Caspase Enzyme Activity Is Not Essential for Apoptosis During Thymocyte Development

Abstract

Caspases, a family of cysteine proteases, are critical mediators of apoptosis. To address the importance of caspases in thymocyte development, we have generated transgenic mice that express the baculovirus protein p35, a viral caspase inhibitor, specifically in the thymus. p35 expression inhibited Fas (CD95)-, CD3-, or peptide-induced caspase activity in vitro and conferred resistance to Fas-induced apoptosis. However, p35 did not block specific peptide-induced negative selection in OT1 and HY TCR transgenic mouse models. Even the potent pharmacological caspase inhibitor zVAD-FMK (benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl-ketone) could not prevent peptide-induced deletion 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.

Uptake of apoptotic DC converts immature DC into tolerogenic DC that induce differentiation of Foxp3+ Treg

“…Systemic autoimmune diseases can be modeled in transgenic mice harboring defects in DC apoptosis [10] but not in mice with apoptosis defects in T and B cells [11][12][13]. Our study shows that in addition to the dogma of DC apoptosis as a mechanism to eliminate activated DC to prevent hyperactivation of the immune response, DC apoptosis also plays an active role in induction and maintenance of tolerance through induction of Treg, whereby defects in DC apoptosis may trigger autoimmunity.…”

“…DC apoptosis in itself is an important event for maintenance of tolerance. Defects in DC apoptosis have been linked to the development of autoimmunity with systemic autoimmune diseases modeled in transgenic mice harboring defects in DC apoptosis [10] but not in mice with apoptosis defects in T and B cells [11][12][13]. However, it is unclear how defects in DC apoptosis can trigger autoimmune responses.…”

“…We need to be cautious in interpreting our findings because our data indicates that several fold higher amounts of apoptotic DC are required than live DC for tolerance induction. However, in certain pathologies, such as sepsis, there could be local environments within lymphoid organs where apoptotic DC could potentially outnumber live DC.Systemic autoimmune diseases can be modeled in transgenic mice harboring defects in DC apoptosis [10] but not in mice with apoptosis defects in T and B cells [11][12][13]. Our study shows that in addition to the dogma of DC apoptosis as a mechanism to eliminate activated DC to prevent hyperactivation of the immune response, DC apoptosis also plays an active role in induction and maintenance of tolerance through induction of Treg, whereby defects in DC apoptosis may trigger autoimmunity.…”

The role of apoptosis in the development and function of T lymphocytes

“…p35 transgenic thymocytes are also resistant to anti-CD3 induced apoptosis in vitro [158]. However, in another study, the p35 transgene could only rescue 7-10% thymocytes from anti-CD3 induced apoptosis in vitro [159]. Furthermore, the p35 transgene has no effect on thymocyte deletion in H-Y and OT-1 transgenic mice [159].…”

“…However, Apaf-1 -/-thymocytes have no defect in negative selection [157]. Pan-caspase inhibitor p35 T cell specific transgenic mice led to contradictory results in terms of negative selection [158,159]. In one study, the p35 transgene rescues specific peptide and superantigen induced thymocyte apoptosis in F5 TCR transgenic mice in vivo [158].…”