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Contents

Introduction

This image shows a colored SEM T lymphocyte [1]

T lymphocytes are a special class of lymphocyte possessing T-cell receptor (TCR) on their cell surfaces. This is the defining characteristic of T cells, setting them apart from other lymphocytes like B lymphocytes or Natural Killer cells. T-cells earn their name from their main organ of maturation, the thymus, which provides the means for their selection and differentiation, as shown in [2][3][4] and reviewed in [5]. Some T cells also mature within the human tonsil[6].

T cells are a diverse population of cells which enables the immune system to be modulated and regulated very precisely to fit the necessary host response. It is well known that T cells play a crucial role in the immune system, particularly pertaining to adaptive immune responses and cell-mediated immunity. As reviewed in [7], T-cells are able to determine the presence of immunogenic material by recognizing fragments of target antigens. The fragments are displayed as part of the MHC cell surface receptors of antigen presenting cells (APCs). Antibodies, on the other hand, as reviewed in [7], have binding specificity for intact antigen.

As reviewed in [7], the majority (~95%) of T cells are αβ T cells, named after their T cell receptor subtype. As part of the host immune response, αβ T cells recognise antigen fragments, usually peptides incorporated into the class I or class II MHC molecules of the infected cell, and evoke an adaptive immune response. On the other hand, as reviewed in [8], γδ T cells seem to initiate more rapid immune responses but make up only a small fraction of total human T cells and function more invariably as part of the innate immune system.

Since established acceptance of T-cells as a distinct type of lymphocyte in 1970[9], new classifications and characteristics of T cells have been discovered relevant to their structure, function, and roles in current research and clinical implications. This wiki will serve as a systematic overview of current information about T cells.

The following is a YouTube video by Handwritten Tutorials outlining the path by which T lymphocytes can develop from lymphoid progenitor cells:

Cells of the Immune System  

YouTube Link

History

Year Finding
1961 Found that the thymus plays a role in immune function, [2]
1964 Found that small lymphocytes continuously re-circulate through blood, lymphactics and secondary lymphoid tissue, [10]
1966 Found small lymphocytes have immunological memory potential, [11]
1967 Proposition of two major subsets of Lymphocytes, [12]
1970 Became widely accepted that Thymus derived (T cells) are distinct from antibody-forming (B cells), [9]
1972 T cells are found responsible for allogeneic cytotoxicity, [13]
1979 Generation of first monoclonal T cell, [14]
1983 Discovery of T cell antigen receptor (TCR) that consists of a disulfide linked heterodimer with both constant and variable regions, [15] , [16]
1984 Discovery of the TCR b locus and the basis for TCR diversity, [17], [18], [19]
1987 3D crystalline structure of class 1 MHC determined, [20]
1989 Discovery that CD8 glycoproteins expressed on cytotoxic T lymphocytes recognise antigens presented by class 1 MHC, [21]
1994 Demonstrated the CD8 T cell memory could be maintained in the absence of antigen [22]
1995 Regulatory T cells discovered [23]
1999 Found that memory CD4 T cells could persist long-term in the absence of MHC molecules[24]
2001 Discovery of FOXP3 gene and its involvement with T regulatory cell development [25]
2002 - 2003 Notion that T cells differentiate into memory T cells as a continuum [26]
2006 Role of Interleukins 7 and 15 in memory T cell maintenance and division through homeostatic proliferation [27]
2007 Discovery of the interplay between CD4 T cells and CD8 T cell in the development of memory CD8 T cells [28]

Commonalities between T cell subtypes

Functions

Model Figure of T Cell Migration 
Migratory challenges faced by T cells [29]

As reviewed in[30] and [31], there are three main T cell types that take part in the immune response: CD4+ T Helper Cells, CD8+ Cytotoxic T Cells and T Regulatory cells. The former two are often summarized as effector T cells, since they take effect against a pathogen. This terminology of effector versus regulatory T cells is widely used in the field and should be familiar when proceeding to subsequent paragraphs. Each subtype has very different functions. What all have in common, however, is their ability to regulate and orchestrate immune responses. Whereas Makrophages and B cells directly attack a pathogen, T cells always modulate other cells and indirectly combat pathogens. They can activate B cells, suppress general immunoactivity or lead to cell death of infected cells (reviewed in [32]). The latter eventually will cause phagocytosis by makrophages and thus presentation of the infecting pathogen to the immune system. So even despite the fact that T cells do not undertake actual pathogen killing (makrophages) or inhibiting (B cell antibodies), no immune response can take place without them.

Activation

Dendritic cells (DC) play the main part in T-cell activation. DCs find and internalize antigens to present them as part of the MHC receptor complex on their surface. They are the most important APCs (antigen presenting cell) and basically all adaptive immune responses are initially triggered by presentation of antigens by APC. Every T cell has a unique receptor (TCR), which allows binding of one exact amino acid sequence (with the exception of NKT, which are also able to bind lipids, as elaborated in a later section). This means, T cells, just as B cells, will only be activated if this exact sequence is present on the pathogen. With an extreme abundance of unique T cells, our body is able to detect nearly every possible antigen. Before T cells bind to their specific antigen presented by an APC, they are called naïve T-cells, and after the activation process they are referred to as active T cells. It is very important to emphasize that TCR (reviewed in [30])

Migration and the Cytoskeleton

Lymphocyte rosettes [33]

The cytoskeleton of a T cell is made of a dynamic filament meshwork that forms the structure of the cell and maintains the cell's essential features. The cytoskeleton gives the T cell mobility that allows circulating within the blood stream, migrating through tissues, and interacting with the APC[34]. In order to achieve mobility, the physical structure of the T cell has to be continuously remodeled. Circulating T cells in the blood stream can perform movement by short microvilli. These microvilli consist of bundles of highly dynamic actin filaments[35]. When the T cell interacts with soluble and endothelium-displayed chemokines, they squeeze between endothelial junctions to enter the underlying tissues(as reviewed in[36]). When the T-cell is inside, the morphology of the cell changes to s “hand mirror” structure. The name originates from the fact that the nucleus will form a flat surface on one end of an elongated cell (mirror surface) while the other end is thin and elongated (the mirror handle), giving it the appearance of a hand mirror. The motility of the T-cell in the tissue is performed by F actin filaments which push the cell forward. The speed of the T-cell movement is much faster than most nonhematopoietic cells, averaging a speed of 10 μm/min, and reaching rates as high as 25 μm/min[34]. For the last, the T-cell forms a tight contact rich in branched actin filaments when T-cell recognises an APC. As reviewed in[37], The contact between T-cell and APC forms an immunological synapse (IS) and also makes a scaffold for protein sequestration at the distal pole complex (DPC).

Structure

There are three main classes of lymphocytes, T cells, B cells and Natural Killer cells. T Cells comprise about 70% of all lymphocytes. T-cells are located in various places. Lymphoid organs often have a fairly structured morphology. T cells are generally concentrated in T cell specific areas of those Lymphoid organs, such as T cell dependent areas (TDA), which can be seen in the figure of a lymph node in the development subsection. T cells are also found patrolling through the blood and loosely accumulated in connective tissues of barrier organs such as the skin or gut lining. There even are T cells which are integrated into the epithelial linings of barrier organs, such as the intraepithelial leukocytes. They are called T cells since, in contrast to B cells, they mature in the Thymus after being created in the bone marrow. B cells, in contrast, not only develop in the bone marrow, but also fully develop there (reviewed in [38]).

T Cell Morphology

B and T cell comparison with ER rich B cells on the right. [39]

T cells typically have roundly shaped, fairly small (7 to 10µm) cells with typical lymphocyte morphology. Without markers, naive T cells cannot be distinguished from the other lymphocyte cell type (B cells) in histology slides. Approximately 10%, however, take the morphology of Large Granular Lymphocytes (LGL) with a size of 10-14µm. Granulocytes and monocytes are typically between 14-20µm. Small lymphocytes may thus be distinguished from granulocytes by size, yet LGL can have approximately the same size. Another difference between Granulo-/monocytes and lymphocytes is nucleus shape. Lymphocyte nuclei are generally evenly round, whereas granulo-/monocyte nuclei have more complicated shapes. The T cells with LGL morphology, in turn, cannot be distinguished from Natural Killer cells in unlabeled histology slides. Natural Killer cells comprise the bigger part of LGL morphology cells (reviewed in[40]).

