Emergence of CD4 independence envelopes and astrocyte infection in R5 simian-human immunodeficiency virus model of encephalitis

SHIV-C109p5 NHP induces rapid disease progression in elderly macaques with extensive GI viral replication

CCR5-tropic simian/human immunodeficiency viruses (SHIV) with clade C transmitted/founder envelopes represent a critical tool for the investigation of HIV experimental vaccines and microbicides in nonhuman primates, although many such isolates lead to spontaneous viral control post infection. Here, we generated a high-titer stock of pathogenic SHIV-C109p5 by serial passage in two rhesus macaques (RM) and tested its virulence in aged monkeys. The co-receptor usage was confirmed before infecting five geriatric rhesus macaques (four female and one male). Plasma viral loads were monitored by reverse transcriptase-quantitative PCR (RT-qPCR), cytokines by multiplex analysis, and biomarkers of gastrointestinal damage by enzyme-linked immunosorbent assay. Antibodies and cell-mediated responses were also measured. Viral dissemination into tissues was determined by RNAscope. Intravenous SHIV-C109p5 infection of aged RMs leads to high plasma viremia and rapid disease progression; rapid decrease in CD4+ T cells, CD4+CD8+ T cells, and plasmacytoid dendritic cells; and wasting necessitating euthanasia between 3 and 12 weeks post infection. Virus-specific cellular immune responses were detected only in the two monkeys that survived 4 weeks post infection. These were Gag-specific TNFα+CD8+, MIP1β+CD4+, Env-specific IFN-γ+CD4+, and CD107a+ T cell responses. Four out of five monkeys had elevated intestinal fatty acid binding protein levels at the viral peak, while regenerating islet-derived protein 3α showed marked increases at later time points in the three animals surviving the longest, suggesting gut antimicrobial peptide production in response to microbial translocation post infection. Plasma levels of monocyte chemoattractant protein-1, interleukin-15, and interleukin-12/23 were also elevated. Viral replication in gut and secondary lymphoid tissues was extensive.IMPORTANCESimian/human immunodeficiency viruses (SHIV) are important reagents to study prevention of virus acquisition in nonhuman primate models of HIV infection, especially those representing transmitted/founder (T/F) viruses. However, many R5-tropic SHIV have limited fitness in vivo leading to many monkeys spontaneously controlling the virus post acute infection. Here, we report the generation of a pathogenic SHIV clade C T/F stock by in vivo passage leading to sustained viral load set points, a necessity to study pathogenicity. Unexpectedly, administration of this SHIV to elderly rhesus macaques led to extensive viral replication and fast disease progression, despite maintenance of a strict R5 tropism. Such age-dependent rapid disease progression had previously been reported for simian immunodeficiency virus but not for R5-tropic SHIV infections.

Transmitted/founder SHIV.D replicates in the brain, causes neuropathogenesis, and persists on combination antiretroviral therapy in rhesus macaques

A biologically relevant non-human primate (NHP) model of HIV persistence in the central nervous system (CNS) is necessary. Most current NHP/SIV models of HIV infection fail to recapitulate viral persistence in the CNS without encephalitis or fail to employ viruses that authentically represent the ongoing HIV-1 pandemic. Here, we demonstrate viral replication in the brain and neuropathogenesis after combination antiretroviral therapy (ART) in rhesus macaques (RMs) using novel macrophage-tropic transmitted/founder (TF) simian-human immunodeficiency virus SHIV.D.191,859 (SHIV.D). Quantitative immunohistochemistry (IHC) and DNA/RNAscope in situ hybridization (ISH) were performed on three brain regions from six SHIV.D-infected RMs; two necropsied while viremic, two during analytical treatment interruptions, and two on suppressive ART. We demonstrated myeloid-mediated neuroinflammation, viral replication, and proviral DNA in the brain in all animals. These results demonstrate that TF SHIV.D models native HIV-1 CNS replication, pathogenesis, and persistence on ART in rhesus macaques.

