A structurally novel exopolysaccharide DSP-1 from the deep-sea-derived fungus Didymella keratinophila with potent immunostimulatory activity.

Recently marine fungal polysaccharides have attracted increasing attention and interest; however, most reported sources originate from shallow or coastal environments, and knowledge regarding those derived from deep-sea fungi remains limited. In this study, we firstly isolated a structurally novel exopolysaccharide DSP-1 from the deep-sea-derived fungus Didymella keratinophilaone SCAU273. This polysaccharide consists of glucose and mannose residues, contains four distinct glycosidic linkages and has a molecular weight of 4,116 Da. In functional assays, DSP-1 consistently promoted macrophage proliferation and induced pro-inflammatory cytokine release at all concentration levels tested. Nitric oxide (NO) production reached highest at over 10 μM when DSP-1 was administered at 200 μg/mL. Meanwhile, at the concentration of 100 μg/mL, DSP-1 induced maximal secretion levels of IL-6 (100 pg/mL), IL-1β (50 pg/mL), and TNF-α (50,000 pg/mL). Complexity in the polysaccharide structure was conferred by a branch emanating from the C-6 carbon of a →4,6)-Glcp-(1→ residue along the backbone, resulting in the structure [α-D-Glcp-(1]2 → 4,6)-α-D-Glcp-(1 → 4)-α-D-Glcp-(1 → . Metabolomics analysis showed that DSP-1 may modulate immune function primarily via a sequential impact on cysteine and methionine metabolism, glutathione depletion, and a ferroptosis-like stress signature. Our results reveal the potential of deep-sea bioactive polysaccharides in biotechnological applications and call for in-depth mechanistic investigations to elucidate their molecular targets and pathways.