SIV-specific antibodies protect against inflammasome-driven encephalitis in untreated macaques.

Viral encephalitis is a growing public health threat with limited diagnostic and treatment options. Simian immunodeficiency virus (SIV)-infected macaques are an established model for human immunodeficiency virus (HIV), and approximately 60% of untreated pigtail macaques rapidly progress to characteristic SIV encephalitis (SIVE). The immune responses of SIV-infected macaques are investigated in plasma, cerebrospinal fluid (CSF), and brain tissue to determine correlates with SIVE pathology. Macaques with SIVE show myeloid-dominant brain lesions with inflammasome activation in infected and bystander cells, as assessed by interleukin (IL)-1ß, IL-18, and apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC), and elevations in monocyte chemoattractant protein (MCP)-1, macrophage inflammatory protein (MIP)-1α, and tumor necrosis factor alpha (TNF-α). SIV-specific immunoglobulin (Ig)G in plasma and CSF is predictive of SIVE as early as 21 days post-inoculation; animals with SIVE continue to show negligible seroconversion 3 months after infection. This dichotomy in immune responses, wherein some macaques fail to initiate robust IgG responses and subsequently develop SIVE, provides insight into the pathogenesis and heterogeneous outcomes in viral encephalitis.

A Switch from Glial to Neuronal Gene Expression Alterations in the Spinal Cord of SIV-infected Macaques on Antiretroviral Therapy.

Despite antiretroviral therapy (ART), HIV-associated peripheral neuropathy remains one of the most prevalent neurologic manifestations of HIV infection. The spinal cord is an essential component of sensory pathways, but spinal cord sampling and evaluation in people with HIV has been very limited, especially in those on ART. The SIV/macaque model allows for assessment of the spinal cord at key time points throughout infection with and without ART. In this study, RNA was isolated from the spinal cord of uninfected, SIV+, and SIV + ART animals to track alterations in gene expression using global RNA-seq. Next, the SeqSeek platform was used to map changes in gene expression to specific cell types. Pathway analysis of differentially expressed genes demonstrated that highly upregulated genes in SIV-infected spinal cord aligned with interferon and viral response pathways. Additionally, this upregulated gene set significantly overlapped with those expressed in myeloid-derived cells including microglia. Downregulated genes were involved in cholesterol and collagen biosynthesis, and TGF-b regulation of extracellular matrix. In contrast, enriched pathways identified in SIV + ART animals included neurotransmitter receptors and post synaptic signaling regulators, and transmission across chemical synapses. SeqSeek analysis showed that upregulated genes were primarily expressed by neurons rather than glia. These findings indicate that pathways activated in the spinal cord of SIV + ART macaques are predominantly involved in neuronal signaling rather than proinflammatory pathways. This study provides the basis for further evaluation of mechanisms of SIV infection + ART within the spinal cord with a focus on therapeutic interventions to maintain synaptodendritic homeostasis.

An ex vivo model of interactions between extracellular vesicles and peripheral mononuclear blood cells in whole blood.

Extracellular vesicles (EVs) can be loaded with therapeutic cargo and engineered for retention by specific body sites; therefore, they have great potential for targeted delivery of biomolecules to treat diseases. However, the pharmacokinetics and biodistribution of EVs in large animals remain relatively unknown, especially in primates. We recently reported that when cell culture-derived EVs are administered intravenously to Macaca nemestrina (pig-tailed macaques), they differentially associate with specific subsets of peripheral blood mononuclear cells (PBMCs). More than 60% of CD20+ B cells were observed to associate with EVs for up to 1 h post-intravenous administration. To investigate these associations further, we developed an ex vivo model of whole blood collected from healthy pig-tailed macaques. Using this ex vivo system, we found that labelled EVs preferentially associate with B cells in whole blood at levels similar to those detected in vivo. This study demonstrates that ex vivo blood can be used to study EV-blood cell interactions.

Effect of Single Housing on Innate Immune Activation in Immunodeficiency Virus-Infected Pigtail Macaques ( Macaca nemestrina ) as a Model of Psychosocial Stress in Acute HIV Infection.

