Enterotoxigenic Escherichia coli Vaccine Candidate MecVax With Protein Antigens Prepared From Animal-Free Media Is Equally Immunogenic and Protective Against Adhesins CFA/I, CS1-CS6 and Toxins LT and STa.

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of diarrhea in young children in low- and middle-income countries and the most common cause of diarrhea in international travelers. Currently, there are no vaccines licensed to prevent ETEC-associated diarrhea. MecVax, a protein-based, multivalent ETEC vaccine candidate, has been demonstrated to be broadly immunogenic and cross-protective in preclinical studies. However, the two recombinant proteins of MecVax were prepared from 2× Yeast Extract Tryptone medium (2× YT, which contains animal materials) and are therefore unsuitable for a human vaccine. In this study, we prepared MecVax recombinant proteins using two animal material-free media (Terrific Broth Complete with animal-free Soytone and 2× YT with animal-free Soytone) and comparatively evaluated the vaccine products for immunogenicity and antibody functions against ETEC bacterial adherence and toxin enterotoxicity. Data showed that MecVax protein antigens prepared from a culture medium without or with animal materials showed no differences in protein yield, reactivity with specific antibodies, and induction of antigen-specific IgG responses in mice. Moreover, the mouse serum antibodies exhibited the same level of inhibition of adherence from ETEC bacteria expressing CFA/I, CS1-CS6, as well as neutralization of ETEC heat-stable toxin or heat-labile toxin. These results indicate that MecVax can be prepared using an animal-free Soytone medium, thereby accelerating the development of this vaccine product against ETEC-associated diarrhea in children and travelers. Additionally, we conducted an additional MecVax dosage study in mouse immunization and refined the minimum effective dose to 2 μg of each protein antigen.