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

Nuclear import defect of human immunodeficiency virus type 1 DNA flap mutants is not dependent on the viral strain or target cell type

We have previously established, using human immunodeficiency virus type 1 (HIV-1) strain LAI, that the HIV-1 central DNA Flap acts as a cis determinant of viral genome nuclear import. Although the impact of the DNA Flap on nuclear import has already found numerous independent confirmations in the context of lentivirus vectors, it has been claimed that it may be nonessential for infectious virus strains LAI, YU-2 (J. D. Dvorin et al., J. Virol. 76:12087-12096, 2002), HXB2, and NL4-3 (A. Limon et al., J. Virol. 76:12078-12086, 2002). We conducted a detailed analysis of virus infectivity using the provirus clones provided by the authors and analogous target cells. In contrast to published data, our results show that all cPPT mutant viruses exhibit reduced infectivity corresponding to a nuclear import defect irrespective of the viral genetic background or target cell.

Inefficient human immunodeficiency virus replication in mobile lymphocytes

Cell-to-cell viral transfer facilitates the spread of lymphotropic retroviruses such as human immunodeficiency virus (HIV) and human T-cell leukemia virus (HTLV), likely through the formation of "virological synapses" between donor and target cells. Regarding HIV replication, the importance of cell contacts has been demonstrated, but this phenomenon remains only partly characterized. In order to alter cell-to-cell HIV transmission, we have maintained cultures under continuous gentle shaking and followed viral replication in this experimental system. In lymphoid cell lines, as well as in primary lymphocytes, viral replication was dramatically reduced in shaken cultures. To document this phenomenon, we have developed an assay to assess the relative contributions of free and cell-associated virions in HIV propagation. Acutely infected donor cells were mixed with carboxyfluorescein diacetate succinimidyl ester-labeled lymphocytes as targets, and viral production was followed by measuring HIV Gag expression at different time points by flow cytometry. We report that cellular contacts drastically enhance productive viral transfer compared to what is seen with infection with free virus. Productive cell-to-cell viral transmission required fusogenic viral envelope glycoproteins on donor cells and adequate receptors on targets. Only a few syncytia were observed in this coculture system. Virus release from donor cells was unaffected when cultures were gently shaken, whereas virus transfer to recipient cells was severely impaired. Altogether, these results indicate that cell-to-cell transfer is the predominant mode of HIV spread and help to explain why this virus replicates so efficiently in lymphoid organs.

DC-SIGN and CLEC-2 mediate human immunodeficiency virus type 1 capture by platelets

Platelets can engulf human immunodeficiency virus type 1 (HIV-1), and a significant amount of HIV-1 in the blood of infected individuals is associated with these cells. However, it is unclear how platelets capture HIV-1 and whether platelet-associated virus remains infectious. DC-SIGN and other lectins contribute to capture of HIV-1 by dendritic cells (DCs) and facilitate HIV-1 spread in DC/T-cell cocultures. Here, we show that platelets express both the C-type lectin-like receptor 2 (CLEC-2) and low levels of DC-SIGN. CLEC-2 bound to HIV-1, irrespective of the presence of the viral envelope protein, and facilitated HIV-1 capture by platelets. However, a substantial fraction of the HIV-1 binding activity of platelets was dependent on DC-SIGN. A combination of DC-SIGN and CLEC-2 inhibitors strongly reduced HIV-1 association with platelets, indicating that these lectins are required for efficient HIV-1 binding to platelets. Captured HIV-1 was maintained in an infectious state over several days, suggesting that HIV-1 can escape degradation by platelets and might use these cells to promote its spread. Our results identify CLEC-2 as a novel HIV-1 attachment factor and provide evidence that platelets capture and transfer infectious HIV-1 via DC-SIGN and CLEC-2, thereby possibly facilitating HIV-1 dissemination in infected patients.

Autophagy is involved in T cell death after binding of HIV-1 envelope proteins to CXCR4

HIV-1 envelope glycoproteins (Env), expressed at the cell surface, induce apoptosis of uninfected CD4+ T cells, contributing to the development of AIDS. Here we demonstrate that, independently of HIV replication, transfected or HIV-infected cells that express Env induced autophagy and accumulation of Beclin 1 in uninfected CD4+ T lymphocytes via CXCR4. The same phenomena occurred in a T cell line and in transfected HEK.293 cells that expressed both wild-type CXCR4 and a truncated form of CD4 that is unable to bind the lymphocyte-specific protein kinase Lck. Env-mediated autophagy is required to trigger CD4+ T cell apoptosis since blockade of autophagy at different steps, by either drugs (3-methyladenine and bafilomycin A1) or siRNAs specific for Beclin 1/Atg6 and Atg7 genes, totally inhibited the apoptotic process. Furthermore, CD4+ T cells still underwent Env-mediated cell death with autophagic features when apoptosis was inhibited. These results suggest that HIV-infected cells can induce autophagy in bystander CD4+ T lymphocytes through contact of Env with CXCR4, leading to apoptotic cell death, a mechanism most likely contributing to immunodeficiency.

