dendritic cells

Dendritic cells and their immature counterparts, Langerhans cells (LC), are highly specialized, professional antigen-presenting cells (APC) located in the skin, mucosa, and lymphoid tissues.

▼ adhesion : APC activities : clonal expansion B cells : cytokines : DC types : disorders : ectopic FDC-formation : follicular dendritic cells (FDC) : FDC networks : generating germinal centers : germinal centers : immature dendritic cells : immune regulators : immunological synapse : interferon producing cells : lymphoid dendritic cells : maturation : morphology : myeloid dendritic cells : pDC : plasmacytoid dendritic cells (PDC, IPC) : precursor dendritic cells : regulators : Th1 and Th2 stimulation : TLRs : types of DC : veiled cells ▼

Immune dendritic cells are named for their morphology (long surface projections), and bear no relationship to neurons. Immature dendritic cells are also called 'veiled cells' because they display large cytoplasmic 'veils' rather than dendrites. DC and LC play a key role in the induction phase of contact allergenicity.

As key regulators of immune responses, dendritic cells (DC) stimulate lymphocytes to perform cell-mediated and humoral immune responses against pathogens and tumor cells. DCs can also educate T cells to tolerate self-antigens, thereby minimizing autoimmune reactions.

Types of dendritic cell
● follicular dendritic cells (origin?) – FDC
● lymphoid dendritic cells (lymphopoiesis) –
● myeloid dendritic cells (monocytopoiesis) – MDC1, MDC2
● plasmacytoid dendritic cells – PDC, IPC – the major producers of type I interferon (IFN)

Immature, precursor dendritic cells (pDC) circulate throughout the body, migrating to lymphocyte rich tissues (such as spleen and lymph nodes) upon stimulating encounter with antigen. The dendritic cells internalize the antigen, then digest, and externalize the fragmented antigen that they present to lymphocytes in MHC-peptide complexes, expressing markers that stimulate lymphocyte activation. Dendritic cells are the most effective antigen presenting cells. Follicular dendritic cells stimulate differentiation of B cells, monocytopoietic lineages (pDC1) stimulate differentiation of Th1 cells, lymphopoietic dendritic cells (pDC2) induce differentiation of Th2 cells. Plasmacytoid cells produce type 1 interferon (IFN-α, β, Ω) and can mature into dendritic cells that link innate and adaptive immune responses.

A variety of factors operate in antigen recognition and processing by immature (precursor) dendritic cells and in the maturation of immature cells. Toll-like receptors on the surfaces of precurson dendritic cells recognize microbial components and induce the differentiation of dendritic cell precursors. GM-CSF and IL-4 stimulate the maturation of monocytopoietic pDC1, while IL-3 stimulates the differentiation of pDC2. The transition to mature dendritic cells down-regulates those factors that were involved in antigen internalization, while up-regulating the expression of MHC, costimulatory molecules that participate in lymphocyte activation, adhesion molecules, and specific cytokines and chemokines.

Adhesion molecules enhance direct interactions between T cells and dendritic cells (immunological synapse). Dendritic cell stimulation of formation of Th1 and Th2 cells appears to be regulated by negative feedback. Th1 production of interferon-γ blocks the further stimulation of Th1 differentiation by DC1 cells. Th2 production of IL-4 kills the dendritic cell precursors that contribute to Th2 cell creation. Thus, although IL-4 stimulates Th2 differentiation, the promotion of Th2 cell formation by DC2 cells does not appear to involve IL-4. Costimulatory receptors CD80 and CD86 expressed by mature dendritic cells activate T cells in concert with the recognition of antigen/MHC by the T cell receptor. The secretion of IL-12 by dendritic cells stimulates T cell responses, in particular the differentiation of Th1 cells, which produce interferon-γ and other inflammatory cytokines.

Follicular dendritic cells are stromal cells unique to primary and secondary lymphoid follicles. FDCs express all three types of complement receptors as well as Ig-Fc receptors, through which antigen-antibody immune complexes are retained. FDCs present native antigens to potential memory B cells, of which only those coated with high affinity B cell receptors (BCR) are able to bind.