Tunneling nanotubes, TNT, communicate glioblastoma with surrounding non-tumor astrocytes to adapt them to hypoxic and metabolic tumor conditions

Cell-to-cell communication is essential for the development and proper function of multicellular systems. We and others demonstrated that tunneling nanotubes (TNT) proliferate in several pathological conditions such as HIV, cancer, and neurodegenerative diseases. However, the nature, function, and contribution of TNT to cancer pathogenesis are poorly understood. Our analyses demonstrate that TNT structures are induced between glioblastoma (GBM) cells and surrounding non-tumor astrocytes to transfer tumor-derived mitochondria. The mitochondrial transfer mediated by TNT resulted in the adaptation of non-tumor astrocytes to tumor-like metabolism and hypoxia conditions. In conclusion, TNT are an efficient cell-to-cell communication system used by cancer cells to adapt the microenvironment to the invasive nature of the tumor.

Astrocytes are HIV reservoirs in the brain: A cell type with poor HIV infectivity and replication but efficient cell-to-cell viral transfer

The major barrier to eradicating Human immunodeficiency virus-1 (HIV) infection is the generation of tissue-associated quiescent long-lasting viral reservoirs refractory to therapy. Upon interruption of anti-retroviral therapy (ART), HIV replication can be reactivated. Within the brain, microglia/macrophages and a small population of astrocytes are infected with HIV. However, the role of astrocytes as a potential viral reservoir is becoming more recognized because of the improved detection and quantification of HIV viral reservoirs. In this report, we examined the infectivity of human primary astrocytes in vivo and in vitro, and their capacity to maintain HIV infection, become latently infected, be reactivated, and transfer new HIV virions into neighboring cells. Analysis of human brain tissue sections obtained from HIV-infected individuals under effective and prolonged ART indicates that a small population of astrocytes has integrated HIV-DNA. In vitro experiments using HIV-infected human primary astrocyte cultures confirmed a low percentage of astrocytes had integrated HIV-DNA, with poor to undetectable replication. Even in the absence of ART, long-term culture results in latency that could be transiently reactivated with histone deacetylase inhibitor, tumor necrosis factor-alpha (TNF-α), or methamphetamine. Reactivation resulted in poor viral production but efficient cell-to-cell viral transfer into cells that support high viral replication. Together, our data provide a new understanding of astrocytes' role as viral reservoirs within the central nervous system (CNS).

CD4+ T cell depletion does not affect the level of viremia in chronically SHIVSF162P3N-infected Chinese cynomolgus monkeys

Chronically SHIV_SF162P3N-infected cynomolgus monkeys were used to determine the effects of the antibody-mediated acute CD4⁺ T cell depletion on viral load as well as on the immunological factors associated with disease progression. Compared with the control animals, CD4⁺ T cell-depleted animals with SHIV infection showed (i) little alteration in plasma viral load over the period of 22 weeks after the depletion; (ii) increased CD4⁺ T cell proliferation and turnover of macrophages at the early phase of the depletion, but subsequent decline to the basal levels; and (iii) little impact on the expression of the inflammatory cytokines and CC chemokines associated with disease progression. These findings indicate that the antibody-mediated acute CD4⁺ T cell depletion had minimal impact on plasma viral load and disease progression in chronically SHIV_SF162P3N-infected cynomolgus monkeys. Future investigations are necessary to identify the key factor(s) related to the immune activation and macrophage infection during the CD4 deletion in chronic viral infection.

Productive HIV infection in astrocytes can be established via a nonclassical mechanism

OBJECTIVE

Astrocytes are proposed to be a critical reservoir of HIV in the brain. However, HIV infection of astrocytes is inefficient in vitro except for cell-to-cell transmission from HIV-infected cells. Here, we explore mechanisms by which cell-free HIV bypasses entry and postentry barriers leading to a productive infection.

METHODS

HIV infection of astrocytes was investigated by a variety of techniques including transfection of CD4-expressing plasmid, treatment with lysosomotropic agents or using a transwell culture system loaded with HIV-infected lymphocytes. Infection was monitored by HIV-1 p24 in culture supernatants and integrated proviral DNA was quantified by Alu-PCR.

