CD4 and infection: To investigate potential synergies between cytokines in driving CD25 expression on NK cells, PBMCs were stimulated with combinations of IL-12, IL-15, and IL-18, with or without varying concentrations of IL-2, to model early NK cell activation in response to primary pathogen infection (innate cytokines only, no IL-2) and secondary infection (innate cytokines plus IL-2 from memory CD4+ T cells).