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Why do dendritic cells have CD4/CD8 on their surface?

Why do dendritic cells have CD4/CD8 on their surface?


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Why do dendritic cells have CD4 or CD8 antigens on their surface? What is their function without the presence of a T-cell receptor?


There is no direct connection between CD (cluster of differentiation) receptors and T-cell receptor (TCR).

CD-receptors are used to label and distinguish different cells belonging to immune system: macrophages, T- and B-cells etc. Dendtritic cells play a significant role as antigen presenting cells and belong to vertebrate (and human) immune system and thus bear certain CD receptor.

These cells participate in the process of learning a new antigen by the immune system. During the very first phase the antigen is taken up by these cells, cleaved and processed. Depending upon the antigenic properties of the molecule it can be recognized as a valid antigen. In this case dentritic cells participate in the process of antigen presentation, where this new antigen bound to the MHC receptor is expressed on the cell membrane.

TCR is the protein that binds to this complex (MHC + antigen) and as long as this binding takes place the signal about antigen is propagated further to the immune system (leading to formation or augmentation of immune response depending upon the cell type which connects to this complex).


I believe rather than canonical involvement in T cell activation the question was what role does CD4 and CD8 have in a non-T cell population. Here is an article that provides some interesting insight: CD4 and CD8: an inside-out coreceptor model for innate immune cells http://www.jleukbio.org/content/86/2/251.full


Dendritic Cells

Dendritic cells (DCs), together with monocytes and macrophages, comprise the mononuclear phagocyte system.

DCs are professional antigen-presenting cells. They are abundant at body surfaces and within tissues, where they sense and sample the environment for self- and non–self-antigens.

Three major subsets of DCs—plasmacytoid DCs, conventional DCs, and monocyte-derived DCs—are characterized by distinct origins, receptors, and functions.

Upon antigen capture, DCs undergo a process of “maturation” exemplified by enhanced antigen processing, induction of major histocompatibility complex molecules, co-stimulatory molecules (CD80/86), and cytokine production. DCs migrate to primary and secondary lymphoid organs, where they present processed antigens to naïve T cells to induce immunity or tolerance.

Mature DCs acquire the ability to differentiate naïve T cells into T helper (Th) 1, Th2, or Th17 cells, T follicular helper cells, or regulatory T cells. Maturing DCs also express cytokines that enable the activation of B cells and natural killer cells and promote the recruitment of other innate immune effector cells.

DCs are innate immune phagocytic cells that sense the environment through membrane and cytosolic pattern recognition receptors. DCs contribute to the maintenance of tolerance in the thymus and in the periphery, although their functions are only partially understood.

There is a fine equilibrium in DC regulation between tolerogenic and inflammatory states, which can contribute to autoimmune diseases or to tolerance of tumors.

DCs, as one the main regulators of the immune response, can either be targeted or used as adjuvants to contain immunopathologies such as autoimmune diseases and cancers, respectively.


Contents

The immune system must recognize millions of 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. [2]

TCRs have two parts, usually an alpha and a beta chain. (Some TCRs have a gamma and a delta chain.) Hematopoietic stem cells in the bone marrow migrate into the thymus, where they undergo V(D)J 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 (αβ T cells), but some T cells in epithelial tissues (like the gut) express gamma-delta TCRs (gamma delta T cells), which recognize non-protein antigens.

T cells with functionally stable TCRs express both the CD4 and CD8 co-receptors and are therefore termed "double-positive" (DP) T cells (CD4+CD8+). The double-positive T cells are exposed to a wide variety of self-antigens in the thymus and undergo two selection criteria:

  1. 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 (MHC1 for CD8, MHC2 for CD4). 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.
  2. 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 (either CD4+ or CD8+), depending on whether their TCR recognizes an MHC class I-presented antigen (CD8) or an MHC class II-presented antigen (CD4). 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.

With the exception of some cell types, such as non-nucleated cells (including erythrocytes), Class I MHC is expressed by all host cells. When these cells are infected with a virus (or another 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 (TCR) 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 (APC). For instance, consider the two signal model for TC cell activation.

Signal T cell APC Description
First Signal TCR peptide-bound MHC class I molecule There is a second interaction between the CD8 coreceptor and the class I MHC molecule to stabilize this signal.
Second Signal CD28 molecule on the T cell either CD80 or CD86 (also called B7-1 and B7-2) CD80 and CD86 are known as costimulators for T cell activation. This second signal can be assisted (or replaced) by stimulating the TC cell with cytokines released from T helper cells.

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. [3] [4] During this process, the CD4 + helper T cells "license" the dendritic cells to give a potent activating signal to the naive CD8 + T cells. [5]

Furthermore, maturation of CD8+ T cells is mediated by CD40 signalling. [6] Once the naïve CD8+ T cell is bound to the infected cell, the infected cell is triggered to release CD40. [6] This CD40 release, with the aid of helper T cells, will trigger differentiation of the naïve CD8+ T cells to mature CD8+ T cells. [6]

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. [7]

Once activated, the TC cell undergoes clonal expansion with the help of the cytokine Interleukin-2 (IL-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.