There are, nevertheless, methods that remain for distinguishing T cells from B cells. B-cells are filled with largely rough endoplasmic reticulum, but T cells have very little endoplasmic reticulum (as seen in the picture on the right) [41]. This seems logical, as key feature of B Cells is production of high amounts of BCR/antibody proteins, whereas T Cells are instead focused on cytokine action. For both B and T cells, the nucleus fills the majority of the cell and the ratio of nucleus to cell diameter is similar in both T cell and B cell with a ratio of 0.9 nucleus diameter divided by cell diameter[41].

Location

T Cells are found in the bloodstream as circulating T Cells and embedded in connective tissue, such as the lamina propria of the gut. However, the greatest density is found within secondary lymphoid organs, such as the tonsils, spleen, lymph nodes and Peyer's patches in the gut, as reviewed by [42]. There is a specific tissue within the secondary lymphoid organs, surrounding the medullary sinus, that hosts mainly T cells. B cells and T cells are, thus, normally separated and unable to invade their counterpart's specific tissue. This is important as B cells are most often, in fact, dependent on T cell activation. T cells must be specifically designed to invade B cell specific tissues, an aspect involved with T Follicular Helper Cells to be covered in more depth later.

Structure of TCR as it is interacting with a MHCII receptor presenting an antigen on an antigen presenting gene. It is further presented, how the gene structure of the B locus of the TCR gene is comprised[43]

T Cell Receptor and Co-receptors

The T cell receptor (TCR) structure (reviewed in [44]) is analogous to a Fab fragment, which is one of the extracellular oriented sides of the Y-shaped antibody/BCR. TCR are often accompanied by co-receptors which are necessary for activation. Two protein chains, called heavy and light chain, are connected by disulfide bonds to form the TCR hetero-dimer receptor. Equivalent to its Fab counterpart, TCR heavy and light chains are comprised of a constant (C) region and a variable region (V). The variable region contains the so called CDR, complementary determining region. The CDR is the part of the TCR that actually interacts with an antigen. As such, they must be individual for each T Cell. This is achieved by VDJ-gene rearrangement, which is further elaborated in the development subsection. Due to its variability, the variable region received its name. the C-Region, in contrast, does not display such heterogeneity, hence it is called Constant Region. Two subtypes of TCR monomers exist, which dimerize to form the TCR hetero-dimer. The alpha (α) and beta (β) monomers account for about 95% of the human TCRs, while 5% consist of gamma and delta (γ/δ) chains. Most of the γ/δ-TCR are found in the gut. No difference in function has been described yet, however γδ seem to lead to the faster production of less specific and variable IgM[45].

Morphological Changes during Activation

The T cell cytoskeleton is highly dynamic and drastically changes shape during activation. The TCR is directly linked to the actin cytoskeleton via the signaling adapters Nck (non-catalytic region of tyrosine kinase), LAT (linker for activation of T Cells) and SLP76 (SH2 domain-containing leukocyte protein, 76kDA molecular weight) and others. They are closely connected to several actin polymerization regulatory proteins like cofilin, Arp2/3 and others. This will lead to polarization of the microtubule organizing center (MTOC) towards the TCR in case of antigen activation [46]. This actin and microtubule rearrangement eventually leads to generation of a structure called the immunological synapse, the MHC/TCR complex[47][48][49][50].

Development

Diagram of production site and lineage of lymphoid progenitor offspring. Lymphoid progenitor cells are created in t.he bone marrow and mature either in the thymus or bone marrow[51]
T Cell Development  

YouTube Link

Bone Marrow

Analogous to all blood cells, T Cell development begins with hematopoietic stem cells in the bone marrow (from greek, haime: blood, poenin: to create).

Bloodstream

T cells begin to differentiate making their way to the thymus through the bloodstream [52]. Settling of these lymphoid progenitor cells in the thymus is a continous regulated [53] and periodic process [54]. The mechanism by which these progenitor cells enter the thymus is mostly unknown, yet it is believed to be analogous to other leukocytes. The process, reviewed by von Adrian, UH et. al. in [55] begins with the loose adherence to blood vasculature via selectins and integrins which results in rolling, as the result of a chemokine gradient. This initial loose adherence is followed by tight adhesion and eventually entrance into the stroma across the endothelium.

Thymus

As soon as lymphoid progenitors enter the thymus, they are termed thymocytes. Entering at the cortico-medullary junction of the thymus, the thymocytes travel through the cortex. The cortex is the outermost layer of connective tissue, into which lymphocytes are embedded. After traveling through the cortex they arrive at their final destination, the subcapsular zone of the thymus. There, the thymocytes subsequently run through several stages of differentiation. T1 cells can be found in macrophage vesicles, and helper T2 cell can be found in extracellular fluid.
This image shows fluorescently labeled cell types in a lymph node, showing typical structure of a secondary lymphoid organ. More specifically, it is a Transverse mouse lymph node section depicting a ring of subcapsular sinus macrophages (CX3CR1-GFP, green) wrapped around numerous B cell follicles (anti-B220, blue) with the T cell zone (T cell-specific dsRed, red) located centrally.[56]

Double Negative stage

At first, the thymocytes do not possess CD4 and CD8 and are referred to as double negative (DN, CD4-/CD8-). The expression of CD4/8 marks the point in T Cell development, at which they are able to be differentiated from B Cells. DN T Cells pass through four stages of selection before they start to express all the corresponding receptors and surface proteins. The fourth stage of DN differentiation is where the αβ or γδ pre-TCR actually form. [57].

V(D)J Rearrangement

During those DN selection steps, the pre-TCR undergoes V(D)J recombination, a process that occurs both in B cells and T cells. V, D, and J are gene regions in the β locus of the TCR gene and make up the variable region of the TCR (see T Cell structure for TCR structure). There are various copies of each of the V,D and J genes in the β locus. From every gene, only one gene copy gets selected, and the rest are spliced out by RAG1 and RAG1 (recombination activating genes) activity. This means, T and B cells are the big exception of the central dogma that every cell in the body carries the same genome (not taking mutations into account). Every single B and T cell has different genetic information after VDJ rearrangement. Thus, in contrast to all other somatic cells, they are not just different in gene transcription. The VDJ gene rearrangement is induced by signals from an already assembled receptor (reviewed in [58])[59]. This allows a great deal of different combinations, leading to highly unique receptors. Besides V(D)J rearrangement, somatic hypermutation, and mutations during the V(D)J rearrangement provide an incredible variety of TCR (reviewed in [60]). Following this tightly regulated process, T cells are generated with a single, specific TCR protein for each specific antigen [61].

Double Positive stage

Cells then move into the double positive (DP) stage, where CD8 and CD4 transmembrane proteins are expressed on the surface. This order of events is mandatory, as the TCR is required for double positive selection. During this stage 'positive' and 'negative selection' occurs (reviewed in [58]).

Positive Selection

Positive selection allows survival only of cells that are able to trigger a viable, positive immune response. The key step for the initiation of an immune response is significant MHC/TCR binding. Cells that have TCRs that are not able to readily attach to an MHC undergo apoptosis. In fact, 90% of the thymocytes that pass through the DN selection stages have excessively weak binding and then undergo apoptosis [62] [63].

Negative Selection

Negative selection, the most critical step, purges cells that are self-reactive. Self-reactivity is tested in a highly orchestrated fashion: cortical epithelial cells of the thymus present self-antigens, which should not trigger an immune response. Any T cell that interacts with the self-antigen undergoes apoptosis. This stage is essential: self-reactive cells are the hallmark of all autoimmune diseases, such as multiple sclerosis (reviewed in [64]). Approximately 5% of all cells are auto-reactive. Negative selection is a biological process that is prone to errors. To counteract self-reactive T Cells, T regulatory cells are employed [62]. The many models in which negative selection occurs are reviewed in [58].

As reviewed in [65] the CD4/CD8 linage is determined through a multitude of complex signalling. The exact mechanisms are not fully understood yet with many models proposed. Ultimately T cells that bind to MHC-class-II-peptide complexes differentiate into CD4+ T cells, whilst DP cells that bind to MHC-class-I-peptide complex differentiate into CD8+ T cells.

Only 2-4% of of intrathymic lymphocytes that undergo V(D)J recombination get released as peripheral T lymphocytes [66].