Recirculating resting B cells migrate through the FDC networks. Antigen-activated B cells undergo clonal expansion within the FDC networks in a T cell-dependent fashion, generating germinal centers. Evidence suggests the presence of two types of dendritic cells within human germinal centers: (i) the classic FDCs that express DRC-1, KiM4, and 7D6 antigens represent stromal cells; and (ii) the newly identified CD3-CD4-CD11c- germinal center dendritic cells (GCDC) represent hematopoietic cells that may be analogous to antigen-transporting cells of mice.

Within germinal centers, B cells undergo somatic hypermutation, positive and negative clonal selection, isotype switching and differentiation into high-affinity plasma cells and memory B cells. Adhesion between FDCs and B cells is mediated by ICAM-1 (CD54)-LFA-1(CD11a) and VCAM-VLA-4. T cells may interact with FDCs in a CD40/CD40-ligand-dependent fashion.

Ectopic FDC-formation is found in a number of autoimmune diseases and/or chronic inflammatory situations, suggesting that FDC development is not restricted to secondary lymphoid organs, but rather that local conditions drives a precursor cell type into FDC-maturation. The precursor of FDCs has presently not been identified, but data suggests a close relation to fibroblast-like cells. [s] It was initially believed that all DCs were of myeloid origin until several recent studies demonstrated that some DCs could also be efficiently generated from lymphoid-restricted precursors. FDCs appear to be involved in the growth of follicular lymphomas and in the pathogenesis of HIV infection.[pm]

Lymphoid dendritic cells are of lymphopoietic origin, and IL-3 stimulates the differentiation of pDC2 cells into DC2 cells, which stimulates differentiation of Th2 cells, which secrete the lymphokine interleukins 4, 5, 10, and 13. (IL-4, IL-5, IL-10, IL-13)

Myeloid dendritic cells are of monocytopoietic origin, and the maturation of precursor cells (pDC1) is stimulated by GM-CSF, and IL-4. Mature DC1 cells secrete interleukin 12 (IL-12), which acts through the JAK-STAT pathway to induce Th1 cells to secrete TNF-β (lymphotoxin) and IFN-γ. MDC-1 is the more common subtype, and is a major stimulator of Th1 cell differentiation. MDC-2 is rare, and may function in response to wound infection.

Plasmacytoid dendritic cells (pDC=IPC) are the major producers of type I interferon (IFN) and exhibit the unique ability to link innate and adaptive immune responses, by differentiating into DC capable of stimulating naive T cells and modulating the adaptive immune response. Human plasmacytoid DCs (PDCs) can induce either Th1- or Th2-type immune responses upon exposure to viruses or IL-3, respectively.

Plasmacytoid dendritic cell precursors (pDC) are type 1 interferon-(α, β, Ω)-producing cells (IPCs) that comprise 0.2%-0.8% of peripheral blood mononuclear cells (humans, mice). IPCs display plasma cell morphology, selectively express Toll-like receptor (TLR)-7 and TLR9, and rapidly secrete massive amounts of type 1 interferon following viral stimulation. IPCs promote the function of natural killer cells, B cells, T cells, and myeloid DCs through type 1 interferons (IFN) during an antiviral immune response. Later in viral infection, IPCs differentiate into a unique type of mature dendritic cell, which directly regulates the function of T cells and thus links innate and adaptive immune responses. [s, ↓] [fft]

images [] sem dendritic cell and T cell [] micrograph Langerhans cells [] PKC bII signaling in dendritic cells [] micrograph gallery dendritic cells [] photomicrograph dendritic cells interacting with yeast (lilac) [] photomicrograph Human Dendritic cell (labelled with anti MHC class-I FITC) presenting Influenza antigens to T-lymphocytes.
videos Џ Nature video library index Q Џ

ф activation ф affinity maturation ф anergy ф antibodies ф antigen ф APCs ф autoimmunity ф B cells ф blood ọ bone marrow ф CD ф cellular response ф class-switch recombination ф clonal selection ф complement system ф costimulation ф helper T cell ф hematopoiesis ф humoral immunity ф immune cytokines ф immune response ф immune tolerance ф inflammatory response ф interferons ф isotype switching ф killer T cells ф lymphocytes ф lymphokines ф lymphoid system ф lymphopoiesis ф macrophages ф MHC ф monocytopoiesis ф pattern-recognition receptors ф phagocyte ф plasma cells ф receptors ф signaling ф somatic hypermutation ф surface receptors ф T cells ф thymus

