Immune cell trafficking

Question 1

Initiating the adaptive immune response:

What happens early in infection?

Answer 1

• Early in infection, the necessary players (T cells and B cells) are scarce
• Naïve T and B cells circulate around all the the secondary lymphoid organs at random looking for what antigen the dendritic cells are presenting in their MHC
• If the naïve T cell doesnt see its antigen it will move out through the efferent lymphatics into the afferent lymphatics of the next lymph node to sample that envronment- this will kneep happenng untill that naïve T cell finds what its looking for or dies
• Naïve cells are low in number, and chances are slight that TH will meet the appropriate B cell

Question 2

How can we greatly increase the chances of T_H encountering the B cell?

Is this what usually happens?

Answer 2

• If the T_H proliferate first to build up their numbers and then go looking for their antigen, recirculating around the lymph nodes, we greatly increase the chances of TH encountering the B cell.
• What usually happens is the naiive T cell comes into the lymph node, detects its antigen and then goes through proliferationa and those progeny cells also go into circulation- they can all circulate through the secondary lymphatic organs and be stimulated by dendrtitic cells.

Question 3

Explain how Naive T cells can enter lymph nodes from the blood?

Answer 3

1. T cells enters a lymph node across high endothelial venules in the cortex
2. T cells monitor non-selft antigen and dendritic cells
3. T cells that do not encounter soecific antigen leave the node in the efferent lymph
4. T cells that encounter specific antigen proliferate and differentiate to effector cells
5. One dendritic cell can stimulate many T cells at the same time

Question 4

TRUE or FALSE:

Naive T cells can only enter lymph nodes from the blood

Answer 4

FALSE

Naive T cells can enter lymph nodes from the lymph and from the blood

Question 5

Explain Lymphocyte recirculation

How many lymphocytes move through the lymphatics every day?

How many lymphocytes are in the blood at any one time?

Answer 5

• Traffic between lymphoid/non-lymphoid tissues involves~ 5 x 10¹¹ cells per day
• Only ~2% (1 x 10¹⁰) of these cells are in the blood at any one time
• Lymphocytes only stay in the blood for ~30 minutes
• Circulating blood pool of lymphocytes is exchanged 48 times a day

Question 6

Infection increases the rate of lymphocyte entry to lymph node by how much?

Answer 6

Infection increases the rate of lymphocyte entry to lymph node four-fold with no change in rate of exit

Question 7

Where are dendritic cells most concentrated?

Dendritic cells go to which lymph node?

Answer 7

On vulnerable areas of the body where pathogen can potentially enter e.g. under epithelial surfaces- gut, respitorytract, genitounrinary tract and skin

Dendritic cells go to the nearest draining lymph node by responding to chemical signals from lymph nodes

Question 8

Describe how High endothelial venules let lymphocytes in

Answer 8

1. Circulating lymphocyte enters the high endothelial venule in the lymph node
2. Binding of L-selectin to GLYCAM-1 and CD34 allows rolling interaction
3. LFA-1 is activated by chemokinesbound to extracelular matrix
4. Activated LFA-1 binds tightly to ICAM-1
5. Lymphocyte migrates into the lymph node by diaedesis

Question 9

What is the difference betwen normal endothelial cells and high endothelial venules

Answer 9

Unlike normal endothlial cells which are usually very flat and tightly adhered to each other, high endothelial venules are much taller and not as tightly connected to each other so they let lymphocytes through more easily

Question 10

Lymphocyte recirculation has two functions:

Answer 10

1. maximise cell contact of cells that need to be in the same small location at the same time
2. maintain T_H activation (e.g. with follicualr dendritic cells)

Question 11

Explain what happens after recirculating T_H cells enter the lymph node where their cognate antigen is presented and are re-stimulated

Answer 11

• Some of these proliferate more – recirculate to spread the help
• Some move to the lymphoid follicles to give the activated B cells help
• Some leave the blood to provide the help (cytokines) to the ‘warriors’ - killer T cells, NK and macrophages in the tissue

Question 12

B cells copy the T cells and their cycles of activation, proliferation, recirculation and restimulation

What will some B cells become?

How are these cells terminally different?

What do these cells produce?

