Towards a Vaccine for Enterotoxigenic E. coli (ETEC): Comparison of Protection Following Intragastric (IG) or Intranasal (IN) Immunization with an Attenuated O157:H7 E. coli Strain (ZCR533) Expressing ETEC Antigens

Abstract

Purpose: Enterotoxigenic E. coli (ETEC) strains are a principal cause of diarrhea in travelers from industrialized nations to developing countries, and they are a frequent cause of infantile diarrhea in the developing world. Although the virulence mechansims of ETEC diarrhea are well understood and vaccine targets are well defined, there are currently no approved vaccines to prevent ETEC diarrhea in adults or infants. Our long term goal is to develop a live, attenuated oral vaccine for ETEC infections. Methods: To deliver ETEC antigens, we have developed an attenuated E. coli strain to serve as a live vector for orogastric administration. We have previously shown that truncation of the adhesin intimin in a rabbit enteropathogenic E. coli (REPEC) strain yields an attenuated strain which is immunogenic and protective against homologous REPEC challenge(1). We have developed a similar intimin mutant (designated ZCR533) in an O157:H7 shiga toxin producing E. coli (STEC) strain to serve not only as an STEC vaccine, but also for use as a vector to express antigens of other pathogens such as ETEC. To develop a safe, live attenuated vaccine against enterotoxigenic E. coli (ETEC), we expressed two critical ETEC antigens, colonization factor antigen I (CFA/I) and detoxified heart labile enterotoxin (LT), in ZCR533 and then tested the resulting strains for immunogenicity and protective efficacy in a streptomycin-treated treated mouse model of intestinal ETEC infection. BALB/c mice were vaccinated by the IG or IN routes, then challenged with ETEC H10407 or purified LT. Results: IG delivery of three doses, or IN delivery of two doses, of the vaccine strain to BALB/C mice induced serum IgG and mucosal IgA antibodies to CFA/I and LT. Both IG and IN administration of the ETEC vaccine significantly reduced intestinal bacterial colonization after an oral challenge with ETEC (vac. vs. sham, P<0.05). However, decreased intestinal fluid accumulation following oral challenge with ETEC, or with purified LT, was seen only in the IG vaccinated mice (vac. vs. sham, P<0.05), but not in those vaccinated IN. Conclusion: These studies suggest that both IG and IN administered vaccine led to protective serum and mucosal responses against colonization, but IG vaccination may better protect against toxin-induced fluid accumulation.