The role of DC-SIGN and DC-SIGNR in HIV and SIV attachment, infection, and transmission

Viral and host cofactors facilitate HIV-1 replication in macrophages

Human immunodeficiency virus type 1 (HIV-1) infection of CD4+ T lymphocytes leads to their progressive loss, whereas HIV-1-infected macrophages appear to resist HIV-1-mediated apoptotic death. The differential response of these two host-cell populations may be critical in the development of immunodeficiency and long-term persistence of the virus. Multiple contributing factors may favor the macrophage as a resilient host, not only supporting infection by HIV-1 but also promoting replication and persistence of this member of the lentivirus subfamily of primate retroviruses. An encounter between macrophages and R5 virus engages a signal cascade eventuating in transcriptional regulation of multiple genes including those associated with host defense, cell cycle, nuclear factor-kappaB regulation, and apoptosis. It is important that enhanced gene expression is transient, declining to near control levels, and during this quiescent state, the virus continues its life cycle unimpeded. However, when viral replication becomes prominent, an increase in host genes again occurs under the orchestration of viral gene products. This biphasic host response must fulfill the needs of the parasitic virus as viral replication activity occurs and leads to intracellular and cell surface-associated viral budding. Inroads into understanding how HIV-1 co-opts host factors to generate a permissive environment for viral replication and transmission to new viral hosts may provide opportunities for targeted interruption of this lethal process.

HIV-1 transmission and cytokine-induced expression of DC-SIGN in human monocyte-derived macrophages

Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) has been described as an attachment molecule for human immunodeficiency virus type 1 (HIV-1) with the potential to mediate its transmission. We examined DC-SIGN expression in monocyte-derived macrophages (MDM) and its role in viral transmission when MDM were exposed to interleukin (IL)-13, IL-4, or interferon-gamma (IFN-gamma). We show that IL-13 and IL-4 increase transcripts, total protein, and cell-surface expression of DC-SIGN in all MDM tested, IFN-gamma results ranged from no change to up-regulation of surface expression, and message and total protein were, respectively, induced in all and 86% of donors tested. Transmission experiments of HIV-1 X4 between cytokine-treated MDM to Sup-T1 cells showed no association between total transmission and DC-SIGN up-regulation. IL-4 but not IL-13 resulted in a less than twofold increase in MDM viral transmission to CD4+ T cells in spite of a fourfold up-regulation in DC-SIGN expression by either cytokine. In contrast, IFN-gamma treatment induced a decrease in total transmission by at least two-thirds, despite its induction of DC-SIGN. Soluble mannan resulted in a greater inhibition of viral transmission to CD4+ T cells than neutralizing anti-DC-SIGN monoclonal antibody (67-75% vs. 39-48%), supporting the role of mannose-binding receptors in viral transmission. Taken together, results show that DC-SIGN regulation in MDM does not singly predict the transmission potential of this cell type.

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.

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.

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

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

Simian immunodeficiency virus dramatically alters expression of homeostatic chemokines and dendritic cell markers during infection in vivo

Dendritic cells (DCs) are potent antigen-presenting cells that likely play multiple roles in human immunodeficiency virus type 1 (HIV-1) pathogenesis. We used the simian immunodeficiency virus (SIV)/macaque model to study the effects of infection on homeostatic chemokine expression and DC localization directly in secondary lymphoid tissues. SIV infection altered the expression of chemokines (CCL19/MIP-3beta, CCL21/ 6Ckine, and CCL20/MIP-3alpha) and of chemokine receptors (CCR7 and CCR6) that drive DC trafficking. CCL19/MIP-3beta, CCL20/MIP-3alpha, CCR6, and CCR7 expression increased in lymph nodes during the early systemic burst of viral replication (acute infection), whereas CCL21/6Ckine expression progressively decreased throughout disease to AIDS. Parallel with the SIV-induced perturbations in chemokine expression were changes in the expression of the DC-associated markers, DC-SIGN, DC-LAMP, and DECTIN-1. During AIDS, DC-LAMP mRNA expression levels were significantly reduced in lymph nodes and spleen, and DC-SIGN levels were significantly reduced in spleen. These findings suggest that the disruption of homeostatic chemokine expression is responsible, in part, for alterations in the networks of antigen-presenting cells in lymphoid tissues, ultimately contributing to systemic immunodeficiency.