Tables  Complement Receptors  Cytokines  Fc receptors  Immune Cytokines  Immunoglobulins  Interferons  Scavenger Receptors  Toll-like Receptors

IPC: professional type 1 interferon-producing cells and plasmacytoid dendritic cell precursors. [Annu Rev Immunol. 2005]
Plasmacytoid dendritic cell precursors/type I interferon-producing cells sense viral infection by Toll-like receptor (TLR) 7 and TLR9. [Springer Semin Immunopathol. 2005] PMID: 15592841 [Free Full Text]
Natural type I interferon-producing cells as a link between innate and adaptive immunity. [Hum Immunol. 2002] PMID: 12480256
Thrombopoietin cooperates with FLT3-ligand in the generation of plasmacytoid dendritic cell precursors from human hematopoietic progenitors. [Blood. 2004] PMID: 14670916
Flexibility of mouse classical and plasmacytoid-derived dendritic cells in directing T helper type 1 and 2 cell development: dependency on antigen dose and differential toll-like receptor ligation. [J Exp Med. 2003] PMID: 12515817
Roles of toll-like receptors in natural interferon-producing cells as sensors in immune surveillance. [Hum Immunol. 2002]

tags [Immunology] [dendritic+cell] [APC] [cytokine] [Toll-like+receptor]

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helper T cell

Helper T cells, T helper cells (Th), effector T cells circulate throughout the body where they interface with MHC class II protein on other cells, determining whether the MHC class II is presenting 'self' or 'non-self protein' (antigen). MHC class II receptors are located on the surface of professional antigen presenting cells (APCs), which display epitope proteins – exogenous antigen or fragmented angtigen from phagocytosed cells – on their surfaces.

When a helper T cell is activated by contact with antigen, it enters the cell cycle in addition to producing lymphokines and chemokines. Th cells direct antibody class switching in B lymphocytes, orchestrate activation and growth of cytotoxic T cells, and maximize the bactericidal activity of phagocytes (macrophages).

Naïve B lymphocytes each have one of millions of distinct surface antigen-specific surface receptors, yet have not encountered their specific, cognate antigen. With a life-span of only a few days, many B cells die without ever encountering their cognate antigen. Naïve B cells are stimulated when the BCR binds to its cognate antigen. This antigen-Ig binding must be coupled with a signal from a helper T cell in order to activate the B cell.

Helper T cells mostly carry the CD4 surface protein, though a few carry CD8. The CD4 receptor triggers targetting by HIV, which determines the crippling effect of HIV on the immune system.
Subsets of Th cells are defined by the class of cytokine that they secrete upon activation:
Th1 – produce copious amounts of IL-2 and IFN-γ.
Th2 – particularly effective at stimulating B cells through secretion of IL-4, IL-5, and IL-6.
Th3 – produce cytokine transforming growth factor-beta (TGF-β) and IL-10.


Th1 cells are the more effective antiviral agents by virtue of their secretion of interferons (IFN-γ).  Immune Cytokines  Interferons

The cytokines produced by the two Th subsets perform cross-regulatory role. An activated Th2 cell secreting IL-4,-5,and 6 downregulates local Th1 cells in the neighborhood, whereas Th1 cytokines downregulate Th2 responses.

Tables  Fc receptors  Immune Cytokines  Immunoglobulins

[] diagram - helper T cells and phagocytic response to tumor cells [] diagram - HIV binding via CD4 receptors [] micrograph germinal center with helper T cells [] tem - helper T & B cell []

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receptors

The functionality of cells of the immune system is particularly dependent on signal pathways, and the various lymphoid cell types sport an array of receptors.

