• Dendritic cells;
  • DC-SIGN;
  • HIV-1;
  • Mycobacteria;
  • pathogens;
  • C-type lectins;
  • immune escape


  1. Top of page
  2. Abstract

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.


  1. Top of page
  2. Abstract

Dendritic cells (DC) are important sentinels of the immune system that defend against invading pathogens. These professional antigen-presenting cells are seeded throughout peripheral tissues to monitor for pathogens, which they capture and process to antigenic fragments (1). After microbial or inflammatory stimuli, immature DC undergo a process of maturation and migrate to secondary lymphoid organs to present processed antigens to naïve T cells. Antigen-specific T cells differentiate into effector T cells that are instrumental in combating infections. Although the function of DC as an antigen-presenting cell is pivotal for the initiation of the immune response against pathogens, certain pathogens have evolved to subvert the DC functions so that they can survive and infect the host. One of these pathogens is HIV-1 and it is now generally accepted that DC participate in the dissemination of HIV-1. Immature DC are one of the first cell types that interact with HIV-1 at sites of infection. DC capture HIV-1 and are thought to subsequently transport it from the periphery into the lymphoid tissues, where DC-bound HIV-1 is efficiently transmitted to CD4+ T cells. The role of DC in HIV-1 dissemination has been known for some time, but it was not until the discovery of the DC-specific C-type lectin DC-SIGN (DC-specific intercellular adhesion molecule (ICAM)-3 grabbing non-integrin) that the molecular mechanism was more clearly understood (2). DC-SIGN functions as a novel kind of HIV-1 receptor: DC-SIGN captures HIV-1 and transmits the virus to recipient T cells by a mechanism that is not yet fully understood. Recent studies have demonstrated that other pathogens can also interact with DC-SIGN to further their own ends (3–6). Some of these pathogens, such as mycobacteria, use a different mechanism than HIV-1 to escape immune surveillance. This chapter describes the function of DC-SIGN as a receptor for pathogens and its possible role in the pathogenesis of infectious diseases caused by these pathogens.


  1. Top of page
  2. Abstract

DC-SIGN (CD209) is a type II transmembrane protein that, based on its structure, belongs to the C-type lectin family (Fig. 1) (2–7). DC-SIGN contains a short cytoplasmic N-terminal domain with several intracellular sorting motifs, an extracellular stalk of seven complete and one partial tandem repeat, and a C-terminal lectin domain with affinity for mannose-containing carbohydrates (Fig. 1) (2). Initially, DC-SIGN was identified as a novel DC-specific adhesion receptor by its high affinity for the intercellular adhesion molecule (ICAM)-3 (2). This study initiated our search for other counterstructures, in particular those that share structural identity with ICAM-3, namely two other members of the immunoglobulin superfamily: ICAM-1 (CD54) and ICAM-2 (CD102). Strikingly, DC-SIGN binds ICAM-2 but not ICAM-1, demonstrating a highly regulated recognition of its immunoglobulin ligands (8). Both ICAM-2 and ICAM-3 function as ligands for the β2 integrin LFA-1 (CD11a/CD18), but although DC express LFA-1 the primary receptor on DC for these ligands is DC-SIGN (2, 9). Studies into the immunological function of DC-SIGN were directed by previous results that demonstrated that ICAM-2 was involved in leukocyte migration and ICAM-3 in DC-T cell clustering.


Figure 1. Structure of the C-type lectin DC-SIGN. A. Schematic structure of DC-SIGN. DC-SIGN is a type II trans-membrane C-type lectin; the extracellular N-terminus contains the C-type lectin domain. The intracellular C-terminus contains three internalization motifs. B. A ribbon diagram of the C-type lectin domain of DC-SIGN. The two α-helices are shown in blue, the β-strands in red, the calcium ions at site 1 and 2 in green, and the three disulphide bridges in yellow. Both the N- and the C-termini are at the bottom of the image. The structure was determined by molecular modeling. The structure were generated using YASARA (

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2.1. DC-SIGN mediates migration of DC from blood into tissues

The capacity to migrate is a fundamental aspect of DC function in controlling immunity. It allows DC to exert a continuous surveillance for incoming foreign antigens and a prompt response to present the encountered antigens to T cells (1). Precursor and immature DC migrate from the blood into peripheral tissues either to replenish resident DC or in response to inflammatory signals. The egress from blood into tissues is tightly regulated and is mediated by a multistep process involving: 1. leukocyte rolling, 2. rapid activation of leukocytes, 3. adhesion to endothelial ligands, and 4. diapedesis (10) (Fig. 2). The interaction with the endothelial linings of the blood vessels is an important control point in the egress. ICAM-2 is constitutively expressed on the endothelium of both blood and lymphatic vessels, as well as on high endothelial vascular cells. Strikingly, DC-SIGN mediates the tethering and rolling of DC-SIGN-positive cells along ICAM-2-expressing surfaces (Fig. 2) (8). Although DC-SIGN binds to both ICAM-2 and ICAM-3 under static conditions, only the DC-SIGN-ICAM-2 interaction resists shear stresses encountered under physiological flow conditions (8). Thus, DC-SIGN behaves as a DC-specific rolling receptor for ICAM-2 and is functionally similar to the selectins, which are well known for their regulation of leukocyte rolling upon carbohydrate recognition (Fig. 2) (11). The DC-SIGN-ICAM-2 interaction is also involved in the adhesion of DC to endothelium and the subsequent transendothelial migration (8), steps 2 and 3 in the multistep paradigm, respectively (Fig. 2). The presence of DC-SIGN-positive DC precursors in blood suggests that these cells could be poised to exit the blood at inflammatory sites, allowing rapid recruitment of these cells to sites where their surveillance function is needed (8, 12). Moreover, DC-SIGN is rapidly upregulated on monocytes in the presence of GM-CSF and IL-4 (2), suggesting that DC-SIGN upregulation by cytokine mediators in response to inflammation may induce migration of precursor DC from blood into the periphery. Thus, these data support the hypothesis that, under physiological circumstances, DC-SIGN-ICAM-2 interactions mediate rolling along endothelial linings, and migration of DC into the periphery and via the lymph into lymphatic tissues (Fig. 2) (8).