Types of T-Cells

There is a plethora of different T Cell subtypes. Each of those subtype appears then again to have even more different subpopulations. Defining the different subtypes and distinguishing them from one another are some of the key achievements that are continuously made in T cell research. Before going into the different T Cell subtypes, one should become aware of a fact that is ever-present in the immune system and only further highlighted by the extensive individuality of the many T cell subtypes: the immune system is incredibly adaptable. Each and every situation seems to have a tailor fit solution comprised of different types of T cells, forming subtypes and subpopulations dependent on the specific requirements of every individual case. Besides that, just like B Cells, T Cells possess an incredibly individual TCR created by V(D)J recombination. The following subtypes should be taken as they are: a model to simulate and understand and, at some point in the future, possibly even exploit the complexity of the immune system. Most T cell subtypes are distinguished through surface markers and the majority seems to have one or a combination of transcription factors specific to that subset. It is, however, an ongoing discussion to what extent T cells may be subtyped and what a T cell needs to display to be considered a distinguished subtype. Just as any model, the accuracy of the different T cell subtypes is constrained and evolving and is by far not able to account for everything in the incredible interactions of T cell subtypes with each other as well as other cells and systems.

The following table provides a quick summary of the following paragraphs. All information and their respective references are found in the corresponding subsections:

Subtype Function Markers
T Helper Cells Helper cells may be imagined as the generals of the immune system commanding innate and adaptive immune cells. Their main task is secretion of cytokines and activation of B Cells to produce antibodies. CD4
Cytotoxic T Cells Cytotoxic T cells patrol the ithe body to detect affected cells. They are able to kill any nucleated infected cell of the body to interrupt with the pathogen proliferation. CD8
Regulatory T Cells Cytotoxic T cells patrol the ithe body to detect affected cells. They are able to kill any nucleated infected cell of the body to interrupt with the pathogen proliferation. CD4+,CD25+,FoxP3+
NKT Cells N Natural Killer T Cells are extremely unique in two features. Firstly, their TCR recognizes lipids which makes them the only T cell type able to recognize non-protein antigens. Secondly, their TCR can bind to a non-MHC receptor, CD1d, on APCs which present the lipid antigen. IL12R, CD40L
Memory T Cells Every T Cell subtype possesses it's own memory cell pool. Memory cells most often travel back to the bone marrow where they reside quiescently and provide immunity against antibodies that have already affected the body once. Dependening on the pathogen and location, memory cells can become decades old. CD45R


T-Helper Cells

This image shows a colored SEM T Helper Cell surface (blue) covered by HIV-1 (yellow)[67]
Helper T Cell Tutorial  

YouTube Link

Function

T helper cells play a critical role in the adaptive immune system. They assist in the activation of B cells, macrophages and cytotoxic T cells so they can perform their function in a coordinated manner [31]. T helper cells are essentially a cytokine factory that release information to orchestrate an immune response. This chemically encoded information shapes the way the immune system responds to any pathogen (as reviewed in [68]. [69]).

Further information on T-Helper Cells

Development and activation

During the double positive stage of differentiation, thymocytes interact with cortical epithelial cells. The double positive thymocytes interacting with MHCII develop into naïve CD4+ T-cells and migrate to peripheral lymphoid organs.

There are 4 stages of the activation of naïve helper T cells.

  • Recognition stage

AP dendritic cells play a role in activating T-cells. DC in mucosa encounters antigens and digests them. Once digested they present a specific portion of the protein on the MHC class II on their surface. Activation of T-cell then occurs when naïve helper T cells interact with the MHC class II of APC [70]. The CD45 leukocyte antigen acts to phosphorylate the TCR, assisting in the interaction between a naïve T cell and APC [71]. When CD45 shortens, it is easier to interact and activate an effector helper T cell.

  • Verification stage

Once CD4+ T cells interact with MHCII, naïve T cells need a second independent biochemical pathway. The second signal is connection between CD28 on the CD4+ T cell and the proteins CD80 or CD86 on the APC. When the naïve T cell has both pathways activated, the only first signal is necessary for future activation. Without this second signal, the T-cell assumes that it is auto-reactive. In contrast, if the T cell does not respond to any antigen, it is anergic and will die through apoptosis [72].

  • Proliferation.

T cell proliferation occurs by the release of a potent T cell growth factor, IL-2. Th0 cells result from helper T cell receiving both signals of activation and proliferation. These Th0 cells also secrete IL-4 and interferon gamma (IFN-γ). IFN-γ causes Th1 cell production and inhibits Th2 cell production. Whilst IL-4 inhibits Th1 and leads to Th2 cell production [73].

  • Maturation

Here the T cell can then differentiate into a memory helper T-cell, effector helper T-cell and regulatory helper T cell. The effector helper T-cell produces cytokines to activate other leukocytes, including helper T-cell. The memory helper T-cell contains the antigen that actually activates the T-cell, and this memory helper T-cell is used for the second immune response[74]. The regulatory helper T cell does not help immune response, but it down regulates the response.

Mechanisms of action

There are several helper T-cell subtypes including Th1, Th2 and Th17 that play a different role in the immune response. Th1 cells respond to IL-2 and IL-12, and secrete the cytokine, IFN-γ. This IFN-γ in turn actives macrophages, CD8 T cells, IgG B cells and IFN-γ CD4 T cells[75]. The second subtype Th2 is trigged by IL-4, causing the secretion cytokines; IL-4, IL-5, IL-9, IL-10 and IL-13 [76][77]. Th17, secrete IL-17 to provide protection at the mucosal surface. [78]. Th17 are linked to autoimmune and inflammatory disorders [79].

Subsets

T Cell subtypes, functions and differentiation factors[80].

There are various T Helper Cell subsets with follicular B Helper T cells playing a critical role in the main T cell function, activation of B cells. They are therefore discussed below in more detail.

Follicular B helper T cells

Follicular B helper T Cells, as their name would indicate, are able to enter the B cell follicles of lymphoid organs. This is crucial as it allows coordinated interaction between T and B cells within secondary lymphoid organs, where they are spatially separated. B cells most often require T cell activation before acquiring plasma B cell morphology and being able to retort any humoral immune attack.

Tfh are exclusively selected to enter follicular zones within secondary lymphoid organs by expression of CXCR5. Within these follicular areas germinal centers form upon antigen contact. Germinal centers are areas of activated B Cells, producing cytokines and antibodies. They are requiring Tfh, as they are able to get close enough with their TCR and CD4 co-receptor. Here they are able to bind to MHCII antigen presenting receptors on the B cell surface and activate follicular B Cells (FoB) through IL-4 and IL-21. IL-21 is crucial in plasma cell differentiation and one of its key drivers (reviewed in [81], [82]).

Just as other T Helper Cell subsets, Tfh possess a master regulatory gene, which will induce most of phenotype. This master regulatory gene is Bcl-6. It affects microRNA expression; miRNA is a key driver of differentiation processes. Besides follicular B cells, also follicular Dendritic cells (fDC) are in direct contact with Fth. fDC form the antigen presenting entity within follicles, that are required for both T and B cell activation (reviewed in [81][83]).

Structure

Helper T-cell express CD4 co-receptors on their surface [84]. Follicular B Helper T cells (Tfh) characterized by expression of the CXCR5 surface receptor. CXCR5 allows to binding to CXCL10, expressed by fibroblasts in follicular stroma serving as signpost for follicular B cells[85].

Tfh also posses the CD40 ligand, which allows binding to the CD40 receptor on follicular B cells. This enables Tfh to connect enough closely to B cells to activate them.

Also crucial is the Inducible T Cell Costimulator (ICOS) expressed by Tfh, which binds to the ICOS ligand on Follicular B Cell (reviewed in [86]).

Clinical Implications and Disease

Autoimmune Demyelination

Interaction has been found between sulfatides (glycolipids with a sulfur group) and autoimmune demyelination present in multiple sclerosis. Myelin in the CNS contains proteins and lipids, both potentially can be recognized as foreign by the immune system. The sulfatides form a major element of myelin glycolipids that can lead to this response. The sulfatide stimulation in this study resulted in anergy of the T helper cells, particularly in naïve T helper cell, rather than necrosis or apoptosis. Sulfatide stimulation also caused prominent suppression of Th17, a known effector for autoimmune demyelination. From this, potential treatment for MS could arise knowing that sulfatides can prevent autoimmune demyelination[87].


