KILLING BY CYTOTOXIC T-CELLS AND NATURAL-KILLER-CELLS - MULTIPLE GRANULE SERINE PROTEASES AS INITIATORS OF DNA FRAGMENTATION

The vectorial secretion of the contents of highly specialized cytoplas mic granules is of pivotal importance to the killing by cytotoxic T ce lls and natural killer cells. The purification and biochemical charact erization of some of the granule constituents, in particular the pore- forming protein perforin, had engendered the notion that the ki ling o f cellular targets was largely an osmotic phenomenon analogous to the insult delivered by complement attack. However, the apparent absence o f membrane perforation in various examples of lymphocyte-mediated kill ing, and the observation that perforin alone could not account for apo ptosis associated with programmed cell death, suggested that perforin activity represented, at best, only a part of the whole mechanism. Mor e recently, the characterization of a large family of granule serine p roteases (granzymes) has provided evidence that these molecules may co llaborate in the killing process by inducing a 'suicide' pathway in ta rget cells, resulting in DNA fragmentation. However, the serine protea ses are inactive alone, their natural substrates have not been defined and they require access into the target cell cytoplasm via perforin-i nduced pores to exert their deleterious effects. Thus, we propose that the cytotoxic granule-mediated mechanism comprises at least two inter dependent arms, perforin and serine proteases, that together are capab le of inflicting cell death by osmotic shock and/or nuclear collapse.