DC-SIGN (CD209) and its homolog L-SIGN (also called DC-SIGN-R, CD209L) belong to the C-type (calcium-dependent) lectin family. This large group of proteins which includes the mannose receptor, DEC-205 or langerin, is specialized in the recognition of carbohydrate structures present on cellular and viral proteins and is implicated in several processes such as cell adhesion and antigen presentation (1,2). DC-SIGN and L-SIGN are constitutively expressed by specific cell populations that play a key role in the activation of the innate and adaptive immune responses. DC-SIGN is highly expressed at the surface of dendritic cells (DCs) localized in the lymphoid tissues (thymus, tonsils, or lymph nodes), mucosal surface, and in the dermis (3) (4) (5) . DC-SIGN is not expressed by Langerhans cells, which are a unique DC subset residing in epidermis. Certain macrophages such as Hofbauer cells in the placenta, Kupffer cells in the liver sinusoids, and alveolar macrophages have been shown to express DC -SIGN (3,4) . L-SIGN expression is restricted to endothelial cells such as those in lymph nodes, placenta, and, particularly, liver sinusoidal endothelial cells (LSECs) (6,7) .
DC-SIGN and L-SIGN share nearly 77% amino acid identity and are closely related in global architecture. Both lectins are type II transmembrane proteins composed of a short cytoplasmic tail responsible for signalling and internalization, a transmembrane region, a neck domain consisting of eight repeat regions of 23 amino acids and a carbohydrate recognition domain (CRD) which binds carbohydrate ligands in a calcium-dependent manner (1,3,4) (Fig. 1) . The DC-SIGN CRD recognizes different mannose or fucose-based carbohydrates whereas the L-SIGN CRD appears to interact only with mannose resi- dues on N-glycans (Man5GlucNac2 to Man9GlucNac2) (8, 9) . The repeat regions within the neck domain permit the oligomerization of the lectins which is critical for their biological activities, because only tetramers efficiently capture glycosylated ligands ( [10] [11] [12] .
DC-SIGN was originally cloned for its ability to bind and internalize the heavily glycosylated human immunodeficiency virus (HIV) gp120 protein (13). DC-SIGN strongly binds all HIV and simian immunodeficiency virus (SIV) strains examined to date and plays an important role in virus adhesion to DC (14) . These studies have paved the way for further investigations into interactions between DC-SIGN and pathogens and it has become clear that many viruses target DC-SIGN and L-SIGN to bind DCs and endothelial cells, respectively. Both lectins recognize high mannose oligosaccharides present on viral glycoproteins and thus function as attachment factors for several viruses including cytomegalovirus (CMV), dengue (DV), Ebola, severe acute respiratory syndrome (SARS), hepatitis C (HCV), Marburg and Sindbis viruses (Table 1 ) (10, [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] . Differential glycosylation of viral envelope glycoproteins strongly influences the efficiency of viral capture by DC-SIGN and L-SIGN (10, 19, 22) . For example, the DV envelope glycoprotein E has two conserved N-linked glycosylation sites at Asn-67 and Asn-153 that mediate DV binding to DCs (22). Only mannosylated E glycoproteins (which are exposed at the surface of DV virions transmitted to humans by infected mosquitoes), and not E proteins with complex glycosylation (produced in mammalian cells), have been shown to interact with DC-SIGN-expressing cells (22) .
DC-SIGN and L-SIGN are endocytic receptors and their cytoplasmic tails carry putative internalisation signals such as a dileucine (LL) motif (which is present in both DC-SIGN and L-SIGN) and a tri-acidic cluster that is believed to be involved in intracellular trafficking (Fig. 1) (3) . Despite findings showing that a large fraction of viral particles captured by DC-SIGN are rapidly internalized and degraded (28,29), viruses are nevertheless able to hijack DC-SIGN and L-SIGN functions to spread infection. For the viruses examined to date, DC-SIGN and L-SIGN have been shown to act as attachment factors rather than authentic entry receptors involved in membrane fusion. This does not exclude the possibility that some viruses, and particularly those that require targeting to acidified endosomes for membrane fusion, use these lectins as primary entry receptors. DC-SIGN and L-SIGN have also been shown to function as "cis-receptors" that enhance infection of target cells. This mechanism, known as cis-infection, has been described for DV, CMV, and HIV, and probably relies on the capacity of these lectins to concentrate viral particles at the cell surface, allowing optimal interaction with their cognate receptors and enhanced viral entry (Table 1) (18, 22, 30) . Viruses captured by DC-SIGN or L-SIGN can also be transmitted in trans to target cells expressing the entry receptors ("trans-infection" ), as has been proposed for HIV, HCV, and SARS virus (14, 16, 26, 31) . For HIV and CMV, DC-SIGN enhances infection of target cells at a low multiplicity of infection (MOI). DC-SIGN-bound viruses infect target cells more efficiently than free viruses and remain infectious for several days (14,18). The molecular mechanisms underlying these processes remain poorly understood.