When exposed to infected/dysfunctional somatic cells, TC 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 (programmed cell death). Due to high lipid order and negatively charged phosphatidylserine present in their plasma membrane, TC cells are resistant to the effects of their perforin and granzyme cytotoxins. [8]

A second way to induce apoptosis is via cell-surface interaction between the TC and the infected cell. When a TC is activated it starts to express the surface protein FAS ligand (FasL)(Apo1L)(CD95L), which can bind to Fas (Apo1)(CD95) 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 TH cells than it is to the cytolytic activity of TC effector cells. Engagement of Fas with FasL allows for recruitment of the death-induced signaling complex (DISC). [9] The Fas-associated death domain (FADD) translocates with the DISC, allowing recruitment of procaspases 8 and 10. [9] 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 (poly ADP ribose polymerase), and DNA-PKcs (DNA-activated protein kinase). The final result is apoptosis of the cell that expressed Fas.

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. [10] 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. [10]

Unlike antibodies, which are effective against both viral and bacterial infections, cytotoxic T cells are mostly effective against viruses. [11]

During hepatitis B virus (HBV) infection cytotoxic T cells kill infected cells and produce antiviral cytokines capable of purging HBV from viable hepatocytes. They also play an important pathogenic role, contributing to nearly all of the liver injury associated with HBV infection. [12] Platelets have been shown to facilitate the accumulation of virus-specific cytotoxic T cells into the infected liver. [13]

Cytotoxic T cells have been implicated in the progression of arthritis: depletion of knee joint cartilage macromolecules such as glycosaminoglycans by cytotoxic T cells and macrophages has been observed in a rat model of the disease. [14]

CD8+ T cells have been found to play a role in HIV infection. HIV over time has developed many strategies to evade the host cell immune system. For example, HIV has adopted very high mutation rates to allow them to escape recognition by CD8+ T cells. [15] They are also able to down-regulate expression of surface MHC Class I proteins of cells that they infect, in order to further evade destruction by CD8+ T cells. [15] If CD8+ T cells cannot find, recognize and bind to infected cells, the virus will not be destroyed and will continue to grow.

Furthermore, CD8+ T cells may be involved in Type 1 diabetes. [16] In addition to the role of CD4+ cells in this autoimmune disease, studies in a diabetic mouse model showed that CD8+ T cells may also play a role by destroying insulin-producing pancreatic cells. [16]

CD8+ T cells may be necessary to resolve chemotherapy-induced peripheral neuropathy (CIPN). [17] [18] Mice without CD8+ T cells show prolonged CIPN compared to normal mice and injection of educated CD8+ T cells resolve or prevent CIPN.


Cell Surface Antigens: Meaning, Types and Functions | Cell Biology

Cells are antigenic. This means that when cells of one species are injected into another species, the recipient will first identify the injected cells as being of foreign origin and start producing antibodies to interact specifically with the alien cells. Therefore, the foreign cell that provokes antibody production in the recipient is called antigen.

If whole cells are injected, many cell surface components—like protein, carbo­hydrates or some combination of the two— may act as cellular antigen of the foreign cell. Several cell surface antigens have been studied in detail—the ABO blood group antigens, the MN blood group antigens, histocompatibility antigens etc. The most commonly studied cell surface anti­gens are discussed below.

Types of Surface Antigens:

(i) The ABO Blood Group Antigens:

The recognition of any blood grouping or spe­cific type within it depends on the ability to de­tect the presence or absence of specific antigens on the red blood cells. Such antigens are found in the membrane of red cells or erythrocytes. In the human, classification of a person as blood type A, B, etc. is possible because of the presence of the detectable antigens.

If a certain antigen is present in the red blood cells, these may be clumped by the correspond­ing antibody when it is present. If the specific antigen is not present in the red blood cells, the corresponding antibody anti-A or anti-B is present in the blood serum.

A person of type AB blood is born with both A and B antigens in the red blood cells but no anti-A or anti-B antibody are found in the serum. The type O person lacks both A and B antigens into the membrane of the red blood cells, but the serum contains both antibodies anti-A and anti-B.

All these antigens are under genetic control so that an individual may possess either A or B antigen, or both, or neither. When neither antigen is present, the individual is called O type. However, individuals with O type erythrocytes possess an antigen called H which is the structural foundation on which A and B antigens are built.

The antigens of ABO system are glycolipids, with the oligosaccharide portion of the glycolipid responsible for antigenic properties. The oligosaccharide antigen is covalently attached to sphingolipids which are immersed in the bi-molecular lipid leaflet of the plasma membrane. There are two types of oligosaccharide core chains which, ultimately, give rise to two sub­groups of each antigenic type.

The difference is due to the type of linkage between galactose and N-acetylgalactosamine at the core of the oligosaccharide. In type I chain, galactose is attached with either N-acetyl glucosamine or N- acetylgalactosamine by β-1, 3 linkage, whereas in type II chain galactose is attached with N- acetyl glucosamine by β-1, 4 linkage.

Erythrocytes that are O type, does not con­tain galactose or N-acetyl glucosamine or N- acetylgalactosamine as side chain and fucose is linked a-1, 2 galactose, a structure that constitutes the H antigen. From this unit the A and B antigens are enzymatically generated by glycosyltransferases that attach either on N- acetylgalactosamine unit (type A) or a galactose unit (type B).

The level of genetic control is exercised by presence or absence of glycosyl transferees that modify the H antigen adding either an A or B determinant.

(ii) The MN Blood Group Antigens:

The second major blood group system in hu­man is MN system. In this case, the oligosac­charide is linked to a protein, glycophorin. It has two units. One “type of unit (A) is attached through asparagine, and the other unit (B) through serine or threonine. Vari­ations on these structures exist.

