Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are toxic metals, toxic chemicals, microbe neurotoxins, radiation particles and even specific neurotransmitters when the system is out of balance. Also some types of venom, e.g. from the puff adder (Bitis arietans) or brown recluse spider (Loxosceles reclusa) are toxic to cells.
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Cytotoxic T cells | Immune system physiology | NCLEX-RN | Khan Academy
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Spike Protein Cytotoxicity?
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Cell Viability and Cytotoxicity
Transcription
When we learned about antigen presenting cells, we learned that they can first digest something-- let me draw a dendritic cell right here-- my best version of a dendritic cell. Maybe I should draw them simpler than that. A dendritic cell is a phagocyte and it is an antigen presenting cell. So after phagocytoses some type of a pathogen, it'll cut it all up, and then it'll display-- it'll present the antigen on its surface on a protein complex here and the part of the pathogen that it cut up, it'll put up right here. And we learned on the antigen presenting cell video that this complex right here was an MHC type II complex, where MHC stands for major histocompatibility complex. Where histocompatibility just means tissue compatibility. And this was the case on antigen presenting cells. So even B cells did this. Let me draw a B cell. So a B cell-- it has its membrane bound antibody, just like that. It actually has many, many thousands of these. I could keep drawing a bunch of them, but just so you know there's more than one. Maybe one of these get triggered or get attached to some type of virus or protein or bacteria floating around. And what it'll do is it'll take this in and cut it up again and do the same thing as what the dendritic cell did. It'll cut up a part of this and present it on its surface in conjunction with an MHC II complex. So once again, this is an MHC II complex. So these professional antigen presenting cells that go out and take things out of the fluid, out of the humoral parts of our body, things just floating around. They'll take them in, they'll say, this is bad, cut them up, and then present them on these MHC II. That's why we call them professional antigen presenting cells. Now, it turns out that pretty much all cells in our bodies-- when I say almost all cells, it's actually all nucleated cells. So all cells that have a nucleus in the human body-- so the only cells in our human body that don't have nucleuses are red blood cells, which I find fascinating-- so that they can have more space for storing hemoglobin. But all nucleated cells in our bodies have another major histocompatibility complex on it and it's called an MHC I-- major histocompatibility type I. And just so you know, these are also nucleated cells. So they're also going to have an MHC type I complex on them right here. Now the interesting thing about the MHC type I complex is because it's on every cell in our human body-- so pretty much everything but the red blood cells have an MHC I-- this is where if anything wacky is going on inside the cell. Maybe the cell is cancerous and producing crazy proteins. Maybe it's been infected with a virus. Maybe some type of bacteria or some type of weird protein has gotten in here-- any cell in the human body can cut those up, even if it's malfunctioning, and it will present them. So let's say the cell is cancerous. So this cell's cancerous and it has all these wacky proteins that only cancer cells present that is not normal for a normal cell-- that will be presented on the MHC I. Let's say that I have some other cell in my body that's a different type of cell. It's nucleated. Let's say it's been infected with a virus. So it's turning into this virus factory. Same thing-- there are mechanisms in a cell that will take some of the proteins that make up those viruses and present them on the MHC I complex. So in the case of MHC II, this is what triggered helper T cells to say, hey, you know what? I found something floating out here. Here's a little piece of it, Mr. Helper T cell. Why don't you bond to this and raise the alarm system? Now the MHC I system says, this isn't stuff floating around. I've been infected. I am cancerous. I'm going nuts. You better kill me. I'm a virus, I'm a virus-making machine. You better kill me. And that message goes to the cytotoxic T cells and that's really the topic of this video. So just to make sure you understand the difference-- so T cells. They both have T cell receptors, but the helper T cells bond to MHC II complexes. Let's say that this is a helper T cell right here. It would want to-- not all helper T cells will. Only the ones that have the right combination, the right variable portion right here that just perfectly bonds to this combination of an antigen and the MHC II complex-- this type of helper T cell will bond here, get activated, and start differentiating. And the effector versions of them will start raising the alarms and the memory versions of them will stick around in case this type of thing needs to happen again. With MHC I, instead of attracting a helper T cell, it will attract a cytotoxic T cell. So like helper T cells, the T cell receptor has a non-variable portion, but it also has a variable portion that is specific to this combination of antigen and MHC II. So maybe this cytotoxic T cell will be involved when this cell goes cancerous. This cytotoxic T cell would be of no use-- or it won't bond to this one that was attracted to a virus. It's going to have to be another cytotoxic T cell that does that. And the mechanism where we get this variability in the helper T cells or the cytotoxic T cells or you saw in the B cells on their membrane bound antibodies, that all comes from when-- in their development stage or in the maturation process, the DNA that codes for these variable portions gets shuffled around intentionally. So normally, we're always trying to preserve DNA information, here it gets shuffled around. But anyway, once a cytotoxic T cell finds one of these guys on an MHC