OBJECTIVE: Simian immunodeficiency virus (SIV) infection of macaques recapitulates many aspects of HIV pathogenesis and is similarly affected by both genetic and environmental factors. Psychosocial stress is associated with immune system dysregulation and worse clinical outcomes in people with HIV. This study assessed the impact of single housing, as a model of psychosocial stress, on innate immune responses of pigtailed macaques ( Macaca nemestrina ) during acute SIV infection. METHODS: A retrospective analysis of acute SIV infection of 2- to si6-year-old male pigtailed macaques was performed to compare the innate immune responses of socially ( n = 41) and singly ( n = 35) housed animals. Measures included absolute monocyte count and subsets, and in a subset ( n ≤ 18) platelet counts and activation data. RESULTS: SIV infection resulted in the expected innate immune parameter changes with a modulating effect from housing condition. Monocyte number increased after infection for both groups, driven by classical monocytes (CD14 + CD16 - ), with a greater increase in socially housed animals (227%, p < .001, by day 14 compared with preinoculation time points). Platelet numbers recovered more quickly in the socially housed animals. Platelet activation (P-selectin) increased by 65% ( p = .004) and major histocompatibility complex class I surface expression by 40% ( p = .009) from preinoculation only in socially housed animals, whereas no change in these measures occurred in singly housed animals. CONCLUSIONS: Chronic psychosocial stress produced by single housing may play an immunomodulatory role in the innate immune response to acute retroviral infection. Dysregulated innate immunity could be one of the pathways by which psychosocial stress contributes to immune suppression and increased disease severity in people with HIV.

Pharmacokinetics and biodistribution of extracellular vesicles administered intravenously and intranasally to Macaca nemestrina.

Extracellular vesicles (EVs) have potential in disease treatment since they can be loaded with therapeutic molecules and engineered for retention by specific tissues. However, questions remain on optimal dosing, administration, and pharmacokinetics. Previous studies have addressed biodistribution and pharmacokinetics in rodents, but little evidence is available for larger animals. Here, we investigated the pharmacokinetics and biodistribution of Expi293F-derived EVs labelled with a highly sensitive nanoluciferase reporter (palmGRET) in a non-human primate model (Macaca nemestrina), comparing intravenous (IV) and intranasal (IN) administration over a 125-fold dose range. We report that EVs administered IV had longer circulation times in plasma than previously reported in mice and were detectable in cerebrospinal fluid (CSF) after 30-60 minutes. EV association with PBMCs, especially B-cells, was observed as early as one minute post-administration. EVs were detected in liver and spleen within one hour of IV administration. However, IN delivery was minimal, suggesting that pretreatment approaches may be needed in large animals. Furthermore, EV circulation times strongly decreased after repeated IV administration, possibly due to immune responses and with clear implications for xenogeneic EV-based therapeutics. We hope that our findings from this baseline study in macaques will help to inform future research and therapeutic development of EVs.

Progression and Resolution of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Infection in Golden Syrian Hamsters.

To catalyze severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) research, including development of novel interventive and preventive strategies, the progression of disease was characterized in a robust coronavirus disease 2019 (COVID-19) animal model. In this model, male and female golden Syrian hamsters were inoculated intranasally with SARS-CoV-2 USA-WA1/2020. Groups of inoculated and mock-inoculated uninfected control animals were euthanized at 2, 4, 7, 14, and 28 days after inoculation to track multiple clinical, pathology, virology, and immunology outcomes. SARS-CoV-2-inoculated animals consistently lost body weight during the first week of infection, had higher lung weights at terminal time points, and developed lung consolidation per histopathology and quantitative image analysis measurements. High levels of infectious virus and viral RNA were reliably present in the respiratory tract at days 2 and 4 after inoculation, corresponding with widespread necrosis and inflammation. At day 7, when the presence of infectious virus was rare, interstitial and alveolar macrophage infiltrates and marked reparative epithelial responses (type II hyperplasia) dominated in the lung. These lesions resolved over time, with only residual epithelial repair evident by day 28 after inoculation. The use of quantitative approaches to measure cellular and morphologic alterations in the lung provides valuable outcome measures for developing therapeutic and preventive interventions for COVID-19 using the hamster COVID-19 model.