Cytotoxic T Cells

Lattice Light-Sheet Microscopy Reveals Actin Dynamics in Conjugating T Cells as attacking a cancerous cell.[88]

Function

The role of cytotoxic T Cells (CTL) is to kill virally or bacterially infected or tumorigenic cells. As such, CTL must be able to do three things.

Firstly, they must be able to bind to any cell within the body. This is achieved by the CD8 co-receptor. The CD8 co-receptor allows CTLs to bind to MHCI, which is expressed on all nucleated cells.

Secondly, CTL must be able to first recognise infected cells in order to destroy them. This is achieved by the presentation of proteins from the pathogen (viral/bacterial proteins) on MHCI receptors. CTL will, thus, only bind to infected and therefore death-sentenced cells.

Lastly, they must be able to kill the infected cells. Cytolytic proteins contained in vesicles that get secreted by CTLs do this. The main killing mechanisms are perforins, granzymes, and FAS dependent death signalling [89].

Further Information on CTLs

Development

Dendritic cells directly stimulate naïve CD8 T cells by presenting antigen fragments in complex with MHC class 1 surface proteins. This antigen-MHC complex binds to the CD8 and TCR receptors that recognised the specific peptide fragment, and the cell becomes an antigen-specific CTL.

CD4+ helper T cells must also recognise the antigen-MHC class 2 complex on the same dendritic cell, this leads to generation of memory T cells and allow repetitive stimulation of cytotoxic T cells (reviewed in [90], [91]).

Inflammatory cytokines, in particular IFN-γ, IL-12 and type I IFNs (IFN-α/β) play additional signalling roles in co-stimulating and determining T cell response [92].

Activation

MHCI molecules are found on the surface of all nucleated cells in the body. When a cell is infected with a virus or other pathogen, rather than express ‘self house-keeping’, proteins of this pathogen are fragmented within the host cell, and these peptides become expressed through the MHCI [93].

CTLs are activated upon the binding of both their TCR and CD8 co-receptor to the same peptide-MHCI complex, as reviewed in [94]. The variable region of the TCR recognises the specific peptide antigens presented by the MHCI molecules on the surface of target cell [95], [93]. The CD8 stabilises the bond through contact with the non-polymorphic region of MHCI [95]. This CD8 - MHCI interaction serves as the first of two major signals in activation [96].

The second major signal is triggered by the association of B7-1 (CD80) or B7-2 (CD86) ligands on APC with the CD28 molecule expressed on the T cell [96]. CD28 signals promote production of cytokines, proliferation and survival of CTLs (reviewed in [97]). A CTL response may not always be elicited as many tumour cells will not express the B7-1 ligand, without full co-stimulation the T cell enters a state of hypo-responsiveness [98], leading to a less efficient immune response [99], [100].

Once the CTL is activated it secretes cytokines, with the major one being IL-2 that assist in clonal expansion, resulting in a larger population of cells that can detect specific antigenic somatic cells [101]. These signals also assist in the survival of the cell and memory T cell production [102].

Mechanisms of Action

CTL recognise and kill target cells through their characterised cytolitic activity that respond to APC by polarising and secreting cytotoxic granules leading to rapid lysis [89]. TCR recognition induces the rapid polarisation of the CTL secretory machinery at the point of contact with the target, forming the immunological synapse. Lytic granules are moved along the microtubule network and are secreted. It is at this secretory cleft, formed between the two cells that seal off the external environment, that the lytic granules are released [103].

There are two main contents in the lytic granules, a pore-forming protein known as perforin and granzymes that are a series of serine proteases. Perforin acts by incorporating itself into the lipid bilayer of the target cell through its unique lipid-recognition motif, a calcium dependent C2 domain and then polymerises, creating pores in the membrane [104]. These pores then allow the entrance of the granzymes into the cytosol of the target cell (as reviewed in [105]). These granzymes act variously depending on type; however, all secrete proteins that damage the cell during cytolysis [106]. There have been five granzymes identified in humans all with different specificities [107] and multiple potential substrates [108].

In this process the CTL is protected from the toxic effects of its own proteins through the granule membrane, which is resistant to its contents action due to capthesin B [109]. In addition to this, both perforin and granzymes are synthesised in an inactive pro-form with a short C-terminal amino acid pro-piece, that blocks the C2 chain ([110], reviewed in [105]). These amino acid pro-pieces are cleaved under acidic conditions once in the granule [111]

Within the lytic granules are also Fas ligands, a potential mediator of targeted cell death [112]. This initiates apoptosis by recruiting the Fas-associated death domain protein as a result of cross-linking to Fas transmembrane receptors on target cells surface. A cascade of caspase cleavage events results. Group II caspase proteins, notably caspase 3, play a role in cleaving structural components of the target cell (reviewed in [113]).

Interferon-γ (IFN-γ) is another essential cytokine that is produced in high levels upon TCR stimulation. IFN-γ upregulates the expression of MHCI on cell, facilitating recognition by TCR and CD8 co-receptors [114].

Structure

CD8+ T cell binds with TCR and its CD8 co-receptor to MHC1 on Antigen presenting cell (own creation).

Cytotoxic T Lymphocytes (CTLs), which are, also termed CD8+ T-cells, Killer T cells and Tc have the key structural features of T-cell Receptors (TCR) and surface CD8 glycoproteins that act as a co-receptors [95]. These receptors form what is known as the ‘immunological synapse’, with the MHC1 molecules of the infected peptide-presenting cell [115]. Through this, they function to kill pathogen-infected, tumorigenic and other damaged cells [116], via secretion of cytotoxic mediators leading to apoptosis [103].

CD8

As reviewed in [117] the CD8 transmembrane glycoprotein on the T-cell surface can either be expressed as a αα-homodimer or αβ-heterodimer, with αβ-heterodimers being the most prevalent within T cell populations. The β chain broadens the range of antigen recognition of the T cell, enhancing activation of CTLs [118]. The extracellular domain N-terminal exist on both α and β CD8 and has structural features to immunoglobulin-variable domains [119]. The Ig-like ectodomain binds to MHCI in a similar manner to antibody-antigen binding. This CD8-peptide-MHCI complex is asymmetric with 75% binding to just one of the CD8 subunits [120]. This is linked to the transmembrane component through ‘stalk’ of roughly 48 amino acids long [95].

CD28

CD28 receptor with MYPPPY loop binds with B7-1 or B7-2 Ligand on APC (own creation).

CD28 is a covalent homodimer, it has a pair of immunoglobulin superfamily-like variable domains attached to a transmembrane and cytoplasmic domains possessing signalling motifs that a tyrosine dependent [121]. The MYPPPY loop present on the variable domain is the site of cross linking with the B7-1 or -2 ligand [122].

Clinical Implications and Disease

Hepatitis B virus

Contributors to T cell exhaustion in HBV [123]

Hepatitis B is an infectious disease of the liver that results from the exposure to blood or body fluids containing blood that is infected with the hepatitis B virus. Infection with HBV can be acute resulting in nausea, vomiting, mild fever, and dark urine, and development of jaundice. Most adults clear the virus and chronic infection developing in only 5 – 10% cases. In chronic stages there are minimal symptoms however, cirrhosis and liver cancer often develop in later stages. Infection during early childhood is often asymptomatic as a result up to 90% of cases persist as chronic HBV [124].

HBV-specific CTLs play a crucial role in the clearance of the virus however, these CTLs become exhausted through a variety of mechanisms. These include Treg cells inhibiting CTLs, loss of CD4+ T-cell help, and high viral load, all contributing to the progression of chronic HBV [125]. The resulting uncontrolled HBV leads to liver damage by lysis of hepatocytes. This lysis is carried out by both the CTLs and their recruitment of non-virus specific T cells. The balance between viral replication and the immune defenses independently determine the severity of liver damage (reviewed in [126]).

Multiple sclerosis

MS is a relatively poorly understood neurodegenerative autoimmune disease. The progressive and multifocal demyelination is the result of invasion of inflammatory cells across the blood-brain barrier (BBB). It is also suspected there is some degree of genetics involved in one's susceptibility of developing the disease (reviewed in [127]).

The lesions that are associated with demyelination exhibited clonal expanded CTLs and yd T cells, where normally there is few T cells in the BBB. These CTLs have the potential to damage and destroy all CNS cell subtypes including astrocytes, microglia and neurons (reviewed in [128]).

