Novel pathway for induction of latent virus from resting CD4(+) T cells in the simian immunodeficiency virus/macaque model of human immunodeficiency virus type 1 latency

Suppression of viral rebound by a Rev-dependent lentiviral particle in SIV-infected rhesus macaques

Persistence of human immunodeficiency virus (HIV) reservoirs prevents viral eradication, and consequently HIV-infected patients require lifetime treatment with antiretroviral therapy (ART) [1-5]. Currently, there are no effective therapeutics to prevent HIV rebound upon ART cessation. Here we describe an HIV/SIV Rev-dependent lentiviral particle that can be administered to inhibit viral rebound [6-9]. Using simian immunodeficiency virus (SIV)-infected rhesus macaques as a model, we demonstrate that the administration of pre-assembled SIV Rev-dependent lentiviral particles into SIVmac239-infected Indian rhesus macaques can lead to reduction of viral rebound upon ART termination. One of the injected animals, KC50, controlled plasma and CNS viremia to an undetectable level most of the time for over two years after ART termination. Surprisingly, detailed molecular and immunological characterization revealed that viremia control was concomitant with the induction of neutralizing antibodies (nAbs) following the administration of the Rev-dependent vectors. This study emphasizes the importance of neutralizing antibodies (nAbs) for viremia control [10-15], and also provides proof of concept that the Rev-dependent vector can be used to target viral reservoirs, including the CNS reservoirs, in vivo. However, future large-scale in vivo studies are needed to understand the potential mechanisms of viremia control induced by the Rev-dependent vector.

So Pathogenic or So What?-A Brief Overview of SIV Pathogenesis with an Emphasis on Cure Research

HIV infection requires lifelong antiretroviral therapy (ART) to control disease progression. Although ART has greatly extended the life expectancy of persons living with HIV (PWH), PWH nonetheless suffer from an increase in AIDS-related and non-AIDS related comorbidities resulting from HIV pathogenesis. Thus, an HIV cure is imperative to improve the quality of life of PWH. In this review, we discuss the origins of various SIV strains utilized in cure and comorbidity research as well as their respective animal species used. We briefly detail the life cycle of HIV and describe the pathogenesis of HIV/SIV and the integral role of chronic immune activation and inflammation on disease progression and comorbidities, with comparisons between pathogenic infections and nonpathogenic infections that occur in natural hosts of SIVs. We further discuss the various HIV cure strategies being explored with an emphasis on immunological therapies and "shock and kill".

Quantitative Viral Outgrowth Assay to Measure the Functional SIV Reservoir in Myeloid Cells

The role of CD4+ T cells in HIV infection and the latent reservoir, that is, latently infected cells that harbor replication competent virus, has been rigorously assessed. We have previously reported a quantitative viral outgrowth assay (QVOA) for SIV that demonstrated the frequency of latently infected CD4+ T cells is approximately 1 in a million cells, similar to that of HIV infected individuals on ART. However, the frequency of productively infected monocytes in blood and macrophages in tissues has not been similarly studied. Myeloid cells are infected during acute HIV and SIV infection; however, unlike lymphocytes, they are resistant to the cytopathic effects of the virus. Moreover, tissue-resident macrophages have the ability to self-renew and persist in the body for months to years. Thus, tissue macrophages, once infected, have the characteristics of a stable viral reservoir. A better understanding of the number of productively infected macrophages is critical to understanding the role of infected myeloid cells as a viral reservoir. In order to assess the functional latent reservoir. we have developed specific QVOAs for monocytes in blood, and macrophages in spleen, BAL and brain, which are described in detail in this chapter.

Myeloid and CD4 T Cells Comprise the Latent Reservoir in Antiretroviral Therapy-Suppressed SIVmac251-Infected Macaques

Human immunodeficiency virus (HIV) eradication or long-term suppression in the absence of antiretroviral therapy (ART) requires an understanding of all viral reservoirs that could contribute to viral rebound after ART interruption. CD4 T cells (CD4s) are recognized as the predominant reservoir in HIV type 1 (HIV-1)-infected individuals. However, macrophages are also infected by HIV-1 and simian immunodeficiency virus (SIV) during acute infection and may persist throughout ART, contributing to the size of the latent reservoir. We sought to determine whether tissue macrophages contribute to the SIVmac251 reservoir in suppressed macaques. Using cell-specific quantitative viral outgrowth assays (CD4-QVOA and MΦ-QVOA), we measured functional latent reservoirs in CD4s and macrophages in ART-suppressed SIVmac251-infected macaques. Spleen, lung, and brain in all suppressed animals contained latently infected macrophages, undetectable or low-level SIV RNA, and detectable SIV DNA. Silent viral genomes with potential for reactivation and viral spread were also identified in blood monocytes, although these cells might not be considered reservoirs due to their short life span. Additionally, virus produced in the MΦ-QVOA was capable of infecting healthy activated CD4s. Our results strongly suggest that functional latent reservoirs in CD4s and macrophages can contribute to viral rebound and reestablishment of productive infection after ART interruption. These findings should be considered in the design and implementation of future HIV cure strategies.IMPORTANCE This study provides further evidence that the latent reservoir is comprised of both CD4+ T cells and myeloid cells. The data presented here suggest that CD4+ T cells and macrophages found throughout tissues in the body can contain replication-competent SIV and contribute to rebound of the virus after treatment interruption. Additionally, we have shown that monocytes in blood contain latent virus and, though not considered a reservoir themselves due to their short life span, could contribute to the size of the latent reservoir upon entering the tissue and differentiating into long-lived macrophages. These new insights into the size and location of the SIV reservoir using a model that is heavily studied in the HIV field could have great implications for HIV-infected individuals and should be taken into consideration with the development of future HIV cure strategies.

