Dectin-1 is a natural killer (NK)-cell-receptor-like C-type lectin that is thought to be involved in innate immune responses to fungal pathogens. This transmembrane signalling receptor mediates various cellular functions, from fungal binding, uptake and killing, to inducing the production of cytokines and chemokines. These activities could influence the resultant immune response and can, in certain circumstances, lead to autoimmunity and disease. As I discuss here, understanding the molecular mechanisms behind these functions has revealed new concepts, including collaborative signalling with the Toll-like receptors (TLRs) and the use of spleen tyrosine kinase (SYK), that have implications for the role of other non-TLR pattern-recognition receptors in immunity.

Dectin-1: a signalling non-TLR pattern-recognition receptor.

Fungal β-Glucans and Mammalian Immunity

The recognition of conserved microbial structures is a key aspect of metazoan immunity, and beta-glucans are emerging as a major target for the recognition of fungal pathogens. A number of receptors for these carbohydrates have been identified, which upon recognition, trigger a variety of immune responses. In contrast to many other systems, there is little apparent conservation in these mechanisms between vertebrates and invertebrates. In this review, we will highlight all the known receptors for beta-glucans and will discuss the various immune responses they can initiate, with reference to fungal infection, in both vertebrates and invertebrates.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1462-5822.2005.00505.x

Immune recognition of fungal beta-glucans.

Glucans are (1-3)-beta-D-linked polymers of glucose that are produced as fungal cell wall constituents and are also released into the extracellular milieu. Glucans modulate immune function via macrophage participation. The first step in macrophage activation by (1-3)-beta-D-glucans is thought to be the binding of the polymer to specific macrophage receptors. We examined the binding/uptake of a variety of water soluble (1-3)-beta-D-glucans and control polymers with different physicochemical properties to investigate the relationship between polymer structure and receptor binding in the CR3- human promonocytic cell line, U937. We observed that the U937 receptors were specific for (1-->3)-beta-D-glucan binding, since mannan, dextran, or barley glucan did not bind. Scleroglucan exhibited the highest binding affinity with an IC(50)of 23 nM, three orders of magnitude greater than the other (1-->3)-beta-D-glucan polymers examined. The rank order competitive binding affinities for the glucan polymers were scleroglucan>>>schizophyllan > laminarin > glucan phosphate > glucan sulfate. Scleroglucan also exhibited a triple helical solution structure (nu = 1.82, beta = 0.8). There were two different binding/uptake sites on U937 cells. Glucan phosphate and schizophyllan interacted nonselectively with the two sites. Scleroglucan and glucan sulfate interacted preferentially with one site, while laminarin interacted preferentially with the other site. These data indicate that U937 cells have at least two non-CR3 receptor(s) which specifically interact with (1-->3)-beta-D-glucans and that the triple helical solution conformation, molecular weight and charge of the glucan polymer may be important determinants in receptor ligand interaction.

The influence of glucan polymer structure and solution conformation on binding to (1→3)-β-d-glucan receptors in a human monocyte-like cell line

http://www.vitawithimmunity.com/wp-content/uploads/2014/11/ZimmermanJBiolChem1998.pdf

A novel carbohydrate-glycosphingolipid interaction between a beta-(1-3)-glucan immunomodulator, PGG-glucan, and lactosylceramide of human leukocytes.

PGG-Glucan (Betafectin) is a novel soluble beta-glucan immunomodulator that enhances leukocyte microbicidal activities without inducing inflammatory cytokines. Although several different receptors for soluble and particulate beta-glucans have been described, the signal transduction pathway(s) used by soluble beta-glucans have not been elucidated. We report that in a murine monocytic cell line (BMC2.3) PGG-Glucan activates nuclear factor-kappaB (NF-kappaB)-like and NF-interleukin-6 (IL-6)-like transcription factors. Electrophoretic mobility shift assays showed that PGG-Glucan activation of the factors is time- and concentration-dependent. The NF-kappaB-like complex includes subunit p65 (rel-A) as one of its components, but apparently not p50 (kappaB1), p52 (kappaB2), p68 (rel-B), or p75 (C-rel) family members. The NF-IL-6-like complex contains subunit C/EBP-beta (NF-IL-6alpha) as one of its components, but apparently not C/EBP-alpha or C/EBP-delta (NF-IL-6beta). As expected, lipopolysaccharide (LPS) activated p65/p50 NF-kappaB and C/EBP-beta NF-IL-6 complexes, increased the nuclear titer of p65 and p50 antigens, and increased cytokine (IL-1beta, tumor necrosis factor alpha) mRNA production. In contrast, PGG-Glucan increased the nuclear titer of p65, but apparently not p50, and did not induce cytokine mRNA production. These data demonstrate that PGG-Glucan utilizes signal transduction pathways different from those used by LPS. The data suggest that activation of the PGG-Glucan-stimulated factors is not sufficient to stimulate cytokine mRNA transcription.

https://jlb.onlinelibrary.wiley.com/doi/pdf/10.1002/jlb.62.6.865

PGG-Glucan activates NF-kappaB-like and NF-IL-6-like transcription factor complexes in a murine monocytic cell line.

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PGG-Glucan activates NF-kappaB-like and NF-IL-6-like transcription factor complexes in a murine monocytic cell line.