Evaluation of the Safety, Tolerability, and Immunogenicity of an Oral, Inactivated Whole-Cell Shigella flexneri 2a Vaccine in Healthy Adult Subjects

Abstract

Shigella causes high morbidity and mortality worldwide, but there is no licensed vaccine for shigellosis yet. We evaluated the safety and immunogenicity of a formalin-inactivated whole-cell Shigella flexneri2a vaccine, Sf2aWC, given orally to adult volunteers. In a double-blind, placebo-controlled trial, 82 subjects were randomized to receive three doses of vaccine in dose escalation (2.6 ± 0.8 × 10(8), × 10(9), × 10(10), and × 10(11)vaccine particles/ml). Vaccine safety was actively monitored, and antigen-specific systemic and mucosal immune responses were determined in serum, antibody in lymphocyte supernatant (ALS), and fecal samples. Cytokines were measured in the serum. Sf2aWC was well tolerated and generally safe at all four dose levels. The vaccine resulted in a dose-dependent immune response. At the highest dose, the vaccine induced robust responses to lipopolysaccharide (LPS) in both serum and ALS samples. The highest magnitude and frequency of responses occurred after the first dose in almost all samples but was delayed for IgG in serum. Fifty percent of the vaccinees had a >4-fold increase in anti-LPS fecal antibody titers. Responses to invasion plasmid antigens (Ipa) were low. The levels of interleukin-17 (IL-17), IL-2, gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and IL-10 were increased, and IL-8 was decreased immediately after first dose, but these changes were very transient. This phase I trial demonstrated that the Sf2aWC vaccine, a relatively simple vaccine concept, was safe and immunogenic. The vaccine elicited immune responses which were comparable to those induced by a live, attenuated Shigella vaccine that was protective in prior human challenge studies.

Copyright © 2016, American Society for Microbiology. All Rights Reserved.

Figures

FIG 1

Subject enrollment and retention throughout the study. The flow diagram outlines the status (screened, excluded, enrolled, lost to follow-up, withdrawn) of all study participants.

FIG 2

IgA (A)- and IgG (B)-to-LPS geometric mean (95% confidence interval) titer increase in serum after immunization with either 2.6 ± 0.8 ×1010 or 2.6 ± 0.8 ×1011 vp/ml of Sf2aWC or placebo. The x axis indicates days of immune responses measured: day 0 (day before first dose), day 7 (7 days after first dose), day 28 (day of the second dose), day 35 (7 days after second dose), day 56 (day of the third dose), day 63 (7 days after third dose), and day 84 and day 112.

FIG 3

IgA- and IgG-to-LPS geometric mean (95% confidence interval) fold increase of titers compared to baseline in ALS after immunization with either 2.6 ± 0.8 ×1010 or 2.6 ± 0.8 ×1011 vp/ml of Sf2aWC or placebo. The x axis indicates days of immune responses measured: day 7 (7 days after first dose), day 28 (day of the second dose), day 35 (7 days after second dose), day 56 (day of the third dose), or day 63 (7 days after third dose). Data are presented in logarithmic scale (log10). The dotted line shows 4-fold increase. **, P < 0.001 (responses compared to baseline).

FIG 4

IgA-to-LPS geometric mean (95% confidence interval) fold increase of titers compared to baseline in fecal samples after immunization with either 2.6 ± 0.8 ×1010 or 2.6 ± 0.8 ×1011 vp/ml of Sf2aWC or placebo. The x axis indicates days of immune responses measured: day 7 (7 days after first dose), day 35 (7 days after second dose), or day 63 (7 days after third dose). The dotted line shows 4-fold increase. Data are presented in logarithmic scale (log10). *, P < 0.05 (responses compared to baseline).

FIG 5

Shigella-specific cytokine production in serum after immunization with three doses of 2.6 ± 0.8 ×1011 vp/ml. Cytokines IL-17 (A), IL-2 (B), IFN-γ (C), and IL-10 and IL-8 (D) were measured before vaccination (0 h), at 8, 24, and 72 h after the first dose, at day 56 (28 days after the second dose), and at day 63 (7 days after the third dose). *, P < 0.05; **, P < 0.001 (responses compared to baseline).