Immune Cell Migration Flashcards
what is the afferent lymphatic?
into the lymph nodes
what is the efferent lymphatic?
out of the lymph nodes
what is a key problem for the immune system?
- pathogens can establish anywhere
- Needs T and B cells to detect the pathogens
- these cells are highly specific and therefore are very rare
- to solve this immune cells circulate the blood and the lymph
- to get cells to the right place you need chemokines and adhesion molecules
what are chemokines?
chemotactic cytokines
- secreted molecules that bind to cell surface GAGs
- chemokines bind back to the cells that produce
how is a chemokine gradient formed?
- have the highest chemokine concentration on its surface
- any that dont bind become tethered to the ECM proteins
- tethered chemokines form immobilised concentration gradient
how are chemokines sense?
- target cells express chemokine receptors
- chemokine receptors are coupled to heterotrimeric G proteins
what is the structure of a chemokine receptor?
- alpha, beta and gamma
- alpha is bound to GDP
- when chemokine binds it causes a conformational shift, causes activation
- G proteins associate
- exchange GDP for GTP which initiates a signalling cascade
what happens when a chemokine binds to a receptor?
there is cytoskeletal rearrangement and the cell migrates
what is a CC chemokine?
- towards the end terminus they have cysteine residues
- involved in disulphide bonds
what is a CXC chemokine?
got cysteine residues but with any amino acid in between
what kind of leukocytes can be found in the blood?
T cells; CD4 and CD8, B cells, neutrophils, monocytes
what is lymphatic circulation?
- lymph transmits signals from tissues to draining lymph nodes
- carry lymph from peripheral tissues to draining lymph nodes
- transports immune cells, intact pathogens, free antigens
where are adaptive immune responses initiated?
in draining lymph nodes
why does the lymph node also have blood vessels?
- T cells can either circulate or they can interact and migrate into the lymph node
- this would be the HEV (high endothelial vessel)
- carry signals, dendritic cells which interact with T cells
how do T cells exit the blood and enter the lymph node?
- blood vessels convert into HEV
- T cells can interact
- EXTRAVASATION
- blood moves quickly, cells need to slow down
- rolling, activation, firm adhesion, transmitgration
what is rolling (stage 1 of extravasation)?
- lectin dependent
- Naive T cells express cell surface protein L selectin (CDG2L)
- binds to carbohydrates on endothelial cells on HEV, recognises the glycoproteins
- different lymph nodes can have different glycoproteins
what are HEV surface glycoproteins called in rolling?
‘addressins’
what are the addressins found on HEV in rolling?
GlyCAM
CD34
what are PNAd in rolling?
- peripheral node addressins
- addressins for peripheral lymph nodes are PNAd
in rolling what is the interaction between L selectin and PNAd like?
weak/transient (on/off kinetics)
what is activation and firm adhesion (stages 2 and 3 of extravasation)?
- chemokines convert rolling to firm adhesion
- have chemokines on the surface of HEV
- display 2 chemokines on its surface GAGs (CCL19 and CCL21)
- naive T cell express CCR7, the receptor for CCL19 and CCL21
how does chemokine signalling lead to integrin activation in activation and firm adhesion?
- T cells have integrin LFA1 on their surface
- endothelial cells have adhesion molecule ICAM1
- LFA1 has alpha1 and beta2
- LFA1 binds to ICAM1
- chemokines signalling activates and causes a conformational shift in surface integrin’s
- conversion to high affinity ICAM1 binding = firm adhesion
what structure do integrins have?
heterodimers
alpha and beta subunits
why does activation and firm adhesion in extravasation need signalling?
in the absence of signalling LFA1 and ICAM1 the interaction is very weak, LFA1 starts in a low affinity conformation
what is transmigration in extravasation?
- naive T cells produce proteases to transmit between endothelial cells and through the basement membrane
- metalloproteases
how do T cells have directed movement in the lymph node?
- T cells need to get to the T cell zone
- T cell zone contains specialised fibroblasts that produce CCL19 and CCL21
- CCL19/21 gradient attracts CCR7+ T cells to the T cell area
how do dendritic cells have directed movement in the lymph node?
- tissue denritic cells upregulate CCR7 in response to PAMPs and inflammatory cytokines
- responsive to the same chemokine as T cells
- will go to the same area as the T cells and can interact
how do B cell have directed movement in the lymph node?
- B cell zone contains fibroblasts that produce CXCL13
- CXCL13 gradient attracts CXCR5 B cells to B cell follicles
what happens when there is prolonged exposure to chemokines in the lymph node?
causes transient down-regulation of chemokine receptors
how do leukocytes leave the lymph node?
- leaving the lymph node (S1P gradient)
- become responsive to SIP
- high outside the lymph node and forms a gradient
- exit via efferent
what is normal unifected tissue like?
- under homeostatic conditions blood immune cells rarely enter normal tissues
- unlike HEV normal blood vessel endothelial cells have low levels of adhesion molecules and chemokines
how is change in blood vessels during infection initaited?
- pathogen recognised by macrophages
- PAMPs recognised through PRR
- macrophages produce cytokines
- blood vessels have TNF receptors and become activated
what molecules are produced to attract immune cells through the blood vessels during infection?
- TNF upregulates adhesion molecules on endothelial cells
- activated tissue resident macrophage produce inflammatory cytokines
- chemokines transcytose the endothelial cell barrier
- neutrophils express CXCR1 and CXCR2, receptors bind CXCL8
- monocytes expressed CCR2, receptor binds CCL2
- chemokine receptor signalling causes the activation of integrins
what happens when blood vessels have TNF receptors and become activated?
- causes blood vessels to start expressing high levels of selectin adhesion molecules
- neutrophils and monocytes have glycoproteins on their surface which bind to the selectins
- result in the rolling of leukocytes
what inflammatory cytokines will resident macrophage produce?
CXCL8
CCL2
what happens when integrins are activated through chemokine receptor signalling?
- integrin LFA1 is expressed by neutrophils and monoccytes when the receptors and ligands bind
- recognise strongly and get firm adhesion
- migration across
how are pathogens killed?
- neutrophils phagocytose pathogens and secrete toxic granules
- netocytosis
- monocytes develop into macrophages
what are homeostatic chemokines?
constitutively expressed to promote immune cell rolling to target organs
what are inflammatory chemokines?
induced by infections/inflammation to recruit immune cells to sites of inflammation
what do you need chemokine signatures for different infections?
different pathogens need to be controlled with different immune response
what chemokines are produced for a viral infections?
- induce interferom gamma production
- sitmulate macrophages to make CXCL9/CXCL10/CXCL11
- they bind CXCR3
- recruit antiviral T cells and NK cells
what chemokines are produced for parasitic worm infections?
- induce interleukin-13 production
- IL-13 stimulates macrophages to make EOTAtuxius
- Eotaxius bind to CCR3 to recruit anti-parasite eosinophils
