Syndecans serve as attachment receptors for human immunodeficiency virus type 1 on macrophages

Proteoglycans in host-pathogen interactions: molecular mechanisms and therapeutic implications

Many microbial pathogens subvert proteoglycans for their adhesion to host tissues, invasion of host cells, infection of neighbouring cells, dissemination into the systemic circulation, and evasion of host defence mechanisms. Where studied, specific virulence factors mediate these proteoglycan-pathogen interactions, which are thus thought to affect the onset, progression and outcome of infection. Proteoglycans are composites of glycosaminoglycan (GAG) chains attached covalently to specific core proteins. Proteoglycans are expressed ubiquitously on the cell surface, in intracellular compartments, and in the extracellular matrix. GAGs mediate the majority of ligand-binding activities of proteoglycans, and many microbial pathogens elaborate cell-surface and secreted factors that interact with GAGs. Some pathogens also modulate the expression and function of proteoglycans through known virulence factors. Several GAG-binding pathogens can no longer attach to and invade host cells whose GAG expression has been reduced by mutagenesis or enzymatic treatment. Furthermore, GAG antagonists have been shown to inhibit microbial attachment and host cell entry in vitro and reduce virulence in vivo. Together, these observations underscore the biological significance of proteoglycan-pathogen interactions in infectious diseases.

Galectin-1 and HIV-1 Infection

Initial binding of human immunodeficiency virus-1 (HIV-1) to its susceptible CD4(+) cells is the limiting step for the establishment of infection as the avidity of viral envelope gp120 for CD4 is not high and the number of viral envelope spikes on the surface is found to be low compared to highly infectious viruses. Several host factors, such as C-type lectins, are listed as being able to enforce or facilitate the crucial interaction of HIV-1 to the susceptible cell. Recent works suggest that a host soluble beta-galactoside-binding lectin, galectin-1, also facilitates both virion binding and the infection of target cells in a manner dependent on lactose but not mannose, suggesting that this soluble galectin can be considered as a host factor that influences HIV-1 pathogenesis. In this chapter, we describe methods used to investigate the potential role of the galectin family in HIV-1-mediated disease progression.

Diverse functions of glycosaminoglycans in infectious diseases

Glycosaminoglycans (GAGs) are complex carbohydrates that are expressed ubiquitously and abundantly on the cell surface and in the extracellular matrix (ECM). The extraordinary structural diversity of GAGs enables them to interact with a wide variety of biological molecules. Through these interactions, GAGs modulate various biological processes, such as cell adhesion, proliferation and migration, ECM assembly, tissue repair, coagulation, and immune responses, among many others. Studies during the last several decades have indicated that GAGs also interact with microbial pathogens. GAG-pathogen interactions affect most, if not all, the key steps of microbial pathogenesis, including host cell attachment and invasion, cell-cell transmission, systemic dissemination and infection of secondary organs, and evasion of host defense mechanisms. These observations indicate that GAG-pathogen interactions serve diverse functions that affect the pathogenesis of infectious diseases.

Alternate receptor usage of neuropilin-1 and glucose transporter protein 1 by the human T cell leukemia virus type 1

Recent studies have demonstrated that neuropilin 1 (NP-1) is involved in HTLV-1 entry; however, the role NP-1 plays in this process is not understood. We demonstrated that ectopic expression of human NP-1 but not NP-2 cDNA increased susceptibility to HTLV-1. SiRNA-mediated inhibition of NP-1 expression correlated with significant reduction of HTLV-1 Env-mediated fusion. The vascular endothelial growth factor (VEGF(165)) caused downmodulation of surface NP-1 and inhibited HTLV-1 infection of U87 cells. In contrast, VEGF(165) partially inhibited infection of primary astrocytes and had no significant effect on infection of HeLa cells. VEGF(165) and antibodies to the glucose transporter protein 1 (anti-GLUT-1) were both needed to block infection of primary astrocytes, however, only anti-GLUT-1 antibodies were sufficient to block infection of HeLa cells. HTLV-1 Env forms complexes with both NP-1 and GLUT-1 in primary human astrocytes. The alternate usage of these two cellular receptors may have important implications regarding HTLV-1 neuro-tropism.

Spermatozoa capture HIV-1 through heparan sulfate and efficiently transmit the virus to dendritic cells

Semen is the main vector for HIV-1 dissemination worldwide. It contains three major sources of infectious virus: free virions, infected leukocytes, and spermatozoa-associated virions. We focused on the interaction of HIV-1 with human spermatozoa and dendritic cells (DCs). We report that heparan sulfate is expressed in spermatozoa and plays an important role in the capture of HIV-1. Spermatozoa-attached virus is efficiently transmitted to DCs, macrophages, and T cells. Interaction of spermatozoa with DCs not only leads to the transmission of HIV-1 and the internalization of the spermatozoa but also results in the phenotypic maturation of DCs and the production of IL-10 but not IL-12p70. At low values of extracellular pH (∼6.5 pH units), similar to those found in the vaginal mucosa after sexual intercourse, the binding of HIV-1 to the spermatozoa and the consequent transmission of HIV-1 to DCs were strongly enhanced. Our observations support the notion that far from being a passive carrier, spermatozoa acting in concert with DCs might affect the early course of sexual transmission of HIV-1 infection.

Microbicides: an emerging science of HIV-1 prevention in women-15th Conference on Retroviruses and Opportunistic Infections, Boston, USA, 3-6 February 2008

Women account for almost 60% of human immunodeficiency virus type 1 (HIV-1) infections in Sub-Saharan Africa. HIV-1 prevention tools such as condoms, abstinence and monogamy are not always feasible options for women due to various socio-economic and cultural factors. Microbicides are anti-microbial medications formulated for topical administration to prevent the sexual transmission of HIV-1 and other pathogens. Ideally, they will afford bidirectional protection to both men and women who are engaged in vaginal or anal sex. Since the use of condom is often difficult or impossible, this multifunctional role of microbicides will be crucial in the fight against AIDS. The 15th Conference on Retroviruses and Opportunistic Infections (CROI) was recently held in Boston, USA, where one of the most interesting subject area discussed by researchers from all around the world was the latest developments and understandings in microbicide-related basic science and pre-clinical product development as well as in product manufacturing and formulation that address the issue of user adherence.

Early events in vaginal HIV transmission: implications in microbicide development

In vitro models for HIV crossing through mucosae include direct infection of epithelial cells, transcytosis through epithelial cells, epithelial transmigration of infected donor cells, uptake by intraepithelial dendritic cells, and circumvention of the epithelial barrier through physical breaches. Mucosal crossing of HIV for further reaching of the submucosal target cells (macrophages, lymphocytes and dendritic cells) may be modulated by supraepithelial factors, such as seminal complement components (opsonized HIV), by epithelial factors released in the submucosal microenvironment, such as antimicrobial soluble factors, cytokines and chemokines, and by the potent intraepithelial and submucosal innate immunity. Poor understanding of the subtle and complex orchestration of the numerous virus and cell factors involved in HIV mucosal crossing renders the design of effective microbicide formulations difficult. Thus, there is currently no clear relationship between the success of preclinical development of microbicide formulations, using the available assays of anti-HIV efficacy and mucosal toxicity, and its efficacy against HIV acquisition in women enrolled in a large-scale Phase III trial. In addition, the proof of concept that a microbicide formulation may be efficient outside the laboratory has not yet been clearly demonstrated. Finally, there is an urgent need to better understand and modelize the early events occurring during the first hours of HIV contact with the female genital mucosae, especially considering the enormous gaps of knowledge in the understanding of the mechanisms of HIV mucosal crossing through female genital mucosae.

