Dendritic Cells Flashcards
What is the role of Dendritic cells?
Include their family
- Family of leukocytes
- Evolved to translate innate recognition into adaptive immunity
- Primary function is to capture and present protein antigens to naïve T-lymphocytes
- “Antigen presenting cells”
List two major functions of DCs during adaptive immunity
– Capture and process antigens for presentation to T-lymphocytes
– Produce signals required for proliferation and differentiation of lymphocytes
Describe the Dendritic cell life cycle
- Generated from hematopoietic stem cells in the
bone marrow - Differentiate under the control of a complex
network of soluble growth factors produced by
bone-marrow stroma and direct cell-cell contact
with bone marrow stromal cells
– e.g. GM-CSF, IL-3, FLT3L - Give rise to circulating precursors that home to
tissues where they reside as immature cells
*As they mature, dendritic cells express CCR7 which allows them to localize to the lymphoid tissues
*They pick up antigens in peripheral tissues, then migrate to lymph nodes, where they express high levels of adhesion, costimulatory molecules, and MHC class II molecules
*Once they have migrated, DCs stop synthesizing MHC class II molecules, but maintain high levels of MHC class II molecules containing peptides from antigens derived from the tissue where they originated
*As they mature, DCs also increase expression of key costimulatory molecules, including CD40, CD80 and CD86 (B7-1 and B7-2
Describe Immature dendritic cells
- Located throughout epithelium of the skin, the respiratory tract, and the gastrointestinal tract
- Recruited to sites of inflammation in peripheral tissue
– By chemokines - MIP-3alpha (CCL20), RANTES (CCL5) and MIP-1alpha (CCL3)
– Immature DCs express chemokine receptors – CCR1, CCR2, CCR5, CCR6 and CXCR1
– DCs accumulate at the site of infection very rapidly – within an hour
What are the different antigen materials DCs can uptake
– Apoptotic bodies
– Bacterial material
– Material from virally infected cells
– Hsp/antigen complexes
– Immunoglobulin cross-linked material
– Extracellular fluid
– Material from healthy cells
List and describe the different Antigen uptake pathways
- Receptor mediated endocytosis
– C-type lectin receptors: mannose receptor,
DEC-205
– Fcgamma receptor types I (CD64) and types II
(CD32)
– CD91 α2-macroglobulin receptor (hsp)- Shrivastava and tumour peptide delivery - Phagocytosis of particulate material
– Apoptotic and necrotic cell fragments
– Bacteria including mycobacteria
– Intracellular parasites such as Leishmania major
– Viruses
– latex beads - Macropinocytosis
– Aquaporins – water channels involved in regulating osmotic pressure, may be responsible for constitutive macropinocytosis
Describe maturation of DCs
- Dendritic cells need to receive a maturation stimulus to trigger their transition from immature antigen capturing cells to mature antigen presenting cells
- Under steady-state conditions only a small
number of tissue-resident DCs ‘spontaneously’ mature and migrate to the lymph nodes
Describe maturation stimuli
- Pathogenic molecules as a consequence of
infection
– lipopolysaccharide (LPS)
– Bacterial DNA
– dsRNA - Balance between pro- and anti-inflammatory
signals in local environment
– TNF-α, IL-1, IL-6, IL-10, TGF- and prostaglandins - T cell derived signals
– CD40
Describe the changes during DC maturation (What is up/downregulated?)
- Down regulation of receptors for inflammatory
chemokines - Down regulation of antigen capture
– Loss of endocytic and phagocytic receptors - Change in morphology
– Loss of adhesive structures
– Cytoskeleton remodelling
– Acquisition of high cellular motility - Up regulation of receptors for homing to lymphoid tissue
– CCR7 - Up regulation of antigen presentation
- Up regulation of co-stimulatory molecules
– CD40, CD58, CD80, CD86
Describe Dendritic cell migration
- Migration of Dendritic cells to the lymphoid
tissues is regulated by chemokine - chemokine
receptor interactions - Upon maturation, surface expression of CCR7 is
up regulated - DCs become responsive to CCL19/MIP-3β and
CCL21/ 6Ckine
– CCL19 is expressed in the afferent lymph ducts
– CCL21 is expressed in high endothelial venules of
lymph nodes and in the T-cell areas of spleen and
lymph nodes - CCL19 and CCL21 guide DCs from the tissue to
the T cell areas of lymph nodes
For all chemokine receptors expressed by dendritc cells, what are their ligads?
