During the course of an immune response, antigen-reactive T cells clonally expand and then are removed by apoptosis to maintain immune homeostasis. Life and death of T cells is determined by multiple factors, such as T-cell receptor triggering, co-stimulation or cytokine signalling, and by molecules, such as caspase-8 (FLICE)-like inhibitory protein (FLIP) and haematopoietic progenitor kinase 1 (HPK1), which regulate the nuclear factor-kappaB (NF-kappaB) pathway. Here, we discuss the concepts of activation-induced cell death (AICD) and activated cell-autonomous death (ACAD) in the regulation of life and death in T cells.

Caspases, a family of cysteine proteases, play a central role in apoptosis. During the last decade, major progress has been made to further understand caspase structure and function, providing a unique basis for drug design. This Review gives an overview of caspases and their classification, structure, and substrate specificity. We also describe the current knowledge of how interference with caspase signaling can be used to pharmacologically manipulate cell death.

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Mathematical modeling is required for understanding the complex behavior of large signal transduction networks. Previous attempts to model signal transduction pathways were often limited to small systems or based on qualitative data only. Here, we developed a mathematical modeling framework for understanding the complex signaling behavior of CD95(APO-1/Fas)-mediated apoptosis. Defects in the regulation of apoptosis result in serious diseases such as cancer, autoimmunity, and neurodegeneration. During the last decade many of the molecular mechanisms of apoptosis signaling have been examined and elucidated. A systemic understanding of apoptosis is, however, still missing. To address the complexity of apoptotic signaling we subdivided this system into subsystems of different information qualities. A new approach for sensitivity analysis within the mathematical model was key for the identification of critical system parameters and two essential system properties: modularity and robustness. Our model describes the regulation of apoptosis on a systems level and resolves the important question of a threshold mechanism for the regulation of apoptosis.

CD95 (APO-1/Fas) is a member of the death receptor (DR) family. Stimulation of CD95 leads to induction of apoptotic and nonapoptotic signaling pathways. The formation of the CD95 death-inducing signaling complex (DISC) is the initial step of CD95 signaling. Activation of procaspase-8 at the DISC leads to the induction of DR-mediated apoptosis. The activation of procaspase-8 is blocked by cellular FLICE-inhibitory proteins (c-FLIP). This review is focused on the role in the CD95-mediated signaling of the death effector domain-containing proteins procaspase-8 and c-FLIP. We discuss how dynamic cross-talk between procaspase-8 and c-FLIP at the DISC regulates life/death decisions at CD95. Open QuestionsThe exact mechanism of CD95-mediated non-apoptotic signaling is not established. The stoichiometry of the CD95 DISC is a question of future studies. New molecules may be found to be associated with the DISC. CD95Signaling CD95 (also called APO-1; Fas; fas antigen; tumor necrosis factor receptor superfamily member 6, TNFRSF6) is a member of the DR family, a subfamily of the tumor necrosis factor receptor superfamily. 1 All members of the DR family are characterized by a cytoplasmic region termed death domain (DD). 2,3 DD are 80-100 amino-acid long motifs involved in the transduction of the apoptotic signal. Crosslinking of CD95 with its natural ligand (L), CD95L (CD178) 4 or with agonistic antibodies such as anti-APO-1 5 induces apoptosis in sensitive cells.Stimulation of CD95 has been also reported to trigger nonapoptotic pathways. [6][7][8][9][10][11][12] However, details of CD95-mediated non-apoptotic pathways remain largely unknown. Importantly, it has been shown that membrane-bound CD95L is essential for the cytotoxic activity, whereas soluble CD95L appears to promote autoimmunity and tumorigenesis via induction of non-apoptotic pathways, in particular NF-kB. 13 Future studies should elucidate more details on the mechanism of nonapoptotic action of CD95L.Binding of CD95L or agonistic antibodies to CD95 leads to formation of a receptor complex at the cellular membrane, which was named DISC. 14 The DISC consists of oligomerized receptors, the DD-containing adaptor molecule FADD/MORT1 (Fas-associated DD), procaspase-8 (FLICE, MACHa, Mch5), procaspase-10 and the c-FLIP (Figure 1). [15][16][17] The interactions between the molecules at the DISC are based on homotypic contacts. The DD of the receptor interacts with the DD of FADD, whereas the death effector domain (DED) of FADD interacts with the N-terminal tandem DEDs of procaspases-8, -10 and c-FLIP. As a result of DISC formation procaspase-8 is activated at the DISC resulting in the formation of the active caspase-8, which leads to apoptosis.The initial events of DISC formation have not been clarified yet. Pre-oligomerization of CD95 via the pre-ligand assembly domain has been reported to have an important role in apoptosis initiation. 18 Recently, there have been several new reports on the X-ray structure of the complex formed by isolated CD95 and FADD DDs. 19,20 ...

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The CD95 (Fas/APO-1) death-inducing signaling complex (DISC) is essential for the initiation of CD95-mediated apoptotic and nonapoptotic responses. The CD95 DISC comprises CD95, FADD, procaspase-8, procaspase-10, and c-FLIP proteins. Procaspase-8 and procaspase-10 are activated at the DISC, leading to the formation of active caspases and apoptosis initiation. In this study we analyzed the stoichiometry of the CD95 DISC. Using quantitative western blots, mass spectrometry, and mathematical modeling, we reveal that the amount of DED proteins procaspase-8/procaspase-10 and c-FLIP at the DISC exceeds that of FADD by several-fold. Furthermore, our findings imply that procaspase-8, procaspase-10, and c-FLIP could form DED chains at the DISC, enabling the formation of dimers and efficient activation of caspase-8. Taken together, our findings provide an enhanced understanding of caspase-8 activation and initiation of apoptosis at the DISC.

c-FLIP proteins (isoforms: c-FLIPL, c-FLIPS, and c-FLIPR) play an essential role in the regulation of death receptor–induced apoptosis. Here, we demonstrate that the cytoplasmic NH2-terminal procaspase-8 cleavage product of c-FLIP (p22-FLIP) found in nonapoptotic malignant cells, primary T and B cells, and mature dendritic cells (DCs) strongly induces nuclear factor κB (NF-κB) activity by interacting with the IκB kinase (IKK) complex via the IKKγ subunit. Thus, in addition to inhibiting apoptosis by binding to the death-inducing signaling complex, our data demonstrate a novel mechanism by which c-FLIP controls NF-κB activation and life/death decisions in lymphocytes and DCs.