored potent reductions in viral loads, tissue viral RNA and numbers of HIV-1p24+ cells in infected and treated animals. We conclude that FA-P407 coating of ART nanoparticles readily facilitates drug carriage and antiretroviral responses.

Recent developments in animal models for human herpesvirus 6A and 6B

Progress in the identification of suitable animal models for human herpesvirus (HHV)-6A and HHV-6B infections has been slow. Recently, new models have been established, mainly for HHV-6A, which reproduce some pathological features seen in humans. Neuroinflammatory signs were observed in infected marmosets and CD46-transgenic mice; although viral replication was not prominent, persistence of viral DNA and specific immunologic responses were detected, suggesting an immune-mediated pathogenic mechanism. Pig-tailed macaques showed robust viral replication concomitant with acute-phase symptoms, and provided a model to study the effects of HHV-6A on AIDS progression. In humanized mice, viral replication was less evident, but infection led to T-cell alterations. Altogether, these recent developments have opened new perspectives for studying the pathogenic role of HHV-6A in humans.

Cell-type-specific aptamer and aptamer-small interfering RNA conjugates for targeted human immunodeficiency virus type 1 therapy

Human immunodeficiency virus (HIV) is a virus that causes acquired immunodeficiency syndrome, a chronic and incurable disease of the human immune system. As the standard of care for the patients with HIV-1, current highly active antiretroviral treatment has been therapeutically effective in most patients; however, it is not curative, and highly active antiretroviral treatment is intolerable because of severe adverse effects. Therefore, nucleic acid-based therapeutics, such as antisense oligonucleotide, ribozyme, messenger RNA, RNA interference (RNAi)-based therapeutics, aptamer, and so on, have been actively developed as alternative or adjuvant agents for those chemical antiviral drugs to surmount those drawbacks. The combinatorial use of various antiviral nucleic acids could be more efficacious in blocking viral replication and preventing the emergence of resistant variants. In this regard, RNAi can function as a gene-specific therapeutic option for controlling HIV-1 replication. Another type of therapeutic nucleic acid--aptamers--shows promise as a new and potent class of anti-HIV agent and can additionally function as a cell-type-specific delivery vehicle for targeted RNAi. The combined use of small interfering RNA (siRNAs) and aptamers could effectively block viral replication and prevent the emergence of resistant variants. The present review offers a brief overview of the use of cell-type-specific aptamer and aptamer-siRNA conjugates' development in our group for the treatment of HIV-1. Their potentials for targeted delivering RNAi therapeutics (eg, siRNA) and suppressing HIV-1 replication in vitro and in humanized animal model will be highlighted here.

Adding new dimensions: towards an integrative understanding of HIV-1 spread

In vitro studies in primary or immortalized cells continue to be used to elucidate the essential principles that govern the interactions between HIV-1 and isolated target cells. However, until recently, substantial technical barriers prevented this information from being efficiently translated to the more complex scenario of HIV-1 spread in the host in vivo, which has limited our understanding of the impact of host physiological parameters on the spread of HIV-1. In this Review, we discuss the recent development of imaging approaches to visualize HIV-1 spread and the adaptation of these approaches to organotypic ex vivo models and animal models. We focus on new concepts, including the mechanisms and in vivo relevance of cell-cell transmission for HIV-1 spread and the function of the HIV-1 pathogenesis factor Nef, which have emerged from the application of these integrative approaches in complex cell systems.

Characterization of HIV-1 infection in the humanized Rag2-/-γc-/- mouse model

Engraftment of immunodeficient mice with a human immune system (humanized mice) provides a model system to study pathogens that target human immune cells. Humanized Rag2(-/-)γc(-/-) mice produce the major target cells of HIV-1 and these cells can be detected in primary and secondary lymphoid tissues, as well as in the vaginal and rectal mucosa and brain tissues. This humanized model has already yielded important findings on HIV-1 transmission, mechanisms of pathogenesis, and testing of novel antiviral strategies in vivo. Here, we describe the methods used to infect humanized mice with HIV-1 and to characterize plasma viral load and blood CD4(+) T cell depletion.

