The decreased replicative capacity of simian immunodeficiency virus SIVmac239Delta(nef) is manifest in cultures of immature dendritic cellsand T cells

Halophilic Archaea Halorhabdus Rudnickae and Natrinema Salaciae Activate Human Dendritic Cells and Orient T Helper Cell Responses

Halophilic archaea are procaryotic organisms distinct from bacteria, known to thrive in hypersaline environments, including salt lakes, salterns, brines and salty food. They have also been identified in the human microbiome. The biological significance of halophiles for human health has rarely been examined. The interactions between halophilic archaea and human dendritic cells (DCs) and T cells have not been identified so far. Here, we show for the first time that the halophilic archaea Halorhabdus rudnickae and Natrinema salaciae activate human monocyte-derived DCs, induce DC maturation, cytokine production and autologous T cell activation. In vitro both strains induced DC up-regulation of the cell-surface receptors CD86, CD80 and CD83, and cytokine production, including IL-12p40, IL-10 and TNF-α, but not IL-23 and IL-12p70. Furthermore, autologous CD4⁺ T cells produced significantly higher amounts of IFN-γ and IL-13, but not IL-17A when co-cultured with halophile-stimulated DCs in comparison to T cells co-cultured with unstimulated DCs. IFN-γ was almost exclusively produced by naïve T cells, while IL-13 was produced by both naïve and memory CD4⁺ T cells. Our findings thus show that halophilic archaea are recognized by human DCs and are able to induce a balanced cytokine response. The immunomodulatory functions of halophilic archaea and their potential ability to re-establish the immune balance may perhaps participate in the beneficial effects of halotherapies.

PolyICLC Exerts Pro- and Anti-HIV Effects on the DC-T Cell Milieu In Vitro and In Vivo

Myeloid dendritic cells (mDCs) contribute to both HIV pathogenesis and elicitation of antiviral immunity. Understanding how mDC responses to stimuli shape HIV infection outcomes will inform HIV prevention and treatment strategies. The long double-stranded RNA (dsRNA) viral mimic, polyinosinic polycytidylic acid (polyIC, PIC) potently stimulates DCs to focus Th1 responses, triggers direct antiviral activity in vitro, and boosts anti-HIV responses in vivo. Stabilized polyICLC (PICLC) is being developed for vaccine adjuvant applications in humans, making it critical to understand how mDC sensing of PICLC influences HIV infection. Using the monocyte-derived DC (moDC) model, we sought to describe how PICLC (vs. other dsRNAs) impacts HIV infection within DCs and DC-T cell mixtures. We extended this work to in vivo macaque rectal transmission studies by administering PICLC with or before rectal SIVmac239 (SIVwt) or SIVmac239ΔNef (SIVΔNef) challenge. Like PIC, PICLC activated DCs and T cells, increased expression of α₄β₇ and CD169, and induced type I IFN responses in vitro. The type of dsRNA and timing of dsRNA exposure differentially impacted in vitro DC-driven HIV infection. Rectal PICLC treatment similarly induced DC and T cell activation and pro- and anti-HIV factors locally and systemically. Importantly, this did not enhance SIV transmission in vivo. Instead, SIV acquisition was marginally reduced after a single high dose challenge. Interestingly, in the PICLC-treated, SIVΔNef-infected animals, SIVΔNef viremia was higher, in line with the importance of DC and T cell activation in SIVΔNef replication. In the right combination anti-HIV strategy, PICLC has the potential to limit HIV infection and boost HIV immunity.

Adenovirus-based vaccines against rhesus lymphocryptovirus EBNA-1 induce expansion of specific CD8+ and CD4+ T cells in persistently infected rhesus macaques

The impact of Epstein-Barr virus (EBV) on human health is substantial, but vaccines that prevent primary EBV infections or treat EBV-associated diseases are not yet available. The Epstein-Barr nuclear antigen 1 (EBNA-1) is an important target for vaccination because it is the only protein expressed in all EBV-associated malignancies. We have designed and tested two therapeutic EBV vaccines that target the rhesus (rh) lymphocryptovirus (LCV) EBNA-1 to determine if ongoing T cell responses during persistent rhLCV infection in rhesus macaques can be expanded upon vaccination. Vaccines were based on two serotypes of E1-deleted simian adenovirus and were administered in a prime-boost regimen. To further modulate the response, rhEBNA-1 was fused to herpes simplex virus glycoprotein D (HSV-gD), which acts to block an inhibitory signaling pathway during T cell activation. We found that vaccines expressing rhEBNA-1 with or without functional HSV-gD led to expansion of rhEBNA-1-specific CD8(+) and CD4(+) T cells in 33% and 83% of the vaccinated animals, respectively. Additional animals developed significant changes within T cell subsets without changes in total numbers. Vaccination did not increase T cell responses to rhBZLF-1, an immediate early lytic phase antigen of rhLCV, thus indicating that increases of rhEBNA-1-specific responses were a direct result of vaccination. Vaccine-induced rhEBNA-1-specific T cells were highly functional and produced various combinations of cytokines as well as the cytolytic molecule granzyme B. These results serve as an important proof of principle that functional EBNA-1-specific T cells can be expanded by vaccination.

IMPORTANCE

EBV is a common human pathogen that establishes a persistent infection through latency in B cells, where it occasionally reactivates. EBV infection is typically benign and is well controlled by the host adaptive immune system; however, it is considered carcinogenic due to its strong association with lymphoid and epithelial cell malignancies. Latent EBNA-1 is a promising target for a therapeutic vaccine, as it is the only antigen expressed in all EBV-associated malignancies. The goal was to determine if rhEBNA-1-specific T cells could be expanded upon vaccination of infected animals. Results were obtained with vaccines that target EBNA-1 of rhLCV, a virus closely related to EBV. We found that vaccination led to expansion of rhEBNA-1 immune cells that exhibited functions fit for controlling viral infection. This confirms that rhEBNA-1 is a suitable target for therapeutic vaccines. Future work should aim to generate more-robust T cell responses through modified vaccines.

Contributions of HIV-1 Nef to immune dysregulation in HIV-infected patients: a therapeutic target?

INTRODUCTION

HIV accessory protein Nef is a factor responsible for many of the viral pathogenic effects. Progression to AIDS is dramatically delayed and in some well-documented cases completely abolished on infection with naturally occurring HIV strains lacking intact nef sequences in their genomes. The topic of this review is the contribution of Nef to the immune pathology as a possible target in HIV-infected patients.

AREAS COVERED

An overview of known Nef functions accounting for its role in pathogenesis is presented, emphasizing interactions with dendritic cells and macrophages, and Nef-induced exosome secretion, all involved in immune dysregulation during the course of HIV infection. Current approaches to Nef inhibition by different classes of compounds are reviewed.

EXPERT OPINION

Blocking Nef for therapeutic purposes is a challenging endeavor mainly due to intrinsic properties of this HIV accessory protein. Nef has multiple interfaces to interact with host proteins and lacks a catalytic domain. Potential benefits arising from the development of successful inhibitors could however prove beneficial for reducing gradual deterioration of immune system in chronically infected patients in absence of functional cure.

Mobilization of HIV spread by diaphanous 2 dependent filopodia in infected dendritic cells

Paramount to the success of persistent viral infection is the ability of viruses to navigate hostile environments en route to future targets. In response to such obstacles, many viruses have developed the ability of establishing actin rich-membrane bridges to aid in future infections. Herein through dynamic imaging of HIV infected dendritic cells, we have observed how viral high-jacking of the actin/membrane network facilitates one of the most efficient forms of HIV spread. Within infected DC, viral egress is coupled to viral filopodia formation, with more than 90% of filopodia bearing immature HIV on their tips at extensions of 10 to 20 µm. Live imaging showed HIV filopodia routinely pivoting at their base, and projecting HIV virions at µm.sec⁻¹ along repetitive arc trajectories. HIV filopodial dynamics lead to up to 800 DC to CD4 T cell contacts per hour, with selection of T cells culminating in multiple filopodia tethering and converging to envelope the CD4 T-cell membrane with budding HIV particles. Long viral filopodial formation was dependent on the formin diaphanous 2 (Diaph2), and not a dominant Arp2/3 filopodial pathway often associated with pathogenic actin polymerization. Manipulation of HIV Nef reduced HIV transfer 25-fold by reducing viral filopodia frequency, supporting the potency of DC HIV transfer was dependent on viral filopodia abundance. Thus our observations show HIV corrupts DC to CD4 T cell interactions by physically embedding at the leading edge contacts of long DC filopodial networks.

