Systemic administration of antiretrovirals prior to exposure prevents rectal and intravenous HIV-1 transmission in humanized BLT mice

A germ-free humanized mouse model shows the contribution of resident microbiota to human-specific pathogen infection

Germ-free (GF) mice, which are depleted of their resident microbiota, are the gold standard for exploring the role of the microbiome in health and disease; however, they are of limited value in the study of human-specific pathogens because they do not support their replication. Here, we develop GF mice systemically reconstituted with human immune cells and use them to evaluate the role of the resident microbiome in the acquisition, replication and pathogenesis of two human-specific pathogens, Epstein-Barr virus (EBV) and human immunodeficiency virus (HIV). Comparison with conventional (CV) humanized mice showed that resident microbiota enhance the establishment of EBV infection and EBV-induced tumorigenesis and increase mucosal HIV acquisition and replication. HIV RNA levels were higher in plasma and tissues of CV humanized mice compared with GF humanized mice. The frequency of CCR5+ CD4+ T cells throughout the intestine was also higher in CV humanized mice, indicating that resident microbiota govern levels of HIV target cells. Thus, resident microbiota promote the acquisition and pathogenesis of two clinically relevant human-specific pathogens.

Transient CD4+ T cell depletion during suppressive ART reduces the HIV reservoir in humanized mice

Lifelong treatment is required for people living with HIV as current antiretroviral therapy (ART) does not eradicate HIV infection. Latently infected cells are essentially indistinguishable from uninfected cells and cannot be depleted by currently available approaches. This study evaluated antibody mediated transient CD4+ T cell depletion as a strategy to reduce the latent HIV reservoir. Anti-CD4 antibodies effectively depleted CD4+ T cells in the peripheral blood and tissues of humanized mice. We then demonstrate that antibody-mediated CD4+ T cell depletion of HIV infected ART-suppressed animals results in substantial reductions in cell-associated viral RNA and DNA levels in peripheral blood cells over the course of anti-CD4 antibody treatment. Recovery of CD4+ T cells was observed in all tissues analyzed except for the lung 26 days after cessation of antibody treatment. After CD4+ T cell recovery, significantly lower levels of cell-associated viral RNA and DNA were detected in the tissues of anti-CD4 antibody-treated animals. Further, an 8.5-fold reduction in the levels of intact HIV proviral DNA and a 3.1-fold reduction in the number of latently infected cells were observed in anti-CD4-antibody-treated animals compared with controls. However, there was no delay in viral rebound when ART was discontinued in anti-CD4 antibody-treated animals following CD4+ T cell recovery compared with controls. Our results suggest that transient CD4+ T cell depletion, a long-standing clinical intervention that might have an acceptable safety profile, during suppressive ART can reduce the size of the HIV reservoir in humanized mice.

EFdA efficiently suppresses HIV replication in the male genital tract and prevents penile HIV acquisition

Sexually transmitted HIV infections in heterosexual men are acquired through the penis. Low adherence to condom usage and the fact that 40% of circumcised men are not protected indicate the need for additional prevention strategies. Here, we describe a new approach to evaluate the prevention of penile HIV transmission. We demonstrated that the entire male genital tract (MGT) of bone marrow/liver/thymus (BLT) humanized mice is repopulated with human T and myeloid cells. The majority of the human T cells in the MGT express CD4 and CCR5. Direct penile exposure to HIV leads to systemic infection including all tissues of the MGT. HIV replication throughout the MGT was reduced 100-1,000-fold by treatment with 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA), resulting in the restoration of CD4+ T cell levels. Importantly, systemic preexposure prophylaxis with EFdA effectively protects from penile HIV acquisition. IMPORTANCE Over 84.2 million people have been infected by the human immunodeficiency virus type 1 (HIV-1) during the past 40 years, most through sexual transmission. Men comprise approximately half of the HIV-infected population worldwide. Sexually transmitted HIV infections in exclusively heterosexual men are acquired through the penis. However, direct evaluation of HIV infection throughout the human male genital tract (MGT) is not possible. Here, we developed a new in vivo model that permits, for the first time, the detail analysis of HIV infection. Using BLT humanized mice, we showed that productive HIV infection occurs throughout the entire MGT and induces a dramatic reduction in human CD4 T cells compromising immune responses in this organ. Antiretroviral treatment with novel drug EFdA suppresses HIV replication in all tissues of the MGT, restores normal levels of CD4 T cells and is highly efficient at preventing penile transmission.

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

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

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

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

Loss of In Vivo Replication Fitness of HIV-1 Variants Resistant to the Tat Inhibitor, dCA

HIV resistance to the Tat inhibitor didehydro-cortistatin A (dCA) in vitro correlates with higher levels of Tat-independent viral transcription and a seeming inability to enter latency, which rendered resistant isolates more susceptible to CTL-mediated immune clearance. Here, we investigated the ability of dCA-resistant viruses to replicate in vivo using a humanized mouse model of HIV infection. Animals were infected with WT or two dCA-resistant HIV-1 isolates in the absence of dCA and followed for 5 weeks. dCA-resistant viruses exhibited lower replication rates compared to WT. Viral replication was suppressed early after infection, with viral emergence at later time points. Multiplex analysis of cytokine and chemokines from plasma samples early after infection revealed no differences in expression levels between groups, suggesting that dCA-resistance viruses did not elicit potent innate immune responses capable of blocking the establishment of infection. Viral single genome sequencing results from plasma samples collected at euthanasia revealed that at least half of the total number of mutations in the LTR region of the HIV genome considered essential for dCA evasion reverted to WT. These results suggest that dCA-resistant viruses identified in vitro suffer a fitness cost in vivo, with mutations in LTR and Nef pressured to revert to wild type.

Acute antagonism in three-drug combinations for vaginal HIV prevention in humanized mice

Adolescent girls and young women in low- to middle-income countries are disproportionately at risk of becoming HIV-1 infected. New non-vaccine biomedical products aimed at overcoming this global health challenge need to provide a range of safe, effective, and discreet dosage forms based on the delivery of one or more antiviral compounds. An overarching strategy involves vaginal drug administration through inserts/tablets, gels, films, and intravaginal rings. The approach derives its appeal from being women-controlled and topical, there-by potentially minimizing systemic exposure to the agents and their metabolites. Oral regimens based on tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) are established and effective in HIV-1 pre-exposure prophylaxis (PrEP), and form a promising basis for vaginal PrEP. Here, we used bone marrow/liver/thymus humanized mice to measure the in vivo efficacy against HIV-1 of single and combination antiviral compounds applied vaginally, coupled with data analysis using the Chou-Talalay mathematical model to study the dose-effect characteristics. Unexpectedly, strong antagonism was observed in drug combinations composed of TDF-FTC coupled with a third agent using a different mode of action against HIV-1. The antagonistic effect was remedied when TDF was omitted from the regimen. Our approach provides a translational template for the preclinical, rational, and systematic evaluation of drug combinations for the prevention of HIV-1, and other viral diseases.