Elucidation of the Central Serotonin Metabolism Pathway in Rhesus Macaques (Macaca mulatta) with Self-injurious Behavior.

Macaques with self-injurious behavior (SIB) have been used as a model of human SIB and have previously been shown to respond to treatments targeting enhancement of central serotonin signaling, whether by supplementation with tryptophan, or by inhibiting synaptic reuptake. Decreased serotonin signaling in the brain has also been implicated in many human psychopathologies including major depression disorder. A disturbance in tryptophan metabolism that moves away from the production of serotonin and toward the production of kynurenine has been proposed as a major etiological factor of depression. We hypothesized that in macaques with SIB, central tryptophan metabolism would be shifted toward kynurenine production, leading to lower central serotonin (5-hydroxytryptamine). We analyzed tryptophan metabolites in the cerebral spinal fluid (CSF) of macaques with and without SIB to determine whether and where tryptophan metabolism is altered in affected animals as compared with behaviorally normal controls. We found that macaques with SIB had lower CSF concentrations of serotonin than did behaviorally normal macaques, and that these deficits were inversely correlated with the severity of abnormal behavior. However, our results suggest that this decrease is not due to shifting of the tryptophan metabolic pathway toward kynurenine, as concentrations of kynurenine were also low. Concentrations of IL6 were elevated, suggesting central inflammation. Determining the mechanism by which serotonin function is altered in self-injurious macaques could shed light on novel therapies for SIB and other disorders of serotonin signaling.

Psychosocial Stress Alters the Immune Response and Results in Higher Viral Load During Acute Simian Immunodeficiency Virus Infection in a Pigtailed Macaque Model of Human Immunodeficiency Virus.

BACKGROUND: Although social distancing is a key public health response during viral pandemics, psychosocial stressors, such as social isolation, have been implicated in adverse health outcomes in general [1] and in the context of infectious disease, such as human immunodeficiency virus (HIV) [2, 3]. A comprehensive understanding of the direct pathophysiologic effects of psychosocial stress on viral pathogenesis is needed to provide strategic and comprehensive care to patients with viral infection. METHODS: To determine the effect of psychosocial stress on HIV pathogenesis during acute viral infection without sociobehavioral confounders inherent in human cohorts, we compared commonly measured parameters of HIV progression between singly (n = 35) and socially (n = 41) housed simian immunodeficiency virus (SIV)-infected pigtailed macaques (Macaca nemestrina). RESULTS: Singly housed macaques had a higher viral load in the plasma and cerebrospinal fluid and demonstrated greater CD4 T-cell declines and more CD4 and CD8 T-cell activation compared with socially housed macaques throughout acute SIV infection. CONCLUSIONS: These data demonstrate that psychosocial stress directly impacts the pathogenesis of acute SIV infection and imply that it may act as an integral variable in the progression of HIV infection and potentially of other viral infections.

Combining In Vivo Corneal Confocal Microscopy With Deep Learning-Based Analysis Reveals Sensory Nerve Fiber Loss in Acute Simian Immunodeficiency Virus Infection.

PURPOSE: To characterize corneal subbasal nerve plexus features of normal and simian immunodeficiency virus (SIV)-infected macaques by combining in vivo corneal confocal microscopy (IVCM) with automated assessments using deep learning-based methods customized for macaques. METHODS: IVCM images were collected from both male and female age-matched rhesus and pigtailed macaques housed at the Johns Hopkins University breeding colony using the Heidelberg HRTIII with Rostock Corneal Module. We also obtained repeat IVCM images of 12 SIV-infected animals including preinfection and 10-day post-SIV infection time points. All IVCM images were analyzed using a deep convolutional neural network architecture developed specifically for macaque studies. RESULTS: Deep learning-based segmentation of subbasal nerves in IVCM images from macaques demonstrated that corneal nerve fiber length and fractal dimension measurements did not differ between species, but pigtailed macaques had significantly higher baseline corneal nerve fiber tortuosity than rhesus macaques (P = 0.005). Neither sex nor age of macaques was associated with differences in any of the assessed corneal subbasal nerve parameters. In the SIV/macaque model of human immunodeficiency virus, acute SIV infection induced significant decreases in both corneal nerve fiber length and fractal dimension (P = 0.01 and P = 0.008, respectively). CONCLUSIONS: The combination of IVCM and robust objective deep learning analysis is a powerful tool to track sensory nerve damage, enabling early detection of neuropathy. Adapting deep learning analyses to clinical corneal nerve assessments will improve monitoring of small sensory nerve fiber damage in numerous clinical settings including human immunodeficiency virus.