Dendritic cells represent a unique cell type with respect to HIV, as they are the first point of contact for the virus in the genital mucosa and have the ability to spread virus efficiently in very low numbers to the primary HIV target, CD4 T cells. During the primary immune response, dendritic cells work in small numbers to make numerous and repetitive contacts, in order to filter and communicate with appropriate CD4 T cells. Thus HIV is hypothesized to be hijacking the same DC-CD4 T cell communication. Attempts to observe how HIV would achieve this have largely been limited, as introduction of imaging markers in the virus has often led to significant viral attenuation. Herein by using novel HIV constructs that permit imaging of HIV in infected dendritic cells, we observed newly forming HIV virions on the tips of long finger-like projections known as filopodia. In real-time imaging filopodia pivoted at their base and moved virions along trajectories that led to numerous CD4 T cell contacts. By manipulating filopodial formation we conclude the location of the virus on long filopodial tips allows the virus to corrupt the promiscuous dendritic cell to CD4 T cell contacts for efficient viral spread.

Modulation of HIV pathogenesis and T-cell signaling by HIV-1 Nef

HIV-1 Nef protein is an approximately 27-kDa myristoylated protein that is a virulence factor essential for efficient viral replication and infection in CD4(+) T cells. The functions of CD4(+) T cells are directly impeded after HIV infection. HIV-1 Nef plays a crucial role in manipulating host cellular machinery and in HIV pathogenesis by reducing the ability of infected lymphocytes to form immunological synapses by promoting virological synapses with APCs, and by affecting T-cell stimulation. This article reviews the current status of the efficient Nef-mediated spread of virus in the unreceptive environment of the immune system by altering CD4(+) T-lymphocyte signaling, intracellular trafficking, cell migration and apoptotic pathways.

Simian immunodeficiency virus interactions with macaque dendritic cells

This chapter summarizes advances in the following areas: (1) dendritic cell (DC)-mediated simian immunodeficiency virus (SIV) transmission, (2) role of DCs in innate and adaptive immunity against SIV, and (3) approaches to harness DC function to induce anti-SIV responses. The nonhuman primate (NHP) model of human immunodeficiency virus (HIV) infection in rhesus macaques and other Asian NHP species is highly relevant to advance the understanding of virus-host interactions critical for transmission and disease pathogenesis. HIV infection is associated with changes in frequency, phenotype, and function of the two principal subsets of DCs, myeloid DCs and plasmacytoid DCs. DC biology during pathogenic SIV infection is strikingly similar to that observed in HIV-infected patients. The NHP models provide an opportunity to dissect the requirements for DC-driven SIV infection and to understand how SIV distorts the DC system to its advantage. Furthermore, the SIV model of mucosal transmission enables the study of the earliest events of infection at the portal of entry that cannot be studied in humans, and, importantly, the involvement of DCs. Nonpathogenic infection in African NHP hosts allows investigations into the role of DCs in disease control. Understanding how DCs are altered during SIV infection is critical to the design of therapeutic and preventative strategies against HIV.

Attachment and fusion inhibitors potently prevent dendritic cell-driven HIV infection

Dendritic cells (DCs) efficiently transfer captured (trans) or de novo-produced (cis) virus to CD4 T cells. Using monocyte-derived DCs, we evaluated entry inhibitors targeting HIV envelope (BMS-C, T-1249) or CCR5 (CMPD167) for their potency to prevent DC infection, DC-driven infection in T cells in trans and cis, and direct infection of DC-T-cell mixtures. Immature DC-T-cell cultures with distinct mechanisms of viral transfer yielded similar levels of infection and produced more proviral DNA compared with matched mature DC-T-cell cultures or infected immature DCs. Although all compounds completely blocked HIV replication, 16 times more of each inhibitor (250 vs 15.6 nM) was required to prevent low-level infection of DCs compared with the productive DC-T-cell cocultures. Across all cell systems tested, BMS-C blocked infection most potently. BMS-C was significantly more effective than CMPD167 at preventing DC infection. In fact, low doses of CMPD167 significantly enhanced DC infection. Elevated levels of CCL4 were observed when immature DCs were cultured with CMPD167. Viral entry inhibitors did not interfere with Candida albicans-specific DC cytokine/chemokine responses. These findings indicate that an envelope-binding small molecule is a promising tool for topical microbicide design to prevent the infection of early targets needed to establish and disseminate HIV infection.

Nef does not inhibit F-actin remodelling and HIV-1 cell-cell transmission at the T lymphocyte virological synapse

Nef, a HIV-1 pathogenesis factor, elevates virus replication in vivo and thus progression to AIDS by incompletely defined mechanisms. As one of its biological properties, Nef enhances the infectivity of cell-free HIV-1 particles in single round infections, however it fails to provide a significant and amplifying growth advantage for HIV-1 on such virus producing cells. A major difference between HIV-1 cell-free single round infections and virus replication kinetics on T lymphocytes consists in the predominant role of cell-associated virus transmission rather than cell-free infection during multiple round virus replication. HIV-1 cell-to-cell transmission occurs across close cell contacts also referred to as virological synapse (VS) and involves polarization of the F-actin cytoskeleton, formation of F-actin rich membrane bridges as well as virus budding to cell-cell contacts. Since Nef potently interferes with triggered actin remodelling in several cell systems to reduce e.g. cell motility and signal transduction, we set out here to address whether Nef also affects organization and possibly function of the T lymphocyte VS. We find that in addition to increasing infectivity of cell-free virions, Nef can also moderately enhance single rounds of HIV-1 cell-cell transmission between Jurkat T lymphocytes. This occurs without affecting cell conjugation efficiencies or polarization of F-actin and HIV-1 p24Gag at the VS, identifying actin remodelling at the VS as an example of Nef-insensitive host cell actin rearrangements. However, Nef-mediated enhancement of single round cell-free infection or cell-to-cell transmission does not potentiate over multiple rounds of infection. These results suggest that Nef affects cell-free and cell-associated HIV-1 infection by the same mechanism acting on the intrinsic infectivity of HIV-1 particles. They further indicate that the high efficacy of cell-to-cell transmission can compensate such infectivity defects. Nef therefore selectively interferes with actin remodelling processes involved in antiviral host cell defense while actin driven processes that promote virus propagation remain unaltered.

HIV-1 at the immunological and T-lymphocytic virological synapse

Cell-cell transmission of human immunodeficiency virus type 1 (HIV-1) is considered the most effective mode of viral spread in T-lymphocyte cultures. Evidence has accumulated that HIV-1 assembles polarized synaptic-like structures, referred to as virological synapses, as specialized sites of viral transfer. Interestingly, it was recently also discovered that HIV-1 impairs the formation of the structurally similar immunological synapse, thereby modulating exogenous T-lymphocyte stimulation to yield an optimal activation state for productive HIV-1 infection. The careful dissection of these opposing effects will contribute to our understanding of retroviral spread and cellular signal transduction machineries.

