Immunogenic cell death

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Immunogenic cell death is any type of cell death eliciting an immune response. Both accidental cell death and regulated cell death can result in immune response. Immunogenic cell death contrasts to forms of cell death (apoptosis, autophagy or others) that do not elicit any response or even mediate immune tolerance.

Contents

The name 'immunogenic cell death' is also used for one specific type of regulated cell death that initiates an immune response after stress to endoplasmic reticulum.

Types

Immunogenic cell death types are divided according to molecular mechanisms leading up to, during and following the death event. The immunogenicity of a specific cell death is determined by antigens and adjuvant released during the process. [1]

Accidental cell death

Accidental cell death is the result of physical, chemical or mechanical damage to a cell, which exceeds its repair capacity. It is an uncontrollable process, leading to loss of membrane integrity. The result is the spilling of intracellular components, which may mediate an immune response. [2]

Immunogenic cell death or ICD

ICD or immunogenic apoptosis is a form of cell death resulting in a regulated activation of the immune response. This cell death is characterized by apoptotic morphology, [3] maintaining membrane integrity. Endoplasmic reticulum (ER) stress is generally recognised as a causative agent for ICD, with high production of reactive oxygen species (ROS). Two groups of ICD inducers are recognised. Type I inducers cause stress to the ER only as collateral damage, mainly targeting DNA or chromatin maintenance apparatus or membrane components. Type II inducers target the ER specifically. [3] ICD is induced by some cytostatic agents such as anthracyclines, [4] oxaliplatin and bortezomib, or radiotherapy and photodynamic therapy (PDT). [5] Some viruses can be listed among biological causes of ICD. [6] Just as immunogenic death of infected cells induces immune response to the infectious agent, immunogenic death of cancer cells can induce an effective antitumor immune response through activation of dendritic cells (DCs) and consequent activation of specific T cell response. [7] [6] This effect is used in antitumor therapy.

ICD is characterized by secretion of damage-associated molecular patterns (DAMPs).There are three most important DAMPs which are exposed to the cell surface during ICD. Calreticulin (CRT), one of the DAMP molecules which is normally in the lumen of the endoplasmic reticulum, is translocated after the induction of immunogenic death to the surface of dying cell. There it functions as an "eat me" signal for professional phagocytes. Other important surface exposed DAMPs are heat-shock proteins (HSPs), namely HSP70 and HSP90, which under stress condition also translocate to the plasma membrane. On the cell surface they have an immunostimulatory effect, based on their interaction with number of antigen-presenting cell (APC) surface receptors like CD91 and CD40 and also facilitate crosspresentation of antigens derived from tumour cells on MHC class I molecule, which then leads to the CD8+ T cell response. Other important DAMPs, characteristic for ICD are secreted HMGB1 and ATP. [2] HMGB1 is considered to be a marker of late ICD and its release to the extracellular space seems to be required for the optimal presentation of antigens by dendritic cells. It binds to several pattern recognition receptors (PRRs) such as Toll-like receptors (TLR) 2 and 4, which are expressed on APCs. ATP released during immunogenic cell death functions as a "find-me" signal for phagocytes when secreted and induces their attraction to the site of ICD. Also, binding of ATP to purinergic receptors on target cells has immunostimulatory effect through inflammasome activation. DNA and RNA molecules released during ICD activate TLR3 and cGAS responses, both in the dying cell and in phagocytes.

The concept of using ICD in antitumor therapy has started taking shape with the identification of some inducers mentioned above, which have a potential as anti-tumor vaccination strategies. The use of ICD inducers alone or in combination with other anticancer therapies (targeted therapies, immunotherapies [8] ) has been effective in mouse models of cancer [9] and is being tested in the clinic. [10]

Necroptosis

Another type of regulated cell death that induces an immune response is necroptosis. Necroptosis is characterized by necrotic morphology. [2] This type of cell death is induced by extracellular and intracellular microtraumas detected by death or damage receptors. For example, FAS, TNFR1 and pattern recognition receptors may initiate necroptosis. These activation inducers converge on receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and mixed lineage kinase domain like pseudokinase (MLKL). Sequential activation of these proteins leads to membrane permeabilization. [2] [1]