Upregulation of Superoxide Dismutase 2 by Astrocytes in the SIV/Macaque Model of HIV-Associated Neurologic Disease.

HIV-associated neurocognitive disorders (HAND) remain prevalent despite implementation of antiretroviral therapy (ART). Development of HAND is linked to mitochondrial dysfunction and oxidative stress in the brain; therefore, upregulation of antioxidant defenses is critical to curtail neuronal damage. Superoxide dismutase 2 (SOD2) is a mitochondrial antioxidant enzyme essential for maintaining cellular viability. We hypothesized that SOD2 was upregulated during retroviral infection. Using a simian immunodeficiency virus (SIV)-infected macaque model of HIV, quantitative PCR showed elevated SOD2 mRNA in cortical gray ([GM], 7.6-fold for SIV vs uninfected) and white matter ([WM], 77-fold for SIV vs uninfected) during SIV infection. Further, SOD2 immunostaining was enhanced in GM and WM from SIV-infected animals. Double immunofluorescence labeling illustrated that SOD2 primarily colocalized with astrocyte marker glial fibrillary acidic protein (GFAP) in SIV-infected animals. Interestingly, in ART-treated SIV-infected animals, brain SOD2 RNA levels were similar to uninfected animals. Additionally, using principal component analysis in a transcriptomic approach, SOD2 and GFAP expression separated SIV-infected from uninfected brain tissue. Projection of these data into a HIV dataset revealed similar expression changes, thereby validating the clinical relevance. Together, our findings suggest that novel SOD2-enhancing therapies may reduce neuroinflammation in ART-treated HIV-infected patients.

Differential regulation of TREM2 and CSF1R in CNS macrophages in an SIV/macaque model of HIV CNS disease.

HIV-associated neuroinflammation is primarily driven by CNS macrophages including microglia. Regulation of these immune responses, however, remains to be characterized in detail. Using the SIV/macaque model of HIV, we evaluated CNS expression of triggering receptor expressed on myeloid cells 2 (TREM2) which is constitutively expressed by microglia and contributes to cell survival, proliferation, and differentiation. Loss-of-function mutations in TREM2 are recognized risk factors for neurodegenerative diseases including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), and Nasu-Hakola disease (NHD); recent reports have also indicated a role for TREM2 in HIV-associated neuroinflammation. Using in situ hybridization (ISH) and qRT-PCR, TREM2 mRNA levels were found to be significantly elevated in frontal cortex of macaques with SIV encephalitis compared with uninfected controls (P = 0.02). TREM2 protein levels were also elevated as measured by ELISA of frontal cortex tissue homogenates in these animals. Previously, we characterized the expression of CSF1R (colony-stimulating factor 1 receptor) in this model; the TREM2 and CSF1R promoters both contain a PU.1 binding site. While TREM2 and CSF1R mRNA levels in the frontal cortex were highly correlated (Spearman R = 0.79, P < 0.001), protein levels were not well correlated. In SIV-infected macaques released from ART to study viral rebound, neither TREM2 nor CSF1R mRNA increased with rebound viremia. However, CSF1R protein levels remained significantly elevated unlike TREM2 (P = 0.02). This differential expression suggests that TREM2 and CSF1R play unique, distinct roles in the pathogenesis of HIV CNS disease.

