Safety and Immunogenicity of a Parenterally Administered, Structure-Based Rationally Modified Recombinant Staphylococcal Enterotoxin B Protein Vaccine, STEBVax

Abstract

Staphylococcus aureus produces several enterotoxins and superantigens, exposure to which can elicit profound toxic shock. A recombinant staphylococcal enterotoxin B (rSEB) containing 3 distinct mutations in the major histocompatibility complex class II binding site was combined with an alum adjuvant (Alhydrogel) and used as a potential parenteral vaccine named STEBVax. Consenting healthy adult volunteers (age range, 23 to 38 years) participated in a first-in-human open-label dose escalation study of parenteral doses of STEBVax ranging from 0.01 μg up to 20 μg. Safety was assessed by determination of the frequency of adverse events and reactogenicity. Immune responses to the vaccination were determined by measurement of anti-staphylococcal enterotoxin B (anti-SEB) IgG by enzyme-linked immunosorbent assay and a toxin neutralization assay (TNA). Twenty-eight participants were enrolled in 7 dosing cohorts. All doses were well tolerated. The participants exhibited heterogeneous baseline antibody titers. More seroconversions and a faster onset of serum anti-SEB IgG toxin-neutralizing antibodies were observed by TNA with increasing doses of STEBVax. There was a trend for a plateau in antibody responses with doses of STEBVax of between 2.5 and 20 μg. Among the participants vaccinated with 2.5 μg to 20 μg of STEBVax, ∼93% seroconverted for SEB toxin-neutralizing antibody. A strong correlation between individual SEB-specific serum IgG antibody titers and the neutralization of gamma interferon production was found in vitro STEBvax appeared to be safe and immunogenic, inducing functional toxin-neutralizing antibodies. These data support its continued clinical development. (This study has been registered at ClinicalTrials.gov under registration no. NCT00974935.).

FIG 1

Serum anti-SEB IgG antibody detection by ELISA. Individual serum anti-SEB IgG titers were measured by ELISA subsequent to administration of a single dose of 0.01 μg, 0.1 μg, 0.5 μg, 2.5 μg, 10 μg, or 20 μg or two doses of 20 μg of the STEBVax vaccine and are expressed as the number of ELISA units (EU) per milliliter. For the recipients of single doses, the circles denote (from left to right) data from the following six time points: the baseline and 7, 14, 21, 28, and 56 days postvaccination. For the recipients of two doses, the vertical dotted lines indicate the times of administration of the first and second doses, which were day 0 (baseline) and day 21, respectively, and the circles denote (from left to right) data from the following time points: the baseline and 7, 14, 21, 28, 35, 42, 49, and 77 days after administration of the first dose of vaccine. The baseline antibody level and any postvaccination antibody level which failed to achieve a 4-fold increase from the baseline are indicated by open circles. Positive antibody responses, defined by achievement of a ≥4-fold increase in titer compared to that at the baseline, are indicated by shaded circles.

FIG 2

Serum toxin-neutralizing antibody responses. Individual toxin-neutralizing antibody titer subsequent to administration of a single dose of 0.01 μg, 0.1 μg, 0.5 μg, 1 μg, 5 μg, 10 μg, or 20 μg or two doses of 20 μg of vaccine. The neutralization of IFN-γ production, expressed as the median (50%) inhibitory concentration (IC₅₀), in the supernatant of human PBMCs stimulated with 0.1 ng of SEB and blocked with serially diluted serum was measured by ELISA. For the recipients of single doses, the circles denote (from left to right) data from the following six time points: the baseline and 7, 14, 21, 28, and 56 days postvaccination. For the recipients of two doses, the vertical dotted lines indicate the times of administration of the first and second doses, which were day 0 (baseline) and day 21, respectively, and the circles denote (from left to right) data from the following time points: the baseline and 7, 14, 21, 28, 35, 42, 49, and 77 days after administration of the first dose of vaccine. The baseline antibody level and any postvaccination antibody level which failed to achieve a 4-fold increase from that at the baseline are indicated by open circles. Positive responses, defined by ≥4-fold increases in titer compared to that at the baseline, are indicated by shaded circles. One subject in the 10-μg-dose cohort did not consent to storage of that subject's research specimens for future use, and thus, specimens from that subject were not available for assessment of toxin-neutralizing activity.

FIG 3

Correlation of antibody titers by ELISA and TNA. A scatter plot of individual serum anti-SEB ELISA IgG titers (x axis) versus the SEB toxin-neutralizing antibody titer (y axis) is shown. (A) Representative results for all subjects and time points (n = 174; r = 0.94, 95% CI, 0.92 to 0.95); (B) representative results for all subjects at the baseline and at the times of the postvaccination peaks (n = 27; r = 0.71, 95% CI, 0.44 to 0.86).