Figure 2. DC-SIGN controls many functions of DC to elicit immune responses. The egress of precursor DC from blood into tissues is partly mediated by DC-SIGN. DC-SIGN facilitates both rolling and trans-endothelial migration of DC-SIGN+ precursor DC, whereas arrest is mediated by integrin-mediated interactions. Initial DC-T cell clustering, necessary for an efficient immune response, is mediated by transient interactions between DC-SIGN and ICAM-3. This interaction facilitates the formation of low-avidity LFA-1/ICAM-1 interaction and scanning of the antigen-MHC repertoire. DC-SIGN also functions as an antigen receptor. DC-SIGN rapidly internalizes upon binding soluble ligand and is targeted to late endosomes/lysosomes, where antigens are processed and presented by MHC class II molecules.

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2.2. DC-SIGN is involved in the initiation of an immune response

The initial interaction of T cells with DC is antigen-independent and allows scanning of the peptide-MHC II complex-repertoire by the T cell receptor (TCR). This requires the formation of a specialized junction between these cells, the immunological synapse, that is generated by the carefully orchestrated recruitment of specific adhesion receptors into the contact site to strengthen DC-T cell contact (13–15). The abundance of appropriate MHC-peptide complexes is too low to mediate by itself significant adhesion and thus adhesion molecules are essential for an efficient TCR engagement. The initial interaction is transient and allows rapid scanning of the MHC-peptide complexes by T cells. Several studies have suggested that ICAM-3, which is expressed at high levels on resting T cells, might be important in establishing these initial DC-T cell interactions (2, 16–20). Recently, Montoya et al. demonstrated that ICAM-3 is recruited in the contact region of APC with T cells. The observed clustering occurred very rapidly after cell-cell contact, supporting a role for ICAM-3 in the early adhesive events (20). The counter-receptor for ICAM-3 on DC was originally thought to be LFA-1. However, the β2 integrin LFA-1 is inactive on DC and only after activation interacts with ICAM-3 at low affinity (2). In contrast, DC-SIGN has a high affinity for ICAM-3 and is fully active on DC, indicating that it is the primary receptor for ICAM-3 on DC (2). The importance of DC-SIGN-ICAM-3 interactions in the initial DC-T cell contact is emphasized by the potency of anti-DC-SIGN antibodies to inhibit DC-T cell clustering and DC-induced proliferation of resting T cells. The interaction of DC-SIGN with ICAM-3 is transient, allowing screening of the MHC-peptide complexes (Fig. 2) (2). The initial DC-SIGN-ICAM-3 interaction would stabilize intimate DC-T cell membrane contact, enabling efficient TCR engagement. The transient nature of the DC-SIGN-ICAM-3 interactions enables DC to interact with a large number of resting T cells, until a productive TCR engagement is obtained. TCR signaling increases the avidity of LFA-1 and CD2 on T cells, thereby strengthening the interaction between DC and T cell via multiple adhesive contacts through LFA-1 and LFA-3 that provide further positional stability and full activation of the T cell by the DC (Fig. 2) (15, 21).

2.3. DC-SIGN functions as an antigen receptor

DC express other C-type lectins, such as the mannose receptor (MR) (22), DEC-205 (23), BCDA-2 (24) and DC-ASGPR (25) that may function as pathogen receptors. These lectins may interact with conserved molecular patterns shared by a large group of microbes and internalize these pathogens for processing and antigen presentation, thus initiating immune responses against a diversity of microorganisms (26). Strikingly, the cytoplasmic tail of DC-SIGN contains putative internalization motifs (Fig. 1a) and DC-SIGN can function as an endocytic receptor (27). Binding of soluble ligands induces rapid internalization of DC-SIGN from the cell surface, which is mediated by the di-leucine motif in the cytoplasmic region (Fig. 2). Moreover, the cytoplasmic region contains a tri-acidic cluster, which is a signal for lysosomal targeting, and accordingly DC-SIGN-ligand complexes are targeted to lysosomal compartments where ligands are processed for MHC class II presentation to T cells (Fig. 2) (27). Thus, DC-SIGN not only functions as an adhesion receptor but also as an antigen receptor for pathogenic antigens similar to other C-type lectins.


  1. Top of page
  2. Abstract

The identification of DC-SIGN as a DC-specific adhesion receptor (2) revealed its 100% identity to the previously cloned HIV-1 envelope-binding C-type lectin (7), and initiated a detailed investigation into the function of DC-SIGN as an HIV-1 receptor on DC (9).

3.1. DC-SIGN as a HIV-1 trans-receptor

Early work revealed that DC pulsed with HIV-1 promote a robust infection of co-cultivated T cells (28, 29). This ability of DC to enhance infection of T cells was molecularly defined 8 years later when DC-SIGN was identified as the HIV-1 trans-receptor on DC (9). The affinity of DC-SIGN for the HIV-1 envelope glycoprotein gp120 exceeds that of CD4 (7) and, in contrast to CD4, DC-SIGN does not function as a classical HIV-1 entry receptor; co-expression of DC-SIGN with either CD4 or CCR5 does not enable HIV-1 entry into HIV-1 nonpermissive cells (9). In contrast, DC-SIGN acts as a novel HIV-1 trans-receptor that binds HIV-1, and transmits it very efficiently to neighboring permissive target cells (9). DC-SIGN expressed on DC, and on transfectants, binds both M- and T-tropic HIV-1, HIV-2 and SIV, and transmits these viruses to recipient T cells to result in an efficient infection of T cells (Fig. 3) (9, 30). Strikingly, DC-SIGN does not only capture and transmit HIV-1, but also enhances infection of T cells; at low virus titers, CD4/CCR5-expressing cells are not detectably infected without the assistance of DC-SIGN in trans (9). Conditions in which the number of HIV-1 particles is limiting are likely to resemble those found early during infection in vivo, and thus suggest that DC-SIGN is required not only for HIV-1 transmission from mucosa to lymphoid tissues, but also for efficient infection of T cells (Fig. 3). These data suggest that DC-SIGN is a unique receptor for HIV-1.