They char­acteristically contain terminal sialic acid (N-acetylneuraminic acid). Both M and N deter­minants are destroyed when erythrocytes are treated with neuraminidase, an enzyme that removes sialic acid.

(iii) Histocompatibility Antigens:

Histocompatibility antigens are tissue cell sur­face proteins that differ from individual to indi­vidual. They are recognised by the mechanism of tissue graft rejection. There are different number of antigenic combinations possible on tissue surfaces and this is the basis for the difficulties faced in getting a good tissue match for skin or organ transplantation surgery.

Some of the histocompatibility antigens are strong and others are weak. H-2 antigens in mice and HLA-antigen in humans have been studied in detail. Both these systems have many genetic variants. The H-2 antigen in mice is strongly antigenic. The intact H-2 antigen is made of two chains of which one is heavy or long and other is light or short.

The light claim is actually a protein and it is called β-2-micro-globulin. The heavy and light chains interact but not by covalent bonds. The proteolytic enzyme papain cleaves the heavy chain in such a way that a water-soluble portion called the Fs fragment, is released and a small piece, the Fm fragment is left in the membrane.

Partial sequences have been worked out for the heavy chains of both H-2 and HLA antigens and similarities are seen between the two types of heavy chains.

Functions of Surface Antigens:

The chemical nature of several types of surface antigen are now known. Surface antigens have some biological importance and are involved in many functions. For examples, H-2 anti­gens may function during the mechanism of immunological surveillance.

If any change or modification in cell surface structures takes place—either by mutation or other means— the surface structures become abnormal. The abnormalities arising in cell surface are read­ily recognised by wandering lymphocytes in the immune system.

When a defective cell is found by the lymphocytes, a message is sent to the lymph modes where a class of lymphocytes called killer T cells are activated and, ultimately, destroy the abnormal target cell. The target cell must have an H-2 antigen plus an abnormal or modified antigen.

The normal (H-2) and the abnormal antigens may be present independently, or they may form a hybrid surface structure. In any case they are then recognised or they may attract the lymphocytes without being physically attached to one another. In any event, in the absence of H-2 antigen, an abnormal cell will avoid destruction and proliferate.


CD4/CD8-p56 lck and the Initiation of TCR Signaling

Despite this important work, a critical missing area was the possible involvement of protein tyrosine phosphorylation in T-cells. Emerging data had underscored the importance of this type of phosphorylation in regulating multiple events in other mammalian cells. Most phosphorylation occurs on serine and threonine with ρ% on tyrosine residues. Tony Hunter had described phosphorylation on tyrosine residues in the late 1970s, working on middle T-antigen (Eckhart et al., 1979). Transmembrane receptors such as the platelet-derived growth factor receptor (PDGF-R) and the insulin receptor were then found to have intrinsic protein-tyrosine kinase domains in their cytoplasmic tails (Rudd, 1990 Hunter, 2007). However, another family of soluble protein-tyrosine kinases had also been defined with the prototype pp60 src . Notably, a truncated form of the kinase termed pp60 v−src had been identified in the Rous sarcoma virus which acted as an oncogene (Parker et al., 1981). Michael Bishop and Harold Varmus had won the 1989 Nobel Prize for showing that the oncogene in the virus was an altered version of a gene derived from the normal cellular gene of normal cells. However, the cellular homolog pp60 src had no apparent function in mammalian cells. A role for src family members in normal cell function had been unclear. The src family of non-receptor tyrosine kinases (SFKs) include Src, Fyn, Yes, Lck, Hck, Blk, Fgr, Lyn, and Yrk (Neet and Hunter, 1996 Serfas and Tyner, 2003). Src, Yes, Lyn, and Fyn are widely expressed in cells, while Blk, Fgr, Hck, and Lck are expressed primarily in hematopoietic cells (Thomas and Brugge, 1997). T cells express predominantly Lck and Fyn that include an alternatively spliced isoform of Fyn termed Fyn T .

In immunology, there was a major gap in knowing whether protein-tyrosine kinases, or a potential phosphorylation cascade operated in T-cells and other immune cells. There were no known surface receptors with endogenous protein-kinase domains connected to the antigen-receptor (TCR/CD3 complex) and little evidence of tyrosine phosphorylation in immune cells. The main evidence came from studies on LSTRA cells, T-cell lymphoma transformed by the Moloney Murine Leukemia Virus that showed elevated tyrosine phosphorylation of intracellular proteins (Casnellie et al., 1982 Gacon et al., 1982 Voronova et al., 1984). However, it was unclear whether this was an anomaly and whether receptors on normal T-cells engage tyrosine kinases to evoke a phosphorylation cascade. The lab of Larry Samelson and Richard Klausner provided some of the first hints by showing that a p21 chain associated with the T cell antigen receptor underwent tyrosine phosphorylation of 294 hybridoma T-cells (Samelson et al., 1986b).

The central problem was that neither the TCR itself nor its associated CD3 γ/ε, δ/ε, or ζ chains showed sequence homology with known protein-tyrosine kinases. Given this situation, it seemed a reasonable possibility to us that the TCR might be coupled to an unidentified transmembrane tyrosine kinase receptor, an activator of a kinase protein tyrosine kinase, or in some unusual manner, might bind to a protein-tyrosine kinase. Our initial studies initially showed little endogenous kinase activity co-precipitated with the anti-CD3 precipitated TCR complex in auto-phosphorylation kinase assays. This observation shifted our attention to the co-receptors CD4 and CD8, which had recently been shown to bind to non-polymorphic regions of the major histocompatibility complex (MHC) (Meuer et al., 1982). For example, the α chain of the CD8 complex binds to HLA's 㬒 and 㬓 domains of MHC class 1 antigens (Gao et al., 1997). We envisioned that a situation where a kinase associated with CD4 and CD8 might be brought into physical proximity with the TCR complex for its phosphorylation.