Persistent Viral Reservoirs in Lymphoid Tissues in SIV-Infected Rhesus Macaques of Chinese-Origin on Suppressive Antiretroviral Therapy

Understanding HIV latent reservoirs in tissues is essential for the development of new strategies targeting these sites for eradication. Here, we assessed the size of latent reservoirs and the source of residual viruses in multiple lymphoid tissues of SIV-infected and fully suppressed rhesus macaques of Chinese-origin (cRMs). Eight cRMs were infected with SIVmac251 and treated with tenofovir and emtricitabine daily for 24 weeks initiated 4 weeks post-infection. Four of the eight animals reached sustained full viral suppression with undetectable viremia. The levels of cell-associated SIV DNA varied in peripheral blood mononuclear cells (PBMCs) and multiple lymphoid tissues, but with higher levels in the mesenteric lymph nodes (MesLNs). The levels of cell-associated SIV RNA also varied in different tissues. The higher frequency of viral RNA detection in the MesLNs was also observed by in situ hybridization. Consistently, the infection unit per million cells (IUPM) in the MesLNs was higher than in PBMCs and other tested lymphoid tissues by quantitative viral outgrowth assay (QVOA). Furthermore, env gp120 from tissue SIV RNA was amplified by single genome amplification. Phylogenetic analysis revealed diverse variants from tissues parallel to the viral inoculum in all viral suppressed animals. These results demonstrate that the latency and viral reservoirs in the lymphoid tissues still exist in aviremic macaques under full suppressive therapy. Moreover, the size of viral latent reservoirs differs in various lymphoid tissues with a relatively larger size in the MesLNs.

Monkey Models and HIV Vaccine Research

Since the discovery of acquired immunodeficiency syndrome (AIDS) in 1981, it has been extremely difficult to develop an effective vaccine or a therapeutic cure despite over 36 years of global efforts. One of the major reasons is due to the lack of an immune-competent animal model that supports live human immunodeficiency virus (HIV) infection and disease progression such that vaccine-induced correlates of protection and efficacy can be determined clearly before human trials. Nevertheless, rhesus macaques infected with simian immunodeficiency virus (SIV) and chimeric simian human immunodeficiency virus (SHIV) have served as invaluable models not only for understanding AIDS pathogenesis but also for studying HIV vaccine and cure. In this chapter, therefore, we summarize major scientific evidence generated in these models since the beginning of the AIDS pandemic. Hopefully, the accumulated knowledge and lessons contributed by thousands of scientists will be useful in promoting the search of an ultimate solution to end HIV/AIDS.

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.

Insulin Treatment Prevents Neuroinflammation and Neuronal Injury with Restored Neurobehavioral Function in Models of HIV/AIDS Neurodegeneration

HIV-1 infection of the brain causes the neurodegenerative syndrome HIV-associated neurocognitive disorders (HAND), for which there is no specific treatment. Herein, we investigated the actions of insulin using ex vivo and in vivo models of HAND. Increased neuroinflammatory gene expression was observed in brains from patients with HIV/AIDS. The insulin receptor was detected on both neurons and glia, but its expression was unaffected by HIV-1 infection. Insulin treatment of HIV-infected primary human microglia suppressed supernatant HIV-1 p24 levels, reduced CXCL10 and IL-6 transcript levels, and induced peroxisome proliferator-activated receptor gamma (PPAR-γ) expression. Insulin treatment of primary human neurons prevented HIV-1 Vpr-mediated cell process retraction and death. In feline immunodeficiency virus (FIV) infected cats, daily intranasal insulin treatment (20.0 IU/200 μl for 6 weeks) reduced CXCL10, IL-6, and FIV RNA detection in brain, although PPAR-γ in glia was increased compared with PBS-treated FIV⁺ control animals. These molecular changes were accompanied by diminished glial activation in cerebral cortex and white matter of insulin-treated FIV⁺ animals, with associated preservation of cortical neurons. Neuronal counts in parietal cortex, striatum, and hippocampus were higher in the FIV⁺/insulin-treated group compared with the FIV⁺/PBS-treated group. Moreover, intranasal insulin treatment improved neurobehavioral performance, including both memory and motor functions, in FIV⁺ animals. Therefore, insulin exerted ex vivo and in vivo antiviral, anti-inflammatory, and neuroprotective effects in models of HAND, representing a new therapeutic option for patients with inflammatory or infectious neurodegenerative disorders including HAND.

