Structural analysis and adjuvant activity of a polysaccharide from Urtica macrorrhiza

Developing new vaccine adjuvants for clinical use remains a significant challenge. Herein, we reported a polysaccharide (UMRG) from Urtica macrorrhiza. It has a molecular weight of 743.35 kDa and is composed of rhamnose (Rha), glucuronic acid (GlcA), galacturonic acid (GalA), and galactose (Gal) in a molar ratio of 1.94: 1.00: 4.17: 1.79. Structural analysis revealed that UMRG contains a rhamnogalacturonan I backbone with short side chains of β-Galp-(1→4)-β-GlcAp-(1→4)-β-Glap-(1→ linked at the C-4 position of →2,4)-α-Rhap-(1→. In vivo, UMRG significantly increased the production of antigen-specific IgG, IgG1, and IgG2a by 1.91-, 2.09-, and 3.43-fold, respectively, on day 42 post-immunization. It also promoted the proliferation of splenic lymphocytes, increasing the proportion of CD3⁺ and CD3⁺CD4⁺ T lymphocytes from 32.63 ± 1.13 % to 38.13 ± 2.03 % and from 21.05 ± 0.93 % to 24.34 ± 1.21 %, respectively. Further investigation demonstrated that UMRG promoted the phagocytosis of antigens by dendritic cells, improved their maturation, and stimulated the secretion of the cytokines TNF-α, IL-12, and IL-6. Additionally, both in vitro and in vivo experiments demonstrated that UMRG displayed good biosafety. Our results suggested the Urtica macrorrhiza polysaccharide may exhibit the potential to be developed as a highly efficient and low-toxicity immune adjuvant.