Polyvalent Protein Adhesin MEFA-II Induces Functional Antibodies against Enterotoxigenic Escherichia coli (ETEC) Adhesins CS7, CS12, CS14, CS17, and CS21 and Heat-Stable Toxin (STa)

An effective vaccine is lacking against ETEC, a primary cause of children's diarrhea and traveler's diarrhea and a threat to global health. The key challenge in ETEC vaccine development is that ETEC bacteria express heterogeneous virulence determinants (>25 adhesins and two toxins). There are no licensed vaccines for enterotoxigenic Escherichia coli (ETEC), a common cause of children's diarrhea and travelers' diarrhea. ETEC strains producing enterotoxins (heat-labile toxin, LT; heat-stable toxin, STa) and adhesins CFA/I, CFA/II (CS1-CS3) or CFA/IV (CS4-CS6) attributed to a majority of ETEC-associated diarrheal cases, thus the two toxins (STa, LT) and the seven adhesins (CFA/I, CS1 to CS6) are historically the primary targets in ETEC vaccine development. Recent studies, however, revealed that ETEC strains with adhesins CS14, CS21, CS7, CS17, and CS12 are also prevalent and cause moderate-to-severe diarrhea; these adhesins are now considered antigen targets as well for ETEC vaccines. In this study, we applied the epitope- and structure-based multiepitope-fusion-antigen (MEFA) vaccinology platform and constructed a polyvalent protein to present immuno-dominant continuous B-cell epitopes of these five adhesins (also an STa toxoid); we then characterized this protein antigen's (termed as adhesin MEFA-II) broad immunogenicity and evaluated antibody functions against each targeted adhesin and STa toxin. Data showed that mice intramuscularly immunized with adhesin MEFA-II protein developed robust IgG to the targeted adhesins and toxin STa. Importantly, the antigen-derived antibodies significantly inhibited adherence of ETEC bacteria expressing adhesin CS7, CS12, CS14, CS17, or CS21 and reduced STa enterotoxicity. These results indicated that adhesin MEFA-II protein is broadly immunogenic and induces cross-functional antibodies, suggesting adhesin MEFA-II can be an effective ETEC vaccine antigen; if included in an ETEC vaccine candidate, adhesin MEFA-II can expand vaccine coverage and increase efficacy against ETEC-associated children's diarrhea and travelers' diarrhea.IMPORTANCE An effective vaccine is lacking against ETEC, a primary cause of children's diarrhea and traveler's diarrhea and a threat to global health. The key challenge in ETEC vaccine development is that ETEC bacteria express heterogeneous virulence determinants (>25 adhesins and two toxins). While the current strategy to target the seven most prevalent ETEC adhesins (CFA/I, CS1 to CS6) potentially lead to a vaccine against many clinical cases, the prevalence of ETEC strains shifts chronically and geographically, and ETEC expressing other adhesins, mainly CS7, CS12, CS14, CS17, and CS21, also cause moderate-to-severe diarrhea. However, it is impossible to develop an ETEC vaccine to target as many as 12 adhesins under conventional approaches. This study used a unique vaccinology platform to create a polyvalent antigen and demonstrated the antigen's broad immunogenicity and functions against the targeted ETEC adhesins, enabling the development of a broadly protective vaccine essentially against all of the important ETEC strains.