Normal human CD4⁺ memory T cells display broad heterogeneity in their activation threshold for cytokine synthesis

CD4⁺ memory T cells coordinate immune responses against viruses and other pathogens via the Ag-induced secretion of potent effector cytokines. The efficacy of these responses depends on both the overall number of pathogen-specific memory T cells and the particular array of cytokines that these cells are programmed to secrete. Here, we provide evidence that heterogeneity in Ag triggering thresholds constitutes an additional critical determinant of memory T cell function. Using a novel assay that allows single-cell detection of Ag-specific T cell cytokine production, we demonstrate that CMV-specific CD4⁺ memory cells from human peripheral blood display pronounced differences in their costimulatory requirements for Ag- induced triggering of IFN-γ and IL-2 secretion, ranging from cells that trigger with little costimulation (e.g., resting APC alone) to cells requiring potent costimulation through multiple pathways (resting APC plus multiple costimulatory mAbs, or activated APC). These differences in costimulatory requirements are independent of clonal differences in TCR signaling intensity, consistent with an intrinsic activation-threshold heterogeneity that is 'downstream' from the TCR. Thus, 'effective' frequencies of Ag-specific CD4⁺ memory T cells appear to depend on the activation status of available APC, a dependence that would allow the immune system to rapidly adjust the number of functional Ag-specific memory T cells in a particular effector site according to local conditions.