Anatomical and physiological aspects of the HIV infection pathogenesis in animal models

Understanding the entire pathogenesis of HIV infection, from penetration at the gates of infection to the induction of severe immunodeficiency, is an essential tool for the development of new treatment methods. Less than 40 years of research into the mechanisms of HIV infection that lead to the development of acquired immunodeficiency syndrome have accumulated a huge amount of information, but HIV's own unique variability identifies new whitespaces. Despite the constant improvement of the protocols of antiretroviral therapy and the success of its use, it has not yet been possible to stop the spread of HIV infection. The development of new protocols and the testing of new groups of antiretroviral drugs is possible, first of all, due to the improvement of animal models of the HIV infection pathogenesis. Their relevance, undoubtedly increases, but still depends on specific research tasks, since none of the in vivo models can comprehensively simulate the mechanism of the infection pathology in humans which leads to multi-organ damage. The aim of the review was to provide up-to-date information on known animal models of HIV infection, focusing on the method of their infection and anatomical, physiological and pathological features.

Translational Models to Predict Target Concentrations for Pre-Exposure Prophylaxis in Women

The HIV epidemic remains a significant public health burden. Women represent half of the global HIV epidemic, yet there is an urgent need for a variety of prevention options to meet the needs of more women. Pre-exposure prophylaxis (PrEP) is a valuable prevention tool that uses antiretrovirals before a potential HIV exposure to prevent virus transmission. Development of effective preventive drug regimens for women is dependent on convenient dosing schedules and routes of administration, and on identifying defined target concentrations in mucosal tissues that provide complete protection against HIV transmission. There is a critical need for a translational model that can accurately predict in vivo target concentrations that are completely protective against HIV infection. There is no gold-standard preclinical model to predict PrEP efficacy. In this study, we review the strengths and limitations of three different preclinical models and their utility in predicting target concentrations in the female genital tract: humanized mice, non-human primates, and the ex vivo tissue model.

Fundamental aspects of long-acting tenofovir alafenamide delivery from subdermal implants for HIV prophylaxis

Global efforts aimed at preventing human immunodeficiency virus type one (HIV-1) infection in vulnerable populations appear to be stalling, limiting our ability to control the epidemic. Long-acting, controlled drug administration from subdermal implants holds significant potential by reducing the compliance burden associated with frequent dosing. We, and others, are exploring the development of complementary subdermal implant technologies delivering the potent prodrug, tenofovir alafenamide (TAF). The current report addresses knowledge gaps in the preclinical pharmacology of long-acting, subdermal TAF delivery using several mouse models. Systemic drug disposition during TAF implant dosing was explained by a multi-compartment pharmacokinetic (PK) model. Imaging mass spectrometry was employed to characterize the spatial distribution of TAF and its principal five metabolites in local tissues surrounding the implant. Humanized mouse studies determined the effective TAF dose for preventing vaginal and rectal HIV-1 acquisition. Our results represent an important step in the development of a safe and effective TAF implant for HIV-1 prevention.

Autologous humanized mouse models of iPSC-derived tumors enable characterization and modulation of cancer-immune cell interactions

Modeling the tumor-immune cell interactions in humanized mice is complex and limits drug development. Here, we generated easily accessible tumor models by transforming either primary skin fibroblasts or induced pluripotent stem cell-derived cell lines injected in immune-deficient mice reconstituted with human autologous immune cells. Our results showed that fibroblastic, hepatic, or neural tumors were all efficiently infiltrated and partially or totally rejected by autologous immune cells in humanized mice. Characterization of tumor-immune infiltrates revealed high expression levels of the dysfunction markers Tim3 and PD-1 in T cells and an enrichment in regulatory T cells, suggesting rapid establishment of immunomodulatory phenotypes. Inhibition of PD-1 by Nivolumab in humanized mice resulted in increased immune cell infiltration and a slight decrease in tumor growth. We expect that these versatile and accessible cancer models will facilitate preclinical studies and the evaluation of autologous cancer immunotherapies across a range of different tumor cell types.

Long-acting and extended-release implant and nanoformulations with a synergistic antiretroviral two-drug combination controls HIV-1 infection in a humanized mouse model

The HIV pandemic has affected over 38 million people worldwide with close to 26 million currently accessing antiretroviral therapy (ART). A major challenge in the long-term treatment of HIV-1 infection is nonadherence to ART. Long-acting antiretroviral (LA-ARV) formulations, that reduce dosing frequency to less than once a day, are an urgent need that could tackle the adherence issue. Here, we have developed two LA-ART interventions, one an injectable nanoformulation, and the other, a removable implant, for the delivery of a synergistic two-drug ARV combination comprising a pre-clinical nonnucleoside reverse transcriptase inhibitor (NNRTI), Compound I, and the nucleoside reverse transcriptase inhibitor (NRTI), 4'-ethynyl-2-fluoro-2'-deoxyadenosine. The nanoformulation is poly(lactide-co-glycolide)-based and the implant is a copolymer of ω-pentadecalactone and p-dioxanone, poly(PDL-co-DO), a novel class of biocompatible, biodegradable materials. Both the interventions, packaged independently with each ARV, released sustained levels of the drugs, maintaining plasma therapeutic indices for over a month, and suppressed viremia in HIV-1-infected humanized mice for up to 42 days with maintenance of CD4+ T cells. These data suggest promise in the use of these new drugs as LA-ART formulations in subdermal implant and injectable mode.

