Structural Elucidation of a Novel Highly Branched Galactoglucomannan from Bamboo Parasitic Fungus Hypocrella bambusae and Its MAPK-Mediated Immunostimulatory Activity

Hypocrella bambusae is a special medical fungus known for its valuable bioactive pigments, yet its polysaccharide components have remained underexplored. This study aimed to investigate the structure of a heteropolysaccharide (HB0-1) isolated and purified from the stromata of H. bambusae and to evaluate its immunomodulatory activity. The analysis revealed that HB0-1 was composed of galactose, glucose, and mannose with a (1→6)-α-D-mannopyranose backbone. Its side chains include β-D-galactofuranose, α-D-glucopyranose, and β-D-galactopyranose residues. In cellular assays, HB0-1 significantly increased the viability, phagocytic activity, reactive oxygen species (ROS) production, and the secretion of pro-inflammatory cytokines—interleukin-6 (IL-6), interleukin-12 (IL-12), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α)—as well as the expression of inducible nitric oxide synthase (iNOS) in RAW264.7 macrophages. Further mechanistic studies demonstrated that HB0-1 activated the MAPK signaling pathway in a concentration-dependent manner, thereby enhancing immune responses. These results revealed that the highly branched architecture of HB0-1, featuring a (1→6)-α-D-mannan backbone densely substituted with glucose- and galactose-rich side chains (including furanose motifs), is responsible for its potent MAPK-mediated macrophage activation. This structural–functional relationship positions HB0-1 as a promising immunomodulatory candidate for functional food and therapeutic applications.