Genetic fusions of heat-labile toxoid (LT) and heat-stable toxin b (STb) of porcine enterotoxigenic Escherichia coli elicit protective anti-LT and anti-STb antibodies

Enterotoxigenic Escherichia coli (ETEC)-associated diarrhea causes a substantial economic loss to swine producers worldwide. The majority of ETEC strains causing porcine diarrhea, especially postweaning diarrhea (PWD), produce heat-labile toxin (LT) and heat-stable toxin b (STb). LT is commonly used in vaccine development, but STb has not been included because of its poor immunogenicity. As a virulence factor in porcine diarrhea, STb needs to be included as an antigen for development of broad-spectrum vaccines. In this study, we used an LT toxoid (LT_R192G [hereafter, LT₁₉₂]) derived from porcine ETEC to carry a mature STb peptide for LT₁₉₂-STb fusions to enhance STb immunogenicity for potential vaccine application. Anti-LT and anti-STb antibodies were detected in immunized rabbits and pigs. In addition, when challenged with an STb-positive ETEC strain, all 10 suckling piglets borne by immunized gilts remained healthy, whereas 7 out 9 piglets borne by unimmunized gilts developed moderate diarrhea. This study indicates that the LT₁₉₂-STb fusion enhanced anti-STb immunogenicity and suggests the LT₁₉₂-STb fusion antigen can be used in future vaccine development against porcine ETEC diarrhea.