Leukocyte Circulation and Migration into Tissue Flashcards
What is migration or recruitment?
The general process of leukocyte movement from blood into tissues
What is recirculation?
The ability of lymphocytes to repeatedly go to secondary lymphoid organs(home), reside there transiently, and return to the blood.
What is leukocyte homing?
The migration of a leukocyte out of the blood and into a particular tissue, or to a site of an infection or injury
What is a major part of the process of inflammation?
The recruitment of leukocytes and plasma proteins from the blood to sites of infection and tissue injury
What is inflammation triggered by?
Recognition of microbes and dead tissues in innate immune responses and is refined an prolonged during adaptive immune responses
What is the inflammatory response?
A response that delivers the cells and molecules of host defense to the sites where offending agents need to be combated
What process is responsible for causing tissue damage and underlies many important diseases?
Inflammatory response
What underlies the trafficking of all subsets of circulating leukocytes?
A cascade of adhesive and activation events
What can circulating leukocytes do through transient interaction of selectins and integrin with their receptors?
They can tether to and roll on the endothelium
What does rolling allow leukocytes to do?
To sample the endothelium for chemokines which activate integrins and trigger adhesion to the endothelium
What happens to adherent leukocytes?
They migrate across the endothelium into the extravascular space, once there they can migrate down chemokine gradients towards specific areas
What constantly circulates through the blood, into tissues and often back into the blood again?
Immune cells and soluble components
What happens to endothelial cells at sites of infection and tissue injury?
They are activated by cytokines secreted by resident macrophages and mast cells at these sites
What is the consequence of activated endothelial cells?
The consequence is increased adhesiveness of the endothelial cells for circulating myeloid leukocytes and Ag-activated effector and memory lymphocytes
What do leukocyte/lymphocyte homing and recruitment require?
The temporary adhesion of the leukocyte to the endothelial cells of blood vessels
What does homing involve?
It involves molecules on the surfaces of both the leukocytes (hooming receptors and chemokine receptors) and endothelial cells (chemokines)
What do addressins comprise of?
A set of several adhesion molecules on the surface of endothelial cells which regulate lymphocyte recirculation via HEV
What do endothelial cells express?
P-selectin and E-selectin
What causes endothelium to express P-selectin?
When endothelium is activated by histamine or thrombin
When is E-selecting synthesized and expressed on endothelial cell surface?
Within 1 to 2 hours in response to the cytokines IL-1 and TNF produced by tissue macrophages in response to infection
What are the ligands for E-selectin and P-selectin?
Complex sialylated carbohydrates related to the Lewis X or Lewis family
What is only expressed on leukocytes and lymphocytes but not on endothelial cells?
L-selectin
What is L-selectin expression activated by?
IL-1 and TNF at sites of inflammation
What is L-selectin important for?
For naive T and B lymphocytes for homing into LNs by interacting with high endothelial venules (HEV)
What are peripheral node addressins (PNAd)?
L-selectin ligands expressed on HEV
What are integrins?
Integrins are heterodimeric cell surface proteins composed of two noncovalently linked polypeptide chains
How many types of integrins are there?
More than 30 different integrins, all with the same basic structure
What do integrins mediate?
Mediate adhesion of cells to other cells or to extracellular matrix via various specific ligands
What is the function of integrins?
To integrate signals triggered by extracellular ligands with cytoskeleton-dependent motility, shape change, and phagocytic responses
Leukocytes that bind to ligand intercellular adhesion molecule 1 expressed on cytokine-activated endothelial cells express what?
LFA-1
Leukocytes that bind to vascular cell adhesion molecule 1 VCAM-1, expressed on cytokine-activated endothelial cells, express what?
VLA-4
What do leukocytes express that binds to VCAM-1 and MadCAM (that is expressed on endothelial cells)?
alpha4 beta7
How are integrins able to respond to intracellular signals?
By rapidly increasing their affinity for their ligands (integrin activation)
What occurs in all leukocytes in response to chemokine binding to chemokine receptors?
Integrin activation
When does Integrin activation in T lymphocytes occur?
When Ag binds to TCRs
What is integrin activation mediated by?
