Immunomodulation is the primary biological activity of polysaccharides. We have previously demonstrated that Bangia fusco-purpurea polysaccharide (BFP) can modulate the immune function of immunosuppressed mice and increase the abundance of Lactobacillus murinus. However, the underlying mechanisms remain unclear. In the present study, we aimed to elucidate the L. murinus-mediated immunomodulatory effect of BFP. Co-culture of L. murinus and BFP revealed that BFP promoted the proliferation of L. murinus at the concentrations of 0.5%–2.0%. We established a mouse L. murinus depletion model, in which L. murinus was administered via gavage. Treatment with L. murinus significantly increased macrophage phagocytosis and the levels of immune-related factors in mice, including interleukin (IL)-2, tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ). These results suggested that L. murinus contributes to the immunomodulatory effect of BFP. Given the important role of macrophages in innate and adaptive immunity, we further confirmed that L. murinus can enhance immune function. Under lipopolysaccharide (LPS)-induced inflammatory conditions, L. murinus stimulated the elevation of IL-10 expression, consequently promoted macrophages polarization toward the anti-inflammatory M2 type, which in turn prevented the overactivation of the inflammatory response and reversed the LPS-induced inflammation. It also maintained the balance between pro- and anti-inflammatory responses, thus playing a role in regulating immune function. Finally, transcriptomic sequencing and pathway validation revealed that L. murinus regulates IL-10 through the macrophage/mitogen-activated protein kinase/nuclear factor-κB (MAPK/NF-κB) signaling axis. Overall, these results suggest that the immunomodulatory mechanism of the BFP may involve L. murinus-induced regulation of IL-10 through the macrophage/MAPK/NF-κB signaling axis.

The immunomodulatory effects of Bangia fusco-purpurea polysaccharides (BFP) in a mouse model of cyclophosphamide (CTX)-induced immunosuppression were investigated. As a result, BFP intervention significantly improved natural killer (NK) cell viability, macrophage phagocytosis and carbon clearance capacity, enhanced T-lymphocyte proliferation, up-regulated the proportions of CD4⁺, down-regulated the proportions of CD8⁺ T cells, increased the ratio of CD4⁺/CD8⁺ cells, reduced the proportion of Th17 cells and the ratio of CD3^-CD19⁺ B cells, and increased the levels of serum hemolysin, immunoglobulin (Ig) A, IgG and immune-related cytokines such as interleukin (IL)-2, IL-6, tumor necrosis factor-α (TNF-α), and interferon-γ (INF-γ), which demonstrated its potent regulatory effects on nonspecific immunity, cellular immunity and humoral immunity in immunosuppressed mice. Furthermore, the effects of BFP on the surface receptors of intestinal immune cells in immunosuppressed mice were analyzed. It was found that BFP significantly downregulated the mRNA (P < 0.05) and protein (P < 0.01) expression of Toll-like receptor 2 (TLR2), TLR4, IL-6 and TNF-α. Based on this, we speculated that BFP can enhance the immunity through the regulation axis of the TLR2/TLR4 downstream signaling pathway. This study provides scientific support for in-depth understanding of the nutritional value of sea foods and promoting the incorporation of sea foods into residents' diets.

In this study, peptide fractions (F1-F4) with different molecular masses were obtained from Bangia fusco-purpurea through enzymatic hydrolysis and ultrafiltration. F2, with molecular masses of 800-2000 Da, exhibited the highest in vitro angiotensin-converting enzyme (ACE) inhibitory activity as determined by high performance liquid chromatography (HPLC). The amino acid sequence of F2 was identified through liquid chromatography-tandem mass spectrometry (LC-MS/MS) and de novo sequencing using PEAKS Studio software. Six ACE inhibitory peptides that stably bind to ACE were selected through molecular docking. The predicted peptides were synthesized by solid-phase synthesis and their in vitro ACE inhibitory activity was verified. Among them, L1 (LVLLFLFGE) showed the highest ACE inhibitory activity with a half maximal inhibitory concentration (IC₅₀) value of 14.22 µg/mL. Molecular docking results indicated that the inhibition of ACE by L1 was mainly attributed to its ability to form hydrogen bond interactions with the active site of ACE. Finally, the effects of temperature, pH, metal ions, light exposure, and simulated gastrointestinal digestion on the stability of L1 were investigated. The results revealed that L1 was highly stable to heat and ionic strength. However, its activity gradually decreased at pH > 2, and was affected by ultraviolet treatment. The ACE inhibitory activity of L1 decreased after simulated gastric and intestinal digestion, but was still significant.