Figure 3. HIV-1 hijacks DC through DC-SIGN to infiltrate lymphoid tissues. DC-SIGN is expressed by both immature DC in mucosal tissues and by DC precursors in blood. HIV-1 is captured by DC-SIGN on DC precursors in blood after infection or on immature DC at mucosal entry sites during sexual transmission. DC-SIGN-bound HIV-1 is protected intracellularly during migration to the lymphoid tissues. Once arrived, DC-SIGN transmits HIV-1 to CD4+ T cells in trans, resulting in a productive HIV-1 infection of the CD4+ T cells.

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Some studies suggest that DC-SIGN can also function as a cis-receptor for HIV-1 since co-expression of DC-SIGN, CD4 and CCR5 increased the infection of target cells (31). However, these studies did not exclude the possibility that cells which appear to be cis-infected by DC-SIGN are actually trans-infected. Therefore, more research needs to be done to demonstrate that DC-SIGN can mediate cis-infection of DC.

To date, DC-SIGN is unique in this function since it not only binds HIV-1 gp120 but also enhances the HIV-1 infection of T cells in trans (9). The process through which DC-SIGN promotes efficient infection in trans of cells through their CD4/chemokine receptor complex remains unclear. Binding of the viral envelope glycoprotein to DC-SIGN may induce a conformational change in gp120 that enables a more efficient interaction with CD4 and/or the chemokine receptor, and subsequent membrane fusion with T cells. Alternatively, binding of viral particles to DC-SIGN may focus or concentrate the virus particles at the surface of the DC, and may thus increase the probability that entry will occur after binding to the CD4 and co-receptor complex on target cells. The potency of DC-SIGN to capture and transmit HIV to T cells may largely depend on the membrane organization of DC-SIGN in rafts or its capacity to multimerize (32). Future experiments will determine the molecular mechanism by which DC-SIGN enhances the infection of T cells, and will elucidate whether in multimerization and membrane organization of DC-SIGN is instrumental for its function as an HIV-1 trans-receptor.

3.2. DC-SIGN protects HIV-1 from degradation

Whereas DC-SIGN-bound ligands are internalized for processing in degradative compartments (27), whole DC-SIGN-bound HIV-1 particles are remarkably stable and retain infectivity for prolonged periods (9). These studies suggest that DC-SIGN ‘hides’ within the cell close to the cell membrane without being degraded (Fig. 4). Recently it was demonstrated that HIV-1 is internalized upon binding to DC-SIGN into non-lysosomal acidic organelles and that internalization is crucial for DC-SIGN-mediated enhancement of the infection of T cells (33). The question remains as to how intact HIV-1 virions escape targeting to lysosomes as occurs for other DC-SIGN-ligands (27) and how they can protect themselves against processing and presentation (Fig. 4). Interestingly, in mature DC, DC-SIGN is targeted to early endosomal compartments, in which HIV-1 would be protected against degradation (27), suggesting that maturation of DC by HIV-1 may lead to its altered internalization. Identification of the mechanism by which HIV-1 prevents degradation and remains highly infectious may lead to the development of successful strategies to combat HIV-1 dissemination. Moreover, finding a way to override this mechanism to target internalized DC-SIGN-HIV complexes to lysosomes would greatly facilitate HIV-1 processing in DC, and would enhance specific anti-HIV-1 immune responses while reducing infection of T cells (27, 33).


Figure 4. Both M. tuberculosis and HIV-1 target DC-SIGN to subvert DC function. DC-SIGN captures both mycobacteria and HIV-1. Mycobacteria are targeted to the late endosomes/lysosomes, whereas HIV-1 escapes internalization and presumably recycles back to the cell surface. HIV-1 bound by DC-SIGN efficiently infects CD4 T cells in trans. Internalized mycobacteria are processed and antigens are presented by MHC II and MHC-like molecules (CD1 family). It is possible that some bacilli escape the lysosomal pathway to infect DC.

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In order to transmit HIV-1 to T lymphocytes, virions internalized by DC-SIGN would have to recycle back to the cell surface to contact entry receptors on the target cell (Fig. 4). The mechanism of virus recycling, and whether this process is constitutively active or regulated, remains to be determined. Interestingly, the intracellular trafficking of DC-SIGN is affected by the Nef protein of HIV-1. Nef is crucial for viral replication, including DC-T cell binding (34). In addition, Nef can interact with the cell sorting machinery to downregulate expression levels of CD4 and MHC class I and thus facilitate immune evasion (35). Expression of Nef in immature DC reduces DC-SIGN internalization and increases cell surface expression, thereby facilitating increased cell adhesion and virus transmission to T cells (36). Redistribution of DC-SIGN requires the di-leucine motif in the cytoplasmic tail of DC-SIGN as well as a di-leucine motif in Nef, indicating that Nef interferes with recognition of DC-SIGN by the sorting machinery (36). This mechanism could increase HIV-1 transmission, since the increase in DC-SIGN expression would increase virus transmission (36).

