DC-SIGNR, a DC-SIGN homologue expressed in endothelial cells, binds to human and simian immunodeficiency viruses and activates infection in trans

Characterization of a novel C-type lectin-like gene, LSECtin: demonstration of carbohydrate binding and expression in sinusoidal endothelial cells of liver and lymph node

A new C-type lectin-like gene encodes 293 amino acids and maps to chromosome 19p13.3 adjacent to the previously described C-type lectin genes, CD23, dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), and DC-SIGN-related protein (DC-SIGNR). The four genes form a tight cluster in an insert size of 105 kb and have analogous genomic structures. The new C-type lectin-like molecule, designated liver and lymph node sinusoidal endothelial cell C-type lectin (LSECtin), is a type II integral membrane protein of approximately 40 kDa in size with a single C-type lectin-like domain at the COOH terminus, closest in homology to DC-SIGNR, DC-SIGN, and CD23. LSECtin mRNA was only expressed in liver and lymph node among 15 human tissues tested, intriguingly neither expressed on hematopoietic cell lines nor on monocyte-derived dendritic cells (DCs). Moreover, LSECtin is expressed predominantly by sinusoidal endothelial cells of human liver and lymph node and co-expressed with DC-SIGNR. LSECtin binds to mannose, GlcNAc, and fucose in a Ca(2+)-dependent manner but not to galactose. Our results indicate that LSECtin is a novel member of a family of proteins comprising CD23, DC-SIGN, and DC-SIGNR and might function in vivo as a lectin receptor.

Binding of human immunodeficiency virus type 1 to immature dendritic cells can occur independently of DC-SIGN and mannose binding C-type lectin receptors via a cholesterol-dependent pathway

Interactions of human immunodeficiency virus type 1 (HIV-1) with immature dendritic cells (DC) are believed to be multifactorial and involve binding to the CD4 antigen, DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), mannose binding C-type lectin receptors (MCLR), and heparan sulfate proteoglycans (HSPG). In this study we assessed the relative contributions of these previously defined virus attachment factors to HIV binding and accumulation in DC and the subsequent transfer of the bound virus particle to CD4(+) T cells. Using competitive inhibitors of HIV-1 attachment to DC, we have identified the existence of DC-SIGN-, MCLR-, and HSPG-independent mechanism(s) of HIV attachment and internalization. Furthermore, virus particles bound by DC independently of CD4, DC-SIGN, MCLR, and HSPG are efficiently transmitted to T cells. Treatment of virus particles with the protease subtilisin or treatment of immature DC with trypsin significantly reduced virus binding, thus demonstrating the role of HIV envelope glycoprotein interactions with unidentified DC-surface factor(s). Finally, this DC-mediated virus binding and internalization are dependent on lipid rafts. We propose that pathways to HIV-1 attachment and uptake in DC exhibit functional redundancy; that is, they are made up of multiple independent activities that can, at least in part, compensate for one another.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

HIV-1 entry and its inhibition

Entry of HIV-1 virions into cells is a complex and dynamic process carried out by envelope (Env) glycoproteins on the surface of the virion that promote the thermodynamically unfavorable fusion of highly stable viral and target cell membranes. Insight gained from studies of the mechanism of viral entry allowed insight into the design of novel inhibitors of HIV-1 entry, several of which are now in clinical trials. This review highlights the mechanism by which viral and cellular proteins mediate entry of HIV-1 into permissive cells, with an emphasis on targeting this process in the design of novel therapies that target distinct steps of the entry process, including antagonizing receptor binding events and blocking conformational changes intimately involved in membrane fusion.

DC-SIGN and L-SIGN can act as attachment receptors for alphaviruses and distinguish between mosquito cell- and mammalian cell-derived viruses

C-type lectins such as DC-SIGN and L-SIGN, which bind mannose-enriched carbohydrate modifications of host and pathogen proteins, have been shown to bind glycoproteins of several viruses and facilitate either cis or trans infection. DC-SIGN and L-SIGN are expressed in several early targets of arbovirus infection, including dendritic cells (DCs) and cells of the reticuloendothelial system. In the present study, we show that DC-SIGN and L-SIGN can function as attachment receptors for Sindbis (SB) virus, an arbovirus of the Alphavirus genus. Human monocytic THP-1 cells stably transfected with DC-SIGN or L-SIGN were permissive for SB virus replication, while untransfected controls were essentially nonpermissive. The majority of control THP-1 cells were permissive when attachment and entry steps were eliminated through electroporation of virus transcripts. Infectivity for the DC-SIGN/L-SIGN-expressing cells was largely blocked by yeast mannan, EDTA, or a DC-SIGN/L-SIGN-specific monoclonal antibody. Infection of primary human DCs by SB virus was also dependent upon SIGN expression by similar criteria. Furthermore, production of virus particles in either C6/36 mosquito cells or CHO mammalian cells under conditions that limited complex carbohydrate content greatly increased SB virus binding to and infection of THP-1 cells expressing these lectins. C6/36-derived virus also was much more infectious for primary human DCs than CHO-derived virus. These results suggest that (i) lectin molecules such as DC-SIGN and L-SIGN may represent common attachment receptor molecules for arthropod-borne viruses, (ii) arbovirus particles produced in and delivered by arthropod vectors may preferentially target vertebrate host cells bearing these or similar lectin molecules, and (iii) a cell line has been identified that can productively replicate alphaviruses but is deficient in attachment receptors.

Transfer of neutralizing IgG to macaques 6 h but not 24 h after SHIV infection confers sterilizing protection: implications for HIV-1 vaccine development

Passive transfer of high-titered antiviral neutralizing IgG, known to confer sterilizing immunity in pig-tailed monkeys, has been used to determine how soon after virus exposure neutralizing antibodies (NAbs) must be present to block a simian immunodeficiency virus (SIV)/HIV chimeric virus infection. Sterilizing protection was achieved in three of four macaques receiving neutralizing IgG 6 h after intravenous SIV/HIV chimeric virus inoculation as monitored by PCR analyses of and attempted virus isolations from plasma, peripheral blood mononuclear cell, and lymph node specimens. In the fourth animal, the production of progeny virus was suppressed for >4 weeks. A delay in transferring NAbs until 24 h after virus challenge resulted in infection in two of two monkeys. These results suggest that even if a vaccine capable of eliciting broadly reactive NAbs against primary HIV-1 were at hand, the Abs generated must remain at, or rapidly achieve, high levels within a relatively short period after exposure to virus to prevent the establishment of a primate lentivirus infection.

Isolation and characterization of the human DC-SIGN and DC-SIGNR promoters

DC-SIGN is a C-type lectin expressed on the surface of dendritic cells (DCs) that is used by a number of human pathogens to disseminate infection in the host. In the human genome, there is a gene closely related to DC-SIGN, termed DC-SIGNR (also L-SIGN, DC-SIGN2), which likely arose through gene duplication. DC-SIGN protein and RNA expression is largely restricted to DCs and some specialized macrophages in lung and placenta, while DC-SIGNR expression is largely restricted to lymph nodes and liver sinusoidal endothelial cells. To begin to investigate the cell type-restricted expression of these closely related genes, we isolated the human DC-SIGN and DC-SIGNR promoters. They were found to be relatively weak promoters that express similarly in plasmid transfection assays in several transformed cell lines, suggesting that the cis-regulatory elements that confer cell-type restricted expression of these two genes are located outside of the promoters. The DC-SIGN gene contains four major transcriptional start sites at +1, +271, +364, and +435, with the +364 site being the most abundantly expressed in DCs. The DC-SIGN promoter is contained within nucleotides +251 to +487. AP-1, Sp1, Ets-1, and NF-kappaB binding sites in the DC-SIGN promoter appear to be important for function. Thus, despite its highly restricted pattern of expression, the DC-SIGN promoter has features common to promoters that are active in other cell types.