I-- remember, every nucleated cell in the body has an MHC I-- then what it does is, it gets activated. So let's say this guy says, hey, that looks shady. You need to die. So this guy gets activated and just like all other activated cells, he starts to divide and divide and divide and divide and differentiate. And he divides and he differentiates into memory, just in case you're going to need me again, just in case this type of cancer shows up again. And then also into effector T cells, which are the ones that do the killing. So this is an effector. So let's say one of these effectors-- they'll also bind to cancerous molecules, cancerous cells, just like this one. So let's say this cell has split and there's another version of it right here. That's what cancer does. It divides aggressively. It's producing wacky proteins. It presents the wacky proteins on its MCH-- major histocompatibility type I complex-- it displays the wacky proteins and then one of these effector cytotoxic T cells will be attracted to it just like that. And I'm not going into details on what necessarily does the attractions and all the membrane bound proteins. If you take an immunology class, you'll see more on that. So this is a cytotoxic T cell and it essentially forces this cell to kill itself in a couple of different ways. One, it actually can exocytose a bunch of proteins. They're call perforins-- that make little holes in the membrane of the cell. And it has other proteins that it releases called granzymes that go in here and essentially start mechanisms that make this cell want to kill itself. So the big picture is, if you want to just take 20,000 feet, these cells are very effective at produces-- so when a B cell gets activated, it produces antibodies that kill things that are floating around, right? Once a B cell gets activated, it starts producing a bunch of antibodies. These antibodies float around and then they can bond up to viruses, make them ineffective, or essentially tag them for pickup from macrophages or dendritic cells, or other types of phagocytes-- while cytotoxic T cells-- these are used to essentially kill cells that have gone awry. For example, a cancer cell that's presenting weird proteins or once the virus has entered the cell, then the antibodies are really of no use. The antibodies aren't going to be able to get into those cells. In that case, instead of cleaning up the virus itself, a cytotoxic T cell will come here and just kill this cell because this cell is a virus factory. So you have to get it out of the way.
Cell physiology
Treating cells with the cytotoxic compound can result in a variety of prognoses. The cells may undergo necrosis, in which they lose membrane integrity and die rapidly as a result of cell lysis. The cells can stop actively growing and dividing (a decrease in cell viability), or the cells can activate a genetic program of controlled cell death (apoptosis).
Cells undergoing necrosis typically exhibit rapid swelling, lose membrane integrity, shut down metabolism, and release their contents into the environment. Cells that undergo rapid necrosis in vitro do not have sufficient time or energy to activate apoptotic machinery and will not express apoptotic markers. Apoptosis is characterized by well defined cytological and molecular events including a change in the refractive index of the cell, cytoplasmic shrinkage, nuclear condensation and cleavage of DNA into regularly sized fragments. Cells in culture that are undergoing apoptosis eventually undergo secondary necrosis. They will shut down metabolism, lose membrane integrity and lyse.[1]
Measurement
Cytotoxicity assays are widely used by the pharmaceutical industry to screen for cytotoxicity in compound libraries. Researchers can either look for cytotoxic compounds, if they are interested in developing a therapeutic that targets rapidly dividing cancer cells, for instance; or they can screen "hits" from initial high-throughput drug screens for unwanted cytotoxic effects before investing in their development as a pharmaceutical.[2]
Assessing cell membrane integrity is one of the most common ways to measure cell viability and cytotoxic effects. Compounds that have cytotoxic effects often compromise cell membrane integrity. Vital dyes, such as trypan blue or propidium iodide are normally excluded from the inside of healthy cells; however, if the cell membrane has been compromised, they freely cross the membrane and stain intracellular components.[1] Alternatively, membrane integrity can be assessed by monitoring the passage of substances that are normally sequestered inside cells to the outside. One molecule, lactate dehydrogenase (LDH), is commonly measured using LDH assay. LDH reduces NAD to NADH which elicits a colour change by interaction with a specific probe.[3] Protease biomarkers have been identified that allow researchers to measure relative numbers of live and dead cells within the same cell population. The live-cell protease is only active in cells that have a healthy cell membrane, and loses activity once the cell is compromised and the protease is exposed to the external environment. The dead-cell protease cannot cross the cell membrane, and can only be measured in culture media after cells have lost their membrane integrity.[4]
Cytotoxicity can also be monitored using 3-(4, 5-Dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide (MTT) or with 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT), which yields a water-soluble product, or the MTS assay. This assay measures the reducing potential of the cell using a colorimetric reaction. Viable cells will reduce the MTS reagent to a colored formazan product. A similar redox-based assay has also been developed using the fluorescent dye, resazurin. In addition to using dyes to indicate the redox potential of cells in order to monitor their viability, researchers have developed assays that use ATP content as a marker of viability.[1] Such ATP-based assays include bioluminescent assays in which ATP is the limiting reagent for the luciferase reaction.[5]
Cytotoxicity can also be measured by the sulforhodamine B (SRB) assay, WST assay and clonogenic assay.
