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

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.

TZM-gfp cells: a tractable fluorescent tool for analysis of rare and early HIV-1 infection

Here we describe TZM-gfp, a novel HIV-1 reporter cell derived from the same parental clone JC.53, used previously to generate the widely-utilized indicator cell line TZM-bl. We re-engineered JC.53 cells to express GFP under regulation of HIV Tat and Rev. We characterize the new reporter cell line to show that TZM-gfp cells are equally susceptible to HIV infection, exhibit minimal background signal, and can report HIV infection in rare cells from a bulk population of experimentally-infected human monocyte-derived macrophages. We demonstrate the utility and sensitivity of the cells in detection of even a single HIV-positive macrophage by fluorescence-assisted correlative electron microscopy, using the GFP signal to guide imaging of HIV virions in primary co-culture. Finally, we used TZM-gfp cells for viral capture during co-culture with human peripheral blood mononuclear cells, showing that TZM-gfp can support outgrowth and analyses of patient-derived primary HIV-1 isolates.

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.

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.

New Frontiers in Measuring and Characterizing the HIV Reservoir

A cure for HIV infection remains elusive due to the persistence of replication-competent HIV proviral DNA during suppressive antiretroviral therapy (ART). With the exception of rare elite or post-treatment controllers of viremia, withdrawal of ART invariably results in the rebound of viremia and progression of HIV disease. A thorough understanding of the reservoir is necessary to develop new strategies in order to reduce or eliminate the reservoir. However, there is significant heterogeneity in the sequence composition, genomic location, stability, and expression of the HIV reservoir both within and across individuals, and a majority of proviral sequences are replication-defective. These factors, and the low frequency of persistently infected cells in individuals on suppressive ART, make understanding the reservoir and its response to experimental reservoir reduction interventions challenging. Here, we review the characteristics of the HIV reservoir, state-of-the-art assays to measure and characterize the reservoir, and how these assays can be applied to accurately detect reductions in reservoir during efforts to develop a cure for HIV infection. In particular, we highlight recent advances in the development of direct measures of provirus, including intact proviral DNA assays and full-length HIV DNA sequencing with integration site analysis. We also focus on novel techniques to quantitate persistent and inducible HIV, including RNA sequencing and RNA/gag protein staining techniques, as well as modified viral outgrowth methods that seek to improve upon throughput, sensitivity and dynamic range.

Relationship between intact HIV-1 proviruses in circulating CD4+ T cells and rebound viruses emerging during treatment interruption

Combination antiretroviral therapy controls but does not cure HIV-1 infection because a small fraction of cells harbor latent viruses that can produce rebound viremia when therapy is interrupted. The circulating latent virus reservoir has been documented by a variety of methods, most prominently by viral outgrowth assays (VOAs) in which CD4⁺ T cells are activated to produce virus in vitro, or more recently by amplifying proviral near full-length (NFL) sequences from DNA. Analysis of samples obtained in clinical studies in which individuals underwent analytical treatment interruption (ATI), showed little if any overlap between circulating latent viruses obtained from outgrowth cultures and rebound viruses from plasma. To determine whether intact proviruses amplified from DNA are more closely related to rebound viruses than those obtained from VOAs, we assayed 12 individuals who underwent ATI after infusion of a combination of two monoclonal anti-HIV-1 antibodies. A total of 435 intact proviruses obtained by NFL sequencing were compared with 650 latent viruses from VOAs and 246 plasma rebound viruses. Although, intact NFL and outgrowth culture sequences showed similar levels of stability and diversity with 39% overlap, the size of the reservoir estimated from NFL sequencing was larger than and did not correlate with VOAs. Finally, intact proviruses documented by NFL sequencing showed no sequence overlap with rebound viruses; however, they appear to contribute to recombinant viruses found in plasma during rebound.

Tackling HIV Persistence: Pharmacological versus CRISPR-Based Shock Strategies

Jan Svoboda studied aspects of viral latency, in particular with respect to disease induction by avian RNA tumor viruses, which were later renamed as part of the extended retrovirus family. The course of retroviral pathogenesis is intrinsically linked to their unique property of integrating the DNA copy of the retroviral genome into that of the host cell, thus forming the provirus. Retroviral latency has recently become of major clinical interest to allow a better understanding of why we can effectively block the human immunodeficiency virus type 1 (HIV-1) in infected individuals with antiviral drugs, yet never reach a cure. We will discuss HIV-1 latency and its direct consequence—the formation of long-lasting HIV-1 reservoirs. We next focus on one of the most explored strategies in tackling HIV-1 reservoirs—the “shock and kill” strategy—which describes the broadly explored pharmacological way of kicking the latent provirus, with subsequent killing of the virus-producing cell by the immune system. We furthermore present how the clustered regularly interspaced palindromic repeats (CRISPR) and associated protein (Cas) system can be harnessed to reach the same objective by reactivating HIV-1 gene expression from latency. We will review the benefits and drawbacks of these different cure strategies.