Immunoregulatory effects of HIV-1 Nef protein

During HIV infection, the perturbation of the adaptive and innate immune responses contributes to the progressive immunosuppression leading to an increased susceptibility to opportunistic infections and neoplastic diseases. Several impairments observed in HIV-infected patients include a gradual loss of CD4(+) T cells, CD8(+) T cell dysfunction, and a decreased number and function of natural killer (NK) cells. Moreover, a functional impairment and variation in the number of DC and B cells were observed during HIV infection. HIV-1 codes for proteins, including the accessory Nef proteins, that interacting with immune cells may contribute to AIDS pathogenesis. Here, we review the recent progress on the immunomodulatory effect of the accessory Nef protein and its role in the pathogenesis of HIV-1 infection. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Determinants in HIV-1 Nef for enhancement of virus replication and depletion of CD4+ T lymphocytes in human lymphoid tissue ex vivo

Background

HIV-1 Nef critically contributes to AIDS in part by augmenting virus titers in infected individuals. Analyzing which of Nef's activities contribute to HIV pathogenesis has been hampered by the lack of a cell culture model in which Nef exerts pronounced effects on HIV replication. The human lymphoid aggregate culture (HLAC) from tonsil maintains the cell populations and cytokine milieu found in vivo, supports a productive infection without exogenous stimulation, and Nef contributes to efficient HIV-1 replication as well as CD4⁺ T cell depletion in this experimental ex vivo-model.

Results

To identify determinants in Nef that mediate these activities, we infected HLAC with a panel of isogenic HIV-1_NL4-3 strains that encode for well-characterized mutants of HIV-1_SF2 Nef. Determination of HIV-1 replication revealed that enhancement of the virus spread by Nef is governed by a complex set of protein interaction surfaces. In contrast, increased CD4⁺ T lymphocyte depletion depended on only two protein interaction surfaces in Nef that mediate either downregulation of cell surface CD4 or interaction with the NAKC signalosome. Consistently, in HLAC from 9 out of 14 donors, Nef enhanced CD4⁺ T cell depletion in the absence of a significant effect on virus replication. Moreover, our results suggest that this Nef-dependent enhancement in depletion occurred predominately in uninfected bystander CD4⁺ T cells.

Conclusion

Our findings suggest that Nef facilitates depletion of CD4⁺ T lymphocytes in HIV-1-infected lymphoid tissue ex vivo by increasing the pool of productively infected cells and by sensitizing bystander cells for killing. This ability might contribute to Nef's pathogenic potential in vivo.

Feline immunodeficiency virus dendritic cell infection and transfer

Feline immunodeficiency virus (FIV) interacts with dendritic cells (DC) during initiation of infection, but whether DC support or transfer FIV infection remains unclear. To address this issue, we studied the susceptibility of feline myeloid DC to FIV infection and assessed potential transfer of infection from DC to CD4(+) T cells. FIV was detected in membrane-bound vesicles of DC within 2 h of inoculation, although only low concentrations of FIV DNA were found in virus-exposed isolated DC. Addition of resting CD4(+) T cells increased viral DNA levels; however, addition of activated CD4(+) T cells resulted in a burst of viral replication manifested by FIV p27 capsid antigen generation. To determine whether transfer of FIV infection required productively infected DC (vs virus bound to DC but not internalized), virus-exposed DC were cultured for 2 days to allow for degradation of uninternalized virus and initiation of infection in the DC, then CD4(+) T blasts were added. Infection of T cells remained robust, indicating that T-cell infection is likely to be mediated by de novo viral infection of DC followed by viral transfer during normal DC/T-cell interactions. We conclude that feline DC support restricted FIV infection, which nevertheless is sufficient to efficiently transfer infection to susceptible T cells and trigger the major burst of viral replication. Feline DC/FIV/T-cell interactions (similar to those believed to occur in human immunodeficiency virus and simian immunodeficiency virus infections) highlight the means by which immunodeficiency-inducing lentiviruses exploit normal DC/T-cell interactions to transfer and amplify virus infection.

Changes in simian immunodeficiency virus reverse transcriptase alleles that appear during infection of macaques enhance infectivity and replication in CD4+ T cells

We previously showed that a slowly replicating, minimally pathogenic clone of simian immunodeficiency virus (SIV), SIVmneCl8, evolves increased ability to replicate in T cells with the onset of AIDS in pig-tailed macaques. Moreover, molecular clones derived from late stages of infection (SIVmne170 and SIVmne027) replicate to high levels in vivo compared to SIVmneCl8. Here, we investigated the role of rt mutations in SIVmne variant replication. We demonstrate selection for rt alleles that enhance viral infectivity and replication capacity in CD4(+) T cells. Moreover, the ability of SIVmne to be induced from resting CD4(+) T cells by anti-CD3/CD28 stimulation is more strongly influenced by the variant rt alleles than nef alleles. Taken together, our data underscore the importance of RT determinants for pathogenicity of SIV and for the capacity to replicate in CD4(+) T cell populations.

Macrophage Tropism and Cytopathicity of HIV-1 Variants Isolated Sequentially from a Long-Term Survivor Infected with nef-Deleted Virus

Long-term survival of human immunodeficiency virus type 1 (HIV-1) infection has been noted in rare cohorts of individuals infected with nef-deleted virus. Enhanced macrophage tropism and cytopathicity contribute to pathogenicity of wild type HIV-1. To better understand the pathogenesis of nef-deleted HIV-1, we analyzed the replication capacity and macrophage cytopathicity of nef-deleted HIV-1 isolated sequentially from a long-term survivor during progression to AIDS (n=6 isolates). Compared with controls, all nef-deleted viruses replicated to low levels in peripheral blood mononu-clear cells and monocyte-derived macrophages (MDM). One nef-deleted virus that was isolated on the development of AIDS caused high levels of syncytia in MDM similar to control viruses, but five viruses isolated from earlier times prior to AIDS onset caused only minimal cytopathicity. Together, these results suggest that enhanced cytopathicity of nef-deleted HIV-1 for MDM can occur independently of replication capacity, and may contribute to the pathogenesis of nef-deleted HIV-1 infection.

Modulation of the immunological synapse: a key to HIV-1 pathogenesis?

AIDS is the result of a constant struggle between the lentivirus HIV and the immune system. Infection with HIV interferes directly with the function of CD4(+) T cells and manipulates the host immune response to the virus. Recent studies indicate that the viral protein Nef, a central player in HIV pathogenesis, impairs the ability of infected lymphocytes to form immunological synapses with antigen-presenting cells and affects T-cell-receptor-mediated stimulation. An integrative picture of the abnormal behaviour of HIV-infected lymphocytes is therefore emerging. We propose that modulating lymphocyte signalling, apoptosis and intracellular trafficking ensures efficient spread of the virus in the hostile environment of the immune system.

Feline immunodeficiency virus infection is enhanced by feline bone marrow-derived dendritic cells

In the pathogenesis of feline immunodeficiency virus (FIV) infection, feline dendritic cells (feDCs) are thought to play an important role. As with DCs in other species, feDCs are believed to transport virus particles to lymph nodes and transfer them to lymphocytes. Our investigation has focused on the ability of feDCs to influence the infection of syngeneic peripheral blood mononuclear cells (PBMCs) and allogeneic thymocytes. feDCs were derived from bone marrow mononuclear cells that were cultured under the influence of feline interleukin-4 and feline granulocyte–macrophage colony-stimulating factor. By using these feDCs in co-culture with resting PBMCs, an upregulation of FIV replication was shown. An enhancement of FIV infection was also detected when co-cultures of feDCs/feline thymocytes were infected. To obtain this enhancement, direct contact of the cells in the co-culture was necessary; transwell cultures showed that the involvement of only soluble factors produced by feDCs in this process is not likely. These feDCs were also able to induce the proliferation of resting thymocytes, which might explain the enhanced FIV replication observed. Together, these data suggest that feDCs have abilities similar to those shown for simian and human DCs in the interaction with leukocytes. This system is suitable for further investigations of the interplay of DC and T cells during FIV infection in vitro.