Brain macrophages harbor latent, infectious simian immunodeficiency virus.

: The current review examines the role of brain macrophages, that is perivascular macrophages and microglia, as a potential viral reservoir in antiretroviral therapy (ART) treated, simian immunodeficiency virus (SIV)-infected macaques. The role, if any, of latent viral reservoirs of HIV and SIV in the central nervous system during ART suppression is an unresolved issue. HIV and SIV infect both CD4 lymphocytes and myeloid cells in blood and tissues during acute and chronic infection. HIV spread to the brain occurs during acute infection by the infiltration of activated CD4 lymphocytes and monocytes from blood and is established in both embryonically derived resident microglia and monocyte-derived perivascular macrophages. ART controls viral replication in peripheral blood and cerebrospinal fluid in HIV-infected individuals but does not directly eliminate infected cells in blood, tissues or brain. Latently infected resting CD4 lymphocytes in blood and lymphoid tissues are a well recognized viral reservoir that can rebound once ART is withdrawn. In contrast, central nervous system resident microglia and perivascular macrophages in brain have not been examined as potential reservoirs for HIV during suppressive ART. Macrophages in tissues are long-lived cells that are HIV and SIV infected in tissues such as gut, lung, spleen, lymph node and brain and contribute to ongoing inflammation in tissues. However, their potential role in viral persistence and latency or their potential to rebound in the absence ART has not been examined. It has been shown that measurement of HIV latency by HIV DNA PCR in CD4 lymphocytes overestimates the size of the latent reservoirs of HIV that contribute to rebound that is cells containing the genomes of replicative viruses. Thus, the quantitative viral outgrowth assay has been used as a reliable measure of the number of latent cells that harbor infectious viral DNA and, may constitute a functional latent reservoir. Using quantitative viral outgrowth assays specifically designed to quantitate latently infected CD4 lymphocytes and myeloid cells in an SIV macaque model, we demonstrated that macrophages in brain harbor SIV genomes that reactivate and produce infectious virus in this assay, demonstrating that these cells have the potential to be a reservoir.

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.

Infectious Virus Persists in CD4+ T Cells and Macrophages in Antiretroviral Therapy-Suppressed Simian Immunodeficiency Virus-Infected Macaques.

Understanding the cellular and anatomical sites of latent virus that contribute to human immunodeficiency virus (HIV) rebound is essential for eradication. In HIV-positive patients, CD4+ T lymphocytes comprise a well-defined functional latent reservoir, defined as cells containing transcriptionally silent genomes able to produce infectious virus once reactivated. However, the persistence of infectious latent virus in CD4+ T cells in compartments other than blood and lymph nodes is unclear. Macrophages (Mϕ) are infected by HIV/simian immunodeficiency virus (SIV) and are likely to carry latent viral genomes during antiretroviral therapy (ART), contributing to the reservoir. Currently, the gold standard assay used to measure reservoirs containing replication-competent virus is the quantitative viral outgrowth assay (QVOA). Using an SIV-macaque model, the CD4+ T cell and Mϕ functional latent reservoirs were measured in various tissues using cell-specific QVOAs. Our results showed that blood, spleen, and lung in the majority of suppressed animals contain latently infected Mϕs. Surprisingly, the numbers of CD4+ T cells, monocytes, and Mϕs carrying infectious genomes in blood and spleen were at comparable frequencies (∼1 infected cell per million). We also demonstrate that ex vivo viruses produced in the Mϕ QVOA are capable of infecting activated CD4+ T cells. These results strongly suggest that latently infected tissue Mϕs can reestablish productive infection upon treatment interruption. This study provides the first comparison of CD4+ T cell and Mϕ functional reservoirs in a macaque model. It is the first confirmation of the persistence of latent genomes in monocytes in blood and Mϕs in the spleen and lung of SIV-infected ART-suppressed macaques. Our results demonstrate that transcriptionally silent genomes in Mϕs can contribute to viral rebound after ART interruption and should be considered in future HIV cure strategies.IMPORTANCE This study suggests that CD4+ T cells found throughout tissues in the body can contain replication-competent SIV and contribute to rebound of the virus after treatment interruption. In addition, this study demonstrates that macrophages in tissues are another cellular reservoir for SIV and may contribute to viral rebound after treatment interruption. This new insight into the size and location of the SIV reservoir could have great implications for HIV-infected individuals and should be taken into consideration for the development of future HIV cure strategies.

