Immunology (Liz - Extravasation) Flashcards
What is the first action when a bacteria invades a tissue?
Tissue macrophages release cytokines and chemokines
- Vasodilation and increased vascular permeability
- Inflammatory cells migrate to the tissue
What are the 4 signs of inflammation? What are they caused by?
- Redness
- Heat
- Swelling
- Pain
First 3 causes by vasodilation and increased vascular permeaility
Pain causes by release of inflammatory mediators by inflammatory cells
Why do lymphocytes continually circulate around the body?
To give maximum chance of interacting with foreign antigens and microorganisms within the tissue or on APCs in the lymph nodes
How many lymphocytes are specific for and will bind to a particular antigen?
1 in 100,000
Essential that lymphocytes travel widely in body
What is needed before a lymphocyte can enter an inflammed tissue or a peripheral lymphoid organ?
Must ADHERE to and PENETRATE the layer of endothelial cells lining the wall of blood vessels
What is extravasation?
The process by which lymphocytes pass through the walls of blood vessels
Where is the only place that lymphocytes can migrate from the blood into the lymph nodes?
- Specialised areas in the post capillary venules called high-endothelial venules (HEVs)
What is the role of cell adhesion molecules (CAMs)?
- Vital for allowing cells to adhere to correct endothelium
Where are CAMs expressed and what do they bind to?
- Expressed on the endothelial cells lining the blood vessels
- Capable of binding to ligands found on the surface of leukocytes
Are CAMs expressed constitutively, only during an inflammatory response or both?
Both
- Some CAMs expressed all the time
- Some expressed in response to localised concentrations of cytokines produced during an inflammatory response
What cells have CAMs that bind to vascular endothelium?
Recirculating lymphocytes, monocytes and granulocytes
What other immune interactions are CAMs used in?
- Th cell binding to an APC
- Th cell interacting with a B cell
- Tc cell binding with a target virally infected cell
The ‘glue’ that holds cells together while antigen presentation takes place
What are the 4 families of CAMs?
1) The SELECTIN family
2) MUCIN-LIKE
3) INTEGRIN
4) IMMUNOGLOBULIN super-family
What are SELECTINS and what do they bind to?
- Family of membrane glycoproteins containing a lectin-like domain
- Lectin-like domain binds to specific carbohydrate (CHO) group on Mucin-like CAMs
What selectin is found on leukocytes?
L-selectin
What selectins are found on vascular endothelial cells during an inflammatory respose?
E-selectin and P-selectin
What Mucin-like CAMS on endothelial cells do L-selectins bind to?
- CD34
- GlyCAM-1
What is the first step of extravasation and what adhesion interaction is responsible?
- Adhesion between a fast-moving lymphocyte or neutrophil and the static endothelial layer
- Selectin/Mucin CAM interaction
What are MUCIN-LIKE CAMs and what do they bind to?
- Group of serine/threonine rich proteins
- Heavily glycosylated
- Many carbohydrate side chains that bind to SELECTINS
Give some examples of MUCIN-LIKE CAMs and what they bind to?
CD34 and GlyCAM-1 on endothelial cells both bind to L-selectin of leukocytes
What are INTEGRIN CAMs and where are they found?
Group of heterodimeric proteins
- consist of alpha and beta chain
- Expressed on surface of leukocytes
Integrins are grouped into categories based on what?
Which beta subunit they contain
What influences tight binding of leukocytes to endothelial surfaces?
Chemokines that activate the intergrin expression (e.g LFA-1) on the leukocyte
What do integrins bind to?
IMMUNOGLOBULIN-LIKE CAMs
What are IMMUNOGLOBULIN LIKE CAMs? Where are they found?
Group of CAMs that contain a variable number of immunoglobulin-like domains
Expressed on vascular endothelial cells
What do immunoglobulin-like CAMs bind to? Give some example molecules
Bind specifically and tightly to INTEGRINS on the surface of leukocytes
e.g.