T cell dynamics in HIV-1 infection

In the absence of antiretroviral treatment, HIV-1 establishes a chronic, progressive infection of the human immune system that invariably, over the course of years, leads to its destruction and fatal immunodeficiency. Paradoxically, while viral replication is extensive throughout the course of infection, deterioration of conventional measures of immunity is slow, including the characteristic loss of CD4(+) T cells that is thought to play a key role in the development of immunodeficiency. This conundrum suggests that CD4(+) T cell-directed viral cytopathicity alone cannot explain the course of disease. Indeed, recent advances now indicate that HIV-1 pathogenesis is likely to result from a complex interplay between the virus and the immune system, particularly the mechanisms responsible for T cell homeostasis and regeneration. We review these data and present a model of HIV-1 pathogenesis in which the protracted loss of CD4(+) T cells results from early viral destruction of selected memory T cell populations, followed by a combination of profound increases in overall memory T cell turnover, damage to the thymus and other lymphoid tissues, and physiological limitations in peripheral CD4(+) T cell renewal.

DC-SIGN: a novel HIV receptor on DCs that mediates HIV-1 transmission

The dendritic cell (DC)-specific HIV-1 receptor DC-SIGN plays a key-role in the dissemination of HIV-1 by DCs. DC-SIGN captures HIV-1 at sites of entry, enabling its transport to lymphoid tissues, where DC-SIGN efficiently transmits low amounts of HIV-1 to T cells. The expression pattern of DC-SIGN in mucosal tissue, lymph nodes, placenta and blood suggests a function for DC-SIGN in both horizontal and vertical transmission of HIV-1. Moreover, the efficiency of DC-SIGN+ blood DC to transmit HIV-1 to T cells supports a role in HIV-1 transmission via blood. To date, DC-SIGN represents a novel class of HIV-1 receptor, because it does not allow viral infection but binds HIV-1 and enhances its infection of T cells in trans. Its unique function is further underscored by its restricted expression on DCs. Although DC-SIGN is a C-type lectin with an affinity for carbohydrates exemplified by its interaction with its immunological ligand ICAM-3, recent evidence demonstrates that glycosylation of gp120 is not necessary for its interaction with DC-SIGN. Moreover, mutational analysis demonstrates that the HIV-1 gp120 binding site in DC-SIGN is different from that of ICAM-3. Besides its role in DC-mediated adhesion processes, DC-SIGN also functions as an antigen receptor that captures and internalises antigens for presentation by DC. Strikingly, HIV-1 circumvents processing after binding DC-SIGN and remains infectious for several days after capture. A better understanding of the action of this novel HIV receptor in initial viral infection and subsequent transmission will provide a basis for the design of drugs that inhibit or alter interactions of DC-SIGN with gp120, interfering with HIV-1 dissemination and that may have a therapeutic value in both immunological diseases and/or HIV-1 infections.

Pathogens target DC-SIGN to influence their fate DC-SIGN functions as a pathogen receptor with broad specificity

Dendritic cells (DC) are vital in the defense against pathogens. To sense pathogens DC express pathogen recognition receptors such as toll-like receptors (TLR) and C-type lectins that recognize different fragments of pathogens, and subsequently activate or present pathogen fragments to T cells. It is now becoming evident that some pathogens subvert DC functions to escape immune surveillance. HIV-1 targets the DC-specific C-type lectin DC-SIGN to hijack DC for viral dissemination. HIV-1 binding to DC-SIGN protects HIV-1 from antigen processing and facilitates its transport to lymphoid tissues, where DC-SIGN promotes HIV-1 infection of T cells. Recent studies demonstrate that DC-SIGN is a more universal pathogen receptor that also recognizes Ebola, cytomegalovirus and mycobacteria. Mycobacterium tuberculosis targets DC-SIGN by a mechanism that is distinct from that of HIV-1, leading to inhibition of the immunostimulatory function of DC and pathogen survival. Thus, a better understanding of DC-SIGN-pathogen interactions and their effects on DC function is necessary to combat infections.

DC-SIGN and L-SIGN are high affinity binding receptors for hepatitis C virus glycoprotein E2

The hepatitis C virus (HCV) genome codes for highly mannosylated envelope proteins, which are naturally retained in the endoplasmic reticulum. We found that the HCV envelope glycoprotein E2 binds the dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) and the related liver endothelial cell lectin L-SIGN through high-mannose N-glycans. Competing ligands such as mannan and an antibody directed against the carbohydrate recognition domains (CRD) abrogated binding. While no E2 interaction with distant monomeric CRDs on biosensor chips could be detected, binding is observed if CRDs are closely seeded (Kd = 48 nm) and if the CRD is part of the oligomeric-soluble extracellular domain of DC-SIGN (Kd = 30 nm). The highest affinity is seen for plasma membrane-expressed DC-SIGN and L-SIGN (Kd = 3 and 6 nm, respectively). These results indicate that several high-mannose N-glycans in a structurally defined cluster on E2 bind to several subunits of the oligomeric lectin CRD. High affinity interaction of viral glycoproteins with oligomeric lectins might represent a strategy by which HCV targets to and concentrates in the liver and infects dendritic cells.

L-SIGN (CD 209L) is a liver-specific capture receptor for hepatitis C virus

Hepatitis C virus (HCV) infects nearly 3% of the population of the world and is a major cause of liver disease. However, the mechanism whereby the virus targets the liver for infection remains unknown, because none of the putative cellular receptors for HCV are both expressed specifically in the liver and capable of binding HCV envelope glycoproteins. Liver/lymph node-specific intercellular adhesion molecule-3-grabbing integrin (L-SIGN) is a calcium-dependent lectin expressed on endothelial cells of liver and lymph nodes. Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), a homologous molecule expressed on dendritic cells, binds HIV and promotes infection. By using a virus-binding assay, we demonstrate that L-SIGN and DC-SIGN specifically bind naturally occurring HCV present in the sera of infected individuals. Further studies demonstrate that binding is mediated by the HCV envelope glycoprotein E2 and is blocked by specific inhibitors, including mannan, calcium chelators, and Abs to the lectin domain of the SIGN molecules. Thus, L-SIGN represents a liver-specific receptor for HCV, and L-SIGN and DC-SIGN may play important roles in HCV infection and immunity.

DC-SIGN (CD209) mediates dengue virus infection of human dendritic cells

Dengue virus is a single-stranded, enveloped RNA virus that productively infects human dendritic cells (DCs) primarily at the immature stage of their differentiation. We now find that all four serotypes of dengue use DC-SIGN (CD209), a C-type lectin, to infect dendritic cells. THP-1 cells become susceptible to dengue infection after transfection of DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN), or its homologue L-SIGN, whereas the infection of dendritic cells is blocked by anti-DC-SIGN antibodies and not by antibodies to other molecules on these cells. Viruses produced by dendritic cells are infectious for DC-SIGN- and L-SIGN-bearing THP-1 cells and other permissive cell lines. Therefore, DC-SIGN may be considered as a new target for designing therapies that block dengue infection.