RESULTS

Persistent HIV infection could be established in astrocytes by transfection of proviral DNA, transduction with VSV-G-pseudotyped viruses, transient expression of CD4 followed by HIV infection, or simultaneous treatment with lysosomotropic chloroquine or Tat-HA2 peptide with HIV infection. In absence of these treatments, HIV entered via endocytosis as seen by electronmicroscopy and underwent lysosomal degradation without proviral integration, indicating endocytosis is a dead end for HIV in astrocytes. Nevertheless, productive infection was observed when astrocytes were in close proximity but physically separated from HIV-infected lymphocytes in the transwell cultures. This occurred with X4 or dual tropic R5X4 viruses and was blocked by an antibody or antagonist to CXCR4.

CONCLUSION

A CD4-independent, CXCR4-dependent mechanism of viral entry is proposed, by which immature HIV particles from infected lymphocytes might directly bind to CXCR4 on astrocytes and trigger virus--cell fusion during or after the process of viral maturation. This mechanism may contribute to the formation of brain HIV reservoirs.

Simian-Human Immunodeficiency Virus SHIV.CH505 Infection of Rhesus Macaques Results in Persistent Viral Replication and Induces Intestinal Immunopathology

Simian-human immunodeficiency viruses (SHIVs) have been utilized to test vaccine efficacy and characterize mechanisms of viral transmission and pathogenesis. However, the majority of SHIVs currently available have significant limitations in that they were developed using sequences from chronically HIV-infected individuals or uncommon HIV subtypes or were optimized for the macaque model by serially passaging the engineered virus in vitro or in vivo Recently, a newly developed SHIV, SHIV.C.CH505.375H.dCT (SHIV.CH505), which incorporates vpu-env (gp140) sequences from a transmitted/founder HIV-1 subtype C strain, was shown to retain attributes of primary HIV-1 strains. However, a comprehensive analysis of the immunopathology that results from infection with this virus, especially in critical tissue compartments like the intestinal mucosa, has not been completed. In this study, we evaluated the viral dynamics and immunopathology of SHIV.CH505 in rhesus macaques. In line with previous findings, we found that SHIV.CH505 is capable of infecting and replicating efficiently in rhesus macaques, resulting in peripheral viral kinetics similar to that seen in pathogenic SIV and HIV infection. Furthermore, we observed significant and persistent depletions of CCR5⁺ and CCR6⁺ CD4⁺ T cells in mucosal tissues, decreases in CD4⁺ T cells producing Th17 cell-associated cytokines, CD8⁺ T cell dysfunction, and alterations of B cell and innate immune cell function, indicating that SHIV.CH505 elicits intestinal immunopathology typical of SIV/HIV infection. These findings suggest that SHIV.CH505 recapitulates the early viral replication dynamics and immunopathogenesis of HIV-1 infection of humans and thus can serve as a new model for HIV-1 pathogenesis, treatment, and prevention research.IMPORTANCE The development of chimeric SHIVs has been instrumental in advancing our understanding of HIV-host interactions and allowing for in vivo testing of novel treatments. However, many of the currently available SHIVs have distinct drawbacks and are unable to fully reflect the features characteristic of primary SIV and HIV strains. Here, we utilize rhesus macaques to define the immunopathogenesis of the recently developed SHIV.CH505, which was designed without many of the limitations of previous SHIVs. We observed that infection with SHIV.CH505 leads to peripheral viral kinetics and mucosal immunopathogenesis comparable with those caused by pathogenic SIV and HIV. Overall, these data provide evidence of the value of SHIV.CH505 as an effective model of SIV/HIV infection and an important tool that can be used in future studies, including preclinical testing of new therapies or prevention strategies.

Friends Turn Foe-Astrocytes Contribute to Neuronal Damage in NeuroAIDS

Astrocytes play a wide variety of roles in the central nervous system (CNS). Various facets of astrocyte-neuron interplay, investigated for the past few decades, have placed these most abundant and important glial cell types to be of supreme importance for the maintenance of the healthy CNS. Interestingly, glial dysfunctions have proven to be the major contributor to neuronal loss in several CNS disorders and pathologies. Specifically, in the field of neuroAIDS, glial dysfunction-mediated neuronal stress is a major factor contributing to the HIV-1 neuropathogenesis. As there is increasing evidence that astrocytes harbor HIV-1 and serve as "safe haven" for the dormant virus in the brain, the indirect pathway of neuronal damage has taken over the direct neuronal damage in its contribution to HIV-1 neuropathogenesis. In this review, we provide a brief insight into the astrocyte functions and dysfunctions in different CNS conditions with an elaborated insight into neuroAIDS. Detailed understanding of the role of astrocytes in neuroAIDS will help in the better therapeutic management of the neurological problems associated with HIV-1 patients.