To date, the function of DC-SIGN in HIV-1 infection is mainly based on in vitro models. Rhesus macaque DC-SIGN has a high homology with the human homologue and the macaque homologues can act as a HIV-1 trans-receptor similar to human DC-SIGN (37–39). Similarly to human DC-SIGN, the primate homologues are abundantly expressed in lymphoid tissues such as lymph nodes, as well as in mucosal tissues involved in sexual transmission of HIV-1 (37–40), indicating that primate models may be suitable for further dissecting the role of DC-SIGN in the transmission and pathogenesis of infection with immunodeficiency viruses.

3.3. DC-SIGN expression and other possible HIV-1 receptors

Although it remains to be determined whether DC-SIGN has a significant role in HIV-1 pathogenesis in vivo, the pattern of expression of DC-SIGN in mucosal tissues is consistent with it having a key function in the early stages of viral infection.

DC-SIGN is present on DC that are localized at the initial sites of HIV-1 infection during sexual transmission. DC-SIGN is expressed by immature DC in the lamina propria of mucosal tissues (rectum, cervix and uterus) (9, 41). Moreover, DC-SIGN is expressed in vivo by dermal DC of the skin as well as by DC present in lymphoid tissues such as lymph nodes, tonsils and spleen, but not by Langerhans cells in the epidermis (2).

Recently, a DC-SIGN+ DC subset in blood was identified that may be involved in HIV-1 infection through blood (12). This study demonstrated that DC-SIGN is expressed by a subset of myeloid CD14+ DC in human blood (8, 12), but not by plasmacytoid DC as previously suggested (42). These DC-SIGN+ blood cells display a DC-like morphology and express markers present on DC, such as CD1c, CD11b, CD11c, CD86, and high levels of MHC class I and II molecules. Moreover, these blood cells efficiently stimulate proliferation of allogeneic T cells (12). Strikingly, upon incubation with low amounts of HIV-1 these blood DC-SIGN+ DC were able to enhance infection of T lymphocytes in trans, whereas blood monocytes and CD14 blood DC were not capable of transmitting HIV-1 (12). Therefore, these DC-SIGN+ blood DC can be important in the capture and transmission of HIV-1 after HIV-1 infection via blood (12).

Although DC-SIGN is not the only C-type lectin that is able to bind HIV-1, so far it is unique in its function as an HIV-1 trans-receptor. Previous studies demonstrated that both MR and Langerin are able to bind HIV-1 gp120 (43) and it was suggested that these C-type lectins may function similarly to DC-SIGN in the HIV-1 transmission. However, so far experiments have been lacking to demonstrate that other C-type lectins can transmit HIV-1 in trans. This is further underscored by the results that DC-SIGN blood DC are not able to transmit HIV-1, in contrast to DC-SIGN+ blood DC (12), whereas the former blood DC do express other C-type lectins that were suggested to bind HIV-1 (43).

3.4. The structure of DC-SIGN and distinct binding sites for gp120 and ICAM-3

DC-SIGN binds two Ca2+ ions that are necessary for binding (2, 7); one calcium ion is essential for the tertiary structure and the other calcium ion coordinates ligand binding (44, 45), similar to other C-type lectins (26). Recent elucidation of the three-dimensional structure of DC-SIGN co-crystallized with an oligosaccharide identified several important features of ligand binding by DC-SIGN (45). Part of the pentasaccharide ligand forms coordination bonds with the Ca2+ at the principal site 2 (45). Carbohydrate binding by DC-SIGN is mediated by the amino acid residues that are also in close contact with this Ca2+ and which form the core of the carbohydrate-binding site. This type of binding is a hallmark of C-type lectin-carbohydrate interactions (46). In contrast to most C-type lectins, DC-SIGN binds an internal mannose of the oligosaccharide, and the external saccharides also interact with the surface of DC-SIGN (45). Although the three-dimensional structure of DC-SIGN with an oligosaccharide revealed several important features of DC-SIGN binding (45), the interaction with its natural ligands is more complex since it is not only mediated by carbohydrate structures (44).

Both HIV-1 and ICAM-3 are heavily glycosylated and contain high mannose-type oligosaccharides (47, 48). Enzymatic removal of the N-linked carbohydrates from ICAM-3 completely abrogates its binding to DC-SIGN (44). Although glycosylation seems to enhance the affinity of gp120 binding, neither O- nor N-linked glycosylations are vital for the interaction of HIV-1 gp120 with DC-SIGN; DC-SIGN interacts with both enzymatically deglycosylated and non-glycosylated gp120 (44). In contrast, Lee demonstrated that recombinant deglycosylated gp120-Fc is not bound by DC-SIGN (49). A different binding assay was used in this study and the conflicting results could be due to different sensitivities between the two assays (44, 49). However, these data demonstrate that carbohydrates are necessary for high-affinity binding. Collectively, these data suggest that DC-SIGN has different binding sites for ICAM-3 and HIV-1 gp120 in DC-SIGN; the C-type lectin DC-SIGN interacts with its ligands either through carbohydrate interactions, such as those with mannan and ICAM-3, or through protein-carbohydrate interactions, such as with gp120. The differences in carbohydrate dependency suggest that gp120 binding by DC-SIGN differs from ICAM-3 binding. The three-dimensional structure of DC-SIGN with the pentasaccharide shows that the Val351 residue in DC-SIGN participates in carbohydrate binding through van der Waals' interactions (45). Strikingly, this Val351 in DC-SIGN denotes the difference between the interaction of DC-SIGN with gp120 and ICAM-3 (44). The Val351 residue is important in binding of both carbohydrates and ICAM-3, but it is not essential for gp120 binding, since the V351G mutant of DC-SIGN still interacts with gp120 but is unable to bind ICAM-3 (44). These data support the hypothesis that DC-SIGN has distinct binding sites for HIV-1 gp120 and ICAM-3 (44). These findings further indicate that, in addition to the ligands interacting with the primary binding site centered around the Ca2+ at site 2, the ligands form additional contacts with the surface of DC-SIGN, thereby creating different ligand binding sites for HIV-1 gp120 and ICAM-3. Clustering of DC-SIGN at the cell surface in a novel tetrameric coiled-coil motif is suggested to contribute to a further increase in ligand binding specificity (32).