DC-SIGN (CD209) Mediates Dengue Virus Infection of Human Dendritic Cells

Dengue virus is a single-stranded, enveloped RNA virus that productively infects human dendritic cells (DCs) primarily at the immature stage of their differentiation. We now find that all four serotypes of dengue use DC-SIGN (CD209), a C-type lectin, to infect dendritic cells. THP-1 cells become susceptible to dengue infection after transfection of DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN), or its homologue L-SIGN, whereas the infection of dendritic cells is blocked by anti–DC-SIGN antibodies and not by antibodies to other molecules on these cells. Viruses produced by dendritic cells are infectious for DC-SIGN– and L-SIGN–bearing THP-1 cells and other permissive cell lines. Therefore, DC-SIGN may be considered as a new target for designing therapies that block dengue infection.

Hepatitis C virus glycoproteins interact with DC-SIGN and DC-SIGNR

DC-SIGN and DC-SIGNR are two closely related membrane-associated C-type lectins that bind human immunodeficiency virus (HIV) envelope glycoprotein with high affinity. Binding of HIV to cells expressing DC-SIGN or DC-SIGNR can enhance the efficiency of infection of cells coexpressing the specific HIV receptors. DC-SIGN is expressed on some dendritic cells, while DC-SIGNR is localized to certain endothelial cell populations, including hepatic sinusoidal endothelial cells. We found that soluble versions of the hepatitis C virus (HCV) E2 glycoprotein and retrovirus pseudotypes expressing chimeric forms of both HCV E1 and E2 glycoproteins bound efficiently to DC-SIGN and DC-SIGNR expressed on cell lines and primary human endothelial cells but not to other C-type lectins tested. Soluble E2 bound to immature and mature human monocyte-derived dendritic cells (MDDCs). Binding of E2 to immature MDDCs was dependent on DC-SIGN interactions, while binding to mature MDDCs was partly independent of DC-SIGN, suggesting that other cell surface molecules may mediate HCV glycoprotein interactions. HCV interactions with DC-SIGN and DC-SIGNR may contribute to the establishment or persistence of infection both by the capture and delivery of virus to the liver and by modulating dendritic cell function.

The tandem-repeat polymorphism of the DC-SIGNR gene does not affect the susceptibility to HIV infection and the progression to AIDS

DC-SIGNR is a C-type lectin that functions as a transreceptor for HIV-1. The exon 4 of the DC-SIGNR gene comprises a variable number of 69-bp tandem repeats, encoding for parts of the extracellular protein domain. Here, we analyzed the relevance of this gene polymorphism for the interindividual transmission of HIV-1 and the progression to AIDS. A cross-sectional comparison between HIV-1-infected patients (n = 391) and healthy volunteers (n = 134) did not reveal significant differences with regard to the DC-SIGNR allele distribution. Moreover, DC-SIGNR allele frequencies were similar in slowly progressing HIV patients (n = 31) and patients who rapidly progressed to AIDS (n = 46). Additionally, in a cohort of 149 newly HIV-infected patients, no relationship was found between HIV set point viremia and DC-SIGNR genotypes. Thus, the DC-SIGNR tandem-repeat polymorphism in exon 4 does not have a significant impact on the host's susceptibility to HIV and the clinical progression to AIDS.

HIV-1 entry and entry inhibitors as therapeutic agents

Defining the mechanisms of HIV-1 entry has enabled the rational design of strategies aimed at interfering with the process. This article delineates what is currently understood about HIV-1 entry, as a window through which to understand what will likely be the next major group of antiretroviral therapeutics. These exciting new approaches offer the promise of adding viral entry to reverse transcription and protein processing as steps to block in the viral life cycle. Several principles learned with other antiretroviral drugs are sure to be valid for entry antagonists, whereas other considerations may be unique to this group of agents. There is no agent to which HIV-1 has not been able to acquire resistance and this is likely to remain the case. Multiple rounds of viral replication are required to generate the genetic diversity that forms the basis of resistance. Combination therapy in which replication is maximally suppressed will remain a cornerstone of treatment with entry inhibitors, as with other agents. Furthermore, the coreceptor specificity of some entry and fusion inhibitors argues that combinations will likely be needed to broaden the effective range of susceptible viral variants. Finally, the targeting of multiple steps within the entry process has the potential for synergy. The fusion inhibitor T20 and CXCR4 antagonist AMD3100 are synergistic in vitro at blocking infection of PBMC with clinical isolates [115] and T20 combined with the CD4 inhibitor PRO 542 have synergistic in vitro effects, with more than 10-fold greater inhibition of R5, X4, and R5X4 strains than either agent alone [116]. Entry antagonists raise other, unique issues. As discussed previously, the theoretic concern exists that blocking CCR5 could enhance the emergence of CXCR4-using variants and possibly accelerate disease. So far, in vitro selection for variants resistant to the CCR5 antagonist SCH-C in PBMC (which express both CCR5 and CXCR4) has resulted in mutants that were resistant to the blocker but still used CCR5. Alternatively, because many HIV-1 strains have the capacity to use several other chemokine or orphan receptors for entry, blocking both CCR5 and CXCR could lead to a variant that uses one of these other molecules in place of the principal coreceptors, although data in vitro so far suggest that this is unlikely [13,14]. This new class of antiviral drugs offers great promise but also novel concerns, and careful analysis of viruses that arise with their use in vivo is essential.

Transplacental transmission of HIV: a potential role for HIV binding lectins

Although the majority of vertical transmission of HIV occurs around the time of birth, 1.5-2% of pregnancies in HIV-positive women appear to result in the vertical transmission of HIV across the placenta. HIV infection of a number of placental cell types has been demonstrated, but the exact mechanisms of intrauterine vertical transmission remain obscure. The recent discovery of the HIV binding lectins dendritic cell-specific ICAM-grabbing non-integrin (DC-SIGN) and DC-SIGN-related molecule (DC-SIGNR) provides one possible explanation. Cells expressing these lectins are able to adsorb the virus and mediate high efficiency HIV infection of other cell types. Both lectins are expressed by the placenta, with DC-SIGN expression also being present on maternal cells intimately associated with the placenta. This review focuses on possible mechanisms by which these lectins may potentiate the intrauterine vertical transmission of HIV.

Rapid monoclonal antibody generation via dendritic cell targeting in vivo

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. Previous studies have demonstrated that targeting foreign antigens to DC leads to enhanced antigen (Ag)-specific responses in vivo. However, the utility of this strategy for the generation of MAbs has not been investigated. To address this question we immunized mice with IgG-peptide conjugates prepared with the hamster anti-murine CD11c MAb N418. Synthetic peptides corresponding to two different exposed regions of DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN), a human C-type lectin, were conjugated to N418 using thiol-based chemistry. The N418 MAb served as the targeting molecule and synthetic peptides as the Ag (MAb-Ag). A rapid and peptide specific serum IgG response was produced by Day 7 when the synthetic peptides were linked to the N418 MAb, compared to peptide co-delivered with the N418 without linkage. Spleen cells from N418-peptide immunized mice were fused on Day 10, and three IgG1/k monoclonal antibodies (MAbs) were selected to one of the peptide epitopes (MID-peptide). One of the MAbs, Novik 2, bound to two forms of recombinant DC-SIGN protein in enzyme-linked immunosorbent assay (ELISA), and was specifically inhibited by the MID-peptide in solution. Two of these MAbs show specific binding to DC-SIGN expressed by cultured human primary DC. We conclude that in vivo DC targeting enhances the immunogenicity of synthetic peptides and is an effective method for the rapid generation of MAbs to predetermined epitopes.