T Regulatory Cells

Function

T Regulatory cells and their many subtypes differ from other T cell types in their function of only regulating other cells opposed to causing a direct effect. The inhomogeneity throughout subtypes present difficulties in finding adequate markers to clearly establish differentiating factors from effector T cells. It is important to distinguish Treg from Treg17, regulatory T Helper cells producing IL-17, also having immunomodulatory capacity [129]. Treg are called 'regulatory' for their ability to suppress and hence regulate CD4+ T Helper, CD8+ T Killer, B Cells, Natural Killer Cells and dendritic Cells ([130] and reviewed in [131][132][133] [134]).

Treg cells work to balance out the errors that have been made during the T Cell development process. They recognize circulating, self-reactive T Cells and eliminate them with their negative effects to correct any erroneous development and build a second line of defence against autoimmunity [135][136]. Gene defects involved with Treg development are correlated with susceptibility for autoimmune disorders, with complete inactivation resulting in fatal autoimmune diseases.

They are also unequivocally important for the preservation of tissues against immune-induced tissue damage. Tissue damage becomes very present in arthritis and other degenerative diseases which can cause catastrophic destruction of the motile system[137]. Immune-induced tissue damage is achieved mostly through the NFkB pathway, activated through pro-inflammatory cytokines, and the anti-inflammatory TGFβ and IL10.

Further information about T Regulatory Cells  

Development

Tregs can stem from two different sources. Natural, thymic generated T regulatory cells, making up 70-90% of population and peripheral induced Treg cells (reviewed in [138][139] and [140]). Peripheral, induced Treg cells do not develop as Tregs, but start expressing Treg markers after differential events, with CD4+ T cells as progenitors.

IL-10 is the main anti-inflammatory cytokine acting in Treg cell development along with TGFbeta. High levels of TGFbeta secretions lead to or contribute to inducing Treg differentiation of normal FoxP3 negative CD4+ cells.


Expression of the Helios marker is used to differentiate between induced and natural Treg cells, with the latter lacking. Induced Tregs have a considerable heterogeneity compared to natural Tregs, emphasizing their adaptive yet highly regulated homeostasis. Interaction with APC during development is another point of difference. Induced Tregs are more strongly influenced by APC, whereas thymic, natural Tregs are less geared towards APC activation [141].

Locations

Tregs are found in both lymphoid and non-lymphoid tissues, with or without the presence of a current immune response. If inflammation occurs locally or systemically, Tregs are rapidly recruited to the site of inflammation, increasing their numbers dramatically. The amount of T regulatory cells within a particular organ correlates with its immunoactivity.

Tregs are in higher concentrations within the skin predominantly in the vicinity of the invaginations of hair follicles. Here they serve as a protection barrier, with an abundant presence even in absence of obvious inflammation. [142].


Additionally, the intestine hosts an abundance of Tregs, with Treg deficiency being highly associated with intestinal inflammation. As most T Cells, adult tissue specific Treg cells are not particularly motile and will not circulate in an extensive manner[143]. These intestine specific Treg cells differ from other Treg cells in their phenotypical properties, such as expression of specific surface proteins (homing proteins), which allow Tregs to always finding back to their place of origin after entering circulation.

Activation

Treg activation differs from other T cells in that they show anergy. That is, they will not proliferate, even if confronted with their specific TCR antigen ([133] and reviewed in [138])

The transcription factor, Interferon regulatory factor 4 (IRF4) is crucial to Treg activation. Additionally, more typical T cell activation factors, such as IL-2, are still involved in altering gene expression within the cell. [144].

Mechanism of Action

Several Pathways are important for Treg regulation. Mechanisms of action include immunosuppressive cytokines, such as IL-10 from Treg Memory cells, paracrine signaling via cell-cell interaction or modulation of antigen presenting cells. Disruption as hypoactivity can lead to autoimmune conditions and hyperactivity resulting in chronic inflammation involving diseases (reviewed in [133] and [145]).

TGFbeta and IL10

These two anti-inflammatory cytokines have been postulated as main immunosuppressive forces. TGFbeta and IL10 result in a decrease in CD4/8+ cell proliferation, making activation of T effector cells impossible.

Cytolytic Activity

Suppression through cytolytic activity occurs under certain circumstances using killing mechanisms are normally only observed in CD8+ cells. CD4+ 25+ FoxP T Cells express perforin and granzymes, which leads to apoptosis (process discussed in CTLs) [146][110]. This control in the population of effector cells, such as CD4+ and CD8+ T Helper cells serves to confine immune responses.

Galectin-1 mediated mechanism

Galectin-1 acts to arrest the effector cells cycle by binding to various glycoproteins such as CD45/43/7, expressed on effector cells. This binding and arresting of the cell cycle will result in apoptosis. Galectin-1 bound Teff cells will also not be able to produce inflammatory cytokines, further combating inflammatory state.

Mechanistic interaction between Tregs and target cells

The suppression of CD4/8+ cell proliferation requires direct contact of Tregs and its target cells. This is partially achieved by TCR/CD3-Ligand interaction. [133]

Regulation of Treg supression

When Tregs are suppressed there is a decline in immunosuppression. This occurs when APC activated through lipopolysaccharide (LPS, found in the outer bacterial membrane of gram negative bacteria) binding to their TLR receptors. In addition to this, activated T effector cells will increase their expression of IL6R and GITR, two cytokine receptors, making them insusceptible to Treg suppression.

Clonal amplification and memory

Treg cells activated by APC have the potential to clonally amplify and for a memory T reg subpopulation [135].

Treg cells expand after successfully overcoming acute viral infections, to form a memory pool by decreasing in concentration (to 5-10% of peak). Subsequently, in a secondary immune response Tregs will expand more rapidly and secrete large quantities of IL-10. The function of Treg in the secondary immune response is to suppress the overreaction of T effector cells. Hence, Treg concentrations fluctuate with the effector T Cells, further underlining their importance as immune reaction gatekeepers.

Most research however, focuses on long term Treg activity rather than acute, since immunomodulatory function is more relevant in the context of chronic inflammation conditions such as HIV, MS and cancer.

Structure

There is a big inhomogenity in definitions of what characterizes a Treg in humans. They are commonly defined by expression of the surface markers CD3, CD4, CD25 and the transcription factor FoxP3[147]. Also used is negative expression of CD127 and less often, CD45RA. These six markers are generally considered to be the "backbone" markers to define Treg Cells[148][149].

CD25 is a component of the high-affinity IL-2 receptor complex [23].

CD4+ CD25+ Tregs normally represent about 2-4% of the CD4+ T Cell population[150].

Clinical Implications and Disease

Cancer

The role of T Regulatory Cells in cancer is twofold, either boosting or halting tumour development and growth (reviewed in[151]).

Generally, in the main growth phase of cancers, hyperplasia and dysplasia, tumours are supported by inflammation. Metastases result from this inflammation due to better perfusion and destruction and invasion of surrounding connective tissue including the basal lamina (reviewed in [152]). Conversely, once a tumour has formed, the immune system will attempt to destroy it. As Treg have a predominant immunosuppressive function, it is desirable to have overactive Tregs during initial tumour growth, but hypoactive or depleted during solid tumour state [151].

T Regulatory Cells are often increased in cancer patients and correlates with an inferior clinical outcome [153][154][155][156]. Treg cells therefore, became a target of chemotherapy due to their increased susceptibility, seen through increased proliferative potential [156][157]. However, as most agents are not specific enough this approach hasn't been successful. Attempts to inhibit Treg prolifertive signaling more specifically incurred problems as Tregs and T effector cells share many signaling pathways [158].

A promising target for manipulation is the IL-2 induced STAT5 pathway which increases the FoxP3 levels and thus repressive potential [151] [147].

Another signaling pathway targeted is the PI3k/Akt pathway. Activation of PI3K/Akt leads to delocalization of FoxO1 and FoxO3, which are crucial for Treg induction. PTEN, in turn, suppresses PI3K/Akt, which will lead to less Treg induction and fewer total Treg cells. Suppression of PTEN or activation of PI3K/Akt thus may be a suitable therapy strategy against Treg overpopulation in cancer [151].

It is evident that Treg modulation is a promising area of cancer research with many potential pathways to be explored.

HIV/AIDS

The implications of Treg function in HIV/AIDS is of equally monumental interest. A review is found in [159], [160] and [161].