3.5. DC-SIGN also functions as a viral receptor for Ebola and CMV

Due to their strategic localization and their function, immature DC have been implicated in the pathogenesis of other viruses than HIV-1. Human cytomegalovirus (CMV) is a ubiquitous pathogen in humans that causes lifelong infection and reactivation episodes. CMV is asymptomatic in most immunocompetent individuals because of an efficient antiviral immune response. In contrast, CMV remains a major cause of morbidity and mortality in newborn and immunocompromised patients such as organ-transplanted recipients and AIDS patients. Both HIV-1 and CMV are known to induce immunosuppression and recently it was demonstrated that DC are a target for CMV (50, 51). In addition, CMV-infection DC displayed decreased antigen presentation and differentiation capacities (50, 51). Thus, the observed transient immunosuppression in CMV infections may result from viral interference with DC function. A recent study demonstrated that human CMV is captured by DC through the binding by DC-SIGN and subsequently transmitted to permissive cells (4). Moreover, DC infection by primary CMV isolates was blocked by antibodies against DC-SIGN and expression of DC-SIGN rendered cells permissive for CMV (4). These results demonstrate that DC-SIGN interacts similarly with CMV as with HIV-1. This is further demonstrated by the results that CMV binding to DC-SIGN protects the virus from degradation by DC (4). Hence, by promoting DC-SIGN-mediated trans-infection of target cells as well as infection of DC, DC-SIGN may be involved, apart from virus propagation, in CMV-mediated altered immune responses. Further experiments will determine whether DC-SIGN is involved in the impaired functions of DC following infection.

Alvarez et al. (5) demonstrated that DC-SIGN can act as a cellular entry factor for Ebola virus, a highly lethal pathogen responsible for several outbreaks of hemorrhagic fever. Using lentivirus particles pseudotyped with Ebola GP envelopes the authors demonstrated that DC-SIGN-positive transfectants were more susceptible to Ebola than mock transfectants. Moreover, they demonstrated that immature DC captured the Ebola pseudotyped particles through DC-SIGN and were able to transmit the virus to recipient cells.

Thus, DC-SIGN acts as a viral receptor for HIV-1, CMV and Ebola that efficiently captures these pathogens and facilitates virus infection. These data suggest that perhaps also other viruses or pathogens may utilize DC-SIGN or related receptors for transmission. If so, DC-SIGN could represent a new class of therapeutic targets for antiviral therapy for several viral diseases. Therefore, a detailed understanding of the interaction of DC-SIGN with its ligands is necessary and will contribute to the development of potential prevention of virus transmission and pathogenesis.


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  2. Abstract

The identification of two binding sites for HIV-1 and ICAM and its ability to bind specifically to HIV-1, Ebola and CMV, but not HSV-1 and VSV, lead us to investigate its binding specificity in more detail. A chimeric protein of DC-SIGN with a human IgG1 Fc tag (DC-SIGN-Fc) was used to screen a panel of synthetic glycoconjugates containing mannose or fucose residues and their derivatives in multimeric form (52). Strikingly, besides binding to the previously identified DC-SIGN-ligands ICAM-2, ICAM-3 and gp120 (2, 9), and several less complex mannose-containing glycoconjugates. i.e. mannose and α1[RIGHTWARDS ARROW]3, α1[RIGHTWARDS ARROW]6mannotriose (32, 45), DC-SIGN-Fc also demonstrated a high affinity for Lewis blood group antigens (Lex, Ley, Lea, Leb) that contain fucose residues in different anomeric linkages (52). Sialylation of Lex (yielding sialyl-Lex), an L-E- and P-selectin ligand, completely abrogated recognition by DC-SIGN, indicating that DC-SIGN has a carbohydrate specificity that is distinct from that of the selectins that mediate leukocyte rolling. DC-SIGN has a much higher affinity for the fucose-containing carbohydrate Lex than for mannotriose. Even though DC-SIGN-Fc is a recombinant protein, it exhibits a carbohydrate recognition profile similar to that of cell-surface expressed DC-SIGN. Both DC-SIGN transfectants and monocyte-derived dendritic cells bound similarly to the glycoconjugates as DC-SIGN-Fc. Even though DC express, apart from DC-SIGN, many other C-type lectins on their cellsurface, the glycoconjugates containing either Lex and α1[RIGHTWARDS ARROW]3 or α1[RIGHTWARDS ARROW]6mannotriose are preferentially bound by DC-SIGN. This illustrates that DC-SIGN is the major receptor on DC for these carbohydrate structures. Binding of sulfo-Lea to DC could only be partially blocked by anti-DC-SIGN antibodies, indicating that other C-type lectins on DC compete with DC-SIGN for binding of sulfo-Lea (52). This demonstrates that DC-SIGN recognizes a wider range of glycan structures, including Lewis blood group antigens, than hitherto realized. Thus, DC-SIGN may be an important receptor for recognition of novel biologically relevant targets expressed by the host, or alternatively by human pathogens. The blood group antigen Lex (CD15) is expressed by gastric mucosal epithelial cells, and by polymorphonuclear leukocytes, whereas Ley expression is increased on many carcinomas. This indicates that DC-SIGN may mediate interactions of DC with epithelial cells, polymorphonuclear leukocytes and tumor cells through these carbohydrate structures.