The immunoregulatory effects of HIV‐1 Nef on dendritic cells and the pathogenesis of AIDS

Dendritic cells (DC) play a crucial role in the generation and regulation of immunity, and their interaction with HIV is relevant in the pathogenesis of AIDS favoring both the initial establishment and spread of the infection and the development of antiviral immunity. HIV-1 Nef is an essential factor for efficient viral replication and pathogenesis, and several studies have been addressed to assess the possible influence of endogenous or exogenous Nef on DC biology. Our findings and other reported data described in this review demonstrate that Nef subverts DC biology interfering with phenotypical, morphological, and functional DC developmental programs, thus representing a viral tool underlying AIDS pathogenesis. This review provides an overview on the mechanism by which Nef, hijacking DC functional activity, may favor both the replication of HIV-1 and the escape from immune surveillance. Overall, the findings described here may contribute to the understanding of Nef function, mechanism of action, and cellular partners. Further elucidation of genes induced through Nef signaling in DC could reveal pathways used by DC to drive HIV spread and will be critical to identify therapeutic strategies to bias the DC system toward activation of antiviral immunity instead of facilitating virus dissemination.

Transcriptional activity of blood-and cerebrospinal fluid-derived nef/long-terminal repeat sequences isolated from a slow progressor infected with nef-deleted human immunodeficiency virus type 1 (HIV-1) who developed HIV-associated dementia

The authors studied the transcriptional activity of blood-and cerebrospinal fluid (CSF)-derived nef/long-terminal repeat (LTR) sequences isolated from a slow progressor infected with nef-deleted human immunodeficiency virus type 1 (HIV-1) who developed HIV-associated dementia (HIVD). The transcriptional activity of CSF-derived nef/LTR clones isolated during HIVD was up to 4.5-fold higher than blood-derived clones isolated before and during HIVD when tested under basal, phorbol 12-myristate 13-acetate-(PMA-), and Tat-activated conditions, and was associated with the presence of duplicated nuclear factor (NF)-kappaB and specificity factor-1 (Sp-1) binding sites coupled with a truncated nef sequence, increased replication capacity, and high CSF viral load. Thus, nef and LTR mutations that augment transcription may contribute to neuropathogenesis of nef-deleted HIV-1.

The HIV-1 pathogenicity factor Nef interferes with maturation of stimulatory T-lymphocyte contacts by modulation of N-Wasp activity

The Nef protein is a key determinant of human immunodeficiency virus (HIV) pathogenicity that, among other activities, sensitizes T-lymphocytes for optimal virus production. The initial events by which Nef modulates the T-cell receptor (TCR) cascade are poorly understood. TCR engagement triggers actin rearrangements that control receptor clustering for signal initiation and dynamic organization of signaling protein complexes to form an immunological synapse. Here we report that Nef potently interferes with cell spreading and formation of actin-rich circumferential rings in T-lymphocytes upon surface-supported TCR stimulation. These effects were conserved among Nef proteins from different lentiviruses and occurred in HIV-1-infected primary human T-lymphocytes. This novel Nef activity critically depended on its Src homology 3 domain binding motif and required efficient association with Pak2 activity. Notably, whereas overall signaling microcluster formation immediately following TCR engagement occurred normally in Nef-expressing cells, the viral protein inhibited the concomitant activation of the actin organizer N-Wasp. During the subsequent maturation phase of the stimulatory contact, Nef interfered with the translocation of N-Wasp to the cell periphery, the overall induction of tyrosine phosphorylation, and the selective recruitment of phosphorylated LAT to stimulatory contacts. Consistent with such a critical role of N-Wasp in this process, Nef also blocked morphological changes induced by the known N-Wasp regulators Rac1 and Cdc42. Together, our results demonstrate that Nef alters both the amount and composition of signaling microclusters. We propose modulation of actin dynamics as an important mechanism for Nef-induced alterations of TCR signaling.

Longitudinal analysis of human immunodeficiency virus type 1 nef/long terminal repeat sequences in a cohort of long-term survivors infected from a single source

We studied the evolution of human immunodeficiency virus type 1 (HIV-1) in a cohort of long-term survivors infected with an attenuated strain of HIV-1 acquired from a single source. Although the cohort members experienced differing clinical courses, we demonstrate similar evolution of HIV-1 nef/long-terminal repeat (LTR) sequences, characterized by progressive sequence deletions tending toward a minimal nef/LTR structure that retains only sequence elements required for viral replication. The in vivo pathogenicity of attenuated HIV-1 is therefore dictated by viral and/or host factors other than those that impose a unidirectional selection pressure on the nef/LTR region of the HIV-1 genome.

Modulation of specific surface receptors and activation sensitization in primary resting CD4+ T lymphocytes by the Nef protein of HIV-1

The human immunodeficiency virus type 1 (HIV-1) pathogenicity factor Nef increases viral replication in vivo. In immortalized cell lines, Nef affects the cell surface levels of multiple receptors and signal transduction pathways. Resting CD4+ T lymphocytes are important targets for HIV-1 infection in vivo-they actively transcribe and express HIV-1 genes and contribute to the local viral burden and long-lived viral reservoirs in patients undergoing antiretroviral therapy. In vitro, this primary cell type has, however, thus far been highly refractory to experimental manipulation, and the biological activities exerted by HIV-1 Nef in these cells are largely unknown. Using nucleofection for gene delivery, we find that Nef induces a drastic and moderate down-regulation of CD4 and major histocompatibility complex type 1 (MHC-I), respectively, but does not alter surface levels of other receptors, the down-modulation of which has been reported in cell line studies. In contrast, Nef markedly up-regulated cell surface levels of the MHC-II invariant chain CD74. The effect of Nef on these three surface receptors was also detected upon HIV-1 infection of activated primary CD4+ T lymphocytes. Nef expression alone was insufficient to activate resting CD4+ T lymphocytes, but Nef modestly enhanced the responsiveness of cells to exogenous T cell activation. Consistent with such a signal transduction activity, a subpopulation of Nef localized to lipid raft clusters at the plasma membrane. This study establishes the analysis of Nef functions in these primary HIV target cells. Our data support the involvement of modulation of a defined set of cell surface receptors and sensitization to activation rather than an autonomous activation function in the role of Nef in HIV-1 pathogenesis.

Covert human immunodeficiency virus replication in dendritic cells and in DC-SIGN-expressing cells promotes long-term transmission to lymphocytes

HIV-1 virions are efficiently captured by monocyte-derived immature dendritic cells (iDCs), as well as by cell lines expressing the lectin DC-SIGN. Viral infectivity can be retained for several days, and even enhanced, before transmission to CD4+ lymphocytes. The role of DC-SIGN in viral retention and enhancement of infection is not fully understood and varies according to the cell line expressing the lectin. We studied here the mechanisms underlying this process. We focused our study on X4-tropic human immunodeficiency virus (HIV) strains, since they were widely believed not to replicate in iDCs. However, we first show that X4 HIV replicates covertly and slowly in iDCs. This is also the case in Raji-DC-SIGN cells, which are classically used to study HIV transmission. We used either single-cycle or replicative HIV and measured viral RT and replication to further demonstrate that transfer of incoming virions from iDCs or DC-SIGN+ cells occurs only on the short-term (i.e., a few hours after viral exposure). There is no long-term storage of original HIV particles in these cells. A few days after viral exposure, replicative viruses, and not single-cycle virions, are transmitted to CD4+ cells. The cell-type-dependent activity of DC-SIGN reflects the ability of HIV to replicate covertly in some cells, and not in others.

