NK cell cytotoxicity against tumor spheroids of cervical carcinoma. Tumor spheroids grown from CFSE-labeled CaSki or SiHa cells (d0) were co-cultured with primary human NK cells at different effector-to-target (E:T) ratio for 48 h. Tumor spheroids without NK cells served as controls (CO). (A) Tumor spheroid destruction was monitored by transmission and fluorescence microscopy at 100× magnification. Size bars correspond to 200 μm. (B) Flow cytometric analysis of residual viable target cells. Individual tumor spheroids subjected to cytotoxicity assays with NK cells were disintegrated, and NK cells were stained for CD45. The percentage of residual viable cells was calculated by evaluation of the gates for NK cells (CD45+/SytoxBlue−) and target cells (CFSE+/SytoxBlue−). Dot blots show the percentage of viable target cells in individual tumor spheroids. Data are shown as mean ± SEM of a representative experiment.