HIV is generally characterized with a decreased number of lymphocytes, i.e. lymphopenia. As in cancer, this may present positive or detrimental consequences.

Regulation of effector T cells by Tregs in HIV infection can result in the destruction of anti-HIV specific lymphocytes causing persistence of infection. The balance between Treg and Th17 serves a function in HIV progression.

The roles Tregs play in HIV are somewhat analogous to the role they play in cancer, with both diseases exhibiting chronic inflammation. Particularly CD39+ Tregs play a major role in both conditions [162].

Difficulties arise in developing strategies with regard to manipulating Treg activity against either of the two diseases, as simple deactivation may lead to autoimmune episodes.

Natural Killer T-Cells

NKT cells, by which is meant specifically iNKT cells, interact with a variety of innate and adaptive immune cells to regulate the immune response. NKT cells interact with macrophages, neutrophils, B cells, NK cells, dentritic cells, and other T cells.[163]

Function

NKT (reviewed in [164] and [165]) are true T cells (CD3+, CD56-) and are thus fundamentally different from NK cells, despite a very similar name. They do, however, share certain functions (reviewed in [163]). NKT cells are extremely unique. Normal T cells are only able to recognize protein antigens. NKT cells, however, can recognize lipid antigens. The MHC receptor is only able to present protein fragments, hence NKT cells must able to bind to a different receptor on APCs, to which normal T cells cannot bind. This receptor is called CD1d. NKT cells also releases cytokines to activate or regulate these cells as well as activate MHC-restricted T cells and NK cells (as reviewed in [163]). The ability to bind to CD1d is owed to the presence of a special, semi-invariant αβ TCR, as reviewed in [166]. The term "semi-invariant" indicates a reduced variance within the TCR. As the variance in TCR is mainly due to the great variance in antigen-proteins, lipid-recognizing TCRs do not need to posses this degree of variance. CD1d presents both self, foreign (e.g. isoglobotrihexosylceramide) and glycolipids,(e.g. α-glycuronylceramides, within cell walls of Gram negative bacteria) (reviewed in [166]). As reviewed in [167], NKT cells are now broadly accepted as Cd1-restricted T cells which typically express both a semi-invariant T-cell receptor as well as many NK cell markers.

As reviewed in [164] and [165], NKT cells crucially regulate immune responses regarding microbial infection, autoimmunity, and cancer by connecting the innate and adaptive immune systems. As reviewed in [165], when the MHC I complex presents lipid antigens of CD1d molecules of bacteria, fungi, and parasites, T cell receptors of NKT cells can be activated in order to evoke an immune response. However, as reviewed in [165], NKT cells are also capable of activation by self-recognition of lipids and/or proinflammatory cytokines generated by one another during an infection.

As reviewed in [164], NKT cells, much like cells of the innate immune system, are some of the first cells to play a role in infectious and inflammatory responses, all the while assisting in the preparation of the adaptive immune response to follow. As reviewed in [164], they have been found to play an important role in autoimmune disease, regulating transplantation tolerance, inflammatory responses, asthma and allergic disease, and a variety of infectious diseases due to bacteria, viruses, fungi, and parasites.

Further Information on Natural Killer T Cells
NKT cells, by which is meant specifically iNKT cells, interact with a variety of innate and adaptive immune cells to regulate the immune response. The picture depicts during infection the reciprocal activation of the NKT cell with the APC by lipid antigens and pro-inflammatory cytokines in order to activate members of the innate and adaptive immune system, including macrophages, neutrophils, B cells, NK cells, dentritic cells, and other T cells. The cell products of these activated NKT cells that serve to activate other immune cells include IFNγ, IL-4, IL-13, IL-17A, as well as other chemokines and cytokines.[163]

Development

As reviewed in [168], NKT cells are developed in the thymus, but a mere fraction of NKT cells actually frequent the lymph nodes during typical conditions as opposed to other T cells. As reviewed in [168], the most significant proportions of NKT cell populations are found in the liver, lungs, spleen, and bone marrow, and this localisation is thought to be a consequence of differing chemokine receptor expression among the separate localising populations.

Structure

Classfication As reviewed in [169], NKT cells are comprised of type 1 NKT cells, which are CD1d restricted and also known as invariant NKT cells, and type 2 NKT cells, which are also CD1d restricted and display a diversity of TCRs. As reviewed in [167], a third group of NKT-like cells not CD1d restricted and possessing a diversity of TCRs has also been classified. As reviewed in [169], typically unqualified statements of NKT cells are understood to refer to type 1 NKT cells, i.e. iNKT cells, but this has been a source of early inconsistency in the study of NKT cells.

Type 1 and Type 2 NT

As reviewed in [166], the best understood NKT cells, known as ‘type 1’ NKT cells or ‘invariant’ NKT cells, are a subclass of T cells that possess a very invariant TCR, but they are capable of recognition of lipids and glycolipids in the antigen presenting molecule CD1d, as opposed to recognition of peptide-major histocompatibility complexes (MHCs) typical of other T-cells. As reviewed in [166], ‘type 2’ NKT cells also bind and detect lipids and glycolipids presented by the molecule CD1d.

Since NKT cells possess this crucial characteristic of being able to recognize glycolipids, they have been found to be a very useful area of study in addressing pathogens such as mycobacterium, the bacterial agent responsible for tuberculosis. Specifically, incorporation of specific glycolipid activators, such as α-galactosylceramide (αGalCer), into an attenuated strain of mycobacterium causes a more robust recruitment NKT cells and therefore a more effective vaccination[170].

Markers

NKT cells can be positive or negative for NK1.1, CD4, CD8, CD25, CD56, and CD161, as found in [171] and reviewed in [172].

Further special aspects

As reviewed in [164], NKT cells possess cytokine mRNA, enabling them to generate cytokines rapidly after being activated, and these cytokines play a crucial role in determining the T cell response by the host. NKT cells have an antigen-specific TCRs in the manner of true T cells, and these receptors enable the cell to detect self-antigens as well as foreign antigens. Additionally, the TCRs of NKTs provide the immune system with the ability to detect lipid antigens otherwise undetected by conventional T cells. Invariant natural killer T (iNKT) cells require[173] and express greatly the transcriptional factor promyelocytic leukemia zinc finger (PLZF) for their development, and PLZF appears to regulate the differentiation developing within the thymus into cells with seemingly innate effector characteristics[174].

Even as NKT cells were first being discovered as a separate classification of T cells, there were indications of their potential regulatory role within the immune system. For instance, a collection of suppressor T cell hybridomas were discovered to have a common Vα chain (Vα14), and common Jα segment (Jα18, at the time known as Jα281), and common glycine residue in the N-region, giving evidence for the existence of an NKT cell subset of immune cells as well as suggesting potential regulatory role.[175][176]

Clinical Implications and Disease

Immune System Diseases

There is evidence to show that microbial exposure at a young age is critical to the function of NKT cells, crucial to addressing diseases relating to the immune system such inflammatory bowel disease (IBD) and asthma. Germ-free (GF) mice tend to accumulate invariant natural killer T (iNKT) cells in the lungs and in the lamina propria of colon, increasing morbidity as compared with specific pathogen-free mice. Also, the GF mice have increased intestinal and pulmonary expression of CXCL16, a chemokine ligand associated with increased iNKT cells of the mucosa.[177]

Neonatal GF mice colonized with conventional microbiota, the mice tend to be protected from iNKT accumulation within the mucosa and associated pathogenicity. Contact with commensal microbes by a critical early age is crucial to developing a healthy levels of mucosal iNKT. Additionally, iNKT cells are valuable in providing resistance to environmental exposures, but failure of these cells to further mature by interaction with any conventional microbiota during the early stages of life results in the natural killer T-cells themselves to be a potential source of pathogenicity.[177]

Cancer

Originally, NKT cells were discovered to play a protective role against cancer, but recent studies have shown that NKT cells are able to inhibit tumor immunosurveillance are well as promote it. There paradoxical results have been determined to be a consequence of different classes of NKT cells governing different roles.[164] Importantly, NKT cells with an invariant TCR (Type 1) and also NKT cells with variable TCRs (Type 2) have been discovered to play a regulatory role for one another enabling the modulation of the following immune responses. Additionally, NKT cells are capable of regulating other cells involved in host innate immunity, including cells such as myeloid-derived suppressor cells, NK cells, and dendritic cells.[164]

Asthma

As reviewed in [178], it is likely that Th2 cells and iNKT cells complement one another to result in asthma and that different forms of asthma can result from varying routes of pathogenesis by which the two types of cells interact.

