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L Is for Lytic Granules: Lysosomes That Kill

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L Is for Lytic Granules: Lysosomes That Kill Biochimica et Biophysica Acta 1401Ž. 1998 146±156 View metadata, citation and similar papers at core.ac.uk brought to you by CORE Review provided by Elsevier - Publisher Connector L is for lytic granules: lysosomes that kill Lesley J. Page 1, Alison J. Darmon 2, Ruth Uellner, Gillian M. Griffiths ) MRC Lab for Molecular Cell Biology, UniÕersity College London, Gower St, London WC1E 6BT, UK Received 23 June 1997; revised 30 October 1997; accepted 31 October 1997 Contents 1. Introduction ................................................... 147 2. CTL function and biochemistry ....................................... 147 2.1. The function of CTL .......................................... 147 2.2. Mechanisms of killing.......................................... 147 2.3. Cytolytic proteins ............................................ 148 2.4. Target cell death ............................................. 149 2.5. Serial killing ............................................... 150 3. The lytic granule as a lysosome ....................................... 150 3.1. Lysosomal contents ........................................... 150 3.2. Lytic granules are acidic compartments ............................... 150 3.3. The lytic granule as an endocytic compartment ........................... 151 4. Lytic granules as secretory organelles ................................... 152 4.1. Sorting of lysosomal proteins ..................................... 152 4.2. Sorting of granzymes and perforin .................................. 152 5. Secretory lysosomes in the haemopoietic lineage ............................. 153 5.1. Other cell types with secretory lysosomes ............................. 153 5.2. Chediak Higashi Syndrome ...................................... 153 6. Summary and conclusion ........................................... 154 References ..................................................... 154 ) Corresponding author. Fax: 1865 275 515. 1 Present address: ICRF Laboratories, Lincoln Inn Fields, London WC2A 3PX, UK. 2 Present address: Ludwig Institute for Cancer Research, Courtauld Building, 91 Riding House St., London W1P 8BT, UK. 0167-4889r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S0167-4889Ž. 97 00138-9 L.J. Page et al.rBiochimica et Biophysica Acta 1401() 1998 146±156 147 1. Introduction patibility ComplexŽ. MHC proteins Ž reviewedwx 1. The MHC proteins are highly polymorphic enabling Lysosomes are found in all eukaryotic cell types, CTL to distinguish between self and non-self MHC. and function as organelles specialised in protein The MHC-peptide complex serves as an indicator of degradation. A series of hydrolases packaged within both cell typeŽ. self or foreign and cell status Ž ``nor- r the lysosomes are able to degrade proteins delivered mal'' or transformed infected. Thus, CTL patrol the to this acidic compartment. Proteins can be delivered body to check for foreign cells, by engaging MHC- to the lysosome via both biosynthetic and endocytic peptide complexes with the T cell receptorŽ. TcR . routes. In addition, lysosomes are able to engulf other Normal self cells are ``ignored'' during this process - intracellular organelles by a process called au- they do not activate the CTL and therefore are al- tophagy. lowed to live. However, as a result of either transfor- Proteins delivered to lysosomes are rapidly de- mation or viral infection, non-self proteins are pre- graded, and these organelles form an essential part in sented in conjunction with a cell's MHC proteins, maintaining proper function in all cell types. In the and this cell is subsequently recognised as foreign by majority of cells the lysosome functions only as an the CTL. Once activated, the CTL is able to initiate a intracellular organelle. However, in cytotoxic T lym- series of events which culminate in death of the phocytesŽ. CTL and a small number of other cell recognised cell. Should a CTL be able to recognise a types, the lysosome is also a secretory organelle that self protein-MHC complex, autoimmune disorders contains specialised secretory proteins in addition to may ensue, in which CTL destroy normal self cells. the lysosomal hydrolases. In the case of CTL, the Alternatively, if cells presenting non-self-MHC pro- secretory proteins are specialised for the destruction teins are introduced to the body, for example during of other cells: so that by secreting it's lysosomal organ transplantation, these cells can also be recog- contents the CTL is now able to kill other cells. nised as foreign and destroyed. These specialised lysosomes of the CTL are known as lytic granules. They are dual functional organelles 2.2. Mechanisms of killing which can not only degrade