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Immunological Memory

Dalia Rawashdeh

Ziad Al-Nasser

Sunday, 24/7/2011

30

15

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Immunology - Lecture 15

Sunday, 24/7/2011

Done By: Dalia Rawashdeh

Immunological Memory

Last time we were talking about immunological memory, cell-cell

interaction and immunological synapse and what happens when B or T

cells go to the periphery, and how nave B cells can be changed into

primed cells when it meets its antigen, and how the markers will appear

on its surface and the significance of them in signaling.

When signaling takes place a B cell is either converted into an effector

cell then a plasma cell that produces antibodies orto a memory cell.

Then we started talking about memory cells and how they differ from

nave cells qualitatively and quantitatively, how they increase in number

and where they are located.

1. Primary response:Nave B cells: they have lower affinity, slower response, production of

IgM. No production of effector cells yet. The response time to the

antigen will be slower compared to memory cells (meaning they will

take a longer time) .Naive cells circulates from blood to lymph nodes to

secondary lymphoid organs where they need antigen presentation and

cytokines in order to develop into memory cells. If the nave cell doesnt

encounter the cognate antigen it goes into apoptosis and dies.

Nave T cell: they have to have more than one signal; a signal coming

from binding to the T cell receptor, the other one comes from the

cytokines. They will be activated to effector T cell or memory cell. (Note:

a memory T cell, whether it is T-helper 1, TH2, or cytotoxic can change

into effector cell. We mean by effector that it is going to perform the

function that it is designed to do, for example T cytotoxic performs the

killing and so on.)Note: Plasma cell doesn't have an antigen receptor and if it is not

nourished by cytokines it will die.

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2. Secondary response (or subsequent response):

Memory Cells: we don't see them in the secondary lymphoid organs, we

can see them in tissues and in the circulation. They respond faster to the

antigen compared to the primary immune response. They produce IgH(which could be IgA or IgG, dependant on the need for them. in the book

IgH could be IgA, IgE, or IgG.) The type of immunoglobulin that they will

produce is much better and has higher affinity compared with nave B

cells.

In the secondary immune response the antigen binds to a memory cell

rather than a nave cell because they are already in the tissue and in the

circulation where nave cell are in the secondary lymphoid organs. SO amemory cell is a FASTER and a more EFFEICINT immune response.

Memory cell is derived from activated B cell that has undergone genetic

changes in the variable region gene. Here the genes that code for and

are going to be deleted and replaced by either 1 , 2 , 1,2, 3 or 4.

Note: memory cells develop when we are exposed to antigens either

naturally or by vaccines. These processes for the formation of memory

cells and keeping them for a longer period of time need cytokines.

Vaccines can activate memory cells; increase their numbers and their

efficiency.

T cell is more difficult than B cell because T cell does not go into genetic

changes. The B cell receptor has lower requirement for co-stimulation

compared with T cell because of lymphokines. So we find that the T cell

receptors are high affinitylike immunoglobulins.

In the T memory cell, the surface markers that are needed in nave T-

cells, for rolling and then going to the assigned area into the lymph node

are no more needed. This is because T memory cells can go directly to

the tissues (T memory cells circulate from blood to tissue and vice versa)

while nave T cells circulate from blood to lymph nodes to scan for

antigens. SOL-selectin are more in the nave cell compared to the

memory cell.

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So the primary immune response will change nave B cell to a plasma

cell then the plasma cell will produce IgM antibodies with low affinity.

The secondary immune response we have IgG or IgA antibodies and they

will form the receptor and they have higher affinity.

Note: Antibodies that are produced in the secondary have high affinity

and this is the qualitative difference.

Tetramer Assay:We have an assay called the tetramer assay. In this assay we can see

how memory cells can respond and proliferate and change to effector

cells and more memory cells.In the tetramer assay, we can see how the memory cells can increase in

number when we are exposed to the antigen for the second time.

The tetramer assay is simply an immune fluorescent testing. First we

know that a T cell has a receptor for MHC class 1 and an antigen.

We add bioten to MHC-peptide complex then we get fluorescenated

sterptavidin and put them in the cells(note: bioten binds to

sterptavidin)CD8+ T cells with the appropriate antigen specificity bind

tetramer, of those cells we want to see if they are memory cells or not:

the memory cells will bind to the complex we can calculate them by

flow cytometry.The persistence of the antigen and memory is to retain antigen

developed memory. Ya3niii, the longer you have the antigen the more

memory that you are going to have. And this is the principle of life

attenuated vaccines. If I am using life attenuated vaccines then the

chance to maintain and to keep memory cells for longer periods of time

is much more compared to inactivated vaccines.