No detection of CD4-independent human immunodeficiency virus 1 envelope glycoproteins in brain tissue of patients with or without neurological complications

Macrophage (mac)-tropic human immnunodeficiency virus type 1 (HIV-1) and simian immnunodeficiency virus (SIV) in brain are associated with neurological disease. Mac-tropic HIV-1 evolves enhanced CD4 interactions that enable macrophage infection via CD4, which is in low abundance. In contrast, mac-tropic SIV is associated with CD4-independent infection via direct CCR5 binding. Recently, mac-tropic simian-human immunodeficiency virus (SHIV) from macaque brain was also reported to infect cells via CCR5 without CD4. Since SHIV envelope proteins (Envs) are derived from HIV-1, we tested more than 100 HIV-1 clade B Envs for infection of CD4-negative, CCR5⁺ Cf2Th/CCR5 cells. However, no infection was detected. Our data suggest that there are differences in the evolution of mac-tropism in SIV and SHIV compared to HIV-1 clade B due to enhanced interactions with CCR5 and CD4, respectively.

Gp120 V5 Is Targeted by the First Wave of Sequential Neutralizing Antibodies in SHIVSF162P3N-Infected Rhesus Macaques

Simian-human immunodeficiency virus (SHIV) infection provides a relevant animal model to study HIV-1 neutralization breadth. With previously identified SHIV_SF162P3N infected rhesus macaques that did or did not develop neutralization breadth, we characterized the transmitted/founder viruses and initial autologous/homologous neutralizing antibodies in these animals. The plasma viral load and blood CD4 count did not distinguish macaques with and without breadth, and only one tested homologous envelope clone revealed a trend for macaques with breadth to favor an early homologous response. In two macaques with breadth, GB40 and FF69, infected with uncloned SHIV_SF162P3N, multiple viral variants were transmitted, and the transmitted variants were not equal in neutralization sensitivity. The targets of initial autologous neutralizing antibodies, arising between 10 and 20 weeks post infection, were mapped to N462 glycan and G460a in gp120 V5 in GB40 and FF69, respectively. Although it is unclear whether these targets are related to later neutralization breadth development, the G460a target but not N462 glycan appeared more common in macaques with breadth than those without. Longitudinal plasmas revealed 2⁻3 sequential waves of neutralizing antibodies in macaques with breadth, implicating that 3 sequential envelope variants, if not more, may be required for the broadening of HIV-1 neutralizing antibodies.

Astrocytes sustain long-term productive HIV-1 infection without establishment of reactivable viral latency

The "shock and kill" HIV-1 cure strategy proposes eradication of stable cellular reservoirs by clinical treatment with latency-reversing agents (LRAs). Although resting CD4⁺ T cells latently infected with HIV-1 constitute the main reservoir that is targeted by these approaches, their consequences on other reservoirs such as the central nervous system are still unknown and should be taken into consideration. We performed experiments aimed at defining the possible role of astrocytes in HIV-1 persistence in the brain and the effect of LRA treatments on this viral sanctuary. We first demonstrate that the diminished HIV-1 production in a proliferating astrocyte culture is due to a reduced proliferative capacity of virus-infected cells compared with uninfected astrocytes. In contrast, infection of non-proliferating astrocytes led to a robust HIV-1 infection that was sustained for over 60 days. To identify astrocytes latently infected with HIV-1, we designed a new dual-color reporter virus called NL4.3 eGFP-IRES-Crimson that is fully infectious and encodes for all viral proteins. Although we detected a small fraction of astrocytes carrying silent HIV-1 proviruses, we did not observe any reactivation using various LRAs and even strong inducers such as tumor necrosis factor, thus suggesting that these proviruses were either not transcriptionally competent or in a state of deep latency. Our findings imply that astrocytes might not constitute a latent reservoir per se but that relentless virus production by this brain cell population could contribute to the neurological disorders seen in HIV-1-infected persons subjected to combination antiretroviral therapy.

