CD40L⁺ CD8⁺ T cells in human antigen–specific responses. (A) Human PBMCs from healthy donors were stimulated with peptide pools from different virus antigens. The dot-plots show representative intracellular cytokine and CD40L staining of CD3⁺ CD8⁺ CD4⁻–gated lymphocytes. (B) The frequencies of CD40L⁺ and IFN-γ⁺ virus-specific CD8⁺ T cells and the corresponding ratio of CD40L⁺ cells among virus-specific IFN-γ⁺ CD8⁺ T cells are summarized for CMV (n = 16), EBV (n = 17), and flu (n = 26) (mean ± SEM). (C) At different time points after YFV vaccination, PBMCs from healthy donors were stimulated with the immunodominant peptide YFV-NS4B. The dot-plots show the intracellular IFN-γ and CD40L staining of CD3⁺ CD8⁺ CD4⁻–gated lymphocytes from 1 representative donor (n = 6).