From the outset of our work in 1986, we found that immune precipitates of CD4 and CD8 possessed an unusually high level of endogenous tyrosine kinase activity that was not observed in the precipitates of other receptors. Further, in addition to the phosphorylation of the exogenously added substrate, enolase, we observed a well-labeled band in the 56� Kd range in anti-CD4 and CD8 precipitates that was labeled on tyrosine residues (Rudd et al., 1988 Barber et al., 1989). Two other bands in the 30� Kd and 75� Kd range were also labeled in the anti-CD4 and CD8 precipitates (Rudd et al., 1988 Barber et al., 1989). None of these bands corresponded to CD4 or CD8 indicating that the co-receptors themselves were unlikely to be substrates of the endogenous co-precipitated kinase.

Independent work on pp60 src had shown that src-related kinases could phosphorylate themselves in a process termed auto-phosphorylation. This occurs when a kinase's active site catalyzes its own phosphorylation (cis autophosphorylation), or when a kinase provides the active site of an adjacent kinase (trans autophosphorylation). It did not escape our notice that the band at 55� kd was of a similar size as pp60 c−src , although src was poorly expressed in T-cells. Perhaps a related kinase might be phosphorylating itself in precipitates, and perhaps it was immune cell-specific mirroring the cell-specific nature of receptors on the surface of immune cells. It may seem self-evident now, with the available information, but at the time this was a rather grand conceptional jump. In this context, a protein at 56 Kd, originally termed LSTRA protein-tyrosine kinase had been seen in LSTRA lymphoma T-cells by the labs of Bart Sefton and Edwin Krebs (Casnellie et al., 1982 Gacon et al., 1982 Voronova et al., 1984). The kinase was subsequently cloned by Jamey Marth in the lab of Roger Perlmutter [encoded by a genetic locus defined as lsk T ] and found to be a T-cell-specific member of the pp60 src family, LCK or p56 lck (Marth et al., 1985). However, as in the case of the parental kinase pp60 src , no function for p56 lck had been identified in normal T-cells. The idea that src kinases could in some manner interact with surface receptors, rather than interacting solely with intracellular components such as middle T-antigen, had not been established.

Using an anti-p56 lck sera from Jim Trevillyan at the University of Texas, we showed that it reacted with our 56Kd protein that had been labeled in vitro kinase assays using a combination of blotting and re-precipitation analysis (Rudd et al., 1988 Barber et al., 1989). This clearly showed that the CD4 and CD8 receptors interacted with the src family member called p56 lck . In our original paper, we stated: “the association appears to represent the only known case of an association between a receptor on the surface of T cells and a member of a family of intracellular mediators with an established ability to activate and transform cells.” The fact that both CD4 and CD8 bound to p56 lck was consistent with their similar, but complementary roles in binding to non-polymorphic regions of MHC class II and class 1 antigens, respectively. CD4 binds to p56 lck in a monomeric form, although in certain contexts, the receptor may form dimers or multimers (Lynch et al., 1999 Matthias et al., 2002 Figure 1A). By contrast, CD8 exists as a α/β heterodimer or a α/α homodimer within which the p56 lck binds to the CD8α subunit. The homodimer can recruit two p56 lck molecules, while the CD8α/β heterodimer binds a single p56 lck (Figure 1A).

Figure 1. A tale of three CD4 and CD8-p56 lck complexes and the structure of pp60 src and p56 lck . (A) The model of three CD4 and CD8-p56 lck complexes in T-cells. CD8 is expressed as a CD8α homodimer as well as a CD8α/β heterodimer. p56 lck binds to the α subunit but not the β subunit. CD8α homodimer has two p56 lck bound molecules and the CD8α/β heterodimer has a single p56 lck bound. CD4 binds to p56 lck in a monomeric form. (B) Structure of pp60 src and p56 lck . p56 lck is an immune cell-enriched member of the pp60 src family of protein-tyrosine kinases. p56 lck is myristoylated and palmitoylated at the N-terminus, while Src lacks palmitoylation sites. This region is followed by poorly conserved unique SH4 region which in the case of p56lck binds to the cytoplasmic tails of CD4 and CD8, an SH3 domain that binds to proline-rich residues, an SH2 domain that binds to specific sites that are tyrosine phosphorylated, an SH2-kinase linker region, an SH1 kinase domain followed by a C-terminal negative regulatory region. The C-terminal tail has an inhibitory Y-527 site when phosphorylated, in the case of pp60 src and a Y-505 site in p56 lck . pp60 src and p56 lck also possess an auto-phosphorylation site in the kinase domain of each kinase corresponding to Y-416 in the case of pp60 src and Y-394 in the case of p56 lck .

The CD4 and CD8-p56 lck complexes were the first examples of a protein-tyrosine kinase to associate with a surface receptor. They were also the first case of an interaction with an SFK and explained how receptors that lack intrinsic catalytic activity could transduce activation signals. The interaction provided a mechanism by which the antigen receptor could induce a possible tyrosine phosphorylation cascade in T-cells and put the focus on p56 lck as the central player of T-cell activation, some of which is receptor associated and the rest of which exists in a receptor-free form in cells.

