Botanical polysaccharides as veterinary vaccine adjuvants: structure, receptors, and translational limits

Abstract

Intensive livestock production depends critically on vaccination, yet the licensed veterinary adjuvant repertoire remains narrow, being dominated by aluminium salts and mineral-oil emulsions that offer limited control over response quality and cause injection-site reactions and carcass losses. Polysaccharides from Traditional Chinese medicine (TCM), here referred to as TCM polysaccharides (TCMPs), have attracted attention as candidate adjuvants owing to their structural diversity, low toxicity, and mimicry of pathogen-associated molecular patterns. This review surveys immunomodulatory TCMPs by botanical origin and medicinal part, then reinterprets them structurally: adjuvanticity tracks glycosidic backbone type, molecular weight, branching, and substitution rather than taxonomic provenance. Mechanistically, adjuvant activity engages a multi-tiered cascade — pattern-recognition receptor activation (TLR2/TLR4, Dectin-1–Syk, NLRP3 inflammasome), dendritic-cell programming, modulation of the gut–immune axis and mucosal immunity, and engagement of T follicular helper-dependent humoral responses and, tentatively and as a hypothesis, tissue-resident memory T cells. Species-specific features are emphasised as potential, host-relevant mechanisms rather than established TCMP effects — including proposed Dectin-1-driven repolarisation of porcine alveolar macrophages, WC1+ γδ T cell activation in ruminants, and gut-associated lymphoid tissue in poultry — aspects poorly predicted by murine models. Critical appraisal nevertheless reveals substantial limitations: effects are most often reported for chemically undefined crude extracts, endotoxin contamination is rarely excluded, dose–response relationships are frequently non-monotonic, comparisons with licensed adjuvants are scarce, and batch-to-batch variability compromises reproducibility; biosafety claims rest largely on short-term observations. Translation to the field will depend on standardisation of structurally defined preparations, validated structure–activity relationships, rational particulate and mucosal delivery, DIVA compatibility, and economically viable extraction. The evidence positions TCMPs as a mechanistically attractive but analytically immature adjuvant class, whose contribution to antibiotic-free animal production will be determined less by new bioactive extracts than by analytical and regulatory rigour.