Memory T-Cells

This image shows the effect of antigen exposure on memory T cells[179]

Function

As reviewed in [180], Memory T cells, also known as Memory T lymphocytes, are a special subset of T cells that have previously experienced a particular antigen, from sources such as infection and cancer, and have developed specific recognition and a prepared response for a second encounter with the antigen in question. As reviewed in [180], Previous pathogen exposure, but importantly also vaccination, increase the number of pathogen-specific memory T cells in circulation and also affect their migratory patterns. As reviewed in [180], A second encounter with the antigen produces a secondary immune response, characterized by a faster and stronger showing of immune cells in response to the antigen than generated in the primary exposure.

Further Information on Memory T cells

Development

Generation and maintenance of resident memory T-cell subsets.[181]

As depicted in the accompanying image from the review[181], it has been proposed that resident memory T cells situated in the mucosa develop from effector T cells which in turn developed in lymphoid organs. There effector cells migrated to particular tissues as directed by specific chemokine receptors imprinted in the cell. While the majority of effector T cells perish, some of these T cells develop into resting memory T cells, which are capable of markedly extended life (reviewed in [181]).

As reviewed in [182][181], the memory T cell population in circulation is commonly broken up into two main subsets, namely central memory T cells (TCM) and effector memory T cells (TEM). As reviewed in [182][181], each of these populations are specific to a particular localization within the body, namely TCM within the secondary lymphoid organs and TEM circulating throughout non-lymphoid perpheral tissues.

However, as reviewed in [182][181], a recently classified third subset, known as tissue-resident memory (TRM) cells has been discovered to migrate and take up long-term residence within peripheral mucosal tissues demanding particular chemo-attractants and T cell recruitment, including the skin and genital tract.

As reviewed in [181], it is believed that the capability of extended residence of TRM within peripheral tissues is determined by the fixing of the cell in place through S1PR1 and by the adhesion between cells promoted by the expression of integrin. As reviewed in [181], possible factors contributing to the homeostasis and longevity of TRM in mucosal tissues include the prolonged presence of antigen in the tissue, cytokines inducing cell survival, and low levels of inflammation in the tissue.

As reviewed in [183], Naive T cells are developed in the thymus but remain relatively inactive as they circulate sparsely through secondary lymphoid tissues. As reviewed in [184], once a T cell binds to antigen by means of the major histocompatibility complex (MHC) and peptide associate, the T cell is then primed (activated). These primed naive T cells begin the expansion phase by dividing and differentiating into a diverse set of effector T cells. As reviewed in [184], following this expansion phase, a contraction phase is initiated in which only select number (only about 5-10%) of this effector T-cell population following this expansion phase are differentiated into diverse memory T cells. The rest of the activated T-cells are eliminated. As reviewed in [185], this functional model of memory T cells, however, is most accurate when considering acute infections, as chronic infections can amount to T cell dysfunction and rampant pathogen due to abnormal development of memory T cells and strain on the remaining T cells.

Sub-populations

Memory T cells are commonly split into at least three subtypes, as reviewed in [182]:

• central memory T cells (TCM cells),

• effector memory T cells (TEM cells and TEMRA cells) [186] and

• resident memory T cells (TRM)

Other new sub populations of memory T cells are being analysed through the use of co stimulatory molecular markers such as CCR7, CD27, CD28, and CD45RA[187].

Maintenance of population

One of the major limiting factors to memory CD8(+) T cell populations is the stability of the cell lysosomes. The genes IL-15 and IL-7 contribute to the production of serine protease inhibitor (Spi) 2A, which inhibits cytosolic cathepsin and thus inhibits apoptosis. In animal models, Spi2A inhibition results in less CD8+ T lymphocyte maturation and less homeostatic proliferation as well. Spi2A is required in order for the memory CD8(+) T lymphocytes to maintain population numbers after viral infection and that one specific mechanism of action is protection from lysosomal breakdown via cathepsin B inhibition. Furthermore, T cells deficient in Spi2A can have their homeostatis restored by concurrently blocking cathepsin B, which strongly suggests that the physiological target of Spi2A is indeed cathepsin B.[188]

Structure

Memory T cells are able to be CD4+, as reviewed in [189], or CD8+, as reviewed in [190], and memory T cells tend to express CD45R antibodies, which is then lost while UCHL1 reactivity is gained in the priming of the T cells [191].

Clinical Implications and Disease

HIV

More than 80% of total HIV-1 DNA has been found to be in non-gut-homing CD45RO+ memory T lymphocytes, while less than 10% found in regulatory T cells and CD38+ activated memory cells, and most of the HIV-1 DNA is present in non-gut homing resting CD(+) T cells.[192]

Current Research and Future Directions

T Cells and Cancer

Sean Parker, founder of Napser and Co-founder of Facebook, has invested 250M USD in a project he calls the "cancer dreamteam", which will heavily focus on immunotherapy of cancer. Instead of having competing research labs, Parker has formed a massive group of scientists from the Memorial Sloan Kettering Cancer Center, Stanford University, UCLA, UCSF, the University of Texas MD Anderson Cancer Center, and the University of Pennsylvania to work synergistically. The different labs focus on different approaches, the Sean N. Parker Autoimmune Research Laboratory at UCSF for example, works on an approach to boost T Regulatory Cells in order to fight cancer. [193]

The CAR Race

Flowchart of CAR generation procedure. Generation of scFV by through mAb, genetic Engineering of CAR sequence, introduction into viral vector and transfection of T. Own creation.

Genetically engineered T Cell Receptors specific against cancer specific antigens, called Chimeric Antigen Receptors (CARs)[194], are extremely promising and several companies, including Juno Therapeutics, Kite Pharma and Novartis are competing to provide the first marketable solution[195]. This competition has reached extents, that it is sometimes called the CAR T-Cell Race[196]. The promise is huge, as immunotherapy often is the last rescue if all current therapies have failed. The FDA recognized this and awarded the first 'breakthrough therapy' designation to the new drug approval (NDA) filling for CTL019, the anti-CD19 chimeric antigen receptor T-cell therapy developed at the University of Pennsylvania on July 1, 2014[197].

CARs will reprogram the human immune system to specifically recognize cancer cell surface markers and launch an immune response against the tumor cells. In most cases, CD8+ Killer T Cells are equipped with the CARs, but current clinical trails are exploring further options for carriers, such as CD4+ T-Helper cells [198]. It is often forgotten that most commercialized products are originally based on academic basic research, such as the one performed by Phillip Greenberg, working at the Fred Hutchinson Cancer Research Center (FHCRC). [199] He has recently founded Juno Therapeutics Last Accessed 19/05/16. Most therapies CAR focus on CD19 [200][201][202], but Greenberg, or more precisely the FHCRC has recently filed a patent for CARs against WT-1 [203]. As patents become obsolete as soon as the underlining research has been published, it is to be expected that the according research will be published soon, most likely together with Juno Therapeutics Phase I/II trial results in 2016/17 [204].

CARs, reviewed in [197] and [205], are basically comprised of an extracellular tumor binding domain, the signaling domain of a T Cell and co-stimulatory domains, designed to improve the function. Oddly, despite being called chimeric T cell receptors, often only the signaling domain is actually of TCR origin. The tumor binding domain normally is a single-chain variable fragment (scFV), which combines the two variable regions of heavy and light chain immunoglobulin fragments with a linker protein[206]. The T Cell signaling domain ensures proper signaling within the host T Cell, normally a zeta-chain is used[207]. The Zeta-Chain forms a complex with CD3 and the TCR alpha/beta subunit and is crucial for TCR signaling. Lastly, the co-stimulatory domains often are isolated from other receptors, such as CD27, a TNFalpha-superfamily receptor [208].

The process of CAR therapy, reviewed in [209], is a patient individual, genetic engineering approach. First, patient T Cells are isolated from the blood. A viral vector containing the CAR sequence is introduced into the T Cell, which is subsequently reintroduced into the organism. The genetically altered T Cells will start to transcribe and express the CAR, the CAR will recognize its cancer specific epitope. Consequently, this will lead to clonal proliferation of the CAR engineered T Cell, creating a massive immune reaction against the targeted cancer. In theory, no regrowth of the cancer can occur, as T memory cells will have formed and thus provide immunity against the cancer.

