Cytotoxic T cell

A killer T cell is a T lymphocyte that kills cancer cells, cells that are infected by intracellular pathogens such as viruses or bacteria, or cells that are damaged in other ways.
Most cytotoxic T cells express T-cell receptors that can recognize a specific antigen. An antigen is a molecule capable of stimulating an immune response and is often produced by cancer cells, viruses, bacteria or intracellular signals. Antigens inside a cell are bound to class I MHC molecules, and brought to the surface of the cell by the class I MHC molecule, where they can be recognized by the T cell. If the TCR is specific for that antigen, it binds to the complex of the class I MHC molecule and the antigen, and the T cell destroys the cell.
In order for the TCR to bind to the class I MHC molecule, the former must be accompanied by a glycoprotein called CD8, which binds to the constant portion of the class I MHC molecule. Therefore, these T cells are called CD8⁺ T cells.
The affinity between CD8 and the MHC molecule keeps the T_C cell and the target cell bound closely together during antigen-specific activation. CD8⁺ T cells are recognized as T_C cells once they become activated and are generally classified as having a pre-defined cytotoxic role within the immune system. However, CD8⁺ T cells also have the ability to make some cytokines, such as TNF-α and IFN-γ, with antitumour and antimicrobial effects.

Development

The immune system must recognize more than a billion potential antigens. There are fewer than 30,000 genes in the human body, so it is impossible to have one gene for every antigen. Instead, the DNA in millions of white blood cells in the bone marrow is shuffled to create cells with unique receptors, each of which can bind to a different antigen. Some receptors bind to tissues in the human body itself, so to prevent the body from attacking itself, those self-reactive white blood cells are destroyed during further development in the thymus, in which iodine is necessary for its development and activity.
TCRs have two parts, usually an alpha and a beta chain.. Hematopoietic stem cells in the bone marrow migrate into the thymus, where they undergo VJ recombination of their beta-chain TCR DNA to form a developmental form of the TCR protein, known as pre-TCR. If that rearrangement is successful, the cells then rearrange their alpha-chain TCR DNA to create a functional alpha-beta TCR complex. This highly-variable genetic rearrangement product in the TCR genes helps create millions of different T cells with different TCRs, helping the body's immune system respond to virtually any protein of an invader. The vast majority of T cells express alpha-beta TCRs, but some T cells in epithelial tissues express gamma-delta TCRs, which recognize non-protein antigens. The latter are characterised by their ability to recognise antigens that are not presented. In addition, they can recognise microbial toxic shock proteins and self-cell stress proteins. T γδ cells possess a wide functional plasticity after recognising infected or transformed cells, as they are able to produce cytokines and chemokines, trigger cytolysis of target cells, and interact with other cells, such as epithelial cells, monocytes, dendritic cells, neutrophils and B cells. In some infections, such as human cytomegalovirus, there is a clonal expansion of peripheral γδ T cells that have specific TCRs, indicating the adaptive nature of the immune response mediated by these cells.
T cells with functionally stable TCRs express both the CD4 and CD8 co-receptors and are therefore termed "double-positive" T cells. The double-positive T cells are exposed to a wide variety of self-antigens in the thymus and undergo two selection criteria:

positive selection, in which those double-positive T cells that bind to foreign antigen in the presence of self MHC. They will differentiate into either CD4⁺ or CD8⁺ depending on which MHC is associated with the antigen presented. In this case, the cells would have been presented antigen in the context of MHC1. Positive selection means selecting those TCRs capable of recognizing self MHC molecules.
negative selection, in which those double-positive T cells that bind too strongly to MHC-presented self antigens undergo apoptosis because they could otherwise become autoreactive, leading to autoimmunity.

Only those T cells that bind to the MHC-self-antigen complexes weakly are positively selected. Those cells that survive positive and negative selection differentiate into single-positive T cells, depending on whether their TCR recognizes an MHC class I-presented antigen or an MHC class II-presented antigen. It is the CD8⁺ T-cells that will mature and go on to become cytotoxic T cells following their activation with a class I-restricted antigen.