Novel humanized-PBMC mouse model with delayed onset of GVHD for preclinical HIV research

Humanized mouse models are based on the engraftment of human cells in immunodeficient mouse strains, most notably the NSG strain. Most used models have a major limitation in common, the development of graft-versus-host disease (GVHD). GVHD not only introduces variabilities into the research data but also leads to animal welfare concerns. A new mouse strain, B6.129S-Rag2^tm1Fwa CD47^tm1Fpl Il2rg^tm1Wjl/J which lacks Rag1, IL2rg, and CD47 (triple knockout or TKO), is resistant to GVHD development. We transplanted TKO mice with human peripheral blood mononuclear cells (PBMCs) to establish a new humanized PBMC (hu-PBMC) mouse model. A cohort of these mice was infected with HIV-1 and monitored for plasma HIV viremia and CD4⁺ T cell depletion. The onset and progression of GVHD were monitored by clinical signs. This study demonstrates that TKO mice transplanted with human PBMCs support engraftment of human immune cells in primary and secondary lymphoid tissues, rectum, and brain. Moreover, the TKO hu-PBMC model supports HIV-1 infection via intraperitoneal, rectal, or vaginal routes, as confirmed by robust plasma HIV viremia and CD4⁺ T cell depletion. Lastly, TKO mice showed a delayed onset of GVHD clinical signs (∼28 days) and exhibited significant decreases in plasma levels of TNFβ. Based on these results, the TKO hu-PBMC mouse model not only supports humanization and HIV-1 infection but also has a delayed onset of GVHD development, making this model a valuable tool in HIV research. Importance Currently, there is no cure or vaccine for HIV infection, thus continued research is needed to end the HIV pandemic. While many animal models are used in HIV research, none is used more than the humanized mouse model. A major limitation with current humanized mouse models is the development of graft-versus-host disease (GVHD). Here, we show a novel humanized-PBMC mouse model that has a delayed onset GVHD development and supports and models HIV infection comparable to well-established humanized mouse models.

Advancing our understanding of HIV co-infections and neurological disease using the humanized mouse

Humanized mice have become an important workhorse model for HIV research. Advances that enabled development of a human immune system in immune deficient mouse strains have aided new basic research in HIV pathogenesis and immune dysfunction. The small animal features facilitate development of clinical interventions that are difficult to study in clinical cohorts, and avoid the high cost and regulatory burdens of using non-human primates. The model also overcomes the host restriction of HIV for human immune cells which limits discovery and translational research related to important co-infections of people living with HIV. In this review we emphasize recent advances in modeling bacterial and viral co-infections in the setting of HIV in humanized mice, especially neurological disease, and Mycobacterium tuberculosis and HIV co-infections. Applications of current and future co-infection models to address important clinical and research questions are further discussed.

Modulating HIV-1 envelope glycoprotein conformation to decrease the HIV-1 reservoir

Small CD4-mimetic compounds (CD4mc) sensitize HIV-1-infected cells to antibody-dependent cellular cytotoxicity (ADCC) by facilitating antibody recognition of epitopes that are otherwise occluded on the unliganded viral envelope (Env). Combining CD4mc with two families of CD4-induced (CD4i) antibodies, which are frequently found in plasma of HIV-1-infected individuals, stabilizes Env in a conformation that is vulnerable to ADCC. We employed new-generation SRG-15 humanized mice, supporting natural killer (NK) cell and Fc-effector functions to demonstrate that brief treatment with CD4mc and CD4i-Abs significantly decreases HIV-1 replication, the virus reservoir and viral rebound after ART interruption. These effects required Fc-effector functions and NK cells, highlighting the importance of ADCC. Viral rebound was also suppressed in HIV-1+-donor cell-derived humanized mice supplemented with autologous HIV-1+-donor-derived plasma and CD4mc. These results indicate that CD4mc could have therapeutic utility in infected individuals for decreasing the size of the HIV-1 reservoir and/or achieving a functional cure.

Advances in Humanized Mouse Models to Improve Understanding of HIV-1 Pathogenesis and Immune Responses

Although antiretroviral therapy has transformed human immunodeficiency virus-type 1 (HIV-1) from a deadly infection into a chronic disease, it does not clear the viral reservoir, leaving HIV-1 as an uncurable infection. Currently, 1.2 million new HIV-1 infections occur globally each year, with little decrease over many years. Therefore, additional research is required to advance the current state of HIV management, find potential therapeutic strategies, and further understand the mechanisms of HIV pathogenesis and prevention strategies. Non-human primates (NHP) have been used extensively in HIV research and have provided critical advances within the field, but there are several issues that limit their use. Humanized mouse (Hu-mouse) models, or immunodeficient mice engrafted with human immune cells and/or tissues, provide a cost-effective and practical approach to create models for HIV research. Hu-mice closely parallel multiple aspects of human HIV infection and disease progression. Here, we highlight how innovations in Hu-mouse models have advanced HIV-1 research in the past decade. We discuss the effect of different background strains of mice, of modifications on the reconstitution of the immune cells, and the pros and cons of different human cells and/or tissue engraftment methods, on the ability to examine HIV-1 infection and immune response. Finally, we consider the newest advances in the Hu-mouse models and their potential to advance research in emerging areas of mucosal infections, understand the role of microbiota and the complex issues in HIV-TB co-infection. These innovations in Hu-mouse models hold the potential to significantly enhance mechanistic research to develop novel strategies for HIV prevention and therapeutics.

Antiretroviral Penetration and Drug Transporter Concentrations in the Spleens of Three Preclinical Animal Models and Humans

Adequate antiretroviral (ARV) concentrations in lymphoid tissues are critical for optimal antiretroviral therapy (ART). While the spleen contains 25% of the body's lymphocytes, there are minimal data on ARV penetration in this organ. This study quantified total and protein-unbound splenic ARV concentrations and determined whether drug transporters, sex, or infection status were modifiers of these concentrations in animal models and humans. Two humanized mice models (hu-HSC-Rag [n = 36; 18 HIV-positive (HIV⁺) and 18 HIV-negative (HIV^-)] and bone marrow-liver-thymus [n = 13; 7 HIV⁺ and 6 HIV^-]) and one nonhuman primate (NHP) model (rhesus macaque [n = 18; 10 SHIV⁺ and 8 SHIV^-]) were dosed to steady state with ARV combinations. HIV⁺ human spleens (n = 14) from the National NeuroAIDS Tissue Consortium were analyzed postmortem (up to 24 h postdose). ARV concentrations were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS), drug transporter concentrations were measured with LC-MS proteomics, and protein binding in NHP spleens was determined by rapid equilibrium dialysis. Mice generally had the lowest splenic concentrations of the three species. Protein binding in splenic tissue was 6 to 96%, compared to 76 to 99% in blood plasma. NHPs had quantifiable Mrp4, Bcrp, and Ent1 concentrations, and humans had quantifiable ENT1 concentrations. None significantly correlated with tissue ARV concentrations. There was also no observable influence of infection status or sex. With these dosing strategies, NHP splenic penetration most closely resembled that of humans. These data can inform tissue pharmacokinetic scaling to humans to target HIV reservoirs by identifying important species-related differences.

