Evaluation of therapeutic effects of natural killer (NK) cell-based immunotherapy in mice using in vivo apoptosis bioimaging with a caspase-3 sensor

Natural killer (NK) cell-based immunotherapy is a promising strategy for cancer treatment, and caspase-3 is an important effector molecule in NK cell-mediated apoptosis in cancers. Here, we evaluated the antitumor effects of NK cell-based immunotherapy by serial noninvasive imaging of apoptosis using a caspase-3 sensor in mice with human glioma xeno-grafts. Human glioma cells expressing both a caspase-3 sensor as a surrogate marker for caspase-3 activation and Renilla luciferase (Rluc) as a surrogate marker for cell viability were established and referred to as D54-CR cells. Human NK92 cells were used as effector cells. Treatment with NK92 cells resulted in a time- and effector number-dependent increase in bioluminescence imaging (BLI) activity of the caspase-3 sensor in D54-CR cells in vitro. Caspase-3 activation by NK92 treatment was blocked by Z-VAD treatment in D54-CR cells. Transfusion of NK92 cells induced an increase of the BLI signal by caspase-3 activation in a dose- and time-dependent manner in D54-CR tumor-bearing mice but not in PBS-treated mice. Accordingly, sequential BLI with the Rluc reporter gene revealed marked retardation of tumor growth in the NK92-treatment group but not in the PBS-treatment group. These data suggest that noninvasive imaging of apoptosis with a caspase-3 sensor can be used as an effective tool for evaluation of therapeutic efficacy as well as for optimization of NK cell-based immunotherapy.