Immunology 09 Putting It Together

Front 1

Explain naive lymphocyte homing

Back 1

The naïve lymphocytes exit from the circulation into the lymphatics at high endothelial venules in the cortex of the lymph node. These lymphocytes have homing signals on them that allow them to traffic to these particular areas. CD34 (binds L-selectin) is a receptor that allows exiting of the cell. CCL19 and 21 are chemokines that allow homing of the cell. When dendritic cells begin to mature they also express CCR7 which allows them to home to the lymph node (CCR7 is the allows homing to CCL19 and 21).

Front 2

Where are HEV located?

Back 2

Paracortex, between the medulla and the cortex.

Front 3

Describe the path of T cells that do not find the proper antigen

Back 3

They enter in the HEV and exit through the efferent lymph in the medullary sinus

Front 4

Describe the path of T cells that do find the proper antigen

Back 4

They enter in the HEV and when they find the proper antigen they lose their ability to leave the lymph node. They anchor in the lymph node and will continue to replicate and differentiate until they are effector cells.

Front 5

What are the homing receptors on effector T cells?

Back 5

LFA-1 and VLA-4 are for integrins.

Front 6

What do LFA-1 and VLA-4 bind? What increases the strength of this binding?

Back 6

LFA-1 binds to ICAM-1 and VLA-4 binds to VCAM-1. Activation of the T cell increases this binding affinity 100-1000x.

Front 7

What upregulates ICAM-1 and VCAM-1 on peripheral tissue cells?

Back 7

IF-1 and TNF-alpha

Front 8

Where do T cells go to after activation?

Back 8

They return to their site of origin. If they were activated in the skin, they will return to the skin. If they were activated in the mucosa then they will return to the mucosa.

Front 9

Describe antigen receptor signaling in both B and T cells.

Back 9

The actual receptors that contact the antigen have little cytoplasmic tail an d no signaling machinery. They have to complex with signaling proteins in order to conduct. In the B cell the two main signaling molecules are Igalpha and Igbeta. In T cells it is the CD3 complex made of gamma, delta, epsilon, and zeta chains. ITAMs are amino acid motifs that appear in the tail molecules. There are two tyrosines, spaced by 10 aa. These tyrosines have a phenol ring which is a good site for phosphoylation. These tyrosoines become phosphorylated and become a docking site for downstream signaling proteins.

Front 10

What are ITAMs?

Back 10

Immunoreceptor Tyrosine-based Activating Motifs

Front 11

Explain kinase signaling with CD4 T cells

Back 11

A peptide comes in from above and is recognized by the TCR. The CD4 is complexed continuously with a kinase called Lck. The binding of antigen and MHC causes clustering of multiple TCRs and CD4s. ZAP-70 is a kinase zeta associated protein and is the first T cell specific signaling molecule recruited on T cell activation. LAT is a membrane associated scaffolding protein. In the signaling process you have kinases and scaffolding molecules that serve as docking sites that bring other cytoplasmic molecules into play and then let them go again. The CD4 comes close to the TCR, which results in the Lck phosphorylating the ITAMs. That serves as a docking site for ZAP-70. ZAP-70 is phosphorylated by Lck. This causes ZAP-70 to change conformation and become active. It releases the ITAMs and moves onto other local sites. It can bind to LAT which phosphorlyates LAT. Now LAT can serve as a downstream docking site for signaling mediators.

Front 12

What does NFkappaB do?

Back 12

Initiates a whole series of immune response genes

Front 13

What does Zap70 do?

Back 13

Initiates a series of reactions that leads to the activation of phospholipase-gamma (PLC-gamma)

Front 14

What does PLC-gamma do?

Back 14

Cleaves phosphatidylinostiol bisphospate (PIP2) into diacylglycerol (DAG) and inositol triphosphage (IP3)

Front 15

What does DAG do?

Back 15

Activated protein kinase C-theta, which then actives TF NFkappaB

Activates RasGRP, which in turn activates a MAP kinase cascade. The Ras-induced kinase cascade induces and activates Fos, a component of the AP-1 transcription factor.

Front 16

What does IP3 do?