▼ antigenic determinant : APC costimulation : BCR: complement receptors : cytokines : epitope : FcR : Ig-Fc : IgG : opsonins : pathogen associated molecular patterns : pattern recognition receptors : phagocyte receptors : respiratory burst complement : respiratory burst Fc : : scavenger receptors : TCR : TLR : Toll-like receptors : VDJ recombination ▼

Phagocytes

Phagocytic cells detect infectious agents that bind to a variety of receptors on the phagocytes cell membranes, including:

● Fc receptors (FcR, Ig-Fc) – the constant region (Fc) of IgG on bacterial surfaces can bind to the Fc receptor on phagocytes. Such binding to the Fc receptor requires prior antibody-antigen interaction. The binding of IgG-coated bacteria to phagocytic Fc receptors stimulates both metabolic activity in the phagocytes (respiratory burst) and phagocytic activity. Fc receptors include the clusters of differentiation, CD16 (Fcγ RIII), CD32 (Fcγ RII-A, Fcγ RII-B2, Fcγ RII-B1), and CD64 (Fcγ RI), Fcε RI, and Fcα RI. All FcR are stimulatory except inhibitory Fcγ RII-B1 and B2, which contain immunoreceptor tyrosine based inhibition motifs (ITIMs) in their cytoplasmic tail. Table  Fc receptors

● Complement receptors – Phagocytic cells possess a receptor for the C3b complement opsonins, and binding of C3b-coated bacteria to this receptor stimulates enhanced phagocytosis and the respiratory burst. Table  Complement Receptors.

● Scavenger receptors bind a variety of polyanions on bacterial surfaces, stimulating phagocytosis of the polyanion-coated bacteria. Macrophage scavenger receptors appear to mediate important, conserved functions, so it was likely pattern-recognition receptors that arose early in the evolution of host-defense mechanisms. Table  Scavenger Receptors

● Toll-like receptors are a variety of pattern recognition receptors (PRR) that recognize pathogen associated molecular patterns (PAMP) on infectious agents. Binding of the infectious agents to Toll-like receptors stimulates phagocytosis and the release of inflammatory cytokines (IL-1, TNF-α, IL-6) from the phagocytes. Table  Toll-like Receptors

Tables  Complement Receptors  Fc receptors  Immune Cytokines  Immunoglobulins  Interferons  Scavenger Receptors  Toll-like Receptors .

Lymphocytes

The surfaces of B cells and T cells are coated with thousands of identical copies of different integral membrane receptors (BCRs, TCRs), each capable of binding with a different antigen.

Receptor characteristics
● thousands of copies of integral membrane proteins with unique antigen binding sites
● encoded by genes assembled by VDJ recombination produced without antigen encounter
● the antigen binding site recognizes an antigenic determinant or epitope on the antigen
● binding, by non-covalent forces, is based on complementarity of the surface of the receptor and the surface of the epitope

Binding of receptor to epitope, when accompanied by APC-costimulation, leads to:
● stimulation of the B or T cell to leave the G0 phase and enter the cell cycle
● repeated mitosis generates a clone of cells of identical specificity, each coated with an identical antigen receptor.

Cytokine receptors:
● Hematopoietin family receptors are dimers or trimers with conserved cysteines in their extracellular domains and a conserved Trp-Ser-X-Trp-Ser sequence. The two subunits are i) cytokine-specific, and ii) signal transducing. Examples are receptors for IL-2 through IL-7 and GM-CSF.
___ ● Colony-stimulating factors (CSFs) are glycoprotein molecules that support growth of hematopoietic colonies. Examples are receptors for interleukin 3 (IL-3), G-CSF, GM-CSF, M-CSF.

● Interferon family receptors
Interferons are immune cytokines that are classified, as type I, II, or III, according to the receptors through which they signal. Interferon (INF) family receptors have conserved cysteine residues and include the receptors for IFNα, IFNβ, and IFNγ.

● Tumor Necrosis Factor family receptors possess four extracellular domains. Examples are receptors for TNFα, TNFβ (lymphotoxin β, LT), CD40, CD27, CD30, and Fas.

● Chemokine family receptors have seven transmembrane helices (serpentine, GRCRs) and interact with G protein. This family includes receptors for IL-8, MIP-1, MCP (monocyte chemoattractant protein), and RANTES (regulated upon activation normal T cell expressed and secreted). Chemokine receptors CCR5 and CXCR4 are used by HIV to preferentially enter either macrophages or T cells.