Development of humanized mouse and rat models with full-thickness human skin and autologous immune cells

The human skin is a significant barrier for protection against pathogen transmission. Rodent models used to investigate human-specific pathogens that target the skin are generated by introducing human skin grafts to immunocompromised rodent strains. Infection-induced immunopathogenesis has been separately studied in humanized rodent models developed with human lymphoid tissue and hematopoietic stem cell transplants. Successful co-engraftment of human skin, autologous lymphoid tissues, and autologous immune cells in a rodent model has not yet been achieved, though it could provide a means of studying the human immune response to infection in the human skin. Here, we introduce the human Skin and Immune System (hSIS)-humanized NOD-scid IL2Rγ^null (NSG) mouse and Sprague–Dawley-Rag2^tm2hera Il2rγ^tm1hera (SRG) rat models, co-engrafted with human full-thickness fetal skin, autologous fetal lymphoid tissues, and autologous fetal liver-derived hematopoietic stem cells. hSIS-humanized rodents demonstrate the development of human full-thickness skin, along with autologous lymphoid tissues, and autologous immune cells. These models also support human skin infection following intradermal inoculation with community-associated methicillin-resistant Staphylococcus aureus. The co-engraftment of these human skin and immune system components into a single humanized rodent model could provide a platform for studying human skin infections.

Highly synergistic drug combination prevents vaginal HIV infection in humanized mice

The HIV-1 epidemic remains an urgent global health concern. Young women are disproportionately at risk of acquiring the virus. A range of highly effective, female-controlled, discrete vaginal products therefore is needed to help curb the epidemic. Oral tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) are effective in HIV-1 pre-exposure prophylaxis (PrEP) and form a promising basis for a vaginal product. Here, we evaluate TDF and FTC in combination with the broadly neutralizing antibody VRC01-N using a highly reproducible humanized mouse model. The agents were vaginally dosed individually and in combination, and the efficacy of HIV-1 prevention was analyzed using the established, rigorous median-effect model. Surprisingly, the triple combination showed a high degree of synergism, unprecedented for in vivo HIV-1 PrEP, leading to a possible fivefold dose reduction for some of the agents. Vaginal administration of the TDF-FTC-VRC01-N combination holds significant promise for HIV-1 PrEP.

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

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

Benefits and limitations of humanized mice in HIV persistence studies

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

Curing HIV: Seeking to Target and Clear Persistent Infection

Human immunodeficiency virus type 1 (HIV-1) infection persists despite years of antiretroviral therapy (ART). To remove the stigma and burden of chronic infection, approaches to eradicate or cure HIV infection are desired. Attempts to augment ART with therapies that reverse viral latency, paired with immunotherapies to clear infection, have advanced into the clinic, but the field is still in its infancy. We review foundational studies and highlight new insights in HIV cure research. Together with advances in ART delivery and HIV prevention strategies, future therapies that clear HIV infection may relieve society of the affliction of the HIV pandemic.

Antiretroviral Penetration across Three Preclinical Animal Models and Humans in Eight Putative HIV Viral Reservoirs

For HIV cure strategies like "kick and kill" to succeed, antiretroviral (ARV) drugs must reach effective concentrations in putative viral reservoirs. We characterize penetration of six ARVs in three preclinical animal models and humans. We found that standard dosing strategies in preclinical species closely mimicked tissue concentrations in humans for some, but not all, ARVs. These results have implications for interpreting HIV treatment, prevention, or cure interventions between preclinical and clinical models.

Antiretroviral Drug Concentrations in Lymph Nodes: A Cross-Species Comparison of the Effect of Drug Transporter Expression, Viral Infection, and Sex in Humanized Mice, Nonhuman Primates, and Humans

In a "kick and kill" strategy for human immunodeficiency virus (HIV) eradication, protective concentrations of antiretrovirals (ARVs) in the lymph node are important to prevent vulnerable cells from further HIV infection. However, the factors responsible for drug distribution and concentration into these tissues are largely unknown. Although humanized mice and nonhuman primates (NHPs) are crucial to HIV research, ARV tissue pharmacology has not been well characterized across species. This study investigated the influence of drug transporter expression, viral infection, and sex on ARV penetration within lymph nodes of animal models and humans. Six ARVs were dosed for 10 days in humanized mice and NHPs. Plasma and lymph nodes were collected at necropsy, 24 hours after the last dose. Human lymph node tissue and plasma from deceased patients were collected from tissue banks. ARV, active metabolite, and endogenous nucleotide concentrations were measured by liquid chromatography-tandem mass spectrometry, and drug transporter expression was measured using quantitative polymerase chain reaction and quantitative targeted absolute proteomics. In NHPs and humans, lymph node ARV concentrations were greater than or equal to plasma, and tenofovir diphosphate/deoxyadenosine triphosphate concentration ratios achieved efficacy targets in lymph nodes from all three species. There was no effect of infection or sex on ARV concentrations. Low drug transporter expression existed in lymph nodes from all species, and no predictive relationships were found between transporter gene/protein expression and ARV penetration. Overall, common preclinical models of HIV infection were well suited to predict human ARV exposure in lymph nodes, and low transporter expression suggests primarily passive drug distribution in these tissues. SIGNIFICANCE STATEMENT: During human immunodeficiency virus (HIV) eradication strategies, protective concentrations of antiretrovirals (ARVs) in the lymph node prevent vulnerable cells from further HIV infection. However, ARV tissue pharmacology has not been well characterized across preclinical species used for HIV eradication research, and the influence of drug transporters, HIV infection, and sex on ARV distribution and concentration into the lymph node is largely unknown. Here we show that two animal models of HIV infection (humanized mice and nonhuman primates) were well suited to predict human ARV exposure in lymph nodes. Additionally, we found that drug transporter expression was minimal and-along with viral infection and sex-did not affect ARV penetration into lymph nodes from any species.

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.