Glycosphingolipid composition of human immunodeficiency virus type 1 (HIV-1) particles is a crucial determinant for dendritic cell-mediated HIV-1 trans-infection

Interactions of human immunodeficiency virus type 1 (HIV-1) with dendritic cells (DCs) are multifactorial and presumably require nonredundant interactions between the HIV-1 envelope glycoprotein gp120 and molecules expressed on the DC surface that define the cellular fate of the virus particle. Surprisingly, neutralization of HIV-1 gp120-dependent binding interactions with DCs was insufficient to prevent HIV-1 attachment. Besides gp120, HIV-1 particles also incorporate host cell-derived proteins and lipids in their particle membrane. In this study, we demonstrate a crucial role for host cell-derived glycosphingolipids (GSLs) for the initial interactions of HIV-1 particles with both immature and mature DCs. Production of HIV-1 particles from virus producer cells treated with ceramide synthase inhibitor fumonisin B1 or glucosylceramide synthase inhibitor 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) resulted in the production of virus particles that, although capable of binding previously defined HIV-1 gp120-specific attachment factors CD4, DC-SIGN, and syndecans, were attenuated in their ability to be captured by both immature and mature DCs. Furthermore, GSL-deficient HIV-1 particles were inhibited in their ability to establish productive infections in DC-T-cell cocultures. These studies provide initial evidence for the role of HIV-1 particle membrane-associated GSLs in virus invasion of DCs and also provide additional novel cellular targets, GSL biosynthetic pathways and GSL-dependent HIV-1 interactions with DCs, for development of antiviral therapy.

HIV entry in macrophages is dependent on intact lipid rafts

Macrophages are an important natural target cell for HIV-1, but previous studies of virus entry into these cells are limited, and the involvement of membrane cholesterol and lipid rafts is unknown. Cholesterol disruption of macrophage membranes using four pharmacological agents acting by different mechanisms: methyl-β cyclodextrin, nystatin, filipin complex and Lovastatin, all significantly inhibited productive HIV entry and reverse transcription. The inhibitory effects of these drugs resulted in decreased virus release from infected cells, and could be substantially reversed by the addition of water-soluble cholesterol. The virus bound equally to cholesterol-disrupted cells even though HIV receptor expression levels were significantly reduced. Macrophage CD4 and CCR5 were found to partition with the detergent-resistant membranes with a typical raft-associating protein flotillin-1. HIV particles were observed co-localising with a marker of lipid rafts (CTB-FITC) early post infection. These data suggest that macrophage membrane cholesterol is essential for HIV entry, and implicate lipid raft involvement.

Dendritic cells and macrophages in the genitourinary tract

Dendritic cells (DCs) and macrophages are antigen-presenting cells (APCs) that are important in innate immune defense as well as in the generation and regulation of adaptive immunity against a wide array of pathogens. The genitourinary (GU) tract, which serves an important reproductive function, is constantly exposed to numerous agents of sexually transmitted infections (STIs). To combat these STIs, several subsets of DCs and macrophages are strategically localized within the GU tract. In the female genital mucosa, recruitment and function of these APCs are uniquely governed by sex hormones. This review summarizes the latest advances in our understanding of DCs and macrophages in the GU tract with respect to their subsets, lineage, and function. In addition, we discuss the divergent roles of these cells in immune defense against STIs as well as in maternal tolerance to the fetus.

Focus on antivirally active sulfated polysaccharides: from structure-activity analysis to clinical evaluation

In recent years, many compounds having potent antiviral activity in cell culture have been detected and some of these compounds are currently undergoing either preclinical or clinical evaluation. Among these antiviral substances, naturally occurring sulfated polysaccharides and those from synthetic origin are noteworthy. Recently, several controversies over the molecular structures of sulfated polysaccharides, viral glycoproteins, and cell-surface receptors have been resolved, and many aspects of their antiviral activity have been elucidated. It has become clear that the antiviral properties of sulfated polysaccharides are not only a simple function of their charge density and chain length but also their detailed structural features. The in vivo efficacy of these compounds mostly corresponds to their ability to inhibit the attachment of the virion to the host cell surface although in some cases virucidal activity plays an additional role. This review summarizes experimental evidence indicating that sulfated polysaccharides might become increasingly important in drug development for the prevention of sexually transmitted diseases in the near future.

HIV envelope binding by macrophage-expressed gp340 promotes HIV-1 infection

The scavenger receptor cysteine-rich protein gp340 functions as part of the host innate immune defense system at mucosal surfaces. In the genital tract, its expression by cervical and vaginal epithelial cells promotes HIV trans-infection and may play a role in sexual transmission. Gp340 is an alternatively spliced product of the deleted in malignant brain tumors 1 (DMBT1) gene. In addition to its innate immune system activity, DMBT1 demonstrates instability in multiple types of cancer and plays a role in epithelial cell differentiation. We demonstrate that monocyte-derived macrophages express gp340 and that HIV-1 infection is decreased when envelope cannot bind it. Inhibition of infection occurred at the level of fusion of M-, T-, and dual-tropic envelopes. Additional HIV-1 envelope binding molecules, such as dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN), mannose-binding lectin, and heparan sulfate, enhance the efficiency of infection of the cells that express them by increasing the local concentration of infectious virus. Our data suggest that gp340, which is expressed by macrophages in vivo, may function to enhance infection in much the same manner. Its expression on tissue macrophages and epithelial cells suggests important new opportunities for HIV-1 pathogenesis investigation and therapy.

Donor variability in HIV binding to peripheral blood mononuclear cells

BACKGROUND

HIV infection of cells varies greatly between individuals, with multiple steps in the replication cycle potentially contributing to the variability. Although entry and post-entry variability of HIV infection levels in cells has been demonstrated, variability in HIV binding has not been examined. In this study, we examined variability of HIV binding to peripheral blood mononuclear cells (PBMC) from different donors.

RESULTS

HIV binding to PBMC varied up to 3.9-fold between individuals and was independent of CD4. Replication of HIV in donor PBMC required CD4 and paralleled virus binding trends of donor PBMC. To assess the stability of virus binding phenotypes over time, HIV was bound to donors with low- and high-binding phenotypes. The binding phenotypes were maintained when tested weekly over a 4-week period for 3 of 4 donors, while one high-binding donor decreased to lower binding on the 4th week. The low- and high-binding phenotypes were also preserved across different HIV strains. Experiments performed to determine if there was an association between HIV binding levels and specific cell subset levels within PBMC showed no correlation, suggesting that HIV binds to multiple cell subsets.

CONCLUSION

These results show that differences exist in HIV binding to donor PBMC. Our data also show that HIV binding to donor PBMC is CD4-independent and can change over time, suggesting that virus binding variability is due to differences in the expression of changeable cell-surface host factors. Taken together, this study highlights the impact of cell-surface factors in HIV binding to, and infection of, PBMC which likely represents an important step in HIV infection in vivo.

Oral keratinocytes support non-replicative infection and transfer of harbored HIV-1 to permissive cells

Background

Oral keratinocytes on the mucosal surface are frequently exposed to HIV-1 through contact with infected sexual partners or nursing mothers. To determine the plausibility that oral keratinocytes are primary targets of HIV-1, we tested the hypothesis that HIV-1 infects oral keratinocytes in a restricted manner.