SIGNIFICANCE STATEMENT

HIV-associated neurocognitive disorders (HAND) represent a spectrum disorder of neurocognitive dysfunctions resulting from HIV-1 infection. Although the exact mechanisms causing HAND are unknown, productive HIV-1 infection in the brain with associated neuroinflammation is a potential pathogenic mechanism resulting in neuronal damage and death. We report that, in HIV-infected microglia cultures, insulin treatment led to reduced viral replication and inflammatory gene expression. In addition, intranasal insulin treatment of experimentally feline immunodeficiency virus-infected animals resulted in improved motor and memory performances. We show that insulin restored expression of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-γ), which is suppressed by HIV-1 replication. Our findings indicate a unique function for insulin in improving neurological outcomes in lentiviral infections, implicating insulin as a therapeutic intervention for HAND.

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.

Nonhuman Primate Models for Studies of AIDS Virus Persistence During Suppressive Combination Antiretroviral Therapy

Nonhuman primate (NHP) models of AIDS represent a potentially powerful component of the effort to understand in vivo sources of AIDS virus that persist in the setting of suppressive combination antiretroviral therapy (cART) and to develop and evaluate novel strategies for more definitive treatment of HIV infection (i.e., viral eradication "cure", or sustained off-cART remission). Multiple different NHP models are available, each characterized by a particular NHP species, infecting virus, and cART regimen, and each with a distinct capacity to recapitulate different aspects of HIV infection. Given these different biological characteristics, and their associated strengths and limitations, different models may be preferred to address different questions pertaining to virus persistence and cure research, or to evaluate different candidate intervention approaches. Recent developments in improved cART regimens for use in NHPs, new viruses, a wider array of sensitive virologic assay approaches, and a better understanding of pathogenesis should allow even greater contributions from NHP models to this important area of HIV research in the future.

Quantitation of Productively Infected Monocytes and Macrophages of Simian Immunodeficiency Virus-Infected Macaques

Despite the success of combined antiretroviral therapy (ART), human immunodeficiency virus (HIV) infection remains a lifelong infection because of latent viral reservoirs in infected patients. The contribution of CD4⁺ T cells to infection and disease progression has been extensively studied. However, during early HIV infection, macrophages in brain and other tissues are infected and contribute to tissue-specific diseases, such as encephalitis and dementia in brain and pneumonia in lung. The extent of infection of monocytes and macrophages has not been rigorously assessed with assays comparable to those used to study infection of CD4⁺ T cells and to evaluate the number of CD4⁺ T cells that harbor infectious viral genomes. To assess the contribution of productively infected monocytes and macrophages to HIV- and simian immunodeficiency virus (SIV)-infected cells in vivo, we developed a quantitative virus outgrowth assay (QVOA) based on similar assays used to quantitate CD4⁺ T cell latent reservoirs in HIV- and SIV-infected individuals in whom the infection is suppressed by ART. Myeloid cells expressing CD11b were serially diluted and cocultured with susceptible cells to amplify virus. T cell receptor β RNA was measured as a control to assess the potential contribution of CD4⁺ T cells in the assay. Virus production in the supernatant was quantitated by quantitative reverse transcription-PCR. Productively infected myeloid cells were detected in blood, bronchoalveolar lavage fluid, lungs, spleen, and brain, demonstrating that these cells persist throughout SIV infection and have the potential to contribute to the viral reservoir during ART.

IMPORTANCE Infection of CD4⁺ T cells and their role as latent reservoirs have been rigorously assessed; however, the frequency of productively infected monocytes and macrophages in vivo has not been similarly studied. Myeloid cells, unlike lymphocytes, are resistant to the cytopathic effects of HIV. Moreover, tissue-resident macrophages have the ability to self-renew and persist in the body for months to years. Thus, tissue macrophages, once infected, have the characteristics of a potentially stable viral reservoir. A better understanding of the number of productively infected macrophages is crucial to further evaluate the role of infected myeloid cells as a potential viral reservoir. In the study described here we compared the frequency of productively infected CD4⁺ T cells and macrophages in an SIV-infected macaque model. We developed a critical assay that will allow us to quantitate myeloid cells containing viral genomes that lead to productive infection in SIV-infected macaques and assess the role of macrophages as potential reservoirs.

Infectious SIV resides in adipose tissue and induces metabolic defects in chronically infected rhesus macaques

Background

HIV reservoirs pose major challenges to viral eradication. The main cellular reservoirs include CD4 T cells and macrophages, whereas anatomic reservoirs are thought to be primarily lymphoid tissues. Adipose tissue represents a potentially important non-lymphoid location for HIV replication and persistence because the stromal-vascular-fraction (AT-SVF) contains activated innate and adaptive immune cells that increase in number during infections, obesity, and chronic inflammation.

Results

Adipose tissue from two groups of SHIV-SF162p3-infected (~4 weeks acute infection) or SIVmac251-infected (~38 weeks chronic infection) rhesus macaques (N = 8 for each group) were studied for immune cell content, viral infectiousness, and metabolic health. The AT-SVF cells from SHIV-infected monkeys contained abundant memory CD4 and CD8 T cells, with fewer NKT cells and macrophages, and no B cells. Proviral DNA (Gag and Env) was readily detectable by nested PCR in AT-SVF cells from multiple adipose depots (subcutaneous and visceral) of acutely infected monkeys, but mostly from visceral fat. More importantly, viral outgrowth assays using input CD4 T cells derived from AT-SVF cells or peripheral blood of chronically infected monkeys resulted in robust replication of infectious virus from both AT-SVF and peripheral blood CD4 T cells. Chronically infected monkeys also experienced adipocyte dysfunction (suppression of major adipogenic genes) and systemic dyslipidemia (decreased serum total cholesterol and free fatty acids, and increased triglycerides), similar to metabolic abnormalities of HIV patients.