Dendritic cell-mediated viral transmission: a potential drug target?

Dendritic cells (DCs) are important in the sexual transmission of HIV-1, the most common route of acquiring HIV-1. HIV-1 subverts the biological function of DCs to facilitate its transport from site of entry at mucosal tissues to lymphoid tissues to infect T cells. Recent data have furthered our understanding of how DCs mediate viral transmission to T cells. DCs capture HIV-1 through specific attachment receptors, such as DC-SIGN, which not only facilitate HIV-1 transmission, but also infection of DCs. Therefore, these receptors are very promising targets for the design of inhibitors or vaccination strategies to prevent mucosal HIV-1 transmission. It is becoming evident that other viruses also use DCs for their transmission. This review will discuss the mechanism of HIV-1 transmission and potential intervention strategies.

A nonneutralizing anti-HIV Type 1 antibody turns into a broad neutralizing antibody when expressed on the surface of HIV type 1-susceptible cells. II. Inhibition of HIV type 1 captured and transferred by DC-SIGN

Previously, we demonstrated that the expression of a nonneutralizing human anti-HIV-1 gp41 scFv on the surface of HIV-1-susceptible cells markedly inhibits HIV-1 replication and HIV-1 envelope-mediated cell-cell fusion. The inhibition is at the level of viral entry, specific for the HIV-1 envelope, and independent of virus tropism. In the previous studies, cell-free viruses of laboratory-adapted HIV-1 strains from subtype B were used to infect human CD4 T cell lines. To further test the effectiveness of this membrane-bound scFv (m-scFv) on HIV-1 infection, in this study, we carried out experiments to determine whether the m-scFv can neutralize infection of primary isolates from various HIV-1 subtypes and whether the m-scFv can neutralize HIV-1 captured and transferred by DC-SIGN on the surface of monocytic cell lines or DCs. We demonstrated that the m-scFv markedly inhibits primary isolates derived from various subtypes and significantly blocks HIV-1 captured and transferred by DC-SIGN on monocytic cell lines and on human DCs. Therefore, a nonneutralizing antibody acts as a broad neutralizing antibody when expressed on the cell surface, which significantly inhibits infection of both cell-free and DC-SIGN-captured and transferred virus. Our studies further point out the potential use of m-scFv as a inhibitor against HIV-1 transmission as well as a tool to dissect the mechanism of HIV-1 entry via DC-SIGN capture and transfer to CD4 T cells.

HIV interactions with dendritic cells: has our focus been too narrow?

Although few in number, dendritic cells (DCs) are heterogeneous, ubiquitous, and are crucial for protection against pathogens. In this review, the different DC subpopulations have been described and aspects of DC biology are discussed. DCs are important, not only in the pathogenesis of HIV, but also in the generation of anti-HIV immune responses. This review describes the roles that DC are thought to play in HIV pathogenesis, including uptake and transport of virus. We have also discussed the effects that the virus exerts on DCs such as infection and dysfunction. Then we proceed to focus on DC subsets in different organs and show how widespread the effects of HIV are on DC populations. It is clear that the small number of studies on tissue-derived DCs limits current research into the pathogenesis of HIV.

Differential susceptibility of naïve, central memory and effector memory T cells to dendritic cell-mediated HIV-1 transmission

Background

Dendritic cells (DC) have been proposed to facilitate sexual transmission of HIV-1 by capture of the virus in the mucosa and subsequent transmission to CD4⁺ T cells. Several T cell subsets can be identified in humans: naïve T cells (T_N) that initiate an immune response to new antigens, and memory T cells that respond to previously encountered pathogens. The memory T cell pool comprises central memory (T_CM) and effector memory cells (T_EM), which are characterized by distinct homing and effector functions. The T_EM cell subset, which can be further divided into effector Th1 and Th2 cells, has been shown to be the prime target for viral replication after HIV-1 infection, and is abundantly present in mucosal tissues.

