Evaluating the Immunogenicity of a Recombinant Bacillus subtilis Expressing LTB-Fused Protective Antigen of Transmissible Gastroenteritis Virus in a Murine Model

Transmissible gastroenteritis (TGE), caused by the TGE virus (TGEV), is a highly contagious enteric disease characterized by vomiting, dehydration, and watery diarrhea. It mainly endangers piglets within two weeks of age, with a 100% mortality rate, inflicting severe economic losses on the global swine industry. Since enteric tropism of the virus and mucosa serves as the first line of defense against viral invasion, an oral vaccine inducing sufficient secretory immunoglobulin A (SIgA) antibodies in animals should be developed. Being a generally recognized as safe (GRAS) microorganism, <i>Bacillus subtilis</i> can form endospores under extreme environmental conditions, which confer resistance to the hostile gastric environment and have been widely employed as delivery vehicles for oral vaccines owing to their immunoadjuvant activity and non-specific antidiarrheal effects. In this study, the AD antigenic epitope of the TGEV S protein was selected as the immunogen. The mature peptide of the B subunit of the heat-labile enterotoxin from enterotoxigenic <i>Escherichia coli</i> served as a mucosal adjuvant, and <i>B. subtilis WB800N</i> was used as the delivery host to construct the recombinant strain pHT43-LTB-AD/<i>WB800N</i>. After confirming the successful expression of the target protein, oral immunization was performed using mice as a model. The results demonstrated that this recombinant strain induced robust mucosal, humoral, and cellular immunity, along with considerable levels of neutralizing antibodies. These findings indicate that recombinant <i>B. subtilis</i> could serve as an oral vaccine candidate to combat TGEV infections.