Rodent cells support key functions of the human immunodeficiency virus type 1 pathogenicity factor Nef

After infection with human immunodeficiency virus (HIV), progression toward immunodeficiency is governed by a complex interplay of viral and host determinants. The viral accessory protein Nef is a key factor for the development of AIDS. Strains of HIV and simian immunodeficiency virus that lack functional nef genes either do not induce AIDS or do so only after a significant delay. The validity of a transgenic-small-animal model for de novo infection by HIV will depend on its ability to recapitulate the actions of critical factors of viral pathogenicity, such as Nef. We assessed the ability of rat, mouse, and hamster cells to support key effector functions of Nef. In cell lines from rodents, the subcellular distribution of wild-type HIV type 1 strain SF2 Nef and mutants was comparable to that in human cells. Nef downregulated human CD4 from the cell surface, was associated with p21-activated kinase activity, and enhanced the infectivity of HIV-1 virions. Importantly, these Nef-induced effects, as well as the downregulation of rat CD4 and major histocompatibility complex class I molecules, could also be demonstrated in primary T lymphocytes and macrophages from human CD4-transgenic rats. Thus, HIV-1 Nef exerts key functions in rodent cells. In line with our ongoing efforts to establish a transgenic-rat model of HIV disease, these results indicate that important aspects of viral pathogenesis could be addressed in a transgenic-rodent model permissive for de novo infection and that such a model would be valuable for evaluating the function of Nef in vivo.

Genetic analysis of human immunodeficiency virus type 1nef in portugal: Subtyping, identification of mosaic genes, and amino acid sequence variability

Extending our previous genetic characterization of human immunodeficiency virus type 1 (HIV-1) strains circulating in Portugal, we here report the first phylogenetic and putative amino acid sequence variability analyses of nef accessory gene. Viral sequences (n = 53) were amplified by nested PCR from proviral DNA purified from peripheral blood mononuclear cells of HIV-1 infected individuals (n = 49). Phylogenetic inference analysis demonstrated a distribution of the viral sequences between subtypes A (sub-subtype A1), B, D, F (sub-subtype F1), G, H, and J, with subtypes G and B accounting altogether for more than half of the genotypes found. A significant number of the proviral DNA sequences analyzed (18.4%) were shown to correspond to intragenic nef recombinants, with the majority having the typical CRF02_AG nef structure. In addition, three novel intragenic recombinant structures were found (B/G/B, CRF02_AG/H, and D/G). From phylogenetic analysis, it was concluded that part of the non-recombinant nef genes might have actually been amplified from mosaic viruses: CRF06_cpx, CRF14_BG, and a new envA/nefJ recombinant. While comparing all the putative Nef sequences, significant amino acid sequence variability was observed. However, most of the described nef functional motifs were relatively well conserved in the majority of the sequences analyzed and numerous amino acid changes fell outside these regions. The results presented unambiguously endorse the high level of complexity of HIV-1 epidemics in Portugal.

HIV-1 and the hijacking of dendritic cells: a tug of war

Dendritic cells are critical for host immunity and are involved both in the innate and adaptive immune responses. They are among the first cells targeted by HIV-1 in vivo at mucosal sites. Dendritic cells can sequester HIV-1 in endosomal compartments for several days and transmit infectious HIV-1 to interacting T cells in the lymph node, which is the most important site for viral replication and spread. Initially, the cellular immune response developed against HIV-1 is strong, but eventually it fails to control and resolve the infection. The most dramatic effect seen on the immune system during untreated HIV-1 infection is the destruction of helper CD4(+) T cells, which leads to subsequent immune deficiency. However, the immunomodulatory effects of HIV-1 on different dendritic cell subpopulations may also play an important role in the pathogenesis of HIV-1. This review discusses the effects HIV-1 exerts on dendritic cells in vivo and in vitro, including the binding and uptake of HIV by dendritic cells, the formation of infectious synapses, infection, and the role of dendritic cells in HIV-1 pathogenesis.

The increased replicative capacity of a late-stage simian immunodeficiency virus mne variant is evident in macrophage- or dendritic cell-T-cell cocultures

Human and simian immunodeficiency virus (HIV and SIV) may co-opt antigen capture and presentation functions of antigen presenting cells (APCs) to facilitate infection of CD4+ T-cells. To address whether the replicative capacity of SIV in the host may be associated with the extent of viral replication in response to APC-T-cell interactions, we compared the replicative phenotypes of cloned early and late-stage SIVmne variants of known pathogenicity. Here, we show that the highly pathogenic late variant SIVmne027 replicates more efficiently in both macrophage- and dendritic cell (DC)-T-cell cocultures than the minimally pathogenic early virus SIVmneCl8. Contact between either macrophages or DC and T-cells increases replication of SIVmne027. Our analysis also demonstrates that mutations in pol and nef contribute to the greater replicative capacity of SIVmne027 in DC- or macrophage-T-cell cocultures. Together, these data suggest that variant viruses that evolve to replicate vigorously in response to APC-T-cell interactions may have increased replicative capacity in vivo.

Endogenously expressed HIV-1 nef down-regulates antigen-presenting molecules, not only class I MHC but also CD1a, in immature dendritic cells

The effects of Nef molecules on immature dendritic cells (iDCs) were analyzed using recombinant human immunodeficiency virus type 1 (HIV-1) with intact nef gene, pseudotyped with vesicular stomatitis virus glycoprotein, HIV/VSV-G/+Nef. When iDCs were infected with HIV/VSV-G/+Nef, the surface expression of CD1a, a molecule for presenting glycolipid/lipid antigens, was selectively down-regulated among CD1 molecules (CD1a, -b, -c, and -d) as well as class I MHC. Moreover, the CD1a molecules were also down-modulated and co-localized with DsRed2-tagged-Nef in CD1a-transfected cells. Their co-localization was dependent upon CD1a cytoplasmic tail and the CD1a was redistributed from cell surface to LAMP-1+ late endosomal/lysosomal compartment. These findings reveal that the HIV-1-Nef interferes with the intracellular trafficking of CD1a, and suggest the involvement of CD1a-restricted immune effectors in the protective immunity against HIV-1 infection, which implicates the feasibility of virus-derived glycolipid/lipid antigens together with epitope peptides for the vaccine development.

Human immunodeficiency virus type 1 Nef activates p21-activated kinase via recruitment into lipid rafts

The Nef protein of human immunodeficiency virus type 1 is an important factor in AIDS pathogenesis. In addition to downregulating CD4 and major histocompatibility complex class I molecules from the cell surface, as well as increasing virion infectivity, Nef triggers activation of the T-cell receptor (TCR) cascade to facilitate virus spread. Signaling pathways that are induced by Nef have been identified; however, it is unclear how and in which subcellular compartment Nef triggers signaling. Nef recruits a multiprotein complex to activate the cellular Pak kinase that mediates downstream effector functions. Since a subpopulation of Nef is present in detergent-insoluble microdomains (lipid rafts) from where physiological TCR signaling is initiated, we tested whether lipid rafts are instrumental for Nef-mediated Pak activation. In flotation analysis, Nef-associated Pak activity exclusively fractionated with lipid rafts. Activation of Pak in the presence of Nef coincided with lipid raft recruitment of the kinase, which was otherwise excluded from detergent-insoluble microdomains. Experimental solubilization of lipid rafts interfered with the association of Pak activity with Nef. To analyze the importance of the raft localization for Nef function more rigorously, we generated a palmitoylated Nef (PalmNef). PalmNef was highly enriched in lipid rafts and associated with significantly higher levels of Pak activity than Nef. Notably, activation of Pak by its physiological activators, Cdc42 and Rac, also occurred in lipid rafts and required raft integrity. Together, these data suggest that Nef induces signal transduction via the recruitment of a signaling machinery including Pak into lipid rafts, thereby mimicking a physiological cellular mechanism to initiate the TCR cascade.