Determinants of HIV-1 CD4-Independent Brain Adaptation

BACKGROUND

HIV-1 is known to adapt to the local environment in its usage of receptors, and it can become CD4 independent in the brain where the receptor is scarce. This adaptation is through amino acid variations, but the patterns of such variation are not yet well understood. Given that infection of long-lived CD4-low and CD4-negative cells in anatomical compartments such as the brain expands cell tropism in vivo and may serve as potential viral reservoirs that pose challenge for HIV eradication, understanding the evolution to CD4 independence and envelope conformation associated with infection in the absence of CD4 will not only broaden our insights into HIV pathogenesis but may guide functional cure strategies as well.

METHODS

We characterize, by site-directed mutagenesis, neutralization assay, and structural analysis, a pair of CD4-dependent (cl2) and CD4-independent (cl20) envelopes concurrently isolated from the cerebral spinal fluid of an SHIV-infected macaque with neurological AIDS and with minimum sequence differences.

RESULTS

Residues different between cl2 and cl20 are mapped to the V1V2 and surrounding regions. Mutations of these residues in cl2 increased its CD4 independence in infection, and the effects are cumulative and likely structural.

CONCLUSIONS

Our data suggested that the determinants of CD4 independence in vivo mapped principally to V1V2 of gp120 that can destabilize the apex of the envelope spike, with an additional change in V4 that abrogated a potential N-linked glycan to facilitate movement of the V1V2 domain and further expose the coreceptor-binding site.

Guidelines for cloning, expression, purification and functional characterization of primary HIV-1 envelope glycoproteins

The trimeric HIV-1 envelope (Env) glycoproteins gp120 and gp41 mediate virus entry into target cells by engaging CD4 and the coreceptors CCR5 or CXCR4 at the cell surface and driving membrane fusion. Receptor/gp120 interactions regulate the virus life cycle, HIV infection transmission and pathogenesis. Env is also the target of neutralizing antibodies. Efforts have thus been made to produce soluble HIV-1 glycoproteins to develop vaccines and study the role and mechanisms of HIV/receptor interactions. However, production and purification of Env glycoproteins and their functional assessment has to cope with multiple obstacles. These include difficulties in amplifying and cloning env sequences and setting up receptor binding assays that are suitable for studies on large collections of glycoproteins, flexible enough to adapt to Env and receptor structural heterogeneities, and allow recapitulating the receptor binding properties of virion-associated Env trimers. Here we identify these difficulties and present protocols to produce primary gp120 and determination of their binding properties to receptors. The receptor binding assays confirmed that the produced glycoproteins are competent for binding CD4 and undergo proper CD4-induced conformational changes required for interaction with CCR5. These assays may help elucidate the role of gp120/receptor interactions in the pathophysiology of HIV infection and develop HIV-1 entry inhibitors.

Quantifying the Sensitivity of HIV-1 Viral Entry to Receptor and Coreceptor Expression

Infection by many viruses begins with fusion of viral and cellular lipid membranes, followed by entry of viral contents into the target cell and ultimately, after many biochemical steps, integration of viral DNA into that of the host cell. The early steps of membrane fusion and viral capsid entry are mediated by adsorption to the cell surface, and receptor and coreceptor binding. HIV-1 specifically targets CD4+ helper T-cells of the human immune system and binds to the receptor CD4 and coreceptor CCR5 before fusion is initiated. Previous experiments have been performed using a cell line (293-Affinofile) in which the expressions of CD4 and CCR5 concentration were independently controlled. After exposure to HIV-1 of various strains, the resulting infectivity was measured through the fraction of infected cells. To design and evaluate the effectiveness of drug therapies that target the inhibition of the entry processes, an accurate functional relationship between the CD4/CCR5 concentrations and infectivity is desired in order to more quantitatively analyze experimental data. We propose three kinetic models describing the possible mechanistic processes involved in HIV entry and fit their predictions to infectivity measurements, contrasting and comparing different outcomes. Our approach allows interpretation of the clustering of infectivity of different strains of HIV-1 in the space of mechanistic kinetic parameters. Our model fitting also allows inference of nontrivial stoichiometries of receptor and coreceptor binding and provides a framework through which to quantitatively investigate the effectiveness of fusion inhibitors and neutralizing antibodies.