Antiretroviral concentrations and surrogate measures of efficacy in the brain tissue and CSF of preclinical species

1. Antiretroviral concentrations in cerebrospinal fluid (CSF) are used as surrogate for brain tissue, although sparse data support this. We quantified antiretrovirals in brain tissue across preclinical models, compared them to CSF, and calculated 90% inhibitory quotients (IQ₉₀) for nonhuman primate (NHP) brain tissue. Spatial distribution of efavirenz was performed by mass-spectrometry imaging (MSI). 2. HIV or RT-SHIV-infected and uninfected animals from two humanized mouse models (hemopoietic-stem cell/RAG2-, n = 36; bone marrow-liver-thymus/BLT, n =13) and an NHP model (rhesus macaque, n =18) were dosed with six antiretrovirals. Brain tissue, CSF (NHPs), and plasma were collected at necropsy. Drug concentrations were measured by LC-MS/MS. Rapid equilibrium dialysis determined protein binding in NHP brain. 3. Brain tissue penetration of most antiretrovirals were >10-fold lower (p < 0.02) in humanized mice than NHPs. NHP CSF concentrations were >13-fold lower (p <0.02) than brain tissue with poor agreement except for efavirenz (r = 0.91, p = 0.001). Despite 97% brain tissue protein binding, efavirenz achieved IQ₉₀>1 in all animals and 2-fold greater white versus gray matter concentration. 4. Brain tissue penetration varied across animal models for all antiretrovirals except raltegravir, and extrapolating brain tissue concentrations between models should be avoided. With the exception of efavirenz, CSF is not a surrogate for brain tissue concentrations.

Human immunodeficiency virus infection induces lymphoid fibrosis in the BM-liver-thymus-spleen humanized mouse model

A major pathogenic feature associated with HIV infection is lymphoid fibrosis, which persists during antiretroviral therapy (ART). Lymphoid tissues play critical roles in the generation of antigen-specific immune response, and fibrosis disrupts the stromal network of lymphoid tissues, resulting in impaired immune cell trafficking and function, as well as immunodeficiency. Developing an animal model for investigating the impact of HIV infection-induced lymphoid tissue fibrosis on immunodeficiency and immune cell impairment is critical for therapeutics development and clinical translation. Said model will enable in vivo mechanistic studies, thus complementing the well-established surrogate model of SIV infection-induced lymphoid tissue fibrosis in macaques. We developed a potentially novel human immune system-humanized mouse model by coengrafting autologous fetal thymus, spleen, and liver organoids under the kidney capsule, along with i.v. injection of autologous fetal liver-derived hematopoietic stem cells, thus termed the BM-liver-thymus-spleen (BLTS) humanized mouse model. BLTS humanized mouse model supports development of human immune cells and human lymphoid organoids (human thymus and spleen organoids). HIV infection in BLTS humanized mice results in progressive fibrosis in human lymphoid tissues, which was associated with immunodeficiency in the lymphoid tissues, and lymphoid tissue fibrosis persists during ART, thus recapitulating clinical outcomes.

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.

HIV Replication and Latency in a Humanized NSG Mouse Model during Suppressive Oral Combinational Antiretroviral Therapy

Although current combinatorial antiretroviral therapy (cART) is therapeutically effective in the majority of HIV patients, interruption of therapy can cause a rapid rebound in viremia, demonstrating the existence of a stable reservoir of latently infected cells. HIV latency is therefore considered a primary barrier to HIV eradication. Identifying, quantifying, and purging the HIV reservoir is crucial to effectively curing patients and relieving them from the lifelong requirement for therapy. Latently infected transformed cell models have been used to investigate HIV latency; however, these models cannot accurately represent the quiescent cellular environment of primary latently infected cells in vivo For this reason, in vivo humanized murine models have been developed for screening antiviral agents, identifying latently infected T cells, and establishing treatment approaches for HIV research. Such models include humanized bone marrow/liver/thymus mice and SCID-hu-thy/liv mice, which are repopulated with human immune cells and implanted human tissues through laborious surgical manipulation. However, no one has utilized the human hematopoietic stem cell-engrafted NOD/SCID/IL2rγ^null (NSG) model (hu-NSG) for this purpose. Therefore, in the present study, we used the HIV-infected hu-NSG mouse to recapitulate the key aspects of HIV infection and pathogenesis in vivo Moreover, we evaluated the ability of HIV-infected human cells isolated from HIV-infected hu-NSG mice on suppressive cART to act as a latent HIV reservoir. Our results demonstrate that the hu-NSG model is an effective surgery-free in vivo system in which to efficiently evaluate HIV replication, antiretroviral therapy, latency and persistence, and eradication interventions.IMPORTANCE HIV can establish a stably integrated, nonproductive state of infection at the level of individual cells, known as HIV latency, which is considered a primary barrier to curing HIV. A complete understanding of the establishment and role of HIV latency in vivo would greatly enhance attempts to develop novel HIV purging strategies. An ideal animal model for this purpose should be easy to work with, should have a shortened disease course so that efficacy testing can be completed in a reasonable time, and should have immune correlates that are easily translatable to humans. We therefore describe a novel application of the hematopoietic stem cell-transplanted humanized NSG model for dynamically testing antiretroviral treatment, supporting HIV infection, establishing HIV latency in vivo The hu-NSG model could be a facile alternative to humanized bone marrow/liver/thymus or SCID-hu-thy/liv mice in which laborious surgical manipulation and time-consuming human cell reconstitution is required.

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.

Protection Efficacy of C5A Against Vaginal and Rectal HIV Challenges in Humanized Mice

Introduction:

In the absence of a vaccine, there is an urgent need for the identification of effective agents that prevent HIV transmission in uninfected individuals. Non-vaccine Biomedical Prevention (nBP) methods, such as topical or systemic pre-exposure prophylaxis (PrEP), are promising strategies to slow down the spread of AIDS.

Methods:

In this study, we investigated the microbicidal efficacy of the viral membrane-disrupting amphipathic SWLRDIWDWICEVLSDFK peptide called C5A. We chose the bone marrow/liver/thymus (BLT) humanized mouse model as vaginal and rectal HIV transmission models.

Results:

We found that the topical administration of C5A offers complete protection against vaginal and rectal HIV challenges in humanized mice. After demonstrating that C5A blocks genital HIV transmission in humanized mice, we examined the molecular requirements for its microbicidal property. We found that the removal of four amino acids on either end of C5A does not diminish its microbicidal efficacy. However, the removal of four amino acids at both the ends, abolishes its capacity to prevent vaginal or rectal HIV transmission, suggesting that the length of the peptide is a critical parameter for the microbicidal activity of C5A. Moreover, we demonstrated that the amphipathicity of the helical peptide as well as its hydrophobic surface represents key factors for the microbicidal activity of C5A in humanized mice.

Conclusion:

With its noncellular cytotoxic activity, its property of neutralizing both HSV and HIV, and its unique mechanism of action that disrupts the stability of the viral membrane, C5A represents an attractive multipurpose microbicidal candidate to be combined with other anti-HIV agents including antiretrovirals.

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.