Conclusions

Adipose tissues of SIV-infected rhesus macaques become major compartments for infected immune cells, which in turn induce defects in adipose tissue metabolism.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-016-0260-2) contains supplementary material, which is available to authorized users.

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

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

Animal Models for HIV Cure Research

The HIV-1/AIDS pandemic continues to spread unabated worldwide, and no vaccine exists within our grasp. Effective antiretroviral therapy (ART) has been developed, but ART cannot clear the virus from the infected patient. A cure for HIV-1 is badly needed to stop both the spread of the virus in human populations and disease progression in infected individuals. A safe and effective cure strategy for human immunodeficiency virus (HIV) infection will require multiple tools, and appropriate animal models are tools that are central to cure research. An ideal animal model should recapitulate the essential aspects of HIV pathogenesis and associated immune responses, while permitting invasive studies, thus allowing a thorough evaluation of strategies aimed at reducing the size of the reservoir (functional cure) or eliminating the reservoir altogether (sterilizing cure). Since there is no perfect animal model for cure research, multiple models have been tailored and tested to address specific quintessential questions of virus persistence and eradication. The development of new non-human primate and mouse models, along with a certain interest in the feline model, has the potential to fuel cure research. In this review, we highlight the major animal models currently utilized for cure research and the contributions of each model to this goal.

Modulation of apoptosis and viral latency - an axis to be well understood for successful cure of human immunodeficiency virus

Human immunodeficiency virus (HIV) is the causative agent of the deadly disease AIDS, which is characterized by the progressive decline of CD4(+)T-cells. HIV-1-encoded proteins such as envelope gp120 (glycoprotein gp120), Tat (trans-activator of transcription), Nef (negative regulatory factor), Vpr (viral protein R), Vpu (viral protein unique) and protease are known to be effective in modulating host cell signalling pathways that lead to an alteration in apoptosis of both HIV-infected and uninfected bystander cells. Depending on the stage of the virus life cycle and host cell type, these viral proteins act as mediators of pro- or anti-apoptotic signals. HIV latency in viral reservoirs is a persistent phenomenon that has remained beyond the control of the human immune system. To cure HIV infections completely, it is crucial to reactivate latent HIV from cellular pools and to drive these apoptosis-resistant cells towards death. Several previous studies have reported the role of HIV-encoded proteins in apoptosis modulation, but the molecular basis for apoptosis evasion of some chronically HIV-infected cells and reactivated latently HIV-infected cells still needs to be elucidated. The current review summarizes our present understanding of apoptosis modulation in HIV-infected cells, uninfected bystander cells and latently infected cells, with a focus on highlighting strategies to activate the apoptotic pathway to kill latently infected cells.

Interleukin-21 combined with ART reduces inflammation and viral reservoir in SIV-infected macaques

Despite successful control of viremia, many HIV-infected individuals given antiretroviral therapy (ART) exhibit residual inflammation, which is associated with non-AIDS-related morbidity and mortality and may contribute to virus persistence during ART. Here, we investigated the effects of IL-21 administration on both inflammation and virus persistence in ART-treated, SIV-infected rhesus macaques (RMs). Compared with SIV-infected animals only given ART, SIV-infected RMs given both ART and IL-21 showed improved restoration of intestinal Th17 and Th22 cells and a more effective reduction of immune activation in blood and intestinal mucosa, with the latter maintained through 8 months after ART interruption. Additionally, IL-21, in combination with ART, was associated with reduced levels of SIV RNA in plasma and decreased CD4(+) T cell levels harboring replication-competent virus during ART. At the latest experimental time points, which were up to 8 months after ART interruption, plasma viremia and cell-associated SIV DNA levels remained substantially lower than those before ART initiation in IL-21-treated animals but not in controls. Together, these data suggest that IL-21 supplementation of ART reduces residual inflammation and virus persistence in a relevant model of lentiviral disease and warrants further investigation as a potential intervention for HIV infection.

Simian Immunodeficiency Virus SIVsab Infection of Rhesus Macaques as a Model of Complete Immunological Suppression with Persistent Reservoirs of Replication-Competent Virus: Implications for Cure Research

Simian immunodeficiency virus SIVsab infection is completely controlled in rhesus macaques (RMs) through functional immune responses. We report that in SIVsab-infected RMs, (i) viral replication is controlled to <0 to 3 copies/ml, (ii) about one-third of the virus strains in reservoirs are replication incompetent, and (iii) rebounding virus after CD8(+) cell depletion is replication competent and genetically similar to the original virus stock, suggesting early reservoir seeding. This model permits assessment of strategies aimed at depleting the reservoir without multidrug antiretroviral therapy.

