A combination vaccine consisting of three live attenuated enterotoxigenic Escherichia coli strains expressing a range of colonization factors and heat-labile toxin subunit B is well tolerated and immunogenic in a placebo-controlled double-blind phase I trial in healthy adults

Abstract

Immune responses against colonization factors (CFs) and the nontoxic B component of the enterotoxigenic Escherichia coli (ETEC) heat-labile toxin (LTB) are considered to be important for immunity against diarrhea caused by ETEC. Individual live attenuated ETEC derivatives that have had their toxin genes removed and whose aroC, ompC, and ompF genes are deleted have shown promise as vaccines against ETEC. The development of such strains has culminated in the testing of a three-strain-combination live attenuated vaccine known as ACE527, comprised of strains ACAM2025 expressing colonization factor antigen I (CFA/I) and LTB; ACAM2022, expressing CS5, CS6, and LTB; and ACAM2027, expressing CS1, CS2, CS3, and LTB. The recombinant CF and LTB genes expressed in the three strains were inserted into the bacterial chromosome to ensure their stable inheritance and expression without the requirement for any selection. ACE527 has been tested in a randomized placebo-controlled, double-blind, phase I safety and immunogenicity study in healthy adult volunteers and proved to be well tolerated and immunogenic at dose levels of 10(10) and 10(11) total CFU. There was no indication of strain interference on the basis of fecal shedding patterns, with all three being detected in the feces of 50% and 83% of low- and high-dose vaccine recipients, respectively. Similarly, strong immune responses to LTB and to CFs expressed on all three constituent strains were induced, with at least 50% of subjects in the high-dose group responding to LTB, CFA/I, CS3, and CS6.

Fig. 1.

Subject disposition from screening.

Fig. 2.

Serum antibody responses to LTB. Graphs show geometric means on a log₂ scale of the fold increase over preimmunization titers, with error bars representing 95% confidence intervals. (A) Serum IgG titers; (B) serum IgA titers. Filled circles, 10¹¹ CFU ACE527; open circles, 10¹⁰ CFU ACE527; open diamonds with dashed line, placebo.

Fig. 3.

Magnitude of ALS responses to LTB and colonization factors. Graphs show the mean specific IgA titer in ALSs on a log₂ scale for all subjects in a group, with error bars indicating standard deviations. Responses are plotted for each individual day (days 0, 7, 10, 21, 28, and 31), as is the peak response for each subject on any day. (A) LTB responses; (B) CFA/I responses; (C) CS3 responses; (D) CS6 responses. Empty bars, hatched bars, and solid bars, data for placebo, low-dose, and high-dose groups, respectively; solid horizontal lines, mean preimmunization titers for all subjects for each specific antigen; dotted horizontal lines, four times the mean preimmunization titers for all subjects for each specific antigen.