The suicide in the thymus, a twisted trail

Cell Death in the Origin and Treatment of Cancer

Cell death, or, more specifically, cell suicide, is a process of fundamental importance to human health. Throughout our lives, over a million cells are produced every second. When organismal growth has stopped, to balance cell division, a similar number of cells must be removed. This is achieved by activation of molecular mechanisms that have evolved so that cells can destroy themselves. The first clues regarding the nature of one of these mechanisms came from studying genes associated with cancer, in particular the gene for BCL-2. Subsequent studies revealed that mutations or other defects that inhibit cell death allow cells to accumulate, prevent removal of cells with damaged DNA, and increase the resistance of malignant cells to chemotherapy. Knowledge of this mechanism has allowed development of drugs that kill cancer cells by directly activating the cell death machinery and by synergizing with conventional chemotherapy as well as targeted agents to achieve improved outcomes for cancer patients.

T Cells and Regulated Cell Death: Kill or Be Killed

Cell death plays two major complementary roles in T cell biology: mediating the removal of cells that are targeted by T cells and the removal of T cells themselves. T cells serve as major actors in the adaptive immune response and function by selectively killing cells which are infected or dysfunctional. This feature is highly involved during homeostatic maintenance, and is relied upon and modulated in the context of cancer immunotherapy. The vital recognition and elimination of both autoreactive T cells and cells which are unable to recognize threats is a highly selective and regulated process. Moreover, detection of potential threats will result in the activation and expansion of T cells, which on resolution of the immune response will need to be eliminated. The culling of these T cells can be executed via a multitude of cell death pathways which are used in context-specific manners. Failure of these processes may result in an accumulation of misdirected or dysfunctional T cells, leading to complications such as autoimmunity or cancer. This review will focus on the role of cell death regulation in the maintenance of T cell homeostasis, as well as T cell-mediated elimination of infected or dysfunctional cells, and will summarize and discuss the current knowledge of the cellular mechanisms which are implicated in these processes.

Defects in the Bcl-2-regulated apoptotic pathway lead to preferential increase of CD25 low Foxp3+ anergic CD4+ T cells

Defects in the Bcl-2-regulated apoptotic pathway inhibit the deletion of self-reactive T cells. What is unresolved, however, is the nature and fate of such self-reactive T cells escaping deletion. In this study, we report that mice with such defects contained increased numbers of CD25(low)Foxp3(+) cells in the thymus and peripheral lymph tissues. The increased CD25(low)Foxp3(+) population contained a large fraction of cells bearing self-reactive TCRs, evident from a prominent increase in self-superantigen-specific Foxp3(+)Vβ5(+)CD4(+) T cells in BALB/c Bim(-/-) mice compared with control animals. The survival rate of the expanded CD25(low)Foxp3(+) cells was similar to that of CD25(high)Foxp3(+) CD4 T cells in vitro and in vivo. IL-2R stimulation, but not TCR ligation, upregulated CD25 on CD25(low)Foxp3(+)CD4(+) T cells in vitro and in vivo. The expanded CD25(low)Foxp3(+)CD4(+) T cells from Bim(-/-) mice were anergic but also had weaker regulatory function than CD25(high)Foxp3(+) CD4(+) T cells from the same mice. Analysis of Bim(-/-) mice that also lacked Fas showed that the peripheral homeostasis of this expanded population was in part regulated by this death receptor. In conclusion, these results show that self-reactive T cell escapes from thymic deletion in mice defective in the Bcl-2-regulated apoptotic pathway upregulate Foxp3 and become unresponsive upon encountering self-Ag without necessarily gaining potent regulatory function. This clonal functional diversion may help to curtail autoaggressiveness of escaped self-reactive CD4(+) T cells and thereby safeguard immunological tolerance.

Apoptotic signal transduction and T cell tolerance

The healthy immune system makes use of a variety of surveillance mechanisms at different stages of lymphoid development to prevent the occurrence and expansion of potentially harmful autoreactive T cell clones. Disruption of these mechanisms may lead to inappropriate activation of T cells and the development of autoimmune and lymphoproliferative diseases [such as multiple sclerosis, rheumatoid arthritis, lupus erythematosus, diabetes and autoimmune lymphoproliferative syndrome (ALPS)]. Clonal deletion of T cells with high affinities for self-peptide-MHC via programmed cell death (apoptosis) is an essential mechanism leading to self-tolerance. Referred to as negative selection, central tolerance in the thymus serves as the first checkpoint for the developing T cell repertoire and involves the apoptotic elimination of potentially autoreactive T cells clones bearing high affinity T cell receptors (TCR) that recognize autoantigens presented by thymic epithelial cells. Autoreactive T cells that escape negative selection are held in check in the periphery by either functional inactivation ("anergy") or extrathymic clonal deletion, both of which are dependent on the strength and frequency of the TCR signal and the costimulatory context, or by regulatory T cells. This review provides an overview of the different molecular executioners of cell death programs that are vital to intrathymic or extrathymic clonal deletion of T cells. Further, the potential involvement of various apoptotic signaling paradigms are discussed with respect to the genesis and pathophysiology of autoimmune disease.

