Reactivation of HIV-1 proviruses in immune-compromised mice engrafted with human VOA-negative CD4+ T cells

Bispecific antibodies promote natural killer cell-mediated elimination of HIV-1 reservoir cells

The persistence of CD4+ T cells carrying latent human immunodeficiency virus-1 (HIV-1) proviruses is the main barrier to a cure. New therapeutics to enhance HIV-1-specific immune responses and clear infected cells will probably be necessary to achieve reduction of the latent reservoir. In the present study, we report two single-chain diabodies (scDbs) that target the HIV-1 envelope protein (Env) and the human type III Fcγ receptor (CD16). We show that the scDbs promoted robust and HIV-1-specific natural killer (NK) cell activation and NK cell-mediated lysis of infected cells. Cocultures of CD4+ T cells from people with HIV-1 on antiretroviral therapy (ART) with autologous NK cells and the scDbs resulted in marked elimination of reservoir cells that was dependent on latency reversal. Treatment of human interleukin-15 transgenic NSG mice with one of the scDbs after ART initiation enhanced NK cell activity and reduced reservoir size. Thus, HIV-1-specific scDbs merit further evaluation as potential therapeutics for clearance of the latent reservoir.

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.

Amplification of Replication Competent HIV-1 by Adoptive Transfer of Human Cells From Infected Humanized Mice

Detection of latent human immunodeficiency virus type 1 (HIV-1) in "putative" infectious reservoirs is required for determining treatment efficiency and for viral elimination strategies. Such tests require induction of replication competent provirus and quantitative testing of viral load for validation. Recently, humanized mice were employed in the development of such tests by employing a murine viral outgrowth assay (mVOA). Here blood cells were recovered from virus infected antiretroviral therapy suppressed patients. These cells were adoptively transferred to uninfected humanized mice where replication competent virus was recovered. Prior reports supported the notion that an mVOA assay provides greater sensitivity than cell culture-based quantitative VOA tests for detection of latent virus. In the current study, the mVOA assays was adapted using donor human hematopoietic stem cells-reconstituted mice to affirm research into HIV-1 elimination. We simulated an antiretroviral therapy (ART)-treated virus-infected human by maintaining the infected humanized mice under suppressive treatment. This was operative prior to human cell adoptive transfers. Replication-competent HIV-1 was easily detected in recipient animals from donors with undetectable virus in plasma. Moreover, when the assay was used to investigate viral presence in tissue reservoirs, quantitative endpoints were determined in "putative" viral reservoirs not possible in human sample analyses. We conclude that adoptive transfer of cells between humanized mice is a sensitive and specific assay system for detection of replication competent latent HIV-1.

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

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

Recent Updates on Mouse Models for Human Immunodeficiency, Influenza, and Dengue Viral Infections

Well-developed mouse models are important for understanding the pathogenesis and progression of immunological response to viral infections in humans. Moreover, to test vaccines, anti-viral drugs and therapeutic agents, mouse models are fundamental for preclinical investigations. Human viruses, however, seldom infect mice due to differences in the cellular receptors used by the viruses for entry, as well as in the innate immune responses in mice and humans. In other words, a species barrier exists when using mouse models for investigating human viral infections. Developing transgenic (Tg) mice models expressing the human genes coding for viral entry receptors and knock-out (KO) mice models devoid of components involved in the innate immune response have, to some extent, overcome this barrier. Humanized mouse models are a third approach, developed by engrafting functional human cells and tissues into immunodeficient mice. They are becoming indispensable for analyzing human viral diseases since they nearly recapitulate the human disease. These mouse models also serve to test the efficacy of vaccines and antiviral agents. This review provides an update on the Tg, KO, and humanized mouse models that are used in studies investigating the pathogenesis of three important human-specific viruses, namely human immunodeficiency (HIV) virus 1, influenza, and dengue.

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.

Innovations in the quantitative virus outgrowth assay and its use in clinical trials

A robust measure of the size of the latent HIV reservoir is essential to quantifying the effect of interventions designed to deplete the pool of reactivatable, replication competent proviruses. In addition to the ability to measure a biologically relevant parameter, any assay designed to be used in a clinical trial needs to be reproducible and scalable. The need to quantify the number of resting CD4+ T cells capable of releasing infectious virus has led to the development of the quantitative viral outgrowth assay (VOA). The assay as originally described has a number of features that limit its scalability for use in clinical trials; however recent developments reducing the time and manpower requirements of the assay, while importantly improving reproducibility mean that it is becoming much more practical for it to enter into more widespread use. This review describes the background to VOA development and the practical issues that they present in utilising them in clinical trials. It describes the innovations that have made their usage more practical and the limitations that still exist.

The mouse viral outgrowth assay: avatars for the detection of HIV-1 reservoirs

Sensitive assays are needed for the detection of residual viral reservoirs in HIV-1-infected subjects on suppressive combination antiretroviral therapy regimens to determine whether eradication strategies are effective. Mouse viral outgrowth assays have recently been developed and have the potential to be more sensitive than traditional in vitro quantitative viral outgrowth assays. In this article we describe these assays and review several studies that have used them to measure the latent reservoir.

An SIV/macaque model targeted to study HIV-associated neurocognitive disorders

Simian immunodeficiency virus (SIV) infection of pigtailed macaques is a highly representative and well-characterized animal model for HIV neuropathogenesis studies that provides an excellent opportunity to study and develop prognostic markers of HIV-associated neurocognitive disorders (HAND) for HIV-infected individuals. SIV studies can be performed in a controlled setting that enhances reproducibility and offers high-translational value. Similar to observations in HIV-infected patients receiving antiretroviral therapy (ART), ongoing neurodegeneration and inflammation are present in SIV-infected pigtailed macaques treated with suppressive ART. By developing quantitative viral outgrowth assays that measure both CD4+ T cells and macrophages harboring replication competent SIV as well as a highly sensitive mouse-based viral outgrowth assay, we have positioned the SIV/pigtailed macaque model to advance our understanding of latent cellular reservoirs, including potential CNS reservoirs, to promote HIV cure. In addition to contributing to our understanding of the pathogenesis of HAND, the SIV/pigtailed macaque model also provides an excellent opportunity to test innovative approaches to eliminate the latent HIV reservoir in the brain.

Humanized mouse models of latent HIV infection

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

A humanized mouse-based HIV-1 viral outgrowth assay with higher sensitivity than in vitro qVOA in detecting latently infected cells from individuals on ART with undetectable viral loads

Assays that can verify full viral eradication are essential in the context of achieving a cure for HIV/AIDS. In vitro quantitative viral out growth assays (qVOA) are currently the gold standard for measuring latent HIV-1 but these assays often fail to detect very low levels of replication-competent virus. Here we investigated an alternative in vivo approach for sensitive viral detection using humanized mice (hmVOA). Peripheral blood CD4+ T cell samples from HIV subjects on stable ART with undetectable viral loads by RT-PCR were first assayed by in vitro qVOA. Corresponding patient samples in which no virus was detected by qVOA were injected into humanized mice to allow viral outgrowth. Of the five qVOA virus negative samples, four gave positive viral outgrowth in the hmVOA assay suggesting that it is more sensitive in detecting latent HIV-1.