Pyroptosis

Pyroptosis is a distinct type of regulated cell death, exhibiting a necrotic morphology and cellular content spilling. [2] This type of cell death is induced most commonly in response to microbial pathogen infection, such as infection with Salmonella , Francisella , or Legionella. Host factors, such as those produced during myocardial infarction, may also induce pyroptosis. [11] Cytosolic presence of bacterial metabolites or structures, termed pathogen associated molecular patterns (PAMPs), initiates the pyroptotic response. Detection of such PAMPs by some members of Nod-like receptor family (NLRs), absent in melanoma 2 (AIM2) or pyrin leads to the assembly of an inflammasome structure and caspase 1 activation.

So far, the cytosolic PRRs that are known to induce inflammasome formation are NLRP3, NLRP1, NLRC4, AIM2 and Pyrin. These proteins contain oligomerization NACHT domains, CARD domains and some also contain similar pyrin (PYR) domains. Caspase 1, the central activator protease of pyroptosis, attaches to the inflammasome via the CARD domains or a CARD/PYR-containing adaptor protein called apoptosis-associated speck-like protein (ASC). [12] Activation of caspase 1 (CASP1) is central to pyroptosis and when activated mediates the proteolytic activation of other caspases. In humans, other involved caspases are CASP3, CASP4 and CASP5, in mice CASP3 and CASP11. [2] Precursors of IL-1β and IL-18 are among the most important CASP1 substrates, and the secretion of the cleavage products induces the potent immune response to pyroptosis. The release of IL-1β and IL-18 occurs before any morphological changes occur in the cell. [13] The cell dies by spilling its contents, mediating the distribution of further immunogenic molecules. Among these, HMGB1, S100 proteins and IL-1α are important DAMPs. [12]

Pyroptosis has some characteristics similar with apoptosis, an immunologically inert cell death. Primarily, both these processes are caspase-dependent, although each process utilizes specific caspases. Chromatin condensation and fragmentation occurs during pyroptosis, but the mechanisms and outcome differ from those during apoptosis. Contrasting with apoptosis, membrane integrity is not maintained in pyroptosis, [2] [13] while mitochondrial membrane integrity is maintained and no spilling of cytochrome c occurs. [11]

Ferroptosis

Ferroptosis is also a regulated form of cell death. The process is initiated in response to oxidative stress and lipid peroxidation and is dependent on iron availability. Necrotic morphology is typical of ferroptotic cells. Peroxidation of lipids is catalyzed mainly by lipoxygenases, but also by cyclooxygenases. Lipid peroxidation can be inhibited in the cell by glutathione peroxidase 4 (GPX4), making the balance of these enzymes a central regulator of ferroptosis. Chelation of iron also inhibits ferroptosis, possibly by depleting iron from lipoxygenases. Spilling of cytoplasmic components during cell death mediates the immunogenicity of this process. [2]

MPT-driven necrosis

Mitochondria permeability transition (MPT)- driven cell death is also a form of regulated cell death and manifests a necrotic morphology. Oxidative stress or Ca2+ imbalance are important causes for MPT-driven necrosis. The main event in this process is the loss of inner mitochondrial membrane (IMM) impermeability. The precise mechanisms leading to the formation of permeability-transition pore complexes, which assemble between the inner and outer mitochondrial membranes, are still unknown. Peptidylprolyl isomerase F (CYPD) is the only known required protein for MPT-driven necrosis. The loss of IMM impermeability is followed by membrane potential dissipation and disintegration of both mitochondrial membranes. [2]

Parthanatos

Parthanatos is also a regulated form of cell demise with necrotic morphology. It is induced under a variety of stressing conditions, but most importantly as a result of long-term alkylating DNA damage, oxidative stress, hypoxia, hypoglycemia and inflammatory environment. This cell death is initiated by the DNA damage response components, mainly poly(ADP-ribose) polymerase 1(PARP1). PARP1 hyperactivation leads to ATP depletion, redox and bioenergetic collapse as well as accumulation of poly(ADPribose) polymers and poly(ADP-ribosyl)ated proteins, which bind to apoptosis inducing factor mitochondria associated 1 (AIF). The outcome is membrane potential dissipation and mitochondrial outer membrane permeabilization. Chromatin condensation and fragmentation by AIF is characteristic of parthanatos. Interconnection of the prathanotic process with some members of the necroptotic apparatus has been proposed, as RIPK3 stimulates PARP1 activity. [2]

