Michael J. Lenardo

The TNF and TNF Receptor Superfamilies: Integrating Mammalian Biology

The authors regret the inability to cite all of the primary literature contributing to this review due to length considerations. The authors thank F. Chan, T. Migone, M. Peter, J. Puck, R. Siegel, H. Walczak, and J. Wang for insightful comments on the manuscript. N. K. is a Scholar of the Leukemia Society. Supported in part by grants from the National Institutes of Health (R. M. L., N. K.).

Dominant interfering fas gene mutations impair apoptosis in a human autoimmune lymphoproliferative syndrome

Five unrelated children are described with a rare autoimmune lymphoproliferative syndrome (ALPS) characterized by massive nonmalignant lymphadenopathy, autoimmune phenomena, and expanded populations of TCR-CD3+CD4-CD8- lymphocytes. These findings, suggesting a genetic defect in the ability of T lymphocytes to respond to normal immunoregulatory mechanisms, prompted an evaluation of lymphocyte apoptosis. Each child had defective Fas-mediated T lymphocyte apoptosis associated with a unique, deleterious Fas gene mutation. One mutation appeared to cause a simple loss of function; however, four others had a dominant negative phenotype when coexpressed with normal Fas. Family studies demonstrated the inheritance of the mutant Fas alleles. The occurrence of Fas mutations together with abnormal T cell apoptosis in ALPS patients suggests an involvement of Fas in this recently recognized disorder of lymphocyte homeostasis and peripheral self-tolerance.

IMF-κB and Rel: Participants in a Multiform Transcriptional Regulatory System

Publisher Summary This chapter highlights NF-кB/Rel protein complex. Associations of the binding and regulatory subunits of NF-кB operate at the center of a variety of different signal pathways. NF-кB can process and integrate instructions that come from the extracellular environment and detect intracellular events originating in either the cytoplasm or the nucleus. NF-кB then transmits this information with astonishing rapidity to the transcriptional machinery by directly binding to a range of different DNA sequences in gene control regions. NF-кB comprises members of a family of dimer-forming proteins with homology to the rel oncogene. NF-кB has the striking appearance of being constitutively present only in B cells of the “appropriate” stage for Ig к light chain gene expression. The functional role of NF-кB is demonstrated by mutational analysis of the к intronic enhancer. The property that gives NF-кB a widespread significance in cellular regulation is its role as a mediator of inducible gene transcription. The key features of NF-кB transcriptional control are that it is fast and versatile and is used in many different gene systems. It also has the important ability to carry signals from the cytoplasm into the nucleus and trans -activate specific genes by binding directly to their promoters.

CD4+CD25+Foxp3+ regulatory T cells induce cytokine deprivation-mediated apoptosis of effector CD4+ T cells.

A key issue in mammalian immunology is how CD4+CD25+Foxp3+ regulatory T cells (Treg cells) suppress immune responses. Here we show that Treg cells induced apoptosis of effector CD4+ T cells in vitro and in vivo in a mouse model of inflammatory bowel disease. Treg cells did not affect the early activation or proliferation of effector CD4+ T cells. Cytokines that signal through the common γ-chain suppressed Treg cell–induced apoptosis. Treg cell–induced effector CD4+ T cell death required the proapoptotic protein Bim, and effector CD4+ T cells incubated with Treg cells showed less activation of the prosurvival kinase Akt and less phosphorylation of the proapoptotic protein Bad. Thus, cytokine deprivation–induced apoptosis is a prominent mechanism by which Treg cells inhibit effector T cell responses.

Termination of autophagy and reformation of lysosomes regulated by mTOR

Autophagy is an evolutionarily conserved process by which cytoplasmic proteins and organelles are catabolized. During starvation, the protein TOR (target of rapamycin), a nutrient-responsive kinase, is inhibited, and this induces autophagy. In autophagy, double-membrane autophagosomes envelop and sequester intracellular components and then fuse with lysosomes to form autolysosomes, which degrade their contents to regenerate nutrients. Current models of autophagy terminate with the degradation of the autophagosome cargo in autolysosomes, but the regulation of autophagy in response to nutrients and the subsequent fate of the autolysosome are poorly understood. Here we show that mTOR signalling in rat kidney cells is inhibited during initiation of autophagy, but reactivated by prolonged starvation. Reactivation of mTOR is autophagy-dependent and requires the degradation of autolysosomal products. Increased mTOR activity attenuates autophagy and generates proto-lysosomal tubules and vesicles that extrude from autolysosomes and ultimately mature into functional lysosomes, thereby restoring the full complement of lysosomes in the cell—a process we identify in multiple animal species. Thus, an evolutionarily conserved cycle in autophagy governs nutrient sensing and lysosome homeostasis during starvation.Autophagy is an evolutionarily conserved process to catabolize cytoplasmic proteins and organelles1, 2. During starvation, the target of rapamycin (TOR), a nutrient-responsive kinase, is inhibited, thereby inducing autophagy. In autophagy, double-membrane autophagosomes envelop and sequester intracellular components and then fuse with lysosomes to form autolysosomes which degrade their contents to regenerate nutrients. Current models of autophagy terminate with the degradation of autophagosome cargo in autolysosomes3-5, but the regulation of autophagy in response to nutrients and the subsequent fate of the autolysosome are poorly defined. Here we show that mTOR signaling is inhibited during autophagy initiation, but reactivated with prolonged starvation. mTOR reactivation is autophagy-dependent, and requires the degradation of autolysosomal products. Increased mTOR activity attenuates autophagy and generates proto-lysosomal tubules and vesicles that extrude from autolysosomes and ultimately mature into functional lysosomes, thereby restoring the full complement of lysosomes in the cell – a process we identify in multiple animal species. Thus, an evolutionarily-conserved cycle in autophagy governs nutrient sensing and lysosome homeostasis during starvation.