T Cells and Space

The scope of T cell research has broadened remarkable, even to space and the effects of micro-gravity. The Hughes-Fulford Laboratory is currently researching T Cells on the ISS as part of their research of ageing and T Cells [210]. Interestingly, this will be the first NIH, i.e. governmental, funded research project in space that is carried out by a commercial space travel company, SpaceX, founded by Elon Musk, the founder of paypal, Tesla and SolarCity[211]. According to NASA, APOLLO astronauts have shown several unusual immunological phenomena, including the occurrence of a disease that normally only occurs under immune suppression. Hence, the reasoning is that space must have had an influence on the T Cells. The experiment ran from 2014-2015, the results are pending[212].

Research at the UNSW and Active Labs

Here is an incomplete list of Research at the UNSW and Labs currently performing research about T Cells for further interest
  • UNSW, Dr Cindy Ma, Garvan Institute of Medical Research: Using human primary immunodeficiencies to investigate CD4+ T cell differentiation
  • UNSW, Prof Pageon and Dr Gaus Singular Molecule Science: Investigating the dynamics of T cell signalling during immune recognition
  • UNSW, Kirby Institute, Dr. Turtville: HIV and T Cells
  • Monash University, Melbourne: Professor Frank Alderuccio, tried to exploit Treg cells against Multiple Sclerosis, with little success so far. His research continues.
  • The University of Melbourne, Mackay Lab: Tissue-Resident Memory T cells; Lymphocyte Differentiation; Peripheral Immunity
  • Stanford, The Goronzy Lab: T cell homeostasis, T cell receptor calibration with age, T cell receptor calibration in autoimmunity, metabolism and others
  • Ragon Institute of MGH, MIT and Harvard, Walker Lab: HIV and CD8+ Cells
  • Yale, Hafler Lab: Autoreactive CD4+ T Cells in Multiple Sclerosis, Th1 Tregs, Cancer, and T-Cell co-stimulatory pathways
  • McMaster University, McMaster T-cell Epitope Centre (MTEC): Allergen Specific Immunotherapy (SIT)
  • La Trobe University, Chen Lab: Influenza and T Cells
  • Peripheral Immunity University of California, San Francisco, Ansel Lab: T Cell genetics, especially allergy and asthma

Final thoughts

T Cells have an incredible variety of subtypes, many of which are yet to be classified. The complexity of each individual and distinctively functioning type and their interaction is mind boggling. The most serious and challenging diseases our society faces nowadays, such as HIV, MS and Cancer, are all caused or arise in close relation with T Cells. Fundamental phenomena, such as aging, are equally related to T cells. As seen with CARs, T Cells can be hijacked and turned into effective patrolling and immunizing soldiers against cancer and possibly a huge amount other types of diseases. The boundaries of this technology are unable to be gauged yet. Research, particularly in conjunction with genetic engineering, thus holds unprecedented promises. Whether the boundary to the immortal superhuman actually may be crossed, seems unclear, and beside science's child-like curiosity, caution and ethical questioning will have to be science's companion in the future. With all that in mind, T cells certainly are and will be one of the most interesting fields of research, and many great discoveries are yet to come.

Glossary

A Glossary in chronological order. References are found within the respective paragraphs
Term Meaning
antigen Polymer that triggers an adaptive immune response. Can be nucleic acid, carbohydrate, lipid or protein
TCR T Cell Receptor, is found on T Cell surface
MHC Major Histocompatibility Complex, Receptor found on all nucleated cells to present antigens
αβ Subunits of the TCR heterodimer. TCR is either αβ or γδ. No functional implication known
γδ Subunits of the TCR heterodimer. TCR is either αβ or γδ. No functional implication known
Heterodimer Aggregate of two unequal protein chains
Lymphoid Tissue tissue with primary immunological function and high leukocyte count. Connected to lymphoid ducts and nodes
CD Cluster of Differentiation. Surface glycoproteins, display cell identity to the outside of the cell and used as markers to determine cell type.
CD8 Surface protein on T Cells and Natural Killer Cells, acts as a co-receptor for TCR/MHCI
B locus Locus: unique position within genetic code. B locus is the position on the genome carrying T Cell receptor genes
FOXP3 Forkhead box Protein 3. Transcription factor from the FOX family
disulfie bond covalent chemical bond created by oxidation of two cystein amino acid residues. Post translational modification.
CD4 Surface protein on helper T cells and T regulatory cells. Co-receptor for TCR/MHCII
Cytokine Cytos: Cell. Kinesis: Movement. Cell signaling molecule
Interleukin Inter: between, leukos: white. Chemical signaling molecule to signal between leukocytes
APC Antigen presenting cell. Present internalized portions of pathogens on MHCII molecules and activate adaptive immune system
Th1 T Helper Cell subtype. Mainly against intracellular pathogens (bacteria, virus)
Th2 T Helper Cell subtype. Mainly against extracellular pathogens (Helminths, bacteria)
Th17 T Helper Cell subtype. Secretes IL17 ( Th17 specific). Mainly extracellular pathogens and involved in autoimmunity
TGFβ Transforming Growth Factor, cytokine. One of the two major anti-inflammatory cytokines (besides IL10)
IL10 Interleukin10, One of the two major anti-inflammatory cytokines (besides TGFβ)
BCR Locus: unique position within genetic code. B locus is the position on the genome carrying T Cell receptor genes
B locus B Cell receptor. Same structure as antibody/immunoglobuline molecules
Primary lymphoid organs where leukocytes originate and maturate, i.e. Thymus and Bone Marrow
secondary lymphoid organ where the majority of leukocytes reside after maturation
CDR Complementary Determining Region. Contains antigen binding sites and is withing variable region
VDJ Variable Diverse Joining gene clusters within B locus and BCR/Antibody locus that determine TCR/BCR/Antibody diversity
Cortex latin for bark. Outermost layer of connective tissue of an organ
Medulla latin for pulp. Inner layer of connective tissue of an organ
DN Double Negative. Stage within T Cell development. Neither CD8 nor CD4 expressed, hence double negative for both
DP Double Positive. Stage within T Cell development. Both CD8 and CD4 expressed
Dendritic Cell Protein tyrosine phosphatase, a surface protein on the surface of T Cells
CD45 Protein tyrosine phosphatase, a surface protein on the surface of T Cells
CD28 TCR co-receptor, specific marker just for T Cells
CD80 Found on activated B Cells, specific for B cells
CD86 Found on antigen presenting cells
IFN Interferon, a cytokine
Tfh' Follicular T Helper Cells, T cells able to migrate into secondary lymphoid organ follicles
FoB Follicular B Cells, B cells residing in secondary lymphoid organ follicles
CXCR Chemokine Receptors
CD40L on follicular T Helper cells, allows binding to CD40
CD40 on antigen presenting cells and follicular B cells, acts as costimulatory receptor
fDC fDC follicular dendritic cells. Special population of DC within secondary lymhpoid organ follicles
C2 Domain A protein domain contained in various receptors involved in cell membrane targeting
MYPPPY loop A Met-Tyr-Pro-Pro-Pro-Tyr loop domain, found in CD28 molecules
HBV Hepatitis B Virus
Tregs T Regulatory cells
NKT Natural Killer T Cells
Hybridoma Hybris was a creature in ancient greek mythology with two heads. Hybridoma cells are cell lines that are immortalized through fusion with cancer cell lines
CD1 Expressed on the surface of antigen presenting cells. Related to MHC molecule, involved in presentation of lipid-antigens
PLZF Promyelocytic leukemia zinc finger. Zinc finger are DNA binding protein domains that are comprised of a helix-turn-helix motive complexed with a zinc ion in the middle. they bind to the major groove of DNA
IBD Inflammatory Bowel Disease. Chronic inflammation of the bowel, with various subtypes such as Crohn's Disease and others
CXCL16 Cytokine, binds do CXCR6. Produced by dendritic cells in follicular T cell region of secondary lymphoid organs
iNKT invariant Natural Killer T Cells, subtypes of NKT found in mice
CAR chimeric antigen receptor. Genetically engineered T Cell receptor. Chimera, a beast in ancient greek mythology, is a fire-breathing hybrid between a lion, snake and a goat.

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