Envelope residue 375 substitutions in simian-human immunodeficiency viruses enhance CD4 binding and replication in rhesus macaques

Most simian-human immunodeficiency viruses (SHIVs) bearing envelope (Env) glycoproteins from primary HIV-1 strains fail to infect rhesus macaques (RMs). We hypothesized that inefficient Env binding to rhesus CD4 (rhCD4) limits virus entry and replication and could be enhanced by substituting naturally occurring simian immunodeficiency virus Env residues at position 375, which resides at a critical location in the CD4-binding pocket and is under strong positive evolutionary pressure across the broad spectrum of primate lentiviruses. SHIVs containing primary or transmitted/founder HIV-1 subtype A, B, C, or D Envs with genotypic variants at residue 375 were constructed and analyzed in vitro and in vivo. Bulky hydrophobic or basic amino acids substituted for serine-375 enhanced Env affinity for rhCD4, virus entry into cells bearing rhCD4, and virus replication in primary rhCD4 T cells without appreciably affecting antigenicity or antibody-mediated neutralization sensitivity. Twenty-four RMs inoculated with subtype A, B, C, or D SHIVs all became productively infected with different Env375 variants-S, M, Y, H, W, or F-that were differentially selected in different Env backbones. Notably, SHIVs replicated persistently at titers comparable to HIV-1 in humans and elicited autologous neutralizing antibody responses typical of HIV-1. Seven animals succumbed to AIDS. These findings identify Env-rhCD4 binding as a critical determinant for productive SHIV infection in RMs and validate a novel and generalizable strategy for constructing SHIVs with Env glycoproteins of interest, including those that in humans elicit broadly neutralizing antibodies or bind particular Ig germ-line B-cell receptors.

SIV Infection Impairs the Central Nervous System in Chinese Rhesus Macaques

The central nervous system (CNS) impairment is a consequence seen in SIV infection of rhesus macaques of Indian-origin, which is more common in infected macaques with rapid disease progression than in those with conventional disease progression. Here, we investigated the CNS damages in SIVmac239-infected Chinese rhesus macaques. We demonstrated that SIV infection of Chinese macaques could cause neuropathological impairments, which was evidenced by appearance of SIV-RNA positive cells, the infiltration of activated macrophages and abundant multinucleated giant cells (MNGCs) in the different regions of the brains. The animals with high viremia and short survival time (average of 16 weeks, rapid progression, RP) had severer neuropathological changes than those with conventional progression (CP). As compared with the RP animals, CP macaques had lower viremia and much longer survival time (average of 154 weeks). These findings indicate that SIVmac239 infection of Chinese rhesus macaque can be used as a suitable animal model and alternative resource for nueroAIDS research.

Brain Invasion by CD4(+) T Cells Infected with a Transmitted/Founder HIV-1BJZS7 During Acute Stage in Humanized Mice

Human immunodeficiency virus (HIV)-associated neurocognitive disorder (HAND) is one of the common causes of cognitive dysfunction and morbidity among infected patients. However, to date, it remains unknown if a transmitted/founder (T/F) HIV-1 leads to neurological disorders during acute phase of infection. Since it is impossible to answer this question in humans, we studied NOD.Cg-Prkdc scid Il2rgtm1Wjl/SzJ mice (NSG) reconstituted with human PBMC (NSG-HuPBL), followed by the peritoneal challenge with the chronic HIV-1JR-FL and the T/F HIV-1BJZS7, respectively. By measuring viral load, P24 antigenemia and P24(+) cells in peripheral blood and various tissue compartments, we found that systemic infections were rapidly established in NSG-HuPBL mice by both HIV-1 strains. Although comparable peripheral viral loads were detected during acute infection, the T/F virus appeared to cause less CD4(+) T cell loss and less numbers of infected cells in different organs and tissue compartments. Both viruses, however, invaded brains with P24(+)/CD3(+) T cells detected primarily in meninges, cerebral cortex and perivascular areas. Critically, brain infections with HIV-1JR-FL but not with HIV-1BJZS7 resulted in damaged neurons together with activated microgliosis and astrocytosis as determined by significantly increased numbers of Iba1(+) microglial cells and GFAP(+) astrocytes, respectively. The increased Iba1(+) microglia was correlated positively with levels of P24 antigenemia and negatively with numbers of NeuN(+) neurons in brains of infected animals. Our findings, therefore, indicate the establishment of two useful NSG-HuPBL models, which may facilitate future investigation of mechanisms underlying HIV-1-induced microgliosis and astrocytosis.

