Modification of Brassica rapa L. Polysaccharide by Selenylation and Its Immune-Enhancing Activity When Combined with a Live-Attenuated Newcastle Disease Vaccine in Poultry

<b>Background</b>: The quest for effective immunoenhancers is central to improving vaccine efficacy, especially against avian viruses such as Newcastle disease (ND) virus. Selenized polysaccharides integrate bioactive polysaccharides with selenium's immunoenhancing properties while reducing selenium toxicity, making them promising candidates for the development of a novel vaccine immunoenhancer. <b>Aim</b>: This study aimed to develop an efficient selenized <i>Brassica rapa</i> L. polysaccharide (sBRP) and evaluate its potential to enhance the immunogenicity of a live-attenuated ND vaccine in poultry. <b>Methods</b>: Selenization was achieved via nitrite-assisted selenization of <i>Brassica rapa</i> L. polysaccharide (BRP). In vivo, 180 yellow-feathered broilers were divided into six groups: control (Con), vaccine-only (Vac), BRP (20 mg/kg), and low/medium/high-dose sBRP (sBRP-L/M/H: 5/10/20 mg/kg). On days 14 and 28, all groups except Con were vaccinated against ND via drinking water. Concurrently, the BRP and sBRP-L/M/H groups received their respective polysaccharides via oral gavage. Parameters assessed included immune organ indices, lymphocyte proliferation, serum antibody titers (HI), cytokine levels (IL-2/IL-6/IFN-γ), and densities of intestinal intraepithelial lymphocytes (IELs) and goblet cells (GCs). <b>Results</b>: sBRP exhibited a selenium content of 30.6 mg/g, with Se-O-C covalent modification confirmed. The sBRP-H group significantly enhanced immune organ indices, lymphocyte proliferation, Newcastle disease virus HI antibody titers, and serum IL-2/IL-6/IFN-γ levels. The sBRP-M group increased IEL and GC densities in the intestine. <b>Conclusions</b>: sBRP acts synergistically with the vaccine to enhance vaccine-induced cellular, humoral, and mucosal immunity, demonstrating promise as a novel oral vaccine immunoenhancer.