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.