Inherited Human Caspase 10 Mutations Underlie Defective Lymphocyte and Dendritic Cell Apoptosis in Autoimmune Lymphoproliferative Syndrome Type II

Caspases are cysteine proteases that mediate programmed cell death in phylogenetically diverse multicellular organisms. We report here two kindreds with autoimmune lymphoproliferative syndrome (ALPS) type II, characterized by abnormal lymphocyte and dendritic cell homeostasis and immune regulatory defects, that harbor independent missense mutations in Caspase 10. These encode amino acid substitutions that decrease caspase activity and interfere with death receptor-induced apoptosis, particularly that stimulated by Fas ligand and TRAIL. These results provide evidence that inherited nonlethal caspase abnormalities cause pleiotropic apoptosis defects underlying autoimmunity in ALPS type II.

Pleiotropic defects in lymphocyte activation caused by caspase-8 mutations lead to human immunodeficiency

Apoptosis is a form of programmed cell death that is controlled by aspartate-specific cysteine proteases called caspases. In the immune system, apoptosis counters the proliferation of lymphocytes to achieve a homeostatic balance, which allows potent responses to pathogens but avoids autoimmunity. The CD95 (Fas, Apo-1) receptor triggers lymphocyte apoptosis by recruiting Fas-associated death domain (FADD), caspase-8 and caspase-10 proteins into a death-inducing signalling complex. Heterozygous mutations in CD95, CD95 ligand or caspase-10 underlie most cases of autoimmune lymphoproliferative syndrome (ALPS), a human disorder that is characterized by defective lymphocyte apoptosis, lymphadenopathy, splenomegaly and autoimmunity. Mutations in caspase-8 have not been described in ALPS, and homozygous caspase-8 deficiency causes embryonic lethality in mice. Here we describe a human kindred with an inherited genetic deficiency of caspase-8. Homozygous individuals manifest defective lymphocyte apoptosis and homeostasis but, unlike individuals affected with ALPS, also have defects in their activation of T lymphocytes, B lymphocytes and natural killer cells, which leads to immunodeficiency. Thus, caspase-8 deficiency in humans is compatible with normal development and shows that caspase-8 has a postnatal role in immune activation of naive lymphocytes.

The multifaceted role of Fas signaling in immune cell homeostasis and autoimmunity.

Originally identified as a cell surface receptor that triggered the death of lymphocytes and tumor cells, it is now recognized that Fas (also known as CD95 or Apo-1) has distinct functions in the life and death of different cell types in the immune system. Fas signaling may also be involved in T cell costimulation and proliferation. Although Fas deficiency in humans and mice predisposes them towards systemic autoimmunity, Fas-FasL interactions can also facilitate organ-specific immunopathology. Proximal signaling by Fas and related receptors depends on subunit preassembly, which accounts for the dominant-negative effect of pathogenic receptor mutants and natural splice variants.

A Role for Tumor Necrosis Factor Receptor-2 and Receptor-interacting Protein in Programmed Necrosis and Antiviral Responses

Members of the tumor necrosis factor (TNF) receptor (TNFR) superfamily are potent regulators of apoptosis, a process that is important for the maintenance of immune homeostasis. Recent evidence suggests that TNFR-1 and Fas and TRAIL receptors can also trigger an alternative form of cell death that is morphologically distinct from apoptosis. Because distinct molecular components including the serine/threonine protein kinase receptor-interacting protein (RIP) are required, we have referred to this alternative form of cell death as “programmed necrosis.” We show that TNFR-2 signaling can potentiate programmed necrosis via TNFR-1. When cells were pre-stimulated through TNFR-2 prior to subsequent activation of TNFR-1, enhanced cell death and recruitment of RIP to the TNFR-1 complex were observed. However, TNF-induced programmed necrosis was normally inhibited by caspase-8 cleavage of RIP. To ascertain the physiological significance of RIP and programmed necrosis, we infected Jurkat cells with vaccinia virus (VV) and found that VV-infected cells underwent programmed necrosis in response to TNF, but deficiency of RIP rescued the infected cells from TNF-induced cytotoxicity. Moreover, TNFR-2–/– mice exhibited reduced inflammation in the liver and defective viral clearance during VV infection. Interestingly, death effector domain-containing proteins such as MC159, E8, K13, and cellular FLIP, but not the apoptosis inhibitors Bcl-xL, p35, and XIAP, potently suppressed programmed necrosis. Thus, TNF-induced programmed necrosis is facilitated by TNFR-2 signaling and caspase inhibition and may play a role in controlling viral infection.

Caspase-10 is an initiator caspase in death receptor signaling.

A role for caspase-10, previously implicated in the autoimmune lymphoproliferative syndrome, in death receptor signaling has not been directly shown. Here we show that caspase-10 can function independently of caspase-8 in initiating Fas- and tumor necrosis factor-related apoptosis-inducing ligand-receptor-mediated apoptosis. Moreover, Fas crosslinking in primary human T cells leads to the recruitment and activation of caspase-10. Fluorescent resonance energy transfer analysis indicates that the death-effector domains of caspase-8 and -10 both interact with the death-effector domain of FADD. Nonetheless, we find that caspase-8 and -10 may have different apoptosis substrates and therefore potentially distinct roles in death receptor signaling or other cellular processes.