Reading 1 - Apoptosis Flashcards
Programmed cell death and the
immune system
NATURE REVIEWS | IMMUNOLOGY
The acquired immune system has developed multiple mechanisms for efficiently deleting self-reactive immune cells.
In addition, activated immune cells need to be eliminated soon after they have accomplished their task in order to prevent excessive immune reactions that can cause host pathology.
Furthermore, during an infection, pathogen-infected cells actively kill themselves before the pathogens
multiply and kill cells in order to spread.
In each of these immunological settings,
programmed cell death has an essential role.
Apoptosis in the immune system
In 1972, Kerr et al.2 observed a specific type of cell death in human tissues in which the cells and nuclei
became condensed and fragmented, and they called this cell death process ‘apoptosis’.
They proposed that apoptosis is crucial for regulating cell populations during tissue development and turnover.
At around the same time, cytotoxic T lymphocytes (CTLs) were shown to induce cell death in their target cells or in virus-infected cells.
Around 1990, apoptosis was reported to have important roles in the adaptive immune system in the deletion
of thymocytes that express autoreactive or non-reactive T cell receptors (TCRs)4,5 and in the deletion of autoreactive immature B cells
Two major apoptosis pathways
EXTRINSIC: known as the ‘death receptor’ or ‘extrinsic’ pathway, the binding of FASL to FAS causes a conformational change of the FAS trimer. FAS then forms a multiprotein complex (known as the death-inducing signalling complex (DISC)) with the adaptor protein FAS-associated death domain protein (FADD) and pro-caspase 8. Pro-caspase 8 is processed into mature caspase 8 in the DISC, and the activated mature caspase 8 then processes pro-caspase 3 to form mature caspase 3.
INTRINSIC: a developmental signal or genotoxic agent
activates a pro-apoptotic member of the B cell lymphoma 2 (BCL-2) family. The pro-apoptotic members of BCL-2 family stimulate mitochondria to release many
molecules that can regulate apoptosis.
Among these molecules, cytochrome c forms a multiprotein complex (known as the apoptosome) together with pro-caspase 9 and apoptotic protease-activating factor 1 (APAF1), and the complex processes
pro-caspase 9 to form mature caspase 9. The activated caspase 9 then processes pro-caspase 3 to form mature caspase 3.
Caspase 3, which is activated in both the extrinsic and intrinsic pathways, subsequently cleaves more than 500 cellular substrates to execute the
apoptosis programme. The involvement of cytochrome c and APAF1 in apoptotic cell death in vivo was confirmed in mice and in Drosophila melanogaster.
Defects in apoptosis result in autoimmunity
It was shown in humans that patients with autoimmune
lymphoproliferative syndrome also carry somatic or germline mutations in the genes that encode FAS or FASL. These results, together with the finding that the FASL–FAS system is indispensable for activation-induced cell death in T cells, suggested that the
FAS-mediated extrinsic apoptosis pathway is responsible for the deletion of peripheral T cells.
The FAS system has a role in eliminating autoreactive B cells.
Mice deficient in Bim (also known as Bcl2l11), a pro-apoptotic member of the BCL-2 family that is involved in the intrinsic apoptotic pathway, also develop lymphoproliferation and suffer from an SLE-type autoimmune disease.
This suggested that the intrinsic and extrinsic
apoptotic pathways have collaborative roles in maintaining lymphocyte homeostasis.
BIM-mediated apoptosis is involved in regulating the lifespan of short-lived myeloid cells such as eosinophils,
neutrophils and monocytes.
Expression of FAS and FASL
FASL expression is restricted to specific
lymphocyte populations, such as CTLs, T helper 1 (TH1) cells and natural killer cells
FAS is widely expressed by most cell types in various tissues
FASL-mediated apoptosis maintains tissue homeostasis, for example, by inducing cell death to resolve post-injury fibrosis or to prevent excessive myofibroblast proliferation in the lung
FASL is involved in the development of graft-versus-host disease, whereas perforin produced by CTLs can mediate the graft-versus-leukaemia effect. If this relationship holds true in humans, blocking the FAS death system could be beneficial for patients who
are undergoing cancer therapies and require
bone marrow transplantation.
The ‘eat me’ signal, PtdSer
Almost all of the dead cells were inside ‘histiocytes’, indicating that apoptotic cells are quickly engulfed by phagocytes. This rapid phagocytosis of dying cells prevents inflammation that could be caused by noxious materials released from the dead cells. This process is now referred to as ‘efferocytosis’ to emphasize its uniqueness as a form of phagocytosis
Macrophages engulf apoptotic cells, but not healthy live cells, led to the identification of ‘eat me’ signals that are exposed by apoptotic cells
Phosphatidylserine (PtdSer) exposed on the surface of
apoptotic cells triggers efferocytosis
PtdSer is localized to the inner leaflet of the plasma membrane in healthy cells
Annexin V, which specifically binds to PtdSer, is used as
a marker of apoptotic cells
The exposure of PtdSer depends on caspase activation
In healthy cells, ATP11A and ATP11C, which are phospholipid-transporting ATPases at the plasma membrane, actively translocate or flip PtdSer from the outer leaflet to the inner leaflet to confine PtdSer to
the inner leaflet of the plasma membrane. In cells undergoing apoptosis, active caspase 3 cleaves and inactivates these ATPases. At the same time, XK-related protein 8 (XKR8), a transmembrane protein, is cleaved by caspase 3 at its carboxy-terminal tail region
and functions as a phospholipid scramblase to scramble phospholipids between the inner and outer plasma-membrane leaflets, thus quickly exposing PtdSer.
