Induction of apoptosis by canine natural killer cells

Besides a secretory pathway of canine natural killer (NK) cells, which results in necrosis of the target cell, a second pathway was demonstrated, which results in apoptosis of the target cell. Comparing the Chromium Release Assay (CRA) and the Rose Bengal Assay (RBA) for quantification of in vitro canine NK cell activity, a constant 10% higher NK cell activity was found in the RBA compared with the CRA. To find out the mechanism responsible for the different results of both tests, morphological studies of in vitro canine NK cell activity against epithelial and mesenchymal allogenic target cell lines were performed. Most target cells were undergoing necrosis as a result of NK cell killing, which was evidenced by transmission electron microscopy. However, besides necrotic target cells, shrunken target cells with dense cytoplasm, fragmented nuclei and disruption into membrane-bound bodies were detected, which are known as signs of apoptosis. Additionally, using the terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) method, 13-23% of target cells presented a positive staining, indicative of apoptosis. These findings give evidence for the ability of canine NK cells to kill their target cells via two different pathways, which results either in apoptosis or necrosis.