Human C-type lectin domain family 4, member C (CLEC4C/BDCA-2/CD303) is a receptor for asialo-galactosyl-oligosaccharides.

Plasmacytoid dendritic cells are specialized in the production of type I interferon (type I IFN), which promotes antiviral and antitumor responses, as well as autoimmune disorders. Activation of type I IFN secretion depends on the pattern recognition receptors TLR7 and TLR9, which sense microbial RNA and DNA, respectively. Type I IFN production is modulated by several receptors, including the type II C-type lectin domain family 4, member C (CLEC4C). The natural ligand of CLEC4C is unknown. To identify it, here we probed a glycan array with a soluble form of the CLEC4C ectodomain. We found that CLEC4C recognizes complex type sugars with terminal galactose. Importantly, soluble CLEC4C bound peripheral blood leukocytes and tumor cells that express glycans with galactose residues at the non-reducing ends. The positive and negative modulation of galactose residues on cell membranes was paralleled by the regulation of type I IFN secretion by plasmacytoid dendritic cells in co-culture experiments in vitro. These results suggest that the modulation in the expression of non-sialylated oligosaccharides by invading pathogens or transformed cells may affect type I IFN response and immune surveillance.

Patterns of nonclassical MHC antigen presentation.

The identification of pattern-recognition receptors that selectively respond to evolutionarily conserved chemical (often pathogen-derived) moieties has provided key insight into how innate immune cells facilitate rapid and relatively specific antimicrobial immune activity. In contrast, relatively slower adaptive immune responses rely on T cell clonal expansion that develops in response to variable peptides bound to the groove of classical major histocompatibility complex (MHC) proteins. For certain nonclassical 'MHC-like' class Ib proteins, such as H2-M3 and CD1d, their respective binding grooves seem to have been adapted to present to T cells unique molecular patterns analogous to those involved in innate signaling. Here we propose that another MHC class Ib protein, MR1, which is required for the gut flora-dependent development of mucosa-associated invariant T cells, presents either a microbe-produced or a microbe-induced pattern.

Structural determinants of chemokine binding by an Ectromelia virus-encoded decoy receptor.

The EVM1 protein encoded by Ectromelia virus is a member of a highly conserved family of poxvirus chemokine binding proteins that interfere with host immune surveillance processes. EVM1 is abundantly expressed early during mousepox infection and is able to selectively bind CC chemokines and inhibit their interactions with host receptors. Here, we characterize the interaction between EVM1 and the human and murine chemokines CCL3 (MIP-1alpha), CCL2 (MCP-1), and CCL5 (RANTES). Each of these CC chemokines binds EVM1 with 1:1 stoichiometry and equilibrium dissociation constants ranging from 29 pM to 20 nM. The interactions are characterized by rapid-association kinetics between acidic EVM1 and generally basic chemokines with half-lives enduring up to 30 min. The 2.6-A crystal structure of EVM1 reveals a globular beta sandwich with a large, sequence-conserved surface patch encircled by acidic residues on one face of the protein. To determine whether this conserved cluster of residues is involved in chemokine engagement, a structure-based mutational analysis of EVM1 was employed. Mapping of the mutational results onto the surface of EVM1 reveals that a cluster of five residues (I173, S171, S134, N136, and Y69) emanating from one beta sheet is critical for CCL2 and CCL3 sequestration. Additionally, we find that the extended beta2-beta4 loop flanking this conserved cluster is also essential for high-affinity, lasting interactions with chemokines. This analysis provides insight into the mechanism of CC-chemokine inhibition employed by the poxvirus family of chemokine decoy receptors.