Granzyme H induces apoptosis of target tumor cells characterized by DNA fragmentation and Bid-dependent mitochondrial damage.

Natural killer (NK) cells are the effectors of innate immunity to act as the first line of defense against viruses and tumors. Granzyme H (GzmH) is predicted to evolve from GzmB and constitutively expressed at a high level in human NK cells. It indicates GzmH plays a pivotal role in NK cell mediated cytolysis. However GzmH is defined as an orphan granzyme and its function has less been defined. Here we demonstrate GzmH can induce rapid apoptosis of target cells, which is dependent on caspase activation and mitochondrial damage. GzmH-induced death is characterized by phophatidylserine externalization, nuclear condensation, DNA fragmentation, caspase activation and cytochrome c release that are hallmarks of typical apoptosis. GzmH can directly cleave ICAD to unleash CAD for DNA fragmentation. Moreover, GzmH directly processes Bid to produce the active form tBid leading to cytochrome c release. Therefore, GzmH may play an essential role in caspase-dependent pathogen clearance in the innate immunity that may complement the proapoptotic function of GzmB in human NK cells.