Our original submitted paper languished for over a year with Nature from 1986 to 1987, at which time we decided to re-submit to PNAS for publication and to file patents, which were filed and granted several years later (Nos. 5,250,431, 1993, US5432076 EP0347143A2, 1988). I also began to discuss our unpublished findings openly with colleagues at the Dana-Farber Cancer Institute which led to a contact from Andre Veillette in the lab of Joseph Bolen at the National Institutes of Health. After some discussion, they agreed to collaborate showing the presence of the CD4 and CD8-p56 lck complexes in mouse cells (Veillette et al., 1988). This collaborative work was very important and helpful to us, given that, at the time, my group was comprised of a young technician and myself, without an established reputation in the field of protein-tyrosine kinases. The work in our first paper was supported by the Cancer Research Institute (NY), an organization whose founding was based on the work of Dr. William B. Coley in the late 1800s to treat cancer patients with immunotherapy. We were gratified that our CD4 and CD8-p56 lck complexes as initiators of the activation cascade in human T-cells are the same signal mediators that stimulate T-cells to react and kill tumors in immunotherapy. Our first paper was recognized as “Pillars of Immunology” paper by the American Association of Immunologists together with a paper from our collaborators in the Bolen lab (Rudd et al., 2010 Veillette et al., 2010).

CD4 and CD8-p56 lck complexes became models for how other immune receptors employ SFKs in immune cell activation. Lyn and Fyn were subsequently found to associate with the Igα/Igβ heterodimer subunits of the B cell receptor in B-cells (Gauld and Cambier, 2004), Src and Lyn to the Fc receptor (FCR) (Wu et al., 2001) and Fyn and Lyn to the glycoprotein VI (GPVI)-FcR gamma-chain complex, a key receptor for collagen on platelets (Suzuki-Inoue et al., 2002). In fact, a single Lyn single molecule may be sufficient to initiate phosphorylation of multiple aggregated high-affinity IgE receptors (Wofsy et al., 1999). Further, pp60 Src is activated by binding the integrin β cytoplasmic domain (Arias-Salgado et al., 2003), while in T-cells, p59 fyn , and p56 lck associates, albeit with lower stoichiometry, with the CD3 subunits of the TCR receptor (Hartl et al., 2020). p56 lck was also been found to associate with the co-receptor CD28 by using its SH2 domain to bind to a phospho-specific site (Kong et al., 2011).

With an emphasis placed on p56 lck , it was subsequently ablated in mice and found to be needed for the early and late stages of thymic differentiation (using proximal and distal Lck promoters) (Teh et al., 1991), naive T cell survival (Seddon and Zamoyska, 2002), and T-cell activation. Lck/Fyn double deficient mice show a 3 stage (DN3) block in the thymus which requires pre-TCR signaling (Liao et al., 1997). Similarly, B-cells require Lyn kinase activity for B-cell receptor phosphorylation and function (Fujimoto et al., 1999). Likewise, macrophages lacking the Hck and Lyn are defective in IgG-mediated phagocytosis (Fitzer-Attas et al., 2000). Other examples exist.

In the field of cancer biology, as mentioned, previous seminal work had documented how truncated forms of pp60 v−src transformed cells however, a role for non-oncogenic src-related kinases had been missing. Other non-lymphoid surface receptors, such as the platelet-derived growth factor receptor (PDGF-R) were eventually also shown to bind and generate signals via SFKs (Thomas and Brugge, 1997 Rudd, 1999).

Lastly, our studies impinged on the field of acquired immunodeficiency syndrome (AIDS) and the human immunodeficiency virus (HIV-1), being the first example of a mediator to associate with the HIV-1 receptor, CD4 (Rudd et al., 1988). p56 lck and its binding to CD4 were later shown to provide signals that regulate HIV-1 transcription in T-cells (Tremblay et al., 1994). HIV-1 induced apoptosis is accelerated by interaction of CD4 with p56 lck (Corbeil et al., 1996).


Dendritic Cell Subsets

While all DCs share certain features, they actually represent a variety of cell types with different differentiation histories, phenotypic traits and, as outlined above, different effector functions.

Conventional Dendritic Cells (cDCs)

Conventional dendritic cells are also called Myeloid Dendritic Cells (mDCs). As that name implies, these cells are derived from the same myeloid progenitors in the bone marrow that give rise to granulocytes and monocytes [View].

In humans, the majority of our cDCs belong to a subset designated conventional type 1 dendritic cells (cDC1). They present antigen to T cells and activate the T cells by secreting large amounts of IL-12 [View]. They are the major stimulators of Th1 helper T cells.

  • TLR2 which responds to the peptidoglycan of Gram-positive bacteria and
  • TLR4 which responds to the LPS of Gram-negative bacteria

Conventional DC1 cells can present antigen not only to CD4 + T cells but also to CD8 + T cells (cross-presentation).

Plasmacytoid Dendritic Cells

These cells ("pDCs") get their name from their extensive endoplasmic reticulum which resembles that of plasma cells. However, unlike plasma cells that are machines for pumping out antibodies, pDCs secrete huge amounts of interferon-alpha especially in response to viral infections.