Plasmodium infection reduces the volume of the viral reservoir in SIV-infected rhesus macaques receiving antiretroviral therapy

Background

Previous studies indicated that Plasmodium infection activates the immune system, including memory CD4+ T cells, which constitute the reservoir of human immunodeficiency virus type-1 (HIV-1). Therefore, we postulated that co-infection with malaria might activate the reservoir of HIV-1. To test this hypothesis, we used a rhesus macaque model of co-infection with malaria and simian immunodeficiency virus (SIV), along with antiretroviral therapy (ART).

Results

Our results showed that Plasmodium infection reduced both the replication-competent virus pool in resting CD4+ T cells and the integrated virus DNA (iDNA) load in peripheral blood mononuclear cells in the monkeys. This reduction might be attributable to malaria-mediated activation and apoptotic induction of memory CD4+ T cells. Further studies indicated that histone acetylation and NF-kappaB (NF-κB) activation in resting CD4+ T cells may also play an important role in this reduction.

Conclusions

The findings of this work expand our knowledge of the interaction between these two diseases. As more HIV-1-infected individuals in malaria-endemic areas receive ART, we should explore whether any of the patients co-infected with Plasmodium experience virologic benefits.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0112-x) contains supplementary material, which is available to authorized users.

Immune clearance of highly pathogenic SIV infection

Established infections with the human and simian immunodeficiency viruses (HIV and SIV, respectively) are thought to be permanent with even the most effective immune responses and antiretroviral therapies only able to control, but not clear, these infections. Whether the residual virus that maintains these infections is vulnerable to clearance is a question of central importance to the future management of millions of HIV-infected individuals. We recently reported that approximately 50% of rhesus macaques (RM; Macaca mulatta) vaccinated with SIV protein-expressing rhesus cytomegalovirus (RhCMV/SIV) vectors manifest durable, aviraemic control of infection with the highly pathogenic strain SIVmac239 (ref. 5). Here we show that regardless of the route of challenge, RhCMV/SIV vector-elicited immune responses control SIVmac239 after demonstrable lymphatic and haematogenous viral dissemination, and that replication-competent SIV persists in several sites for weeks to months. Over time, however, protected RM lost signs of SIV infection, showing a consistent lack of measurable plasma- or tissue-associated virus using ultrasensitive assays, and a loss of T-cell reactivity to SIV determinants not in the vaccine. Extensive ultrasensitive quantitative PCR and quantitative PCR with reverse transcription analyses of tissues from RhCMV/SIV vector-protected RM necropsied 69-172 weeks after challenge did not detect SIV RNA or DNA sequences above background levels, and replication-competent SIV was not detected in these RM by extensive co-culture analysis of tissues or by adoptive transfer of 60 million haematolymphoid cells to naive RM. These data provide compelling evidence for progressive clearance of a pathogenic lentiviral infection, and suggest that some lentiviral reservoirs may be susceptible to the continuous effector memory T-cell-mediated immune surveillance elicited and maintained by cytomegalovirus vectors.

Endothelial cell stimulation overcomes restriction and promotes productive and latent HIV-1 infection of resting CD4+ T cells

Highly active antiretroviral therapy (HAART) is able to suppress human immunodeficiency virus type 1 (HIV-1) to undetectable levels in the majority of patients, but eradication has not been achieved because latent viral reservoirs persist, particularly in resting CD4(+) T lymphocytes. It is generally understood that HIV-1 does not efficiently infect resting CD4(+) T cells, and latent infection in those cells may arise when infected CD4(+) T lymphoblasts return to resting state. In this study, we found that stimulation by endothelial cells can render resting CD4(+) T cells permissible for direct HIV infection, including both productive and latent infection. These stimulated T cells remain largely phenotypically unactivated and show a lower death rate than activated T cells, which promotes the survival of infected cells. The stimulation by endothelial cells does not involve interleukin 7 (IL-7), IL-15, CCL19, or CCL21. Endothelial cells line the lymphatic vessels in the lymphoid tissues and have frequent interactions with T cells in vivo. Our study proposes a new mechanism for infection of resting CD4(+) T cells in vivo and a new mechanism for latent infection in resting CD4(+) T cells.

Considerations in the development of nonhuman primate models of combination antiretroviral therapy for studies of AIDS virus suppression, residual virus, and curative strategies

PURPOSE OF REVIEW

Animal models will be critical for preclinical evaluations of novel HIV eradication and/or functional cure strategies in the setting of suppressive combination antiretroviral therapy (cART). Here, the strengths, limitations, and challenges of recent efforts to develop nonhuman primate (NHP) models of cART-mediated suppression for use in studies of persistent virus and curative approaches are discussed.

RECENT FINDINGS

Several combinations of NHP species and viruses that recapitulate key aspects of human HIV infection have been adapted for cART-mediated suppression studies. Different cART regimens incorporating drugs targeting multiple different steps of the viral replication cycle have provided varying levels of virologic suppression, dependent in part upon the host species, virus, drug regimen and timing, and virologic monitoring assay sensitivity. New, increasingly sensitive virologic monitoring approaches for measurements of plasma viral RNA, cell-associated and tissue-associated viral RNA and DNA, and the replication-competent residual viral pool in the setting of cART in NHP models are being developed to allow for the assessment of persistent virus on cART and to evaluate the impact of viral induction/eradication strategies in vivo.

