DC-SIGN, a dendritic cell surface C-type lectin receptor, has been proposed to enhance HIV-1 infection of T cells in trans. The physiological role of DC-SIGN was thought to bind adhesion molecules, ICAM-2 and ICAM-3, to initiate a dendritic cell and T cell contact to facilitate the T cell activation and priming by dendritic cells. We expressed various extracellular truncations of DC-SIGN and DC-SIGNR as recombinant proteins and showed that 1) DC-SIGN/R forms a tetramer through the extracellular repeat region; 2) the receptor-gp120 binding affinity depends on the oligomerization of the receptor; 3) DC-SIGN/R recognition of gp120 is pH dependent such that the affinity reduced drastically under the endosomal pH condition; and 4) DC-SIGN recognizes ICAM-3 with much lower affinity than it does gp120. The structural study of DC-SIGN/R has resulted in a 1.5 ? resolution crystal structure for DC-SIGN R8 and a molecular model for the tetrameric extracellular DC-SIGN/R. Based on the model, a prediction scheme was developed to evaluate potential ligands for DC-SIGN. A ligand search among the sequences of the human genome and genomes of human viruses revealed a diverse list of potential host and viral ligands of the receptor. The host ligands include not only adhesion molecules but also a family of mucin-like proteins, the extracellular matrix proteins and some tumor markers. Most of the enveloped RNA viruses appear to have ligands for the receptor. Using a bovine submaxillary mucin as the example, we showed both in solution and on the cell surface that DC-SIGN and DC-SIGNR recognize the mucin family of proteins with sub-micromolar affinity. The receptor binding to mucin competes with the receptor binding to HIV-1 gp120. Using FACS analysis and confocal microscopy, we showed that DC-SIGN preferentially recognizes MUC1-transfected cells as well as an MUC1-expressing cancer cell line Colo205. The binding of DC-SIGN/R to Colo205 cells can be blocked with mannan in a dose-dependent manner. Thus, we propose that DC-SIGN functions as an antigen- capturing receptor for both viral glycoprotein and cellular tumor and mucin-associated antigens. The receptor internalization would lead to the degradation and antigen presentation.? ? SIGLECs (sialic-acid Ig-like binding lectins) are a family of adhesion and signaling receptors that specifically recognize sialylated carbohydrate moieties. The potential involvement of members of SIGLEC family receptors in HIV pathogenesis remain to be determined. To understand the molecular basis for the sialic acid-dependent adhesion implemented by SIGLECs and to get an insight into receptor specificity, structural studies have been carried out using two Ig-like N-terminal domains of SIGLEC-5. X-ray structure solution using molecular replacement with phased translation function uncovered unparalleled features not seen in other one-domain structures of related SIGLECs, including unusual conformation of variable loop C-C? of the ligand-binding domain and a unique interdomain disulfide bond. To get an insight into receptor specificity, we have crystallized several receptor-ligand complexes using sialylated oligosaccharides commonly found at cell surfaces and in the extracellular milieu. In combination with binding assays, these structural studies have given us an insight into what governs ligand recognition and receptor specificity in SIGLEC family of lectins.