Raltegravir blocks the infectivity of red-fluorescent-protein (mCherry)-labeled HIV-1JR-FL in the setting of post-exposure prophylaxis in NOD/SCID/Jak3-/- mice transplanted with human PBMCs

Employing NOD/SCID/Jak3-/- mice transplanted with human PBMCs (hNOJ mice) and replication-competent, red-fluorescent-protein (mCherry; mC)-labeled HIV-1JR-FL (HIVmC), we examined whether early antiretroviral treatment blocked the establishment of HIV-1 infection. The use of hNOJ mice and HIVmC enabled us to visually locate infection foci and to examine the early dynamics of HIVmC infection without using a large amount of antiretroviral unlike in non-human primate models. Although when raltegravir (RAL) administration was begun 1 day after intraperitoneal (ip) inoculation of HIVmC, no plasma p24 or plasma HIV-1-RNA (pRNA) were detected in 10 of 12 hNOJ (hNOJmCRAL+) mice as assessed on the last day of the 14-day continuous twice-daily RAL administration, all 10 untreated hNOJmC (hNOJmCRAL-) mice became positive for p24 and pRNA and had significantly swollen lymph nodes in peritoneal cavity and abundant p24+/mC+/CD3+/CD4+ T cells and p24+/mC+/CD68+ monocytes/macrophages were identified in their omenta and mesenteric lymphoid tissues/lymph nodes upon necropsy of the mice on day 14. In 12 hNOJmCRAL+ mice, no significantly swollen lymph nodes were seen compared to hNOJmCRAL- mice; however, in the omentum of the 2 hNOJmCRAL+ mice that were positive for pRNA and in site RNA, mC+/p24+/CD3+/CD83+ cells were identified, suggesting that viral breakthrough occurred later in the observation period. The present data suggest that the use of hNOJ mouse model and HIVmC may shed light on the study of early-phase dynamics of HIV-1 infection and cellular events in post-exposure/pre-exposure prophylaxis.

Prevention of vaginal and rectal HIV transmission by antiretroviral combinations in humanized mice

With more than 7,000 new HIV infections daily worldwide, there is an urgent need for non-vaccine biomedical prevention (nBP) strategies that are safe, effective, and acceptable. Clinical trials have demonstrated that pre-exposure prophylaxis (PrEP) with antiretrovirals (ARVs) can be effective at preventing HIV infection. In contrast, other trials using the same ARVs failed to show consistent efficacy. Topical (vaginal and rectal) dosing is a promising regimen for HIV PrEP as it leads to low systematic drug exposure. A series of titration studies were carried out in bone marrow/liver/thymus (BLT) mice aimed at determining the adequate drug concentrations applied vaginally or rectally that offer protection against rectal or vaginal HIV challenge. The dose-response relationship of these agents was measured and showed that topical tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) can offer 100% protection against rectal or vaginal HIV challenges. From the challenge data, EC50 values of 4.6 μM for TDF and 0.6 μM for FTC for HIV vaginal administration and 6.1 μM TDF and 0.18 μM for FTC for rectal administration were obtained. These findings suggest that the BLT mouse model is highly suitable for studying the dose-response relationship in single and combination ARV studies of vaginal or rectal HIV exposure. Application of this sensitive HIV infection model to more complex binary and ternary ARV combinations, particularly where agents have different mechanisms of action, should allow selection of optimal ARV combinations to be advanced into pre-clinical and clinical development as nBP products.

Humanized mice: models for evaluating NeuroHIV and cure strategies

While the human immunodeficiency virus (HIV) epidemic was initially characterized by a high prevalence of severe and widespread neurological pathologies, the development of better treatments to suppress viremia over years and even decades has mitigated many of the severe neurological pathologies previously observed. Despite effective treatment, mild neurocognitive impairment and premature cognitive aging are observed in HIV-infected individuals, suggesting a changing but ongoing role of HIV infection in the central nervous system (CNS). Although current therapies are effective in suppressing viremia, they are not curative and patients must remain on life-long treatment or risk recrudescence of virus. Important for the development and evaluation of a cure for HIV will be animal models that recapitulate critical aspects of infection in vivo. In the following, we seek to summarize some of the recent developments in humanized mouse models and their usefulness in modeling HIV infection of the CNS and HIV cure strategies.

Early phase dynamics of traceable mCherry fluorescent protein-carrying HIV-1 infection in human peripheral blood mononuclear cells-transplanted NOD/SCID/Jak3-/- mice

We attempted to elucidate early-phase dynamics of HIV-1 infection using replication-competent, red-fluorescent-protein (mCherry)-labeled HIV-1JR-FL (HIVJR-FLmC) and NOD/SCID/Jak3-/- mice transplanted with Individual-A's human peripheral blood mononuclear cells (hPBMC)(hNOJ mice). On day 7 following HIVJR-FLmC inoculation, mCherry-signal-emitting infection foci were readily identified in the subserosa of 10 of 10 HIVJR-FLmC-inoculated hNOJ mice, although infection foci were not located without the mCherry signal in unlabeled HIV-1JR-FL-inoculated mice (n = 6). Even on day 14, infection foci were hardly located in the unlabeled HIV-1JR-FL-inoculated mice, while in all of 7 HIVJR-FLmC-inoculated hNOJ mice examined, mCherry-signal-emitting lymph nodes were easily identified, in which active viral replication was present. On day 14, a significantly larger number of mesenteric lymph nodes were seen in HIVJR-FLmC-exposed hNOJ mice than in HIVJR-FLmC-unexposed mice (P = 0.0025). The weights of mesenteric lymph nodes of those HIVJR-FLmC-exposed hNOJ mice were also greater than those of HIVJR-FLmC-unexposed mice (P = 0.0005). When hNOJ mice were inoculated with HIVJR-FLmC-exposed hPBMC from Individual-B, significantly greater viremia was seen than in cell-free HIVJR-FLmC-inoculated hNOJ mice as examined on day 7. In the lymph nodes of those mice inoculated with HIVJR-FLmC-exposed hPBMC from Individual-B, a substantial number of Individual-B's HIVJR-FLmC-infected cells were identified together with Individual-A's cells as examined on day 14. The present HIVJR-FLmC-infected mouse model represents the first system reported using traceable HIVJR-FLmC and human target cells, not using SIV or simian cells, which should be of utility in studies of early-phases of HIV-1 transmission and in evaluating the effects of potential agents for post-exposure and pre-exposure prophylaxis.

Multimodal analysis of drug transporter expression in gastrointestinal tissue

OBJECTIVES

Drug transporters affect antiretroviral therapy (ART) tissue disposition, but quantitative measures of drug transporter protein expression across preclinical species are not available. Our objective was to use proteomics to obtain absolute transporter concentrations and assess agreement with corresponding gene and immunometric protein data.