Genetic analysis of Indian HIV-1 nef: subtyping, variability and implications

HIV-1 subtype C is predominant in India and globally. In the present study, we analyze HIV-1 subtype C regulatory protein Nef sequences from five recent Indian seroconverters and five long-term survivors (LTSs) for variability at crucial functional domains. Sequence analysis suggested the possibility of using regulatory gene sequences for viral subtyping and evolutionary studies apart from structural genes. In the phylogenetic tree, Indian nef sequences segregated away from other reported subtype C sequences, forming an Indian subclade within subtype C. Our studies also suggested no evidence for the association of truncated Nef with slow progression of disease, as all LTSs had intact Nef. We could identify some variations in juxtapositions to crucial functional domains, especially in seroconverter sequences, when comparing them with others. In phylogenetic analysis, specifically for the base-pair regions 411-428 and 478-525, our seroconverter sequences segregated away from those reported earlier in the literature, indicating specific evolutionary changes in these conserved regions of nef in currently circulating viruses. But the dN/dS ratio for our samples was less than one on comparing them with reported subtype C and representative sequences of different clades, strongly emphasizing the necessity of sequence conservation at different disease stages and even across clades. HLA-I binding epitope predictions for common Indian HLAs indicated that specific mutations in seroconverter Nef may alter the intensity of epitope binding, which may alter the outcome of the immune response. Hence these data would be useful in designing Nef epitopes to be included in multi-epitope HIV-1 vaccine for the Indian population and would also be of immense help in HIV-1 evolutionary studies.

Immunodeficiency virus exploitation of dendritic cells in the early steps of infection

The unique capacity of dendritic cells (DCs) to capture and process pathogens for presentation to the immune system, combined with their capacity to express costimulatory and adhesion molecules as well as cytokines and chemokines, renders them powerful antigen-presenting cells. However, immunodeficiency viruses hijack DCs to facilitate virus dissemination while subverting effective immune activation. Depending on the activation level of the DC subset, human immunodeficiency virus can use different receptors (CD4, chemokine, and C-type lectin receptors) to bind to DCs. These aspects likely impact whether a DC is productively infected by or simply carries virus for transmission to more permissive targets. DCs efficiently transmit virus to CD4+ T cells, driving virus growth as well as providing signals to trigger virus expansion in virus-bearing CD4+ T cells. There is accumulating evidence that viral determinants (nef, tat) selectively modulate immature DC biology, fostering DC-T cell interactions and virus replication without up-regulating costimulatory molecules for effective immune function. In addition, virus-loaded, immature DCs activate CD4+ virus-specific T cells, and mature DCs stimulate CD4+ and CD8+ T cells. Thus, even if immature DCs entrap virus as it crosses the mucosae and initiate a CD4+ T cell response, this is likely insufficient to control infection. Appreciating how virus modulates DC function and what determines whether virus is processed for immune stimulation or transmitted between cells will unveil the exact role of these cells in the onset of infection and advance preventative microbicide and vaccine/therapeutic approaches.

The interaction of immunodeficiency viruses with dendritic cells

Dendritic cells (DCs) can influence HIV-1 and SIV pathogenesis and protective mechanisms at several levels. First, HIV-1 productively infects select populations of DCs in culture, particularly immature DCs derived from blood monocytes and skin (Langerhans cells). However, there exist only a few instances in which HIV-1- or SIV-infected DCs have been identified in vivo in tissue sections. Second, different types of DCs reliably sequester and transmit infectious HIV-1 and SIV in culture, setting up a productive infection in T cells interacting with the DCs. This stimulation of infection in T cells may explain the observation that CD4+ T lymphocytes are the principal cell type observed to be infected with HIV-1 in lymphoid tissues in vivo. DCs express a C-type lectin, DC-SIGN/CD209, that functions to bind HIV-1 (and other infectious agents) and transmit virus to T cells. When transfected into the THP-1 cell line, the cytosolic domain of DC-SIGN is needed for HIV-1 sequestration and transmission. However, DCs lacking DC-SIGN (Langerhans cells) or expressing very low levels of DC-SIGN (rhesus macaque monocyte-derived DCs) may use additional molecules to bind and transmit immunodeficiency viruses to T cells. Third, DCs are efficient antigen-presenting cells for HIV-1 and SIV antigens. Infection with several recombinant viral vectors as well as attenuated virus is followed by antigen presentation to CD4+ and CD8+ T cells. An intriguing pathway that is well developed in DCs is the exogenous pathway for nonreplicating viral antigens to be presented on class I MHC products. This should allow DCs to stimulate CD8+ T cells after uptake of antibody-coated HIV-1 and dying infected T cells. It has been proposed that DCs, in addition to expanding effector helper and killer T cells, induce tolerance through T cell deletion and suppressor T cell formation, but this must be evaluated directly. Fourth, DCs are likely to be valuable in improving vaccine design. Increasing DC uptake of a vaccine, as well as increasing their numbers and maturation, should enhance efficacy. However, DCs can also capture antigens from other cells that are initially transduced with a DNA vaccine or a recombinant viral vector. The interaction of HIV-1 and SIV with DCs is therefore intricate but pertinent to understanding how these viruses disrupt immune function and elicit immune responses.

Expression of simian immunodeficiency virus (SIV) nef in astrocytes during acute and terminal infection and requirement of nef for optimal replication of neurovirulent SIV in vitro

As the most numerous cells in the brain, astrocytes play a critical role in maintaining central nervous system homeostasis, and therefore, infection of astrocytes by human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV) in vivo could have important consequences for the development of HIV encephalitis. In this study, we establish that astrocytes are infected in macaques during acute SIV infection (10 days postinoculation) and during terminal infection when there is evidence of SIV-induced encephalitis. Additionally, with primary adult rhesus macaque astrocytes in vitro, we demonstrate that the macrophage-tropic, neurovirulent viruses SIV/17E-Br and SIV/17E-Fr replicate efficiently in astrocytes, while the lymphocyte-tropic, nonneurovirulent virus SIV(mac)239 open-nef does not establish productive infection. Furthermore, aminoxypentane-RANTES abolishes virus replication, suggesting that these SIV strains utilize the chemokine receptor CCR5 for entry into astrocytes. Importantly, we show that SIV Nef is required for optimal replication in primary rhesus macaque astrocytes and that normalizing input virus by particle number rather than by infectivity reveals a disparity between the ability of a Nef-deficient virus and a virus encoding a nonmyristoylated form of Nef to replicate in these central nervous system cells. Since the myristoylated form of Nef has been implicated in functions such as CD4 and major histocompatibility complex I downregulation, kinase association, and enhancement of virion infectivity, these data suggest that an as yet unidentified function of Nef may exist to facilitate SIV replication in astrocytes that may have important implications for in vivo pathogenesis.

nef gene is required for robust productive infection by simian immunodeficiency virus of T-cell-rich paracortex in lymph nodes

The pathogenesis of AIDS virus infection in a nonhuman primate AIDS model was studied by comparing plasma viral loads, CD4(+) T-cell subpopulations in peripheral blood mononuclear cells, and simian immunodeficiency virus (SIV) infection in lymph nodes for rhesus macaques infected with a pathogenic molecularly cloned SIVmac239 strain and those infected with its nef deletion mutant (Deltanef). In agreement with many reports, whereas SIVmac239 infection induced AIDS and depletion of memory CD4(+) T cells in 2 to 3 years postinfection (p.i.), Deltanef infection did not induce any manifestation associated with AIDS up to 6.5 years p.i. To explore the difference in SIV infection in lymphoid tissues, we biopsied lymph nodes at 2, 8, 72, and 82 weeks p.i. and analyzed them by pathological techniques. Maximal numbers of SIV-infected cells (SIV Gag(+), Env(+), and RNA(+)) were detected at 2 weeks p.i. in both the SIVmac239-infected animals and the Deltanef-infected animals. In the SIVmac239-infected animals, most of the infected cells were localized in the T-cell-rich paracortex, whereas in the Deltanef-infected animals, most were localized in B-cell-rich follicles and in the border region between the paracortex and the follicles. Analyses by double staining of CD68(+) macrophages and SIV Gag(+) cells and by double staining of CD3(+) T cells and SIV Env(+) cells revealed that SIV-infected cells were identified as CD4(+) T cells in either the SIVmac239 or the Deltanef infection. Whereas the many functions of Nef protein were reported from in vitro studies, our finding of SIVmac239 replication in the T-cell-rich paracortex in the lymph nodes supports the reported roles of Nef protein in T-cell activation and enhancement of viral infectivity. Furthermore, the abundance of SIVmac239 infection and the paucity of Deltanef infection in the T-cell-rich paracortex accounted for the differences in viral replication and pathogenicity between SIVmac239 and the Deltanef mutant. Thus, our in vivo study indicated that the nef gene enhances SIV replication by robust productive infection in memory CD4(+) T cells in the T-cell-rich region in lymphoid tissues.