The contribution of DC-SIGN and L-SIGN to viral transmission and dissemination in vivo is currently unknown. Their role as principal attachment factors for a broad range of enveloped viruses and their restricted expression in anatomical site of virus exposure suggest that these two lectins dictate viral tropism for DCs and endothelial cells and consequently may influence viral pathogenesis. DCs are sentinel cells that capture pathogens entering skin or mucosal tissues and then migrate to the lymph nodes where they present processed antigens to T-cells, initiating adaptive immune responses. By interacting with DC-SIGN, viruses that are transmitted sexually (such as HIV) or through introduction into human skin by an insect vector (such as dengue virus or Sindbis virus) may hijack DC function either to modulate the immune response or to assure their dissemination from peripheral tissues to lymphoid organs (32). LSECs also participate in the capture and processing of foreign antigens, in addition to the elimination of undesirable macromolecules from the blood by transporting them to hepatocytes (6,7). LSECs represent a barrier separating the liver and the blood and could be exploited by viruses such as HCV in order to gain access to hepatocytes (16,33,34) . We recommend the two following reviews for an overview of the physiological importance of LSECs in viral infections of the liver (6,7) .
In this chapter, we will provide general protocols to study the molecular interactions between viruses and DC-SIGN or L-SIGN and to investigate the functions of these two lectins in viral infection and transmission. We will first describe the methods used to obtain human dermal-like DCs and cells expressing DC-SIGN and L-SIGN. We next present protocols to produce soluble viral envelope proteins using a Semliki forest virus (SFV) vector (35-37) and to study the molecular basis of viral capture and internalization by DC-SIGN or L-SIGN. Finally, we will describe methods required to assess the contribution of DC-SIGN and L-SIGN to cis-and trans-infection. 
1. Human peripheral blood mononuclear cells (PBMC) are isolated from healthy donors by density gradient centrifugation through Ficoll-Paque Plus (Amersham Biosciences). 2. Lysis buffer: dissolve 8.3 g of NH 4 Cl and 1 g of NaHCO 3 in 1 L of water complemented with 1 mL of EDTA (100 mM, pH ~8.0). Lysis buffer must be autoclaved and can be stored at 4°C for several months. 
1. Soluble viral glycoproteins are produced using the SFV vector in BHK mammalian cells. SFV shuts off the cellular translation machinery and so transduced cells produce only the viral protein of interest, which is secreted and accumulates in the supernatant of infected cells. This allows the production of large amounts of soluble proteins with a high degree of purity. The SFV expression vector was originally described in reviews (35-37). Briefly, the SFV genome is a singlestranded, positive RNA which encodes both structural and nonstructural viral proteins. A signal sequence in the 5' RNA domain permits specific viral genome packaging. The SFV vector is made of two cDNAs (pSFV-∆env and pSFV-helper2) (Fig. 2) . The first codes for the SFV RNA genome in which the sequence corresponding to structural proteins is replaced by the soluble glycoprotein of interest (pSFV-∆env from Calbiochem and Sigma respectively. These molecules specifically inhibit α-manosidase I and II and permit the production of proteins with mannose carbohydrate residues in mammalian cells (Fig. 3) .
