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

Lewis X component in human milk binds DC-SIGN and inhibits HIV-1 transfer to CD4+ T lymphocytes

DC-specific ICAM3-grabbing non-integrin (DC-SIGN), which is expressed on DCs, can interact with a variety of pathogens such as HIV-1, hepatitis C, Ebola, cytomegalovirus, Dengue virus, Mycobacterium, Leishmania, and Candida albicans. We demonstrate that human milk can inhibit the DC-SIGN-mediated transfer of HIV-1 to CD4+ T lymphocytes as well as viral transfer by both immature and mature DCs. The inhibitory factor directly interacted with DC-SIGN and prevented the HIV-1 gp120 envelope protein from binding to the receptor. The human milk proteins lactoferrin, alpha-lactalbumin, lysozyme, beta-casein, and secretory leukocyte protease inhibitor did not bind DC-SIGN or demonstrate inhibition of viral transfer. The inhibitory effect could be fully alleviated with an Ab recognizing the Lewis X (LeX) sugar epitope, commonly found in human milk. LeX in polymeric form or conjugated to protein could mimic the inhibitory activity, whereas free LeX sugar epitopes could not. We reveal that a LeX motif present in human milk can bind to DC-SIGN and thereby prevent the capture and subsequent transfer of HIV-1 to CD4+ T lymphocytes. The presence of such a DC-SIGN-binding molecule in human milk may both influence antigenic presentation and interfere with pathogen transfer in breastfed infants.

Interactions of DC-SIGN with Mac-1 and CEACAM1 regulate contact between dendritic cells and neutrophils

Early during infection neutrophils are the most important immune cells that are involved in killing of pathogenic bacteria and regulation of innate immune responses at the site of infection. It has become clear that neutrophils also modulate adaptive immunity through interactions with dendritic cells (DCs) that are pivotal in the induction of T cell responses. Upon activation, neutrophils release TNF-alpha and induce maturation of DCs that enables these antigen-presenting cells to stimulate T cell proliferation and to induce T helper 1 polarization. DC maturation by neutrophils also requires cellular interactions that are mediated by binding of the DC-specific receptor DC-SIGN to Mac-1 on the neutrophil. Here, we demonstrate that also CEACAM1 is an important ligand for DC-SIGN on neutrophils. Binding of DC-SIGN to both CEACAM1 and Mac-1 is required to establish cellular interactions with neutrophils. DC-SIGN is a C-type lectin that has specificity for Lewis(x), and we show that DC-SIGN mediates binding to CEACAM1 through Lewis(x) moieties that are specifically expressed on CEACAM1 derived from neutrophils. This indicates that glycosylation-driven binding of both Mac-1 and CEACAM1 to DC-SIGN is essential for interactions of neutrophils with DCs and enables neutrophils to modulate T cell responses through interactions with DCs.

DC-SIGN mediates binding of dendritic cells to authentic pseudo-LewisY glycolipids of Schistosoma mansoni cercariae, the first parasite-specific ligand of DC-SIGN

During schistosomiasis, parasite-derived glycoconjugates play a key role in manipulation of the host immune response, associated with persistence of the parasite. Among the candidate host receptors that are triggered by glycoconjugates are C-type lectins (CLRs) on dendritic cells (DCs), which in concerted action with Toll-like receptors determine the balance in DCs between induction of immunity versus tolerance. Here we report that the CLR DC-SIGN mediates adhesion of DCs to authentic glycolipids derived from Schistosoma mansoni cercariae and their excretory/secretory products. Structural characterization of the glycolipids, in combination with solid phase and cellular binding studies revealed that DC-SIGN binds to the carbohydrate moieties of both glycosphingolipid species with Galbeta1-4(Fucalpha1-3)GlcNAc (Lewis(X)) and Fucalpha1-3Galbeta1-4(Fucalpha1-3)GlcNAc (pseudo-Lewis(Y)) determinants. Importantly, these data indicate that surveying DCs in the skin may encounter schistosome-derived glycolipids immediately after infection. Recent analysis of crystals of the carbohydrate binding domain of DC-SIGN bound to Lewis(X) provided insight into the ability of DC-SIGN to bind fucosylated ligands. Using molecular modeling we showed that the observed binding of the schistosome-specific pseudo-Lewis(Y) to DC-SIGN is not directly compatible with the model described. To fit pseudo-Lewis(Y) into the model, the orientation of the side chain of Phe(313) in the secondary binding site of DC-SIGN was slightly changed, which results in a perfect stacking of Phe(313) with the hydrophobic side of the galactose-linked fucose of pseudo-Lewis(Y). We propose that pathogens such as S. mansoni may use the observed flexibility in the secondary binding site of DC-SIGN to target DCs, which may contribute to immune escape.

Dendritic cells recognize tumor-specific glycosylation of carcinoembryonic antigen on colorectal cancer cells through dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin

Dendritic cells play a pivotal role in the induction of antitumor immune responses. Immature dendritic cells are located intratumorally within colorectal cancer and intimately interact with tumor cells, whereas mature dendritic cells are present peripheral to the tumor. The majority of colorectal cancers overexpress carcinoembryonic antigen (CEA), and malignant transformation changes the glycosylation of CEA on colon epithelial cells, resulting in higher levels of Lewis(x) and de novo expression of Lewis(y) on tumor-associated CEA. Dendritic cells express the C-type lectin dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) that has high affinity for nonsialylated Lewis antigens, so we hypothesized that DC-SIGN is involved in recognition of colorectal cancer cells by dendritic cells. We show that immature dendritic cells within colorectal cancer express DC-SIGN and that immature dendritic cells but not mature dendritic cells interact with tumor cells. DC-SIGN mediates these interactions through binding of Lewis(x) and Lewis(y) carbohydrates on CEA of colorectal cancer cells. In contrast, DC-SIGN does not bind CEA expressed on normal colon epithelium that contains low levels of Lewis antigens. This indicates that dendritic cells may recognize colorectal cancer cells through binding of DC-SIGN to tumor-specific glycosylation on CEA. Similar to pathogens that target DC-SIGN to escape immunosurveillance, tumor cells may interact with DC-SIGN to suppress dendritic cell functions.