Results

We determined the susceptibility of T_N, T_CM and T_EM cells to DC-mediated HIV-1 transmission and found that co-receptor expression on the respective T cell subsets is a decisive factor for transmission. Accordingly, CCR5-using (R5) HIV-1 was most efficiently transmitted to T_EM cells, and CXCR4-using (X4) HIV-1 was preferentially transmitted to T_N cells.

Conclusion

The highly efficient R5 transfer to T_EM cells suggests that mucosal T cells are an important target for DC-mediated transmission. This may contribute to the initial burst of virus replication that is observed in these cells. T_N cells, which are the prime target for DC-mediated X4 virus transmission in our study, are considered to inefficiently support HIV-1 replication. Our results thus indicate that DC may play a decisive role in the susceptibility of T_N cells to X4 tropic HIV-1.

DC-SIGN on B lymphocytes is required for transmission of HIV-1 to T lymphocytes

Infection of T cells by HIV-1 can occur through binding of virus to dendritic cell (DC)-specific ICAM-3 grabbing nonintegrin (DC-SIGN) on dendritic cells and transfer of virus to CD4⁺ T cells. Here we show that a subset of B cells in the blood and tonsils of normal donors expressed DC-SIGN, and that this increased after stimulation in vitro with interleukin 4 and CD40 ligand, with enhanced expression of activation and co-stimulatory molecules CD23, CD58, CD80, and CD86, and CD22. The activated B cells captured and internalized X4 and R5 tropic strains of HIV-1, and mediated trans infection of T cells. Pretreatment of the B cells with anti–DC-SIGN monoclonal antibody blocked trans infection of T cells by both strains of HIV-1. These results indicate that DC-SIGN serves as a portal on B cells for HIV-1 infection of T cells in trans. Transmission of HIV-1 from B cells to T cells through this DC-SIGN pathway could be important in the pathogenesis of HIV-1 infection.

A cell surface molecule, DC-SIGN, is known to bind the AIDS virus, human immunodeficiency virus 1 (HIV-1), on dendritic cells. HIV-1 can then be transferred from these dendritic cells to CD4⁺ T cells, in which the virus replicates and kills the T cells. Here, Rappocciolo and colleagues present their findings that DC-SIGN serves a similar function on a subset of B cells of the peripheral blood and tonsils. Although B cells that express DC-SIGN do not replicate HIV-1, they serve as portals for transfer and enhanced HIV-1 infection of CD4⁺ T cells, the major site of virus replication in the host. This newly described pathway for HIV-1 infection of T cells via B cells could be important in the pathogenesis of the virus infection.

Candidate polyanion microbicides inhibit HIV-1 infection and dissemination pathways in human cervical explants

BACKGROUND

Heterosexual intercourse remains the major route of HIV-1 transmission worldwide, with almost 5 million new infections occurring each year. Women increasingly bear a disproportionate burden of the pandemic, thus there is an urgent need to develop new strategies to reduce HIV-1 transmission that could be controlled by women themselves. The potential of topical microbicides to reduce HIV transmission across mucosal surfaces has been clearly identified, and some agents are currently under evaluation in clinical trials. Many of these "first generation" microbicides consist of polyanionic compounds designed to interfere with viral attachment. Here we have evaluated two candidate polyanion compounds in clinical trials, PRO 2000 and dextrin sulphate (DxS) to determine their safety and efficacy against in vitro HIV-1 and HSV-2 infection using cellular and tissue explant models.

RESULTS

PRO 2000 and DxS potently inhibited infection by HIV-1 X4 and R5 isolates when present during viral exposure. However PRO 2000 required 10-fold and DxS 2000-fold more compound to block infection with R5 virus than X4. While both compounds were virucidal for X4 HIV-1, neither was virucidal for R5 virus. PRO 2000 efficiently inhibited infection of cervical explants and dissemination of virus by migratory DC. DxS was less active, able to completely inhibit cervical explant infection, but providing only partial reduction of virus dissemination by DC. PRO 2000, but not DxS, also inhibited HIV-1 binding to DC-SIGN+ cells and trans infection of co-cultured target cells. The inflammatory potential of both compounds was screened by measurement of cytokine production from cervical explants, and statistically significant increases were only observed for IL-1beta and RANTES following treatment with PRO 2000. Both compounds also demonstrated potent activity against HSV-2 infection of cervical epithelial cells.

