Structural characterization and antitumor activity via immune modulation of an α-glucan from Grifola frondosa

Grifola frondosa is a valued edible and medicinal fungus whose polysaccharides are key bioactive components, yet the identification of specific homogeneous effectors and their detailed immunomodulatory mechanisms remains a critical focus of research. In this study, polysaccharide fractions were isolated from G. frondosa fruiting bodies via systematic purification, and the principal bioactive polysaccharide responsible for breast cancer inhibition was identified. A novel homogeneous polysaccharide, GFI-21α, was determined as the primary effector. Structural characterization defined GFI-21α as a high-molecular-weight α-(1 → 4)-D-glucan backbone featuring →3,6)- and →4,6)-linked branch points substituted at the O-6 position with short →3)-α-D-Glcp-(1 → side chains. None of the isolated polysaccharide fractions exhibited direct cytotoxicity in vitro. However, in vivo experiments demonstrated that GFI-21α elicited substantial tumor growth suppression. This efficacy significantly exceeded that of the clinical β-glucan comparator, lentinan, without causing overt toxicity. Mechanistically, distinct from the innate immune modulation typically associated with β-glucans, GFI-21α was associated with enhanced intratumoral CD8⁺ T cell responses and IFN-γ-associated cytotoxic effector activity. These findings establish GFI-21α as a structurally distinct α-glucan with potent antitumor efficacy, supporting its potential as a candidate for cancer immunotherapy.