A Quantitative Approach to SIV Functional Latency in Brain Macrophages.

Lentiviruses are retroviruses that primarily infect myeloid cells, leading to acute inflammatory infections in many tissues particularly, lung, joints and the central nervous system (CNS). Acute infection by lentiviruses is followed by persistent/latent infections that are not cleared by the host immune system. HIV and SIV are lentiviruses that also infect CD4+ lymphocytes as well as myeloid cells in blood and multiple tissues. HIV infection of myeloid cells in brain, lung and heart cause tissue specific diseases as well as infect cells in gut, lymph nodes and spleen. AIDS dementia and other tissue specific disease are observed when infected individuals are immunosuppressed and the number of circulating CD4+ T cells declines to low levels. Antiretroviral therapy (ART) controls viral spread and dramatically changes the course of immunodeficiency and AIDS dementia. However, ART does not eliminate virus-infected cells. Brain macrophages contain HIV DNA and may represent a latent reservoir that persists. HIV latency in CD4+ lymphocytes is the main focus of current research and concern in efforts to eradicate HIV. However, a number of studies have demonstrated that myeloid cells in blood and tissues of ART suppressed individuals harbor HIV DNA. The resident macrophages in tissues such as brain (microglia), spleen (red pulp macrophages) and alveolar macrophages in lung are derived from the yolk sac and can self renew. The question of the latent myeloid reservoir in HIV has not been rigorously examined and its potential as a barrier to eradication been considered. Using a well characterized SIV ART suppressed, non-human primate (NHP) model, our laboratory developed the first quantitative viral outgrowth assay (QVOA) designed to evaluate latently infected CD4+ lymphocytes and more recently developed a similar protocol for the assessment of latently infected myeloid cells in blood and brain. Using an SIV ART model, it was demonstrated that myeloid cells in blood and brain harbor latent SIV that can be reactivated and produce infectious virus in vitro. These studies demonstrate for the first time that myeloid cells have the potential to be a latent reservoir of HIV that produces infectious virus that can be reactivated in the absence of ART and during HIV eradication strategies. Graphical Abstract.

SIV-Induced Immune Activation and Metabolic Alterations in the Dorsal Root Ganglia During Acute Infection.

Human immunodeficiency virus-associated sensory neuropathy (HIV-SN) remains a frequent neurologic complication of HIV infection. Little is known about alterations in the peripheral nervous system during the early stages of HIV, a time when neuroprotective interventions may be most beneficial. We performed Nanostring gene expression analysis on lumbar dorsal root ganglia (DRG) from 6 simian immunodeficiency virus (SIV)-infected pigtailed macaques killed at 7 days post-inoculation and 8 uninfected controls. We found significant upregulation of many genes involved in immune signaling and activation in the DRG. Among genes related to glutamate metabolism, there was significant upregulation of glutamine synthetase (GS), while glutaminase (GLS) was downregulated. Several genes involved in the oxidative stress response also showed significant differential regulation in the DRG of 7d SIV-infected animals, with superoxide dismutase-2 (SOD2) showing the greatest median fold change compared to controls. Novel findings in the DRG were compared to corresponding brain data and further investigated at the protein level by Western blotting and immunohistochemistry. Together with our previous finding of significant epidermal nerve fiber loss at 14 days post-SIV infection, results of this study demonstrate that immune activation and altered cellular metabolism at in the DRG precede and likely contribute to early sensory nerve injury in HIV-SN.

SIV Latency in Macrophages in the CNS.

