Whole Glucan Particles (WGP) - Dectin-1 Ligand

β-Glucans

β-Glucans are carbohydrates consisting of a backbone of glucose residues joined by β-(1➝3) linkages with β(1➝6) linked glucose side-chain residues. These polysaccharides are major cell wall structural components in fungi, including the yeasts Saccharomyces cerevisiae and Candida albicans.  They are also found in plants and some bacteria. Depending on the source, β-glucans vary in the type of linkage, the degree of branching, molecular weight and tertiary structure. They are not synthesized by animals and thus are recognized by the innate immune system as pathogen-associated molecular patterns (PAMPs). This recognition is mediated by pattern recognition receptors (PRRs) and, among them, Dectin-1 has emerged as the primary receptor [1]. 

C-type lectin receptor Dectin-1

C-type lectin receptors (CLRs) comprise a large family of receptors that bind to carbohydrates in a calcium-dependent manner. CLRs include Dectin-1, Mincle, DC-SIGN, DC-SIGNR and MBL. These CLRs share one or more carbohydrate-recognition domains (CRD). They are involved in fungal recognition and the modulation of the innate immune response.
 

Dectin-1 is one member of the CLR family and plays an important role in antifungal innate immunity. It is expressed primarily by cells of myeloid origin, including macrophages, dendritic cells and neutrophils. In human and mice, Dectin-1 is alternatively spliced into 2 major isoforms: a full-length A isoform (Dectin-1a) and a ‘stalkless’ B isoform (Dectin-1b). Dectin-1b does not induce the same response to soluble and particulate β-glucans [2].

Upon binding to its ligand, Dectin-1 triggers phagocytosis and activation of Src and Syk kinases, through its ITAM-like motif. Syk, in turn, induces the CARD9-Bcl10-Malt1 complex leading to the production of reactive oxygen species (ROS), activation of NFκB and subsequent secretion of proinflammatory cytokines [3]. ROS have a direct microbicidal role in the phagosome but also can affect IL-1β secretion by activating the NLRP3 inflammasome, which in turn activates caspase-1 and permits processing of pro-IL-1β [4]. Dectin-1 signaling has been shown to collaborate with Toll-like receptor 2 (TLR2) signaling to enhance the responses triggered by each receptor [5].

Indeed, most published studies focus exclusively on the responses of murine (m)Dectin-1 to β-glucans, which may differ considerably from the response mediated by human (h)Dectin-1. In addition to discrepancies between mDectin-1 and hDectin-1, many inconsistencies exist due to the use of very different, and often impure β-glucans, and the analysis of different cell types and model systems. Recognition of β-glucans by the immune system appears very complex and further studies are required to fully understand the immunomodulating properties of these molecules.

References:

1. Tsoni SV & Brown GD. 2008. β-Glucans and dectin-1. Ann N Y Acad Sci. 1143:45-60.
2. Heinsbroek S.E. et al., 2006. C-type Lectin Receptor: Old Friend and New Player. Med Chem. 2017;13(6):536-543. 
3. Goodridge HS. et al., 2011. Activation of the innate immune receptor Dectin-1 upon formation of a 'phagocytic synapse'. Nature. 472(7344):471-5.
4. Kankkunen P. et al., 2010. (1,3)-b-glucans activate both Dectin-1 and NLRP3 inflammasome in human macrophages. J Immunol. 184;6335-6342. 
5. Gantner BN. et al., 2003. Collaborative induction of inflammatory responses by dectin-1 and Toll-like receptor 2. J Exp Med. 197(9):1107-17.