Calcein release killing assay was performed as described previously (28 (link)). Briefly, NK cells were isolated from naïve C57BL/6 mouse spleens with an NK cell negative selection isolation kit (Miltenyi biotec, Bergisch Gladbach). Target cells, MOC2 tumor cells, were stained with Calcein (ThermoFisher, Waltham MA) in a 2 μg/mL solution in RPMI media with 10% FBS. NK cells were incubated with target cells at an effector:target ratio of 2:1. Stimulation was added as indicated at the following concentrations: IL-2, 1000 U/mL (29 (link)), IL-15, 20 ng/mL (29 (link)), IL-15 SA, 50 ng/mL (30 ), anti-CD25, 10 mg/mL. IL-15SA was created by mixing 0.75 μg rIL-15 (ebioscience) with 7 μg rIL-15RA-FC chimera protein (R&D systems), and incubating for 30 mins at 37°C. Stimulants were added to cultures and incubated for 4 hours. Plates were centrifuged, and supernatant was removed to a flat bottom plate. Fluorescence was read on Tecan Infinite M plex fluorescence plate reader and 485 nm/530 nm ratio was calculated. Specific cell lysis was calculated through the equation: (Test release−Spontaneous release)/(Maximum release−Spontaneous release)]×100. Maximum release was calculated following incubation of target cells with 1% triton X in media, and spontaneous release was calculated following incubation of target cells in 10% RPMI with no stimulus.