Lentiviruses infect myeloid cells, leading to acute infection followed by persistent/latent infections not cleared by the host immune system. HIV and SIV are lentiviruses that infect CD4+ lymphocytes in addition to myeloid cells in blood and tissues. HIV infection of myeloid cells in brain, lung, and heart causes tissue-specific diseases that are mostly observed during severe immunosuppression, when the number of circulating CD4+ T cells declines to exceeding low levels. Antiretroviral therapy (ART) controls viral replication but does not successfully eliminate latent virus, which leads to viral rebound once ART is interrupted. HIV latency in CD4+ lymphocytes is the main focus of research and concern when HIV eradication efforts are considered. However, myeloid cells in tissues are long-lived and have not been routinely examined as a potential reservoir. Based on a quantitative viral outgrowth assay (QVOA) designed to evaluate latently infected CD4+ lymphocytes, a similar protocol was developed for the assessment of latently infected myeloid cells in blood and tissues. Using an SIV ART model, it was demonstrated that myeloid cells in blood and brain harbor latent SIV that can be reactivated and produce infectious virus in vitro, demonstrating that myeloid cells have the potential to be an additional latent reservoir of HIV that should be considered during HIV eradication strategies.

Increased Microglial CSF1R Expression in the SIV/Macaque Model of HIV CNS Disease.

Chronic microglial activation and associated neuroinflammation are key factors in neurodegenerative diseases including HIV-associated neurocognitive disorders. Colony stimulating factor 1 receptor (CSF1R)-mediated signaling is constitutive in cells of the myeloid lineage, including microglia, promoting cell survival, proliferation, and differentiation. In amyotrophic lateral sclerosis and Alzheimers disease, CSF1R is upregulated. Inhibiting CSF1R signaling in animal models of these diseases improved disease outcomes. In our studies, CNS expression of the CSF1R ligand, colony-stimulating factor 1 (CSF1) was significantly increased in a SIV/macaque model of HIV CNS disease. Using a Nanostring nCounter immune panel, we found CSF1 overexpression was strongly correlated with upregulation of microglial genes involved in antiviral and oxidative stress responses. Using in situ hybridization, we found that CSF1R mRNA was only present in Iba-1 positive microglia. By ELISA and immunostaining with digital image analysis, SIV-infected macaques had significantly higher CSF1R levels in frontal cortex than uninfected macaques (p = 0.018 and p = 0.02, respectively). SIV-infected macaques treated with suppressive ART also had persistently elevated CSF1R similar to untreated SIV-infected macaques. Coordinate upregulation of CSF1 and CSF1R expression implicates this signaling pathway in progressive HIV CNS disease.

Lymphocyte-Dominant Encephalitis and Meningitis in Simian Immunodeficiency Virus-Infected Macaques Receiving Antiretroviral Therapy.

A retrospective neuropathologic review of 30 SIV-infected pigtailed macaques receiving combination antiretroviral therapy (cART) was conducted. Seventeen animals with lymphocyte-dominant inflammation in the brain and/or meninges that clearly was morphologically distinct from prototypic SIV encephalitis and human immunodeficiency virus encephalitis were identified. Central nervous system (CNS) infiltrates in cART-treated macaques primarily comprised CD20+ B cells and CD3+ T cells with fewer CD68+ macrophages. Inflammation was associated with low levels of SIV RNA in the brain as shown by in situ hybridization, and generally was observed in animals with episodes of cerebrospinal fluid (CSF) viral rebound or sustained plasma and CSF viremia during treatment. Although the lymphocytic CNS inflammation in these macaques shared morphologic characteristics with uncommon immune-mediated neurologic disorders reported in treated HIV patients, including CNS immune reconstitution inflammatory syndrome and neurosymptomatic CSF escape, the high prevalence of CNS lesions in macaques suggests that persistent adaptive immune responses in the CNS also may develop in neuroasymptomatic or mildly impaired HIV patients yet remain unrecognized given the lack of access to CNS tissue for histopathologic evaluation. Continued investigation into the mechanisms and outcomes of CNS inflammation in cART-treated, SIV-infected macaques will advance our understanding of the consequences of residual CNS HIV replication in patients on cART, including the possible contribution of adaptive immune responses to HIV-associated neurocognitive disorders.

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.