4.1. DC-SIGN is a pathogen receptor with broad specificity

The identification of novel carbohydrate structures recognized by DC-SIGN lead to a more detailed analysis of the binding of DC-SIGN to human pathogens that express mannose or fucose-containing glycans. The gram-negative bacterium Helicobacter pylori, which induces peptic ulcers and gastric carcinoma (53), and the worm parasite Schistosoma mansoni (the causal agent of schistosomiasis) both express Lex (54). In H. pylori, Lex is present on surface-located lipopolysaccharide, while in S. mansoni Lex is expressed at all stages of the parasite, including soluble egg antigen (SEA) (54). Indeed, both Lex-positive pathogen structures, H. pylori LPS and S. mansoni SEA, were strongly bound by DC-SIGN expressed by transfectants and the binding was completely inhibited by anti-DC-SIGN antibodies (52). Moreover, whole H. pylori bacteria were also specifically bound by DC-SIGN (52). Investigation of mannose-containing pathogens demonstrated that the mannose-capped surface lipophosphoglycan (LPG) expressed by Leishmania mexicana, a unicellular parasite that causes leishmaniasis (52), and Mycobacterium tuberculosis antigens also interact with DC-SIGN-Fc, whereas no binding of DC-SIGN to three clinically relevant Gram-negative bacterial human pathogens (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa) or to Gram-positive Staphylococcus aureus was observed (52). These findings indicate that binding of DC-SIGN to pathogens is selective, and that the carbohydrate specificity of DC-SIGN governs a broader pathogen recognition than HIV-1, Ebola and CMV (5, 6, 9).

A common feature of the specific pathogens that interact with DC-SIGN, such as mycobacteria, Leishmania and Helicobacter, is that they cause chronic infections that may last a lifetime, and secondly, that manipulation of the Th1/Th2 balance by these pathogens is central to their persistence. The interaction between these pathogens and DC-SIGN may greatly influence antigen presentation, as well as cytokine secretion by DC, and may thereby contribute to their persistence. For infection with M. tuberculosis it has already been demonstrated that the ManLAM-DC interaction reduces IL-12 production by DC and shifts the immune response toward Th2, which promotes immune evasion and persistence (55). Likewise, a Th1 to Th2 shift, associated with a decrease in IL-12 concentrations, is crucial to virulence and persistence of Leishmania mexicana. Also for Schistosoma mansoni, a Th2 immune response is associated with persistence of the pathogen. SEA and its major glycan antigen Lex are able to cause a switch towards a Th2-mediated immune response (56). Therefore, these pathogens could have evolved to target DC-SIGN not only to infect DC but also to shift the Th1/Th2 balance in favor of persistence.


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  2. Abstract

Mycobacterium tuberculosis represents a world-wide health risk and persistence is a particular problem in M. tuberculosis infections. Macrophages are the primary targets for M. tuberculosis, and the mycobacteria survive within so-called phagosomes of the infected macrophages. Initially, innate immune responses against mycobacteria predominate and are directed by activated macrophages (reviewed in (57)). However, subsequent recruitment of cellular responses that restrict mycobacterial infections are mediated by DC (58). The ability of M. tuberculosis to exist as a latent infection of the host suggests that mycobacteria are able to suppress cellular immune responses. Although DC are not the primary targets for infection by mycobacteria, the specific function of DC in the cellular immune response seems to be modulated by mycobacteria (55). Thus, knowledge about the interaction of DC with mycobacteria and mycobacterial components is essential to fully understand and combat M. tuberculosis infections.

5.1. DC-SIGN interacts with mycobacteria through the cell-wall component ManLAM

Soluble DC-SIGN-Fc, as well as cellular DC-SIGN, binds strongly to viable mycobacteria such as M. tuberculosis and M. bovis bacillus Calmette-Guérin (BCG) (3, 59). Further analysis demonstrated that the mannose-containing phosphorylated glycolipid ManLAM, a major cell-wall component of these mycobacteria strains, specifically interacts with DC-SIGN. In contrast, M. smegmatis only contains uncapped AraLAM and this mycobacterium as well as AraLAM alone did not interact with DC-SIGN (3). ManLAM, in contrast to AraLAM, contains a mannose cap consisting exclusively of mono-, di- and trimers of α-d-mannoses directly linked to the arabinofuranosyl-termini (60). Thus, DC-SIGN binds specifically to the exterior Manα1[RIGHTWARDS ARROW]2Man-linked residues of ManLAM, but not to the mannose-containing core of ManLAM, which is shared with AraLAM. Both viable mycobacteria, containing ManLAM, and ManLAM alone bind specifically to the primary binding site of DC-SIGN, similarly to the other DC-SIGN ligands ICAM-3 and HIV-1 gp120 (3). ICAM-3 and gp120 make distinct additional contacts with DC-SIGN, and Val351 at the edge of the binding pocket denotes the difference between these ligands. The DC-SIGN V351G mutant does not bind ICAM-3, whereas HIV-1 gp120 is still bound by this mutant (44). Both M. bovis BCG and ManLAM are bound by the DC-SIGN V351G mutant, demonstrating that ManLAM is indeed the ligand on mycobacteria that is bound by DC-SIGN (3). Thus, the pathogen structures ManLAM and HIV-1 gp120 occupy a similar binding pocket in DC-SIGN that is distinct from that of the cellular ligand ICAM-3. These observations suggest that DC-SIGN may distinguish between different types of ligand and may tailor its responses specifically.