Suitable assays can be combined and performed sequentially on the same cells in order to reduce assay-specific false positive or false negative results. A possible combination is LDH-XTT-NR (Neutral red assay)-SRB which is also available in a kit format.
A label-free approach to follow the cytotoxic response of adherent animal cells in real-time is based on electric impedance measurements when the cells are grown on gold-film electrodes. This technology is referred to as electric cell-substrate impedance sensing (ECIS). Label-free real-time techniques provide the kinetics of the cytotoxic response rather than just a snapshot like many colorimetric endpoint assays.
Material that has been determined as cytotoxic, typically biomedical waste, is often marked with a symbol that consists of a capital letter "C" inside a triangle.[6][7]
Prediction
A highly important topic is the prediction of cytotoxicity of chemical compounds based on previous measurements, i.e. in-silico testing.[8] For this purpose many QSAR and virtual screening methods have been suggested. An independent comparison of these methods has been done within the "Toxicology in the 21st century" project.[9]
Cancers
Some chemotherapies contain cytotoxic drugs, whose purpose is interfering with the cell division. These drugs cannot distinguish between normal and malignant cells, but they inhibit the overall process of cell division with the purpose to kill the cancers before the hosts.[10][11]
Immune system
Antibody-dependent cell-mediated cytotoxicity (ADCC) describes the cell-killing ability of certain lymphocytes, which requires the target cell being marked by an antibody. Lymphocyte-mediated cytotoxicity, on the other hand, does not have to be mediated by antibodies; nor does complement-dependent cytotoxicity (CDC), which is mediated by the complement system.
Three groups of cytotoxic lymphocytes are distinguished:
See also
References
- ^ a b c Riss TL, Moravec RA (February 2004). "Use of multiple assay endpoints to investigate the effects of incubation time, dose of toxin, and plating density in cell-based cytotoxicity assays". Assay Drug Dev Technol. 2 (1): 51–62. doi:10.1089/154065804322966315. PMID 15090210.
- ^ Gavanji S, Bakhtari A, Famurewa AC, Othman EM (January 2023). "Cytotoxic Activity of Herbal Medicines as Assessed in Vitro: A Review". Chemistry & Biodiversity. 20 (2): 3–27. doi:10.1002/cbdv.202201098. PMID 36595710. S2CID 255473013.
- ^ Decker T, Lohmann-Matthes ML (November 1988). "A quick and simple method for the quantitation of lactate dehydrogenase release in measurements of cellular cytotoxicity and tumor necrosis factor (TNF) activity". J. Immunol. Methods. 115 (1): 61–9. doi:10.1016/0022-1759(88)90310-9. PMID 3192948.
- ^ Niles AL, Moravec RA, Eric Hesselberth P, Scurria MA, Daily WJ, Riss TL (July 2007). "A homogeneous assay to measure live and dead cells in the same sample by detecting different protease markers". Anal. Biochem. 366 (2): 197–206. doi:10.1016/j.ab.2007.04.007. PMID 17512890.
- ^ Fan F, Wood KV (February 2007). "Bioluminescent assays for high-throughput screening". Assay Drug Dev Technol. 5 (1): 127–36. doi:10.1089/adt.2006.053. PMID 17355205. S2CID 10261888.
- ^ Easty, A.C.; Coakley, N.; Cheng, R.; Cividino, M.; Savage, P.; Tozer, R.; White, R.E. (February 2015). "Safe handling of cytotoxics: guideline recommendations". Current Oncology. 22 (1): e27–e37. doi:10.3747/co.21.2151. ISSN 1198-0052. PMC 4324350. PMID 25684994.
- ^ Capoor, Malini R; Bhowmik, Kumar Tapas (2017). "Cytotoxic Drug Dispersal, Cytotoxic Safety, and Cytotoxic Waste Management: Practices and Proposed India-specific Guidelines". Indian Journal of Medical and Paediatric Oncology. 38 (2): 190–197. doi:10.4103/ijmpo.ijmpo_174_16 (inactive 2024-07-11). ISSN 0971-5851. PMC 5582558. PMID 28900329.
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: CS1 maint: DOI inactive as of July 2024 (link) - ^ Dearden, J. C. (2003). "In silico prediction of drug toxicity". Journal of Computer-aided Molecular Design. 17 (2–4): 119–127. Bibcode:2003JCAMD..17..119D. doi:10.1023/A:1025361621494. PMID 13677480. S2CID 21518449.
- ^ "Toxicology in the 21st century Data Challenge" https://tripod.nih.gov/tox21/challenge/leaderboard.jsp
- ^ Priestman, T. J. (1989). Cancer Chemotherapy: an Introduction. doi:10.1007/978-1-4471-1686-8. ISBN 978-3-540-19551-1. S2CID 20058092.
- ^ "How Is Chemotherapy Used to Treat Cancer?". www.cancer.org. Retrieved 2021-06-28.
External links
- Cytotoxins at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
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