Results

To study the fate of HIV-1, immortalized oral keratinocytes (OKF6/TERT-2; TERT-2 cells) were characterized for the fate of HIV-specific RNA and DNA. At 6 h post inoculation with X4 or R5-tropic HIV-1, HIV-1gag RNA was detected maximally within TERT-2 cells. Reverse transcriptase activity in TERT-2 cells was confirmed by VSV-G-mediated infection with HIV-NL4-3Δenv-EGFP. AZT inhibited EGFP expression in a dose-dependent manner, suggesting that viral replication can be supported if receptors are bypassed. Within 3 h post inoculation, integrated HIV-1 DNA was detected in TERT-2 cell nuclei and persisted after subculture. Multiply spliced and unspliced HIV-1 mRNAs were not detectable up to 72 h post inoculation, suggesting that HIV replication may abort and that infection is non-productive. Within 48 h post inoculation, however, virus harbored by CD4 negative TERT-2 cells trans infected co-cultured peripheral blood mononuclear cells (PBMCs) or MOLT4 cells (CD4+ CCR5+) by direct cell-to-cell transfer or by releasing low levels of infectious virions. Primary tonsil epithelial cells also trans infected HIV-1 to permissive cells in a donor-specific manner.

Conclusion

Oral keratinocytes appear, therefore, to support stable non-replicative integration, while harboring and transmitting infectious X4- or R5-tropic HIV-1 to permissive cells for up to 48 h.

Setting the stage: host invasion by HIV

For more than two decades, HIV has infected millions of people worldwide each year through mucosal transmission. Our knowledge of how HIV secures a foothold at both the molecular and cellular levels has been expanded by recent investigations that have applied new technologies and used improved techniques to isolate ex vivo human tissue and generate in vitro cellular models, as well as more relevant in vivo animal challenge systems. Here, we review the current concepts of the immediate events that follow viral exposure at genital mucosal sites where most documented transmissions occur. Furthermore, we discuss the gaps in our knowledge that are relevant to future studies, which will shape strategies for effective HIV prevention.

Monitoring early fusion dynamics of human immunodeficiency virus type 1 at single-molecule resolution

The fusion of human immunodeficiency virus type 1 (HIV-1) to host cells is a dynamic process governed by the interaction between glycoproteins on the viral envelope and the major receptor, CD4, and coreceptor on the surface of the cell. How these receptors organize at the virion-cell interface to promote a fusion-competent site is not well understood. Using single-molecule force spectroscopy, we map the tensile strengths, lifetimes, and energy barriers of individual intermolecular bonds between CCR5-tropic HIV-1 gp120 and its receptors CD4 and CCR5 or CXCR4 as a function of the interaction time with the cell. According to the Bell model, at short times of contact between cell and virion, the gp120-CD4 bond is able to withstand forces up to 35 pN and has an initial lifetime of 0.27 s and an intermolecular length of interaction of 0.34 nm. The initial bond also has an energy barrier of 6.7 k(B)T (where k(B) is Boltzmann's constant and T is absolute temperature). However, within 0.3 s, individual gp120-CD4 bonds undergo rapid destabilization accompanied by a shortened lifetime and a lowered tensile strength. This destabilization is significantly enhanced by the coreceptor CCR5, not by CXCR4 or fusion inhibitors, which suggests that it is directly related to a conformational change in the gp120-CD4 bond. These measurements highlight the instability and low tensile strength of gp120-receptor bonds, uncover a synergistic role for CCR5 in the progression of the gp120-CD4 bond, and suggest that the cell-virus adhesion complex is functionally arranged about a long-lived gp120-coreceptor bond.

Sialoadhesin expressed on IFN-induced monocytes binds HIV-1 and enhances infectivity

Background

HIV-1 infection dysregulates the immune system and alters gene expression in circulating monocytes. Differential gene expression analysis of CD14⁺ monocytes from subjects infected with HIV-1 revealed increased expression of sialoadhesin (Sn, CD169, Siglec 1), a cell adhesion molecule first described in a subset of macrophages activated in chronic inflammatory diseases.

Methodology/Principal Findings

We analyzed sialoadhesin expression on CD14⁺ monocytes by flow cytometry and found significantly higher expression in subjects with elevated viral loads compared to subjects with undetectable viral loads. In cultured CD14⁺ monocytes isolated from healthy individuals, sialoadhesin expression was induced by interferon-α and interferon-γ but not tumor necrosis factor-α. Using a stringent binding assay, sialoadhesin-expressing monocytes adsorbed HIV-1 through interaction with the sialic acid residues on the viral envelope glycoprotein gp120. Furthermore, monocytes expressing sialoadhesin facilitated HIV-1 trans infection of permissive cells, which occurred in the absence of monocyte self-infection.

Conclusions/Significance

Increased sialoadhesin expression on CD14⁺ monocytes occurred in response to HIV-1 infection with maximum expression associated with high viral load. We show that interferons induce sialoadhesin in primary CD14⁺ monocytes, which is consistent with an antiviral response during viremia. Our findings suggest that circulating sialoadhesin-expressing monocytes are capable of binding HIV-1 and effectively delivering virus to target cells thereby enhancing the distribution of HIV-1. Sialoadhesin could disseminate HIV-1 to viral reservoirs during monocyte immunosurveillance or migration to sites of inflammation and then facilitate HIV-1 infection of permissive cells.

The HIV-1 envelope glycoprotein gp120 features four heparan sulfate binding domains, including the co-receptor binding site

It is well established that the human immunodeficiency virus-1 envelope glycoprotein surface unit, gp120, binds to cell-associated heparan sulfate (HS). Virus infectivity is increased by such interaction, and a variety of soluble polyanions efficiently neutralize immunodeficiency virus-1 in vitro. This interaction has been mainly attributed to the gp120 V3 loop. However, although evidence suggested that this particular domain does not fully recapitulate the binding activity of the protein, the ability of HS to bind to other regions of gp120 has not been completely addressed, and the exact localizations of the polysaccharide binding sites are not known. To investigate in more detail the structural basis of the HS-gp120 interaction, we used a mapping strategy and compared the heparin binding activity of wild type and mutant gp120 using surface plasmon resonance-based binding assays. Four heparin binding domains (1-4) were identified in the V2 and V3 loops, in the C-terminal domain, and within the CD4-induced bridging sheet. Interestingly, three of them were found in domains of the protein that undergo structural changes upon binding to CD4 and are involved in co-receptor recognition. In particular, Arg(419), Lys(421), and Lys(432), which directly interact with the co-receptor, are targeted by heparin. This study provides a complete account of the gp120 residues involved in heparin binding and identified several binding surfaces that constitute potential target for viral entry inhibition.

Hepatitis C virus NS5A anchor peptide disrupts human immunodeficiency virus

In the absence of an effective vaccine, there is an urgent need for safe and effective antiviral agents to prevent transmission of HIV. Here, we report that an amphipathic alpha-helical peptide derived from the hepatitis C virus NS5A anchor domain (designated C5A in this article) that has been shown to be virocidal for the hepatitis C virus (HCV) also has potent antiviral activity against HIV. C5A exhibits a broad range of antiviral activity against HIV isolates, and it prevents infection of the three in vivo targets of HIV: CD4(+) T lymphocytes, macrophages, and dendritic cells by disrupting the integrity of the viral membrane and capsid core while preserving the integrity of host membranes. C5A can interrupt an ongoing T cell infection, and it can prevent transmigration of HIV through primary genital epithelial cells, infection of mucosal target cells and transfer from dendritic cells to T cells ex vivo, justifying future experiments to determine whether C5A can prevent HIV transmission in vivo.

