Safety and immunogenicity of a single oral dose of recombinant double mutant heat-labile toxin derived from enterotoxigenic Escherichia coli

Abstract

Enterotoxigenic Escherichia coli (ETEC) is a primary cause of traveler's diarrhea for which there is no licensed vaccine. This phase 1 trial determined the safety and immunogenicity of a recombinantly produced double mutant heat-labile enterotoxin (dmLT) of ETEC. It was administered as a single oral dose of dmLT in escalating doses of 5 μg, 25 μg, 50 μg, and 100 μg, followed by a 72-h inpatient observation, outpatient visits at 8, 14, and 28 days, and telephone calls at 2 and 6 months postvaccination. Safety was assessed by frequency of adverse events, and immune responses determined after immunization included dmLT-specific serum IgA and IgG, fecal IgA, antibody-secreting cells (ASC), and antibodies in lymphocyte supernatant (ALS) responses. All doses were well tolerated by the 36 healthy adults enrolled. Immune responses were limited in the 5- and 25-μg dose recipients. The 50-μg dose recipients trended toward stronger responses than the 100-μg dose recipients by serum IgA (67% versus 33%, P = 0.22), serum IgG (58% versus 33%, P = 0.41), and fecal IgA (58% versus 33%, P = 0.41). By day 14 postvaccination, there were significantly more positive responders (≥4-fold increase from baseline) among the 50- versus 100-μg dose recipients for serum IgA (P = 0.036) but not serum IgG (P = 0.21). In conclusion, a single oral dose of dmLT was well tolerated and immunogenic, with immune responses plateauing at the 50-μg dose. (This clinical trial is registered at www.clinicaltrials.gov, registration number NCT01147445.).

Figures

Fig 1

Individual serum anti-dmLT IgG (top) and IgA (middle) and adjusted fecal IgA (bottom) responses for each of the four doses administered (5 μg, 25 μg, 50 μg, and 100 μg). Within each dose group, the four circles denote (from left to right) the following four time points: baseline and 8, 14, and 28 days postvaccination. A positive response was defined by ≥4-fold increases compared to the baseline (indicated by the filled symbols). The diamond indicates a subject that did not submit a baseline sample (bottom; fecal IgA in the 25-μg dose group), and the square indicates a subject that submitted only the baseline and day 8 samples (100-μg dose group in all 3 panels).

Fig 2

dmLT-specific IgA (top left) and IgG (top right) ASC and IgA (bottom left) and IgG (bottom right) ALS responses at baseline (prevaccination) and 8 days (postvaccination) for each of the four doses administered (5 μg, 25 μg, 50 μg, and 100 μg). A positive ASC response is defined as ≥8 spot-forming cells (SFC)/106 cells, and a positive ALS response is defined as a ≥2-fold increase compared to the baseline (indicated by filled circles).