Polysaccharide Extracted from Longan (Dimocarpus longan Lour.) as Novel Adjuvant to Boost Humoral and Cellular Immune Responses

Adjuvants, which enhance the effectiveness of antigens, are essential for vaccines against infectious or malignant diseases. Currently, the development of adjuvants encounters challenges as highly effective adjuvants tend to be highly toxic, whereas those with lower toxicity often lack efficacy. Polysaccharides have unique advantages as adjuvants due to their multiple immunomodulatory activities and favorable safety profiles. In this study, longan polysaccharide (LP) was characterized physicochemically and identified as an effective adjuvant. LP, consisting of 96.44% glucose, was mainly linked by the α-1,6-glycosidic bond. In vitro experiments revealed that LP could induce the secretion of pro-inflammatory cytokines (TNF-α, IL12, and IL1β) and expression of co-stimulatory molecules (CD80 and CD86) through toll-like receptor 4 (TLR4) activation. More importantly, LP could promote antigen cross-presentation when formulated with a model antigen—ovalbumin (OVA). In vivo experiments indicated that the LP+OVA formulation could boost both humoral and cellular immune responses in immunized C57BL/6J mice. The histopathological evaluation of the major organs showed that LP+OVA was non-toxic. Therefore, our findings suggested that LP is an effective and safe adjuvant for vaccine development.