Intestinal mucosal immune responses induced by oral administration of chitosan nanoparticles encapsulating the PEDV S1 protein

Porcine epidemic diarrhea (PED), a highly contagious intestinal disease caused by PEDV, threatens newborn piglets and causes enormous losses in the swine industry. Intestinal mucosal immunity is vital for defense against PEDV. Chitosan (CS), a mucin-adhesive biodegradable polysaccharide, has emerged as a promising mucosal vaccine delivery vector and antigen adjuvant. To develop a PEDV mucosal vaccine, we prepared S1-CS nanoparticles (NPs) loaded with the S1 domain of the PEDV spike (S) protein. We evaluated their ability to induce mucosal immune responses in mice via different inoculation routes and to confer immune protection in piglets after oral inoculation. The results showed that S1-CS NPs exhibited no cytotoxicity and remained stable in simulated gastric fluid in vitro. Compared with oral administration, intramuscular inoculation of S1-CS NPs induced higher levels of PEDV-specific serum IgG in mice. However, PEDV-specific IgA was detected only in the serum and intestinal lavage fluid of orally vaccinated mice. Oral administration also elicited higher IFN-γ levels than intramuscular injection, suggesting stronger activation of cellular immunity. In piglets, oral S1-CS NPs induced serum neutralizing antibodies and IgA responses in both serum and intestinal mucosa, increased the number of IgA-secreting cells in the intestine, reduced viremia and virus shedding, and improved intestinal villus morphology. These findings indicate that the S1-CS NPs developed in this study represent promising candidates for an oral mucosal PEDV vaccine.