FELASA recommendations for the health monitoring of mouse, rat, hamster, guinea pig and rabbit colonies in breeding and experimental units

The microbiological quality of experimental animals can critically influence animal welfare and the validity and reproducibility of research data. It is therefore important for breeding and experimental facilities to establish a laboratory animal health monitoring (HM) programme as an integrated part of any quality assurance system. FELASA has published recommendations for the HM of rodent and rabbit colonies in breeding and experimental units (Nicklas et al. Laboratory Animals, 2002), with the intention of harmonizing HM programmes. As stated in the preamble, these recommendations need to be adapted periodically to meet current developments in laboratory animal medicine. Accordingly, previous recommendations have been revised and shall be replaced by the present recommendations. These recommendations are aimed at all breeders and users of laboratory mice, rats, Syrian hamsters, guinea pigs and rabbits as well as diagnostic laboratories. They describe essential aspects of HM, such as the choice of agents, selection of animals and tissues for testing, frequency of sampling, commonly used test methods, interpretation of results and HM reporting. Compared with previous recommendations, more emphasis is put on the role of a person with sufficient understanding of the principles of HM, opportunistic agents, the use of sentinel animals (particularly under conditions of cage-level containment) and the interpretation and reporting of HM results. Relevant agents, testing frequencies and literature references are updated. Supplementary information on specific agents and the number of animals to be monitored and an example of a HM programme description is provided in the appendices.

Rectal microbicide development

The last few years have seen important progress in demonstrating the efficacy of oral pre-exposure prophylaxis, vaginal microbicides, and treatment as prevention as effective strategies for reducing the risk of acquiring or transmitting HIV infection. There has also been significant progress in the development of rectal microbicides. Preclinical non-human primate studies have demonstrated that antiretroviral microbicides can provide significant protection from rectal challenge with SIV or SHIV. Recent Phase 1 rectal microbicide studies have characterized the safety, acceptability, compartmental pharmacokinetics (PK), and pharmacodynamics (PD) of both UC781 and tenofovir gels. The tenofovir gel formulation used in vaginal studies was not well tolerated in the rectum and newer rectal-specific formulations have been developed and evaluated in Phase 1 studies. The PK/PD data generated in these Phase 1 studies may reduce the risk of advancing ineffective candidate rectal microbicides into late stage development. Tenofovir gel is currently poised to move into Phase 2 evaluation and it is possible that a Phase 2B/3 effectiveness study with this product could be initiated in the next 2-3 years.

Humoral immunity in humanized mice: a work in progress

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

Pre-clinical modeling of CCR5 knockout in human hematopoietic stem cells by zinc finger nucleases using humanized mice

Genetic strategies to block expression of CCR5, the major co-receptor of human immunodeficiency virus type 1 (HIV-1), are being developed as anti-HIV therapies. For example, human hematopoietic stem/precursor cells (HSPC) can be modified by the transient expression of CCR5-targeted zinc finger nucleases (ZFNs) to generate CCR5-negative cells, which could then give rise to HIV-resistant mature CD4(+) T cells following transplantation into patients. The safety and anti-HIV effects of such treatments can be evaluated by transplanting ZFN-treated HSPC into immunodeficient mice, where the extent of human cell engraftment, lineage differentiation and anti-HIV activity arising from the engineered HSPC can be examined. In this way, humanized mice are providing a powerful small animal model for pre-clinical studies of novel anti-HIV therapies.

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.

Gene therapy strategies for HIV/AIDS: preclinical modeling in humanized mice

In the absence of an effective vaccine and lack of a complete cure, gene therapy approaches to control HIV infection offer feasible alternatives. Due to the chronic nature of infection, a wide window of opportunity exists to gene modify the HIV susceptible cells that continuously arise from the bone marrow source. To evaluate promising gene therapy approaches that employ various anti-HIV therapeutic molecules, an ideal animal model is necessary to generate important efficacy and preclinical data. In this regard, the humanized mouse models that harbor human hematopoietic cells susceptible to HIV infection provide a suitable in vivo system. This review summarizes the currently used humanized mouse models and different anti-HIV molecules utilized for conferring HIV resistance. Humanized mouse models are compared for their utility in this context and provide perspectives for new directions.

HIV-1 Vpr accelerates viral replication during acute infection by exploitation of proliferating CD4+ T cells in vivo

The precise role of viral protein R (Vpr), an HIV-1-encoded protein, during HIV-1 infection and its contribution to the development of AIDS remain unclear. Previous reports have shown that Vpr has the ability to cause G₂ cell cycle arrest and apoptosis in HIV-1-infected cells in vitro. In addition, vpr is highly conserved in transmitted/founder HIV-1s and in all primate lentiviruses, which are evolutionarily related to HIV-1. Although these findings suggest an important role of Vpr in HIV-1 pathogenesis, its direct evidence in vivo has not been shown. Here, by using a human hematopoietic stem cell-transplanted humanized mouse model, we demonstrated that Vpr causes G₂ cell cycle arrest and apoptosis predominantly in proliferating CCR5⁺ CD4⁺ T cells, which mainly consist of regulatory CD4⁺ T cells (Tregs), resulting in Treg depletion and enhanced virus production during acute infection. The Vpr-dependent enhancement of virus replication and Treg depletion is observed in CCR5-tropic but not CXCR4-tropic HIV-1-infected mice, suggesting that these effects are dependent on the coreceptor usage by HIV-1. Immune activation was observed in CCR5-tropic wild-type but not in vpr-deficient HIV-1-infected humanized mice. When humanized mice were treated with denileukin diftitox (DD), to deplete Tregs, DD-treated humanized mice showed massive activation/proliferation of memory T cells compared to the untreated group. This activation/proliferation enhanced CCR5 expression in memory CD4⁺ T cells and rendered them more susceptible to CCR5-tropic wild-type HIV-1 infection than to vpr-deficient virus. Taken together, these results suggest that Vpr takes advantage of proliferating CCR5⁺ CD4⁺ T cells for enhancing viremia of CCR5-tropic HIV-1. Because Tregs exist in a higher cycling state than other T cell subsets, Tregs appear to be more vulnerable to exploitation by Vpr during acute HIV-1 infection.