  • TLR-7 and TLR-8, which bind to the single-stranded RNA (ssRNA) genomes of such viruses as influenza, measles, and mumps.
  • TLR-9, which binds to the unmethylated cytosines in the dinucleotide CpG in the DNA of the pathogen. (The cytosines in the host's CpG dinucleotides often have methyl groups attached.)

Ralph Steinman, the pioneer in the study of dendritic cells, has provided striking visual evidence of the cellular interactions between antigen-presenting dendritic cells, T cells, and B cells. When spleen cells are cultured with antigen, tight clusters of cells form (see figure). The clustering occurs in two phases:

  • an early phase (days 0&ndash2) during which only the dendritic cells and T cells need to be present to form clusters
  • a later phase (days 2&ndash5) when antigen-primed B cells enter the cluster and differentiate into antibody-secreting cells.

What are CD8 Cells?

CD8 is considered as a transmembrane glycoprotein which functions in the immune system. CD8 is also known as a co-receptor of the T cell receptor (TCR). Similar to the TCR, CD8 binds to the major histocompatibility complex (MHC) class I protein specifically. The CD8 are mainly located on the surface of cytotoxic T cells and cortical thymocytes, natural killer cells, and dendritic cells. Just like CD4, CD8 also belongs to the immunoglobulin superfamily. In order to facilitate the function, the CD8 forms a dimer which consists of a CD8 chain pair. The common types of CD8 are CD8-α and CD8-β. It consists of an immunoglobulin variable (IgV) like extracellular domain connecting to the membrane by a stalk and an intracellular tail. Normally, the IgV, like extracellular domain of the type CD8-α, cooperates with the class I MHC molecules. This affinity between the molecules keeps the cytotoxic T cell’s T cell receptor tightly bound together with the target cell during the activation of antigen specificity.

Figure 02: CD8 Cells


CD83: an update on functions and prospects of the maturation marker of dendritic cells

CD83 is one of the most characteristic cell surface markers for fully matured dendritic cells (DCs). In their function as antigen presenting cells they induce T-cell mediated immune responses. In this review we provide an overview on well described and proposed functions of this molecule as well as on very recent insights and new hypothesis. Already the CD83 messenger RNA processing differs remarkably from the processing of other cellular mRNAs: instead of the usual TAP mRNA export pathway, the CD83 mRNA is exported by the specific CRM1-mediated pathway, utilized only by a minority of cellular mRNAs. On the protein level, two different isoforms of CD83 exist: a membrane-bound and a soluble form. The isoforms are generated by different subsets of cells, including DCs, T-cells and B-cells, and also differ in their biological function. While the membrane-bound CD83 is of immune stimulatory capacity, activates T-cells and is important for the generation of thymocytes, the soluble CD83 has the opposite effect and has an immune inhibitory capacity. Due to its immune inhibitory function, CD83 has great potential for treatment of autoimmune diseases, for organ transplantations, and for immunotherapy, just to name a few examples. Moreover, some viruses prevent recognition by the host’s immune system by specifically targeting CD83 surface expression.

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Important factor in the development of dendritic cells identified

The human immune system comprises functionally specialised cellular defence mechanisms that protect the body against disease. These include the dendritic cells. Their main function is to present antigens to other immune cells, especially T cells, thereby activating a primary immune response. Dendritic cells are divided into Type 1 (DC1) and Type 2 (DC2) dendritic cells. Each type fulfils different functions: DC1 provide an immune response to bacteria and viruses, DC2 protect against fungal or parasitic infections. In a recent study conducted at MedUni Vienna's Institute of Cancer Research, researchers found that a particular group of proteins plays a major role in the development of Type 1 dendritic cells. This could open up new therapeutic options in the defence against viruses or bacteria but also for cancer immunity.

Dendritic cells are formed from multipotent progenitor cells in the bone marrow. However, it was hitherto unclear which proteins are responsible for this transition from stem cells to differentiated cells. The study, which has now been published in Cell Death and Differentiation, used animal models and molecular biology techniques such as RNA sequencing to show that a combination of two proteins known as "c-Jun" and "JunB" are essential factors in the development of Type 1 dendritic cells. "Both proteins are transcription factors, DNA-binding molecules belonging to the Activator-Protein-1 (AP-1) family," explains study author Philipp Novoszel, who is also associated with the Comprehensive Cancer Center (CCC) of MedUni Vienna and Vienna General Hospital.

In order to analyse the role of these proteins, the c-Jun- and/or JunB gene was deleted in dendritic cells. "This showed that c-Jun and JunB are jointly, but not individually, essential for DC1 development," says second study author, Barbara Drobits from the Institute of Cancer Research and CCC. The mechanism in detail: working in a never previously described synergy, the c-Jun/JunB transcription factor pair together controls the development of DC1. "An expression analysis of DC1 lacking c-Jun/JunB, showed changes in the cellular identity, and a shift towards DC2." At the same time, the immunological functions of DC1 were greatly reduced when c-Jun/JunB were lacking. Differences were also found in an infection model. In the animal model, deactivation of c-Jun/JunB protected against infection with the bacterium Listeria monocytogenes.

"The results describe a previously unknown function of c-Jun/JunB in the development of dendritic cells. It has already been shown in previous studies that another member of the AP-1 family known as Batf3 is necessary for DC1 development, in that it regulates the expression of the transcription factor IRF8. However, it was not clear with which AP-1 protein Batf3 interacts to perform this function. Our data now provide this "missing link," in that they point to c-Jun/JunB as being Batf3's tango partner," summarise the study authors.