No requirement for TRAIL in intrathymic negative selection

The contribution of the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) pathway to intrathymic negative selection is a controversial subject with two studies suggesting a key role for TRAIL, while others demonstrated normal negative selection, in TRAIL- and TRAIL receptor-deficient mice. The basis of these discrepancies is unclear and may in part reflect differences in the negative selection models under investigation. Considering the importance of the negative selection process in the establishment of a competent immune system, it is essential that these discrepancies be fully resolved. In this study, we failed to identify a role for TRAIL in an acute model of peptide antigen-specific negative selection using a TCR transgenic system as well as antibody-mediated TCR/CD3 ligation in vitro and in vivo. Moreover, thymic dendritic cells, the main cellular mediators of negative selection in the thymus, did not constitutively express TRAIL, and TRAIL receptor (DR5) expression was negative or extremely low on thymocytes. Furthermore, in vitro thymocyte deletion was normal in C57BL/6 TRAIL(-/-) gld mice, suggesting that TRAIL and FasL do not function cooperatively to induce negative selection. These results, combined with the fact that aged C57BL/6 TRAIL(-/-) mice showed no signs of spontaneous autoimmunity, strongly indicate that intrathymic negative selection occurs normally in the absence of TRAIL signaling.

Bid is cleaved upstream of caspase-8 activation during TRAIL-mediated apoptosis in human osteosarcoma cells

TRAIL induces apoptosis in many malignant cell types. In this study, we used the human papilloma virus (HPV) 16 E6 protein as a molecular tool to probe the TRAIL pathway in HCT116 colon carcinoma cells and U2OS osteosarcoma cells. Intriguingly, we found that while E6 protected HCT116 cells from TRAIL, U2OS cells expressing E6 remained sensitive to TRAIL. Furthermore, silencing FADD and procaspase-8 expression with siRNA did not prevent TRAIL-induced apoptosis in U2OS cells. However, siBid provided significant protection from TRAIL, and the cleavage kinetics of Bid and caspase-8 revealed that Bid was cleaved prior to the activation of caspase-8. Cathepsin B activity in U2OS cells was significantly activated shortly after exposure to TRAIL, and the cathepsin B inhibitor, CA074Me, inhibited both TRAIL- and anti-DR5-mediated apoptosis and delayed the cleavage of Bid. These findings suggest that TRAIL activates a pathway dependent on Bid, but largely independent of FADD and caspase-8, in U2OS cells.

TNF-related apoptosis-inducing ligand as a therapeutic agent in autoimmunity and cancer

Recombinant, soluble TNF-related apoptosis-inducing ligand (TRAIL) is currently being developed as a promising natural immune molecule for trial in cancer patients because it selectively induces apoptosis in transformed or stressed cells but not in most normal cells. In cancer patients, phase 1 and 2 clinical trials using agonistic mAbs that engage the human TRAIL receptors DR4 and DR5 have also provided encouraging results. It is now evident that TRAIL suppresses autoimmune disease in various experimental animal models, suggesting that the therapeutic value of recombinant TRAIL and agonistic DR4 and DR5 mAbs might also extend to the suppression of autoimmune disease. This review provides an insight into our current understanding of the role(s) of TRAIL in disease, with a specific focus on cancer and autoimmunity. We also emphasize biological agents and drugs that sensitize tumour cells to TRAIL-mediated apoptosis and discuss the potential molecular basis for their sensitization.

HIV-1 infection and AIDS: consequences for the central nervous system

Infection with the human immunodeficiency virus-1 (HIV-1) can induce severe and debilitating neurological problems that include behavioral abnormalities, motor dysfunction and frank dementia. After infiltrating peripheral immune competent cells, in particular macrophages, HIV-1 provokes a neuropathological response involving all cell types in the brain. HIV-1 also incites activation of chemokine receptors, inflammatory mediators, extracellular matrix-degrading enzymes and glutamate receptor-mediated excitotoxicity, all of which can trigger numerous downstream signaling pathways and disrupt neuronal and glial function. This review will discuss recently uncovered pathologic neuroimmune and degenerative mechanisms contributing to neuronal damage induced by HIV-1 and potential approaches for development of future therapeutic intervention.