DC-SIGN and DC-SIGNR bind ebola glycoproteins and enhance infection of macrophages and endothelial cells

Ebola virus exhibits a broad cellular tropism in vitro. In humans and animal models, virus is found in most tissues and organs during the latter stages of infection. In contrast, a more restricted cell and tissue tropism is exhibited early in infection where macrophages, liver, lymph node, and spleen are major initial targets. This indicates that cellular factors other than the broadly expressed virus receptor(s) modulate Ebola virus tropism. Here we demonstrate that the C-type lectins DC-SIGN and DC-SIGNR avidly bind Ebola glycoproteins and greatly enhance transduction of primary cells by Ebola virus pseudotypes and infection by replication-competent Ebola virus. DC-SIGN and DC-SIGNR are expressed in several early targets for Ebola virus infection, including dendritic cells, alveolar macrophages, and sinusoidal endothelial cells in the liver and lymph node. While DC-SIGN and DC-SIGNR do not directly mediate Ebola virus entry, their pattern of expression in vivo and their ability to efficiently capture virus and to enhance infection indicate that these attachment factors can play an important role in Ebola transmission, tissue tropism, and pathogenesis.

Differential N-linked glycosylation of human immunodeficiency virus and Ebola virus envelope glycoproteins modulates interactions with DC-SIGN and DC-SIGNR

The C-type lectins DC-SIGN and DC-SIGNR [collectively referred to as DC-SIGN(R)] bind and transmit human immunodeficiency virus (HIV) and simian immunodeficiency virus to T cells via the viral envelope glycoprotein (Env). Other viruses containing heavily glycosylated glycoproteins (GPs) fail to interact with DC-SIGN(R), suggesting some degree of specificity in this interaction. We show here that DC-SIGN(R) selectively interact with HIV Env and Ebola virus GPs containing more high-mannose than complex carbohydrate structures. Modulation of N-glycans on Env or GP through production of viruses in different primary cells or in the presence of the mannosidase I inhibitor deoxymannojirimycin dramatically affected DC-SIGN(R) infectivity enhancement. Further, murine leukemia virus, which typically does not interact efficiently with DC-SIGN(R), could do so when produced in the presence of deoxymannojirimycin. We predict that other viruses containing GPs with a large proportion of high-mannose N-glycans will efficiently interact with DC-SIGN(R), whereas those with solely complex N-glycans will not. Thus, the virus-producing cell type is an important factor in dictating both N-glycan status and virus interactions with DC-SIGN(R), which may impact virus tropism and transmissibility in vivo.

Novel member of the CD209 (DC-SIGN) gene family in primates

Two CD209 family genes identified in humans, CD209 (DC-SIGN) and CD209L (DC-SIGNR/L-SIGN), encode C-type lectins that serve as adhesion receptors for ICAM-2 and ICAM-3 and participate in the transmission of human and simian immunodeficiency viruses (HIV and SIV, respectively) to target cells in vitro. Here we characterize the CD209 gene family in nonhuman primates and show that recent evolutionary alterations have occurred in this family across primate species. All of the primate species tested, specifically, Old World monkeys (OWM) and apes, have orthologues of human CD209. In contrast, CD209L is missing in OWM but present in apes. A third family member, that we have named CD209L2, was cloned from rhesus monkey cDNA and subsequently identified in OWM and apes but not in humans. Rhesus CD209L2 mRNA was prominently expressed in the liver and axillary lymph nodes, although preliminary data suggest that levels of expression may vary among individuals. Despite a high level of sequence similarity to both human and rhesus CD209, rhesus CD209L2 was substantially less effective at binding ICAM-3 and poorly transmitted HIV type 1 and SIV to target cells relative to CD209. Our data suggest that the CD209 gene family has undergone recent evolutionary processes involving duplications and deletions, the latter of which may be tolerated because of potentially redundant functional activities of the molecules encoded by these genes.

Syndecan captures, protects, and transmits HIV to T lymphocytes

This study demonstrates that syndecan functions as an in trans HIV receptor. We show that syndecan, when expressed in nonpermissive cells, becomes the major mediator for HIV adsorption. This adsorption is mediated by the binding of gp120 to the heparan sulfate chains of syndecan. Although syndecan does not substitute for HIV entry receptors, it enhances the in trans infectivity of a broad range of primate lentiviruses including primary viruses produced from PBMCs. Furthermore, syndecan preserves virus infectivity for a week, whereas unbound virus loses its infectivity in less than a day. Moreover, we obtain evidence suggesting that the vast syndecan-rich endothelial lining of the vasculature can provide a microenvironment which boosts HIV replication in T cells.

Identification of gp120 binding sites on CXCR4 by using CD4-independent human immunodeficiency virus type 2 Env proteins

Human immunodeficiency virus (HIV) and simian (SIV) immunodeficiency virus entry is mediated by binding of the viral envelope glycoprotein (Env) to CD4 and chemokine receptors, CCR5 and/or CXCR4. CD4 induces extensive conformational changes that expose and/or induce formation of a chemokine receptor binding site on gp120. CD4-independent Env's of HIV type 1 (HIV-1), HIV-2, and SIV have been identified that exhibit exposed chemokine receptor binding sites and can bind directly to CCR5 or CXCR4 in the absence of CD4. While many studies have examined determinants for gp120-CCR5 binding, analysis of gp120-CXCR4 binding has been hindered by the apparently lower affinity of this interaction for X4-tropic HIV-1 isolates. We show here that gp120 proteins from two CD4-independent HIV-2 Env's, VCP and ROD/B, bind directly to CXCR4 with an apparently high affinity. By use of CXCR4 N-terminal deletion constructs, CXCR4-CXCR2 chimeras, and human-rat CXCR4 chimeras, binding determinants were shown to reside in the amino (N) terminus, extracellular loop 2 (ECL2), and ECL3. Alanine-scanning mutagenesis of charged residues, tyrosines, and phenylalanines in extracellular CXCR4 domains implicated multiple amino acids in the N terminus (E14/E15, D20, Y21, and D22), ECL2 (D187, R188, F189, Y190, and D193), and ECL3 (D262, E268, E277, and E282) in binding, although minor differences were noted between VCP and ROD/B. However, mutations in CXCR4 that markedly reduced binding did not necessarily hinder cell-cell fusion by VCP or ROD/B, especially in the presence of CD4. These gp120 proteins will be useful in dissecting determinants for CXCR4 binding and Env triggering and in evaluating pharmacologic inhibitors of the gp120-CXCR4 interaction.

Human atherosclerotic plaques express DC-SIGN, a novel protein found on dendritic cells and macrophages

The association of autoimmune phenomena with atherosclerosis suggests that plaques may contain specialized antigen-presenting cells, dendritic cells (DCs). DC-SIGN is a C-type lectin expressed by DCs. This study assessed whether human atherosclerotic plaques expressed DC-SIGN and several other macrophage/DC markers. Plaques from human coronary and carotid arteries and aorta contained DC-SIGN-immunoreactive cells. Double-labelling showed co-expression of DC-SIGN and macrophage/DC lineage markers CD14, CD68, HLA-DR, and S100. There was no immunoreactivity for the DC activation markers CD83 or CMRF-44. Since DC-SIGN mediates adhesion to T-lymphocytes and endocytosis, its expression in atherosclerotic plaques may have functional implications. Activated DCs migrate quickly from areas of inflammation to regional lymph nodes, possibly explaining the paucity of activated DCs in atherosclerotic plaques. In conclusion, this study has shown that DC-SIGN is expressed in atherosclerosis.