Describe the structure of a lymph node
Describe immune cells interactions in lymph nodes
- Mature DCs migrate to the T cell zones
- Stimulate quiescent, naïve and memory T and B
lymphocytes - Selection of rare CD4+ and CD8+ T cells and B cell
clones - Induce an immune reaction by priming cytoxic T cells and helper T cells
– Antigens presented in the context of Class II prime T helper cells
– Antigens presented in the context of Class I prime Cytotoxic T-cells
Describe DC - T Cell interactions
- DC-SIGN
– (DC-specific intercellular adhesion molecule (ICAM)-3 grabbing non-integrin)
– DC specific ligand for ICAM-3 expressed on naïve T cells
– Promotes transient clustering between a DC and a T cell, this allows a DC to screen numerous T cells for a matched TCR - Dectin-1
– DC specific type II C-type
– Lectin binds T cells promoting their proliferation - CD80 and CD86
– Co-stimulatory molecules expressed on mature DCs
-bind to CD28 (Naive T-cells) and its homolog CTLA-4 (CD152), which is expressed after T cell activation
– Regulate T cell activity - prolongs and augments the production of IL-2 and other cytokines
- is probably important in preventing the induction of tolerance
- CD40 – CD40L
– T cells can activate DCs via CD40L
Describe DC directed B cell activation
- Follicular areas of the lymph node
- DC can directly signal B cells to proliferate
- CD40 : CD40L (T cell) interaction at the
DC (menage a trois) - DC secretes cytokines to cause proliferation of B cells and Ig production
Describe DC interactions with NK cells
- Occurs at site of infection
- Pathogen activated DCs can activate NK cells
through cell-cell contact and soluble signals
– IFNa, IFN-b, IL-2, IL-12, IL-15 and IL-18
– Leads to NK cell secretion of IFNg and cytolytic
activity - Il-12, IL-18 and IL-15 from DC promote NK
proliferation and survival - IFN and TNF release from NK cells can mature
DCs - Non-MHC dependent
What’s the problem with cross-presentation by DCs?
- Cross-presenting DC are valid targets for
CTL, express viral peptides and MHC-I on the cell surface. - DC contain SPI-6 (serine protease inhibitor-6) protect against lysis.
- Th1 cells induce SPI-6 in DC
Why do some DCs die?
- Death of the DC may be a way of self-
limiting the response. - Sufficient clonal expansion
- Too many CTL may be damaging.
- NEEDS TO BE CONTROL
Cross-dressing
- Alternative way of cross-presenting esp
TAA - Transfer of MHC1 molecules from tumour
cell to DC - Well documented for mDC but recently
shown for pDC (acquire cell membrane
‘patches’
How do Dendritic cell subsets differ
- Although all DCs perform essentially the same function to collect, process and present antigen they are a heterogenous group cells
- DCs differ in
– the signals the respond to
– the regulatory signals they transmit
– the precursor cells they derive from
– their location
– stage of maturation
Describe DC1 and DC2
- Early studies divided dendritic cells into two groups called DC1 and DC2 based on their ability to induce either Th1 or Th2 responses
- Largely based on the premise that human pDC did not appear to produce IL-12 and preferentially drove Th2 development whereas Mo-DC produced IL-12 and droveTh1 differentiation
- In the mouse early studies suggested that mouse CD8a+ DC produced IL-12 and direct Th1 responses whereas CD8a- DC did not synthesise IL-12 and preferentially induced Th2
- BUT both mouse and human pDC can produce IL-12 a well as IFN-I and drive Th1 responses
- The DC1 and DC2 division is an oversimplification
Describe Myeloid lineage CD1c+ DC (Dendritic cell subsets)
*Major population of DC in blood, tissues and
lymphoid organs (~1% mononuclear cells)
*Tend to have a more activated phenotype in
tissues than in blood (increased CD80, 86,
83 and 40 and CCR7).
*Form interdigitating DC in LN
*a myeloid progenitor (DC1) that gives rise to myeloid DCs, otherwise called bone-marrow derived or bm-DCs
*mainly produce IL-8, IL-12
*Some derived from monocytes CD14+
(inflammatory dendritic cells?) more like
macrophages, not good at stimulating naïve
T cells. Can be tolerogenic if non-mature.
*Express little CCR7
difference between mDC1 & mDC2
- myeloid DC, most common in PB and
resident in LN, tonsil, spleen and BM - mDC2>mDC1
- mDC1 high ‘intrinsic’ cross-presentation.
- mDC2 can be induce to have high cross-
presentation - mDC1 in anti-viral response; mDC2 in anti
-bacterial as well (responds to PAMPs: flagellin, lps)
Describe Plasmacytoid DCs (pDCs) (Dendritic cell subsets)
- Plasma-cell like morphology
- Minor population of blood DCs
- May have rearranged BCR/TCR
- Some ability to present antigen, express
MHC2, CD80 and 86 - High expression of TLR7 and 9
- Primary role seems to be in production of Type
1 IFN in response to virus - CD34+/CD11clow/CD45RA+
- CD123+ (IL-3R)
- Recent evidence suggests that mDC and pDC
MAY have a common precursor.
a lymphoid progenitor (DC2) that develops into
plasmacytoid DCs
Describe Tissue derived DCs (Dendritic cell subsets)
- Langerhans Cells
– Found in the skin and skin draining lymph nodes
– Express high levels of langerin - Langerhans’ cells have Birbeck granules containing Langerin
Myeloid DCs can also be divided into at least three types:
Langerhans’ cells (LCs), etc.
