A new fucosylated glucuronoxylomannan from the fruit bodies of Tremella aurantia: structural characterization and immunoenhancing activity on seasonal influenza mRNA vaccine

The development of potent adjuvants is crucial for enhancing influenza mRNA vaccine efficacy. Tremella aurantia polysaccharides possess bioactivities, but their structural complexity and adjuvant potential remain unclear. In this study, a high-molecular-weight (Mw: 1178.77 kDa) polysaccharide (TAP) was purified from T. aurantia fruit bodies and hydrolyzed by using an ultrasonic-assisted H 2 O 2 -Cu 2+ method, yielding three products (designated TAP1-TAP3). Structural analyses revealed that TAP1 (426.28 kDa) and TAP2 (16.96 kDa) retained the native monosaccharide composition (Man, GlcA, Xyl, Fuc), whereas TAP3 (21.97 kDa) lost its fucose residues. All polysaccharides shared a 1,3-α-D-mannose backbone with GlcA and Xyl side chains. TAP2, a novel low-Mw fucosylated glucuronoxylomannan, exhibited superior immunostimulatory activity in vitro with minimal cytotoxicity. When used as an adjuvant with a seasonal influenza HA mRNA vaccine in mice, TAP2 significantly enhanced hemagglutination inhibition titers, increased influenza-specific IgG levels, and improved survival rates against a lethal influenza virus challenge. Mechanistically, the immunoenhancing effect of TAP2 was attributed to its optimal molecular size and sulfated fucose branches, which potently promoted dendritic cell maturation primarily via activation of the TLR4/MyD88/NF-κB pathway. This study not only identified TAP2 as a promising adjuvant candidate for mRNA vaccines but also elucidated the structure-activity relationship of Tremella glucuronoxylomannans.