Answer 12

• Some B cells will become plasma cells and move to the bone marrow, mucosa or spleen
• Plasma cells are terminally differentiated, so no longer able to talk to TH cells (no MHC class II expression), and no longer able to class switch or alter the binding affinity of the antibody
• Plasma cells produce vast quantities of soluble antibody: 10,000 antibodies per cell per second

Question 13

Explain B cell activation:

Where does this happen?

Answer 13

• Germinal centres: frantic activity for about three weeks:
• Light zone (follicular dendritic cells, T cells and B cells are all interacting with each other) and dark zone (proliferating B cells) at the edges of the lymph nodes and the medulary sinus in the middle (where T cell and dentritic cells interact)
• Most of the opsonised antigen on FDC has been picked up by B cells
• Most B cells will have either left the follicle or died
• Some mature B cells continue to reside in the secondary lymphatic tissue as memory B cells
• Germinal centres shrink back to primary follicles

Question 14

Why do lymphocytes need to be directed?

What happens when a T cell recognises rotavirus in a gut draining lymph node ?

Answer 14

For effective immunity, lymphocytes activated in a particular region of the body need to preferentially return to the same region.

Mesoteric lymph nodes in the spaces around your intestine are vital for coordinating immune responses in the gut:

1. Virus antigen is being picked up by your dendritic cells
2. They are presnted to your cytotoxic T c ells in the lymph nodes]
3. The cytotoxic T cells need to find the infected cells and kill them before the infection can progress
4. T cells is activated in the gut lymph node and gets a signal that a pathogen is in the gut
5. They will get signals to only circulate through the gut lymhatics so will never go to the skin

Question 15

What is the difference between homing and trafficking?

Answer 15

• Trafficking: Non-random movement of cells from tissues, through the blood or lymph. Includes migration to and from sites of lymphocyte maturation as well as homing.
• Homing: Tendency of lymphocytes activated in a particular region of the body to preferentially return to the same region. Includes localisation of cells in distinct regions of lymphoid tissue

Question 16

Draw a schematic view of a lymph node

Explain the nomenclature for chemokines and their receptors

Answer 16

Chemokines starts with CXC or CC

C stands for cysteine

X stands for any amino acid

L stands for ligand

They bind to receptors on a cells surface, the receptors also have the CC or CXC that tells what kind of chemokine bind to the receptor and they have the letter R in their name

Question 17

Homing- Explain Integrins (αβ dimers) and chemokine receptors (CCR, CXCR) in the small intestine

Answer 17

Gut homing T-cell will express CCR9 and the ligand for that chemokine will be produced in the gut→ integrin (α4β7 and αLβ2) expression on the T cell→ high endothelial vessles express the matching ligands for the integrins= the lymph node the T cell can get into is limited

Question 18

Trafficking: naïve T cells

Where are naive T cells restricted from?

Why does this make sense?

Answer 18

Naïve T cells can visit all the secondary lymphoid organs but aren’t allowed into inflamed tissues

This makes sense for two reasons:

1. This trafficking pattern brings naïve T cells into contact with invaders that might have entered the body at any point

This maximises the chances of a naïve T cell seeing its cognate antigen on the dendritic cell

2. There’s no point sending a naïve T cell into the site of infection because it doesnt know what it needs to do e.g. if it needs to be a TH1 cell and fight off a bacterial infection or a TH2 cell and fight off a parasitic worm because it hasn’t recieved that inforamtion yet

Question 19

Describe the life and times of a naïve T cell

Answer 19

• Born in the bone marrow, educated in the thymus
• Use random gene selection to generate specific receptors
• So their cognate antigen might be ANYWHERE
• Naïve T cells let out into the big wide world express a mixture of adhesion molecules – a global passport
• L-selectin on the T cell binds GlyCAM-1, a cell adhesion molecule on the high endothelial venule (lets T cell into lymph node)
• Integrin α4β7 on the T cell binds MadCAM-1 (lets T cell into Peyer’s patches)
• Dendritic cells produce the ligand for CCR7 on T cells which allows them to follow a chemokine gradient (they will find dendritic cells)
• Naïve T cells can circulate through all the secondary lymphoid organs looking for antigen
• This searching will continue for about 6 weeks, with naïve T cells completing a circuit every 24 hours
• If they haven’t been activated after this long

Question 20

Describe trafficking of experienced T cells

Why does it make sense?