These DCs are rich in class II MHC molecules
Langerhans’ cells in the epidermis and in other squamous epithelia migrate via the afferent lymphatics into the paracortex of the draining lymph nodes (Fig. 2.16). Here, they interact with T cells and are termed interdigitating cells
Describe Dermal/interstitial DCs (Dendritic cell subsets)
– Found in all tissues
– Traffic to draining lymph nodes
- Myeloid DCs can also be divided into at least three types: dermal or interstitial DCs (DDCIDCs) etc.
- Myeloid DCs, but not pDCs express CD1a and CD208, whilst DDC-IDC and moDC express
also CD11b.
Give an overview of DC types
Antigen presenting cells reinforce self
tolerance
Describe this
Dendritic cells can present antigen in a tolerogenic manner
* DCs play a role in both central and
peripheral tolerance
Depending on the information received from the DC in addition to peptide presentation,the TH cells may become:
* activated;
* anergic;
* converted into a regulatory T cell; or
* undergo apoptosis.
The critical importance of DCs for the maintenance of tolerance has also been shown in experiments in which conditional DC depletion in mature mice resulted in spontaneous autoimmunity.
Functional maturation of DCs, characterized by strong expression of MHC and co-stimulatory molecules is induced by microbial or self-derived stimuli, which are sometimes called danger signals. In the absence of such stimuli immature DCs express MHC and costimulatory molecules at low levels and antigen presentation induces T cell anergy or deletion depending upon the expression of high or low levels of self-antigen respectively
Immunity vs. Tolerance by DCs
– All DCs have the capacity to induce tolerance or immunity, the
distinction is depending on their maturation state
* Either immature DCs produce tolerance whereas mature DCs
immunity
* Or Mature DCs need to be activated to produce immunity, quiescent
mature DCs maintain self-tolerance
* “Danger signals” are required to produce immunity
Central tolerance vs Peripheral tolerance by DCs
- Central tolerance
– Mediated by thymic DCs
– Thymic DCs Induce the apoptotic death of potentially self-reactive T cells within the thymus - Peripheral tolerance
– Mediated by DCs
– DCs promote tolerance by killing T cells, paralysing T cells (anergy) or by inducing the generation of antigen specific regulatory T cells
How are danger signals recognised?
- “Danger Signals” are Recognised by Pattern Recognition Receptors
- Toll-Like Receptors are a large family of pattern
recognition receptors expressed on dendritic cells - Recognise conserved molecular products derived from various classes of pathogen including bacteria, viruses, fungi and protozoa
– pathogen associated molecular patterns (PAMPs) - TLR triggering has pleiotrophic effects on DCs
– Promoting survival, chemokine secretion, expression of chemokine receptors, migration, cytoskeletal and shape changes or endocytic re-modelling
What is the effect of danger signals on DCs?
- Danger signals act directly on DCs to drive migration and maturity
- Danger signals affect the DC:T cell interaction
Describe TLR signalling regulation of DCs
- TLR signalling regulates three categories
of signals DCs deliver to T cells
– Antigen (Signal 1)
– Co-stimulation (Signal 2)
– Cytokines (Signal 3)
Antigen: Signal 1
- In vitro treatment of DCs with LPS increases antigen loading onto MHC
class II and surface display of
MHCII:peptide complexes - Poly I:C treatment of mouse DC
enhances MHCI presentation and
cross-presentation.
Co-stimulation: Signal 2
- All TLR agonists tested to date
increase expression of CD40, CD80
and CD86 in at least one DC sub-set
Cytokines: Signal 3
- E.g. CpG and LPS stimulate DCs to
produce IL-6 which makes responding T
cells refractory to suppression by regulatory
T cells - TLR signalling can promote synthesis of IL-
1, IL-2 and/or TNF-a by DCs, all of which
can contribute to clonal expansion of T
cells - TLR ligation can induce production of IL-
12p70 which directs differentiation of CD4+
and CD8+ T cells into type 1 effectors
TLR, Ligand, original ligand
TLR locations and combinations
- TLRs found on cell surfaces
– TLR1, TLR2, TLR4, TLR5, TLR6 - TLRs found in the membranes of the endosomes used to degrade pathogens
– TLR3, TLR7, TLR8, TLR9 - Many of the TLRs signal in pairs e.g.
– TLR1/TLR2 bind bacterial lipopeptides and GPI-anchored proteins in parasites
– TLR-6/TL2 pairs bind lipoteichoic acid from gram-positive cell walls and zymosan from fungi
– TLR-4/TLR-4 pairs bind lipopolysaccharide from gram-negative cell walls pairs
TLR signalling pathways
DCs direct Th1 / Th2 polarisation
- Th1 / Th2 polarisation
– Th1 - Key cytokine in polarising immune response is IL-12p70 produced
by dendritic cells - IL-12p70 produced in response to DCs detecting intracellular
infection
– e.g. signalling through endosomally located TLR3 in response to
dsRNA in apoptotic cell body - IFN-a produced by pDCs also polarises for Th1
– Th2 - IL-4 directs CD4 T cells to promote B cell proliferation
- In response to DC detecting an extracellular infection
– e.g. signalling through cell surface bound TLR5 in response to flagellin
