Early activation of caspases during T lymphocyte stimulation results in selective substrate cleavage in nonapoptotic cells

Apoptosis induced by T cell receptor (TCR) triggering in T lymphocytes invo lves activation of cysteine proteases of the caspase family through their p roteolytic processing. Caspase-3 cleavage was also reported during T cell s timulation in the absence of apoptosis, although the physiological relevanc e of this response remains unclear. We show here that the caspase inhibitor benzyloxycarbonyl (Cbz)-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD) blocks proliferation, major histocompatibility complex class II expression, and bl astic transformation during stimulation of peripheral blood lymphocytes. Mo reover, T cell activation triggers the selective processing and activation of downstream caspases (caspase-3, -6, and -7), but not caspase-1, -2, or - 4, as demonstrated even in intact cells using a cell-permeable fluorescent substrate. Caspase-3 processing occurs in different T cell subsets (CD4(+), CD8(+), CD45RA(+), and CD45RO(+)), and in activated B lymphocytes. The pat hway leading to caspase activation involves death receptors and caspase-8, which is also processed after TCR triggering, but not caspase-9, which rema ins as a proenzyme. Most importantly, caspase activity results in a selecti ve substrate specificity, since poly(ADP-ribose) polymerase (PARP), lamin B , and Wee1 kinase, but not DNA fragmentation factor (DFF45) or replication factor C (RFC140), are processed. Caspase and substrate processing occur in nonapoptotic lymphocytes. Thus, caspase activation is an early and physiol ogical response in viable, stimulated lymphocytes, and appears to be involv ed in early steps of lymphocyte activation.