Most DC-SIGNR transcripts at mucosal HIV transmission sites are alternatively spliced isoforms

The repeat region of DC-SIGNR (CD209L) is polymorphic on the genomic level, and, in a separate study, we observed a correlation between the DC-SIGNR genotype and HIV-1 susceptibility during sexual contact. However, previous investigations using immunohistochemistry failed to detect membrane-bound DC-SIGNR on cells in the genital and rectal mucosa. We therefore explored the presence of DC-SIGNR in these compartments with a more sensitive limiting dilution RT-PCR, which also allowed for quantification of alternatively spliced mRNA isoforms. DC-SIGN (CD209) and DC-SIGNR mRNA transcript isoforms were found in all 12 vaginal and two rectal biopsies obtained from 14 healthy individuals. For DC-SIGNR, we detected significantly more isoform than full-length transcripts (mean copy numbers/mug RNA: 602 vs 26; P=0.0009). Four mucosal samples lacked full-length DC-SIGNR transcripts entirely. Cloning and sequencing of DC-SIGNR mRNA in three additional individuals revealed a diverse repertoire of DC-SIGNR isoforms, many of which encoded for proteins predicted to be soluble and secreted. Indeed, in one vaginal sample, we detected only soluble isoforms. In conjunction with our prior observation that the DC-SIGNR genotype has an effect on HIV-1 transmission in vivo, these findings emphasize that DC-SIGNR, in addition to DC-SIGN, should be considered as a cofactor in sexual HIV-1 transmission. Soluble isoforms, in particular, may modulate the efficiency of viral transmission and dissemination.

Neutrophils mediate immune modulation of dendritic cells through glycosylation-dependent interactions between Mac-1 and DC-SIGN

Neutrophils are key players of the innate immune system that provide a first line of defense against invading pathogens. However, it is unknown whether neutrophils can interact with dendritic cells (DCs) to modulate adaptive immune responses. We demonstrate that neutrophils strongly cluster with immature DCs and that activated, not resting, neutrophils induce maturation of DCs that enables these DCs to trigger strong T cell proliferation and T helper type 1 polarization of T cells. This neutrophil–DC interaction is driven by the binding of the DC-specific, C-type lectin DC-SIGN to the β₂-integrin Mac-1. Strikingly, DC-SIGN only interacts with Mac-1 from neutrophils, but not from other leukocytes, mainly because of specific Lewis^x carbohydrates that are present on the α_M chain of Mac-1 from neutrophils. Furthermore, we show that besides the formation of cellular contact, the tumor necrosis factor-α produced by activated neutrophils is essential for inducing DC maturation. Our data demonstrate that DC-SIGN and Mac-1 define a molecular pathway to establish cellular adhesion between DCs and neutrophils, thereby providing a novel cellular link between innate and adaptive immunity.

HIV-1 induces IL-10 production in human monocytes via a CD4-independent pathway

In HIV-infected patients, increased levels of IL-10, mainly produced by virally infected monocytes, were reported to be associated with impaired cell-mediated immune responses. In this study, we investigated how HIV-1 induces IL-10 production in human monocytes. We found that CD14(+) monocytes infected by either HIV-1(213) (X4) or HIV-1(BaL) (R5) produced IL-10, IL-6, tumor necrosis factor-alpha (TNF-alpha), and to a lesser extent, IFN-gamma. However, the capacity of HIV-1 to induce these cytokines was not dependent on virus replication since UV-inactivated HIV-1 induced similar levels of these cytokines. In addition, soluble HIV-1 gp160 could induce CD14(+) monocytes to produce IL-10 but at lower levels. Cross-linking CD4 molecules (XLCD4) with anti-CD4 mAbs and goat anti-mouse IgG (GAM) resulted in high levels of IL-6, TNF-alpha and IFN-gamma but no IL-10 production by CD14(+) monocytes. Interestingly, neither anti-CD4 mAbs nor recombinant soluble CD4 (sCD4) receptor could block IL-10 secretion induced by HIV-1(213), HIV-1(BaL) or HIV-1 gp160 in CD14(+) monocytes, whereas anti-CD4 mAb or sCD4 almost completely blocked the secretion of the other cytokines. Furthermore, HIV-1(213) could induce IL-10 mRNA expression in CD14(+) monocytes while XLCD4 by anti-CD4 mAb and GAM failed to do so. As with IL-10 protein levels, HIV-1(213)-induced IL-10 mRNA expression in CD14(+) monocytes could not be inhibited by anti-CD4 mAb or sCD4. Taken together, HIV-1 binding to CD14(+) monocytes can induce CD4-independent IL-10 production at both mRNA and protein levels. This finding suggests that HIV induces the immunosuppressive IL-10 production in monocytes and is not dependent on CD4 molecules and that interference with HIV entry through CD4 molecules may have no impact on counteracting the effects of IL-10 during HIV infection.