Apoptosis and autoimmunity

Autoimmune diseases reflect the confluence of genetic, environmental and stochastic events. Recent studies have implicated apoptotic cell death pathways in initiating and propagating autoimmune diseases, as well as in rendering individuals susceptible to such diseases. Similar to autoimmunity, apoptosis is a multistep process, affecting immune and target cells, integrating numerous intrinsic and extrinsic signals, and requiring the actions of multiple gene products. Particularly relevant to the complexity of autoimmunity are the recent observations that apoptotic death might provide a primary source of tolerogen to shape the immune repertoire, or be the target of the immune response in autoimmunity, and that apoptosis is both required for lymphocyte selection and immunoregulation, and is a prominent outcome of immune and inflammatory effector pathways.

The signaling and apoptotic effects of TNF-related apoptosis-inducing ligand in HIV-1 associated dementia

HIV-1 Associated Dementia (HAD) develops during progressive HIV-1 infection and is characterized by cognitive impairments, behavioral disorders and potential progressive motor abnormality. Abnormal inflammation within the central nervous system (CNS), activation of macrophage/microglia and involvement of proinflammatory cytokines have been suggested as primary factors in the pathogenesis of HAD. Impairment of neuronal function and neuronal cell death are believed to be the end pathophysiological result of HAD. TNF-related apoptosis-inducing ligand (TRAIL), a member of the TNF family of cytokines, was suggested to participate in apoptotic cell death during HAD. As a death ligand, TRAIL was originally thought to target only tumor cells. TRAIL is not typically present in CNS; however, emerging data show that TRAIL can be induced by immune stimuli on macrophage and microglia, major disease effector cells during HAD. Upregulated TRAIL may then cause neuronal apoptosis through direct interaction with TRAIL receptors on neurons or through macrophage death-mediated release of neurotoxins. In this review, we summarize the pivotal role of TRAIL in HAD and TRAIL-initiated intracellular death cascades that culminate in neuronal apoptosis as observed in HAD.

Following a TRAIL: update on a ligand and its five receptors

Identification of tumour necrosis factor apoptosis inducing ligand (TRAIL), a TNF family ligand, sparked a torrent of research, following an initial observation that it could kill tumour cells, but spare normal cells. Almost a decade after its discovery, and with five known receptors, the true physiological role of TRAIL is still debated and its anti-tumorigenic properties limited by potential toxicity. This review takes a comprehensive look at the story of this enigmatic ligand, addressing its remaining potential as a therapeutic and providing an overview of the TRAIL receptors themselves.

The "fuzzy logic" of the death-inducing signaling complex in lymphocytes

Receptors belonging to the tumor necrosis factor receptor family have long been thought to play an important role in the regulation of immunity. Although this family is composed of a large number of surface receptors that potentiate myriad functions in vivo, a subset is known to directly convey apoptotic signals. One such molecule belonging to this subset is CD95. Ligation of CD95 instigates the formation of a complex known as the "death-inducing signaling complex" or DISC, which is composed of molecules including FADD (Fas associated with death domain) and RIP (receptor-interacting kinase), as well as procaspases-8 and -10, and a caspase-8-like molecule that lacks proteolytic activity called c-FLIP. Although the DISC was initially thought to serve an exclusively proapoptotic role, humans and mice with defects in various components of this complex demonstrate a variety of developmental and hematopoietic defects that are not apparently due to aberrant apoptosis. These findings paint a far more complex picture of the numerous components of the DISC, and provide evidence that these complexes serve nonapoptotic functions. Herein, we summarize the experimental evidence challenging the notion that the DISC imparts an exclusively apoptotic function and provide hypotheses to account for these alternative roles. Rather than operating as a binary system, we propose that the DISCs formed around various DRs transduce signals leading to a variety of cellular fates.

