Immunity to enterotoxigenic Escherichia coli.

Enterotoxigenic Escherichia coli strains represent the most frequent etiological agent of travelers diarrhea. Challenge studies with several of these strains were undertaken in volunteers to evaluate the mechanisms of disease-induced immunity. Seventeen students and other community volunteers were given 10(6) or 10(8) organisms of E. coli B7A (O148:H28), which produces heat-labile and heat-stable enterotoxins. Ten individuals developed diarrheal illness closely resembling natural travelers diarrhea; of these ten, rises in titer of serum antitoxin and anti-O antibody occurred in eight (80%). Eight of the volunteers who developed diarrhea in the first test agreed to undergo rechallenge 9 weeks later with 10(8) B7A organisms. Only one of these eight "veterans" developed diarrhea versus seven of twelve controls given the same challenge (P = 0.05). Despite clinical protection, all "veterans" excreted B7A after rechallenge. Four controls who developed diarrhea during the homologous B7A rechallenge test were rechallenged 9 weeks later with 10(9) organisms of E. coli strain E2528-C1 (O25:H-), which produces only heat-labile enterotoxin and possesses a different O, H, and pili antigen composition than B7A. Three of four "veterans" and two of six controls developed comparable diarrhea. These studies demonstrate that prior disease due to enterotoxigenic E. coli confers homologous immunity against subsequent challenge, and the operative mechanism apparently is not bactericidal and is not mediated by serum anti-O antibodies. Heterologous protection was not conferred where the only common antigen was heat-labile enterotoxin, indicating that serum infection-derived antitoxin to heat-labile enterotoxin also is not protective.