Characterization of DC-SIGN/R interaction with human immunodeficiency virus type 1 gp120 and ICAM molecules favors the receptor's role as an antigen-capturing rather than an adhesion receptor

The dendritic cell (DC)-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin binding receptor (DC-SIGN) was shown to bind human immunodeficiency virus type 1 (HIV-1) viral envelope protein gp120 and proposed to function as a Trojan horse to enhance trans-virus infection to host T cells. To better understand the mechanism by which DC-SIGN and DC-SIGNR selectively bind HIV-1 gp120, we constructed a series of deletion mutations in the repeat regions of both receptors. Different truncated receptors exist in different oligomeric forms. The carbohydrate binding domain without any repeats was monomeric, whereas the full extracellular receptors existed as tetramers. All reconstituted receptors retained their ability to bind gp120. The dissociation constant, however, differed drastically from micromolar values for the monomeric receptors to nanomolar values for the tetrameric receptors, suggesting that the repeat region of these receptors contributes to the avidity of gp120 binding. Such oligomerization may provide a mechanism for the receptor to selectively recognize pathogens containing multiple high-mannose-concentration carbohydrates. In contrast, the receptors bound to ICAMs with submicromolar affinities that are similar to those of two nonspecific cell surface glycoproteins, FcgammaRIIb and FcgammaRIII, and the oligomerization of DC-SIGNR resulted in no increase in binding affinity to ICAM-3. These findings suggest that DC-SIGN may not discriminate other cell surface glycoproteins from ICAM-3 binding. The pH dependence in DC-SIGN binding to gp120 showed that the receptor retained high-affinity gp120 binding at neutral pH but lost gp120 binding at pH 5, suggesting a release mechanism of HIV in the acidic endosomal compartment by DC-SIGN. Our work contradicts the function of DC-SIGN as a Trojan horse to facilitate HIV-1 infection; rather, it supports the function of DC-SIGN/R (a designation referring to both DC-SIGN and DC-SIGNR) as an antigen-capturing receptor.

Identification of the mycobacterial carbohydrate structure that binds the C-type lectins DC-SIGN, L-SIGN and SIGNR1

Mycobacterium tuberculosis represents a worldwide health risk and although macrophages are primarily infected, dendritic cells (DC) are important in inducing cellular immune responses against M. tuberculosis. Recent studies have demonstrated that M. tuberculosis targets the DC-specific C-type lectin DC-SIGN to inhibit the immuno-stimulatory function of DC through the interaction of the mycobacterial mannosylated lipoarabinomannan (ManLAM) to DC-SIGN, which prevents DC maturation and induces the immuno-suppressive cytokine IL-10. This may contribute to survival and persistence of M. tuberculosis. Here, we have identified the specific pathogen-derived carbohydrate structure on ManLAM that is recognized by DC-SIGN. We have synthesized the mannose-cap oligosaccharides man-ara, (man)2-ara and (man)3-ara, and demonstrate that these neoglycoconjugates are specifically bound by DC-SIGN. Moreover, we demonstrate that the human and murine DC-SIGN homologue L-SIGN and SIGNR1, respectively, also interact with mycobacteria through ManLAM. Both homologues have the highest affinity for the (man)3-ara structure, similar to DC-SIGN. This study provides information about the specific carbohydrate structures on pathogens that are recognized by DC-SIGN, and may provide strategies to develop vaccines against these pathogens. Moreover, the identification of SIGNR1 as a receptor for ManLAM will enable in vivo studies to investigate the role of DC-SIGN in M. tuberculosis pathogenesis.

Lactoferrin prevents dendritic cell-mediated human immunodeficiency virus type 1 transmission by blocking the DC-SIGN--gp120 interaction

One of the cell types first encountered by human immunodeficiency virus type 1 (HIV-1) following sexual transmission are dendritic cells (DC). DC capture HIV-1 through C-type lectin receptors, of which the best studied example is DC-SIGN, which mediates HIV-1 internalization. DC can keep the virus infectious for several days and are able to transmit HIV-1 to CD4(+) T cells. We tested proteins from milk and serum for their ability to block DC-mediated HIV-1 transmission, of which bovine lactoferrin (bLF) is the most potent inhibitor. bLF binds strongly to DC-SIGN, thus preventing virus capture and subsequent transmission. Interestingly, bLF is a much more efficient inhibitor of transmission than human lactoferrin. Since bLF is nontoxic and easy to purify in large quantities, it is an interesting candidate microbicide against HIV-1. Another advantage of bLF is its ability to block HIV-1 replication in T cells. DC-mediated capture of a bLF-resistant HIV-1 variant that was selected during long-term culturing in T cells could still be blocked by bLF. This underscores the usefulness of bLF as a microbicide drug to prevent HIV-1 transmission.

Complement-opsonized HIV: the free rider on its way to infection

The complement system (C) is one of the main humoral components of innate immunity. Three major tasks of C against invading pathogens are: (i) lysis of pathogens by the formation of the membrane attack complex (MAC); (ii) opsonization of pathogens with complement fragments to favor phagocytosis; and (iii) attraction of inflammatory cells by chemotaxis. Like other particles, HIV activates C and becomes opsonized. To escape complement-mediated lysis, HIV has adopted various properties, which include the acquisition of HIV-associated molecules (HAMs) belonging to the family of complement regulators, such as CD46, CD55, CD59, and the interaction with humoral regulatory factors like factor H (fH). Opsonized virus may bind to complement receptor positive cells to infect them more efficiently or to remain bound on the surface of such cells. In the latter case HIV can be transmitted to cells susceptible for infection. This review discusses several aspects of C-HIV interactions and provides a model for the dynamics of this process.

Helicobacter pylori modulates the T helper cell 1/T helper cell 2 balance through phase-variable interaction between lipopolysaccharide and DC-SIGN

The human gastric pathogen Helicobacter pylori spontaneously switches lipopolysaccharide (LPS) Lewis (Le) antigens on and off (phase-variable expression), but the biological significance of this is unclear. Here, we report that Le⁺H. pylori variants are able to bind to the C-type lectin DC-SIGN and present on gastric dendritic cells (DCs), and demonstrate that this interaction blocks T helper cell (Th)1 development. In contrast, Le⁻ variants escape binding to DCs and induce a strong Th1 cell response. In addition, in gastric biopsies challenged ex vivo with Le⁺ variants that bind DC-SIGN, interleukin 6 production is decreased, indicative of increased immune suppression. Our data indicate a role for LPS phase variation and Le antigen expression by H. pylori in suppressing immune responses through DC-SIGN.