13. HIV-1 infection

This review is intended to provide a fundamental perspective on the dynamic interplay between HIV-1 and the immune system, an essential aspect in defining the pathogenesis and treatment of AIDS. HIV-1 infection, the cause of AIDS, is a worldwide pandemic with enormous adverse heath and economic implications, particularly in the developing world. This bloodborne and sexually transmitted disease, which evolved from simian immunodeficiency virus, infects and replicates in helper T cells and macrophages and utilizes CD4 and a chemokine coreceptor for entry. Immune deficiency occurs as a result of virally induced attrition of CD4 T cells, resulting in the development of opportunistic infections and malignancy. Prophylaxis against opportunistic infections is required according to the extent of immune deficiency. HIV-specific immunity can control viral replication and delay disease progression but does not clear infection. Antiretroviral treatment consists of inhibitors that target for viral entry, reverse transcriptase, and viral protease. Therapy can control viral replication, restore immunity, and delay disease progression, but it cannot eliminate infection. Thus chronic infection persists even in treated patients. Antiretroviral drugs have been highly effective in preventing mother-to-child transmission and for postexposure prophylaxis. Several novel vaccines in development hold promise for either effective infection prevention or attenuation of disease progression.

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

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

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

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

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

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

Subset of DC-SIGN(+) dendritic cells in human blood transmits HIV-1 to T lymphocytes

The dendritic cell (DC)-specific molecule DC-SIGN is a receptor for the HIV-1 envelope glycoprotein gp120 and is essential for the dissemination of HIV-1. DC-SIGN is expressed by DCs, both monocyte-derived DCs and DCs in several tissues, including mucosa and lymph nodes. To identify a DC-SIGN(+) DC in blood that may be involved in HIV-1 infection through blood, we have analyzed the expression of DC-SIGN in human blood cells. Here we describe the characterization of a subset of DCs in human blood, isolated from T-/NK-/B-cell-depleted peripheral blood mononuclear cells (PBMCs) on the basis of expression of DC-SIGN. This subset coexpresses CD14, CD16, and CD33 and is thus of myeloid origin. In contrast to CD14(+) monocytes, DC-SIGN(+) blood cells display a DC-like morphology and express markers of antigen-presenting cells, including CD1c, CD11b, CD11c, CD86, and high levels of major histocompatibility complex (MHC) class I and II molecules. This DC population differs from other described CD14(-) blood DC subsets. Functionally, DC-SIGN(+) blood DCs are able to stimulate proliferation of allogeneic T cells and can produce tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) upon activation with lipopolysaccharide (LPS). When they encounter HIV-1, low amounts of these blood DC-SIGN(+) DCs enhance infection of T lymphocytes in trans, whereas blood monocytes and CD14(-) blood DCs are not capable of transmitting HIV-1. Therefore DC-SIGN(+) blood DCs can be the first target for HIV-1 upon transmission via blood; they can capture minute amounts of HIV-1 through DC-SIGN and transfer HIV-1 to infect target T cells in trans.

The mannose-dependent epitope for neutralizing antibody 2G12 on human immunodeficiency virus type 1 glycoprotein gp120