This type of cell death has been linked to some pathologies, such as some cardiovascular and renal disorders, diabetes, cerebral ischemia, and neurodegeneration. [2]

Lysosome-dependent cell death

Lysosome dependent cell death is a type of regulated cell death that is dependent on permeabilization of lysosomal membranes. The morphology of cells dying by this death is variable, with apoptotic, necrotic or intermediate morphologies observed. It is a type of intracellular pathogen defense, but is connected with several pathophysiological processes, like tissue remodeling or inflammation. Lysosome permeabilization initiates the cell death process, sometimes along with mitochondrial membrane permeabilization. [2]

NETotic cell death

NETotic cell death is a specific type of cell death typical for neutrophils, but also observed in basophils and eosinophils. The process is characterized by extrusion of chromatin fibers bound into neutrophil extracellular traps (NETs). NET formation is generally induced in response to microbial infections, but pathologically also in sterile conditions of some inflammatory diseases. ROS inside the cell trigger release of elastase (ELANE) and myeloperoxidase (MPO), their translocation to the nucleus and cytoskeleton remodeling. Some interaction with the necroptotic apparatus (RIPK and MLKL) has been suggested. [2]

Related Research Articles

<span class="mw-page-title-main">Apoptosis</span> Type of programmed cell death in multicellular organisms

Apoptosis is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay. The average adult human loses 50 to 70 billion cells each day due to apoptosis. For the average human child between 8 and 14 years old, each day the approximate loss is 20 to 30 billion cells.

<span class="mw-page-title-main">Caspase</span> Family of cysteine proteases

Caspases are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cysteine protease activity – a cysteine in its active site nucleophilically attacks and cleaves a target protein only after an aspartic acid residue. As of 2009, there are 12 confirmed caspases in humans and 10 in mice, carrying out a variety of cellular functions.

<span class="mw-page-title-main">Cell death</span> Biological cell ceasing to carry out its functions

Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as diseases, localized injury, or the death of the organism of which the cells are part. Apoptosis or Type I cell-death, and autophagy or Type II cell-death are both forms of programmed cell death, while necrosis is a non-physiological process that occurs as a result of infection or injury.

<span class="mw-page-title-main">Fas ligand</span> Protein-coding gene in the species Homo sapiens

Fas ligand is a type-II transmembrane protein expressed on various types of cells, including cytotoxic T lymphocytes, monocytes, neutrophils, breast epithelial cells, vascular endothelial cells and natural killer (NK) cells. It binds with its receptor, called FAS receptor and plays a crucial role in the regulation of the immune system and in induction of apoptosis, a programmed cell death.

<span class="mw-page-title-main">Caspase 1</span> Enzyme found in humans

Caspase-1/Interleukin-1 converting enzyme (ICE) is an evolutionarily conserved enzyme that proteolytically cleaves other proteins, such as the precursors of the inflammatory cytokines interleukin 1β and interleukin 18 as well as the pyroptosis inducer Gasdermin D, into active mature peptides. It plays a central role in cell immunity as an inflammatory response initiator. Once activated through formation of an inflammasome complex, it initiates a proinflammatory response through the cleavage and thus activation of the two inflammatory cytokines, interleukin 1β (IL-1β) and interleukin 18 (IL-18) as well as pyroptosis, a programmed lytic cell death pathway, through cleavage of Gasdermin D. The two inflammatory cytokines activated by Caspase-1 are excreted from the cell to further induce the inflammatory response in neighboring cells.

<span class="mw-page-title-main">Survivin</span> Mammalian protein

Survivin, also called baculoviral inhibitor of apoptosis repeat-containing 5 or BIRC5, is a protein that, in humans, is encoded by the BIRC5 gene.