Note: the minute that the antigen is not there, then the number of

memory cells start to decrease but they might not disappear as long as

we have cytokines to nourish them for a longer period of time.

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The activation of B cellis going to follow more than one mechanism and

it is not simply the binding of the antigen to the BCR but also if you have

immune-complexes as well. Immune-complexes bind to Fc receptor and

to BCR and activate B cells. And this mechanism doesnt apply to all

antigens, so an antigen has to bind to immunoglobulin to form immune-

complexes and these immune-complexes have to bind to the receptor.You will see on the surface of B cells you could have a BCR and you have

beside that an Fc receptor for immunoglobulins. The Fc receptor is

where the free part of an immunoglobulin (the Fc portion) could bind.

And the binding depends on the number of immune-complexes,

whether they are going to activate or suppress an immune response, we

will talk about that later.

We will talk about how these positive signaling and negative signaling

take place when immune-complexes are present, and how the Fc

portion of the immunoglobulin that is bound to immune-complex can

bind the free part of the Fc receptor on the surface of B cells.

Can our body keep the fundamental number of memory cells

generated?!

Yes, if we can take care of them but if not they will die by apoptosis.The balance between the need for clonal expansion and the need to

maintain memory cell is so crucial. And the maintenance of memory

cells depends on cytokines.Most effector cells die as the antigen is eliminated. The minute that the

antigen is taken out of our body we will not need effector cells so theywill die.So how do these cells die?!

They die by a mechanism we call apoptosis or programmed cell death.

So if this mechanism is not activated then cells will survive for ever. For

example malignant cells survive for ever, they will keep growing because

they have something we call it anti-apoptotic pathways.

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So each target cell has a protein substance that acts as a receptor we call

it FAS, and the killer cells (T-cytotoxic) has an anti-FAS (the ligand) for

the FAS we call it FAS-L (FAS-ligand).So the first step in the apoptotic pathway is the activation of the T-cytotoxic cell which will express the FAS-ligand on its surface a signal

is going to be generated inside the cell this signal is going to activate

an enzyme we call theCaspase enzymeThis enzyme will turn from the

inactive state to the active state the activated caspase will activate

CAD (caspase-activatable DNAse) CAD will act on the DNA and

destroy it cell death; and this is how apoptosis takes place.

Apoptosis is a mechanism that the T-cytotoxic cells do to convince cellsto commit suicide.

Now what about the anti-apoptotic pathways (how can cells refuse to

die)?!We have certain genes that are going to inactivate the caspase we call

them anti-apoptotic death or inhibitory genes like for examplebcl-2,

bcl-X, andbcl-w. If the cell has those genes it will persist apoptosis.We also have Pro-apoptotic death promoting genes for example

bax,bak,andbcl-Xs. Those code for the caspase.

So again apoptosis is triggered by FAS-FAS-L interaction, FAS is on the

target cell, FAS-L on the T cytotoxic cells, and when triggered they

activate by biochemical pathways caspase then caspase activate CAD

then CAD breaks DNA into fragments. And we have antiapoptotic genes

and proapoptotic genes that (the latter) are present in malignant cells.*Note: Memory cells have more of inhibitory genes because they need

to stay a longer period of time so they have more inhibitory genes

compared to regular cells.

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CHAPTER 18: its a review!

We talked about primary immune response and the secondary immune

response.Also we talked about the primary lymphoid organs and the secondary

lymphoid organs. And how the bone marrow and thymus produce nave

cells that encounter pathogens and it is processed and presented to the

APC. The adaptive and the innate and their interactions. The innate is

fast, no specificity, and no memory. And how the innate and the

adaptive cooperate.

Cooperation, interaction among APCs. Lymphocytes activate an immuneresponse final outcome antigen activated lymphocyte, that home to

infection sites and eliminate pathogen in the secondary lymphoid

organs, then memory cells will develop and the system will return to

homeostasis after that .Now important points we have talked about:

How a repertoire of antigen receptor is developed.

How a nave cell recognizes an antigen till it goes into an effector

cell, APC, thymus-dependant, thymus-independent.