Back 16

Increases intracellular Ca2+ concentration, activates a phosphatase, calcineurin

Calcineurin activates a TF NFAT (nuclear factor of activates T cells)

Front 17

What do NFkappaB, NFAT, and AP-1 do?

Back 17

Change the pattern of gene expression in T cells leading to cell division, proliferation, and differentiation to effector T cells

Front 18

What are the costimulatory molecules on resting APCs?

Back 18

B7-1/C7-2, CD80/CD86

Front 19

What happens if a T cell receives signal 1 without signal 2?

Back 19

The T cell becomes anergic

Front 20

What are the consequences of APC activation?

Back 20

APC maturation and up-regulation of costimulatory molecules on the surface. This gives signal 1 and 2.

Front 21

What happens when a T cell receives signal 1 and 2 from an APC?

Back 21

There is brisk proliferation of the T cell. Activation of the T cell leads to the release of IL-2 and the high affinity receptor of IL-2 which leads to proliferation. This is an autocrine loop.

Front 22

Which APC is most capable of activating a naive T cell?

Back 22

Dendritic cells

Front 23

What are the effective T cell subtypes?

Back 23

CD8 T cells:
Cytotoxic (killer) T cells

CD4 T cells:
Th1 cells
Th2 cells
Th17 cells
Treg cells

Front 24

What do cytotoxic T cells do?

Back 24

They kill cells. They require CD4 T cell help for optimal maturation. They require good antigen presentation by a dendritic cell. Once they are activated and become a mature cytotoxic T cell, they roam the body looking for virally infected cells.

Front 25

What do Th1 cells do?

Back 25

They produce lots of interferon-gamma.

They are important in activating phagocytes to upregulate superoxide generation and become super killers. They are important in killing phagocytosed bacteria and intracellular bacteria

They can drive B cell proliferation

Front 26

What do Th2 cells do?

Back 26

They are capable of driving B cell proliferation.
Th2 cells make IL4 which is a B cell proliferative factor and drives class switch to IgE. The Th2 arm is antiparasitic and mediates allergy.

Front 27

What do Th17 cells do?

Back 27

Make IL-17. These are important in neutrophil recruitment and killing of extrtracellular pathogens and are important in some autoimmune diseases, like MS.

Front 28

What do Treg cells do?

Back 28

Regulatory T cells are important in maintaining tolerance to self in the periphery. They make TGFbeta and IL-10, which are immunosuppressive cytokines.

Front 29

What is signal 3? What is its importance?

Back 29

The naive T cell come into contact with an APC. Depending on what the APC is secreting, the "signal 3", the T cell will make a decision on how to differentiate.

Front 30

Describe the Th1 cell fate

Back 30

If there is IL-12 and INF-gamma, the T cell becomes a Th1 cell through activation of the transcription factor T-bet. Th1 cells produce IL-2 and IFN-gamma which furthers the development of Th1 cells

Front 31

Describe the Th2 cell fate

Back 31

If there is IL-4, the T cell becomes a Th2 cell through activation of the transcription factor GATA-3. Th2 cells produce IL-4 and IL-5 which furthers the development of Th2 cells.

Front 32

Describe the Th17 cell fate

Back 32

If there is TGFbeta and IL-6, the T cell becomes a Th17 cell through activation of the transcription factor RORgammaT. Th17 cells produce IL-6 and IL-17 which furthers the development of Th17 cells.

Front 33

Describe the Treg cell fate

Back 33

If there is TGF-beta, the T cell becomes a Treg cell through activation of the transcription factor FoxP3. Treg cells produce TGFbeta and IL-10 which furthers the development of Treg cells.

Front 34

What is IL-4 important for?

Back 34

IgE production

Front 35

What is IL-5 important for?

Back 35

It is the major growth factor for eosinophils

Front 36

What is TGFbeta?

Back 36

An immunosuppresive cytokine

Front 37

What is IL-6?

Back 37

An inflammatory cytokine

Front 38

Describe T cell querying

Back 38

The cell is highly organized in how it delivers its signal. The T cell first encounters any of its targets and uses its adhesion receptors. The first interaction is nonspecific and with a relatively loose adhesion. If the T cell receptor comes into contact with MHC and the appropriate peptide, these bonds become much firmer and the cell organizes itself. The cytotoxic granules are brought to bear right at the interface between the two cells. This is the immune synapse. It is a tightly joined junction that is ringed by adhesion receptors on the outside and the antigen receptor is in the middle. The cytokine release is taking place in this space. The cell becomes highly polarized upon contact with the other cell.

