The oral, live attenuated enterotoxigenic Escherichia coli vaccine ACE527 reduces the incidence and severity of diarrhea in a human challenge model of diarrheal disease

Abstract

An oral, live attenuated, three-strain recombinant bacterial vaccine, ACE527, was demonstrated to generate strong immune responses to colonization factor and toxin antigens of enterotoxigenic Escherichia coli (ETEC) in human volunteers. The vaccine was safe and well tolerated at doses of up to 10(11) CFU, administered in each of two doses given 21 days apart. These observations have now been extended in a phase 2b study with a total of 70 subjects. Fifty-six of these subjects were challenged 28 days after the second dose of vaccine with the highly virulent ETEC strain H10407 to obtain preliminary indicators of efficacy against disease and to support further development of the vaccine for both travelers and infants in countries where ETEC is endemic. The vaccine had a significant impact on intestinal colonization by the challenge strain, as measured by quantitative fecal culture 2 days after challenge, demonstrating the induction of a functional immune response to the CFA/I antigen. The incidence and severity of diarrhea were also reduced in vaccinees as measured by a number of secondary and ad hoc endpoints, although the 27% reduction seen in the primary endpoint, moderate to severe diarrhea, was not statistically significant. Together, these observations support the hypothesis that the ACE527 vaccine has a dual mode of action, targeting both colonization factors and the heat-labile enterotoxin (LT), and suggest that it should be further developed for more advanced trials to evaluate its impact on the burden of ETEC disease in field settings.

Fig 1

Subject allocation and retention through study.

Fig 2

Quantitative serum IgG responses to LT-B following vaccination and challenge. Titers are expressed as fold increase relative to prevaccination levels, on a log₂ scale. Solid circles, ACE527 vaccinees; open circles, placebo recipients. Bars show means and standard errors. Vaccination with ACE527 or placebo was on days 0 and 21, and challenge with H10407 was on day 49. The horizontal dashed line at a 2.5-fold increase is the threshold for a positive response.

Fig 3

Quantitative level of H10407 shedding at day 2 postchallenge. Bars show geometric means and 95% confidence intervals.

Fig 4

Frequency distribution of severity of diarrhea based on maximum loose stool output in any 24-hour period within 120 h of challenge. None, 0 g; mild, 1 to 400 g; moderate, 401 to 800 g; severe, 801 to 1,600 g; very severe, >1,600 g. Open bars, ACE527 vaccinees; solid bars, placebo recipients.

Fig 5

Mean accumulation of loose stools with time from the first diarrheal stool experienced by a subject. Solid line, ACE527 vaccinees; dashed line, = placebo recipients.

Fig 6

Kaplan-Meier survival analysis of the time to reach the primary endpoint (moderate or severe diarrhea) according to the quantitative level of shedding of H10407 in stool at 2 days after challenge. All subjects for whom quantitative shedding data were available were sorted by the level of shedding. Those below the median level (6.4 × 10⁶ CFU/g) were considered “low shedders,” represented by filled circles, and those above or at the median were considered “high shedders,” represented by open circles.