Tables  Complement Receptors  Fc receptors  Immune Cytokines  Immunoglobulins  Interferons  Cell Adhesion Molecules  Cell signaling  Receptor Tyrosine Kinases (RTKs)  Receptor Signal Transduction  Second Messengers  Scavenger Receptors  Toll-like Receptors 

▲ф ф antibodies ф antigen : antigenic determinant ф APCs : APC costimulation : BCR ф BCR ф B cells ф CD ф cellular response ф clonal selection ф complement system : complement receptors ф complement system ф costimulation : cytokines ~ cytokines ф dendritic cells : epitope : FcR  Fc receptors ф granulocytes ф helper T cell ф hematopoiesis ф humoral immunity : Ig-Fc : IgG  Immune Cytokines  Immunoglobulins □□ Immunology ~ immunoglobulins ф inflammatory response ф immune cytokines ф immune response ф lymphocytes ф lymphoid system ф macrophages ф MHC : opsonins ф pathogens : pathogen associated molecular patterns (PAMP) : pattern recognition receptors (PRR) ф pattern-recognition receptors : phagocyte receptors ф phagocyte ф plasma cells : respiratory burst complement ››› respiratory burst : respiratory burst Fc : scavenger receptors ф signaling ф surface receptors : TCR ф TCR ф T cells : TLR : Toll-like receptors : VDJ recombination ▲ф

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T cells

T cells or T lymphocytes are specialized lymphocytes distinguished by their TCR surface receptors.

▼ activation T reg : APC : apoptosis : B7-1 : B7-2 : CD8 + : CD28 : CD80 : CD86 : costimulation : CTL : cytotoxic T lymphocytes : Fas-FasL : γδ T cells : granulysin : granzyme : helper T cell : IL-2 : interleukin-2 : natural killer T cells : NKT : perforin : pore : self-protection : serine protease : Tc : Th : Treg : Treg activation/action : T cell types : VDJ recombination ▼

'T' lymphocytes initially develop in the thymus, and differentiate peripherally into several functional subsets of cells:
● cytotoxic T cells – CTL, Tc – CD8 +
● helper T cells, effector T cells – Th – CD4 +
● regulatory T cells –Treg (formerly termed 'suppressor' T cells)
● natural killer T cells – NKT (different than NK cells)
● γδ T cells

Cytotoxic T lymphocytes (CTLs) or killer T cells express antigen-specific, VDJ recombination-generated TCRs plus glycoprotein CD8 on their surfaces. The TCR recognizes specific antigenic proteins that are fragmented into peptides and then are presented in complex with MHC class I molecules (MHC-peptide complex - red).

APCs provide synergistic signaling by costimulatory molecules. APC signaling is necessary to sustain and integrate TCR signaling and to stimulate optimal T cell proliferation and differentiation.

The binding of TCR•antigen•CD8•MHC-I holds the activated CTL (Tc) in proximity to the damaged/infected target cell. Once activated, the Tc cell undergoes clonal expansion with the assistance of a cytokine, interleukin-2 (IL-2) that acts as a growth and differentiation factor for T cells. (Table  Immune Cytokines)

CD28 binds the CD80 (B7-1) and CD86 (B7-2) ligands that are expressed on antigen presenting cells (APCs). When exposed to infected/dysfunctional somatic cells, Tc cells release perforin, which form pores in the target cell's plasma membrane. Also released from Tcs are granulysin and the serine protease granzyme, which transit the perforin pore and induce apoptosis of the target cell.

A second cytolytic mechanism is mediated by the interaction of activated T cell surface FasL with TNFR family Fas receptors (TNFRSF6, Apo_1, CD95) expressed on the surface of the target cell.

Helper T cells (Th), or effector T cells circulate throughout the body where they interface with MHC class II protein on other cells, determining whether the MHC class II is presenting 'self' or 'non-self protein' (antigen). Th cells direct antibody class switching in B lymphocytes.

When the helper T cell is activated by contact with antigen, it enters the cell cycle in addition to producing lymphokines and chemokines. Th cells orchestrate activation and growth of cytotoxic T cells, and maximize the bactericidal activity of phagocytes (macrophages).

Helper T cells mostly carry the CD4 surface protein, though a few carry CD8. The CD4 receptor triggers targetting by HIV, which determines the crippling effect of HIV on the immune system.