Human cytomegalovirus binding to DC-SIGN is required for dendritic cell infection and target cell trans-infection

Cytomegalovirus (CMV) infection is characterized by host immunosuppression and multiorganic involvement. CMV-infected dendritic cells (DC) were recently shown to display reduced immune functions, but their role in virus dissemination is not clear. In this report, we demonstrated that CMV could be captured by DC through binding on DC-SIGN and subsequently transmitted to permissive cells. Moreover, blocking DC-SIGN by specific antibodies inhibited DC infection by primary CMV isolates and expression of DC-SIGN or its homolog DC-SIGNR rendered susceptible cells permissive to CMV infection. We demonstrated that CMV envelope glycoprotein B is a viral ligand for DC-SIGN and DC-SIGNR. These results provide new insights into the molecular interactions contributing to cell infection by CMV and extend DC-SIGN implication in virus propagation.

Dendritic cell-mediated viral transfer to T cells is required for human immunodeficiency virus type 1 persistence in the face of rapid cell turnover

Human immunodeficiency virus type 1 (HIV-1)-infected and activated CD4(+) T cells have short half-lives in vivo (<2 days). We have established an in vitro culture system in which infected T cells are turned over frequently to provide a model system that examines this important facet of in vivo HIV-1 replication. We observed that virus replication in T cells under rapid-turnover conditions was possible only when immature dendritic cells or DC-SIGN-expressing cells mediated HIV-1 transmission to T cells. Virus replication was initiated more rapidly in T cells infected with the cell-associated form of virus compared to infection by the cell-free route. This accelerated transfer of virus required adhesion molecule-mediated interactions between the virus-presenting cell and T cell, but surprisingly, HIV-1 transfer could occur independently of DC-SIGN (DC-specific intracellular adhesion molecule 3 [ICAM-3]-grabbing nonintegrin)in the dendritic-cell-T-cell cocultures. These results suggest that dendritic cell-mediated transmission of HIV-1 enables virus replication under conditions of rapid cell turnover in vivo.

Marginal zone macrophages express a murine homologue of DC-SIGN that captures blood-borne antigens in vivo

Antigen-presenting cells are localized in essentially every tissue, where they operate at the interface of innate and acquired immunity by capturing pathogens and presenting pathogen-derived peptides to T cells. C-type lectins are important pathogen recognition receptors and the C-type lectin, dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN), is unique in that, in addition to pathogen capture, it regulates adhesion processes such as DC trafficking and T-cell synapse formation. We have isolated a murine homologue of DC-SIGN that is identical to the previously reported murine homologue mSIGNR1. mSIGNR1 is more closely related to the human DC-SIGN homologue L-SIGN than to DC-SIGN itself because mSIGNR1 is specifically expressed by liver sinusoidal endothelial cells, similar to L-SIGN, and not by DCs. Moreover, mSIGNR1 is also expressed by medullary and subcapsular macrophages in lymph nodes and by marginal zone macrophages (MZMs) in the spleen. Strikingly, these MZMs are in direct contact with the bloodstream and efficiently capture specific polysaccharide antigens present on the surface of encapsulated bacteria. We have investigated the in vivo function of mSIGNR1 on MZMs in spleen. We demonstrate here that mSIGNR1 functions in vivo as a pathogen recognition receptor on MZMs that capture blood-borne antigens, which are rapidly internalized and targeted to lysosomes for processing. Moreover, the antigen capture is completely blocked in vivo by the blocking mSIGNR1-specific antibodies. Thus, mSIGNR1, a murine homologue of DC-SIGN, is important in the defense against pathogens and this study will facilitate further investigations into the in vivo function of DC-SIGN and its homologues.

Diversity of receptors binding HIV on dendritic cell subsets

The ability of HIV-1 to use dendritic cells (DCs) for transport and to transfer virus to activated T cells in the lymph node may be crucial in early HIV-1 pathogenesis. We have characterized primary DCs for the receptors involved in viral envelope attachment and observed that C-type lectin receptor (CLR) binding was predominant in skin DCs, whereas binding to emigrating and tonsil DCs was CD4-dependent. No one CLR was solely responsible for envelope binding on all skin DC subsets. DC-SIGN (DC-specific ICAM-3-grabbing nonintegrin) was only expressed by CD14(+)CDla(lo) dermal DCs. The mannose receptor was expressed by CD1a(hi) and CD14(+)CDla(lo) dermal DCs, and langerin was expressed by Langerhans cells. The diversity of CLRs able to bind HIV-1 in skin DCs may reflect their ability to bind a range of microbial glycoproteins.

Addition of a single gp120 glycan confers increased binding to dendritic cell-specific ICAM-3-grabbing nonintegrin and neutralization escape to human immunodeficiency virus type 1

The potential role of dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) binding in human immunodeficiency virus transmission across the mucosal barrier was investigated by assessing the ability of simian-human immunodeficiency chimeric viruses (SHIVs) showing varying degrees of mucosal transmissibility to bind the DC-SIGN expressed on the surface of transfected cells. We found that gp120 of the highly transmissible, pathogenic CCR5-tropic SHIV(SF162P3) bound human and rhesus DC-SIGN with an efficiency threefold or greater than that of gp120 of the nonpathogenic, poorly transmissible parental SHIV(SF162), and this increase in binding to the DC-SIGN of the SHIV(SF162P3) envelope gp120 translated into an enhancement of T-cell infection in trans. The presence of an additional glycan at the N-terminal base of the V2 loop of SHIV(SF162P3) gp120 compared to that of the parental virus was shown to be responsible for the increase in binding to DC-SIGN. Interestingly, this glycan also conferred escape from autologous neutralization, raising the possibility that the modification occurred as a result of immune selection. Our data suggest that more-efficient binding of envelope gp120 to DC-SIGN could be relevant to the enhanced mucosal transmissibility of SHIV(SF162P3) compared to that of parental SHIV(SF162).

Distribution and immunophenotype of DC-SIGN-expressing cells in SIV-infected and uninfected macaques

DC-SIGN (dendritic cell-specific ICAM-3 grabbing nonintegrin), an external C-type lectin expressed on dendritic cells (DCs), has been proposed to play a pivotal role in trafficking HIV/SIV from mucosal surfaces to lymphoid tissues. Although the location of DC-SIGN expression has been established in a limited number of human tissues, its distribution in the rhesus macaque has not yet been determined. This study characterized the distribution and immunophenotype of DC-SIGN-expressing cells in SIV-infected and uninfected macaque tissues by immunohistochemistry (IHC) and confocal microscopy. IHC, using monoclonal and polyclonal antibodies against DC-SIGN, was performed on a variety of tissues. To further define the immunophenotype of DC-SIGN(+) cells, double-labeling with antibodies to CD68, fascin, and HLA-DR was done. In both infected and uninfected macaques, DC-SIGN(+) cells were located within the submucosa and lamina propria of tongue, vagina, rectum, and tonsil; however, no positive cells were present within the epithelium of any tissue. Antibodies to DC-SIGN also labeled Kupffer cells within the liver and scattered perivascular cells in the brain. Within lymph nodes, numerous positive cells were present within sinusoids in addition to cells consistent with interdigitating reticular cells in the paracortex and scattered follicular dendritic cells within germinal centers. In spleen of uninfected macaques, there was a similar distribution of DC-SIGN(+) cells with sinusoidal, marginal zone, and interdigitating dendritic cells staining; however, there was a marked paucity of staining in the spleens of SIV-infected macaques. DC-SIGN(+) cells were consistently CD68(+), but fascin(-) and HLA-DR(-). The absence of intraepithelial DC-SIGN-positive cells in mucosal tissues suggests that DC-SIGN does not play a significant role in transmucosal passage of HIV/SIV.

