YF17D vaccination induces specific CD4+ T cells of mixed T helper phenotype. (a) PBMCs from YF17D-vaccinated volunteers (day 28 after vaccination) were stimulated with a group of three immunostimulatory YF17D-derived peptides pools, and then stained with antibodies against CD4 (surface) and CD154, IL-2, and IFN-g (intracellular). The numbers indicate the percentages of cells within the parent population. (b) FACS analysis of PBMCs from day 365 after vaccination. Cells from four volunteers were stimulated (S) for with immunostimulatory YF17D peptide pools, and then restimulated (RS) or not with the same peptide pools before staining. The data were analyzed according to the gating strategy shown in Fig. S7, and are expressed as the percentage of central memory CD4+ T cells (CD45RA2CCR7+) that express the marker for recent activation CD154, and that are either Th1 or Th2 in the RS samples, over background (S). Fig. S7 is available.