Dectin-1 and Dectin-2 in innate immunity against fungi

Dectin-1 and Dectin-2 are type II transmembrane proteins of the C-type lectin family with single carbohydrate recognition domains (CRDs) in their extracellular region. They are expressed mainly in dendritic cells and macrophages. Dectin-1 recognizes beta-glucans with its CRD and transduces signals through its immunoreceptor tyrosine-based activation motif (ITAM)-like motif in the cytoplasmic domain, whereas Dectin-2 recognizes alpha-mannans and transduces its signal through association with the ITAM-containing Fc receptor gamma chain. Upon ligand binding, spleen tyrosine kinase is recruited to the ITAM and activates the caspase recruitment domain family member 9 (CARD9)-nuclear factor-kappa B axis, resulting in the activation of various genes including those encoding pro-inflammatory cytokines. Both beta-glucans and alpha-mannans are major cell wall components of fungi including Candida albicans and Pneumocystis carinii. Recently, it was reported that Dectin-1 is important in protection against P. carinii by inducing reactive oxygen species, whereas both Dectin-1 and Dectin-2 play important roles in defense against C. albicans by preferentially inducing T(h)17 cell differentiation. In this review, we briefly revisit the structures, ligands, signal transduction and functional roles of Dectin-1 and Dectin-2 in host defense against fungal infection.