Potency of HIV-1 envelope glycoprotein gp120 antibodies to inhibit the interaction of DC-SIGN with HIV-1 gp120

The interaction of DC-SIGN with gp120 provides an attractive target for intervention of HIV-1 transmission. Here, we have investigated the potency of gp120 antibodies to inhibit the DC-SIGN-gp120 interaction. We demonstrate that although the V3 loop is not essential for DC-SIGN binding, antibodies against the V3 loop partially inhibit DC-SIGN binding, suggesting that these antibodies sterically hinder DC-SIGN binding to gp120. Polyclonal antibodies raised against non-glycosylated gp120 inhibited both low and high avidity DC-SIGN-gp120 interactions in contrast to polyclonal antibodies raised against glycosylated gp120. Thus, glycans present on gp120 may prevent the generation of antibodies that block the DC-SIGN-gp120 interactions. Moreover, the polyclonal antibodies against non-glycosylated gp120 efficiently inhibited HIV-1 capture by both DC-SIGN transfectants and immature dendritic cells. Therefore, non-glycosylated gp120 may be an attractive immunogen to elicit gp120 antibodies that block the binding to DC-SIGN. Furthermore, we demonstrate that DC-SIGN binding to gp120 enhanced CD4 binding, suggesting that DC-SIGN induces conformational changes in gp120, which may provide new targets for neutralizing antibodies.

Hepatitis C virus targets DC-SIGN and L-SIGN to escape lysosomal degradation

Hepatitis C virus (HCV) is a major health problem. However, the mechanism of hepatocyte infection is largely unknown. We demonstrate that the dendritic cell (DC)-specific C-type lectin DC-SIGN and its liver-expressed homologue L-SIGN/DC-SIGNR are important receptors for HCV envelope glycoproteins E1 and E2. Mutagenesis analyses demonstrated that both HCV E1 and E2 bind the same binding site on DC-SIGN as the pathogens human immunodeficiency virus type 1 (HIV-1) and mycobacteria, which is distinct from the cellular ligand ICAM-3. HCV virus-like particles are efficiently captured and internalized by DCs through binding of DC-SIGN. Antibodies against DC-SIGN specifically block HCV capture by both immature and mature DCs, demonstrating that DC-SIGN is the major receptor on DCs. Interestingly, internalized HCV virus-like particles were targeted to nonlysosomal compartments within immature DCs, where they are protected from lysosomal degradation in a manner similar to that demonstrated for HIV-1. Lewis X antigen, another ligand of DC-SIGN, was internalized to lysosomes, demonstrating that the internalization pathway of DC-SIGN-captured ligands may depend on the structure of the ligand. Our results suggest that HCV may target DC-SIGN to "hide" within DCs and facilitate viral dissemination. L-SIGN, expressed by THP-1 cells, internalized HCV particles into similar nonlysosomal compartments, suggesting that L-SIGN on liver sinusoidal endothelial cells may capture HCV from blood and transmit it to hepatocytes, the primary target for HCV. We therefore conclude that both DCs and liver sinusoidal endothelial cells may act as reservoirs for HCV and that the C-type lectins DC-SIGN and L-SIGN, as important HCV receptors, may represent a molecular target for clinical intervention in HCV infection.

Domains of macaque DC-SIGN essential for capture and transfer of simian immunodeficiency virus

The C-type lectin DC-SIGN mediates the capture and transfer of simian immunodeficiency virus (SIV) from macaque dendritic cells (DCs) to permissive T-cells. To further identify the determinants in macaque DC-SIGN required for capture and transfer of virus, we created mutants containing deletions or point mutations in the extracellular domains, and tested their ability to capture and transmit SIV. We found that SIV bound to the carbohydrate recognition domain (CRD) of macaque DC-SIGN via the envelope protein. In addition, deleting the C-terminal half of the CRD, or mutating amino acids within this region that contact Ca(2+) or mannose, disrupted virion capture activity. However, an N-terminal CRD deletion mutant was capable of binding SIV, indicating that this region was not necessary for binding. Finally, deletion of the neck domain also reduced the capacity for macaque DC-SIGN to capture SIV. Interestingly, ICAM-3, the cellular ligand for DC-SIGN, did not bind to any of the DC-SIGN mutants, including mutants with amino acid changes in the N-terminal region of the CRD. These data suggest that the binding sites for SIV and ICAM-3 may be distinct but overlapping. Together, the data demonstrate the importance of both the neck and the CRD of macaque DC-SIGN for efficient capture of SIV and binding to ICAM-3.

Engagement of ICAM-3 provides a costimulatory signal for human immunodeficiency virus type 1 replication in both activated and quiescent CD4+ T lymphocytes: implications for virus pathogenesis

Human immunodeficiency virus type 1 (HIV-1) replication is regulated by several extracellular signals. We demonstrate that intercellular adhesion molecule 3 (ICAM-3) acts as a costimulating molecule to increase HIV-1 transcription and viral production, a process allowing productive infection of quiescent CD4+ T lymphocytes. The present work suggests an important role for ICAM-3 in HIV-1 replication.

Trans-dominant cellular inhibition of DC-SIGN-mediated HIV-1 transmission

Background

Dendritic cell (DC) transmission of human immunodeficiency virus (HIV) to CD4+ T cells occurs across a point of cell-cell contact referred to as the infectious synapse. The relationship between the infectious synapse and the classically defined immunological synapse is not currently understood. We have recently demonstrated that human B cells expressing exogenous DC-SIGN, DC-specific intercellular adhesion molecule-3 (ICAM-3)-grabbing nonintegrin, efficiently transmit captured HIV type 1 (HIV-1) to CD4+ T cells. K562, another human cell line of hematopoietic origin that has been extensively used in functional analyses of DC-SIGN and related molecules, lacks the principal molecules involved in the formation of immunological synaptic junctions, namely major histocompatibility complex (MHC) class II molecules and leukocyte function-associated antigen-1 (LFA-1). We thus examined whether K562 erythroleukemic cells could recapitulate efficient DC-SIGN-mediated HIV-1 transmission (DMHT).