Pyroptosis is a highly inflammatory form of lytic programmed cell death that occurs most frequently upon infection with intracellular pathogens and is likely to form part of the antimicrobial response. This process promotes the rapid clearance of various bacterial, viral, fungal and protozoan infections by removing intracellular replication niches and enhancing the host's defensive responses. Pyroptosis can take place in immune cells and is also reported to occur in keratinocytes and some epithelial cells.

<span class="mw-page-title-main">Cancer immunology</span> Study of the role of the immune system in cancer

Cancer immunology (immuno-oncology) is an interdisciplinary branch of biology and a sub-discipline of immunology that is concerned with understanding the role of the immune system in the progression and development of cancer; the most well known application is cancer immunotherapy, which utilises the immune system as a treatment for cancer. Cancer immunosurveillance and immunoediting are based on protection against development of tumors in animal systems and (ii) identification of targets for immune recognition of human cancer.

<span class="mw-page-title-main">Diablo homolog</span> Protein-coding gene in the species Homo sapiens

Diablo homolog (DIABLO) is a mitochondrial protein that in humans is encoded by the DIABLO gene on chromosome 12. DIABLO is also referred to as second mitochondria-derived activator of caspases or SMAC. This protein binds inhibitor of apoptosis proteins (IAPs), thus freeing caspases to activate apoptosis. Due to its proapoptotic function, SMAC is implicated in a broad spectrum of tumors, and small molecule SMAC mimetics have been developed to enhance current cancer treatments.

<span class="mw-page-title-main">RIPK1</span> Enzyme found in humans

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) functions in a variety of cellular pathways related to both cell survival and death. In terms of cell death, RIPK1 plays a role in apoptosis, necroptosis, and PANoptosis Some of the cell survival pathways RIPK1 participates in include NF-κB, Akt, and JNK.

Damage-associated molecular patterns (DAMPs) are molecules within cells that are a component of the innate immune response released from damaged or dying cells due to trauma or an infection by a pathogen. They are also known as danger signals, and alarmins because they serve as warning signs to alert the organism to any damage or infection to its cells. DAMPs are endogenous danger signals that are discharged to the extracellular space in response to damage to the cell from mechanical trauma or a pathogen. Once a DAMP is released from the cell, it promotes a noninfectious inflammatory response by binding to a pattern recognition receptor (PRR). Inflammation is a key aspect of the innate immune response; it is used to help mitigate future damage to the organism by removing harmful invaders from the affected area and start the healing process. As an example, the cytokine IL-1α is a DAMP that originates within the nucleus of the cell which, once released to the extracellular space, binds to the PRR IL-1R, which in turn initiates an inflammatory response to the trauma or pathogen that initiated the release of IL-1α. In contrast to the noninfectious inflammatory response produced by DAMPs, pathogen-associated molecular patterns (PAMPs) initiate and perpetuate the infectious pathogen-induced inflammatory response. Many DAMPs are nuclear or cytosolic proteins with defined intracellular function that are released outside the cell following tissue injury. This displacement from the intracellular space to the extracellular space moves the DAMPs from a reducing to an oxidizing environment, causing their functional denaturation, resulting in their loss of function. Outside of the aforementioned nuclear and cytosolic DAMPs, there are other DAMPs originated from different sources, such as mitochondria, granules, the extracellular matrix, the endoplasmic reticulum, and the plasma membrane.

Inflammasomes are cytosolic multiprotein complexes of the innate immune system responsible for the activation of inflammatory responses and cell death. They are formed as a result of specific cytosolic pattern recognition receptors (PRRs) sensing microbe-derived pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs) from the host cell, or homeostatic disruptions. Activation and assembly of the inflammasome promotes the activation of caspase-1, which then proteolytically cleaves pro-inflammatory cytokines, interleukin 1β (IL-1β) and interleukin 18 (IL-18), as well as the pore-forming molecule gasdermin D (GSDMD). The N-terminal GSDMD fragment resulting from this cleavage induces a pro-inflammatory form of programmed cell death distinct from apoptosis, referred to as pyroptosis, which is responsible for the release of mature cytokines. Additionally, inflammasomes can act as integral components of larger cell death-inducing complexes called PANoptosomes, which drive another distinct form of pro-inflammatory cell death called PANoptosis.

