Again, DC-SIGN and WGA retained HCMV particles, suggesting the presence of high-mannose sugars on viral particles. previously showed that gB is a ligand for the C-type lectin dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) contributing to HCMV attachment to and infection of DC-SIGN-expressing cells. However, the features of the SLx-2119 (KD025) DC-SIGN/gB interaction remain unclear. To address this point, the role of glycans on gB and the consequences of mutagenesis and antibody-mediated blockades on both partners were examined in this study. Results We identified DC-SIGN amino acid residues involved in this interaction through an extensive mutagenesis study. We also showed the importance of high-mannose values below or equal to .05 were considered significant. Additional materials and methods are available in Supplementary Materials. RESULTS Dendritic Cell-Specific Intercellular Adhesion Molecule-3-Grabbing Nonintegrin Binds to Glycoprotein B Through Its Carbohydrate Recognition Domain Although HCMV gB is known as a DC-SIGN ligand, it is not clear whether this interaction is restricted to the DC-SIGN CRD [14]. To that purpose, HEK293T cells were modified to express wild-type (WT) DC-SIGN (AA 1C404; UnitProtKB, SLx-2119 (KD025) “type”:”entrez-protein”,”attrs”:”text”:”Q9NNX6″,”term_id”:”46396012″,”term_text”:”Q9NNX6″Q9NNX6) or 2 deletion mutants, respectively, lacking neck repeats (AA 1C80 in frame with AA 253C404, called neck) or the CRD (AA 1C252, called CRD) in fusion with the enhanced green fluorescent protein (eGFP) [29]. All cells expressed comparable eGFP levels and DC-SIGN cell surface expression as well (Figure 1A). We showed that gB interacts with CRD-containing DC-SIGN molecules and does not require the neck repeats (Figure 1A and ?andBB). Open in a separate window Figure 1. Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) binds the glycoprotein B (gB) through its carbohydrate recognition domain. (A) Histograms showing DC-SIGN expression of wild-type (WT) DC-SIGN or deletion mutants lacking the DC-SIGN neck repeat (neck) or the carbohydrate-recognition domain ([CRD] CRD) regions SLx-2119 (KD025) fused to enhanced green fluorescent protein (eGFP). The eGFP allowed a rapid quantitation of the DC-SIGN expression SLx-2119 (KD025) level on stably transfected HEK293T (left panels), except for the pEGFP-transfected cells (first line). The 2 2 centered columns represent extracellular staining of DC-SIGN with an antineck (clone H-200) and an anti-CRD (clone 1B10) antibody, respectively. The ability of DC-SIGN variants to bind recombinant biotinylated human cytomegalovirus (HCMV) gB is represented in right panels. Gray histograms display nontransfected HEK293T cell fluorescence background. (B) Quantitative measurements of the binding of recombinant biotinylated HCMV gB (2 g/mL) onto WT DC-SIGN or neck- and CRD-expressing cells compared with a control cell line (pEGFP). Biotinylated HCMV gB was revealed with 1 g/mL antigen-presenting cell-conjugated streptavidin. Values are expressed as mean fluorescence intensities (n = 4; *, .05; one-way analysis of variance [ANOVA] with multiple comparison tests). (C) Histograms showing the binding of recombinant Alexa Fluor SLx-2119 (KD025) 647-conjugated HCMV gB (4 g/mL, mean fluorescence intensity [MFI]) on HEK293T cell lines expressing WT or mutated DC-SIGN on their surface. Values indicated for each histogram represent MFI. These results are representative of 3 independent experiments. (D) Quantitative results showing the behavior of mutated DC-SIGN compared with the WT form towards the binding of recombinant Alexa Fluor 647-conjugated HCMV gB (n = 3). Statistically significant results were marked by an asterisk (*, .05; one-way ANOVA with multiple comparison tests). Then, we sought to identify CRD AA involved in this interaction. We hypothesized that AA taking part to the calcium ion coordination or sugar binding could be detrimental [20, 30]. Single-point mutants were generated and further expressed in HEK293T cells. Antineck staining showed similar DC-SIGN expression across all cell lines (Supplementary Figure 1). Their ability to bind gB was then assessed by flow cytometry (Figure 1C). E347, N349, E354, N365, and D366 form the Igfbp1 calcium binding site 2 and enable contact with high-mannose sugars as well [30, 31]. Expectedly, mutations at these positions precluded interaction with gB (Figure 1D). Similarly, mutants D320A, E324A, N350A, and D355A lost their ability to optimally bind gB, assuming that it was likely due to substantial fold changes in the calcium binding site 1 as proposed for HIV-1 gp120 [32]. Here, F313Y, Q323E, and K368A DC-SIGN mutations were ineffective (Figure 1D). Moreover, we confirmed that the E354Q within site 2 broke the interaction [33]. The V351 residue was shown to discriminate between endogenous and pathogen-derived ligands such as ICAM-3 and HIV-1 gp120 or hepatitis C virus E1/E2, respectively [32, 34, 35]. In this study, we analyzed 2 mutations, ie, V351G and V351T. The V351G mutant lost its binding capacity to gB, suggesting that this AA is as important as its human herpesvirus (HHV)-8 counterpart and ICAM-3 [36]. It is interesting to note that a methyl group substitution of the V351 by a hydroxyl.
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