Results

Here we demonstrate that DMHT requires cell-cell contact. Despite similar expression of functional DC-SIGN, K562/DC-SIGN cells were inefficient in the transmission of HIV-1 to CD4+ T cells when compared with Raji/DC-SIGN cells. Expression of MHC class II molecules or LFA-1 on K562/DC-SIGN cells was insufficient to rescue HIV-1 transmission efficiency. Strikingly, we observed that co-culture of K562 cells with Raji/DC-SIGN cells impaired DMHT to CD4+ T cells. The K562 cell inhibition of transmission was not directly exerted on the CD4+ T cell targets and required contact between K562 and Raji/DC-SIGN cells.

Conclusions

DMHT is cell type dependent and requires cell-cell contact. We also find that the cellular milieu can negatively regulate DC-SIGN transmission of HIV-1 in trans.

Molecular basis of the differences in binding properties of the highly related C-type lectins DC-SIGN and L-SIGN to Lewis X trisaccharide and Schistosoma mansoni egg antigens

The dendritic cell-specific C-type lectin DC-SIGN functions as a pathogen receptor that recognizes Schistosoma mansoni egg antigens through its major glycan epitope Galbeta1,4(Fucalpha1,3)GlcNAc (Lex). Here we report that L-SIGN, a highly related homologue of DC-SIGN found on liver sinusoidal endothelial cells, binds to S. mansoni egg antigens but not to the Lex epitope. L-SIGN does bind the Lewis antigens Lea, Leb, and Ley, similar as DC-SIGN. A specific mutation in the carbohydrate recognition domain of DC-SIGN (V351G) abrogates binding to all Lewis antigens. In L-SIGN Ser363 is present at the corresponding position of Val351 in DC-SIGN. Replacement of this Ser into Val resulted in a "gain of function" L-SIGN mutant that binds to Lex, and shows increased binding to the other Lewis antigens. These data indicate that Val351 is important for the fucose specificity of DC-SIGN. Molecular modeling and docking of the different Lewis antigens in the carbohydrate recognition domains of L-SIGN, DC-SIGN, and their mutant forms, demonstrate that Val351 in DC-SIGN creates a hydrophobic pocket that strongly interacts with the Fucalpha1,3/4-GlcNAc moiety of the Lewis antigens. The equivalent amino acid residue Ser363 in L-SIGN creates a hydrophilic pocket that prevents interaction with Fucalpha1,3-GlcNAc in Lex but supports interactions with the Fucalpha1,4-GlcNAc moiety in Lea and Leb antigens. These data demonstrate for the first time that DC-SIGN and L-SIGN differ in their carbohydrate binding profiles and will contribute to our understanding of the functional roles of these C-type lectin receptors, both in recognition of pathogen and self-glycan antigens.

Early steps of retrovirus replicative cycle

During the last two decades, the profusion of HIV research due to the urge to identify new therapeutic targets has led to a wealth of information on the retroviral replication cycle. However, while the late stages of the retrovirus life cycle, consisting of virus replication and egress, have been partly unraveled, the early steps remain largely enigmatic. These early steps consist of a long and perilous journey from the cell surface to the nucleus where the proviral DNA integrates into the host genome. Retroviral particles must bind specifically to their target cells, cross the plasma membrane, reverse-transcribe their RNA genome, while uncoating the cores, find their way to the nuclear membrane and penetrate into the nucleus to finally dock and integrate into the cellular genome. Along this journey, retroviruses hijack the cellular machinery, while at the same time counteracting cellular defenses. Elucidating these mechanisms and identifying which cellular factors are exploited by the retroviruses and which hinder their life cycle, will certainly lead to the discovery of new ways to inhibit viral replication and to improve retroviral vectors for gene transfer. Finally, as proven by many examples in the past, progresses in retrovirology will undoubtedly also provide some priceless insights into cell biology.

Self- and nonself-recognition by C-type lectins on dendritic cells

Dendritic cells (DCs) are highly efficient antigen-presenting cells (APCs) that collect antigen in body tissues and transport them to draining lymph nodes. Antigenic peptides are loaded onto major histocompatibility complex (MHC) molecules for presentation to naive T cells, resulting in the induction of cellular and humoral immune responses. DCs take up antigen through phagocytosis, pinocytosis, and endocytosis via different groups of receptor families, such as Fc receptors for antigen-antibody complexes, C-type lectin receptors (CLRs) for glycoproteins, and pattern recognition receptors, such as Toll-like receptors (TLRs), for microbial antigens. Uptake of antigen by CLRs leads to presentation of antigens on MHC class I and II molecules. DCs are well equipped to distinguish between self- and nonself-antigens by the variable expression of cell-surface receptors such as CLRs and TLRs. In the steady state, DCs are not immunologically quiescent but use their antigen-handling capacities to maintain peripheral tolerance. DCs are continuously sampling and presenting self- and harmless environmental proteins to silence immune activation. Uptake of self-components in the intestine and airways are good examples of sites where continuous presentation of self- and foreign antigens occurs without immune activation. In contrast, efficient antigen-specific immune activation occurs upon encounter of DCs with nonself-pathogens. Recognition of pathogens by DCs triggers specific receptors such as TLRs that result in DC maturation and subsequently immune activation. Here we discuss the concept that cross talk between TLRs and CLRs, differentially expressed by subsets of DCs, accounts for the different pathways to peripheral tolerance, such as deletion and suppression, and immune activation.

