Cognate CD4+ T cells induce licensing of DCs for enhanced memory CTL responses. (A to D) Summary and representative plots of CD8+ T-cell activation. (A) MoDCs were loaded with HCMV-derived pp65 and cocultured with 50,000 A2/NLVPMVATV-specific CD8+ T cells in the absence (upper graphs) or presence (lower graphs) of HLA-DRB1*01/KYQEFFWDANDIYRI-specific CD4+ T cells. Freshly thawed T cells were gated based on CD3 and CD8 expression and analyzed for activation-induced production of IFN-γ (left) and TNF (middle) and LAMP-1 surface expression (right). (B) Summary (mean + standard error of the mean [SEM]) of HCMV pp654–503 cross-presentation. Bars represent production of IFN-γ after coculture with MoDCs loaded with 3 μg pp65 in the presence of antigen-specific CD4+ T cells (mean, 120.9%; SEM, 15.8%; n = 6). The black bar shows a maximum response after stimulation with NLV peptide-loaded DCs. (C) Mean fluorescence intensity (MFI) of CD8+ T-cell cytokine production (IFN-γ and TNF) and LAMP-1 surface expression after coculture with pp65-loaded DCs with (red bars) or without (white bars) 100,000 antigen-specific CD4+ T cells (mean + SEM, n = 4). (D) MFI of IFN-γ, gated on IFN-γ-producing CD8+ T cells (mean + SEM, n = 4). (E) MoDCs were loaded with HCMV-derived pp65 and cocultured with 50,000 A2/NLVPMVATV-specific CD8+ T cells in the presence of donor-matched polyclonal CD4+ T cells (mean + SEM, n = 4). Significance in all panels was determined using a nonparametric Mann-Whitney test. *, P < 0.05.