Carbohydrate maturation in mammalian cells. Proteins with NXS or NXT sites that pass through the endoplasmic reticulum can be potentially glycosylated. Glycoproteins are sensitive to EndoH until they are modified by α1,2-mannosidase II. Swainsonine and 1-deoxymannojirimycin hydrochloride (DMJ) block maturation steps of glycoprotein carbohydrates. These drugs permit to produce mannosylated glycoproteins in mammalian cells. 1, internal tri-mannose branch recognized by DC-SIGN; 2, external tri-mannose branch; ER, endoplasmic reticulum; GDP, guanosine biphosphate; UDP, uridine biphosphate.
11. Glycoproteins are concentrated through columns (Biomax, Millipore) with an appropriate molecular weight cut-off for the particular protein being produced. 12. Endoglycosidase H (EndoH) and peptide: N-glycosidase F (PNGase F) are purchased from Roche Applied Science and New England Biolabs respectively. The buffer provided with the PNGase F is used to treat the glycoproteins either with EndoH or with PNGase F. PNGase F is able to digest all N-glycans whereas EndoH digests only N-glycans unmodified by α-manosidase I and II (Figs. 3 and  4A ). Aldrich. Mannan stock solution is dissolved in water (5 mg/mL -1 ) and can be stored at 4°C for several months. 4. Scintillation solution is obtained from Wallac (optiphase supermix solution).
Soluble HIV gp120 is produced in BHK cells in the presence or absence of mannosidase inhibitors (1 mM DMJ and 5 µM swainsonine) (HIV gp120 DMJ or HIV gp120, respectively) as described under Subheading 3.2.3. Secreted proteins were subjected to digestion with EndoH or PNGase F and analyzed by Western blot. Only HIV g120 DMJ is sensitive to EndoH confirming its high mannosylated glycosylation. (B) 35 S-labeled HIV gp120 DMJ and HIV gp120 (20 nM) are bound to Raji and Raji-DC-SIGN cells for 2 h at 4°C. Cells are washed three times before measuring cell-associated radioactivity. Note that only HIV gp120 DMJ , which carries only mannosylated N-glycans, binds to DC-SIGN.
Wild-type viruses or viral particles carrying the reporter genes firefly luciferase (Luc) or green fluorescent protein (GFP) can both be used to study DC-SIGN and L-SIGN-mediated cis-infection and trans-enhancement of target cell infection. These viruses can be prepared using standard methods, and an example is described in Note 7. 2. At 6 h postinfection, cells are washed five times in serum-free GMEM to eliminate BSA and then maintained in serum-free GMEM for protein production. For the production of radiolabeled protein, cells are starved for 1 h in serum-and methionine/cysteine-free DMEM prior to addition of 100 µCi/mL -1 [35S] cysteine and methionine. 3. Synthesis of proteins is continued up to 24 h postinfection in the presence or absence of α-mannosidase I and II inhibitors DMJ (1 mM) and swainsonine (5 µM) . This results in the production of soluble glycoproteins carrying only mannose N-glycans (Figs. 3 and 4) . Supernatants are clarified by centrifugation (10 min, 850 g at room temperature) and concentrated (see Subheading 2.3., item 11). 4. The glycosylation pattern of the proteins produced can be verified by treatment with endoglycosidase H (endoH; 2 mU, Roche) or Peptide: N-Glycosidase F (PNGAse F; 1000 U, Biolabs). An example is given in Fig. 4A . Only HIV gp120 bearing mannose N-glycans (HIV gp120 DMJ produced in presence of DMJ and swainsonine) is sensitive to EndoH. In contrast, PNGaseF treatment allows complete de-glycosylation of HIV gp120 produced either in the presence or absence of inhibitors. The molecular weight of the exclusively mannosylated HIV gp120 is inferior to that of gp120 produced in the absence of mannosidase inhibitors. This is a result of the lower molecular weight of mannose residues compared to complex glycosylation. 
DC-SIGN-or L-SIGN-expressing cells and their parental counterpart (10 5 cells) are exposed to viral particles for 2 h at 37°C at varying MOI in FSC-free medium supplemented with 1% penicillin/streptomycin, pH approx 7.5. Cells are washed three times with complete medium to remove excess virus and incubated at 37°C. Viral replication is evaluated 2 to 3 d later, depending on the readout used (see Note 7). free medium supplemented with 1% penicillin/streptomycin, pH approx 7.5 (see Note 8). Cells exposed to virus are washed extensively with cold PBS to remove unbound viral particles, resuspended in 100 µL of complete medium and co-cultured with an equal number of target cells in 96-well plates. Viral transmission is quantified two or 3 d later.