Mitophagy is the selective degradation of mitochondria by autophagy. It often occurs to defective mitochondria following damage or stress. The process of mitophagy was first described in 1915 by Margaret Reed Lewis and Warren Harmon Lewis. Ashford and Porter used electron microscopy to observe mitochondrial fragments in liver lysosomes by 1962, and a 1977 report suggested that "mitochondria develop functional alterations which would activate autophagy." The term "mitophagy" was in use by 1998.

<span class="mw-page-title-main">Pyrin domain</span>

A pyrin domain is a protein domain and a subclass of protein motif known as the death fold, the 4th and most recently discovered member of the death domain superfamily (DDF). It was originally discovered in the pyrin protein, or marenostrin, encoded by MEFV. The mutation of the MEFV gene is the cause of the disease known as Familial Mediterranean Fever. The domain is encoded in 23 human proteins and at least 31 mouse genes.

<span class="mw-page-title-main">NLRP11</span> Protein-coding gene in the species Homo sapiens

NOD-like receptor family pyrin domain containing 11 is a protein that in humans is encoded by the NLRP11 gene located on the long arm of human chromosome 19q13.42. NLRP11 belongs to the NALP subfamily, part of a large subfamily of CATERPILLER. It is also known as NALP11, PYPAF6, NOD17, PAN10, and CLR19.6

<span class="mw-page-title-main">Necroptosis</span> Programmed form of necrosis, or inflammatory cell death

Necroptosis is a programmed form of necrosis, or inflammatory cell death. Conventionally, necrosis is associated with unprogrammed cell death resulting from cellular damage or infiltration by pathogens, in contrast to orderly, programmed cell death via apoptosis. The discovery of necroptosis showed that cells can execute necrosis in a programmed fashion and that apoptosis is not always the preferred form of cell death. Furthermore, the immunogenic nature of necroptosis favors its participation in certain circumstances, such as aiding in defence against pathogens by the immune system. Necroptosis is well defined as a viral defense mechanism, allowing the cell to undergo "cellular suicide" in a caspase-independent fashion in the presence of viral caspase inhibitors to restrict virus replication. In addition to being a response to disease, necroptosis has also been characterized as a component of inflammatory diseases such as Crohn's disease, pancreatitis, and myocardial infarction.

<span class="mw-page-title-main">GSDMD</span> Protein found in humans

Gasdermin D is a protein that in humans is encoded by the GSDMD gene on chromosome 8. It belongs to the gasdermin family which is conserved among vertebrates and comprises six members in humans, GSDMA, GSDMB, GSDMC, GSDMD, GSDME (DFNA5) and DFNB59 (Pejvakin). Members of the gasdermin family are expressed in a variety of cell types including epithelial cells and immune cells. GSDMA, GSDMB, GSDMC, GSDMD and GSDME have been suggested to act as tumour suppressors.

<span class="mw-page-title-main">Vishva Dixit</span> Kenyan molecular biologist

Vishva Mitra Dixit is a physician of Indian origin who is currently Vice President and Senior Fellow of Physiological Chemistry and Research Biology at Genentech.

PANoptosis is a prominent innate immune, inflammatory, and lytic cell death pathway initiated by innate immune sensors and driven by caspases and receptor-interacting protein kinases (RIPKs) through multiprotein PANoptosome complexes. The assembly of the PANoptosome cell death complex occurs in response to germline-encoded pattern-recognition receptors (PRRs) sensing pathogens, including bacterial, viral, and fungal infections, as well as pathogen-associated molecular patterns, damage-associated molecular patterns, and cytokines that are released during infections, inflammatory conditions, and cancer. Several PANoptosome complexes, such as the ZBP1-, AIM2-, RIPK1-, and NLRC5- and NLRP12-PANoptosomes, have been characterized so far.

Jonathan C. Kagan is an American immunologist and the Marian R. Neutra, Ph.D. Professor of Pediatrics at Harvard Medical School. He is also the director of Basic Research and Shwachman Chair in Gastroenterology at Boston Children's Hospital. Kagan is a world leader in defining the molecular basis of innate immunity and inflammation.

