Lecture 9 - Mobilising immunological army and generation of long term immunological memory Flashcards
Why has a transport system evolved for transport of immune cells?
-stops autoreactive cells from being able to come into contact with receptive ‘self’ cells
What problem does the lymphatic vessel network present for the immune system?
-immune system must be able to efficiently direct the APCs niave T cells and effector cells during an immune response
What three things is the trafficking of immune cells dependent on?
- inflammation
- chemokines
- adhesion molecules
How is inflammation involved in the transport of immune cells?
inflammation involves:
- increase in vascular diameter (faster flow quicker T cells to lymph node)
- decrease in velocity of flow rate allow inner wall of capillaries (allow T cells to exit when recieve correct signal)
- swelling - cells move apart for transport of molecules to/from lymph nodes leading to leakage of contents
How does inflammation initiate the movemement of immune cells
by inducing expression of cytokines
1) pro-inflammatory cytokines e.g. tumour necrosis factor alpha, initates movement of cells and allow endothelial cells to be permissable to T cells
2) chemotactic chemokines = direct the action of cells
What is the structure of chemokine receptors?
- seven transmembrane domains
- G protein coupled
How does chemokine receptor signalling occur?
1- signalling through chemokine receptors mediated by an intracellular signalling molecule (heterotrimeric G protein)
2 - in inactive state G protein has bound GDP, a chemokine binding to its receptor (GPCR) causes conformational change in receptor enabling it to associate with G protein as GDP replaced by GTP
3) G protein dissociates into alpha and gammabeta subunits which activate other proteins: GTpases Rac, Rho and CDC42 which stimulates chemotaxis/respiritory burst
4)alpha subunit cleaves GTP to GDP allowing alpha and gammabeta subunits to reassociate terminating signal
What does the dissociation of alpha and gammabeta heterodimeric G protein activate?
1) enables secretion molecules
2) change in surface molecule structure
3) changes motility of a cell
How do chemokines promote movement of cells?
By building a chemokine gradient, the stronger concentration of chemokines is where cell will move to, the leading edge of the cell
THEREFORE in immune response the concentration of appropriate chemokine highest when immune system wants immune cell to move to
What are the two types of chemokines?
1) Constituative/Basal trafficking or homing chemokine
2) Inducible/Inflammatory Chemokines
What are the features of constituative/basal trafficking or homing chemokines?
- expressed constituatively at homing sites e.g lymph node, skin, mucosa
- receptors mainly on lymphcytes
- regulate adaptive immunity - move cell through lymph nodes to make sure an adaptive response can take place
What are the features of the inflammatory/inducible chemokines?
- only induced when infected
- produced by stomal cells and infiltrating cells at the site of infection
- recruit effector cells: monocytes, granulocytes, effector T cells
- high redundancy, lots of chemokines mediate same effect
What chemokine receptors do niave T cells have?
receptors for constituative chemokines
What chemokine recpetors fo effector T cells have?
inflammatory chemokines
How are chemokines involved in the migration of lymphatics to the lymph nodes?
1) pathogens induce inflammatory cytokines/chemokines from macrophages
2) Immature APC expresses receptors which respond to inflammatory chemokines and move to site of infection and pick up antigen
3) After antigen uptake APC downregulates receptors for inflammatory chemokines and upregulates receptors for constituative trafficking chemokines and migrate to lymph node
What is CCR7?
- receptor for CCL19 and CCL21
- expressed on activated dendritic cells, niave T & B cells and central memory T cells
What are CCL19 andCCL21?
- expressed by high endothelial venules (HEV)
- found in the lymphatic and stroma in lymph nodes
What is Extravasation?
The movement of immune cells out of blood vessels into tissue
What is the process of lymphcyte exit from blood vessels and entry into tissues?
1) Rolling - when near entry point, cells slow down, attach to endothelial cells (primary adhesion) and roll along endothelial cells
2) Leukocyte activation - chemokines trigger conformational change of adhesion molecules on leukocyte
3) Firm adhesion (secondary adhesion) - activation dependent adhesion
4) Diapedesis - movement through basement
What selectins, chemokines and integrins control leukocyte exit from capillaries?
Rolling - controlled by selectins
Leukocyte activation - controlled by chemokines and inflammatory stimuli
Firm adhesion - integrins and inflammatory stimuli
Transmigration and diapedesis - metalloproteinases
What selectins is primary adhesion mediated by?