HIV-1 encodes nine genes, five of which (gag, pol, env, tat, and rev) are essential for viral replication, and four, termed accessory genes (vif, vpu, nef, and vpr), appear to aid virus infection. Of the four accessory proteins, Vpr is the most enigmatic. It is well known that Vpr has the potential to cause G₂ cell cycle arrest and apoptosis in vitro. Moreover, it has been reported that Vpr-mediated G₂ arrest increases HIV-1 production in vitro. However, the role of Vpr in HIV-1 propagation in vivo remains unclear. Here, by using a humanized mouse model, we demonstrate that Vpr enhances CCR5-tropic but not CXCR4-tropic HIV-1 replication in vivo by exploiting Tregs during acute infection. In CCR5-tropic HIV-1-infected humanized mice, Vpr-dependent G₂ cell cycle arrest and apoptosis are predominantly observed in infected Tregs, and wild-type but not vpr-deficient HIV-1-infected mice displayed acute Treg depletion. This Vpr-dependent Treg depletion may lead to immune activation and provide a pool of activated/proliferating CD4⁺ T cells, which supports subsequent HIV-1 expansion in vivo. This is the first report demonstrating the role of Vpr in HIV-1 infection in vivo.

Host specificity of bacterial pathogens

Most pathogens are able to infect multiple hosts but some are highly adapted to a single-host species. A detailed understanding of the basis of host specificity can provide important insights into molecular pathogenesis, the evolution of pathogenic microbes, and the potential for pathogens to cross the species barrier to infect new hosts. Comparative genomics and the development of humanized mouse models have provided important new tools with which to explore the basis of generalism and specialism. This review will examine host specificity of bacterial pathogens with a focus on generalist and specialist serovars of Salmonella enterica.

HIV-1 infection, response to treatment and establishment of viral latency in a novel humanized T cell-only mouse (TOM) model

Background

The major targets of HIV infection in humans are CD4⁺ T cells. CD4⁺ T cell depletion is a hallmark of AIDS. Previously, the SCID-hu thy/liv model was used to study the effect of HIV on thymopoeisis in vivo. However, these mice did not develop high levels of peripheral T cell reconstitution and required invasive surgery for infection and analysis. Here, we describe a novel variant of this model in which thy/liv implantation results in systemic reconstitution with human T cells in the absence of any other human hematopoietic lineages.

Results

NOD/SCID-hu thy/liv and NSG-hu thy/liv mice were created by implanting human fetal thymus and liver tissues under the kidney capsule of either NOD/SCID or NSG mice. In contrast to NOD/SCID-hu thy/liv mice that show little or no human cells in peripheral blood or tissues, substantial systemic human reconstitution occurs in NSG-hu thy/liv. These mice are exclusively reconstituted with human T cells (i.e. T-cell only mice or TOM). Despite substantial levels of human T cells no signs of graft-versus-host disease (GVHD) were noted in these mice over a period of 14 months. TOM are readily infected after parenteral exposure to HIV-1. HIV replication is sustained in peripheral blood at high levels and results in modest reduction of CD4⁺ T cells. HIV-1 replication in TOM responds to daily administration of combination antiretroviral therapy (ART) resulting in strong suppression of virus replication as determined by undetectable viral load in plasma. Latently HIV infected resting CD4⁺ T cells can be isolated from suppressed mice that can be induced to express HIV ex-vivo upon activation demonstrating the establishment of latency in vivo.

Conclusions

NSG-hu thy/liv mice are systemically reconstituted with human T cells. No other human lymphoid lineages are present in these mice (i.e. monocytes/macrophages, B cells and DC are all absent). These T cell only mice do not develop GVHD, are susceptible to HIV-1 infection and can efficiently maintain virus replication. HIV infected TOM undergoing ART harbor latently infected, resting CD4⁺ T cells.