B134495
Describe the main mechanism used by cytotoxic CD8 T cells for killing and discuss benefits and dangers of this pathway.
Word count:
Introduction
Cytotoxic CD+ T cells are able to induce necrosis or apoptosis in a variety of cytolytic and non-cytolytic ways, including the release of cytotoxic granules containing perforin and granzymes, secretion of cytokines, and via Fas/FasL binding. These mechanisms allow for highly specific killing of tumorous and allographic cells along with those infected with intracellular viruses, bacteria, and some protozoan species. Killer T cells are critical because of their ability to target specific cells surrounded by healthy tissue and to swiftly kill them via necrosis or, more frequently and effectively, apoptosis, allowing the infection (if present) to be suppressed. Dangers in the mechanisms used for cell death come from non-specific cell destruction and non complete destruction of infectious DNA during necrosis; there is also research suggesting that immunogenic pressure from high levels of CTLs in the cancer microenvironment can lead to acquired resistance to apoptosis, both immunological and therapeutic.
Mechanisms
The principal mechanism by which cytotoxic CD8 T cells kill infected cells is by secreting granules that are filled with preformed activated forms of proteins within the family perforin and granzyme. A cytotoxic CD8+ T cell receptor recognizes a target presenting its antigen when the CD8 on the CTLs is able to bind to a constant portion of the class I MHC molecule that holds the antigen. The release is calcium dependent and the molecules inside the granules are active but suppressed by the conditions of the granule itself; once released, the two molecules work together. Perforin acts first, forming a pore in the membrane of a target cell by polymerizing and inserting itself into the lipid bilayer. The pore is a cylindrical structure formed by polymerized perforin molecules that is lipophilic around the edges and hydrophilic in the center, which allows the granzymes to enter the cell. The pore is either formed extracellularly and inserted into the membrane as a unit, or it is formed from individual molecules within the membrane. Once a pore is inserted, granzymes are able to pass through and into the cell, where they work by triggering an enzymatic cascade that results in the halting of viral protein production and other molecules vital for cellular function. By suspending cellular function, the CTL is ultimately able to induce apoptosis. An example of a granzyme B target enzyme is CPP-32, which, when turned on by granzyme B, activates caspase-activated deoxyribonuclease, or CAD, by binding to it and cleaving off an inhibitory unit (ICAD) attached to the deoxyribonuclease. Once active, CAD is able to generate double and single stranded 3’-OH breaks of nuclear DNA, which ultimately leads to apoptosis.
The secretion of cytokines is a secondary method that cytotoxic CD8+ T cells use to kill target tumour or virally infected cells. This method is less specific than directional granule release and can be used to kill general areas of cells, rather than infected cells within healthy tissue layers. The most prominent of these cytokines is IFN-γ, which is able to inhibit viral replication by inducing various molecules that work to inactivate eukaryotic protein synthesis, inhibit influenza viruses, and deplete tryptophan stores, leading to cell starvation of intracellular parasites. It is also able to induce cell death via the expression of iNOS, a gene that encodes for the release of nitric oxide synthase. Nitric oxide has the ability to pass from cell to neighboring cell, which can be an advantage and a disadvantage, as it allows for quick spread in an infected area but can also wreak havoc on healthy tissue.
A third mechanism by which cytotoxic CD8+ T cells kill target cells is via Fas/Fas ligand crosslinking. FasL is expressed on activated CD8+ cells, and when they bind to Fas on target cells, the Fas molecules trimerise and start a caspase cascade (Figure X), leading to apoptosis of the target cell. CD8 cells have both Fas and FasL, so this mechanism can also be used as a self check, allowing CD8+ cells to kill one another. This usually happens at the end of an immune response during the contraction phase, when a high population of immune effector cells is no longer necessary.
Benefits
Cytotoxic CD8 T cells killing of infected and cancerous cells is an extremely useful and necessary function of the immune system for a number of reasons: it is precise, quick, and neat. Precision comes from the directional release of the granules and Fas/FasL binding, which allows for targeted cell killing and not broader tissue death. The speed at which the targeted cell is recognized by TC and apoptosis is begun is important in the containment of an infection and of cancerous growth. The induction of apoptosis rather than necrosis also helps with containment, as necrotic viral debris has the potential to infect surrounding and phagocytic cells. These benefits are not always applicable to the release of cytokines by CTLs, but this mechanism of killing has its own benefits (along with its drawbacks).
