High-molecular-weight Fucoidan exerts an immune-enhancing effect in RAW 264.7 cells and cyclophosphamide-induced immunosuppression rat by altering the gut microbiome

•Fucoidan P activated macrophages by inducing NO and cytokine production via NF-κB.•Fucoidan P restored immune markers in CP-induced immunosuppressed rats.•Fucoidan P altered gut microbiota composition in CP-induced immunosuppressed rats.•Fucoidan P shows potential as a therapeutic agent for immune enhancement. High-molecular-weight fucoidan (Fucoidan P), sourced from Undaria pinnatifida exhibits several health benefits, including immunomodulation. However, the mechanisms underlying the immune-enhancing effects of Fucoidan P remain unclear. Here, we investigated the immune-enhancing effects and the potential mechanisms of Fucoidan P using RAW 264.7 macrophages and cyclophosphamide (CP)-induced immunosuppression rat model. In macrophages, Fucoidan P showed dose-dependent stimulation by increasing cell proliferation, nitric oxide production, and gene expression of inducible nitric oxide synthase, cyclooxygenase-2, and proinflammatory cytokines. These effects are mediated through the activation of the nuclear factor-kappa B (NF-κB) signaling pathway. Moreover, orally administered Fucoidan P was evaluated in immunosuppressed rats treated with CP. Fucoidan P administration increased hematological values and natural killer cell activity, and positively affected nitrite and prostaglandin E2 levels. The Fucoidan P treatment groups exhibited improved serum cytokine levels as well as splenic and intestinal cytokine mRNA expression compared to the model group. Fucoidan P also mitigated splenic damage and increased the phosphorylation of NF-κB and NF-κB inhibitor alpha (IκBα). Furthermore, Fucoidan P treatment altered the gut microbiota composition, enhancing the alpha diversity, evenness, and abundance of Bacteroidetes, which are associated with immune function. Taken together, our findings suggest that Fucoidan P exerts beneficial effects on immune function by activating NF-κB and modulating gut microbiota. These findings suggested its potential as a therapeutic agent for immune enhancement.