Cell-to-cell contact facilitates HIV transmission from lymphocytes to astrocytes via CXCR4

OBJECTIVES

HIV reservoir in the brain represents a major barrier for curing HIV infection. As the most abundant, long-lived cell type, astrocytes play a critical role in maintaining the reservoir; however, the mechanism of infection remains unknown. Here, we determine how viral transmission occurs from HIV-infected lymphocytes to astrocytes by cell-to-cell contact.

DESIGN AND METHODS

Human astrocytes were exposed to HIV-infected lymphocytes and monitored by live-imaging, confocal microscopy, transmission and three-dimensional electron microscopy. A panel of receptor antagonists was used to determine the mechanism of viral entry.

RESULTS

We found that cell-to-cell contact resulted in efficient transmission of X4 or X4R5-using viruses from T lymphocytes to astrocytes. In co-cultures of astrocytes with HIV-infected lymphocytes, the interaction occurred through a dynamic process of attachment and detachment of the two cell types. Infected lymphocytes invaginated into astrocytes or the contacts occurred via filopodial extensions from either cell type, leading to the formation of virological synapses. In the synapses, budding of immature or incomplete HIV particles from lymphocytes occurred directly onto the membranes of astrocytes. This cell-to-cell transmission could be almost completely blocked by anti-CXCR4 antibody and its antagonist, but only partially inhibited by anti-CD4, ICAM1 antibodies.

CONCLUSION

Cell-to-cell transmission was mediated by a unique mechanism by which immature viral particles initiated a fusion process in a CXCR4-dependent, CD4-independent manner. These observations have important implications for developing approaches to prevent formation of HIV reservoirs in the brain.

Neuropathogenesis of HIV: from initial neuroinvasion to HIV-associated neurocognitive disorder (HAND)

Early in the HIV epidemic, the central nervous system (CNS) was recognized as a target of infection and injury in the advanced stages of disease. Though the most severe forms of HIV-associated neurocognitive disorder (HAND) related to severe immunosuppression are rare in the current era of widespread combination antiretroviral therapy (cART), evidence now supports pathological involvement of the CNS throughout the course of infection. Recent work suggests that the stage for HIV neuropathogenesis may be set with initial viral entry into the CNS, followed by initiation of pathogenetic processes including neuroinflammation and neurotoxicity, and establishment of local, compartmentalized HIV replication that may reflect a tissue reservoir for HIV. Key questions still exist as to when HIV establishes local infection in the CNS, which CNS cells are the primary targets of HIV, and what mechanistic processes underlie the injury to neurons that produce clinical symptoms of HAND. Advances in these areas will provide opportunities for improved treatment of patients with established HAND, prevention of neurological disease in those with early stage infection, and understanding of HIV tissue reservoirs that will aid efforts at HIV eradication.

HIV-1 endocytosis in astrocytes: a kiss of death or survival of the fittest?

The brain is a target of HIV-1 and serves as an important viral reservoir. Astrocytes, the most abundant glial cell in the human brain, are involved in brain plasticity and neuroprotection. Several studies have reported HIV-1 infection of astrocytes in cell cultures and infected brain tissues. The prevailing concept is that HIV-1 infection of astrocytes leads to latent infection. Here, we provide our perspective on endocytosis-mediated HIV-1 entry and its fate in astrocytes. Natural entry of HIV-1 into astrocytes occurs via endocytosis. However, endocytosis of HIV-1 in astrocytes is a natural death trap where the majority of virus particles are degraded in endosomes and a few which escape intact lead to successful infection. Thus, regardless of artificial fine-tuning (treatment with cytokines or proinflammatory products) done to astrocytes, HIV-1 does not infect them efficiently unless the viral entry route or the endosomal enzymatic machinery has been manipulated.