Activation

Mature T cells go through different stages, depending on the number of times they have been in contact with the antigen. In the first place, naïve T-lymphocytes are an initial stage of T cell residing in the thymus which have not yet encountered an antigen with affinity for its TCR. T cells that have been in contact with the antigen at least once but have subsequently returned to a quiescent or inactive state, are known as memory T cells. This particular stage remains ready to respond again to the specific antigen against which they were stimulated. Finally, when an immune response is triggered, these naive and memory T cells are activated, giving rise to effector T cells that have the capacity to kill pathogens or tumor cells.
The threshold for activation of these cells is very high, and the process can occur via two pathways: thymus-independent or thymus-dependent. In the thymus-independent pathway, because the APC is infected, it is highly activated and expresses a large number of co-receptors for coactivation. If APCs are not infected, CD4 cells need to be involved: either to activate the APC by co-stimulation or to directly activate the Tc cell by secreting IL-2.
If activation occurs, the lymphocyte polarizes its granules towards the site of the synapse and releases them, producing a "lethal hit". At this point, it separates from the target cell, and can move on to another, and another. The target cell dies in about 6 hours, usually by apoptosis.
Class I MHC is expressed by all host cells, except for non-nucleated ones, such as erythrocytes. When these cells are infected with a intracellular pathogen, the cells degrade foreign proteins via antigen processing. These result in peptide fragments, some of which are presented by MHC Class I to the T cell antigen receptor on CD8⁺ T cells.
The activation of cytotoxic T cells is dependent on several simultaneous interactions between molecules expressed on the surface of the T cell and molecules on the surface of the antigen-presenting cell. For instance, consider the two signal model for T_C cell activation.
A simple activation of naive CD8⁺ T cells requires the interaction with professional antigen-presenting cells, mainly with matured dendritic cells. To generate longlasting memory T cells and to allow repetitive stimulation of cytotoxic T cells, dendritic cells have to interact with both, activated CD4⁺ helper T cells and CD8⁺ T cells. During this process, the CD4⁺ helper T cells "license" the dendritic cells to give a potent activating signal to the naive CD8⁺ T cells. This licensing of antigen-presenting cells by the CD4⁺ T helper cells proceeds by signaling between CD154/CD40L on the T helper cell and the CD40 receptor on the antigen-presenting cell during immunological synapse formation.
While in most cases activation is dependent on TCR recognition of antigen, alternative pathways for activation have been described. For example, cytotoxic T cells have been shown to become activated when targeted by other CD8 T cells leading to tolerization of the latter.
Once activated, the T_C cell undergoes clonal expansion with the help of the cytokine interleukin 2, which is a growth and differentiation factor for T cells. This increases the number of cells specific for the target antigen that can then travel throughout the body in search of antigen-positive somatic cells.

Effector functions

When exposed to infected/dysfunctional somatic cells, T_C cells release the cytotoxins perforin, granzymes, and granulysin. Through the action of perforin, granzymes enter the cytoplasm of the target cell and their serine protease function triggers the caspase cascade, which is a series of cysteine proteases that eventually lead to apoptosis. This is called a "lethal hit" and allows to observe a wave-like death of the target cells. Due to high lipid order and negatively charged phosphatidylserine present in their plasma membrane, T_C cells are resistant to the effects of their perforin and granzyme cytotoxins.
A second way to induce apoptosis is via cell-surface interaction between the T_C and the infected cell. When a T_C is activated it starts to express the surface protein FAS ligand, which can bind to Fas molecules expressed on the target cell. However, this Fas-Fas ligand interaction is thought to be more important to the disposal of unwanted T lymphocytes during their development or to the lytic activity of certain T_H cells than it is to the cytolytic activity of T_C effector cells. Engagement of Fas with FasL allows for recruitment of the death-induced signaling complex.
The Fas-associated death domain translocates with the DISC, allowing recruitment of procaspases 8 and 10. These caspases then activate the effector caspases 3, 6, and 7, leading to cleavage of death substrates such as lamin A, lamin B1, lamin B2, PARP, and DNA-PKcs. The final result is apoptosis of the cell that expressed Fas.
CD8 T cells can also show Activation Induced Cell Death or AICD which is mediated by CD3 receptor complex. Recently, a platelet released protein TLT-1 has been shown to induce AICD like cell death in CD8 T cells
The transcription factor Eomesodermin is suggested to play a key role in CD8⁺ T cell function, acting as a regulatory gene in the adaptive immune response. Studies investigating the effect of loss-of-function Eomesodermin found that a decrease in expression of this transcription factor resulted in decreased amount of perforin produced by CD8⁺ T cells.