Dendritic Cells Flashcards
Functions of Dendritic Cells
To activate naive T cells by presenting antigens to them
Two states of DC
Immature and Mature
Immature DC
Found in the periphery
Efficient in antigen uptake
Inefficient in antigen presentation
Mature DC
Stimulated by Innate danger signals (tissue damage/infection)
Migrate to Secondary Lymphoid organs (lymph nodes)
Inefficient antigen uptake
Efficient Antigen presentation
Antigen capture/uptake
Constantly sampling environment using:
- Macropinocytosis
- Receptor Mediated Endocytosed
- Phagocytosis
Macropinocytosis
Constitutive in immature DCs whilst only occuring in other cells upon growth factor stimulation
- enables non-specific uptake of large amounts of surrounding fluid (and suspended antogens)
- requires formations of ruffles on the DC surface (outer projections encapsulate material close to cell surface)
Receptor Mediated endocytosis
C-type lectins
- mannose receptors and DEC205 recognise glycosylated proteins
Fc Receptors
- FcgRI and FceRI bind to Fc portion of antibodies and internalise immune complexes
Scavenger receptors
- CD36 binds to apoptotic bodies
Heat Shock Protein (HSP) receptors
- Bind Hsc70 and gp96 from tumour cells/infected cells and mediate internalisation HSP-peptide complexes
Phagocytosis
Can use the same receptors from receptor-mediated endocytosis to uptake large particulate species.
DCs phagocytose:
- almost all bacteria
- yeast cells and fungal hyphae
- Apoptotic and neurotic bodies
Direct signals inducing maturation of DCs
Directs signals recognised primarily by Toll Like Receptors (TLRs)
- TLR2: G+ve bacterial cell wall (lipoproteins)
- TLR3: dsRNA
- TLR4: Lipopolysaccharide (LPS) G-ve bacteria
- TLR5: Flagellin from G+ve/G-ve bacteria
Activated TLRs immediately increase macropinocytosis
Indirect signals inducing maturation of DCs
Molecules who’s secretion is triggered by pathogens or responses to tissue damage
- Tumour Necrosis Factor a (TNFa)
- Interleukin 1beta (IL-1B)
- Protaglandin E2 (PGE2)
Stages of DC migration to secondary lymphoid organs
DCs in epidermis (Langerhan cells) receive innate danger signals triggering maturation and migration:
Epidermis –> Dermis –> Afferent lymphatic vessel –> lymph node
Lymph node = rich with naive T cells to interact with; initiating a T-cell mediated immune response
Cellular receptors involved in DC migration
1) Langerhan cells interact with keritinocytes via E-cadherins to normally stay in the epidermis
- upon maturation, E-cadherin expression is reduced so it can become mobile
2) Chemokine Receptor 7 (CCR7) expression increases upon maturation enabling homing to lymph nodes
CCR7 has 2 chemokine ligands
1) Chemokine Ligand 21 (CCL21)
- expressed by endothelial cells of lymphatic vessels
- DCs move up the concentration gradient of the CCL21 hence it moves into lymphatic vessel and up along it
2) Chemokine Ligand 19 (CCL19)
- Expressed by stromal cells in the T cell zone of lymph nodes
- DCs home in towards this ligand to reach the T cell zone of lymph node
Antigen presenting cells
DCs are the only known cells able to present antigens to and activate naive CD4 (via MHCII) and CD8 (via MHCI) T cells
- antigen presentation regulated by maturation
The MCHII pathway/activation
Activating CD4 cells:
1) MHCII molecules synthesised in the ER are associated with the Invariant chain (I molecule)
- prevents MHCII binding other molecules in ER
2) The I molecule directs MHCII trafficking via the Golgi to the MHCII Compartment (MIIC), an endosomal compartment enriched in MHCII
3) In the MIIC, the I molecule is removed by proteolysis freeing up the MHCII’s ‘peptide binding groove’
4) Simultaneously, endocytosis of antigens occurs and the antigens are delivered to the MIIC to undergo proteolysis into smaller peptide fragments that bind MHCII
5) MHCII/peptide complex can be displayed on cell surface
MIIC in Immature DCs (antigen presentation = off)
Reduced acidity of MIIC results in low protease activity so I chains & antigens can’t be chopped up
- Reduced acidity due to low activity of Vacuolar proton pump
High levels of protease inhibitor Cystatin C
- inhibits Cathepsin S needed for cleaving I chain as well as chopping certain antigens
Cell surface MHCII molecules rapidly endocytosed
MIIC in Mature DCs (antigen presentation = on)
High activity of vacuolar proton pump increases acidity of MIIC whilst there are reduced levels of Cystatin C
- high protease activity
Limited endocytosis of cell surface MHCII
The MHCI pathway/activation
Activating CD8 cells:
1) In cytosol, antogen processing occurs (tube-like proteosome complex cuts viral antigen)
2) The short peptides are transported into the ER lumen (transporter associated with antigen processing [TAP])
3) MHCI folding with aid of chaperone molecules
4) MHCI associates with tapasin which aids in the binding of antigenic peptides to the MHCI
5) MHCI-antigen complex transported to cell surface to bind with CD8+ T cells
Immature DCs: moderate MHCI expression levels
Mature DCs: high MHCI expression levels
DC cross presentation
‘Breaking the rules’ and presenting exogenous antigens on MCHI molecules:
- important in initiation of adaptive immunity and allows DC to activate CD8 even for infectious agents that can’t infect DCs
1) DCs take up exogenous antigens (even if it wasn’t infected) by macropinocytosis/phagocytosis
2) Merging with classical MHCI pathway to bind antigen to MHCI in ER
T cell activation
Costimulation (antigen signal alone can’t stimulate T cell)
DCs also provide secondary signals to ensure activation:
B7.1 and B7.2 bind to CD28 receptor on naive T cellss
Costimulatory molecule expression in mature and immature DCs
Immature:
- Moderate MHCI and low MHCII expression
- Low expression of B7.1 and B7.2
Mature:
- High MCHI and MHCII expression
- High expression of B7.1 and B7.2
