Mechanism of pathogen recognition by human dectin-2.

Dectin-2, a C-type lectin on macrophages and other cells of the innate immune system, functions in response to pathogens, particularly fungi. The carbohydrate-recognition domain (CRD) in dectin-2 is linked to a transmembrane sequence that interacts with the common Fc receptor γ subunit to initiate immune signaling. The molecular mechanism by which dectin-2 selectively binds to pathogens has been investigated by characterizing the CRD expressed in a bacterial system. Competition binding studies indicated that the CRD binds to monosaccharides with modest affinity and that affinity was greatly enhanced for mannose-linked α1-2 or α1-4 to a second mannose residue. Glycan array analysis confirmed selective binding of the CRD to glycans that contain Manα1-2Man epitopes. Crystals of the CRD in complex with a mammalian-type high-mannose Man9GlcNAc2 oligosaccharide exhibited interaction with Manα1-2Man on two different termini of the glycan, with the reducing-end mannose residue ligated to Ca2+ in a primary binding site and the nonreducing terminal mannose residue occupying an adjacent secondary site. Comparison of the binding sites in DC-SIGN and langerin, two other pathogen-binding receptors of the innate immune system, revealed why these two binding sites accommodate only terminal Manα1-2Man structures, whereas dectin-2 can bind Manα1-2Man in internal positions in mannans and other polysaccharides. The specificity and geometry of the dectin-2-binding site provide the molecular mechanism for binding of dectin-2 to fungal mannans and also to bacterial lipopolysaccharides, capsular polysaccharides, and lipoarabinomannans that contain the Manα1-2Man disaccharide unit.

Defining the conformation of human mincle that interacts with mycobacterial trehalose dimycolate.

Trehalose dimycolate, an unusual glycolipid in the outer membrane of Mycobacterium tuberculosis, stimulates macrophages by binding to the macrophage receptor mincle. This stimulation plays an important role both in infection by mycobacteria and in the use of derivatives of mycobacteria as adjuvants to enhance the immune response. The mechanism of trehalose dimycolate binding to the C-type carbohydrate-recognition domain in human mincle has been investigated using a series of synthetic analogs of trehalose dimycolate and site-directed mutagenesis of the human protein. The results support a mechanism of binding acylated trehalose derivatives to human mincle that is very similar to the mechanism of binding to bovine mincle, in which one glucose residue in the trehalose headgroup of the glycolipid is ligated to the principle Ca(2+)-binding site in the carbohydrate-recognition domain, with specificity for the disaccharide resulting from interactions with the second glucose residue. Acyl chains attached to the 6-OH groups of trehalose enhance affinity, with the affinity dependent on the length of the acyl chains and the presence of a hydrophobic groove adjacent to the sugar-binding sites. The results indicate that the available crystal structure of the carbohydrate-recognition domain of human mincle is unlikely to be in a fully active conformation. Instead, the ligand-binding conformation probably resembles closely the structure observed for bovine mincle in complex with trehalose. These studies provide a basis for targeting human mincle as a means of inhibiting interactions with mycobacteria and as an approach to harnessing the ability of mincle to stimulate the immune response.