And we talked about APC, B and T cells cooperation , two signals

it's not just simply the binding of the antigen to the receptor you

also need cytokines .

We talked about lymphocyte homing and trafficking where we

need those to go to the assigned area.

The role of memory cells and its significance and how to keep

them alive by nourishing them by cytokines.

We talked also about cytosolic and the vesicle-bound pathways

and what determines if cell is to go into the cell mediated

immunity or to the humeral response class 1 and class 2, Class 1

presentation to T-cytotoxic cells and class 2 to T-helper cells.

The structure and function of the B-cell receptor and the TCR and

you have to know the differences and the similarities between

them.

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The role of co-receptor and costimulatory molecules.

The antigen receptor and the signal transduction on both T and B

lymphocytes and what's common between them.

Antigen interaction with the BCR and TCR and how to modify this. We also talked about the APLs which means how the altered

antigen and the significance of that and the cyclosporins and how

we can use it.

The diversity and the clones, we talked about the diversity on

antibodies level and for TCR

B and T cell development, affinity maturation and class switching.

Important points we will talk about: the immune system, how it doesntperform on its own but can interact with other biological systems in our

body for example the endocrine system which plays a major role, the

sympathetic and the parasympathetic, the adrenalin and nor-adrenalin

and epinephrine and how this can modify our immune system, and

steroids from suprarenal glands and how they can suppress our immune

system and we will talk about stress and the immune system for

example chronic stress will suppress our immune system and the acute

stress could modify the immune system.

Introduction to Innate Immune System

We will talk about the innate immune responses and about the pattern-

recognition molecule that is found for example on the macrophage that

recognizes a group of antigens like lipopolysaccarides and how the

recognition of pattern-recognition molecule is going to stimulate that

cell to produce cytokines that will enhance and give signals to theadaptive immune response.

The initiation signal of the adaptive immune response comes from the

innate through pattern-recognition molecule. In the adaptive system the

antigen-recognition molecule is like the pattern-recognition molecule in

the innate immune response. One of them Toll-like receptor of the

innate system. The toll-like receptor is a group of pattern recognition

molecules.

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For example, if we have a microbial pathogen like gram negative

bacteria, it has LPS and LPS is present on so many bacteria (e.g

enterobacteriaceae family like E.coli, proteus, Klebsiella, all of them

have LPS). Also we have LPS-binding protein that is going to bind to LPS

like a complex.And this is going to help to bind to CD14. Now CD14 will not bind to the

Toll-like receptor as a pattern recognition molecule unless LPS is

presented with LPS-binding protein. So this is going to activate the cell to

grow and differentiate and produce transcription factor and cytokines as

well.

So as you can see here it started with pattern recognition moleculesignaling nuclear factor kappa Btranscription cytokines are going

to be produce.Here we are talking about macrophages, so they get stimulated and

produce cytokines like: IL-1 and Tumor necrosis factor-. And those can

affect the adaptive immune system through sending the signal: We

have been attacked!So you can read on the figure, Human Toll-like receptor (TLR) binding to

a ligand created by the pathogen-derived LPS binding initially to LBP and

then to CD14, then Toll-like/IL-1 receptor domain (TIR), finally the

synthesis and release of cytokines and synthesis of adhesion molecules

and enzymes of nitric oxide system that is going to help in the

phagocytosis and killing.IL-1 is one of the most important interleukins and we call it internalor

endogenous pyrogen responsible for the raise of fever with tumor

necrosis factor and others.So the Toll-like receptor concept, first you have to remember it is in the

innate the non-specific immune response. Interleukins like IL-1, IL-6, IL-

8: those are responsible for the raise of fever when somebody is having

what we callpredromal symptoms. So when I have fever, muscle pain

and joint pain this is due to interleukins: IL-1,IL-6 and IL-8. And they are

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produced from the macrophages. And they represent the first signal that

"We have been attacked!" .