Chemokine-induced conformational changes in the extracellular domains of the integrins that lead to increased affinity
What is “inside-out signaling”?
A type of chemokine signaling that induces conformational changes in the extracellular domains of the integrins that mediate integrin activation
The integrins on blood leukocytes are normally in what state?
Low affinity state
When rolling of leukocytes occurs, what can the chemokines do?
Chemokines displayed on the endothelial surface can bind chemokine receptors on the leukocyte
When chemokine receptor signaling occurs what happens to the leukocyte?
The leukocyte integrins are activated, increasing their affinity for their ligands on the endothelial cells
When a ribbon diagram shows a bend conformation of a leukocyte integrin, what state is the integrin in?
Low affinity
When a ribbon diagram shows an extended conformation of a leukocyte integrin, what state is the integrin in?
High affinity
What cells are ready to be recruited into tissue sites of infection or injury without activation?
Neutrophils and monocytes
How do neutrophils and monocytes enter the tissue?
Through post-capillary venules except parenchymal tissues (liver, lungs, kidney) where all the blood cells enter through capillaries
What do myeloid leukocytes do?
Eliminate infectious pathogens, clear dead tissues, and repair the damage
what induces the rapid induction of expression of pre-formed P-selectin on the surface of endothelial cells?
Activated resident macrophages produce cytokines TNF and IL-1 that induce the rapid induction
What else can be induced on the surface of endothelial cells along with P-selectin by cytokines?
E-selectin
What initiates rolling on the endothelium by the leukocytes?
Interaction between P-selectin on endothelial surface and ligands on leukocytes
How is further rolling of leukocyte stabilized?
By L-selecting binding to ligands on endothelial cells.
What do endothelial cells present that is recognized by receptors on neutrophils? What do these do?
A chemokine CXC-chemokine ligand 8 (CXCL8 or IL-8)
Activates leukocyte integrins and results in a more stable leukocyte adhesion
What controls transmigration of monocytes?
CCL2 or MCP-1
What happens once leukocytes have completed trans-endothelial migration?
They interact with chemokines in the underlying basement membrane
What follows diapedesis?
Cells adopt an amoeboid shape
What do the amoeboid shape leukocytes do after entering the tissue?
They migrate in a polarized fashion along collagen fibrils in response to a chemotactic gradient towards the site of infection/inflammation
Where are chemotactic receptors located?
In the leading edge
How do the mechanism of recruitment for neutrophils compare to the mechanism of recruitment of monocytes?
Recruitment of neutrophils require chemokine IL-8 and monocytes require chemokine MCP. Everything else is identical
How do Naive T cells enter the LNs?
Preferentially leave the blood and enter the LNs across the high endothelial venules (HEV)
What enters the LNs through lymphatic vessels?
Dendritic cells bearing Ag
How are T cells activated in the LNs? What follows the activation?
If the T cells recognize Ag, they become activated and they return into the arterial circulation
How do effector and memory T cells travel to sites of inflammation?
They preferentially leave the blood and enter peripheral tissues through venules at sites of inflammation
What is expressed by both T and B lymphocytes?
L-selectin and cell adhesion molecules - but each type utilize a distinct cell adhesion molecule (integrin)
What are chemokines?
A large family of structurally homologous cytokines of 8-to kD that contain two internal disulfide loops
What do chemokines do?
Stimulate leukocyte movement and regulate the migration of leukocytes from the blood to tissues
What is the name chemokine a contraction of?
Chemotactic cytokine
How many human chemokines are there?
50 that are classified into four families on the basis of the number and location of the N-terminal cysteine residues
What are the four families of chemokines in humans?
- CC or beta chemokines: two defining cysteine residues are adjacent
- CXC or alpha chemokines: Residues are separated by one amino acid
- C chemokines: single cysteine
- CXXXC chemokines: two cysteins are separated by three amino acids
How was the biologic importance of L-selectin demonstrated?
By experiments showing the L-selctin deficient lymphocytes are unable to enter peripheral nodes and “see” the Ag
How do Naive lymphocytes enter the LN?