Selection of mutant CHO clones resistant to murine gammaherpesvirus 68 infection

Murine gammaherpesvirus 68 (MHV68) is used as a model to study gammaherpesvirus pathogenesis both in tissue culture systems and in vivo. We used a gene-trapping approach to get insight into cellular factors involved in MHV68 infection. By generating a library of gene-trapped CHO cells, we were able to isolate several clones that exhibited various degrees of resistance to MHV68-induced cytopathic effect. Clones that showed the highest degree of resistance were affected at the early stage of the viral cycle, with the vast majority of these clones being deficient for heparan sulfate (HS) expression at the cell surface. Heparan sulfate expression could be restored in all the HS-deficient clones by expression of EXT1, an enzyme that is essential for the biosynthesis of HS. Consistent with the role of HS in viral entry, HS-deficient CHO cells did not support viral internalization. Cell surface heparan sulfate proteoglycans (HSPG) are mostly composed of HS chains attached to two families of core proteins, the transmembrane syndecans and the GPI-anchored glypicans. Treatment of CHO cells with phosphatidylinositol-specific phospholipase C (PI-PLC) did not significantly affect the level of HS expression, indicating that the glypicans are not a major source of HSPG in CHO cells. By contrast, treatment of CHO cells with PMA, a drug known to accelerate syndecan shedding, resulted in a decrease in both HS expression and susceptibility to MHV68; these effects were abolished by TIMP-3, a specific inhibitor of syndecan shedding. All together, our results confirm the essential role of HS in MHV68 infection and identify the syndecans as a major source of HSPG used by the virus as coreceptors to infect CHO cells.

Syndecan-3 is a dendritic cell-specific attachment receptor for HIV-1

Dendritic cells (DCs) efficiently capture HIV-1 and mediate transmission to T cells, but the underlying molecular mechanism is still being debated. The C-type lectin DC-SIGN is important in HIV-1 transmission by DCs. However, various studies strongly suggest that another HIV-1 receptor on DCs is involved in the capture of HIV-1. Here we have identified syndecan-3 as a major HIV-1 attachment receptor on DCs. Syndecan-3 is a DC-specific heparan sulfate (HS) proteoglycan that captures HIV-1 through interaction with the HIV-1 envelope glycoprotein gp120. Syndecan-3 stabilizes the captured virus, enhances DC infection in cis, and promotes transmission to T cells. Removal of the HSs from the cell surface by heparinase III or by silencing syndecan-3 by siRNA partially inhibited HIV-1 transmission by immature DCs, whereas neutralizing both syndecan-3 and DC-SIGN completely abrogated HIV-1 capture and subsequent transmission. Thus, HIV-1 exploits both syndecan-3 and DC-SIGN to mediate HIV-1 transmission, and an effective microbicide should target both syndecan-3 and DC-SIGN on DCs to prevent transmission.

New world clade B arenaviruses can use transferrin receptor 1 (TfR1)-dependent and -independent entry pathways, and glycoproteins from human pathogenic strains are associated with the use of TfR1

Arenaviruses are rodent-borne viruses, with five members of the family capable of causing severe hemorrhagic fevers if transmitted to humans. To date, two distinct cellular receptors have been identified that are used by different pathogenic viruses, alpha-dystroglycan by Lassa fever virus and transferrin receptor 1 (TfR1) by certain New World clade B viruses. Our previous studies have suggested that other, as-yet-unknown receptors are involved in arenavirus entry. In the present study, we examined the use of TfR1 by the glycoproteins (GPs) from a panel of New World clade B arenaviruses comprising three pathogenic and two nonpathogenic strains. Interestingly, we found that TfR1 was only used by the GPs from the pathogenic viruses, with entry of the nonpathogenic strains being TfR1 independent. The pathogenic GPs could also direct entry into cells by TfR1-independent pathways, albeit less efficiently. A comparison of the abilities of TfR1 orthologs from different species to support arenavirus entry found that the human and feline receptors were able to enhance entry of the pathogenic strains, but that neither the murine or canine forms were functional. Since the ability to use TfR1 is a characteristic feature of the human pathogens, this interaction may represent an important target in the treatment of New World hemorrhagic fevers. In addition, the ability to use TfR1 may be a useful tool to predict the likelihood that any existing or newly discovered viruses in this family could infect humans.

Structural and functional characterization of the interaction between cyclophilin B and a heparin-derived oligosaccharide

The chemotaxis and integrin-mediated adhesion of T lymphocytes triggered by secreted cyclophilin B (CypB) depend on interactions with both cell surface heparan sulfate proteoglycans (HSPG) and the extracellular domain of the CD147 membrane receptor. Here, we use NMR spectroscopy to characterize the interaction of CypB with heparin-derived oligosaccharides. Chemical shift perturbation experiments allowed the precise definition of the heparan sulfate (HS) binding site of CypB. The N-terminal extremity of CypB, which contains a consensus sequence for heparin-binding proteins was modeled on the basis of our experimental NMR data. Because the HS binding site extends toward the CypB catalytic pocket, we measured its peptidyl-prolyl cis-trans isomerase (PPIase) activity in the absence or presence of a HS oligosaccharide toward a CD147-derived peptide. We report the first direct evidence that CypB is enzymatically active on CD147, as it is able to accelerate the cis/trans isomerization of the Asp(179)-Pro(180) bond in a CD147-derived peptide. However, HS binding has no significant influence on this PPIase activity. We thus conclude that the glycanic moiety of HSPG serves as anchor for CypB at the cell surface, and that the signal could be transduced by CypB via its PPIase activity toward CD147.

Apolipoprotein E-derived antimicrobial peptide analogues with altered membrane affinity and increased potency and breadth of activity

Host-derived anti-infective proteins represent an important source of sequences for designing antimicrobial peptides (AMPs). However such sequences are often long and comprise diverse amino acids with uncertain contribution to biological effects. Previously, we identified a simple highly cationic peptide derivative of human apolipoprotein E (apoEdp) that inhibited a range of microorganisms. Here, we have dissected the protein chemistry underlying this activity. We report that basic residues and peptide length of around 18 residues were required for activity; however, the Leu residues can be substituted by several other residues without loss of activity and, when substituted with Phe or Trp, resulted in peptides with increased potency. These apoEdp-derived AMPs (apoE-AMPs) showed no cytotoxicity and minimal haemolytic activity, and were active against HIV and Plasmodium via an extracellular target. CXCR4 and CCR5 strains of HIV were inhibited though an early stage in viral infection upstream of fusion, and a lack of inhibition of vesicular stomatitis virus G protein pseudotyped HIV-1 suggested the anti-HIV activity was relatively selective. Inhibition of Plasmodium invasion of hepatocytes was observed without a direct action on Plasmodium integrity or attachment to cells. The Trp-substituted apoE-AMP adhered to mammalian cells irreversibly, explaining its increased potency; NMR experiments confirmed that the aromatic peptides also showed stronger perturbation of membrane lipids (relative to apoEdp). Our data highlight the contribution of specific amino acids to the activity of apoEdp (and also potentially unrelated AMPs) and suggest that apoE-AMPs may be useful as lead agents for preventing the early stages of HIV and Plasmodium cellular entry.