Endogenously expressed nef uncouples cytokine and chemokine production from membrane phenotypic maturation in dendritic cells

Immature dendritic cells (DCs), unlike mature DCs, require the viral determinant nef to drive immunodeficiency virus (SIV and HIV) replication in coculture with CD4(+) T cells. Since immature DCs may capture and get infected by virus during mucosal transmission, we hypothesized that Nef associated with the virus or produced during early replication might modulate DCs to augment virus dissemination. Adenovirus vectors expressing nef were used to introduce nef into DCs in the absence of other immunodeficiency virus determinants to examine Nef-induced changes that might activate immature DCs to acquire properties of mature DCs and drive virus replication. Nef expression by immature human and macaque DCs triggered IL-6, IL-12, TNF-alpha, CXCL8, CCL3, and CCL4 release, but without up-regulating costimulatory and other molecules characteristic of mature DCs. Coincident with this, nef-expressing immature DCs stimulated stronger autologous CD4(+) T cell responses. Both SIV and HIV nef-expressing DCs complemented defective SIVmac239 delta nef, driving replication in autologous immature DC-T cell cultures. In contrast, if DCs were activated after capturing delta nef, virus growth was not exacerbated. This highlights one way in which nef-defective virus-bearing immature DCs that mature while migrating to draining lymph nodes could induce stronger immune responses in the absence of overwhelming productive infection (unlike nef-containing wild-type virus). Therefore, Nef expressed in immature DCs signals a distinct activation program that promotes virus replication and T cell recruitment but without complete DC maturation, thereby lessening the likelihood that wild-type virus-infected immature DCs would activate virus-specific immunity, but facilitating virus dissemination.

Increased macrophage infection upon subcutaneous inoculation of rhesus macaques with simian immunodeficiency virus-loaded dendritic cells or T cells but not with cell-free virus

Information on the establishment of immunodeficiency virus infection through transmission of infected cells is sparse. Dendritic cells (DCs) and T cells may be central to the onset and subsequent spread of infection following mucosal exposure. To directly investigate the consequences of virus being introduced by DCs or T cells, we reinjected ex vivo simian immunodeficiency virus (SIV)-loaded autologous immature DCs and T cells subcutaneously (s.c.) into healthy macaques. s.c. injection of cell-bound virus was used to mirror what may happen if virus-loaded cells pass through an epithelium or perhaps DCs and T cells that immediately entrap cell-free virus, having just crossed an epithelial barrier. Virus load in the plasma was monitored along with combined in situ hybridization and immunohistochemistry to identify the cells replicating virus in the lymphoid tissues. Both DCs and T cells transmitted infection after being pulsed with either wild-type or nef-defective (delta nef) SIVmac239. As seen in animals infected intravenously, replication of delta nef was attenuated compared to that of wild-type virus when introduced in either cell-bound form. Upon examination of the draining lymph nodes (LNs) during the first days of infection, virus-producing CD4(+) T cells predominated in control animals that received s.c. cell-free virus. In dramatic contrast, both SIV-positive macrophages and T cells were detected in the LNs of monkeys infected with cell-associated SIV. Therefore, although both cell-free and cell-associated viruses are infectious, the initial cells amplifying the virus differ. This may have important implications for the subsequent dissemination of infection and/or induction of antiretroviral immunity.

Human immunodeficiency virus type 1 Nef mediates activation of STAT3 in immature dendritic cells

Replication of immunodeficiency viruses (HIV-1 and SIV) in immature dendritic cell (DC)-T cell cocultures is dependent on Nef. In contrast, mature DCs promote the replication of wild-type and nef-defective SIV in concert with CD4(+) T cells. Transcription factor activation occurs on DC maturation and this study aimed to investigate whether Nef triggers similar events in immature DCs, rendering them more like mature DCs. Recombinant HIV nef-expressing adenovirus was used to selectively introduce nef into immature human or macaque DCs. These data provide the first evidence that the expression of HIV nef in immature DCs induced selective activation of STAT3 and, to a lesser extent, NF-kappaB. This highlights how Nef can signal primary immature DCs, suggesting one way in which Nef may modulate immature DCs to drive virus replication in the DC-T cell milieu.

Subunit H of the V-ATPase binds to the medium chain of adaptor protein complex 2 and connects Nef to the endocytic machinery

Nef is an accessory protein of human and simian immunodeficiency viruses (HIV and SIV) that is required for efficient viral infectivity and pathogenicity. It decreases the expression of CD4 on the surface of infected cells. V1H is the regulatory subunit H of the vacuolar membrane ATPase (V-ATPase). Previously, the interaction between Nef and V1H has been found to facilitate the internalization of CD4, suggesting that V1H could connect Nef to the endocytic machinery. In this study, we demonstrate that V1H binds to the C-terminal flexible loop in Nef from HIV-1 and to the medium chain (mu2) of the adaptor protein complex 2 (AP-2) in vitro and in vivo. The interaction sites of V1H and mu2 were mapped to a central region in V1H from positions 133 to 363, which contains 4 armadillo repeats, and to the N-terminal adaptin-binding domain in mu2 from positions 1 to 145. Fusing Nef to V1H reproduced the appropriate trafficking of Nef. This chimera internalized CD4 even in the absence of the C-terminal flexible loop in Nef. Finally, blocking the expression of V1H decreased the enhancement of virion infectivity by Nef. Thus, V1H can function as an adaptor for interactions between Nef and AP-2.

Enhanced in vitro stimulation of rhesus macaque dendritic cells for activation of SIV-specific T cell responses

The macaque-simian immunodeficiency virus (SIV) system is one of the best animal models available to study the role of dendritic cells (DCs) in transmission and pathogenesis of HIV, as well as to test DC-based vaccine and therapeutic strategies. To better define and optimize this system, the responsiveness of macaque monocyte-derived DCs to a variety of maturation stimuli was examined. Characteristic immunophenotypic and functional DC maturation induced by standard monocyte conditioned medium (MCM) was compared to the activation induced by a panel of stimuli including soluble CD40L, LPS, Poly I:C, PGE(2)/TNFalpha, and a cocktail mixture of PGE(2)/TNFalpha/IL-1beta/IL-6. Immunophenotypic analysis confirmed that all stimuli induced stable up-regulation of CD25, CD40, CD80, CD83, CD86, HLA-DR, DC-LAMP (CD208), and DEC-205 (CD205). In general, macaque DCs exhibited weaker responses to LPS and Poly I:C than human DCs, and soluble CD40L stimulation induced variable expression of CD25. Interestingly, while the endocytic capacity of CD40L-matured cells was down-modulated comparably to DCs matured with MCM or the cocktail, the T cell stimulatory activity was not enhanced to the same extent. The particularly reproducible and potent T cell stimulatory capacity of cocktail-treated DCs correlated with a more homogenous mature DC phenotype, consistently high levels of IL-12 production, and better viability upon reculture compared to DCs activated by other stimuli. Furthermore, cocktail-matured DCs efficiently captured and presented inactivated SIV to SIV-primed T cells in vitro. Thus, the cocktail represents a particularly potent and useful stimulus for the generation of efficacious immunostimulatory macaque DCs.

Nef: agent of cell subversion

Primate lentiviruses encode a small protein designated Nef that has been shown to be a major determinant of virus pathogenicity. Nef regulates multiple host factors in order to optimize the cellular environment for virus replication. The mechanisms by which this small protein modulates distinct host cell properties provide intriguing insight into the intricate interaction between virus and host.

