Background: DC-SIGN, a novel dendritic cell (DC) cell-surface C-type lectin, plays a key role in the dissemination of HIV-1. DC-SIGN captures HIV-1 at mucosal sites of entry through binding of the gp120 envelope glycoprotein of HIV-1, facilitating the transport to lymphoid tissues, where DC-SIGN on the DC can transmit HIV-1 to T-cells in trans. Detailed information on the interaction of DC-SIGN with gp120 is crucial for the development of specific components that can inhibit gp120 binding to DC-SIGN and will set the stage for novel therapies to combat HIV-1.

Methods: We recently identified different binding sites in DC-SIGN for gp120 and its immunological ligand, ICAM-3. Mutational analysis of the ligand-binding site of DC-SIGN was performed to further delineate the differences in binding of ICAM-3 and gp120. DC-SIGN binding assays were used to evaluate the interaction of DC-SIGN with gp120 and glycan mutants of gp120, and to analyze the blocking ability of a panel of anti-gp120 antibodies (Abs). CD4 binding to gp120 was investigated in the presence of DC-SIGN.

Results: Mutational analysis of the region around Val³⁵¹ in the lectin domain of DC-SIGN, demonstrated that binding of gp120 was not altered while ICAM-3 binding was absent. The observed simultaneous binding of CD4 and DC-SIGN to gp120, together with the demonstration that neutralizing anti-gp120 Abs against the CD4-binding site were not able to block the gp120-DC-SIGN interaction, indicated that DC-SIGN and CD4 have distinct binding sites in gp120. Moreover, an increased interaction of CD4 to gp120 was observed in the presence of DC-SIGN. We also showed that several glycan mutants of gp120 bind to DC-SIGN. Additionally, the 2G12 Ab, recognizing a carbohydrate-dependent epitope on gp120, was not able to block binding of gp120 to DC-SIGN. This corroborates our earlier observation that the interaction of gp120 with DC-SIGN does not solely depend on glycosylation.

Conclusions: Crucial amino acids in the ligand binding domain were identified, demonstrating that the gp120-binding site is different from that of ICAM-3. DC-SIGN does not interact with gp120 at its CD4-binding site. Moreover, DC-SIGN binding to gp120 induced conformational changes promoting CD4 binding, conceivably enhancing HIV-1 envelope-mediated cell fusion. In conclusion, our data elucidate on the interaction of DC-SIGN with gp120, which forms the basis for the design of drugs that inhibit this interaction.