DESIGN

In order to make interspecies comparisons, two humanized mouse [hu-HSC-Rag (n = 41); bone marrow-liver-thymus (n = 13)] and one primate [rhesus macaque (nonhuman primate, n = 12)] models were dosed to steady state with combination ART. Ileum and rectum were collected at necropsy and snap frozen for analysis.

METHODS

Tissues were analyzed for gene (quantitative PCR) and protein [liquid chromatography-mass spectrometry (LC-MS) proteomics and western blot] expression and localization (immunohistochemistry) of ART efflux and uptake transporters. Drug concentrations were measured by LC-MS/MS. Multivariable regression was used to determine the ability of transporter data to predict tissue ART penetration.

RESULTS

Analytical methods did not agree, with different trends observed for gene and protein expression. For example, quantitative PCR analysis showed a two-fold increase in permeability glycoprotein expression in nonhuman primates versus mice; however, proteomics showed a 200-fold difference in the opposite direction. Proteomics results were supported by immunohistochemistry staining showing extensive efflux transporter localization on the luminal surface of these tissues. ART tissue concentration was variable between species, and multivariable regression showed poor predictive power of transporter data.

CONCLUSION

Lack of agreement between analytical techniques suggests that resources should be focused on generating downstream measures of protein expression to predict drug exposure. Taken together, these data inform the use of preclinical models for studying ART distribution and the design of targeted therapies for HIV eradication.

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.

HIV persistence in tissue macrophages of humanized myeloid-only mice during antiretroviral therapy

Despite years of fully suppressive antiretroviral therapy (ART), HIV persists in the host and is never eradicated. One major barrier to eradication is that multiple different cell types are infected that may individually contribute to HIV persistence. Tissue macrophages are critical contributors to HIV disease (1–3); however, their specific role in HIV persistence during long-term suppressive ART has not been established (4–6). Using humanized myeloid-only mice (MoM), we demonstrate that HIV infection of tissue macrophages is rapidly suppressed by ART, as determined by a rapid drop in plasma viral load and a dramatic drop in the levels of cell-associated viral RNA and DNA. No virus rebound was observed in the plasma of 67% of the ART treated animals at seven weeks post-ART interruption, and no replication competent virus was rescued from the tissue macrophages obtained from these animals. In contrast, in a subset of animals (~33%), a significantly delayed viral rebound was observed that is consistent with the establishment of persistent infection in tissue macrophages. These observations represent the first direct evidence of HIV persistence in tissue macrophages in vivo.

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.

Predicting HIV Pre-exposure Prophylaxis Efficacy for Women using a Preclinical Pharmacokinetic-Pharmacodynamic In Vivo Model

The efficacy of HIV pre-exposure prophylaxis (PrEP) relies on adherence and may also depend on the route of HIV acquisition. Clinical studies of systemic tenofovir disoproxil fumarate (TDF) PrEP revealed reduced efficacy in women compared to men with similar degrees of adherence. To select the most effective PrEP strategies, preclinical studies are critically needed to establish correlations between drug concentrations (pharmacokinetics [PK]) and protective efficacy (pharmacodynamics [PD]). We utilized an in vivo preclinical model to perform a PK-PD analysis of systemic TDF PrEP for vaginal HIV acquisition. TDF PrEP prevented vaginal HIV acquisition in a dose-dependent manner. PK-PD modeling of tenofovir (TFV) in plasma, female reproductive tract tissue, cervicovaginal lavage fluid and its intracellular metabolite (TFV diphosphate) revealed that TDF PrEP efficacy was best described by plasma TFV levels. When administered at 50 mg/kg, TDF achieved plasma TFV concentrations (370 ng/ml) that closely mimicked those observed in humans and demonstrated the same risk reduction (70%) previously attained in women with high adherence. This PK-PD model mimics the human condition and can be applied to other PrEP approaches and routes of HIV acquisition, accelerating clinical implementation of the most efficacious PrEP strategies.

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.

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.

A long-acting formulation of the integrase inhibitor raltegravir protects humanized BLT mice from repeated high-dose vaginal HIV challenges

OBJECTIVES

Pre-exposure prophylaxis (PrEP) using antiretroviral drugs (ARVs) has been shown to reduce HIV transmission in people at high risk of HIV infection. Adherence to PrEP strongly correlates with the level of HIV protection. Long-acting injectable ARVs provide sustained systemic drug exposures over many weeks and can improve adherence due to infrequent parenteral administration. Here, we evaluated a new long-acting formulation of raltegravir for prevention of vaginal HIV transmission.

METHODS

Long-acting raltegravir was administered subcutaneously to BALB/c, NSG (NOD-scid-gamma) and humanized BLT (bone marrow-liver-thymus) mice and rhesus macaques. Raltegravir concentration in peripheral blood and tissue was analysed. Suppression of HIV replication was assessed in infected BLT mice. Two high-dose HIV vaginal challenges were used to evaluate protection from HIV transmission in BLT mice.

RESULTS

Two weeks after a single subcutaneous injection of long-acting raltegravir in BLT mice (7.5 mg) and rhesus macaques (160 mg), the plasma concentration of raltegravir was comparable to 400 mg orally, twice daily in humans. Serum collected from mice 3 weeks post-administration of long-acting raltegravir efficiently blocked HIV infection of TZM-bl indicator cells in vitro. Administration of long-acting raltegravir suppressed viral RNA in plasma and cervico-vaginal fluids of infected BLT mice, demonstrating penetration of active raltegravir into the female reproductive tract. Using transmitted/founder HIV we observed that BLT mice administered a single subcutaneous dose of long-acting raltegravir were protected from two high-dose HIV vaginal challenges 1 week and 4 weeks after drug administration.

CONCLUSIONS

These preclinical results demonstrated the efficacy of long-acting raltegravir in preventing vaginal HIV transmission.

Nonhuman Primates and Humanized Mice for Studies of HIV-1 Integrase Inhibitors: A Review

Since the discovery of the first inhibitors of HIV replication, drug resistance has been a major problem in HIV therapy due in part to the high mutation rate of HIV. Therefore, the development of a predictive animal model is important to identify impending resistance mutations and to possibly inform treatment decisions. Significant advances have been made possible through use of nonhuman primates infected by SIV, SHIV, and simian-tropic HIV-1 (stHIV-1), and use of humanized mouse models of HIV-1 infections. In this review, we describe some of the findings from animal models used for the preclinical testing of integrase strand transfer inhibitors. These models have led to important findings about the potential role of integrase strand transfer inhibitors in both the prevention and treatment of HIV-1 infection.