5.2. DC-SIGN is the major receptor for M. tuberculosis on immature DC

Immature DC express, besides high levels of DC-SIGN, high levels of the receptors MR, CD11b and CD11c, which have previously been reported to mediate binding of mycobacteria by macrophages (61–63). DC-SIGN is the most important receptor on DC for both M. bovis BCG and ManLAM despite high expression levels of MR on DC (3). DC-SIGN-specific antibodies, in contrast to MR-specific antibodies, inhibit the interaction of DC with both M. bovis BCG and ManLAM by more than 80% (3). Involvement of MR in ManLAM binding by DC has previously been inferred by inhibition studies using mannan (55, 64), since MR has a high affinity for this polycarbohydrate. However, mannan also inhibits the functions of other C-type lectins on DC, such as DC-SIGN (reviewed in (65)), demonstrating that specific blocking antibodies against particular receptors are necessary to accurately determine the involvement of a receptor. ManLAM captured by DC-SIGN was rapidly internalized by DC and targeted to CD107a/Lamp-1+ lysosomes. Uptake of M. tuberculosis LAM by DC results in presentation by CD1b to specific T cells (64) and it is likely that DC-SIGN may be responsible for the delivery of ManLAM to late endosomes/lysosomes for presentation by CD1b (Fig. 4). This is supported by recent findings that DC-SIGN also functions as an antigen receptor that targets internalized antigen to late endosomes/lysosomes for presentation to T cells (27).

Rapid internalization of ManLAM by DC-SIGN suggests that DC-SIGN may also be involved in mycobacterial uptake by DC (3), as was shown for HIV-1 (Fig. 4) (9). Indeed, blocking antibodies against DC-SIGN inhibited the internalization of viable M. bovis BCG by immature DC. Thus, DC-SIGN mediates both capture and internalization of mycobacteria such as M. bovis BCG, which is supported by the co-localization of DC-SIGN staining and FITC-conjugated mycobacteria. Internalized FITC-conjugated mycobacteria are targeted to the lysosomes, since they co-localized with Lamp-1 staining (3). Recently, it was shown that murine DC can act as a reservoir in vivo for mycobacteria (66) and it is possible that some bacilli escape the lysosomal pathway to productively infect human DC. Thus, mycobacteria target DC-SIGN to infect DC and to escape destruction, similar to what was observed for HIV-1.

5.3. ManLAM binding to DC-SIGN suppresses DC function

LAM glycolipids are present in the mycobacterial cell wall, but are also secreted from phagosomes following macrophage ingestion of M. tuberculosis (60, 67, 68). The presence of anti-LAM antibodies in sera of tuberculosis patients suggests that LAM is released in vivo (69). Thus, mycobacteria within macrophages can affect bystander immune cells and modulate the immune response (Fig. 5). DC are critical in that they mediate cellular immune responses against mycobacteria. Strikingly, we demonstrated that secreted ManLAM targets DC-SIGN on DC to suppress DC functions (Fig. 5) (3). Triggering of TLR on DC induces DC maturation, resulting in release of cytokines and upregulation of accessory molecules for efficient stimulation of T lymphocytes (71, 72). DC maturation by LPS is mediated through TLR4, which generates intracellular signaling most notably via the transcription factor NFκB (73). Mycobacteria such as M. bovis BCG also induce DC maturation (74) and M. bovis BCG can mediate the observed maturation through TR2 and TLR4 signaling (74). Strikingly, both M. bovis BCG- and LPS-induced maturation of DC was specifically blocked by ManLAM but not by AraLAM (3). This inhibition by ManLAM is mediated through DC-SIGN, since antibodies against DC-SIGN abrogated this effect and can completely restore strong DC maturation by both M. bovis BCG and LPS. These results suggest that DC-SIGN, upon binding ManLAM, delivers a signal that interferes with the M. bovis BCG-induced signals presumably generated by TLR4 (Fig. 5). These results suggest that pathogen binding to DC-SIGN may mediate intracellular signaling. Moreover, ManLAM binding to DC-SIGN resulted in an induction of IL-10 by LPS-activated DC (3). The ManLAM-induced production of IL-10 could contribute to the virulence of mycobacteria, since IL-10 impairs the ability of DC to generate Th1 responses by blocking upregulation of co-stimulatory molecules and IL-12 production (75). Moreover, M. bovis BCG-infected DC produced high levels of IL-10, demonstrating that mycobacteria induce IL-10 both through direct infection and by influencing bystander DC by ManLAM secretion (Fig. 5) (3). Recently, it was demonstrated that ManLAM inhibits the IL-12 production by LPS-matured DC (55). The authors suggested that MR is involved in ManLAM binding, but DC-SIGN may also be involved in this pathway (3). Nigou et al. hypothesized that pathogen receptors could interfere with TLR signaling upon pathogen recognition, modulating the cellular immune responses against pathogens (55). The data from Geijtenbeek et al. (3) further support this hypothesis. Thus, shifting the balance between TLR and C-type lectin signaling may be a general principle by which pathogens suppress the immune response (27).


Figure 5. M. tuberculosis targets DC-SIGN through ManLAM secretion to suppress cellular immune responses mediated by DC. M. tuberculosis infects macrophages through MR and/or DC through DC-SIGN. Macrophages/DC infected with M. tuberculosis secrete the virulence factor ManLAM that binds to DC-SIGN on DC attracted to the inflammatory site. ManLAM binding to DC-SIGN results in negative signals that interfere with maturation mediated by TLR. The ManLAM-DC-SIGN interaction results in inhibition of DC maturation and an induction of the immunosuppressive cytokine IL-10, thereby preventing an efficient cellular immune response against M. tuberculosis infection.

