FIG. 2.

Purified peripheral blood CD4⁺ (A) or CD8⁺ (B) cells from volunteers vaccinated with serovar Typhi Ty21a were stimulated with 10⁷ heat-killed Ty21a bacteria/ml in the presence of APC at different time points before and after vaccination. Two days after the antigen stimulation, culture supernatants were removed, values for the six replicate wells were pooled, and the IFN-γ levels were measured and normalized by division with the IFN-γ levels of PHA-stimulated cells. The data shown are fold increases of normalized IFN-γ production compared to prevaccination levels. Prevaccination levels were as follows: for CD4⁺ cells, the median rate of production was 390 pg/ml (range, <100 to 2,900 pg/ml); for CD8⁺ cells, the median rate of production was 1,600 pg/ml (range, <100 to 12,100 pg/ml). Values from each of the six individual vaccinees are shown. A paired t test was performed to compare levels at the peak time point with the prevaccination levels. P = 0.027 (A) and 0.045 (B).