APOBEC3G and APOBEC3F Act in Concert To Extinguish HIV-1 Replication

The multifunctional HIV-1 accessory protein Vif counters the antiviral activities of APOBEC3G (A3G) and APOBEC3F (A3F), and some Vifs counter stable alleles of APOBEC3H (A3H). Studies in humanized mice have shown that HIV-1 lacking Vif expression is not viable. Here, we look at the relative contributions of the three APOBEC3s to viral extinction. Inoculation of bone marrow/liver/thymus (BLT) mice with CCR5-tropic HIV-1JRCSF(JRCSF) expressing a vif gene inactive for A3G but not A3F degradation activity (JRCSFvifH42/43D) displayed either no or delayed replication. JRCSF expressing a vif gene mutated to inactivate A3F degradation but not A3G degradation (JRCSFvifW79S) always replicated to high viral loads with variable delays. JRCSF with vif mutated to lack both A3G and A3F degradation activities (JRCSFvifH42/43DW79S) failed to replicate, mimicking JRCSF without Vif expression (JRCSFΔvif). JRCSF and JRCSFvifH42/43D, but not JRCSFvifW79S or JRCSFvifH42/43DW79S, degraded APOBEC3D. With one exception, JRCSFs expressing mutant Vifs that replicated acquired enforced vif mutations. These mutations partially restored A3G or A3F degradation activity and fully replaced JRCSFvifH42/43D or JRCSFvifW79S by 10 weeks. Surprisingly, induced mutations temporally lagged behind high levels of virus in blood. In the exceptional case, JRCSFvifH42/43D replicated after a prolonged delay with no mutations in vif but instead a V27I mutation in the RNase H coding sequence. JRCSFvifH42/43D infections exhibited massive GG/AG mutations in pol viral DNA, but in viral RNA, there were no fixed mutations in the Gag or reverse transcriptase coding sequence. A3H did not contribute to viral extinction but, in combination with A3F, could delay JRCSF replication. A3H was also found to hypermutate viral DNA.

IMPORTANCE

Vif degradation of A3G and A3F enhances viral fitness, as virus with even a partially restored capacity for degradation outgrows JRCSFvifH42/43D and JRCSFvifW79S. Unexpectedly, fixation of mutations that replaced H42/43D or W79S in viral RNA lagged behind the appearance of high viral loads. In one exceptional JRCSFvifH42/43D infection, vif was unchanged but replication proceeded after a long delay. These results suggest that Vif binds and inhibits the non-cytosine deaminase activities of intact A3G and intact A3F, allowing JRCSFvifH42/43D and JRCSFvifW79S to replicate with reduced fitness. Subsequently, enhanced Vif function is acquired by enforced mutations. In infected cells, JRCSFΔvif and JRCSFvifH42/43DW79S are exposed to active A3F and A3G and fail to replicate. JRCSFvifH42/43D Vif degrades A3F and, in some cases, overcomes A3G mutagenic activity to replicate. Vif may have evolved to inhibit A3F and A3G by stoichiometric binding and subsequently acquired the ability to target these proteins to proteasomes.

Macrophages sustain HIV replication in vivo independently of T cells

Macrophages have long been considered to contribute to HIV infection of the CNS; however, a recent study has contradicted this early work and suggests that myeloid cells are not an in vivo source of virus production. Here, we addressed the role of macrophages in HIV infection by first analyzing monocytes isolated from viremic patients and patients undergoing antiretroviral treatment. We were unable to find viral DNA or viral outgrowth in monocytes isolated from peripheral blood. To determine whether tissue macrophages are productively infected, we used 3 different but complementary humanized mouse models. Two of these models (bone marrow/liver/thymus [BLT] mice and T cell-only mice [ToM]) have been previously described, and the third model was generated by reconstituting immunodeficient mice with human CD34+ hematopoietic stem cells that were devoid of human T cells (myeloid-only mice [MoM]) to specifically evaluate HIV replication in this population. Using MoM, we demonstrated that macrophages can sustain HIV replication in the absence of T cells; HIV-infected macrophages are distributed in various tissues including the brain; replication-competent virus can be rescued ex vivo from infected macrophages; and infected macrophages can establish de novo infection. Together, these results demonstrate that macrophages represent a genuine target for HIV infection in vivo that can sustain and transmit infection.

CD19xCD3 DART protein mediates human B-cell depletion in vivo in humanized BLT mice

Novel therapeutic strategies are needed for the treatment of hematologic malignancies; and bispecific antibody-derived molecules, such as dual-affinity re-targeting (DART) proteins, are being developed to redirect T cells to kill target cells expressing tumor or viral antigens. Here we present our findings of specific and systemic human B-cell depletion by a CD19xCD3 DART protein in humanized BLT mice. Administration of the CD19xCD3 DART protein resulted in a dramatic sustained depletion of human CD19⁺ B cells from the peripheral blood, as well as a dramatic systemic reduction of human CD19⁺ B-cell levels in all tissues (bone marrow, spleen, liver, lung) analyzed. When human CD8⁺ T cells were depleted from the mice, no significant B-cell depletion was observed in response to CD19xCD3 DART protein treatment, confirming that human CD8⁺ T cells are the primary effector cells in this in vivo model. These studies validate the use of BLT humanized mice for the in vivo evaluation and preclinical development of bispecific molecules that redirect human T cells to selectively deplete target cells.

ART influences HIV persistence in the female reproductive tract and cervicovaginal secretions

The recently completed HIV prevention trials network study 052 is a landmark collaboration demonstrating that HIV transmission in discordant couples can be dramatically reduced by treating the infected individual with antiretroviral therapy (ART). However, the cellular and virological events that occur in the female reproductive tract (FRT) during ART that result in such a drastic decrease in transmission were not studied and remain unknown. Here, we implemented an in vivo model of ART in BM/liver/thymus (BLT) humanized mice in order to better understand the ability of ART to prevent secondary HIV transmission. We demonstrated that the entire FRT of BLT mice is reconstituted with human CD4+ cells that are shed into cervicovaginal secretions (CVS). A high percentage of the CD4+ T cells in the FRT and CVS expressed CCR5 and therefore are potential HIV target cells. Infection with HIV increased the numbers of CD4+ and CD8+ T cells in CVS of BLT mice. Furthermore, HIV was present in CVS during infection. Finally, we evaluated the effect of ART on HIV levels in the FRT and CVS and demonstrated that ART can efficiently suppress cell-free HIV-RNA in CVS, despite residual levels of HIV-RNA+ cells in both the FRT and CVS.