Fig. 4.

The magnitude of the YFV-effector CD8 T-cell correlates with viral burden. (A) The kinetics for the mean % Ki-67⁺Bcl-2^lo CD8 T cells are dramatically different between vaccinees grouped by high (>100 genomes per mL, dashed blue line) or low (<100 genomes per mL, solid black line) viral load. The former shows significantly more responding T cells than the latter. Error bars show SEM. (B and C) The effector CD8 T-cell response (% Ki-67⁺Bcl-2^lo of CD8 T cells) at day 14 correlates with the peak viral load. (B) We plot (dashed red line) the best fit of a piecewise-linear function (linear up to a threshold, constant above the threshold; Methods) relating the viral load and CD8 T-cell response. When viral load is below a threshold value of ∼225 genome/mL (shaded in gray), viral load strongly predicts the magnitude of the CD8 T-cell response (R² = 0.68 with a 95% bootstrap confidence interval of 0.16–0.87); when viral load is above this value, the CD8 T-cell response saturates. (C) We show a similar result if we use a smooth saturating function in place of the piecewise-linear function. In both B and C we see that the magnitude of the CD8 response increases with viral load, rapidly at first and then saturating at high viral loads. Both piecewise and smooth saturating models fit equally well based on AIC values.