Overexpression of ERK1 within primary human NK cells increases IFN-γ production in response to immobilized IgG and IL-12. Primary human NK cells were transduced with wild-type ERK1 or a DN form of ERK1 using the PINCO retroviral transfection system. Following infection, NK cells were sorted on the basis of GFP expression. (A) Representative flow plots from NK cells transduced with the empty vector or with wild-type ERK1 prior to sorting and following sorting for GFP⁺ cells. (B) Equal numbers of sorted NK cells were cultured separately on immobilized IgG in the presence of 10 ng/mL IL-12. Culture supernatants were harvested at 24 hours for quantification of IFN-γ release by ELISA. (C) Real-time PCR of sorted NK cells for ERK mRNA expression. Results represent the mean plus or minus SEM from n = 3 independent experiments. *P < .01 versus vector control cells.