Front 39

What happens when a T cell recognizes antigen?

Back 39

There is an influx of calcium

Front 40

Describe B cell antigen presentation

Back 40

It is selective. B cells are APCs themselves. They don’t take up antigen nonspecifically. They use their antigen receptor to scan the environment and find things they can bind. They endocytoze, remove the bound antigen, and put the immunoglobulin back on the surface. Whatever was brought in is broken apart and presented on class II peptides.

Front 41

Describe the interaction between B and T cells

Back 41

The naïve B cell is looking for antigen and it binds whole proteins. That is endocytosed, processed in lysosomes and presented on class II MHC. This occurs in secondary lymphoid tissue with T cell around. The T cell will look for a B cell and will scan it. If it finds a specific interaction it is activated. This leads to a program change in the effector T cell where the cell now upregulated the help molecules that it needs. One of these is the CD40 ligand, which is the source of contact dependent help to B cells. The B cell needs crosslinking of the antigen receptor, which is antigen specific, and also a T cell help signal through contact with a T cell expressing CD40 and through cytokines that the T cell makes (IL-4). The second signal is not specific in respect to antigen. Once activated (Specific process), CD40 is turned on and cytokines are produced. This T cell can go off and help any B cell. To keep this antigen specific the CD40 ligand is upregulated rapidly and then downregulated rapidly. The join between the B and T cell lasts for hours. Thus, once activated the T cell only provides help to that particular B cell.

Front 42

How do B cells track the same HEVs as the T cells?

Back 42

They have CCR7 and L-selectin

Front 43

Where do B cells enter a lymph node? Where do they go after? What happens along the way?

Back 43

They enter in the T cell zone and go to the B cell zone (follicle). They encounter helper T cells along the way. T cell help leads to B cell proliferation and the setting up of a primary focus, which is where the B cell differentiates into an effector B cell. Most of these migrate to the follicle to make a germinal center where proliferation, class switching, and affinity maturation take place. This is all orchestrated by homing receptors and HEV receptors.

Front 44

Where is the T cell zone?

Back 44

Paracortex

Front 45

What is the difference in B cells between an unimmunized donor and an immunized donor.

Back 45

The secondary response is after a clonal expansion. You have a 100-1000x higher frequency of response. In the secondary response you have already class switched and you have a higher affinity due to affinity maturation. For the unimmunized donor, most antibodies are IgM. For an immunized donor most are IgG/IgA.

Front 46

Why is there not much affinity maturation take place if a B cell produces IgM?

Back 46

IgM cells arent in the germinal center, they go to the medullary region of the lymph node.

Front 47

How do effector CD4 T cells help provide feedback to the innate system?

Back 47

Th1 cells make IFgamma which is important for activating macrophages and other phagocytes. It increases superoxide generation, phagocytosis, and Fc receptor expression. The T cell provides help not just through the cytokine signal, but through the contact dependent CD40 interacting with the CD40 ligand.

Front 48

Explain granuloma formation

Back 48

When A Th1 cell encounters a macrophage that has digested something that it cant kill, it will try hard to flog the macrophage to kill. If the organism cant be killed the next response is to wall it off. Monocytes will begin to fuse, forming giant cells with living pathogen within them. These are essentially "holding tanks". The T cells still surround it, activating the macrophages. This is an epithelial looking cell called a granuloma.

Front 49

How do effector CD4 T cells help naive CD8 T cells?

Back 49

The APC stimulates effector CD4 T cell, which in turn activates the APC.

The activated APC expresses CD40 and 4-IBBL, which co-stimulates naive CD8 T cells

Front 50

Describe the help that effector CTLs need?

Back 50

They need no help. The naïve CD8 cell needs signal 1 and 2, which it gets from the APC. It then proliferates just like a CD4 cell would and then it goes off on its way looking for cytotoxic targets. At this point it no longer needs all that costimulation.

Front 51

How do CTL kill virally-infected targets?

