CD137 enhances antitumor reactivity of mouse NK cells. (A) CD137L surface expression on P388D.1 cells transfected with mouse CD137L (P388D.1-CD137L) and mock controls (P388D.1-mock) was determined by FACS using CD137L mAb (shaded peaks) and isotype control (open peaks). (B) Mouse splenocytes were cultured for 48 hours in the presence or absence of 250 U/mL human IL-2 or 10 ng/mL human IL-15. Subsequently, CD137 expression was determined using CD137 mAb and rat IgG1 isotype control. Numbers in top right quadrants indicate the percentage of CD137⁺ NK1.1⁺CD3⁻ NK cells. (C-E) Mouse splenocytes were cultured for 48 hours with 250 U/mL human IL-2 and then incubated with P388D.1 transfectants in the presence or absence of 10 μg/mL rat IgG2a or blocking CD137L mAb. (C) Granule mobilization after 3 hours was analyzed by FACS. Numbers in top right quadrants indicate the percentage of CD107a⁺ NK1.1⁺CD3⁻ mouse NK cells. (D) IFN-γ levels in culture supernatants were determined after 24 hours by ELISA. Results represent means of triplicates with SDs. (E) Frequency of IFN-γ–producing mouse NK cells was determined after 8 hours by intracellular FACS. Stimulation with PMA/ionomycin served as positive control. Numbers in top right quadrants indicate percent of IFN-γ⁺ NK1.1⁺CD3⁻ NK cells. One representative experiment each of a total of at least 3 with similar results is shown. *Statistically significant differences.