Loss of the N-linked glycosylation site at position 386 in the HIV envelope V4 region enhances macrophage tropism and is associated with dementia

HIV infects macrophages and microglia in the central nervous system (CNS). Mechanisms that enhance HIV macrophage/microglial tropism are not well understood. Here, we identify an HIV Env variant in the V4 region of gp120, Asp 386 (D386), that eliminates an N-linked glycosylation site at position 386, enhances viral replication in macrophages, and is present at a higher frequency in AIDS patients with HIV-associated dementia (HAD) compared with non-HAD patients. D386 enhances HIV entry and replication in macrophages but not in microglia or peripheral blood mononuclear cells, possibly due to differential glycosylation in these cell types. A D386N mutation in the UK1br Env, which restores the N-linked glycan site, reduced neutralization sensitivity to the IgG1b12 (b12) monoclonal antibody, which recognizes a conserved neutralization epitope that overlaps the CD4 binding site. Molecular modeling suggested that loss of the glycan at position 386 increases exposure of the CD4 and b12 binding sites on gp120. Loss of a glycan at 386 was more frequent in Envs from HAD patients (26%; n=185) compared with non-HAD patients (7%; n=99; p<0.001). The most significant association of these Env variants with HAD was in blood or lymphoid tissue rather than brain. These findings suggest that increased exposure of the b12 epitope overlapping the CD4 binding site via elimination of a glycan at position 386 is associated with enhanced HIV macrophage tropism, and provide evidence that determinants of macrophage and microglia tropism are overlapping but distinct.

Differential in vitro inhibitory activity against HIV-1 of alpha-(1-3)- and alpha-(1-6)-D-mannose specific plant lectins: implication for microbicide development

BACKGROUND

Plant lectins such as Galanthus nivalis agglutinin (GNA) and Hippeastrum hybrid agglutinin (HHA) are natural proteins able to link mannose residues, and therefore inhibit HIV-target cell interactions. Plant lectins are candidate for microbicide development.

OBJECTIVE

To evaluate the activity against HIV of the mannose-specific plant lectins HHA and GNA at the cellular membrane level of epithelial cells and monocyte-derived dendritic cells (MDDC), two potential target cells of HIV at the genital mucosal level.

METHODS

The inhibitory effects of HHA and GNA were evaluated on HIV adsorption to genital epithelial HEC-1A cell line, on HIV transcytosis throughout a monolayer of polarized epithelial HEC-1A cells, on HIV adsorption to MDDC and on transfer of HIV from MDDC to autologous T lymphocytes.

RESULTS

HHA faintly inhibited attachment to HEC-1A cells of the R5-tropic HIV-1Ba-L strain, in a dose-dependent manner, whereas GNA moderately inhibited HIV adsorption in the same context, but only at high drug doses. Only HHA, but not GNA, inhibited HIV-1JR-CSF transcytosis in a dose-dependent manner. By confocal microscopy, HHA, but not GNA, was adsorbed at the epithelial cell surface, suggesting that HHA interacts specifically with receptors mediating HIV-1 transcytosis. Both plant lectins partially inhibited HIV attachment to MDDC. HHA inhibited more efficiently the transfer of HIV from MDDC to T cell, than GNA. Both HHA and GNA lacked toxicity below 200 microg/ml irrespective the cellular system used and do not disturb the monolayer integrity of epithelial cells.

CONCLUSION

These observations demonstrate higher inhibitory activities of the lectin plant HHA by comparison to GNA, on HIV adsorption to HEC-1A cell line, HIV transcytosis through HEC-1A cell line monolayer, HIV adsorption to MDDC and HIV transfer from MDDC to T cells, highlighting the potential interest of HHA as effective microbicide against HIV.

Identification of individual structural fragments of N,N'-(bis-5-nitropyrimidyl)dispirotripiperazine derivatives for cytotoxicity and antiherpetic activity allows the prediction of new highly active compounds

OBJECTIVES

The objectives of this study were (i) to apply computer-based technologies to evaluate the structure of 48 N,N'-(bis-5-nitropyrimidyl)dispirotripiperazines which belong to a new class of highly active antiviral compounds binding to cell surface heparan sulphates, (ii) to understand the chemical- biological interactions governing their activities, and (iii) to design new compounds with strong antiviral activity.

METHODS

The logarithm of 50% cytotoxic concentration (CC(50)) in GMK cells, of 50% inhibitory concentration (IC(50)) against herpes simplex virus type 1, and of selectivity index (SI = CC(50)/IC(50)) was used to develop quantitative structure-activity relationships (QSARs) based on simplex representation of molecular structure. The QSAR model was applied to design new compounds. Two of these compounds were synthesized, physico-chemically characterized and tested for cytotoxicity and antiviral activity.

RESULTS

Statistic characteristics for partial least squares models allow the prediction of CC(50), IC(50) and SI values. The QSAR results demonstrate a high impact of individual structural fragments for antiviral activity. Molecular fragments that promote and interfere with antiviral activity were defined on the basis of the obtained models. Electrostatic factors (38%) and hydrophobicity (34%) were the most important determinants of antiherpetic activity. Using the established method, new potential dispirotripiperazine derivatives were computationally designed. Two of these computationally designed compounds were synthesized. The biological test results confirm the computationally predicted values of these compounds.

CONCLUSIONS

The established QSAR model is suitable for the design of new antiherpetic compounds and prediction of their activity.

Preferential chemotaxis of activated human CD4+ T cells by extracellular cyclophilin A

The recruitment and trafficking of leukocytes are essential aspects of the inflammatory process. Although chemokines are thought to be the main regulators of cell trafficking, extracellular cyclophilins have been shown recently to have potent chemoattracting properties for human leukocytes. Cyclophilins are secreted by a variety of cell types and are detected at high levels in tissues with ongoing inflammation. CD147 has been identified as the main signaling receptor for cyclophilin A (CypA) on human leukocytes. It is interesting that the expression of CD147 is elevated on leukocytes from inflamed tissue, suggesting a correlation among the presence of extracellular cyclophilins, CD147 expression, and inflammatory responses. Thus, cyclophilin-CD147 interactions may contribute directly to the recruitment of leukocytes into inflamed tissues. In the current studies, we show that activated human T lymphocytes express elevated levels of CD147, compared with resting T cells and that these activated T cells migrate more readily to CypA than resting cells. Furthermore, we show that unlike resting CD4+ T cells, the cyclophilin-mediated migration of activated T cells does not require interaction with heparan sulfate receptors but instead, is dependent on CD147 interaction alone. Such findings suggest that cyclophilin-CD147 interactions will be most potent when leukocytes are in an activated state, for example, during inflammatory responses. Thus, targeting cyclophilin-CD147 interactions may provide a novel approach for alleviating tissue inflammation.

