Spirulina platensis Polysaccharides: Evaluation of Immunomodulatory Activity and Mechanism via NF-κB Signaling Pathway Activation in RAW264.7 Cells

Abstract A high-molecular-weight polysaccharide (PSP; 2.54 × 10⁶ Da) was isolated from Spirulina platensis and characterized. PSP consisted of mannose, rhamnose, glucose, galactose, xylose, and arabinose at a molar ratio of 36.32:33.24:20.83:4.52:2.84:2.26. Structural analyses showed that PSP adopted a helical conformation, exhibited a sheet-like morphology, and remained thermally stable below 230 °C. At non-cytotoxic concentrations, PSP significantly enhanced phagocytic activity and nitric oxide (NO) production in RAW264.7 macrophages. PSP also increased the secretion of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), together with the messenger RNA expression of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), TNF-α, and IL-6. The strongest responses were generally observed at lower PSP concentrations, whereas the effects decreased as the concentration increased. Western blot analysis demonstrated increased phosphorylation of inhibitor of nuclear factor kappa B alpha (IκBα) and p65 after PSP treatment. Mechanistic analyses further indicated that PSP activated nuclear factor kappa B (NF-κB) signaling, suggesting that NF-κB activation contributed to PSP-mediated macrophage stimulation. Collectively, these findings establish PSP as a bioactive Arthrospira-derived macromolecular polysaccharide with marked in vitro immunomodulatory activity and support its potential development as a natural immunomodulatory agent.