SUMMARY

Given the vagaries of each specific virus and host species, and cART regimen, each model will require further development and analysis to determine their appropriate application for addressing specific experimental questions.

High levels of CD2 expression identify HIV-1 latently infected resting memory CD4+ T cells in virally suppressed subjects

Resting memory CD4(+) T cells are the largest reservoir of persistent infection in HIV-1-positive subjects. They harbor dormant, stably integrated virus despite suppressive antiretroviral therapy, posing an obstacle to a cure. Surface markers that identify latently infected cells remain unknown. Microarray analyses comparing resting latently infected and uninfected CD4(+) T cells generated in vitro showed profound differences in the expression of gene programs related to transcriptional and posttranscriptional regulation, cell proliferation, survival, cycle progression, and basic metabolism, suggesting that multiple biochemical and metabolic blocks contribute to preventing viral production in latently infected cells. We identified 33 transcripts encoding cell surface markers that are differentially expressed between latently infected and uninfected cells. Quantitative reverse transcriptase PCR (RT-QPCR) and flow cytometry analyses confirmed that the surface marker CD2 was expressed at higher levels on latently infected cells. To validate this result in vivo, we sorted resting memory CD4(+) T cells expressing high and low surface levels of CD2 from six HIV-1-infected subjects successfully treated with antiretroviral drugs for at least 3 years. Resting memory CD4(+) CD2(high) T cells from all subjects harbored higher HIV-1 DNA copy numbers than all other CD4(+) T cell subsets. Moreover, after ex vivo viral reactivation, robust viral RNA production was detected only from resting memory CD4(+) CD2(high) T cells but not from other cell subsets. Altogether, these results show that a high CD2 expression level is a hallmark of latently infected resting memory CD4(+) T cells in vivo.

Lymph node T cell responses predict the efficacy of live attenuated SIV vaccines

Live attenuated simian immunodeficiency virus (SIV) vaccines (LAVs) remain the most efficacious of all vaccines in nonhuman primate models of HIV and AIDS, yet the basis of their robust protection remains poorly understood. Here we show that the degree of LAV-mediated protection against intravenous wild-type SIVmac239 challenge strongly correlates with the magnitude and function of SIV-specific, effector-differentiated T cells in the lymph node but not with the responses of such T cells in the blood or with other cellular, humoral and innate immune parameters. We found that maintenance of protective T cell responses is associated with persistent LAV replication in the lymph node, which occurs almost exclusively in follicular helper T cells. Thus, effective LAVs maintain lymphoid tissue-based, effector-differentiated, SIV-specific T cells that intercept and suppress early wild-type SIV amplification and, if present in sufficient frequencies, can completely control and perhaps clear infection, an observation that provides a rationale for the development of safe, persistent vectors that can elicit and maintain such responses.

HIV-1 infection ex vivo accelerates measles virus infection by upregulating signaling lymphocytic activation molecule (SLAM) in CD4+ T cells

Measles virus (MV) infection in children harboring human immunodeficiency virus type 1 (HIV-1) is often fatal, even in the presence of neutralizing antibodies; however, the underlying mechanisms are unclear. Therefore, the aim of the present study was to examine the interaction between HIV-1 and wild-type MV (MVwt) or an MV vaccine strain (MVvac) during dual infection. The results showed that the frequencies of MVwt- and MVvac-infected CD4(+) T cells within the resting peripheral blood mononuclear cells (PBMCs) were increased 3- to 4-fold after HIV-1 infection, and this was associated with a marked upregulation of signaling lymphocytic activation molecule (SLAM) expression on CD4(+) T cells but not on CD8(+) T cells. SLAM upregulation was induced by infection with a replication-competent HIV-1 isolate comprising both the X4 and R5 types and to a lesser extent by a pseudotyped HIV-1 infection. Notably, SLAM upregulation was observed in HIV-infected as well as -uninfected CD4(+) T cells and was abrogated by the removal of HLA-DR(+) cells from the PBMC culture. Furthermore, SLAM upregulation did not occur in uninfected PBMCs cultured together with HIV-infected PBMCs in compartments separated by a permeable membrane, indicating that no soluble factors were involved. Rather, CD4(+) T cell activation mediated through direct contact with dendritic cells via leukocyte function-associated molecule 1 (LFA-1)/intercellular adhesion molecule 1 (ICAM-1) and LFA-3/CD2 was critical. Thus, HIV-1 infection induces a high level of SLAM expression on CD4(+) T cells, which may enhance their susceptibility to MV and exacerbate measles in coinfected individuals.