CONCLUSION

Our results demonstrate that PRO 2000 is a potent inhibitor of R5 HIV-1 infection and dissemination pathways in human cervical explants. DxS, while demonstrating significant inhibition of R5 infection, was less active against DC mediated dissemination pathways. PRO 2000 has now entered human phase III efficacy trials.

DC-SIGN facilitates fusion of dendritic cells with human T-cell leukemia virus type 1-infected cells

Interactions between the oncogenic retrovirus human T-cell leukemia virus type 1 (HTLV-1) and dendritic cells (DCs) are poorly characterized. We show here that monocyte-derived DCs form syncytia and are infected upon coculture with HTLV-1-infected lymphocytes. We examined the role of DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), a C-type lectin expressed in DCs, in HTLV-1-induced syncytium formation. DC-SIGN is known to bind with high affinity to various viral envelope glycoproteins, including human immunodeficiency virus (HIV) and hepatitis C virus, as well as to the cellular receptors ICAM-2 and ICAM-3. After cocultivating DCs and HTLV-1-infected cells, we found that anti-DC-SIGN monoclonal antibodies (MAbs) were able to decrease the number and size of HTLV-1-induced syncytia. Moreover, expression of the lectin in epithelial-cell lines dramatically enhanced the ability to fuse with HTLV-1-positive cells. Interestingly, in contrast to the envelope (Env) glycoproteins of HIV and other viruses, that of HTLV-1 does not bind directly to DC-SIGN. The facilitating role of the lectin in HTLV-1 syncytium formation is mediated by its interaction with ICAM-2 and ICAM-3, as demonstrated by use of MAbs directed against these adhesion molecules. Altogether, our results indicate that DC-SIGN facilitates HTLV-1 infection and fusion of DCs through an ICAM-dependent mechanism.

Dendritic cells and HIV-specific CD4+ T cells: HIV antigen presentation, T-cell activation, and viral transfer

Human immunodeficiency virus (HIV)-specific CD4+ lymphocytes are preferentially infected in HIV-positive individuals. To study this preferential infection, we have derived several HIV-specific (HS) CD4+ clones. We show that in dendritic cells (DCs), HIV virion capture led to major histocompatibility complex class-II (MHC-II)-restricted viral antigen presentation and to activation of HS cells. In contrast, neither cell-free virions nor infected lymphocytes activated HS cells. In DCs, the dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN/CD209), which internalizes virions, promoted MHC-II presentation of HIV antigens. Activation of HS cells by HIV-exposed DCs triggered an efficient viral spread in lymphocytes. CD4+ clones with irrelevant antigenic specificities were not activated by HIV-exposed DCs and poorly supported viral replication under this setting. Our results unravel the mechanisms of MHC-II-restricted HIV antigen presentation by DCs and describe how HIV gains access to the very cells designed by the immune system to counteract this pathogen.

Infection of dendritic cells (DCs), not DC-SIGN-mediated internalization of human immunodeficiency virus, is required for long-term transfer of virus to T cells

The C-type lectin DC-SIGN expressed on immature dendritic cells (DCs) captures human immunodeficiency virus (HIV) particles and enhances the infection of CD4+ T cells. This process, known as trans-enhancement of T-cell infection, has been related to HIV endocytosis. It has been proposed that DC-SIGN targets HIV to a nondegradative compartment within DCs and DC-SIGN-expressing cells, allowing incoming virus to persist for several days before infecting target cells. In this study, we provide several lines of evidence suggesting that intracellular storage of intact virions does not contribute to HIV transmission. We show that endocytosis-defective DC-SIGN molecules enhance T-cell infection as efficiently as their wild-type counterparts, indicating that DC-SIGN-mediated HIV internalization is dispensable for trans-enhancement. Furthermore, using immature DCs that are genetically resistant to infection, we demonstrate that several days after viral uptake, HIV transfer from DCs to T cells requires viral fusion and occurs exclusively through DC infection and transmission of newly synthesized viral particles. Importantly, our results suggest that DC-SIGN participates in this process by cooperating with the HIV entry receptors to facilitate cis-infection of immature DCs and subsequent viral transfer to T cells. We suggest that such a mechanism, rather than intracellular storage of incoming virus, accounts for the long-term transfer of HIV to CD4+ T cells and may contribute to the spread of infection by DCs.

