LCMV-specific effector and memory Th1 cells rapidly mobilize preformed CD40L upon antigenic stimulation. The surface mobilization assay was combined with intracellular cytokine staining of LCMV-infected splenocytes to detect mobilization of preformed CD40L on the cell surface of LCMV-specific effector (A) (day 9 after infection) and memory (B) (6 months after infection) Th1 cells 30 minutes after stimulation with LCMV GP61 peptide-pulsed APCs as described in “Materials and methods, Flow cytometric analysis of CD40L expression.” The levels of CD40L (shaded) and isotype control (line) for IFN-γ⁺ and IFN-γ⁻ populations are shown. Numbers in plots indicate the percentage of that population among CD4⁺ cells in the sample. Data shown are representative of 3 independent experiments.