Back 51

The immune synapse is formed. Perforin is inserted into the target cell membrane. Enzymes are injected which activate the caspase system leading to apoptosis.

Front 52

What are super antigens?

Back 52

These are proteins made by pathogens that cause tremendous crosslinking of T cell receptors and MHC because they bind to framework regions on the MHC completely independent of the antigen groove.
They crosslink TCR extensively (and overcome the requirement for costimulation), leading to massive TCR activation.

Front 53

What are some of the features of superantigen activation?

Back 53

Superantigen binds framework regions of the TCR encoded by the Valpha and Vbeta gene segment. The result is activation and proliferation of all T cells expressing that V-segment. This is 5-15% of all T cells. This causes a cytokine storm where T cell IFN-gamma leaves to monocyte TNF-alpha and IL-1

Front 54

What are the problems of mucosal surface immunity? What is the solution?

Back 54

The problems are that there are over 300m2 of mucosal surface area to police. There are thousands of benign foreign proteins and thousands of commensal as well as potentially pathogenic organisms. The solution is that the immune system in that area is specially adapted to maintain a state of "calm alert. "Calm alert" means that there are many cells ready to act, but some specialized adaptations that keep them form acting too soon.

Front 55

Describe the organization of Peyer's patches

Back 55

They are organized like a lymph node. There are B cell follicles and T cell zones. These underlie dome structures adjacent to villi. These domes contains an M cell on their surface. The M cell is important in passing antigen from the gut lumen into tissue.

Front 56

What are the inductive sites in the gut?

Back 56

Peyer's patches
Isolated lymphoid follicles

Front 57

What occurs in isolated lymphoid follicles?

Back 57

These are B cell follicles. These are the inductive sites where you generate the pathogen specific immune response and the villi are where you will be fighting the pathogen.

Front 58

What do M cells do?

Back 58

They take up antigen by endocytosis and phagocytosis. Antigen is then transported across the M cell in vesicles and released at the basal surface. Antigen is bound by dendritic cells, which activate T cells.

Front 59

Besides through M cells, how else are dendritic cells exposed to lumenal antigen?

Back 59

Dendritic cells can extend processes across the epithelial layer to capture antigen from the lumen of the gut

Front 60

What is CCR-9?

Back 60

A chemokine receptor in the gut that keeps chemokines coming back to the gut.

Front 61

What happens when a T cell is activated in gut lymphoid tissue?

Back 61

When you have a naïve T cell coming in with CCR-7 and L-selectin, it will get activated by antigen and then drains out of the Peyer's patch lymphatics to mesenteric lymph nodes where proliferation may take place or it may drain through the thoracic duct into the circulation. Once it has been activated here it will tend to recirculate to the gut. CCR-9 is a chemokine receptor in the gut that keeps chemokines coming back to the gut.

Front 62

What cells are in the lamina propria of the healthy intestine?

Back 62

-Effector T cells
Macrophages with decreased TLR expression
-DCs with tolerogenic properties (IL-10>IL-12)
-Mast cells coated with IgE, loaded with histamine
-Effector B cells secreting IgA

Front 63

Describe secretory IgA

Back 63

-IgA can export toxins and pathogens from the lamina propria while being secreted
-IgA is able to bind and neutrolize antigens internalized in endosomes
-Secreted IgA on the gut surface can bind and neutralize pathogens and toxins
-Secreted IgA binds pathogen on M-cell surface and takes it into the lymphoid tissue.

Front 64

How is IgA secreted?

Back 64

IgA is joined as a chain, picked up on the basolateral end of the epithelial cells by a receptor specific for the IgA. It is transcytosed to the lumen to the where it is clipped off with the secretory component. This protects the IgA from digestive enzymes.

Front 65

What are Intra-epithelial lymphocytes?

Back 65

Lymphocytes that lie within the epithelial lining of the gut. They are CD8+ T cells and are in the epithelial layer.

Front 66

Describe intra-epithelial lymphocyte function

Back 66

There are two types:
-Conventional CD8+ CTLs
-patrol mucosa for virally infected cells; kill targets (perforin/granzyme, fas)
-Innate-like CD8+ CTLs
-Limited V-region usage
-Recognize host-encoded surface markers of stress: MIC-A and MIC-B
-Kill stressed target cells by conventional means