Efficient inhibition of HIV-1 replication in human immature monocyte-derived dendritic cells by purified anti-HIV-1 IgG without induction of maturation

During mucosal HIV transmission, immature dendritic cells (DCs) present in the mucosa are among the first cellular targets of the virus. Previous studies have analyzed the inhibition of HIV-1 transfer from human mature DCs to T lymphocytes by neutralizing IgG, but so far no in vitro data regarding the capacity of antibodies to inhibit HIV-1 infection of immature DCs have been reported. Here, we found an increased HIV-inhibitory activity of monoclonal IgG and purified polyclonal IgG when immature monocyte-derived dendritic cells (iMDDCs) were used as target cells instead of autologous blood lymphocytes. We showed that FcgammaRII is involved in the mechanism for inhibiting HIV-1 infection of iMDDCs by IgG, whereas no induction of maturation was detected at concentrations of IgG that result in a 90% reduction of HIV replication. After induction of FcgammaRI expression on iMDDCs by IFN-gamma, an augmentation of the HIV-inhibitory activity of IgG, related to the expression of FcgammaRI, was observed. Taken together, our results demonstrate the participation of FcgammaRs in HIV-1 inhibition by IgG when iMDDCs are the targets. We propose that IgG is able to efficiently inhibit HIV-1 replication in iMDDCs and should be one of the components to be induced by vaccination.

Attachment factors

As obligate intracellular parasites, viruses must bind to, and enter, permissive host cells in order to gain access to the cellular machinery that is required for their replication. The very large number of mammalian viruses identified to date is reflected in the fact that almost every human and animal cell type is a target for infection by one, or commonly more than one, species of virus. As viruses have adapted to target certain cell types for their propagation, there is exquisite specificity in cellular tropism. This specificity is frequently, but not always, mediated by the first step in the viral replication cycle: attachment of viral surface proteins to receptors expressed on susceptible cells. Viral receptors may be protein, carbohydrate, and/or lipid. Many viruses can use more than one attachment receptor, and indeed may sequentially engage multiple receptors to infect a cell. Thus, it is useful to differentiate between attachment receptors, that simply allow viruses a foothold at the limiting membrane of a cell, and entry receptors that mediate delivery the viral genome into the cytoplasm. For some viruses the attachment factors that promote binding to permissive cells are very well defined, but the sequence of events that triggers viral entry is only now beginning to be understood. For other viruses, despite many efforts, the receptors remain elusive. In this chapter we will confine our review to viruses that infect mammals, with particular focus on human pathogens. We do not intend that this will be an exhaustive overview of viral attachment receptors; instead we will take a number of examples of well-characterized virus-receptor interactions, discuss supporting evidence, and highlight any controversies and uncertainties in the field. We will then conclude with a reflection on general principles of viral attachment, consider some exceptions to these principles, and make some suggestion for future research.

Replicative fitness of CCR5-using and CXCR4-using human immunodeficiency virus type 1 biological clones

CCR5-tropic viruses cause the vast majority of new HIV-1 infections while about half of the individuals infected with HIV-1 manifest a co-receptor switch (CCR5 (R5) to CXCR4 (X4)) prior to accelerated disease progression. The underlying biological mechanisms of X4 outgrowth in AIDS patients are still poorly understood. Although X4 viruses have been associated with increased "virulence" in vivo, in vitro replication and cytopathicity studies of X4 and R5 viruses have led to conflicting conclusions. We studied the replicative fitness of HIV-1 biological clones with different co-receptor tropism, isolated from four AIDS patients. On average, R5 and X4 clones replicated equally well in mitogen-activated T cells. In contrast, X4 variants were transferred more efficiently from dendritic cells to autologous CD4+ T cells. These observations suggest that interaction between X4 viruses, DC and T cells might contribute to the preferential outgrowth of X4 viruses in AIDS patients.

HTLV-1 tropism and envelope receptor

The identification of CD4 as the primary receptor for HIV followed shortly after the discovery of the virus, but the HTLV receptor remained long elusive, until its recent identification as the GLUT1 glucose transporter. In the present review, we describe the status of the literature that surrounded this discovery as well as the in vitro and in vivo observations that led to the identification of GLUT1. Also, we will explore a few tracks to conciliate the in vitro and in vivo data on HTLV-1 tropism within the context of the HTLV literature that has accumulated over the past 25 years. A close examination of these data leads us to conclude that the preferential detection of HTLV-1 in CD4+ T lymphocyte subsets in vivo, even in the absence of leukemia, is not likely to be directly related to envelope receptor interactions, but rather to an array of postentry selection bottlenecks in vivo. Furthermore, we propose that infection of other hematopoietic and nonhematopoietic cells is likely to take place during the lifetime of an individual, with a burst early during the infection.