References

  1. 1 2 Galluzzi L, Buqué A, Kepp O, Zitvogel L, Kroemer G (February 2017). "Immunogenic cell death in cancer and infectious disease". Nature Reviews. Immunology. 17 (2): 97–111. doi:10.1038/nri.2016.107. PMID   27748397. S2CID   4045072.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 Galluzzi L, Vitale I, Aaronson SA, Abrams JM, Adam D, Agostinis P, et al. (March 2018). "Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018". Cell Death and Differentiation. 25 (3): 486–541. doi:10.1038/s41418-017-0012-4. PMC   5864239 . PMID   29362479.
  3. 1 2 Garg AD, Dudek-Peric AM, Romano E, Agostinis P (2015). "Immunogenic cell death". The International Journal of Developmental Biology. 59 (1–3): 131–40. doi: 10.1387/ijdb.150061pa . PMID   26374534.
  4. ↑ Garg AD, Galluzzi L, Apetoh L, Baert T, Birge RB, Bravo-San Pedro JM, et al. (2015-11-20). "Molecular and Translational Classifications of DAMPs in Immunogenic Cell Death". Frontiers in Immunology. 6: 588. doi: 10.3389/fimmu.2015.00588 . PMC   4653610 . PMID   26635802.
  5. ↑ Garg AD, Nowis D, Golab J, Vandenabeele P, Krysko DV, Agostinis P (January 2010). "Immunogenic cell death, DAMPs and anticancer therapeutics: an emerging amalgamation". Biochimica et Biophysica Acta (BBA) - Reviews on Cancer. 1805 (1): 53–71. doi:10.1016/j.bbcan.2009.08.003. PMID   19720113.
  6. 1 2 Krysko DV, Garg AD, Kaczmarek A, Krysko O, Agostinis P, Vandenabeele P (December 2012). "Immunogenic cell death and DAMPs in cancer therapy". Nature Reviews. Cancer. 12 (12): 860–75. doi:10.1038/nrc3380. PMID   23151605. S2CID   223813.
  7. ↑ Spisek R, Dhodapkar MV (August 2007). "Towards a better way to die with chemotherapy: role of heat shock protein exposure on dying tumor cells". Cell Cycle. 6 (16): 1962–5. doi: 10.4161/cc.6.16.4601 . PMID   17721082.
  8. ↑ Pfirschke C, Engblom C, Rickelt S, Cortez-Retamozo V, Garris C, Pucci F, et al. (February 2016). "Immunogenic Chemotherapy Sensitizes Tumors to Checkpoint Blockade Therapy". Immunity. 44 (2): 343–54. doi:10.1016/j.immuni.2015.11.024. PMC   4758865 . PMID   26872698.
  9. ↑ Zitvogel L, Galluzzi L, Smyth MJ, Kroemer G (July 2013). "Mechanism of action of conventional and targeted anticancer therapies: reinstating immunosurveillance". Immunity. 39 (1): 74–88. doi: 10.1016/j.immuni.2013.06.014 . PMID   23890065.
  10. ↑ Garg AD, More S, Rufo N, Mece O, Sassano ML, Agostinis P, et al. (4 October 2017). "Trial watch: Immunogenic cell death induction by anticancer chemotherapeutics". Oncoimmunology. 6 (12): e1386829. doi:10.1080/2162402X.2017.1386829. PMC   5706600 . PMID   29209573.
  11. 1 2 Bergsbaken T, Fink SL, Cookson BT (February 2009). "Pyroptosis: host cell death and inflammation". Nature Reviews. Microbiology. 7 (2): 99–109. doi:10.1038/nrmicro2070. PMC   2910423 . PMID   19148178.
  12. 1 2 Vande Walle L, Lamkanfi M (July 2016). "Pyroptosis". Current Biology. 26 (13): R568–R572. doi: 10.1016/j.cub.2016.02.019 . PMID   27404251.
  13. 1 2 Kepp O, Galluzzi L, Zitvogel L, Kroemer G (March 2010). "Pyroptosis - a cell death modality of its kind?". European Journal of Immunology. 40 (3): 627–30. doi: 10.1002/eji.200940160 . PMID   20201017.
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