Differential receptor usage of small ruminant lentiviruses in ovine and caprine cells: host range but not cytopathic phenotype is determined by receptor usage

The ovine maedi-visna (MVV) and caprine arthritis-encephalitis (CAEV) small ruminant lentiviruses (SRLV) exhibit differential species tropism and cytopathic effects in vitro. Icelandic MVV-K1514 is a lytic SRLV which can infect cells from many species in addition to ruminants, whereas a lytic North American MVV strain (85/34) as well as nonlytic MVV strain S93 and CAEV can infect only ruminant cells. In the present study, we determined if differential receptor usage in sheep and goat cells is the basis of differential species tropism or cytopathic phenotype of SRLV. Infection interference assays in sheep and goat synovial membrane cells using pseudotyped CAEV vectors showed that North American MVV strains 85/34 and S93 and CAEV use a common receptor (SRLV receptor A), whereas MVV-K1514 uses a different receptor (SRLV receptor B). In addition, human 293T cells expressing CAEV but not MVV-K1514 envelope glycoproteins fused with a goat cell line persistently infected with MVV-K1514, indicating that MVV-K1514 does not use SRLV receptor A for cell-to-cell fusion. Therefore, our results indicate that the differential species tropism of SRLV is determined by receptor usage. However, receptor usage is unrelated to cytopathic phenotype.

Quantitative expression and virus transmission analysis of DC-SIGN on monocyte-derived dendritic cells

The C-type lectins DC-SIGN and DC-SIGNR efficiently bind human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) strains and can transmit bound virus to adjacent CD4-positive cells. DC-SIGN also binds efficiently to the Ebola virus glycoprotein, enhancing Ebola virus infection. DC-SIGN is thought to be responsible for the ability of dendritic cells (DCs) to capture HIV and transmit it to T cells, thus promoting HIV dissemination in vitro and perhaps in vivo as well. To investigate DC-SIGN function and expression levels on DCs, we characterized a panel of monoclonal antibodies (MAbs) directed against the carbohydrate recognition domain of DC-SIGN. Using quantitative fluorescence-activated cell sorter technology, we found that DC-SIGN is highly expressed on immature monocyte-derived DCs, with at least 100,000 copies and often in excess of 250,000 copies per DC. There was modest variation (three- to fourfold) in DC-SIGN expression levels between individuals and between DCs isolated from the same individual at different times. Several MAbs efficiently blocked virus binding to cell lines expressing human or rhesus DC-SIGN, preventing HIV and SIV transmission. Interactions with Ebola virus pseudotypes were also blocked efficiently. Despite their ability to block virus-DC-SIGN interactions on cell lines, these antibodies only inhibited transmission of virus from DCs by approximately 50% or less. These results indicate that factors other than DC-SIGN may play important roles in the ability of DCs to capture and transmit HIV.

Essential roles for dendritic cells in the pathogenesis and potential treatment of HIV disease

During sexual transmission of HIV, virus crosses mucosal epithelium and eventually reaches lymphoid tissue where it establishes a permanent infection. Evidence has accumulated that infection of Langerhans cells, which are resident dendritic cells in pluristratified epithelia, plays a crucial role in the early events of HIV transmission. HIV infection of Langerhans cells is regulated by surface expression of CD4 and CCR5. Thus, topical microbicides that interfere with HIV infection of Langerhans cells represent an attractive strategy for blocking sexual transmission of virus. Capture and uptake of HIV virions is another major pathway by which HIV interacts with dendritic cells. By contrast, this process is mediated by a newly described C-type lectin, DC-SIGN. It is well established that HIV-exposed dendritic cells transmit virus efficiently to cocultured T cells. Indeed, dendritic cell-T cell interaction, critical in the generation of immune responses, is a rich microenvironment for HIV replication both in vitro and in vivo. Dendritic cells that have captured virus via DC-SIGN, and not HIV-infected dendritic cells, probably facilitate most infection of T cells in chronically infected individuals. Therefore, blocking DC-SIGN-mediated capture of HIV represents a potential therapeutic antiviral strategy for HIV disease. Lastly, dendritic cells have been targeted both ex vivo and in vivo to initiate and enhance HIV-specific immunity. Although these approaches are promising for both therapeutic and prophylactic vaccines, much additional work is needed in order to optimize dendritic-cell-based immunization strategies.

Cell surface receptors, virus entry and tropism of primate lentiviruses

Human immunodeficiency virus (HIV) exploits cell surface receptors to attach to and gain entry into cells. The HIV envelope spike glycoprotein on the surface of virus particles binds both CD4 and a seven-transmembrane coreceptor. These interactions trigger conformational changes in the envelope spike that induce fusion of viral and cellular membranes and entry of the viral core into the cell cytoplasm. Other cell surface receptors also interact with gp120 and aid attachment of virus particles. This review describes these receptors, their roles in HIV entry and their influence on cell tropism.

The role of DC-SIGN and DC-SIGNR in HIV and Ebola virus infection: can potential therapeutics block virus transmission and dissemination?

Sexual transmission of HIV requires that the virus crosses mucosal barriers and disseminates into lymphoid tissue, the major site of viral replication. To achieve this, HIV might engage DC-SIGN, a calcium dependent lectin that is expressed on mucosal dendritic cells (DCs), which binds avidly to HIV. DC-SIGN and other attachment factors are likely to account for the well-known ability of DCs to enhance infection of T cells by HIV. Attachment of HIV to DC-SIGN might thus enhance viral spread in mucosal tissues and, by taking advantage of the inherent capacity of DCs to migrate into lymphoid tissue, might promote viral dissemination within the host. DC-SIGN and a related molecule, termed DC-SIGNR, also enhance infection by Ebola virus. The expression of these lectins on early targets of Ebola virus infection, like liver endothelial cells and alveolar macrophages, suggests an important role for DC-SIGN and DC-SIGNR in the establishment of Ebola infection. This article reviews the interaction of DC-SIGN and DC-SIGNR with HIV and Ebola, discusses the mechanism of DC-SIGN-mediated viral transmission and examines how this process could be inhibited by potential therapeutics.