proteins targeted to them within the cell, but can also destroy whole cells as a consequence of their secretion. This article reviews Following recognition of a target cell through the what is known about the specialised lysosomes of TcR, a number of signals, including protein tyrosine phosphorylation and elevation of intracellular cal- CTL and the mechanisms that are required to modify wx an intracellular lysosome into a destructive secretory cium levelsŽ reviewed 2. , are generated within the granule. CTL. These signals culminate in the induction of transcription of the effector proteins and, after several days, the appearance of electron-dense granules within the CTL. The effector proteins are the mediators of 2. CTL function and biochemistry CTL function and are stored within the granules until required. Subsequent interaction with a target cell results in polarisation of the Golgi apparatus and the 2.1. The function of CTL microtubule organising centres of the CTL towards the point of contact with the target cellwx 3±6 , and CTL represent the body's primary defence against directed exocytosis of the granules towards the target viral-infected and tumourigenic cells and are in- cell. During granule-mediated killing, the cytolytic volved in autoimmune disorders and transplant rejec- proteins released from the granules inflict a ``lethal tion. These functions are possible because of the hit'' on the target cell and the cell diesŽ. Fig. 1 . ability of CTL to recognise other cells as ``foreign'' Alternatively, CTL may utilise a cell surface re- and destroy them. Most cells within the body present ceptor to induce target cell death. A number of cell fragments of proteins, generated within the cell, on types express a surface protein called FasrAPO- the cell surface in the context of the Major Histocom- 1rCD95 which contains a ``death domain'' in its 148 L.J. Page et al.rBiochimica et Biophysica Acta 1401() 1998 146±156 toxicity seems to play a role in down-regulating immune responseswx 10,11 . In this instance, Fas±FasL interactions may mediate the removal of the respond- ing CTL population so that huge numbers of CTL do not accumulate over time. Additionally, the tumour necrosis factor receptor, which is related to Fas and also contains a death domain, has been implicated in the control of CTL number following an immune responsewx 12 . 2.3. Cytolytic proteins As mentioned above, the cytolytic proteins reside in the CTL granules, where they await exocytosis towards a target cell. What are these proteins? The first granule function-related protein identified was the pore-forming protein perforin, which was shown Fig. 1. Mechanisms of CTL-mediated cytotoxicity. At least two to be able to induce calcium-dependent lysis of target cytolytic mechanisms are used by CTLs to destroy target cells. cellswx 13±15 . In the granules, which are acidic, Recognition of a foreign MHC-peptide complex through the T perforin is found in monomeric form in association Ž. cell receptor TcR initiates signals within the CTL that result in with proteoglycans. Granule exocytosis releases per- transcriptional activation of function-related genes. Perforin and the granzymes are stored in lytic granules within the CTL. Upon forin into the intracellular space where it is exposed further stimulation of the TcR, lytic granules are exocytosed to both calcium and neutral pH. The neutral pH towards the target cell. Perforin forms a pore in the target cell causes the release of perforin monomers from proteo- membrane, allowing the granzymes to enter the target cell and glycanswx 16 , allowing the monomers to bind the induce death. During Fas-mediated cytotoxicity, TcR stimulation lipids of the target cell membranewx 17,18 . Perforin results in upregulated FasL expression on CTL surfaces. FasL cross-links Fas on the target cell. A death-inducing signalling then inserts into the cell membrane and aggregates in complexŽ. DISC is recruited to the cytoplasmic tail of Fas, a calcium-dependent manner to form a pore which resulting in transduction of a death signal to the target cell. can be seen by electron microscopywx 19,20 . Since perforin is able to cause lysis of a number of cell types, it was originally believed that formation of the cytoplasmic regionwx 7 . During CTL activation, levels perforin pore was sufficient to induce target cell of a protein named Fas-ligandŽ. FasL are upregulated death. However, perforin alone cannot induce target on the CTL surface. FasL can cross-link target cell cell DNA fragmentationwx 21±23 , suggesting that per- Fas receptors, and the oligomerised receptors can forin cannot mediate all of the events involved in then transduce a death signal to the target cell through CTL-mediated cytotoxicity. It is now believed that a number of associated proteinswx 8,9 . Following the the primary
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