Efferocytosis
Two families of molecules that specifically recognize PtdSer on apoptotic cells (namely, the MFGE8 (lactadherin)–DEL1 (EDIL3) family, and the T cell immunoglobulin and mucin domain-containing protein 4 (TIM4–TIM1 family)
MFGE8 is a soluble protein that is secreted by certain macrophages and serves as a bridging molecule to bring apoptotic cells to integrin-expressing macrophages, and TIM4 and TIM1 are type I membrane proteins that function as PtdSer receptors
As efferocytosis is accompanied by the production of anti-inflammatory cytokines, such as interleukin-10 (IL-10) and transforming growth factor-β (TGFβ)66, it is
possible that when efferocytosis is disrupted, the loss of these anti-inflammatory mediators and the exposure of immune cells to the intracellular components from unengulfed lysed cells contribute to the development of autoimmune diseases.
Degradation of dead cells
After being engulfed by macrophages, apoptotic cells are transported to lysosomes, where their components are degraded into building units (amino acids, nucleotides and monosaccharaides) for re-use. If this process does not proceed efficiently, it will cause a
type of lysosomal storage disease
A caspase 3-activated endonuclease named caspase-activated DNase (CAD; also known as DNA fragmentation factor) was identified71,72, and this enzyme was found to be solely responsible for cell-autonomous apoptotic DNA fragmentation
The DNA of dead cells can leak from lysosomes into the cytoplasm, leading to the activation of the cGAS–STING and AIM2–inflammasome pathways, and induce the innate immunity. In other words, if the DNA of our own dead cells is not properly degraded, it behaves like a pathogen.
Immunogenic cell death
When apoptotic cells are not swiftly engulfed by macrophages, they undergo secondary necrosis during which they swell and the plasma membrane ruptures. In addition to this passive necrotic death, cells can die by programmed necrosis. TNF is one of the inflammatory cytokines that is produced by macrophages infected by a virus or by bacteria, and it usually stimulates the expression of genes that promote inflammation. However, TNF also has the ability to induce apoptosis or necrosis, particularly when it functions in the presence of inhibitors of protein synthesis or RNA
synthesis.
TNF kills cells by inducing necrosis when the apoptosis pathway is inhibited
The necrosis induced by TNF in the presence of a caspase inhibitor is called ‘necroptosis’ and a
compound (necrostatin 1) inhibits this cell death process. Subsequently identified was the receptor-interacting protein kinase 1 (RIPK1) as the target of
necrostatin 1. In necroptosis, the binding of TNF to its receptor stimulates the kinase activity of RIPK1, which activates another kinase, RIPK3 (REFS 90,91). RIPK3 then phosphorylates mixed lineage kinase domain-like protein (MLKL), and the phosphorylated MLKL translocates to the plasma membrane, which it damages to execute necrosis. TNF does not normally kill cells; however, when cells are infected by a virus or by bacteria, their transcriptional or translational machinery is inhibited, which sensitizes the cells to TNF-induced cytotoxicity. Furthermore, viruses and
bacteria often encode molecules that inhibit apoptosis. Thus, cells infected by a virus or by bacteria undergo necroptosis upon their engagement with TNF.
Another type of programmed necrosis is pyroptosis
Two pathways, which are referred to as the canonical and non-canonical pathways, are known to execute pyroptosis. In the canonical pathway, molecules associated with pathogens or released from dead cells stimulate the formation of inflammasomes, which are multiprotein complexes that mediate the processing
and activation of pro-caspase 1. In the non-canonical pathway, endotoxins from Gram-negative bacteria directly bind to human pro-caspase 4 or pro-caspase 5
(or their mouse homologue, pro-caspase 11), and activate these caspases. In both the canonical and non-canonical pathways of pyroptosis, caspase 1, caspase 4 or caspase 5 cleaves a protein called gasdermin D, and processes pro-IL-1β and pro-IL-18 into their mature forms. The approximately 30kDa amino-terminal domain of gasdermin D then translocates to the plasma membrane, where it forms holes that cause necrosis and enable the release of the processed mature forms of IL-1β and IL-18
Thus, pyroptosis has an important role in triggering inflammation by releasing IL-1β and IL-18. In addition, pyroptosis and necrosis are involved in killing pathogen infected cells before the invading virus or bacteria proliferate
These necrotic processes are accompanied by the release of cellular contents from the dying cells, which
act as damage-associated molecular patterns (DAMPs) to stimulate pro-inflammatory processes, including the recruitment and activation of neutrophils, macrophages
and other immune cells. Thus, although these necrosis systems combat pathogens, they also lead to strong, tissue-damaging inflammation and, as described above for defective efferocytosis, the chronic exposure of DAMPs to the immune system may drive autoimmunity
