Enzymatically extracted ulvans restrict viruses via STING signaling and type I interferon after cellular entry

Ulvans, abundant natural polysaccharides produced by Ulvales, have been recognized for antiviral activities, though the underlying mechanisms are not fully understood. In this study, we focused on two polysaccharides and one oligosaccharide, which were extracted enzymatically from Ulva prolifera and named as PR1 (13.5 kDa), PR2 (7.1 kDa) and PR3 (0.6 kDa), respectively. Comprehensive analyses of structures and monosaccharide composition revealed a primary composition of L-rhamnose, D-glucuronic acid and D-xylose. Of particular interest, PR1 showed a pronounced ability to inhibit vesicular stomatitis virus (VSV) in macrophages, demonstrated by an IC₅₀ value of 179.1 ± 29.8 ng/mL. In A549 cells, a human lung carcinoma line, PR1 displayed moderate antiviral activity. However, in IFN-deficient Vero cells, PR1 proved ineffective, suggesting that PR1 might exert antiviral effects through type I interferon. A significant finding of this study is that PR1 is capable of entering cells in an energy-dependent manner, a characteristic previously undocumented. Moreover, PRs were observed to activate the intracellular STING signaling pathway, leading to the phosphorylation and subsequent nuclear translocation of p65 and IRF3. This novel discovery enhances our understanding of ulvan's role in immune modulation, highlighting the importance of considering intracellular proteins and pathways when investigating the mechanisms of polysaccharides.