Comparative toll-like receptor and dectin-1a activation by structurally diverse plant polysaccharides

Naturally derived polysaccharides can activate pattern recognition receptors (PRRs) like Toll-like receptors (TLRs) and Dectin-1, but structure-activity relationships remain unclear. Here, we screened several plant-derived polysaccharides using HEK-Blue™ reporter cells expressing human Toll-like receptor (hTLR) 2, hTLR2/1, hTLR2/6, hTLR3, hTLR4, hTLR5 or hDectin-1a, THP1-Blue™ NF-κB cells and mouse bone marrow-derived macrophages to investigate the correlation between structural features and activity. The polysaccharides included pectins from Gentiana purpurea (GP-AP), Angelica archangelica subsp. archangelica (A-A2-I), Daphne mezereum (DMP-ED-I), Citrullus lanatus (FW), Coffea arabica (CAP) and Citrus sp. (CPA), an arabinogalactan protein (AGP) rich extract from Persea americana (FRAGP), and an arabinoxylan from Livistonia chinensis (LCP). Activation of THP1-Blue™ cells and BMDMs, and interaction with hTLR4 and hTLR2/6 correlated with rhamnogalacturonan-I (RG-I) backbones with galactan, including arabinogalactan-II (AG-II), side chains (GP-AP and DMP-ED-I), and by the presence of AGPs (FRAGP). The presence of 4-O-Me-GlcA in A-A2-I was seemingly negatively correlated with activation of hTLR2/6, despite presence of AG-II structures. The homogalacturonan-rich (HG) polysaccharides investigated in this study, FW, CAP and CPA, did not activate hTLR4 and hTLR2/6, suggesting that HG alone was insufficient for PRR engagement in these models. Several RG-I containing pectins activated hDectin-1a, and higher Mw generally enhanced activity. The AGP and xylose-rich polysaccharides tested in this study did not activate hDectin-1a, and the arabinoxylan LCP was inactive in all assays. None of the polysaccharides activated hTLR3 or hTLR5. In conclusion, several polysaccharides activated multiple PRRs, seemingly associated with presence of RG-I domains with neutral side chains.