DC-SIGN binds to HIV-1 glycoprotein 120 in a distinct but overlapping fashion compared with ICAM-2 and ICAM-3

DC-SIGN is a C-type lectin that binds to endogenous adhesion molecules ICAM-2 and ICAM-3 as well as the viral envelope glycoprotein human immunodeficiency virus, type 1, glycoprotein (gp) 120. We wished to determine whether DC-SIGN binds differently to its endogenous ligands ICAM-2 and ICAM-3 versus HIV-1 gp120. We found that recombinant soluble DC-SIGN bound to gp120-Fc more than 100- and 50-fold better than ICAM-2-Fc and ICAM-3-Fc, respectively. This relative difference was maintained using DC-SIGN expressed on three different CD4-negative cell lines. Although the cell surface affinity for gp120 varied by up to 4-fold on the cell lines examined, the affinity for gp120 was not a correlate of the ability of the cell line to transfer virus. Monosaccharides with equatorial 4-OH groups competed as well as D-mannose for gp120 binding to DC-SIGN, regardless of how the other hydroxyl groups were positioned. Disaccharide competitors and glycan chip analysis showed that DC-SIGN has a preference for oligosaccharides linked in an alpha-anomeric configuration. Alanine-scanning mutagenesis of DC-SIGN revealed that highly conserved residues that coordinate calcium (Asp-366) and/or are involved in both calcium and specific carbohydrate interactions (Glu-347, Asn-349, Glu-354, and Asp-355) significantly compromised binding to all three ligands. Mutating non-conserved residues (Asn-311, Arg-345, Val-351, Gly-352, Glu-353, Ser-360, Gly-361, and Asn-362) minimally affected binding except for the Asp-367 mutant, which enhanced gp120 binding but diminished ICAM-2 and ICAM-3 binding. Conversely, mutating the moderately conserved residue (Gly-346) abrogated gp120 binding but enhanced ICAM-2 and ICAM-3 binding. Thus, DC-SIGN appears to bind in a distinct but overlapping manner to gp120 when compared with ICAM-2 and ICAM-3.

DC-SIGN from African green monkeys is expressed in lymph nodes and mediates infection in trans of simian immunodeficiency virus SIVagm

African green monkeys (AGMs) infected by simian immunodeficiency virus (SIV) SIVagm are resistant to AIDS. SIVagm-infected AGMs exhibit levels of viremia similar to those described during pathogenic human immunodeficiency virus type 1 (HIV-1) and SIVmac infections in humans and macaques, respectively, but contain lower viral loads in their lymph nodes. We addressed the potential role of dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN; CD209) in viral dissemination. In previous studies, it has been shown that human DC-SIGN and macaque DC-SIGN allow transmission of HIV and SIVmac to T cells. Here, we looked at the ability of DC-SIGN derived from AGM lymph nodes to interact with SIVagm. We show that DC-SIGN-expressing cells are present mainly in the medulla and often within the cortex and/or paracortex of AGM lymph nodes. We describe the isolation and characterization of at least three isoforms of dc-sign mRNA in lymph nodes of AGMs. The predicted amino acid sequence from the predominant mRNA isoform, DC-SIGNagm1, is 92 and 99% identical to the corresponding human and rhesus macaque DC-SIGN amino acid sequences, respectively. DC-SIGNagm1 is characterized by the lack of the fourth motif in the repeat domain. This deletion was also detected in the dc-sign gene derived from thirteen animals belonging to five other African monkey species and from four macaques (Macaca fascicularis and M. mulatta). Despite three- to seven-amino-acid modifications compared to DC-SIGNmac, DC-SIGNagm1 allows transmission of SIVagm to T cells. Furthermore, AGM monocyte-derived dendritic cells (MDDC) expressed at least 100,000 DC-SIGN molecules and were able to transmit SIVagm to T cells. At a low multiplicity of infection (10(-5) 50% tissue culture infective doses/cell), viral transmission by AGM MDDC was mainly DC-SIGN dependent. The present study reveals that DC-SIGN from a natural host species of SIV has the ability to act as an efficient attachment and transmission factor for SIVagm and suggests the absence of a direct link between this ability and viral load levels in lymph nodes.

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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.

Dual function of C-type lectin-like receptors in the immune system

Carbohydrate-binding C-type lectin and lectin-like receptors play an important role in the immune system. The large family can be subdivided into subtypes according to their structural similarities and functional differences. The selectins are of major importance in mediating cell adhesion and migration, and the mannose receptor subfamily is specialised in the binding and uptake of pathogens. Recent advances show that some of the type II C-type lectin-like receptors, such as DC-SIGN, can function both as an adhesion receptor and as a phagocytic pathogen-recognition receptor, similar to the Toll-like receptors. Although major differences in the cytoplasmic domains of these receptors might predict their function, recent findings show that differences in glycosylation of ligands can dramatically alter C-type lectin-like receptor usage.

DC-SIGN: escape mechanism for pathogens

Dendritic cells (DCs) are crucial in the defence against pathogens. Invading pathogens are recognized by Toll-like receptors (TLRs) and receptors such as C-type lectins expressed on the surface of DCs. However, it is becoming evident that some pathogens, including viruses, such as HIV-1, and non-viral pathogens, such as Mycobacterium tuberculosis, subvert DC functions to escape immune surveillance by targeting the C-type lectin DC-SIGN (DC-specific intercellular adhesion molecule-grabbing nonintegrin). Notably, these pathogens misuse DC-SIGN by distinct mechanisms that either circumvent antigen processing or alter TLR-mediated signalling, skewing T-cell responses. This implies that adaptation of pathogens to target DC-SIGN might support pathogen survival.

Dendritic cells: interfaces with immunobiology and medicine. A report from the Keystone Symposia Meeting held in Keystone, 3-8 March 2003

Dendritic cells (DCs) are the most potent antigen-presenting cells in vitro and in vivo. They play a key role in the initiation of the immune response and are considered promising targets for immunotherapy. The recent DC Keystone Symposia, organized by Ralph M Steinman, Anne O'Garra and Jacques Banchereau, was held on 3-8 March 2003 in Keystone (CO, USA). This multidisciplinary meeting developed various areas related to the DC biology including: (i) DC and the control of immunity vs tolerance; (ii) DC maturation and manipulation for immunotherapy in vivo; and (iii) antigen capture by DC in situ and interactions with microbial pathogens. The aim of this report is to present some of the highlights developed during the meeting and debated among the DC community.