ICAM-1, -2, -3 (immunoglobulin) binds to VLA-4 (integrin)
LFA-3 binds to LFA-1
Summarise the 4 stages in extravasation, briefly stating the molecules involved
1) Rolling
- L-selectin
2) Activation
- CCL21 (chemokine)
3) Adhesion
- LFA-1 (integrin)
4) Diapedesis (passage of cell through the wall)
- CCL21, CXL12
(chemokines)
Using neutrophils as an example, describe the rolling phase
1) E-selectin is induced on vascular endothelial cells by inflammatory signals
2) E-selectin binds to the sialyl-Lewis-x tetrasaccharide on a mucin-like CAM molecule on the neutrophil
3) Interaction is weak and therefore neutrophil rolls along the endothelium
Using neutrophils as an example, describe the activation phase
1) A chemokine (e.g. CXCL8) binds to its specific receptor (CXCL8R) on the neutrophil
2) This triggers the activation of integrins such as LFA-1
Using neutrophils as an example, describe the adhesion phase
1) TNF-alpha induces expression of ICAM-1 on endothelium
2) LFA-1 expressed on neutrophils binds to ICAM-1
3) Tight binding stops rolling completely
Using neutrophils as an example, describe the diapedesis phase
Neutrophil moves through endothelium using a number of factors:
1) Chemotaxis through tissue CXCL8 and CXCL8R
2) Interactions between CD31 (PE-CAM) on both neutrophil and endothelium physically pulls cell through
3) Metalloproteinases produces by neutrophils cut through the matric of the basement membrane
4) Leukocytes then follow IL-8 chemokine to infection area
How is homing of naive T cells to endothelial tissue achieved?
- L-selectins found on naive T cell surface
- L-selectins recognise carbohydrates on Mucin-like CAMs on endothelial tissue
What Mucin-like CAMs to T Cells bind to on both lymph node and mucosal endothelial?
Lymph node:
- CD34
- GlyCAM-1
Mucosal:
- MAdCAM-1
Where do lymphocytes move from vascular endothelium into lymph nodes?
- Regions of vascular endothelium found in the post-capillary venules of the lymphoid organs called High Endothelial Venules (HEVs)
- They are composed of an area of specialed cells with plump cuboidal shape
How many lymphocytes extravasate every second through HEV areas into a single lymph node?
14,000
What influences the development and maintenance of HEVs?
Influenced by cytokines produced in response to foreign antigens
What is HOMING?
The mechanism by which lymphocytes traffic to the appropriate location
Where do naive T cells home to and why?
Homing to secondary lymphoid tissue in order to become activated via antigen/MHC recognition by APCs
Where do effector T cells home to and why?
Home directly to inflamed vascular endothelium to help fight an infectious agent
What are chemokines?
A family of small polypeptides (90-130 AAs) that selectively control the adhesion, chemotaxis and activation of many types of leukocytes
Signalling pathways stimulated by chemokines cause what 2 changes?
1) Cell adhesion
2) Cytoskeletal changes that allow migration in a paticular direction and then adherence
Inflammatory cytokines are typically produced in response to what?
Infection and/or secretion of pro-inflammatory cytokines e.g. TNF-alpha
How does chemokine signalling occur and what is the result?
- Signals through binding to receptors coupled with heterotrimeric large G proteins
- Results in modifications to target cell allowing chemotaxis towards infected tissue
What dictates the migration pattern of leukocytes?
The pattern of expression of chemokine receptors
Which leukocyte type has the greatest variety of chemokine receptors and why?
T cells, so they can migrate everywhere
Describe the cascade of cells arriving at injected tissue due to chemokine action
1) Neutrophils arrive first and bind to CXCL8 chemokine using CXCL8R and produce a respiratory burst
(Respiratory burst generates ROS and they therefore release their granules to kill tissue microbes)
2) Monocytes and DCs arrive next in response to chemokines
- become activated
- allows then to present antigens to Th cells as they arrive
3) Then Th cells arrive
- attracted via a large range of chemokines via specific chemokine receptors
What are the strucutres of CC and CXC chemokines and what receptors do they bind to?
CC:
- Have 2 cysteine residues next to each other
- BInd to CCR G protein-coupled receptors 1-9
CXC:
- Same cysteine residues but are separated by one AA
- Bind to CXR G protein-coupled receptors 1-6
To summarise …
As different chemokine receptors are found on different types of immune cell, a particular chemokine can be used to attract a particular cell to a particular tissue
How are HEVs and chronic inflammatory disease related?
HEV-like regions (plump endothelial cells) observed in a number of chronic inflammatory diseases
- Allow large scale influx on leukocytes expressing L-selectins, contributing to inflammation of the affected area
What is Leukocyte Adhesion Deficiency (LAD)?
- An inherited disease preventing leukocytes from homing to and entering infected tissue
What causes LAD?
- Deficiency in the leukocyte integrin subunit CD18
- CD18 common beta chain of 2 intergrins: LFA-1, CR3 and CR4
- Levels of leukocytes high in the blood because they cannot migrate to and enter infected tissue to fight infection
What is the result of LAD?
- Repeated bacterial infections
- Problems with wound healing
- Inflammation of the gums (gingivitis)