Answer 20

• Experienced T cells have restricted passports – only lets them visit the secondary lymphoid organ that their antigen was in
• This makes sense: Activated T cells are more likely to meet their cognate antigen again (restimulates them) or to find their corresponding B cells and T_C to help
• Experienced T cells have restricted passports that lets them exit the blood at sites of infection
• This makes sense: Activated T cells (T_C) can kill infected cells or provide help (T_H) to other cells involved in the battle
• Immune response concentrated in a distinc area which means that the cells that have a job to do are at the location where they can do the job most effectively (no longer randomly circulating the secondary lymphoid organs)

Question 21

Explain the life and times of an experienced T cell – part 1

Answer 21

• Experienced T cells have restricted passports (chemokine receptors)
• Activation modulates the expression of adhesion molecules
• The adhesion molecules expressed depends on WHERE the T cell was activated
• T cells activated in a Peyer’s patch (in the gut) will maintain high α4β7 integrin expression but reduce L-selectin expression
• T cells activated in a Peyer’s patch are more likely to return to a Peyer’s patch
• Activated T cells are more likely to go back to where they saw their cognate antigen to be restimulated and remain useful while we need them to be

Question 22

Explain the life and times of an experienced T cell – part 2

Answer 22

• Experienced T cells have specific passports (chemokine receptors)
• You don’t really want useful T cells circulating round and round aimlessly
• Activated T cells need to be able to exit the blood at the site of infection to do their jobs (killing or helping) – chemokines are expressed at the site of inflammation
• Experienced T cells have additional adhesion molecules that let them roll, sniff, stop and exit (like neutrophils)
• e.g. T cells activated in the gut express αEb7 integrin which partners up with an addressin (molecules that allow cells to move into different lymph nodes) found on inflamed mucosal surfaces
• So T cells trained to deal with gut invaders will seek out infected mucosal tissues using chemotaxis (chemokines) and cell-cell contact (integrin) mediated signals

Question 23

Explain the the life and times of an experienced T cell in relation to the different T cell subsets - part 3

Answer 23

• T helper cell differentiation changes chemokine receptor expression
• The matching chemokines are expressed in inflamed tissue to direct the different T cell subsets to where they are needed

Question 24

How is B cell trafficking pretty similar to T cell trafficking?

Answer 24

• Naïve B cells – global passports

access to all the secondary lymphoid organs

• Experienced B cells – not as migratory as T cells

Settle down in the bone marrow or secondary lymphoid organs, produce antibodies and let other cells do all the travelling

• Plasma cells – restricted passports

Home to appropriate sites depending on the type of immunoglobin they produce

Question 25

Describe Plasma cell trafficking

Answer 25

Cells can change what they express on their surface which makes them move into specific regions of the body where they are needed to generate the immune response

Question 26

Give a summary of the lecture

Answer 26

• Activated TH will proliferate, leave the lymph node and recirculate to find the appropriate B cell
• Activated B cells can terminally differentiate to plasma cells and go to spleen or bone marrow
• Naive T/B cells recirculate through the lymphoid organs (not tissues)
• Experienced T cells are directed to the lymphoid tissue where they were activated and can also get into the tissue
• Chemotactic and cell-cell contract mediated signals
• Experienced B cells migrate to bone marrow/spleen
• Cells follow a pattern of recirculation that is appropriate for their jobs (looking for antigen or looking for a fight!)

Question 27

Which chemokine receptor do B cells have on their surface?

Which chemokine receptor do T cells express?

Why do B cells and T cells move to different areas of the lymph node?

Answer 27

• B cells express CXCR5
• T cells express CC47

B cells and T cells respond to different chemokines and this is why they move to different areas of the lymph node. Some cells in the lympjh node produce CXCL-13 which is the ligand for CXCR5 and those cells move towards that signal (gradient)

Dendritic cells in the medullary sinus express high levels of CCL21 which is the ligand for CCR7 to tell the T cells where to go

Question 28

What happens when cells are responding to a chemokine?

Answer 28

They are following a soluble concetration gradient