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In humans, M. tuberculosis may target DC-SIGN to suppress cellular immune responses (Fig. 5), since both immature DC and IL-10–treated DC are not only less efficient at stimulating T cell responses but can also induce a state of antigen-specific tolerance (76, 77). The results obtained with the mildly virulent M. bovis BCG strain indicate that the mechanism of immunosuppression may not directly contribute to the virulence and persistence of virulent M. tuberculosis strains as compared to less virulent strains. However, differences between the interaction of DC-SIGN with virulent and avirulent mycobacteria strains will have to be investigated in more detail, in order to determine whether some strains are more efficient in targeting DC-SIGN and thus suppressing immune responses than others. This hypothesis is supported by our finding that the avirulent strain M. smegmatis does not bind to DC-SIGN.


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  2. Abstract

6.1. L-SIGN, the human DC-SIGN homologue

Initially, analysis of DC-SIGN expression was confused by the presence of the DC-SIGN homologue L-SIGN (78), also called DC-SIGN-related (DC-SIGNR) (79). L-SIGN functions as an HIV-1 trans-receptor similarly to DC-SIGN, but is not expressed by DC (78). L-SIGN is specifically expressed by liver sinusoidal endothelial cells (LSEC) (78, 81), a liver-resident APC population (80), and by a specific subpopulation of non-endothelial macrophage-like cells in lymph nodes (A. Engering, manuscript submitted). Liver sinusoids are specialized capillary vessels characterized by the presence of resident macrophages adhering to the endothelial lining. The LSEC-leukocyte interactions, which require expression of adhesion molecules on the cell surfaces, appear to constitute a central mechanism of peripheral immune surveillance in the liver. The MR, MHC class II, and co-stimulatory receptors such as CD80 and CD86 are known to be expressed on LSEC and to mediate the clearance of many potentially antigenic proteins from the circulation in a manner similar to DC in lymphoid organs (80). L-SIGN may fit into this category of receptors on LSEC, as its tissue location and ligand binding properties strongly implicate a physiological role for this receptor in antigen clearance, as well as in LSEC-leukocyte adhesion. The high expression of ICAM-3 on apoptotic cells (82) may provide the means by which these cells are trapped by L-SIGN-expressing cells in the liver and subsequently cleared.

The expression pattern of L-SIGN in liver sinusoids suggests that LSEC, which are in continual contact with circulating leukocytes, can capture HIV-1 from the blood and promote trans-infection of circulating T cells in the liver. Moreover, prior studies have indicated that LSEC themselves may be susceptible to HIV-1 infection (83). Thus, it is possible that L-SIGN promotes infection of these cells, thereby establishing a reservoir for new virus to pass on to T cells constitutively trafficking through the liver sinusoid. Its expression in lymph nodes suggests that L-SIGN may play an additional role in HIV-1 pathogenesis by promoting HIV-1 infection of T cells in lymph nodes, and thus L-SIGN may be involved in the persistence of chronic HIV-1 infections (78, 81). L-SIGN could also be involved in vertical transmission since it is expressed in placenta (70, 81). Additional functional studies are necessary for understanding the physiological role of L-SIGN and its possible role in HIV-1 pathogenesis. Moreover, L-SIGN interacts with pathogens similar to DC-SIGN, i.e. CMV, Ebola and mycobacteria, suggesting that it may also be involved in the pathogenesis of infections with these pathogens.

6.2. A murine homologue of DC-SIGN captures blood-borne antigens in vivo

Five murine DC-SIGN homologues have been identified, and one, called murine DC-SIGN (mDC-SIGN), is expressed at high mRNA levels in CD11c+ DC (84, 85). In contrast, mRNA of other homologues, designated SIGNR1–4, is hardly detected in DC. We generated antibodies against one of these homologues, SIGNR1, and demonstrated that SIGNR1 is specifically expressed by liver sinusoidal endothelial cells and not by DC (86). Moreover, SIGNR1 is also expressed by medullary and subcapsular macrophages in lymph nodes, and by marginal zone macrophages (MZM) in spleen. mSIGNR1 has a high affinity for human ICAM-3, yet mice do not express ICAM-3 and therefore binding of mSIGNR1 to ICAM-3 is not physiological. However, mSIGNR1 is able to interact with murine ICAM-2 (86), which is widely expressed on murine lymphocytes (87). Thus, ICAM-2 could function as the leukocyte ligand for mSIGNR1 mediating contact between leukocytes and the SIGNR1-positive cells, i.e. MZM and LSEC.

Strikingly, the MZM are in direct contact with the blood stream and efficiently capture specific polysaccharide-antigens present on the surface of encapsulated bacteria (88). Analysis of the in vivo function of SIGNR1 on MZM in spleen demonstrates that SIGNR1 functions in vivo as a pathogen recognition receptor on MZM that captures blood-born antigens, which are rapidly internalized and targeted to lysosomes for processing (86). Antigen capture in vivo is completely blocked by the blocking SIGNR1-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.


  1. Top of page
  2. Abstract

Current data demonstrate that DC-SIGN is not only a receptor for HIV-1 but also for other viruses such as CMV and Ebola. These viruses interact with DC though DC-SIGN and their capture results in transmission of the viruses to other target cells. Thus, DC-SIGN seems to be important in the dissemination of these viruses. Hence, by promoting DC-SIGN-mediated trans-infection of target cells, DC-SIGN may be involved, apart from virus propagation, in virus-mediated altered immune responses. Strikingly, mycobacteria suppress DC function via DC-SIGN differently than these viruses. Further experiments will determine whether DC-SIGN is also involved in the immunosuppression observed in CMV and HIV-1 infections.

DC-SIGN may be the prime target for pathogens such as HIV-1, CMV Leishmania and M. tuberculosis to manipulate the DC function. Therefore, DC-SIGN could be an important target for clinical intervention in infections. The hypothesis of distinct binding sites in DC-SIGN for cellular ligands and pathogen structures could provide the molecular basis for the design of strategies to inhibit DC-SIGN/pathogen interactions without affecting the immunological function of DC-SIGN.


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  2. Abstract
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