A fructan-type garlic polysaccharide upregulates immune responses in macrophage cells and in immunosuppressive mice

The anti-inflammatory effects of plant polysaccharides are well known. However, the stimulatory effects of polysaccharides under immunosuppressive conditions and their link with the polysaccharide structure is underexplored. In this work, the immune modulatory effects of a garlic polysaccharide (GP) are investigated via in vitro and vivo methods. It is observed that GP enhance the immune response of macrophages (RAW264.7) as indicated by the elevated levels of nitric oxide, TNF-α and IL-6. The observation that GP are able to stimulate the immune response in vitro was then explored with the use of an immunosuppressed mouse model. Surprisingly, GP exhibited dose-dependent up-regulatory impacts on the cyclophosphamide (CTX) suppressed levels of cytokines such as IFN-γ and IL-6 and immunoglobulins (e.g. IgA and IgG). The GP intervention reversed histopathological damage to the small intestine and spleen and increased fecal short-chain fatty acid levels. Moreover, GP modulates the gut microbiota dysbiosis by increasing the abundance of immunogenic bacteria such as g__norank_f__Erysipelotrichaceae, while inhibiting the over-abundance of g_Bacteroides. Functional predictions indicated that gut biomarkers of GP possessed the functions of glycoside hydrolase family 32 (GH32) and β-fructofuranosidase. It is concluded that GP is a promising immunostimulant for immune-compromised individuals.