Structural and immunological studies on the polysaccharide from spores of a medicinal entomogenous fungus Paecilomyces cicadae

A neutral branched heteropolysaccharide (Pc0−1) was purified from the spores of Paecilomyces cicadae, which parasitized in the bamboo cicada (Platylomia pieli Kato). The structure of Pc0−1 was analyzed by HPLC, IR, methylation and NMR spectroscopy. The results reveal that Pc0−1, with an average molecular weight of 18 × 10³ kDa, consists of glucose, galactose, mannose and arabinose in the molar ratio of 8:5:4:1. Some of the glucose residues have methyl modification at O-6 position. The Pc0−1 polysaccharide has a core structure containing 1,2-linked α-D-Manp residues as the backbone and branches at the O-3 and O-6 of the α-D-Manp residues. The inner part of the side-chains is comprised of 1,4-linked α-D-Glcp and 1,4-linked 6-O-Me-α-D-Glcp residues. 1,2-linked β-Galf and minor 1,4-linked Arap and 1,3 or 4-linked Arap residues were occasionally linked at the outside of the side-chains. The side-chains have a single terminal residue of α-D-Glcp, α-Manp, β-Galf or minor Arap (minor). Studies on the bioactivity of Pc0−1 on the macrophages show it exhibit moderate immunostimulating activity through increasing the production of nitric oxide (NO) and enhancing the secretion of major inflammatory cytokines by macrophages, such as TNF-α, IL-1β, IL-6, in RAW 264.7 cells. We examined the effect of Pc0−1 on induced NO and cytokine production in macrophages using anti-PRR antibodies to investigate the membrane receptor for the polysaccharide. The results show that Pc0−1 mainly activates macrophages through their mannose receptor (MR). TLR4 and TLR2 also participated in the recognition of Pc0−1.