The release of cytotoxic granules is highly directional, which is part of why this mechanism is so useful. Granules reach their target because they “are released only in the direction of the target cell, aligned along the immune synapse, to avoid non-specific bystander damage to healthy surrounding tissue”. When bound to multiple target cells, the TC will reorient its secretory apparatus (the Golgi apparatus and microtubule-organizing center) towards each target cell individually, releasing the contents of the granules into the synaptic cleft, leading to death one cell at a time; this process of directional successive release is known as serial killing. This polarized release is especially important in areas where regeneration of normal tissue is not possible (ex: neurons) and the death of non-infected cells would be highly detrimental. Likewise, the direct binding of Fas and Fas ligand allows for individual targeting by killer T cells.
The release of cytokines by cytotoxic T cells is not specific, which can be beneficial in that they can reach further and deeper than granules. Once released into the cell environment, these cytokines can attack cells regardless of infectious state, but that is perfectly fine when there is widespread infection of the tissue, or if the target is a cancerous mass. The spread of nitric oxide from neighbor to neighbor, as mentioned above, can be useful when immune-infiltration is tricky physically, as is the case for certain cold tumors, because it allows the chemical to reach into the deeper layers of infected cells.
Quick shittttttt
Either apoptosis or necrosis of a target cell is the end goal for cytotoxic CD8 T cells, but they are not equally useful destruction techniques. While both ultimately end with cell death, dangers lie with necrotic death, as intact pathogenic DNA, which would have been destroyed during apoptosis along with cellular DNA, can be released from the dying cell. This intact pathogen can then go on to infect nearby cells or can parasitize macrophages that phagocytose the dead cell. Because of this risk of further infection, mechanisms such as the release of cytotoxic granules are ultimately superior to the secretion of cytotoxic molecules such as tumor necrosis factor alpha and gamma (TNF-ɑ/β), since these molecules will lead to necrosis, not apoptosis.
Dangers
The danger of cytotoxic CD8+ T cells isn’t that these cells will go wild and become autoimmune or cancerous (or something fun like that); rather, the issue may be that they may be too good at their job and flood a site of tumorous growth so thoroughly that the only cancer cells that are able to survive will have adapted to be able to withstand apoptosis. This evolution of the tumor due to immunogenic pressure may also translate to therapy-induced apoptosis resistance, leading to poor therapeutic results. Ten Berge et al. (1999) found that patients with anaplastic large cell lymphoma and high CTL levels have poor prognosis and suggest that this could be because the surviving cancerous cells could have acquired resistance due to the expression of the gene bcl-2, which can act as an apoptosis inhibitor, regardless of whether apoptosis is being induced via CTLs or therapeutically. Oudejans et al. (1997) also suggest that this resistance to apoptosis could be due to a downregulation of class I MHC molecules, which would prevent infected cells from presenting antigens for recognition by CTLs. Other reasons this research group offers are that the tumor cells are fighting back at CTLs by releasing cytokines such as IL-10 and transforming growth factor-β into the tumor microenvironment and contain perforin neutralizing proteins, which are able to inhibit CTL and perforin function. The findings of these two groups of researchers shows that, while CTLs are not attacking tissue or damaging a host in any way, their reliance on apoptosis leaves them vulnerable when infected cells are able to outsmart them.
The second mechanism I describe above, the release of cytokines into the surrounding environment, also has its dangers in that the cytokines can also be uptaken by healthy cells and can kill them. This could an be an issue because it maycan cause further damage of the tissue that is already susceptible due to infection or starvation because of greedy tumorous cells. Killing healthy cells in these areas may leave room for malignant growth and viral reproduction, even if the cytokine release was meant to rid the area of such attackers. Necrosis initiated by TNFs by this mechanism can also be dangerous, as the processes does not effectively destroy pathogenic DNA, which means there is still the potential for of reinfection.
Conclusion
Cytotoxic CD+ T cells are an incredibly useful tool that allow us to swiftly and neatly get rid of cells infected with intracellular pathogens and tumors presenting mutated self-antigens. The three dominant mechanisms by which these T cells kill their targets —, cytotoxic granules, cytokines, and Fas/FasL binding —, all have their own benefits and drawbacks. Overall, cytotoxic T cells are effective killers because they are able to work swiftly and neatly with specificity (or generally, if need be). However But, their major vulnerability is that they rely heavily on apoptosis, which clever cells, especially those that are able to rapidly grow and evolve (ie: i.e., tumor cells), are able to subvert. The pathways that
References
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https://www.immunology.org/public-information/bitesized-immunology/cells/cd8-t-cells (idk how to cite this)
https://www.ncbi.nlm.nih.gov/books/NBK27101/#A1080 (or this)
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