Molecular characterization of the murine SIGNR1 gene encoding a C-type lectin homologous to human DC-SIGN and DC-SIGNR

The C-type lectin human dendritic cell (DC)-specific intercellular adhesion molecule (ICAM)-3-grabbing non-integrin (DC-SIGN) plays important roles in pattern recognition by dendritic cells in the immune system. In addition to binding human immunodeficiency virus (HIV), this type II membrane protein binds with high affinity to the adhesion molecules ICAM-3 and -2 to promote important dendritic cell interactions with naive T cells and endothelial cells, respectively. DC-SIGNR, a human DC-SIGN homologue expressed on sinusoidal endothelial cells in liver and lymph node, also binds and transmits HIV virus. We describe the cloning and characterization of a family of murine complementary DNAs (cDNAs) called SIGNR1, expressed in skin and spleen, that encode C-type lectins highly related to human DC-SIGN and DC-SIGNR. We also report the genomic structure of the SIGNR1 gene and compare it to that of human DC-SIGN and DC-SIGNR. The different transcripts (alpha, beta, gamma, delta) are generated by differences in 5' untranslated sequences, alternative splicing and/or the use of different polyadenylation sites. The predicted open reading frames encoded by the cDNAs are most closely related to human DC-SIGN and DC-SIGNR in the cytoplasmic domain, the transmembrane region and the carbohydrate recognition domain. Moreover, the alternatively spliced transcripts encode proteins that lack the transmembrane region or have modified carbohydrate recognition domains. Northern hybridization experiments with several different SIGNR1 cDNA probes reveal transcripts of 1.3 and 2.1 kb that are expressed in a tissue-restricted fashion in murine skin, spleen and lung. In situ hybridization and immunocytochemistry experiments demonstrate that, like human DC-SIGN, the murine messenger RNAs are expressed in subsets of dendritic cells in the spleen and skin.

Expression of human immunodeficiency virus (HIV)-binding lectin DC-SIGNR: Consequences for HIV infection and immunity

DC-SIGNR is a human immunodeficiency virus (HIV)-binding C-type lectin that is expressed on endothelium in the hepatic sinusoids, lymph node sinuses and placenta. Like closely related DC-SIGN, DC-SIGNR can bind both ICAM-3 and HIV and can potentiate HIV infection of T lymphocytes in trans. In the present study we have investigated reasons underlying the restricted distribution of DC-SIGNR and have examined DC-SIGNR expression in relation to HIV entry receptors. We show that DC-SIGNR expression does not depend on endothelial cell specialization or on activation state. DC-SIGNR-positive endothelium continues to express DC-SIGNR in conditions of hyperplasia, whereas the molecule is lost after neoplastic transformation, most likely as a result of changes in the microenvironment of the endothelial cells. We have further shown that CCR5, but not CD4, is coexpressed with DC-SIGNR on hepatic sinusoidal and placental capillary endothelial cells. However, CD4-positive CCR5-positive cells, such as hepatic Kupffer cells, placental Hofbauer cells, and CD4-positive T lymphocytes in lymph nodes, can be found adjacent to DC-SIGNR-positive endothelium. Therefore, DC-SIGNR may be able to mediate HIV infection of these cells in trans. Finally, we demonstrate that DC-SIGN and DC-SIGNR can be coexpressed on lymph node sinus endothelial cells, which may lead to modulation of the function of both molecules.

Matrix fibronectin increases HIV stability and infectivity

HIV particles are detected extracellularly in lymphoid tissues, a major reservoir of the virus. We previously reported that a polymerized form of fibronectin (FN), superfibronectin (sFN), as well as a fragment of FN, III1-C, enhanced infection of primary CD4(+) T cells by HIV-1IIIB. We now show that sFN enhances infection of primary CD4(+) T cells by both R5 and X4 strains of HIV-1. Using HIV pseudotyped with different envelope glycoproteins (gp120) and HOS cells transfected with various chemokine receptors alone or in combination with the CD4 molecule, we show that sFN-mediated enhancement requires the CD4 receptor and does not alter the specificity of gp120 for different chemokine receptors. Because the III1-C fragment also resulted in enhancement, we asked whether proteolysis of FN generated fragments capable of enhancing HIV infection. We found that progressive proteolysis of FN by chymotrypsin correlates with an enhancement of HIV infection in both primary CD4(+) T cells and the IG5 reporter cell line. Furthermore, incubation of HIV with sFN significantly prolonged infectivity at 37 degrees C compared with dimeric FN or BSA. In conclusion, these results indicate that polymerized (matrix) or degraded (inflammation-associated), but not dimeric (plasma), FN are capable of enhancing infection by HIV-1, independent of the coreceptor specificity of the strains. Moreover, virions bound to matrix FN maintain infectivity for longer periods of time than do virions in suspension. This study suggests that matrix proteins and their conformational status may play a role in the pathogenesis of HIV.

Functional evaluation of DC-SIGN monoclonal antibodies reveals DC-SIGN interactions with ICAM-3 do not promote human immunodeficiency virus type 1 transmission

DC-SIGN, a type II membrane-spanning C-type lectin that is expressed on the surface of dendritic cells (DC), captures and promotes human and simian immunodeficiency virus (HIV and SIV) infection of CD4(+) T cells in trans. To better understand the mechanism of DC-SIGN-mediated virus transmission, we generated and functionally evaluated a panel of seven monoclonal antibodies (MAbs) against DC-SIGN family molecules. Six of the MAbs reacted with myeloid-lineage DC, whereas one MAb preferentially bound DC-SIGNR/L-SIGN, a homolog of DC-SIGN. Characterization of hematopoietic cells also revealed that stimulation of monocytes with interleukin-4 (IL-4) or IL-13 was sufficient to induce expression of DC-SIGN. All DC-SIGN-reactive MAbs competed with intercellular adhesion molecule 3 (ICAM-3) for adhesion to DC-SIGN and blocked HIV-1 transmission to T cells that was mediated by THP-1 cells expressing DC-SIGN. Similar but less efficient MAb blocking of DC-mediated HIV-1 transmission was observed, indicating that HIV-1 transmission to target cells via DC may not be dependent solely on DC-SIGN. Attempts to neutralize DC-SIGN capture and transmission of HIV-1 with soluble ICAM-3 prophylaxis were limited in success, with a maximal inhibition of 60%. In addition, disrupting DC-SIGN/ICAM-3 interactions between cells with MAbs did not impair DC-SIGN-mediated HIV-1 transmission. Finally, forced expression of ICAM-3 on target cells did not increase their susceptibility to HIV-1 transmission mediated by DC-SIGN. While these findings do not discount the role of intercellular contact in facilitating HIV-1 transmission, our in vitro data indicate that DC-SIGN interactions with ICAM-3 do not promote DC-SIGN-mediated virus transmission.

Identification of different binding sites in the dendritic cell-specific receptor DC-SIGN for intercellular adhesion molecule 3 and HIV-1

The novel dendritic cell (DC)-specific human immunodeficiency virus type 1 (HIV-1) receptor DC-SIGN plays a key role in the dissemination of HIV-1 by DC. DC-SIGN is thought to capture HIV-1 at mucosal sites of entry, facilitating transport to lymphoid tissues, where DC-SIGN efficiently transmits HIV-1 to T cells. DC-SIGN is also important in the initiation of immune responses by regulating DC-T cell interactions through intercellular adhesion molecule 3 (ICAM-3). We have characterized the mechanism of ligand binding by DC-SIGN and identified the crucial amino acids involved in this process. Strikingly, the HIV-1 gp120 binding site in DC-SIGN is different from that of ICAM-3, consistent with the observation that glycosylation of gp120, in contrast to ICAM-3, is not crucial to the interaction with DC-SIGN. A specific mutation in DC-SIGN abrogated ICAM-3 binding, whereas the HIV-1 gp120 interaction was unaffected. This DC-SIGN mutant captured HIV-1 and infected T cells in trans as efficiently as wild-type DC-SIGN, demonstrating that ICAM-3 binding is not necessary for HIV-1 transmission. This study provides a basis for the design of drugs that inhibit or alter interactions of DC-SIGN with gp120 but not with ICAM-3 or vice versa and that have a therapeutic value in immunological diseases and/or HIV-1 infections.