The simian immunodeficiency virus deltaNef vaccine, after application to the tonsils of Rhesus macaques, replicates primarily within CD4(+) T cells and elicits a local perforin-positive CD8(+) T-cell response

Deletion of the nef gene from simian immunodeficiency virus (SIV) strain SIVmac239 yields a virus that undergoes attenuated growth in rhesus macaques and offers substantial protection against a subsequent challenge with some SIV wild-type viruses. We used a recently described model to identify sites in which the SIVDeltanef vaccine strain replicates and elicits immunity in vivo. A high dose of SIVDeltanef was applied to the palatine and lingual tonsils, where it replicated vigorously in this portal of entry at 7 days. Within 2 weeks, the virus had spread and was replicating actively in axillary lymph nodes, primarily in extrafollicular T-cell-rich regions but also in germinal centers. At this time, large numbers of perforin-positive cells, both CD8(+) T cells and CD3-negative presumptive natural killer cells, were found in the tonsil and axillary lymph nodes. The number of infected cells and perforin-positive cells then fell. When autopsy studies were carried out at 26 weeks, only 1 to 3 cells hybridized for viral RNA per section of lymphoid tissue. Nevertheless, infected cells were detected chronically in most lymphoid organs, where the titers of infectious virus could exceed by a log or more the titers in blood. Immunocytochemical labeling at the early active stages of infection showed that cells expressing SIVDeltanef RNA were CD4(+) T lymphocytes. A majority of infected cells were not in the active cell cycle, since 60 to 70% of the RNA-positive cells in tissue sections lacked the Ki-67 cell cycle antigen, and both Ki-67-positive and -negative cells had similar grain counts for viral RNA. Macrophages and dendritic cells, identified with a panel of monoclonal antibodies to these cells, were rarely infected. We conclude that the attenuated growth and protection observed with the SIVDeltanef vaccine strain does not require that the virus shift its characteristic site of replication, the CD4(+) T lymphocyte. In fact, this immunodeficiency virus can replicate actively in CD4(+) T cells prior to being contained by the host, at least in part by a strong killer cell response that is generated acutely in the infected lymph nodes.

A natural variability in the proline-rich motif of Nef modulates HIV-1 replication in primary T cells

In the infected host, the Nef protein of HIV/SIV is required for high viral loads and thus disease progression. Recent evidence indicates that Nef enhances replication in the T cell compartment after the virus is transmitted from dendritic cells (DC). The underlying mechanism, however, is not clear. Here, we report that a natural variability in the proline-rich motif (R71T) profoundly modulated Nef-stimulated viral replication in primary T cells of immature dendritic cell/T cell cocultures. Whereas both Nef variants (R/T-Nef) downregulated CD4, only the isoform supporting viral replication (R-Nef) efficiently interacted with signaling molecules of the T cell receptor (TCR) environment and stimulated cellular activation. Structural analysis suggested that the R to T conversion induces conformational changes, altering the flexibility of the loop containing the PxxP motif and hence its ability to bind cellular partners. Our report suggests that functionally and conformationally distinct Nef isoforms modulate HIV replication on the interaction level with the TCR-signaling environment once the virus enters the T cell compartment.

Dendritic cells transduced with HIV Nef express normal levels of HLA-A and HLA-B class I molecules

HIV Nef protein is important for viral pathogenesis and disease progression. Nef downregulates CD4 and major histocompatibility antigens on the surface of HIV-infected T cells. HIV also infects dendritic cells. We wanted to determine if Nef had a similar function in professional antigen-presenting cells, where downregulation of Class I could have important effects on the initiation of HIV specific cytolytic T cell responses. We infected human dendritic cells with adenovirus expressing Nef. In contrast to T cells and Hela cells, HLA-A and HLA-B molecules are not downregulated nor are other class I molecules increased. We show that, in dendritic cells, HIV Nef has little effect on CD4 or Class I expression.

Nef is required for efficient HIV-1 replication in cocultures of dendritic cells and lymphocytes

Dendritic cells (DCs) are thought to play a crucial role in the pathogenesis of HIV-1 infection. DCs are believed to transport virus particles to lymph nodes before transfer to CD4(+) lymphocytes. We have investigated the role of Nef in these processes. HIV-1 replication was examined in human immature DC-lymphocyte cocultures and in DCs or lymphocytes separately. Using various R5-tropic and X4-tropic HIV-1 strains and their nef-deleted (Deltanef) counterparts, we show that Nef is required for optimal viral replication in immature DC-T cells clusters and in T lymphocytes. Nef exerts only a marginal role on viral replication in immature DCs alone as well as on virion capture by DCs, long-term intracellular accumulation and transmission of X4 strains to lymphocytes. We also show that wild-type and Deltanef virions are similarly processed for MHC-I restricted exogenous presentation by DCs. Taken together, these results help explain how HIV-1 Nef may affect viral spread and immune responses in the infected host.

Novel, live attenuated simian immunodeficiency virus constructs containing major deletions in leader RNA sequences

We have constructed a series of simian immunodeficiency virus (SIV) mutants containing deletions within a 97-nucleotide (nt) region of the leader sequence. Deletions in this region markedly decreased the replication capacity in tissue culture, i.e., in both the C8166 and CEMx174 cell lines, as well as in rhesus macaque peripheral blood mononuclear cells. In addition, these deletions adversely affected the packaging of viral genomic RNA into virions, the processing of Gag precursor proteins, and patterns of viral proteins in virions, as assessed by biochemical labeling and polyacrylamide gel electrophoresis. Different levels of attenuation were achieved by varying the size and position of deletions within this 97-nt region, and among a series of constructs that were generated, it was possible to rank in vitro virulence relative to that of wild-type virus. In all of these cases, the most severe impact on viral replication was observed when the deletions that were made were located at the 3' rather than 5' end of the leader region. The potential of viral reversion over protracted periods was investigated by repeated viral passage in CEMx174 cells. The results showed that several of these constructs showed no signs of reversion after more than 6 months in tissue culture. Thus, a series of novel, attenuated SIV constructs have been developed that are significantly impaired in replication capacity yet retain all viral genes. One of these viruses, termed SD4, may be appropriate for study with rhesus macaques, in order to determine whether reversions will occur in vivo and to further study this virus as a candidate for attenuated vaccination.

Human T-lymphotropic virus type 1 open reading frame I p12(I) is required for efficient viral infectivity in primary lymphocytes

Human T-lymphotropic virus type 1 (HTLV-1) is a complex retrovirus encoding regulatory and accessory genes in four open reading frames (ORF I to IV) of the pX region. Emerging evidence indicates an important role for the pX ORF I-encoded accessory protein p12(I) in viral replication, but its contribution to viral pathogenesis remains to be defined. p12(I) is a conserved, membrane-associated protein containing four SH3-binding motifs (PXXP). Its interaction with the interleukin-2 (IL-2) receptor beta- and gamma-chains implies an involvement of p12(I) in intracellular signaling pathways. In addition, we have demonstrated that expression of pX ORF I p12(I) is essential for persistent infection in rabbits. In contrast, standard in vitro systems have thus far failed to demonstrate a contribution of p12(I) to viral infectivity and ultimately cellular transformation. In this study we developed multiple in vitro coculture assays to evaluate the role of p12(I) in viral infectivity in quiescent peripheral blood mononuclear cells to more accurately reflect the virus-cell interactions as they occur in vivo. Using these assays, we demonstrate a dramatic reduction in viral infectivity in quiescent T lymphocytes for a p12 mutant viral clone (ACH.p12) in comparison to the wild-type clone ACH. Moreover, addition of IL-2 and phytohemagglutinin during the infection completely rescued the ability of ACH.p12 to infect primary lymphocytes. When newly infected primary lymphocytes are used to passage virus, ACH.p12 also exhibited a reduced ability to productively infect activated lymphocytes. Our data are the first to demonstrate a functional role for pX ORF I in the infection of primary lymphocytes and suggest a role for p12(I) in activation of host cells during early stages of infection.