Regulation of cell death in the lymphoid system by Bcl-2 family proteins

Programmed cell death is an ordered process that is essential for the normal development and homeostasis of an organism. Dysregulation of this programmed pathway, resulting in either excess cell numbers or unscheduled cell death, underlies a number of disease states. Bcl-2 family proteins play a key role in regulating cell death and survival, and a number of studies have demonstrated their role as important regulators of cell fate in the lymphoid system. Mice that are genetically deficient or overexpress various Bcl-2 family proteins have provided important clues regarding their roles in lymphocyte development, progression of lymphoid tumors and analogous human disorders. In addition, lymphotropic viruses may trigger cell proliferation and inhibit cell death with the help of their own Bcl-2 homologues. Comparing the shared and distinct functions of viral and cellular Bcl-2-related proteins yields new insight into their fundamental mechanisms.

Kupffer cell engulfment of apoptotic bodies stimulates death ligand and cytokine expression

Hepatocyte apoptosis by death receptors, hepatic inflammation, and fibrosis are prominent features of liver diseases. However, the link between these processes remains unclear. Our aim was to ascertain whether engulfment of apoptotic bodies by Kupffer cells promotes hepatic inflammation and fibrosis. Isolated murine Kupffer cells efficiently engulfed apoptotic bodies generated from UV-treated mouse hepatocytes. Engulfment of the apoptotic bodies, but not latex beads, stimulated Kupffer cell generation of death ligands, including Fas ligand, and tumor necrosis factor alpha (TNF-alpha). Both apoptotic body phagocytosis and death ligand generation were attenuated by gadolinium chloride, a Kupffer cell toxicant. Kupffer cells isolated from 3-day bile duct-ligated (BDL) mice were phenotypically similar to apoptotic body-"fed" Kupffer cells with enhanced death ligand expression; inhibition of hepatocyte apoptosis with a caspase inhibitor prevented this Kupffer cell activation. Consistent with a role for Kupffer cells in liver inflammation and fibrosis, gadolinium chloride attenuated neutrophil infiltration and markers for stellate cell activation. In conclusion, these findings support a model of cholestatic liver injury where Kupffer cell engulfment of apoptotic bodies promotes inflammation and fibrogenesis.

Normal thymocyte negative selection in TRAIL-deficient mice

The molecular basis of thymocyte negative selection, which plays a critical role in establishing and maintaining immunological tolerance, is not yet resolved. In particular, the importance of the death receptor subgroup of the tumor necrosis factor (TNF)-family has been the subject of many investigations, with equivocal results. A recent report suggested that TRAIL was a critical factor in this process, a result that does not fit well with previous studies that excluded a role for the FADD-caspase 8 pathway, which is essential for TRAIL and Fas ligand (FasL) signaling, in negative selection. We have investigated intrathymic negative selection of TRAIL-deficient thymocytes, using four well-established models, including antibody-mediated TCR/CD3 ligation in vitro, stimulation with endogenous superantigen in vitro and in vivo, and treatment with exogenous superantigen in vitro. We were unable to demonstrate a role for TRAIL signaling in any of these models, suggesting that this pathway is not a critical factor for thymocyte negative selection.

Activation-induced cell death in T cells

A properly functioning immune system is dependent on programmed cell death at virtually every stage of lymphocyte development and activity. This review addresses the phenomenon of activation-induced cell death (AICD) in T lymphocytes, in which activation through the T-cell receptor results in apoptosis. AICD can occur in a cell-autonomous manner and is influenced by the nature of the initial T-cell activation events. It plays essential roles in both central and peripheral deletion events involved in tolerance and homeostasis, although it is likely that different forms of AICD proceed via different mechanisms. For example, while AICD in peripheral T cells is often caused by the induction of expression of the death ligand, Fas ligand (CD95 ligand, FasL), it does not appear to be involved in AICD in thymocytes. This and other mechanisms of AICD are discussed. One emerging model that may complement other forms of AICD involves the inducible expression of FasL by nonlymphoid tissues in response to activated T lymphocytes. Induction of nonlymphoid FasL in this manner may serve as a sensing mechanism for immune cell infiltration, which contributes to peripheral deletion.

Negative selection--clearing out the bad apples from the T-cell repertoire

Dead cells are a prominent feature of the thymic landscape as only 5% of developing thymocytes are exported as mature T cells. The remaining thymocytes die by one of two mechanisms; most thymocytes die because they are not positively selected and do not receive a survival signal, whereas a minority of thymocytes undergo T-cell receptor (TCR)-mediated apoptosis, a process known as negative selection. Negative selection is extremely important for establishing a functional immune system, as it provides an efficient mechanism for ridding the T-cell repertoire of self-reactive and potentially autoimmune lymphocytes. This review discusses several cellular and molecular aspects of negative selection.