Conserved changes in envelope function during human immunodeficiency virus type 1 coreceptor switching

We studied the evolution of human immunodeficiency virus type 1 (HIV-1) envelope function during the process of coreceptor switching from CCR5 to CXCR4. Site-directed mutagenesis was used to introduce most of the possible intermediate mutations in the envelope for four distinct coreceptor switch mutants, each with a unique pattern of CCR5 and CXCR4 utilization that extended from highly efficient use of both coreceptors to sole use of CXCR4. Mutated envelopes with some preservation of entry function on either CCR5- or CXCR4-expressing target cells were further characterized for their sensitivity to CCR5 or CXCR4 inhibitors, soluble CD4, and the neutralizing antibodies b12-IgG and 4E10. A subset of mutated envelopes was also studied in direct CD4 or CCR5 binding assays and in envelope-mediated fusion reactions. Coreceptor switch intermediates displayed increased sensitivity to CCR5 inhibitors (except for a few envelopes with mutations in V2 or C2) that correlated with a loss in CCR5 binding. As use of CXCR4 improved, infection mediated by the mutated envelopes became more resistant to soluble CD4 inhibition and direct binding to CD4 increased. These changes were accompanied by increasing resistance to the CXCR4 inhibitor AMD3100. Sensitivity to neutralizing antibody was more variable, although infection of CXCR4-expressing targets was generally more sensitive to neutralization by both b12-IgG and 4E10 than infection of CCR5-expressing target cells. These changes in envelope function were uniform in all four series of envelope mutations and thus were independent of the final use of CCR5 and CXCR4. Decreased CCR5 and increased CD4 binding appear to be common features of coreceptor switch intermediates.

Differential modulation of human lactoferrin activity against both R5 and X4-HIV-1 adsorption on epithelial cells and dendritic cells by natural antibodies

Human lactoferrin (Lf) is an iron binding glycoprotein that is present in several mucosal secretions. Many biological functions have been ascribed to Lf. In the present study, we showed that Lf limited specifically adsorption of R5- and X4-HIV-1-free particles on endometrial epithelial HEC-1A cells, by inhibiting virus adsorption on heparan-sulfated proteoglycans. But, Lf did not interfere with both R5 and X4-HIV transcytosis. We showed also the efficacy of Lf in preventing R5 and X4-HIV capture by dendritic cells. Conversely, we demonstrated that Lf-reacting natural Abs (NAbs) present within i.v. Ig-enhanced HIV attachment on dendritic cells by forming HIV-Lf-NAbs. HIV particles were able to directly interact with Lf following its interaction with NAbs. We also found Lf-reacting natural Abs within cervicovaginal secretions, suggesting the existence of Lf-NAbs complexes in women genital tract in vivo. In conclusion, this study highlights Lf as a potent microbicides and reports new function for NAbs within the genital compartment that may compartment that may abolish the inhibitory activity of microbicide compounds. Thus, we proposed a model in which Lf would appear as a double-edged sword that could have beneficial or detrimental effects depending on both cellular and molecular environments. This study highlights the use of Lf derivates as microbicide candidates to limit such interferences.

Syndecan-1/CD147 association is essential for cyclophilin B-induced activation of p44/42 mitogen-activated protein kinases and promotion of cell adhesion and chemotaxis

Many of the biological functions attributed to cell surface proteoglycans are dependent on the interaction with extracellular mediators through their heparan sulphate (HS) moieties and the participation of their core proteins in signaling events. A class of recently identified inflammatory mediators is secreted cyclophilins, which are mostly known as cyclosporin A-binding proteins. We previously demonstrated that cyclophilin B (CyPB) triggers chemotaxis and integrin-mediated adhesion of T lymphocytes mainly of the CD4+/CD45RO+ phenotype. These activities are related to interactions with two types of binding sites, CD147 and cell surface HS. Here, we demonstrate that CyPB-mediated adhesion of CD4+/CD45RO+ T cells is related to p44/42 mitogen-activated protein kinase (MAPK) activation by a mechanism involving CD147 and HS proteoglycans (HSPG). Although HSPG core proteins are represented by syndecan-1, -2, -4, CD44v3 and betaglycan in CD4+/CD45RO+ T cells, we found that only syndecan-1 is physically associated with CD147. The intensity of the heterocomplex increased in response to CyPB, suggesting a transient enhancement and/or stabilization in the association of CD147 to syndecan-1. Pretreatment with anti-syndecan-1 antibodies or knockdown of syndecan-1 expression by RNA interference dramatically reduced CyPB-induced p44/p42 MAPK activation and consequent migration and adhesion, supporting the model in which syndecan-1 serves as a binding subunit to form the fully active receptor of CyPB. Altogether, our findings provide a novel example of a soluble mediator in which a member of the syndecan family plays a critical role in efficient interaction with signaling receptors and initiation of cellular responses.

Attachment of human immunodeficiency virus to cells and its inhibition

The entry of enveloped viruses involves virus adsorption followed by close apposition of the viral and plasma membranes. This multistep process is initiated by specific binding interactions between glycoproteins in the viral envelope and appropriate receptors on the cell surface. In the case of HIV-1, attachment of virions to the cell surface is attributed to a high affinity interaction between envelope spike glycoproteins (Env, composed of the surface protein gp120 and the transmembrane protein gp41) and a complex made of the primary CD4 receptor and a seven-transmembrane co-receptor (e.g., CXCR4 or CCR5) (reviewed in [1]). Then a chain of dynamic events take place that enable the viral nucleocapsid to penetrate within the target cell following the destabilization of membrane microenvironment and the formation of a fusion pore.

HIV infection of mononuclear cells is calcium-dependent

Strategies that prevent initial HIV infection of cells are greatly needed. In this study, we determined the requirement of divalent cations for HIV infection of and attachment to peripheral blood mononuclear cells (PBMC), which contain several types of HIV-infectable cells-CD4(+) T cells, monocytes and dendritic cells. EDTA, added only during PBMC exposure to HIV, reduced infection by an average of 92%. The reduction of infection by EDTA was accompanied by a reduction in HIV binding to PBMC; R5, X4 and dual-tropic HIV binding to PBMC were inhibited by >85%. EGTA similarly reduced HIV binding to PBMC, while addition of Ca(2+) or Mn(2+), but not Mg(2+), fully restored binding. Virus attachment was inhibited in a dose-dependent manner by trypsin treatment of PBMC, indicating protein involvement in HIV binding. In contrast, mannan or soluble ICAM-1 did not inhibit HIV binding to PBMC. These data indicate that a Ca(2+)-dependent cell-surface protein(s) is responsible for the majority of HIV attachment to and infection of PBMC. Further studies of this are likely to reveal novel strategies to prevent infection of PBMC.

Macrophage entry mediated by HIV Envs from brain and lymphoid tissues is determined by the capacity to use low CD4 levels and overall efficiency of fusion

HIV infects macrophages and microglia in the central nervous system (CNS), which express lower levels of CD4 than CD4+ T cells in peripheral blood. To investigate mechanisms of HIV neurotropism, full-length env genes were cloned from autopsy brain and lymphoid tissues from 4 AIDS patients with HIV-associated dementia (HAD). Characterization of 55 functional Env clones demonstrated that Envs with reduced dependence on CD4 for fusion and viral entry are more frequent in brain compared to lymphoid tissue. Envs that mediated efficient entry into macrophages were frequent in brain but were also present in lymphoid tissue. For most Envs, entry into macrophages correlated with overall fusion activity at all levels of CD4 and CCR5. gp160 nucleotide sequences were compartmentalized in brain versus lymphoid tissue within each patient. Proline at position 308 in the V3 loop of gp120 was associated with brain compartmentalization in 3 patients, but mutagenesis studies suggested that P308 alone does not contribute to reduced CD4 dependence or macrophage-tropism. These results suggest that HIV adaptation to replicate in the CNS selects for Envs with reduced CD4 dependence and increased fusion activity. Macrophage-tropic Envs are frequent in brain but are also present in lymphoid tissues of AIDS patients with HAD, and entry into macrophages in the CNS and other tissues is dependent on the ability to use low receptor levels and overall efficiency of fusion.