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.

Inhibition of human immunodeficiency virus type 1 Env-mediated fusion by DC-SIGN

DC-SIGN, a lectin expressed on dendritic cell and macrophage subsets, binds to human immunodeficiency virus Env glycoproteins, allowing capture of viral particles. Captured virions either infect target cells or are efficiently transmitted to lymphocytes. Cellular mechanisms underlying the effects of DC-SIGN remain poorly understood. Here we have analyzed the effects of DC-SIGN on viral entry and on syncytium formation induced by Env glycoproteins. The lectin enhanced susceptibility to viral infection and dramatically increased virion internalization. Captured virions accumulated in the vesicular pathway, and their access to the cytosol was altered. Strikingly, the presence of DC-SIGN on target cells inhibited their ability to form syncytia with Env-expressing cells. However, increasing CD4 surface levels on target cells alleviated this inhibitory effect of DC-SIGN. Moreover, the potency of the viral fusion inhibitor T-20 was not affected in DC-SIGN-expressing cells. Altogether, our results indicate that DC-SIGN exerts subtle and complex effects during early steps of HIV type 1 replication. DC-SIGN facilitates capture and accumulation of viral particles in a vesicular compartment and inhibits viral fusion. Competition between CD4 and DC-SIGN for Env binding likely affects virus access to the cytosol and syncytium formation.

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.

A fatal attraction: Mycobacterium tuberculosis and HIV-1 target DC-SIGN to escape immune surveillance

Dendritic cells (DCs) are vital in the defense against pathogens. However, it is becoming increasingly clear that some pathogens subvert DC functions to escape immune surveillance. For example, HIV-1 targets the DC-specific C-type lectin DC-SIGN (DC-specific intercellular-adhesion-molecule-3-grabbing nonintegrin) to hijack DCs for viral dissemination. 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 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, hence, promotion of pathogen survival. A better understanding of DC-SIGN-pathogen interactions and their effects on DC function should help to combat infections.

Mycobacteria target DC-SIGN to suppress dendritic cell function

Mycobacterium tuberculosis represents a world-wide health risk and immunosuppression is a particular problem in M. tuberculosis infections. Although macrophages are primarily infected, dendritic cells (DCs) are important in inducing cellular immune responses against M. tuberculosis. We hypothesized that DCs represent a target for M. tuberculosis and that the observed immuno-suppression results from modulation of DC functions. We demonstrate that the DC-specific C-type lectin DC-SIGN is an important receptor on DCs that captures and internalizes intact Mycobacterium bovis bacillus Calmette-Guérin (BCG) through the mycobacterial cell wall component ManLAM. Antibodies against DC-SIGN block M. bovis BCG infection of DCs. ManLAM is also secreted by M. tuberculosis-infected macrophages and has been implicated as a virulence factor. Strikingly, ManLAM binding to DC-SIGN prevents mycobacteria- or LPS-induced DC maturation. Both mycobacteria and LPS induce DC maturation through Toll-like receptor (TLR) signaling, suggesting that DC-SIGN, upon binding of ManLAM, interferes with TLR-mediated signals. Blocking antibodies against DC-SIGN reverse the ManLAM-mediated immunosuppressive effects. Our results suggest that M. tuberculosis targets DC-SIGN both to infect DCs and to down-regulate DC-mediated immune responses. Moreover, we demonstrate that DC-SIGN has a broader pathogen recognition profile than previously shown, suggesting that DC-SIGN may represent a molecular target for clinical intervention in infections other than HIV-1.

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.

Human immunodeficiency virus envelope (gp120) binding to DC-SIGN and primary dendritic cells is carbohydrate dependent but does not involve 2G12 or cyanovirin binding sites: implications for structural analyses of gp120-DC-SIGN binding

The calcium-dependent lectin, DC-SIGN, binds to human immunodeficiency virus (HIV) (and simian immunodeficiency virus) gp120 and mediates the binding and transfer of HIV from monocyte-derived dendritic cells (MDDCs) to permissive T cells. However, it has been recently reported that DC-SIGN binding to HIV gp120 may be carbohydrate independent. Here, we formally demonstrate that gp120 binding to DC-SIGN and MDDCs is largely if not wholly carbohydrate dependent. Endo-beta-N-glucosaminidase H (EndoH) treatment of gp120-Fc under conditions that maintained wild-type CD4 binding-and the full complement of complex glycans-significantly decreased (>90%) binding to DC-SIGN expressing cell lines, as well as to MDDCs. Any residual binding of EndoH-treated gp120-Fc to DC-SIGN was completely competed off with mannan. Mutational analysis indicated that no single glycosylation site affected the ability of gp120-Fc to bind DC-SIGN. To further guide our efforts in mapping the DC-SIGN binding sites on gp120, we used two well-characterized HIV inhibitory agents (2G12 monoclonal antibody and cyanovirin) that bind to high-mannose sugars on gp120. We showed that 2G12 and DC-SIGN bound to nonoverlapping sites in gp120 because (i) 2G12 did not block soluble gp120 or virion binding to DC-SIGN, (ii) 2G12 bound to gp120-Fc that was prebound to cell surface DC-SIGN, and (iii) gp120-Fc mutants that lack glycosylation sites involved in 2G12's epitope were also fully capable of binding DC-SIGN. These data were substantiated by the inability of cyanovirin to block gp120-Fc binding to DC-SIGN. Cyanovirin has been shown to effectively compete for 2G12 binding to gp120. Indeed, high concentrations of cyanovirin dramatically enhanced gp120-Fc binding to cell surfaces in the presence or absence of DC-SIGN. We provide evidence that this enhancement may be due to cyanovirin's ability to bridge gp120 to mannosylated cell surface proteins. These results have implications for antiviral therapeutics and for ongoing efforts to finely map the glycan structures on gp120 responsible for DC-SIGN binding.

