The effects of activin-A on NK-cell proliferation. Purified, CFSE-labeled NK cells were cultured at 10⁵ in 96-well plates in media supplemented with IL-2 (20 U/mL) + IL-12 (10 ng/mL) with or without addition of poly I:C (12.5 μg/mL) or LPS (100 ng/mL) in the absence or presence of activin-A (100 ng/mL) for 5 days. The level of NK-cell proliferation was assessed through the dilution of CFSE intensity by flow cytometry. Data represent the mean plus or minus 1 SD of 3 separate donors. **P < .05 and *P < .01 versus no activin.