Regulatory cells (Treg)
Regulatory T cells were formerly called suppressor T cells because they suppress immune responses. Negative regulatory functions have been ascribed to several T cell types:
● CD4+CD25+
● CD4+CD25–
● interleukin-10-producing TR1 cells
● transforming growth factor-ß-secreting TH3 'regulatory' cells
[v]

Treg cells express the αβ TCR chains and are activated only when the TCR binds to its cognate peptide-class II MHC molecule and receives costimulation from B7 molecules (CD80, CD86) on the antigen-presenting cell. Once activated, Treg secrete interleukin 9 (IL-9), interleukin 10 (IL-10), +/- transforming growth factor-beta (TGF-β), and these lymphokines inhibit Th1 assistance of cell-mediated immunity and inflammation, and
Th2 assistance of antibody production, and may inhibit the action of CD8+ CTLs.

Because the antigenic peptides recognized by Treg TCRs are typically self-peptides, the chief function of Treg cells is provision of self-protection by preventing other T cells from attacking self components, thus protecting against autoimmunity.

CD25, or IL-2 receptor alpha chain, or IL-2R, or Tac antigen is a 55kDa glycoprotein whose transcription is regulated by Stat5, Elf-1, HGG-I(Y), GATA, and Tax from HTLV-1. CD25 is expressed on PHA-stimulated T cells, B cells stimulated with anti-IgM antibody, monocytes/macrophages stimulated with LPS, and highly expressed on HTLV-I-transformed T cell lines. NCBI LocusLink Record: 3559

Natural killer T cells – NKT cells are activated by group 2, CD1d molecules to secrete interferon-γ (IFNγ) and IL-4, which act as Th1 and Th2 cytokines.

γδ T cells carry TCRs encoded by different gene segments than those of αβ T cells. CD27 is cell-surface marker for T-cell progenitors that are beginning to undergo commitment to the αβ or γδ T-cell lineages, and newly emerging αβ and γδ T-lineage cells display early differences in dependence on Notch–Delta signaling and display distinct patterns of gene expression.[npg]

Presentation of peptide antigens for activation of naïve T cells does not reside solely in dendritic cells. A population of γδ T cells can efficiently present peptide antigens to αβT cells, which comprise the predominant mammalian T cell population. Thus, γδ T cells function to bridge the innate and adaptive immune responses.

γδ T cells can form unique memory subsets with distinct adhesion and chemotactic capabilities, determining the tissue homing capabilities of cells. γδ T cells are enriched in areas of inflammation. Human γδ T cells express ligands for E- and P-selectins, so they display specialized homing to epithelial surfaces. The memory γδ T cells express chemokine family receptors not expressed by αβ T cells.

Human γδ T cells that express Vγ2Vδ2 TCRs can recognize nonpeptide antigens and Non-Hodgkin's B cell lymphomas. In particular, recognition of prenyl pyrophosphates is performed by this extracellular presentation pathway, which is distinct from MHC and CD1 presentation, and which is important in many different bacterial and parasitic infections. However, γδ T cells of the major tissue subset recognize self and foreign nonpeptide antigens presented by CD1. These nonpeptide antigens are lipids and glycolipids presented by CD1 proteins, which are MHC-like-molecules specialized for the presentation of lipids.[s].


▲ф ф antibodies ф antigen : APC ф APCs : apoptosis סּ apoptosis : B7-1 : B7-2 ф CD : CD8 + : CD28 : CD80 : CD86 סּ cell growth סּ cell membranes ₪ cellular fate : costimulation ф costimulation : CTL ~ cytokines ф cytolysis : cytotoxic T lymphocytes ₪ differentiation : Fas-FasL : γδ T cells : granulysin § granulysin : granzyme : helper T cell ф helper T cell : IL-2 ~ immunoglobulins : interleukin-2 ф immune cytokines  Immune Cytokines : killer T cells ф killer T cells ф lymphocytes ф lymphokines ф MHC : natural killer T cells : NKT : perforin : pore ф receptors : serine protease ф signaling ф surface receptors: Tc : Th : Treg : T cell types ф thymus : VDJ recombination ф VDJ recombination ▲ф

Tables  Cell Adhesion Molecules  Cell signaling  Complement Receptors  Cytokines  Fc receptors  Immunoglobulins  Interferons  Receptor Tyrosine Kinases (RTKs)  Receptor Signal Transduction  Scavenger Receptors  Second Messengers  Toll-like Receptors
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