Syndecans in wound healing, inflammation and vascular biology

Syndecans are heparan sulphate proteoglycans consisting of a type I transmembrane core protein modified by heparan sulphate and sometimes chondroitin sulphate chains. They are major proteoglycans of many organs including the vasculature, along with glypicans and matrix proteoglycans. Heparan sulphate chains have potential to interact with a wide array of ligands, including many growth factors, cytokines, chemokines and extracellular matrix molecules relevant to growth regulation in vascular repair, hypoxia, angiogenesis and immune cell function. This is consistent with the phenotypes of syndecan knock-out mice, which while viable and fertile, show deficits in tissue repair. Furthermore, there are potentially important changes in syndecan distribution and function described in a variety of human vascular diseases. The purpose of this review is to describe syndecan structure and function, consider the role of syndecan core proteins in transmembrane signalling and also their roles as co-receptors with other major classes of cell surface molecules. Current debates include potential redundancy between syndecan family members, the significance of multiple heparan sulphate interactions, regulation of the cytoskeleton and cell behaviour and the switch between promoter and inhibitor of important cell functions, resulting from protease-mediated shedding of syndecan ectodomains.

Cell-free human immunodeficiency virus type 1 transcytosis through primary genital epithelial cells

Although the transport of human immunodeficiency virus type 1 (HIV-1) through the epithelium is critical for HIV-1 colonization, the mechanisms controlling this process remain obscure. In the present study, we investigated the transcellular migration of HIV-1 as a cell-free virus through primary genital epithelial cells (PGECs). The absence of CD4 on PGECs implicates an unusual entry pathway for HIV-1. We found that syndecans are abundantly expressed on PGECs and promote the initial attachment and subsequent entry of HIV-1 through PGECs. Although CXCR4 and CCR5 do not contribute to HIV-1 attachment, they enhance viral entry and transcytosis through PGECs. Importantly, HIV-1 exploits both syndecans and chemokine receptors to ensure successful cell-free transport through the genital epithelium. HIV-1-syndecan interactions rely on specific residues in the V3 of gp120 and specific sulfations within syndecans. We found no obvious correlation between coreceptor usage and the capacity of the virus to transcytose. Since viruses isolated after sexual transmission are mainly R5 viruses, this suggests that the properties conferring virus replication after transmission are distinct from those conferring cell-free virus transcytosis through the genital epithelium. Although we found that cell-free HIV-1 crosses PGECs as infectious particles, the efficiency of transcytosis is extremely poor (less than 0.02% of the initial inoculum). This demonstrates that the genital epithelium serves as a major barrier against HIV-1. Although one cannot exclude the possibility that limited passage of cell-free HIV-1 transcytosis through an intact genital epithelium occurs in vivo, it is likely that the establishment of infection via cell-free HIV-1 transmigration is a rare event.

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 use different receptor complexes to enter T cells

Studies using adherent cell lines have shown that glucose transporter-1 (GLUT-1) can function as a receptor for human T-cell leukemia virus type 1 (HTLV). In primary CD4(+) T cells, heparan sulfate proteoglycans (HSPGs) are required for efficient entry of HTLV-1. Here, the roles of HSPGs and GLUT-1 in HTLV-1 and HTLV-2 Env-mediated binding and entry into primary T cells were studied. Examination of the cell surface of activated primary T cells revealed that CD4(+) T cells, the primary target of HTLV-1, expressed significantly higher levels of HSPGs than CD8(+) T cells. Conversely, CD8(+) T cells, the primary target of HTLV-2, expressed GLUT-1 at dramatically higher levels than CD4(+) T cells. Under these conditions, the HTLV-2 surface glycoprotein (SU) binding and viral entry were markedly higher on CD8(+) T cells while HTLV-1 SU binding and viral entry were higher on CD4(+) T cells. Binding studies with HTLV-1/HTLV-2 SU recombinants showed that preferential binding to CD4(+) T cells expressing high levels of HSPGs mapped to the C-terminal portion of SU. Transfection studies revealed that overexpression of GLUT-1 in CD4(+) T cells increased HTLV-2 entry, while expression of HSPGs on CD8(+) T cells increased entry of HTLV-1. These studies demonstrate that HTLV-1 and HTLV-2 differ in their T-cell entry requirements and suggest that the differences in the in vitro cellular tropism for transformation and in vivo pathobiology of these viruses reflect different interactions between their Env proteins and molecules on CD4(+) and CD8(+) T cells involved in entry.

IFN-gamma-activated monocytes weakly produce HIV-1 but induce the recruitment of HIV-sensitive T cells and enhance the viral production by these recruited T cells

The ability of macrophages to adapt to changing cytokine environments results in the dominance of a particular functional phenotype of macrophages, which would play a significant role in HIV pathogenesis. In comparison with untreated macrophages (M0), we examined the role of macrophages derived from IFN-gamma-activated monocytes (M1) in the HIV spread. We show that M0 and M1 bind with the same efficiency HIV-1 with a predominant role of C-type lectins in the R5-HIV attachment and of the heparan sulfate proteoglycans in the X4-HIV attachment. Despite similar levels of R5- and X4-HIV DNA, M1 replicates and weakly transmits the virus to activated T cells by releasing CXCR4- and CCR5-interacting chemokines. The blockade of dendritic cell-specific ICAM-3-grabbing nonintegrin expressed on M1 by mAb does not interfere with the viral transfer. Uninfected M1 recruits HIV-sensitive T cells efficiently and releases soluble factors, enhancing the viral production by these recruited cells. This study highlights the role of IFN-gamma to induce a population of macrophages that archive HIV-1 within a latent stage and cause the persistence of the virus by favoring the recruitment of T cells or enhancing the viral replication in infected CD4(+) T cells.

R5- and X4-HIV-1 use differentially the endometrial epithelial cells HEC-1A to ensure their own spread: implication for mechanisms of sexual transmission

The mechanism of viral transmission across the mucosal barrier is poorly understood. Using the endometrial epithelium-derived cell line HEC-1A, we found that the cells are capable of sequestering large numbers of HIV-1 particles but are refractory to cell-free viral infection. The removal of heparan sulfate moieties of cell-surface proteoglycans (HSPG) from the apical pole of HEC-1A accounted for at least 60% of both R5- and X4-HIV-1 attachment, showing their important implication in viral attachment. HEC-1A cells also have the capacity to endocytose a weak proportion of the attached virus and pass it along to underlying cells. Fucose, N-acetylglucosamine and mannosylated-residues inhibited the transcytosis of some virus isolates, suggesting that mannose receptors can be implicated on the both R5- and X4-HIV-1 transcytosis. The inhibition of HIV transcytosis by blocking CCR5 mAb suggests the implication of specific interaction between the viral gp120 and sulfated moiety of syndecans during the transcytosis of mostly R5- and X4-HIV-1. At the basolateral pole of HEC-1A, HSPG sequestered X4- and not R5-HIV-1, highlighting the important role of HEC-1A as an X4 virus reservoir. The cell-free virus particles that have transcytosed could infect activated T cells but with a weaker efficiency than virus that had not transcytosed. The specific stimulation of HEC-1A by R5-HIV-1 increased the release of monocytes/chemokines-attracting chemokines (IL-8 and GR0) and proinflammatory cytokines (TNF-beta and IL-1alpha) that enhanced the production of virus by activated T cells. This study suggests that R5 and X4 viruses can differentially use epithelial cells to ensure their own spread.

