A novel serine esterase expressed by cytotoxic T lymphocytes

Cytotoxic T (Tc) lymphocytes recognize and lyse target cells and are thought to serve as an important defence against viral infections and possibly against neoplasms. The nature of the receptors responsible for antigen recognition by these cells is becoming clearer, but the molecular mechanisms responsible for their cytolytic activity remain largely unknown. The possibility that proteases are involved in this process has been suggested by the effects of certain inhibitors. Here we demonstrate that clones of murine Tc cells possess considerable trypsin-like esterase activity when assayed by a sensitive colorimetric assay. This activity was blocked completely by two serine esterase inhibitors, diisopropylfluorophosphoridate (DFP) and phenylmethylsulphonyl fluoride (PMSF), but not by N alpha-tosyl lysyl chloromethyl ketone (TLCK). The use of 3H-DFP as an affinity-labelling reagent demonstrated that the esterase activity resides in a protein of relative molecular mass (Mr) 28,000 (28K). A wide variety of other lymphocytes, including those from thymus, spleen and lymph node, established lines of B cells and noncytotoxic T cells, and clones of T helper cells, had about 300-fold less esterase activity than the Tc-cell clones and far smaller amounts of the DFP-reactive 28K protein. However, in thymocytes the esterase activity increased 20-50-fold and the 28K protein became more prominent 4 days after these cells had been stimulated in vitro to generate Tc cells.