We will see that we have certain molecules that are going to be

produced in the innate immune system that are going to help in thedefense. And how these carbohydrates, c-reactive protein, and the

complement will help in phagocytosis and get rid of the invading

microorganism.So the innate and the adaptive dont work separately. When the innate

immune system is in use then lymphokines are going to be produce. And

those lymphokines or cytokines are going to work on the adaptive

immune system; they are like warning signals.That was just the introduction to the role of the innate immune system

and defense.***So you remember when we talked about our immune system: the two

arms the innate and the adaptive. And they work interchangeable and

cooperatively together, where the innate help the adaptive system. Andthe final outcome of both of them is to get rid of the infection or foreign

cells.We talked about three lines of defenses: the first line (the physical

barriers mucous membrane), the second line (the blood and its

constituents other than the lymphocytes), and the third line (which is the

specific one , T and B cells.)The innate most of the time can get rid of the microorganism, ya3nii you

dont need to go further into the second and the third lines of defense.

But if the first two fail to get rid of the invading microorganism then the

third should do the job. If the third fails so the consequences are going

to be horrible like those AIDS or inherited immune deficiency diseases

and so on.So physical barriers skin, pH, and cilia, molecular barriers, complement,

and interferon all of those are innate.

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Comparison between Innate and Adaptive

You should remember the differences between the innate and the

adaptive.

We talked about how the innate distinguishes danger from homeostasis.

It starts by giving a warning signal and the adaptive distinguishes self

from non-self.

The innate can only recognize molecules, signaling infection or injury,

while the adaptive distinguish host from pathogens.

Some of the innate can distinguish self from non-self like when we talk

about the complementBUT this is a property design for the adaptivesystem.

Also in the innate there are previously formed components. The

components are already there: the skin, mucous membrane, white

blood cells.

While in the adaptive relies on the genetic events and cellular growth.

Although the cells and components are there but they need the antigen

to interact and then proliferate and produce primary and secondary

immune response.

So the adaptive system takes time, while the innate is faster. So the

innate response works within minutes while the adaptive requires days.

In the innate there is no specificity, the same molecules and cells react to

all pathogens ya3ni the first is like the second and the third. While the

adaptive must have the assigned lymphocyte (clonal selection theory)

In the innate we use pattern-recognition molecule while in the adaptive

we use antigen-recognition molecule. Ya3nii in the adaptive we have

specificity while in the innate group of antigens are dealt with.

The innate uses germline genes to produce collectins( mannan-binding

lectin, surfactant), complement, C-reactive protein and Toll-like

receptor, while in the adaptive we are talking about antigenic

determinant, receptors, lock and key mechanisms.

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In the innate we have probably more than 100 receptors exist because

they are group. While in the adaptive we have probably 10^18 different

receptors of the T-cell receptor, and 10^11 of BCR.

In the innate there is no memory, and it is the older system seen in allmembers of the animal kingdom. While the adaptive is something new,

and has memory. It goes with human development.

The innate rarely malfunctions ya3nii you need physical barriers injury or

genetic deficiency of the enzymes of the complement in order for it to

malfunctions. While in the adaptive we could have autoimmune

diseases, hypersensitivity, immune deficiencies.

So the innate is not specific, very rapid response, no memory, uses

danger signals for the adaptive for example the Toll-like receptor, also

we talked about IL-1,IL6 and IL-8.

Components of the Innate System

The skin which is the largest immune organ we have. It is dry. It has

keratin in it. The keratinocytes can produce IL-8 and tumor necrosis

factor. And the skin has Langerhan cells and they are the macrophagesof the skin.So if you have reached the skin you could have infection. So organisms

and warms must have ways to invade the skin. For example the

schistosomepenetrate through the skin and sometimes they haveenzymes through the skin.So if you get burnt it will facilitate the entry of pathogens to the body,

since the physical barrier has been eliminated.

Respiratory Tract

Now for the respiratory tract we have the: upper (from the nose till the

bronchioles), lower (terminal bronchioles to the alveoli).

So the mucociliary role in the respiratory tract is one of the best non-

specific immune response.

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So anything that goes into our respiratory tract binds to them.

So the cilia acts as a brush border which means washing things out. And

anything that goes inside can bind to mucous (secreted from goblet

cells) and then we cough that into the throat and swallow it into thestomach so we get rid of it.

And always we said that the number one enemy for the cilia is smoking.

Smoking wipes out all of the cilia and we become immune compromised.

And the damage that occurs from smoking is not just for the cilia we

have also carbon particles and the inflammation associated with that will

lead to chronic bronchitis and chronic obstructive lung disease.

When we look at COPD like emphysema, chronic bronchitis all of them

are involved with what the macrophages release. The hydrolytic

enzymes that can destroy cilia and the respiratory lining and change the

way we exchange oxygen.