We have analyzed the unique epitope for the broadly neutralizing human monoclonal antibody (MAb) 2G12 on the gp120 surface glycoprotein of human immunodeficiency virus type 1 (HIV-1). Sequence analysis, focusing on the conservation of relevant residues across multiple HIV-1 isolates, refined the epitope that was defined previously by substitutional mutagenesis (A. Trkola, M. Purtscher, T. Muster, C. Ballaun, A. Buchacher, N. Sullivan, K. Srinivasan, J. Sodroski, J. P. Moore, and H. Katinger, J. Virol. 70:1100-1108, 1996). In a biochemical study, we digested recombinant gp120 with various glycosidase enzymes of known specificities and showed that the 2G12 epitope is lost when gp120 is treated with mannosidases. Computational analyses were used to position the epitope in the context of the virion-associated envelope glycoprotein complex, to determine the variability of the surrounding surface, and to calculate the surface accessibility of possible glycan- and polypeptide-epitope components. Together, these analyses suggest that the 2G12 epitope is centered on the high-mannose and/or hybrid glycans of residues 295, 332, and 392, with peripheral glycans from 386 and 448 on either flank. The epitope is mannose dependent and composed primarily of carbohydrate, with probably no direct involvement of the gp120 polypeptide surface. It resides on a face orthogonal to the CD4 binding face, on a surface proximal to, but distinct from, that implicated in coreceptor binding. Its conservation amidst an otherwise highly variable gp120 surface suggests a functional role for the 2G12 binding site, perhaps related to the mannose-dependent attachment of HIV-1 to DC-SIGN or related lectins that facilitate virus entry into susceptible target cells.

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

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

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

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

DC-SIGN (CD209) expression is IL-4 dependent and is negatively regulated by IFN, TGF-beta, and anti-inflammatory agents

Dendritic cell-specific ICAM-3 grabbing nonintegrin (DC-SIGN) is a monocyte-derived dendritic cell (MDDC)-specific lectin which participates in dendritic cell (DC) migration and DC-T lymphocyte interactions at the initiation of immune responses and enhances trans-infection of T cells through its HIV gp120-binding ability. The generation of a DC-SIGN-specific mAb has allowed us to determine that the acquisition of DC-SIGN expression during the monocyte-DC differentiation pathway is primarily induced by IL-4, and that GM-CSF cooperates with IL-4 to generate a high level of DC-SIGN mRNA and cell surface expression on immature MDDC. IL-4 was capable of inducing DC-SIGN expression on monocytes without affecting the expression of other MDDC differentiation markers. By contrast, IFN-alpha, IFN-gamma, and TGF-beta were identified as negative regulators of DC-SIGN expression, as they prevented the IL-4-dependent induction of DC-SIGN mRNA on monocytes, and a similar inhibitory effect was exerted by dexamethasone, an inhibitor of the monocyte-MDDC differentiation pathway. The relevance of the inhibitory action of dexamethasone, IFN, and TGF-beta on DC-SIGN expression was emphasized by their ability to inhibit the DC-SIGN-dependent HIV-1 binding to differentiating MDDC. These results demonstrate that DC-SIGN, considered as a MDDC differentiation marker, is a molecule specifically expressed on IL-4-treated monocytes, and whose expression is subjected to a tight regulation by numerous cytokines and growth factors. This feature might help in the development of strategies to modulate the DC-SIGN-dependent cell surface attachment of HIV for therapeutic purposes.

Truncation of the cytoplasmic domain induces exposure of conserved regions in the ectodomain of human immunodeficiency virus type 1 envelope protein

We have described a CD4-independent variant of HXBc2, termed 8x, that binds directly to CXCR4 and mediates CD4-independent virus infection. Determinants for CD4 independence map to residues in the V3 and V4-C4 domains together with a single nucleotide deletion in the transmembrane domain which introduces a frameshift (FS) at position 706. This FS results in a truncated cytoplasmic domain of 27 amino acids. We demonstrate here that while introduction of the 8x FS mutation into heterologous R5, X4, or R5X4 Env proteins did not impart CD4 independence, it did affect the conformation of the gp120 surface subunit, exposing highly conserved domains involved in both coreceptor and CD4 binding. In addition, antigenic changes in the gp41 ectodomain were also observed, consistent with the idea that the effects of cytoplasmic domain truncation must in some way be transmitted to the external gp120 subunit. Truncation of gp41 also resulted in the marked neutralization sensitivity of all Env proteins tested to human immunodeficiency virus-positive human sera and monoclonal antibodies directed against the CD4 or coreceptor-binding sites. These results demonstrate a structural interdependence between the cytoplasmic domain of gp41 and the ectodomain of the Env protein. They also may help explain why the length of the gp41 cytoplasmic domain is retained in vivo and may provide a way to genetically trigger the exposure of neutralization determinants in heterologous Env proteins that may prove useful for vaccine development.

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

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