Replication-competent simian immunodeficiency virus (SIV) Gag escape mutations archived in latent reservoirs during antiretroviral treatment of SIV-infected macaques

In response to pressure exerted by major histocompatibility complex (MHC) class I-mediated CD8(+) T cell control, human immunodeficiency virus (HIV) escape mutations often arise in immunodominant epitopes recognized by MHC class I alleles. While the current standard of care for HIV-infected patients is treatment with highly active antiretroviral therapy (HAART), suppression of viral replication in these patients is not absolute and latently infected cells persist as lifelong reservoirs. To determine whether HIV escape from MHC class I-restricted CD8(+) T cell control develops during HAART treatment and then enters latent reservoirs in the periphery and central nervous system (CNS), with the potential to emerge as replication-competent virus, we tracked the longitudinal development of the simian immunodeficiency virus (SIV) Gag escape mutation K165R in HAART-treated SIV-infected pigtailed macaques. Key findings of these studies included: (i) SIV Gag K165R escape mutations emerged in both plasma and cerebrospinal fluid (CSF) during the decaying phase of viremia after HAART initiation before suppression of viral replication, (ii) SIV K165R Gag escape mutations were archived in latent proviral DNA reservoirs, including the brain in animals receiving HAART that suppressed viral replication, and (iii) replication-competent SIV Gag K165R escape mutations were present in the resting CD4(+) T cell reservoir in HAART-treated SIV-infected macaques. Despite early administration of aggressive antiretroviral treatment, HIV immune escape from CD8(+) T cell control can still develop during the decaying phases of viremia and then persist in latent reservoirs, including the brain, with the potential to emerge if HAART therapy is interrupted.

Profound early control of highly pathogenic SIV by an effector memory T-cell vaccine

The acquired immunodeficiency syndrome (AIDS)-causing lentiviruses human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) effectively evade host immunity and, once established, infections with these viruses are only rarely controlled by immunological mechanisms. However, the initial establishment of infection in the first few days after mucosal exposure, before viral dissemination and massive replication, may be more vulnerable to immune control. Here we report that SIV vaccines that include rhesus cytomegalovirus (RhCMV) vectors establish indefinitely persistent, high-frequency, SIV-specific effector memory T-cell (T(EM)) responses at potential sites of SIV replication in rhesus macaques and stringently control highly pathogenic SIV(MAC239) infection early after mucosal challenge. Thirteen of twenty-four rhesus macaques receiving either RhCMV vectors alone or RhCMV vectors followed by adenovirus 5 (Ad5) vectors (versus 0 of 9 DNA/Ad5-vaccinated rhesus macaques) manifested early complete control of SIV (undetectable plasma virus), and in twelve of these thirteen animals we observed long-term (≥1 year) protection. This was characterized by: occasional blips of plasma viraemia that ultimately waned; predominantly undetectable cell-associated viral load in blood and lymph node mononuclear cells; no depletion of effector-site CD4(+) memory T cells; no induction or boosting of SIV Env-specific antibodies; and induction and then loss of T-cell responses to an SIV protein (Vif) not included in the RhCMV vectors. Protection correlated with the magnitude of the peak SIV-specific CD8(+) T-cell responses in the vaccine phase, and occurred without anamnestic T-cell responses. Remarkably, long-term RhCMV vector-associated SIV control was insensitive to either CD8(+) or CD4(+) lymphocyte depletion and, at necropsy, cell-associated SIV was only occasionally measurable at the limit of detection with ultrasensitive assays, observations that indicate the possibility of eventual viral clearance. Thus, persistent vectors such as CMV and their associated T(EM) responses might significantly contribute to an efficacious HIV/AIDS vaccine.

A simian immunodeficiency virus macaque model of highly active antiretroviral treatment: viral latency in the periphery and the central nervous system

PURPOSE OF REVIEW

Here, simian immunodeficiency virus (SIV) macaque models are examined for their strengths in identifying in-vivo sites of HIV latency and persistent virus replication during highly active antiretroviral treatment (HAART). The best characterized HIV reservoir in HAART-treated persons is resting CD4 T cells in blood, although residual virus also comes from other reservoirs. Nonhuman primate/SIV models of HAART have been developed to characterize potential HIV reservoirs, particularly the central nervous system (CNS) and stem cells in bone marrow, known and potential reservoirs of latent virus that are difficult to study in humans.

RECENT FINDINGS

Few SIV macaque models of HAART have examined plasma and cerebrospinal fluid virus decay, the number of resting CD4 T cells harboring replication-competent latent SIV, HAART-treatment effect on the CNS, or residual viral replication or viral DNA levels in that tissue. Using a consistent, accelerated SIV macaque model, we characterized peripheral viral reservoirs, including those in the CNS, among HAART-treated macaques. The SIV model reproduces latency in memory CD4 T cells throughout the body and indicates that the CNS contains a stable SIV DNA reservoir.

SUMMARY

An SIV macaque model of HAART recapitulating viral latency, particularly in the CNS, is required to study therapeutic approaches for a functional HIV cure.

Introduction: challenges to finding a cure for HIV infection

PURPOSE OF REVIEW

Renewed interest has emerged to address the latent reservoir of HIV to achieve a cure.

RECENT FINDINGS

The integrated proviral genome is a fundamental component of the retroviral replication cycle. The establishment of latently infected CD4+ lymphocytes, and perhaps other as yet poorly characterized cells, represents a reservoir of HIV infection that is not appreciably affected by effective antiretroviral chemotherapy. Effective management of HIV infection, thus, will require lifelong treatment unless an approach to purging this reservoir is identified. Although substantial insights about the latent reservoir have been made, our understanding about the details of the reservoirs, the mechanisms of latency and potential targets to eliminate latently infected cells is too primitive to achieve a cure without a great deal of basic research to elucidate some of these areas.

