CD8A and neoplasm: For example, Listeria infection can directly kill tumor cells through the activation of nicotinamide adenine dinucleotide phosphate (NADP+) oxidase and elevated intracellular Ca2+ amount.[26] Both of the two mechanisms can form reactive oxygen species (ROS), which is a highly cytotoxic free radical.[27] ROS can initiate the immunogenic death of tumor cells, then activate CD8+ T cells to eliminate residual tumor cells, and finally prevent metastases.