DC1 are essential for defending against bacteria and viruses as well as for immunity to cancers -- a better understanding of the underlying biology could therefore provide new, promising therapeutic approaches for future clinical application.


Types of dendritic cells

Langerhans cells

The Langerhans cells are the dendritic cells of the skin. They are usually found in stratified epithelia and make up approximately 4% of the epidermal cells where they fulfill their primary defense function. Inside they have some granules called Birbeck.

They were first described by Paul Langerhans in 1868 and were thought to belong to the nervous system, due to its starry shape. Later they were cataloged as macrophages and are the only type of epidermal cell with characteristics of cells of the immune system.

Interdigitating dendritic cells

The interdigitating dendritic cells are widely distributed throughout the body and have a high degree of maturation, which makes them very effective for the activation of virgin T lymphocytes. They are most often found in secondary lymphoid organs, where they exert their lymphocyte activation function.

Anatomically, they have characteristic folds in their cell membrane, which has co-stimulatory molecules They do not have granules.

However, they are essential in the presentation of viral antigens, which are subsequently presented to a type of lymphocytes called T CD4.

Follicular dendritic cells

The follicular dendritic cells are distributed among the lymphatic follicles of the secondary lymphoid organs. Although morphologically they are similar to the other dendritic cells, these cells do not share a common origin.

Follicular dendritic cells do not come from the bone marrow, but from the stroma and mesenchyme. In humans, these cells are found in the spleen and lymph nodes where they gather with other cells called B lymphocytes to present them with the antigen and initiate an adaptive immune response.

Interstitial dendritic cells

Interstitial dendritic cells are located around the vessels and are present in most organs, except the brain. The dendritic cells that are present in the lymph nodes include interstitial, interdigitating and epithelial cells.

Dendritic cells are characterized by being highly efficient antigen presenting cells, which is why they are able to activate different cells that activate the adaptive immune response and, consequently, the production of antibodies.

These cells present the antigens to the T lymphocytes when they are found in the lymph nodes.

Plasmocytoid dendritic cells

Plasmacytoid dendritic cells are a specialized subset of dendritic cells characterized by detecting antigens of viruses and bacteria, and by releasing many molecules of Interferons type I, in response to infection.

An important role has been suggested by these cells in the inflammatory responses caused by the activation of effector T cells, cytotoxic T cells, and other dendritic cells.

In contrast, another group of plasmacytoid dendritic cells participates in processes of suppression of inflammation as a mechanism of regulation.

Veiled cells

The veiled cells of the afferent lymph are classified with dendritic cells on the basis of their morphology, surface markers, staining and cytochemical function.

These cells phagocytose the pathogens and carry the antigens from the peripheral tissues to the paracortical areas, in the lymph nodes. Studies suggest that these veiled cells participate in the presentation of antigens in inflammatory and autoimmune diseases.


DISCUSSION

CD8α – DCs can be divided into functionally distinct subsets based on their level of expression of the CLRs, DCAL2, and DCIR2. Whereas CD8α – DCAL2 + DCs can produce IL-12 and support Th1 cells in response to CpG, CD8α – DCIR2 + DCs can up-regulate OX40L and promote Th2 cells in response to flagellin.

One way to classify mouse DC subsets is based on relative CD8α and CD4 expression splenic DCs can be subdivided into CD8α + CD4 – DEC205 + CD11b lo (also known as CD8α + DCs), CD8α – CD4 + DEC205 – CD11b hi (CD4 + DCs), and CD8α – CD4 – DEC205 – CD11b hi (CD8α/CD4 DN DCs) [7, 39, 40]. These subsets are found in distinct anatomical locations CD8α + DCs reside in splenic T cell areas, whereas CD4 + and DN DCs are found in MZs. Furthermore, these DC subsets appear to play different roles in inducing T cell responses [40]. CD8α + DCs are a major producer of IL-12 [9, 14, 15] and thus, induce Th1 responses, whereas CD4 + DCs are generally low cytokine producers [9, 11]. DN DCs may be tolerogenic and produce TGF-β and induce reguatory T cells [13] or immunogenic and induce Th1 and Th17 cells [41]. Some studies have suggested that CD8α – DCs could also induce Th1 responses, but the precise mechanisms and cells within the CD8α – population responsible were not made clear [13, 42]. For example, CD8α – DCs, which induce Th1 responses, could not be identified based on relative CD4 expression [34]. However, DCAL2 is a marker that helps distinguish a CD8α – DC subpopulation, which produces significant levels of IL-12 and induces Th1 responses. The CD8α/CD4 DN DCs are heterogenous, as they contain DCAL2-high and -low/negative populations. DCAL2 expression is useful for classifying and isolating CD8α – DC subsets, which differentially induce Th1 and Th2 responses. In addition, DCAL2 mAb are useful for isolating CD8α – DCIR2 + DCs, which make up about one-half of all splenic DCs, without ligating DCIR2.