Natural selection on the erythrocyte surface

Surface glycoproteins are principal receptors used by pathogens to invade target cells. It has been suggested that mammalian erythrocyte surface glycoproteins function as decoy receptors attracting pathogens to the anucleated erythrocyte and away from their target tissues. Glycophorin A (GYPA) is solely expressed on the erythrocyte surface where it is the most abundant sialoglycoprotein, although its function is unknown. The pathogen decoy hypothesis may be relevant here, as GYPA has been shown in vitro to bind numerous viruses and bacteria, which do not infect erythrocytes. However, it is also a receptor for erythrocyte invasion by the malarial parasite Plasmodium falciparum. Analyses of gypa sequence variation among six higher primates and within a human population show that there is a large excess of replacement (nonsynonymous) substitutions along each primate lineage (particularly on exons 2-4 encoding the extracellular glycosylated domain of GYPA) and a significant excess of polymorphisms in exon 2 (encoding the terminal portion of the extracellular domain) within humans. These two signatures suggest that there has been exceptionally strong positive selection on this receptor driving GYPA divergence during primate evolution and balancing selection maintaining allelic variation within human populations. The pathogen decoy hypothesis alone is adequate to explain both these signatures of between-species and within-species diversifying selection. This has implications for understanding the functions of erythrocyte surface components and their roles in health and disease.

Rhesus macaque dendritic cells efficiently transmit primate lentiviruses independently of DC-SIGN

Here, we describe the isolation and characterization of the rhesus macaque homolog for human DC-SIGN, a dendritic cell-specific C-type lectin. mac-DC-SIGN is 92% identical to hu-DC-SIGN. mac-DC-SIGN preserves the virus transmission function of hu-DC-SIGN, capturing and efficiently transducing simian and human immunodeficiency virus to target CD4(+) T cells. Surprisingly, however, mac-DC-SIGN plays no discernable role in the ability of rhesus macaque dendritic cells to capture and transmit primate lentiviruses. Expression and neutralization analyses suggest that this process is DC-SIGN independent in macaque, although the participation of other lectin molecules cannot be ruled out. The ability of primate lentiviruses to effectively use human and rhesus dendritic cells in virus transmission without the cells becoming directly infected suggests that these viruses have taken advantage of a conserved dendritic cell mechanism in which DC-SIGN family molecules are significant contributors but not the only participants.

Expression of DC-SIGN by dendritic cells of intestinal and genital mucosae in humans and rhesus macaques

To better understand the role of dendritic cells (DCs) in human immunodeficiency virus (HIV) transmission at mucosal surfaces, we examined the expressions of the HIV adhesion molecule, dendritic-cell-specific ICAM-3 grabbing nonintegrin (DC-SIGN), its closely related homologue DC-SIGNR, and HIV coreceptors by distinct DC populations in the intestinal and genital tracts of humans and rhesus macaques. We also developed monoclonal antibodies (MAbs) specific for DC-SIGN or DC-SIGNR. In the Peyer's patches, DC-SIGN expression was detected in the interfollicular regions and in clusters of cells in the subepithelial dome regions. DC-SIGN expression was not found on plasmacytoid DCs. DC-SIGNR expression was restricted to endothelial cells in approximately one-third of the capillaries in the terminal ileum. In the vaginal epithelium, Langerhans' cells did not express DC-SIGN, whereas subepithelial DCs in the lamina propria expressed moderate levels of DC-SIGN. Finally, the rectum contained cells that expressed high levels of DC-SIGN throughout the entire thickness of the mucosa, while solitary lymphoid nodules within the rectum showed very little staining for DC-SIGN. Triple-color analysis of rectal tissue indicated that CCR5(+) CD4(+) DC-SIGN(+) DCs were localized just beneath the luminal epithelium. These findings suggest that DC-SIGN(+) DCs could play a role in the transmission of primate lentiviruses in the ileum and the rectum whereas accessibility to DC-SIGN(+) cells is limited in an intact vaginal mucosa. Finally, we identified a MAb that blocked simian immunodeficiency virus interactions with rhesus macaque DC-SIGN. This and other specific MAbs may be used to assess the relevance of DC-SIGN in virus transmission in vivo.

C-type lectin receptors on dendritic cells and Langerhans cells

Dendritic cells and Langerhans cells are specialized for the recognition of pathogens and have a pivotal role in the control of immunity. As guardians of the immune system, they are present in essentially every organ and tissue, where they operate at the interface of innate and acquired immunity. Recently, several C-type lectin and lectin-like receptors have been characterized that are expressed abundantly on the surface of these professional antigen-presenting cells. It is now becoming clear that lectin receptors not only serve as antigen receptors but also regulate the migration of dendritic cells and their interaction with lymphocytes.

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

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

Structural basis for selective recognition of oligosaccharides by DC-SIGN and DC-SIGNR

Dendritic cell specific intracellular adhesion molecule-3 (ICAM-3) grabbing nonintegrin (DC-SIGN), a C-type lectin present on the surface of dendritic cells, mediates the initial interaction of dendritic cells with T cells by binding to ICAM-3. DC-SIGN and DC-SIGNR, a related receptor found on the endothelium of liver sinusoids, placental capillaries, and lymph nodes, bind to oligosaccharides that are present on the envelope of human immunodeficiency virus (HIV), an interaction that strongly promotes viral infection of T cells. Crystal structures of carbohydrate-recognition domains of DC-SIGN and of DC-SIGNR bound to oligosaccharide, in combination with binding studies, reveal that these receptors selectively recognize endogenous high-mannose oligosaccharides and may represent a new avenue for developing HIV prophylactics.

cis Expression of DC-SIGN allows for more efficient entry of human and simian immunodeficiency viruses via CD4 and a coreceptor

DC-SIGN is a C-type lectin expressed on dendritic cells and restricted macrophage populations in vivo that binds gp120 and acts in trans to enable efficient infection of T cells by human immunodeficiency virus type 1 (HIV-1). We report here that DC-SIGN, when expressed in cis with CD4 and coreceptors, allowed more efficient infection by both HIV and simian immunodeficiency virus (SIV) strains, although the extent varied from 2- to 40-fold, depending on the virus strain. Expression of DC-SIGN on target cells did not alleviate the requirement for CD4 or coreceptor for viral entry. Stable expression of DC-SIGN on multiple lymphoid lines enabled more efficient entry and replication of R5X4 and X4 viruses. Thus, 10- and 100-fold less 89.6 (R5/X4) and NL4-3 (X4), respectively, were required to achieve productive replication in DC-SIGN-transduced Jurkat cells when compared to the parental cell line. In addition, DC-SIGN expression on T-cell lines that express very low levels of CCR5 enabled entry and replication of R5 viruses in a CCR5-dependent manner, a property not exhibited by the parental cell lines. Therefore, DC-SIGN expression can boost virus infection in cis and can expand viral tropism without affecting coreceptor preference. In addition, coexpression of DC-SIGN enabled some viruses to use alternate coreceptors like STRL33 to infect cells, whereas in its absence, infection was not observed. Immunohistochemical and confocal microscopy data indicated that DC-SIGN was coexpressed and colocalized with CD4 and CCR5 on alveolar macrophages, underscoring the physiological significance of these cis enhancement effects.