Caspase enzyme activity is not essential for apoptosis during thymocyte development

Caspases, a family of cysteine proteases, are critical mediators of apoptos is, To address the importance of caspases in thymocyte development, we have generated transgenic:mice that express the baculovirus protein p35, a vira l caspase inhibitor, specifically in the thymus, p35 expression inhibited F as (CD95)-, CD3-, or peptide-induced caspase activity in vitro and conferre d resistance to Fas-induced apoptosis, However, p35 did not block specific peptide-induced negative selection in OT1 and HY TCR transgenic mouse model s, Even the potent pharmacological caspase inhibitor zVAD-FMK (benzyloxycar bonyl-Val-Ala-Asp-fluoromethyl-ketone) could not prevent peptide-induced de letion of OT1 thymocytes, although it improved basal thymocyte survival in vitro. Moreover, the developmental block observed in rag1(-/-) thymocytes, which lack pre-TCR signaling, was also not rescued by p35 expression. These results indicate that caspase-independent signal transduction pathways can mediate thymocyte death during normal T cell development.