- E selectins (expressed on endothelial cells, mediate neutrophil attraction, upregulated by pro-inflammatory mediators (IL-1, TNF, LPS))
- P-selectins (on activated platelets and activated endothelial cells)
- L-selectins (constitutatively expressed on lymphocytes, down regulated upon activation)
How does L-selectin on niave T cells bind to HEV lining secondary lymphoid tissues?
by binding vascular addressins (ligands contain carbohydrates)
What do vascular addressins do?
Direct entry of leukocytes into particular tissues e.g. MadCAM-1 directs seletin-bearing cells into gut mucosa
What initates the rolling response?
Binding of selectins to addressins
What do integrins do in the migration of immune cells?
Integrins involved in stopping cell rolling and priming to migrate out of blood vessel
What ‘tethers’ cells and how?
chemokines on extracellular matrix (induce conformational change in integrins) e.g. LFA-1
How is firm adhesion mediated?
activation of integrins by chemokines results in a conformational change which transform binding to their ligand from low to high affinity
What is the structure of an integrin?
heterodimeric proteins with alpha and beta subunits
What do integrins bind?
Integrins bind members of the Ig gene superfamily
What endothelial cell does the LFA-1 leukocyte bind to?
ICAM-1, -2, -3
What endothelial cell does the Mac-1 leukocyte bind to?
ICAM-1
What endothelial cell does the VLA-4 leukocyte bind to?
VCAM-1
What is ICAM?
- intracellular adhesion molecule
- expressed on EC and leukocytes at low level
- same structure as IgG
- up regulated by IL-1, TNF
What is VCAM?
- vascular cell adhesion molecule
- expressed on ECs at site of inflammation
- upregulated by IL-1, TNF
How do metalloproteinases mediate diapedesis?
Create a pore
What is diapedesis?
movement of cell through basement membrane
How does an effector T cell get into infected tissue?
1) CCR7 must be lost so can no longer respond to chemokines in lymph node
2) upregulate integrin VLA-4 move out lymph node and go to site of infection
3) T cell becomes activated, cell surface moleules change, express integrins LFA-1 and VLA-4 bind ICAM and VCAM-1 on activated endothelium
4) Leads to extraversion into inflammed/infected site
What is a niave cell?
specific for an MHC/peptide but yet to encounter it
What is an Activated cell?
proliferating and gaining effector function, changing cell surface molecules
What is an Effector cell?
can release all effector molecules necessary to deal with pathogen
Interaction between what triggers T cell apoptosis?
FasL and Fas
What is the process of effector cell death?
1) Fas L binds to Inactivated Fas receptor on T cell surface
2) causes clustering of death domains at cytoplasmic end of Fas which recruits adapter molecules (e.g. FADD Fas associated death doman)
3) Adapter molecules bind to pro-caspases (e.g. procaspase 8)
4) Increasing concentration of procaspases at cytoplasmic tail of Fas results in caspase activation triggers apoptosis
What are the two theories of Memory cell production?
1) Niave cells differentiate into memory and effector cells
OR
2) Activated cells differentiate into memory and effector cells
What are the properties of memory T cells?
- long lived, small ‘resting’ cells
- don’t need presence of pathogen to maintain themselves
- numbers consistant throughout life suggesting balance between proliferation and death
- express survival genes e.g. Bcl-2, antiapoptotic gene
- requires cytokine interleukin 7 for survival -> memory CD8 cells also require IL-15
- have higer levels of integrins LFA-1 and CD2 on surface compared to niave cells
- cytokine or cytolytic cells faster and more powerful than inital activation response
What are the two types of memory cells?
1) effector memory cells
2) central memory T cells
What are the properties of effector memory T cells?
- CCR7 negative
- rapidly mature into effector cell
- secrete large quantites of cytokines early after stimulation
- express receptors for pro-inflammatory chemokines and intergrins
- specialised for rapidly entering inflamed tissue
What are the properties of central memory T cells?
- CCR7 positive (recirculate through lymph nodes)
- take longer to be activated than effector memory T cells
- do not secrete large amounts of cytokines
- upon restimulation differentiate into effector memory T cells
- useful for infection in lymph node
Where will memory/effector T cells first migrate to?
Region of body where first activation