CLRs on DCs play important roles in immunity, such as pathogen-capturing, costimulation, adhesion, and signaling [17, 37, 43]. The function and ligand(s) of DCAL2 remain to be identified. Ligating human DCAL2 is expressed on DCs and can induce protein tyrosine phosphorylation, MAPK activation, and IL-6 and IL-10 production but not full DC maturation [27]. Antibody cross-linking itself did not induce human DC maturation but induced up-regulation of CCR7 expression [27]. The effect of anti-DCAL2 appeared to differ depending on whether DCs were receiving signals from TLR4 or CD40. The ITIM of DCAL2 can recruit protein tyrosine phosphatases SHP-1 and SHP-2 in cell lines expressing DCAL2/Dectin-1 chimeric receptors [29]. However, antibody cross-linking of mouse DCAL2 did not modulate DC maturation or cytokine production (Supplemental Figs. 1 and 2, and see ref. [30]).

Dudziak et al. [24] characterized intrinsic differences of two splenic DC subsets: CD8α + DEC205 + DCs and CD8α – DCIR2 + DCs, both of which are distinct from CD8α – DCAL2 + DCs. CD8α + DEC205 + DCs express MHC-I-associated genes and are efficient in presenting antigen to CD8 T cells, whereas CD8α – DCIR2 + DCs up-regulate MHC-II-related genes and are specialized for activating CD4 T cells [24]. They concluded that these differences in antigen presentation were subset-intrinsic and not dependent on the receptor signaling [24]. The CD8α – DCAL2 + DCs are DEC205 – DCIR2 – and distinct from the two DC populations described by Dudziak et al. [24]. They express different amounts of PRR-high levels of TLR2, -4, and -9 and produce a different set and quantity of cytokines compared with the other subsets. CD8α – DCIR2 + DCs express the highest levels of TLR5 and higher levels of intracellular PRRs, such as Nod1, Ipaf, and RIG-I (Fig. 3B). These differences in PRR expression levels within CD8α – DCs are not evident when CD4 expression is used to subdivide CD8α – DCs [38].

Our data reveal that CD8α – DCAL2 + DCs are as capable of producing IL-12 as CD8α + DCAL2 + DCs (Fig. 4) and can effectively support Th1 responses (Figs. 5 and 6). After treatment with CpG, the CD8α – DCAL2 + DCs produced IL-12p40 in vitro, and the CD8α – DCAL2 + DCs did not (Figs. 3 and 4A). The CD8α – DCIR2 + DCs may require a second signal to induce IL-12p40, as they were able to produce low levels of IL-12p40 after stimulation with CpG in vivo (Fig. 4B). Although CD8α + DCAL2 + DCs and CD8α – DCAL2 + DCs produced detectable levels of IL-12p70 in vitro, we did not detect IL-12p70 production ex vivo under conditions where IL-12p40 was produced (Fig. 4B data not shown). This may be because IL-12-p70 production requires exogenous cytokines [9]. CD8α – DCAL2 + DCs produced lower amounts of IL-12p70 than CD8α + DCAL2 + DCs in response to zymosan, perhaps as they produced IL-10 (Fig. 4B), which can suppress IL-12p70 production from DCs [44]. Interestingly, before this study, there has been little evidence suggesting that CD8α – DCs produce sufficient IL-12 to support Th1 responses [42, 45, 46]. Skokos and Nussenzweig [42] reported a Delta-4-dependent, IL-12-independent, LPS-mediated Th1 induction by CD8α – DCs, but Delta-4 accounted for only 10–15% of the total Th1 responses, as a result of the functional redundancy with IL-12. CD8α – DCAL2 + DCs produced high levels of IL-12 in response to CpG, which is very likely to be contributing to Th1 induction. This discrepancy could be a result of the difference between TLR4 and TLR9 signaling in programming DCs to use Delta-4 for Th1 induction.

The question remains as to why CD8α – DCAL2 + DCs are able to produce more IL-12 than the other DC subsets, although prior literature suggested CD8α – DCs produced less IL-12 than CD8α + DCs. One possibility is that CD8α – DCIR2 + DCs regulate CD8α – DCAL2 + DCs and prevent them from producing IL-12. However, addition of increasing numbers of CD8α – DCIR2 + DCs to CD8α – DCAL2 + DC cultures had no effect on IL-12 production (data not shown). A more likely possibility is that in previous CD8α – DC studies, some cytokine responses by CD8α – DCAL2 + DCs were not detected, as splenic CD8α – DCIR2 + DCs are ∼2.5-fold more frequent than CD8α – DCAL2 + DCs (Fig. 3).

In response to flagellin, CD8α – DCIR2 + DCs up-regulated IL-4-producing cells in vitro and in vivo (Figs. 5 and 6). They appear to be well-equipped for inducing IL-4-producing cells, as they express high levels of the flagellin sensors TLR5 and Ipaf and in response to flagellin, selectively up-regulate OX40L (Figs. 3 and 4), which plays a key role in stimulating primary and memory Th2 responses in vivo [47]. In addition, targeting of antigen to CD8α – DCIR2 + DCs results in induction of Th2 responses in vivo [25]. Moreover, flagellin induces MyD88-dependent, DC-mediated Th2 in vivo by promoting the production of IL-4 and IL-13 from antigen-specific CD4 T cells, as well as IgG1 responses [48]. Thus CD8α – DCIR2 + DCs appear to be designed for responding to pathogens that induce Th2 cells. Although non-DCs, such as basophils, can produce IL-4 and also promote Th2 responses [49, 50], DCs are absolutely required to induce Th2 responses in certain contexts, such as infections with the parasitic helminth, Schistosoma mansoni, as depletion of DCs severely disrupts Th2 responses [51]. Further studies are needed to determine how the CD8α – DCIR2 + subset is programmed and in turn, regulates protective Th2 immunity.



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