Natural type 1 interferon producing cells in HIV infection

Natural type 1 interferon producing cells (IPCs) are in the first line of defense against infectious pathogens. Besides the known properties of type 1 interferons in inhibiting human immunodeficiency virus (HIV) replication, the recent characterization of human IPCs and the possibility to purify them for in vitro studies has greatly accelerated the study of their role in HIV infection. The blood IPC numbers and function are decreased in HIV primary infection and in advanced stages of HIV infection. Loss of circulating IPCs correlates with a high HIV viral load and the occurrence of opportunistic infections. Moreover, HIV can directly infect IPCs in vitro, providing a potential explanation for their in vivo depletion. Thus, the balance between IPCs and HIV replication might be critical in determining the control or progression of HIV infection.

Capture and transfer of simian immunodeficiency virus by macaque dendritic cells is enhanced by DC-SIGN

Dendritic cells (DCs) are among the first cells encountered by human and simian immunodeficiency virus (HIV and SIV) following mucosal infection. Because these cells efficiently capture and transmit virus to T cells, they may play a major role in mediating HIV and SIV infection. Recently, a C-type lectin protein present on DCs, DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), was shown to efficiently bind and present HIV and SIV to CD4(+), coreceptor-positive cells in trans. However, the significance of DC-SIGN for virus transmission and pathogenesis in vivo remains unclear. Because SIV infection of macaques may represent the best model to study the importance of DC-SIGN in HIV infection, we cloned and characterized pig-tailed macaque DC-SIGN and generated monoclonal antibodies (MAbs) against it. We demonstrate that, like human DC-SIGN, pig-tailed macaque DC-SIGN (ptDC-SIGN) is expressed on DCs and macrophages but not on monocytes, T cells, or B cells. Moderate levels of ptDC-SIGN expression were detected on the surface of DCs, and low-level expression was found on macrophages. Additionally, we show that ptDC-SIGN efficiently binds and transmits replication-competent SIVmne variants to CD4(+), coreceptor-positive cells. Moreover, transmission of virus between pig-tailed macaque DCs and CD4(+) T cells is largely ptDC-SIGN dependent. Interestingly, MAbs directed against ptDC-SIGN vary in the capacity to block transmission of different SIVmne variants. These data demonstrate that ptDC-SIGN plays a central role in transmitting virus from macaque DCs to T cells, and they suggest that SIVmne variants may differ in their interactions with ptDC-SIGN. Thus, SIVmne infection of pig-tailed macaques may provide an opportunity to investigate the significance of DC-SIGN in primate lentiviral infections.

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.

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).

Immune escape through C-type lectins on dendritic cells

Dendritic cells (DCs) detect different pathogens and elicit tailored anti-microbial immune responses. They express C-type lectins that recognise carbohydrate profiles on microorganisms, resulting in internalisation, processing and presentation. Intracellular sequences of distinct DC-specific lectins point to differences in intracellular routing that influence antigen presentation. Moreover, putative signalling motifs hint to the activation of DCs on carbohydrate recognition. Recent evidence shows that not only pathogens, but also tumour antigens, exploit C-type lectins to escape intracellular degradation resulting in abortive immunity. More insight into ligand specificity, intracellular targeting and signalling will reveal the pathways by which pathogens modulate immunity through C-type lectins.

Dendritic cell (DC)-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin (DC-SIGN, CD209), a C-type surface lectin in human DCs, is a receptor for Leishmania amastigotes

Dendritic cells (DCs) play a critical role in the initiation of the immunological response against Leishmania parasites. However, the receptors involved in amastigote-dendritic cell interaction are unknown, especially in absence of opsonizing antibodies. We have studied the interaction of Leishmania pifanoi axenic amastigotes with the C-type lectin DC-specific intercellular adhesion molecule (ICAM)-3-grabbing nonintegrin (DC-SIGN, CD209), a receptor for ICAM-2, ICAM-3, human immunodeficiency virus gp120, and Ebola virus. L. pifanoi amastigotes interact with immature human dendritic cells and CD209-transfected K562 cells in a time- and dose-dependent manner. Leishmania amastigote binding to human dendritic cells and DC-SIGN-transfected cells is inhibited by a function-blocking DC-SIGN-specific monoclonal antibody. More importantly, this monoclonal antibody dramatically reduces internalization of Leishmania amastigotes by immature human DCs. These results constitute the first description of a nonviral pathogen ligand for DC-SIGN and provide evidence for a relevant role of DC-SIGN in Leishmania amastigote uptake by dendritic cells. Our finding has important implications for Leishmania host-cell interaction and the immunoregulation of cutaneous leishmaniasis.

A novel adhesion pathway that regulates dendritic cell trafficking and T cell interactions

Dendritic cells (DC) are present in essentially every tissue, where they operate at the interface of innate and acquired immunity by recognizing pathogens and presenting pathogen-derived peptides to T cells. Cell-cell interactions between DC, T cells and endothelial cells are crucial to all immunological processes. Recently, several C-type lectin receptors have been characterized that are abundantly expressed on the surface of DC. It is now becoming clear that these lectin receptors serve not only as antigen-receptors recognizing pathogens, but they may also function as adhesion receptors and/or signaling molecules. In particular the DC specific C-type lectin DC-SIGN (CD209) regulates adhesion processes, such as DC trafficking by interacting with ICAM-2 and T cell synapse formation, upon binding of ICAM-3. C-type lectins such as DC-SIGN contain a lectin domain that recognizes in a Ca2+-dependent manner carbohydrates such as mannose-containing structures presented on the glycoproteins ICAM-2 and ICAM-3. Although the integrin LFA-1 is a counter-receptor for both ICAM-2 and ICAM-3, on DC, DC-SIGN is the high affinity adhesion receptor for ICAM-2/-3. Here we discuss how the heterogeneity of mannose-residues exposed on cellular proteins and pathogens regulates specific binding of a repertoire of DC-expressed C-type lectins that contribute to the diversity of immune responses created by DC.