Polysaccharides from lichen Xanthoria parietina: 1,4/1,6-α-d-glucans and a highly branched galactomannan with macrophage stimulating activity via Dectin-2 activation

Abstract

Hot-water soluble polysaccharides H-1-3 and H-2-1 were isolated from the thalli of the lichen Xanthoria parietina (L.) Th. Fr. and purified by ion exchange and gel permeation chromatography. Structure elucidation was mainly based on 2D-NMR and nano-ESI-Q-TOF MS/MS experiments. H-1-3 (13.7 kDa) was shown to be linear α-glucan with α-d-Glcp-(1 → [→[4)-α-d-Glcp-(1]₂ → [6)-α-d-Glcp-(1]₃ → 4)]_n core backbone. The (1,4)- and (1,6)-α-d-Glcp linkages were in a 2:3 M ratio. H-2-1 (525 kDa) was characterized as a complex branched β-galacto-α-mannan with →[6)-α-d-Manp-(1 → [2,6)-α-d-Manp-(1]₂ → [2)-α-d-Manp-(1]₂→]_n core units and main side chains of (1,3)-β-d-Galf linked at O-6 to →2)-α-d-Manp-(1→, together with minor terminal units of 1,4/1,6-α-D -Glcp units attached to the core chain at O-6 position and α-L-Rhap linked to Galf side chain at O-2 position (Manp: Galf: Glcp: Rhap linkage ratio = 9:3:2:1). H-2-1 exerted strong immunoactivity in vitro and activated murine RAW macrophages 264.7 towards significantly increased phagocytosis, TNF-α and IL-1β secretion. These effects are due to an interaction of the galactomannan with the transmembrane pattern-recognition protein Dectin-2 of the macrophages.

Keywords: Dectin-2; Galactomannan; Glucan; Lichen; Mass spectroscopy; Phagocytosis; Polysaccharides; Silver adducts; Xanthoria parietina.