SUMMARY

A resurgence of interest in latent infection and its treatment promises progress in addressing the challenge of a cure, although, realistically, this will require a prolonged period of investigation in many areas.

HIV reservoirs and latency models

The main impediment to a cure for HIV is the existence of long-lasting treatment resistant viral reservoirs. In this review, we discuss what is currently known about reservoirs, including their formation and maintenance, while focusing on latently infected CD4+ T cells. In addition, we compare several different in vivo and in vitro models of latency. We comment on how each model may reflect the properties of reservoirs in vivo, especially with regard to cell phenotype, since recent studies demonstrate that multiple CD4+ T cell subsets contribute to HIV reservoirs and that with HAART and disease progression the relative contribution of different subsets may change. Finally, we focus on the direct infection of resting CD4+ T cells as a source of reservoir formation and as a model of latency, since recent results help explain the misconception that resting CD4+ T cells appeared to be resistant to HIV in vitro.

Inference of surface membrane factors of HIV-1 infection through functional interaction networks

BACKGROUND

HIV infection affects the populations of T helper cells, dendritic cells and macrophages. Moreover, it has a serious impact on the central nervous system. It is yet not clear whether this list is complete and why specifically those cell types are affected. To address this question, we have developed a method to identify cellular surface proteins that permit, mediate or enhance HIV infection in different cell/tissue types in HIV-infected individuals. Receptors associated with HIV infection share common functions and domains and are involved in similar cellular processes. These properties are exploited by bioinformatics techniques to predict novel cell surface proteins that potentially interact with HIV.

METHODOLOGY/PRINCIPAL FINDINGS

We compiled a set of surface membrane proteins (SMP) that are known to interact with HIV. This set is extended by proteins that have direct interaction and share functional similarity. This resulted in a comprehensive network around the initial SMP set. Using network centrality analysis we predict novel surface membrane factors from the annotated network. We identify 21 surface membrane factors, among which three have confirmed functions in HIV infection, seven have been identified by at least two other studies, and eleven are novel predictions and thus excellent targets for experimental investigation.

CONCLUSIONS

Determining to what extent HIV can interact with human SMPs is an important step towards understanding patient specific disease progression. Using various bioinformatics techniques, we generate a set of surface membrane factors that constitutes a well-founded starting point for experimental testing of cell/tissue susceptibility of different HIV strains as well as for cohort studies evaluating patient specific disease progression.

A simian immunodeficiency virus-infected macaque model to study viral reservoirs that persist during highly active antiretroviral therapy

The treatment of human immunodeficiency virus type 1 (HIV-1) infection with highly active antiretroviral therapy (HAART), a combination of three or more antiretroviral drugs, suppresses viremia below the clinical limit of detection (50 HIV-1 RNA copies/ml), but latently infected resting CD4(+) T cells serve as lifelong reservoirs, and low-level viremia can be detected with special assays. Recent studies have provided evidence for additional reservoirs that contribute to residual viremia but are not present in circulating cells. Identification of all the sources of residual viremia in humans may be difficult. These discoveries highlight the need for a tractable model system to identify additional viral reservoirs that could represent barriers to eradication. In this study, simian immunodeficiency virus (SIV)-infected pig-tailed macaques (Macaca nemestrina) were treated with four antiretroviral drugs to develop an animal model for viral suppression during effective HAART. Treatment led to a biphasic decay in viremia and a significant rise in levels of circulating CD4(+) T cells. At terminal infection time points, the frequency of circulating resting CD4(+) T cells harboring replication-competent virus was reduced to a low steady-state level similar to that observed for HIV-infected patients on HAART. The frequencies of resting CD4(+) T cells harboring replication-competent virus in the pooled head lymph nodes, gut lymph nodes, spleen, and peripheral blood were reduced relative to those for untreated SIV-infected animals. These observations closely parallel findings for HIV-infected humans on suppressive HAART and demonstrate the value of this animal model to identify and characterize viral reservoirs persisting in the setting of suppressive antiretroviral drugs.

An in vitro system to model the establishment and reactivation of HIV-1 latency

HIV-1 establishes latency primarily by infecting activated CD4(+) T cells that later return to quiescence as memory cells. Latency allows HIV-1 to evade immune responses and to persist during antiretroviral therapy, which represents an important problem in clinical practice. The lack of a valid cellular model to study HIV-1 latency has hindered advances in the understanding of its biology. In this study, we attempted to model HIV-1 latency using human primary CD4(+) T cells infected in vitro with HIV-1 after activation with Ag-loaded dendritic cells and then brought back to quiescence through a resting phase in the presence of IL-7. During the resting phase, expression of cellular activation markers disappeared and cell proliferation and viral replication ceased, but resumed following restimulation of rested cells with Ag or mAbs directed to CD3/CD28. In addition, higher cell death rates were observed in HIV-1-infected than uninfected cultures during secondary but not primary stimulation. Thus, this system may allow us to study the biology of HIV-1 latency, as well as the mechanisms of CD4(+) T cell death following HIV-1 reactivation.