The role of heparan sulphate in inflammation

The polysaccharide heparan sulphate is ubiquitously expressed as a proteoglycan in extracellular matrices and on cell surfaces. Heparan sulphate has marked sequence diversity that allows it to specifically interact with many proteins. This Review focuses on the multiple roles of heparan sulphate in inflammatory responses and, in particular, on its participation in almost every stage of leukocyte transmigration through the blood-vessel wall. Heparan sulphate is involved in the initial adhesion of leukocytes to the inflamed endothelium, the subsequent chemokine-mediated transmigration through the vessel wall and the establishment of both acute and chronic inflammatory reactions.

HIV accomplices and adversaries in macrophage infection

Cell surface and intracellular proteins in macrophages influence various steps in the life cycle of lentiviruses. Characterization of these restriction and/or cofactors is essential to understanding how macrophages become unwitting HIV hosts and in fact, can coexist with a heavy viral burden. Although many of the cellular pathways co-opted by HIV in macrophages mimic those seen in CD4+ T cells, emerging evidence reveals cellular constituents of the macrophage, which may be uniquely usurped by HIV. For example, in addition to CD4 and CCR5, membrane annexin II facilitates early steps in infection of macrophages, but not in T cells. Blockade of this pathway effectively diminishes macrophage infection. Viral binding engages a macrophage-centric signaling pathway and a transcriptional profile, including genes such as p21, which benefit the virus. Once inside the cell, multiple host cell molecules are engaged to facilitate virus replication and assembly. Although the macrophage is an enabler, it also possesses innate antiviral mechanisms, including apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3) family DNA-editing enzymes to inhibit replication of HIV. Differential expression of these enzymes, which are largely neutralized by HIV to protect its rebirth, is associated with resistance or susceptibility to the virus. Higher levels of the cytidine deaminases endow potential HIV targets with a viral shield, and IFN-alpha, a natural inducer of macrophage APOBEC expression, renders macrophages tougher combatants to HIV infection. These and other manipulatable pathways may give the macrophage a fighting chance in its battle against the virus.

The role of syndecans in disease and wound healing

Syndecans are a family of transmembrane heparan sulfate proteoglycans widely expressed in both developing and adult tissues. Until recently, their role in pathogenesis was largely unexplored. In this review, we discuss the reported involvement of syndecans in human cancers, infectious diseases, obesity, wound healing and angiogenesis. In some cancers, syndecan expression has been shown to regulate tumor cell function (e.g. proliferation, adhesion, and motility) and serve as a prognostic marker for tumor progression and patient survival. The ectodomains and heparan sulfate glycosaminoglycan chains of syndecans can also act as receptors/co-receptors for some bacterial and viral pathogens, mediating infection. In addition, syndecans bind to obesity-related factors and regulate their signaling, in turn modulating food consumption and weight balance. In vivo animal models of tissue injury and in vitro data also implicate syndecans in processes necessary for wound healing, including fibroblast and endothelial proliferation, cell motility, angiogenesis, and extracellular matrix organization. These new insights into the involvement of syndecans in disease and tissue repair coupled with the emergence of syndecan-specific molecular tools may lead to novel therapies for a variety of human diseases.

Inhibition of HIV Env binding to cellular receptors by monoclonal antibody 2G12 as probed by Fc-tagged gp120

During natural HIV infection, an array of host receptors are thought to influence virus attachment and the kinetics of infection. In this study, to probe the interactions of HIV envelope (Env) with various receptors, we assessed the inhibitory properties of various anti-Env monoclonal antibodies (mAbs) in binding assays. To assist in detecting Env in attachment assays, we generated Fc fusions of full-length wild-type gp120 and several variable loop-deleted gp120s. Through investigation of the inhibition of Env binding to cell lines expressing CD4, CCR5, DC-SIGN, syndecans or combinations thereof, we found that the broadly neutralizing mAb, 2G12, directed to a unique carbohydrate epitope of gp120, inhibited Env-CCR5 binding, partially inhibited Env-DC-SIGN binding, but had no effect on Env-syndecan association. Furthermore, 2G12 inhibited Env attachment to primary monocyte-derived dendritic cells, that expressed CD4 and CCR5 primary HIV receptors, as well as DC-SIGN, and suggested that the dual activities of 2G12 could be valuable in vivo for inhibiting initial virus dissemination and propagation.

Cell surface proteoglycan expression during maturation of human monocytes-derived dendritic cells and macrophages

Cell surface proteoglycans play an important part in the functional and metabolic behaviour of leucocytes. We studied the expression of cell surface proteoglycans in human monocytes, in monocyte-derived immature and mature dendritic cells and in macrophages by metabolic labelling with [(35)S]-sulphate, reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting. Immature dendritic cells had the highest metabolic activity for the synthesis of cell surface proteoglycans. The major part of these proteoglycans was in phosphatidylinositol-anchored form and was released after treatment with phospholipase C. A minor part was released by trypsin. Digestion with chondroitinase ABC and mild HNO(2) treatment showed that cell surface proteoglycans had a higher proportion of chondroitin sulphate, both in the phospholipase C and trypsin fractions, suggesting that at least some glypicans contained chondroitin sulphate chains. RT-PCR detected the transcripts of glypicans 1, 3, 4 and 5 and all syndecans. Immature dendritic cells expressed a most complex spectrum of glypicans and syndecans, glypican-1 and syndecan-1 being expressed preferentially by this type of cells. Mature dendritic cells expressed glypican-3, which was not present in other lineages. These results suggest that different mononuclear cells synthesize cell surface proteoglycans actively with characteristic expression of different syndecans and glypicans genes, depending on the degree of cell differentiation and/or maturation.

The glycosaminoglycan-binding domain of decoy receptor 3 is essential for induction of monocyte adhesion

Decoy receptor 3 (DcR3), a soluble receptor for Fas ligand, LIGHT (homologous to lymphotoxins shows inducible expression and competes with HSV glycoprotein D for herpes virus entry mediator, a receptor expressed by T lymphocytes), and TNF-like molecule 1A, is highly expressed in cancer cells and in tissues affected by autoimmune disease. DcR3.Fc has been shown to stimulate cell adhesion and to modulate cell activation and differentiation by triggering multiple signaling cascades that are independent of its three known ligands. In this study we found that DcR3.Fc-induced cell adhesion was inhibited by heparin and heparan sulfate, and that DcR3.Fc was unable to bind Chinese hamster ovary K1 mutants defective in glycosaminoglycan (GAG) synthesis. Furthermore, the negatively charged, sulfated GAGs of cell surface proteoglycans, but not their core proteins, were identified as the binding sites for DcR3.Fc. A potential GAG-binding site was found in the C-terminal region of DcR3, and the mutation of three basic residues, i.e., K256, R258, and R259, to alanines abolished its ability to trigger cell adhesion. Moreover, a fusion protein comprising the GAG-binding region of DcR3 with an Fc fragment (DcR3_HBD.Fc) has the same effect as DcR3.Fc in activating protein kinase C and inducing cell adhesion. Compared with wild-type THP-1 cells, cell adhesion induced by DcR3.Fc was significantly reduced in both CD44v3 and syndecan-2 knockdown THP-1 cells. Therefore, we propose a model in which DcR3.Fc may bind to and cross-link proteoglycans to induce monocyte adhesion.

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.