Through the HEV
High endothelial venule
What are L-selectin ligands expressed on HEV called?
peripheral node addressins (PNAd)
Why do naive T lymphocytes home to lymph nodes?
Because of the result of L-selectin binding to peripheral lymph node addressin on HEV
Where are HEV present?
Only in secondary lymphoid organs
What are displayed on the surface of HEV?
Chemokines CCL19 and CCL21
Chemokines binding to _________activates integrins?
CCR7 (chemokine receptor on Naive T cell)
What do Naive T cells that have recently entered the LN have? Why?
Low levels of S1PR1 because the receptor is internalized after binding S1P in the blood
What cant Naive T cell, that have recently entered the LN, sense?
Cant sense S1P concentration gradient between the T cell zone of the node and the lymph
Why and how long are naive T cells stuck in the LN if there is no antigen recognition?
They cant exit the node for several hours until S1PR1 is re-expressed and the cell can then exit
What happens if naive T cells recognize antigens in the LN?
They continue to have low levels of S1PR1 and stay in the LN. S1PR1 is re-expressed after several days. Then activated/differentiated effector T cells then sense the S1P gradient and exit the node
Where do activated effector T cells home?
To sites of infection n peripheral tissues
How is T cell migration into tissue mediated?
It is mediated by E-selectin and P-selectin, integrins, and chemokine (CXCL10) that are produced at sites of infection
What are involved in effector/memory T cell migration?
Many chemokines and chemokine receptors
What is the function of the receptor, L-selectin, and ligand, PNAd pair? on naive t cell
Initial weak adhesion of naive T cells to high endothelial venule in lymph node
What is the function of the CCR7 (chemokine receptor on naive T cell) and CCL19 or CCL21?
Activation of integrins and chemokinesis
What is the function of LFA-1 (beta2 integrin) receptor and ICAM-1?
-on naive T cells
Stable arrest on HEV in lymph node
On activated (effector and memory) T cells, what is the initial weak adhesion of the T cells to cytokine activated endothelium at peripheral sit of infection?
E and P selectin ligand on the T cell with the E or P selectin on the endothelium surface
On activated (effector and memory) T cells, what activates integrins and chemokinesis?
CXCR3 or CCR5 with CXCL10 or CCL4 respectively
On activated (effector and memory) T cells, what gives stable arrest on cytokine-activated endothelium at peripheral site of infection?
Beta 1 or 2 integrin with ICAM-1 or VCAM-1
What is CD44 important for?
The mobilization of effector T cells in sites of infection and inflammation
At sites of infection and inflammation, what do endothelial cells secrete?
Secrete chemokines and express E and P selectin and hyaluronic acid (HA)
What can CD44 mediate?
Rolling interactions with vascular endothelial cells that express HA, its natural ligand, or even E-selectin
What does chemokine signaling via GPCRs result in?
Increased integrin affinity, such as VLA-4, which enhances adhesiveness (“inside-out” signaling)
What does the engagement of CD44 with HA also trigger?
GPCR-dependent “inside-out” upregulation of VLA4-VCAM binding
How do Naive B cells home to secondary lymphoid tissues?
Same basic mechanisms as do naive T cells
Once immature B cells leave the bone marrow, where do they go?
Through the blood, enter the red pulp of the spleen, and migrate to the periphery of the white pulp
As B cells mature further, what do they express? What does this do for the B cell?
Chemokine receptor CXCR5, which promotes their movement into the white pulp in response to a chemokine called CXCL13
Once the maturation is completed within the white pulp, what happens to the Naive B cell?
Reenter the circulation and home to lymph nodes and mucosal lymphoid tissues
What does the homing of mature naive B cell from the blood into LNs involve?
Rolling interactions on HEVs, chemokine activation of integrins, and stable arrest.
Same for naive T cells
What chemokines does the homing process of Naive B cells require?
CXCL12, CCL19/CCL21 on HEV and their chemokine receptors CXC$4 and CCR7 on naive B cells
In the stroma, how are B cells migration into follicles driven?
By CXCL13 recognized by CXCR5 expressed on naive b cells
What may B cells encounter in follicles?
Ag and become activated
