Effects of polysaccharide from hot-compressed steamed Rehmannia glutinosa on the immune system and gut microbiota in an immunosuppressed mice model

•HRP obtained by hot-compressed steamed.•HRP exerted immunomodulatory effects by activating macrophage.•HRP acted as an immunopotentiator through the NF-κB signaling pathway.•HRP via alterations in gut microbiota to ameliorate CTX-induced immunocompromised mice. In this study, the immunomodulatory effects of polysaccharide obtained by hot-compressed steaming of Rehmannia glutinosa Libosch (HRP) were investigated using both in vitro and in vivo methods. It was found that HRP activated the TLR4/NF-κB signaling pathway, up-regulated the intracellular expression of TNF-α, IL-6 and IL-1β, and induced of innate immune memory in macrophages. We then investigated the effect of HRP on immunosuppressed mice induced by cyclophosphamide (CTX). Surprisingly, HRP improved CTX-induced weight loss and increased the splenic index, alleviated intestinal mucosal damage and hematopoietic insufficiency caused by CTX, as demonstrated by H&E staining. In addition, HRP promoted the expression of key proteins in the TLR4/NF-κB and autophagy pathways in intestinal tissues, thereby enhancing intestinal immune function. Bacterial 16S rRNA gene sequences of colon contents suggested that HRP may alleviate gut microbiota disruption by increasing the populations of Lachnospiraceae and Erysipelotrichaceae while inhibiting Lactobacillaceae. The results of this study show the potential use of HRP as an immunomodulator in functional foods or pharmaceuticals.