A study of proteolysis as a possible mechanism for T-cell-mediated target cell lysis

The hypothesis that proteolysis is implicated in T-cell-mediated target cell lysis was tested in two ways: first, various inhibitors of protease activity were used either during or before the cytotoxicity test. Ovomucoid, TPCK, and PMSF proved to be not or only marginally inhibitory, whereas the considerable inhibitory action of TLCK was shown to most likely be a toxic effect of the inhibitor. Second, radioactive substrates for cathepsin activity were used, either free in the medium or fixed on the surface of the target cells, to detect a possible breakdown of these substrates by a release of intracellular cathepsins during the cytotoxicity test. Very little dialysable breakdown products were formed, and there was no difference between their amount in either a homologus or a heterologous system. It is concluded that proteolysis probably is not the mechanism by which target cells are killed. Alternative mechanisms are discussed.