Apoptosis is characterized by a series of dramatic perturbations to the cellular architecture that contribute not only to cell death, but also prepare cells for removal by phagocytes and prevent unwanted immune responses. Much of what happens during the demolition phase of apoptosis is orchestrated by members of the caspase family of cysteine proteases. These proteases target several hundred proteins for restricted proteolysis in a controlled manner that minimizes damage and disruption to neighbouring cells and avoids the release of immunostimulatory molecules.

Interleukin-33 (IL-33) is a member of the IL-1 family and is involved in polarization of T cells toward a T helper 2 (Th2) cell phenotype. IL-33 is thought to be activated via caspase-1-dependent proteolysis, similar to the proinflammatory cytokines IL-1 beta and IL-18, but this remains unproven. Here we showed that IL-33 was processed by caspases activated during apoptosis (caspase-3 and -7) but was not a physiological substrate for caspases associated with inflammation (caspase-1, -4, and -5). Furthermore, caspase-dependent processing of IL-33 was not required for ST2 receptor binding or ST2-dependent activation of the NF-kappaB transcription factor. Indeed, caspase-dependent proteolysis of IL-33 dramatically attenuated IL-33 bioactivity in vitro and in vivo. These data suggest that IL-33 does not require proteolysis for activation, but rather, that IL-33 bioactivity is diminished through caspase-dependent proteolysis within apoptotic cells. Thus, caspase-mediated proteolysis acts as a switch to dampen the proinflammatory properties of IL-33.

Members of the caspase family of cysteine proteases play central roles in coordinating the stereotypical events that occur during apoptosis. Because the major executioner caspases, caspase-3 and caspase-7, exhibit almost indistinguishable activity toward certain synthetic peptide substrates, this has led to the widespread view that these proteases occupy functionally redundant roles within the cell death machinery. However, the distinct phenotypes of mice deficient in either of these caspases, as well as mice deficient in both, is at odds with this view. These distinct phenotypes could be related to differences in the relative expression levels of caspase-3 and caspase-7 in vivo, or due to more fundamental differences between these proteases in terms of their ability to cleave natural substrates. Here we show that caspase-3 and caspase-7 exhibit differential activity toward multiple substrate proteins, including Bid, XIAP, gelsolin, caspase-6, and cochaperone p23. Caspase-3 was found to be generally more promiscuous than caspase-7 and appears to be the major executioner caspase during the demolition phase of apoptosis. Our observations provide a molecular basis for the different phenotypes seen in mice lacking either caspase and indicate that these proteases occupy nonredundant roles within the cell death machinery.apoptosis ͉ caspase substrates ͉ proteolysis

Dying cells have an important role in the initiation of CD8 T cell-mediated immunity. The cross-presentation of antigens derived from dying cells enables dendritic cells to present exogenous tissue-restricted or tumour-restricted proteins on MHC class I molecules. Importantly, this pathway has been implicated in multiple autoimmune diseases and accounts for the priming of tumour antigen-specific T cells. Recent data have revealed that in addition to antigen, dying cells provide inflammatory and immunogenic signals that determine the efficiency of CD8 T cell cross-priming. The complexity of these signals has been evidenced by the multiple molecular pathways that result in cell death and that have now been shown to differentially influence antigen transfer and immunity. In this Review, we provide a detailed summary of both the passive and active signals that are generated by dying cells during their initiation of CD8 T cell-mediated immunity. We propose that molecules generated alongside cell death pathways - inducible damage-associated molecular patterns (iDAMPs) - are upstream immunological cues that actively regulate adaptive immunity.

Granzyme B is a cytotoxic lymphocyte-derived protease that plays a central role in promoting apoptosis of virus-infected target cells, through direct proteolysis and activation of constituents of the cell death machinery. However, previous studies have also implicated granzymes A and B in the production of proinflammatory cytokines, via a mechanism that remains undefined. Here we show that IL-1α is a substrate for granzyme B and that proteolysis potently enhanced the biological activity of this cytokine in vitro as well as in vivo. Consistent with this, compared with full-length IL-1α, granzyme B-processed IL-1α exhibited more potent activity as an immunoadjuvant in vivo. Furthermore, proteolysis of IL-1α within the same region, by proteases such as calpain and elastase, was also found to enhance its biological potency. Thus, IL-1α processing by multiple immune-related proteases, including granzyme B, acts as a switch to enhance the proinflammatory properties of this cytokine.

Bax and Bak promote apoptosis by perturbing the permeability of the mitochondrial outer membrane and facilitating the release of cytochrome c by a mechanism that is still poorly defined. During apoptosis, Bax and Bak also promote fragmentation of the mitochondrial network, possibly by activating the mitochondrial fission machinery. It has been proposed that Bax/Bak-induced mitochondrial fission may be required for release of cytochrome c from the mitochondrial intermembrane space, although this has been a subject of debate. Here we show that Bcl-xL, as well as other members of the apoptosis-inhibitory subset of the Bcl-2 family, antagonized Bax and/or Bak-induced cytochrome c release but failed to block mitochondrial fragmentation associated with Bax/Bak activation. These data suggest that Bax/Bak-initiated remodeling of mitochondrial networks and cytochrome c release are separable events and that Bcl-2 family proteins can influence mitochondrial fission-fusion dynamics independent of apoptosis.

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