DC-SIGN-or L-SIGN-expressing cells and their parental counterpart (10 5 cells) are incubated with viral particles at a low MOI (insufficient to directly infect target cells) for 2 h at 37°C and immediately co-cultured with target cells without washing. As an important control, viral particles are incubated with medium alone and 2 h later, transferred to target cells.
This assay permits to determine if viral particles bound to DC-SIGN or L-SIGN remain infectious for several days. The protocol is similar to that described under Subheading 3.4.2. or 3.4.3. except that co-culture with target cells is started several days after exposure of cells expressing DC-SIGN or L-SIGN to virus. Multiple points can be tested to establish the kinetic of the virus protection by cells expressing these lectins.
1. Human primary LSECs expressing L-SIGN are difficult to obtain. For this reason, we use cell lines expressing L-SIGN and we describe only the generation of monocyte-derived DCs that constitutively express DC-SIGN. 2. Alternatively, the SFV particle production can be bypassed. The electroporation of BHK cells with pSFV-∆ env permits to directly produce the recombinant protein coded by the modified SFV plasmid. This alternative protocol is similar to Subheading 3.2.1., steps 1 and 2, except that the amount of pSFV-∆ env RNA transfected is doubled. The quantity of RNA electroporated may require adjustment depending on the gene of interest. The maximum volume of RNA we have electroporated is 150 µL. The next step of the recombinant protein production with this alternative protocol continues at Subheading 3.2.3., step 2. The advantage of production of viral particles is that cells can be infected at equal MOI and hence the quantity of protein produced is more reproducible than direct electroporation with pSFV-∆env RNA. 3. The quality of transcribed RNA is verified by agarose gel (1%) electrophoresis.
Transcribed RNAs can be stored at -20°C before electroporation. 4. Defective SFV particles are used at a MOI varying from 25 to 100. Some adjustments may be required depending on the gene of interest. The number of cells infected correlates with the quantity of protein produced and the efficiency of protein secreted. For information, the best production we observed is for HCV envelope protein E2 (10). We produce approx 20 µg of purified protein per 10 7 cells infected at a MOI of 50. 5. DC-SIGN-or L-SIGN-mediated viral internalization can also be investigated by confocal microscopy using purified viral glycoproteins (see Note 6) or whole virions. HeLa cells expressing DC-SIGN or L-SIGN (5 ϫ 10 4 cells) are seeded on coverslips. The following day, cells are incubated with viral glycoproteins (5 µg/mL -1 ) or wild-type viruses diluted buffer B for 1 h at 4°C. Cells are washed three times with ice cold PBS to remove unbound material and shifted to 37°C for different times to allow endocytosis. Cells are fixed with 3.2% paraformaldehyde for 15 min, washed twice in PBS and treated with PBS 0.2 M glycine for 10 min. Cells are then incubated with 500 µL of PBS containing 0.05% saponin and 0.2% BSA for 30 min. Both DC-SIGN and viral protein trafficking can be followed by using specific mAb. Cells are mounted in Vectashield containing DAPI (Vector Laboratories) and imaged on a Zeiss microscope using a Plan Apochromat ϫ63/1.4 oil immersion objective. 6. Optionally, the soluble glycoprotein can be purified by immuno-affinity when a tag peptide is introduced into its sequence. Several commercial tag peptides are available such as the Flag tag peptide which we used (10). 7. Read-out is specific for the virus studied. We generally use wild-type viruses or viral particles carrying a reporter gene (GFP or Luc). For example, for HIV or HCV, we use single-cycle pseudotyped viral particles that are generated by co-transfecting 293T cells with an HIV-1 NL ∆Env Luc or GFP (a HIV provirus lacking the Env gene and carrying the Luc or GFP gene in the place of Nef) and a cDNA plasmid encoding either HIV or HCV envelope proteins. Viral replication is evaluated by measuring luciferase activity in cell lysates ( 
While this chapter was in press, four new studies were published concerning the interactions between DC-SIGN and viruses, and the role of DC-SIGN in HIV dissemination (40-43).