Engagement of activating receptors differentially recruits Rab27a and Munc13-4 to perforin granules in resting NK cells. Resting NK cells were incubated with S2 cells expressing human ICAM-1, CD48, or ULBP1 singly or in combination or coated with human serum, for 20 minutes at 37°C, followed by fixation, permeabilization, and labeling. Rab27a (A) and Munc13-4 (B) colocalization with perforin was calculated as Pearson correlation coefficient, r, using the JACOP plugin in ImageJ. Ten to 15 NK cells in conjugates were analyzed per condition, and data from 3 or 4 donors were compiled. Mean values are plotted; error bars represent SD. Two-tailed paired Student t tests were performed to assess significance.