Particulate β-glucans synergistically activate TLR4 and Dectin-1 in human dendritic cells

The major receptor for β(1-3)-glucans on immune cells is considered to be Dectin-1 receptor. Particulate β-glucans induce stronger immune responses than soluble β-glucans by clustering of Dectin-1 receptors. Here, it was hypothesized that activation of other pattern recognition receptors such as Toll-like receptor 4 (TLR4) can also contribute to enhanced activity of immune cells after exposure to particulate β-glucans.

To test this hypothesis, reporter cell lines were designed expressing TLR4 with either Dectin-1A or Dectin-1B, that is, one of the two transcript variants of human Dectin-1 receptors. Enhanced NF-κB activation was observed after stimulation with particulate β-glucans in both Dectin-1A-TLR4 and the Dectin-1B-TLR4 cell lines. This was different with soluble β-glucans, which enhanced activation in Dectin-1A-TLR4 cell lines but not in Dectin-1B-TLR4 cells. The synergistic activation of TLR4 and Dectin-1 by particulate β-glucans was confirmed in human dendritic cells. The effects of particulate β-glucan induced TLR4 binding were regulatory as blocking TLR4 enhanced pro-inflammatory cytokine IL-23, IL-4, IL-6, and TNF-α production.

These results suggest that TLR4 and Dectin-1 are synergistically activated by particulate β-glucans, wherein TLR4 activates an immune regulatory pathway in human dendritic cells. Our data suggest that β-glucan is an immune regulatory ligand for TLR4.