Carbohydrate-mediated recognition systems in innate immunity

There is growing interest in carbohydrate-recognizing receptors of the innate immune system. Among them are members of the C-type lectin family, which include the collectins and the selectins and which operate by ligating exogenous (microbial) or endogenous carbohydrates. De novo assignments of the sequences of ligands for carbohydrate-recognizing receptors are among the most challenging topics in cell biology. This is because of the heterogeneity of oligosaccharides on proteins and lipids, and their availability only in limited amounts. To address the need for a microprocedure for direct binding studies with oligosaccharides derived from glycoproteins, we introduced the neoglycolipid technology for generating solid phase oligosaccharide probes for binding experiments. The technology has enabled assignments of unsuspected oligosaccharide ligands for the selectins and given valuable insights into those for the collectins. The ligands so far identified appear not to be unique for a given receptor system; there are considerable cross-reactions. Specificity can be created, however, through different modes of oligosaccharide presentation on macromolecular carriers, or the expression of a particular oligosaccharide sequence on a selected cell type in a given body compartment, and the regulated expression of the receptor protein at the desired location. The existence of unique ligand structures is not ruled out, however. Co-ligation of a receptor may also occur to a second carbohydrate or even to a non-carbohydrate ligand to create a unique assembly. A further group of C-type lectin-like proteins occurs on natural killer (NK) cells and NK T cells, and is associated with activation or inhibition of the cell effector functions. An important challenge is to determine whether carbohydrates are among physiological ligands for this important group of receptors.