In the end of the respiratory tract we have a substance we call it

surfactants and they are so important to the recoiling of the alveoli

otherwise the alveoli will collapse. Premature babies are in danger of

having collapsed lungs because they dont have surfactants so doctors

give them surfactants.

Now these surfactants or collectins act as a pattern-recognition

molecule and they cover cells and microorganisms, and they help in

phagocytosis and get rid of them non-specifically.

pH

The pH is so important for the skin or the stomach for example. It kills

microorganisms because of very high or low pH. In the stomach we have

low pH while in the intestine its high. So the variation kills the

microorganisms.

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Interferons

Now the extracellular molecules of the innate system, like interferon

type 1, interferon-and interferon-.

They are produced from cells that are infected. Interferons protect

against viral infections. Interferon-is produced from TH1 cells and they

activate macrophages, more expression of class 1 MHC. So whenever

you have cell-mediated immunity you have interferon-.

So the outcome of interferon and is getting rid of viruses. They are

produced from cells that are infected to provide protection to other

cells. The mechanism of interferon and is to activate a DNAse

enzyme or an RNAse enzyme to get rid of the mRNA of the virus and

then kill the virus.

The strongest stimulator of interferons and is double stranded RNA.

In our bodies we dont have double stranded RNA we only have single

stranded RNA, like mRNA. We get dsRNA when we get infected with

viruses.

So sometimes pharmaceutical companies they stimulate the productionsof interferons and by exposing us to dsRNA fragments, we call it

AMPLIGEN( known as poly I:poly C12U) . This is sometimes used as a sort of

vaccine.

Interferon is for the treatment of viral hepatitis. Interferon for

multiple sclerosis.

C-Reactive Protein and Erythrocyte Sedimentation Rate

We have a substance from the collectins called the C-reactive protein

(CRP). When we get infected by bacteria, viruses, or any inflammatory

reaction they have noticed the development of CRP. They also noticed

that the erythrocyte sedimentation rate becomes faster. This simply is an

indication of an inflammatory reaction.

The CRP covers the microbes and helps in the phagocytosis. And it can

activate one of the complement pathways.

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We can use CRP and the ESR as markers of inflammation. So if we

measure the CRP at the beginning of infection and it was 30 then after 2-

3 day we measure it again and it is 10 that mean the inflammation is

subsiding and the patient is responding to the treatment, and vice versa.

So if we start with a CRP of 10 and after a few days it becomes 30 then

that means the inflammation is not subsiding and the patient is not

responding to treatment. We use this in cases of acute appendicitis to

see if the patient is responding or not.

Mannan Binding Lectin

The mannan binding lectins (MBL), which we call collectins. They are

proteins , they have a globular component and a tail. The globularcomponent it binds to microorganisms and the tail can bind to

macrophages, activating them when the globular part is bound to a

microbe. The tail part can also bind to the complement, we call them the

mannan binding lectin to complement. They are nonspecific.

FIGURE: This is type 1 interferon, interferon alpha and beta. The

interferon interferes with viral replication, more with alpha and beta

than gamma. Double stranded RNA is a strong stimulator of interferons(including gamma), activates two intracellular enzymes. Viral genome

degradation inhibits mRNA and can activate the natural killer cells,

increase the activity of the transporter proteins TAP1 and TAP2,

macrophage activation, NK cell activation, production of IL-1, IL-6, and

TNF, which we call the acute phase response.

The Complement

What is the complement? Complement is an innate immune system, and

from its name it complements the function of antibodies.

We have three major types of complement: the classical pathway(which

starts with antigen-antibody reaction which is a requirement), the

alternative pathway(mainly triggered by lipopolysaccharides) and the

mannan binding lectin.

The three of them share the C3.

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The complement they are a group of proteins from 1-9, C1, C2, C3C9.

The activation of the complement components follows a cascade kind of

triggering. When one of the complement components is activated it acts

as an enzyme, and activates the one that is next, and this is what we call

cascade.

In the classical pathway we start with C1 then C4 then C2 then C3, C5,

C6, C7, C8, C9.

Why start with C4 then C2? Because they discovered C4 later, and they

figured out that C4 is activated by C1.

The alternative pathway starts with C3, so the LPS activate C3. We donthave C2, C1, C4.

In the MBL we start with C2 and C4 into C3.

What is the outcome of the three pathways of the complement?The

ultimate goal of the complement activation is to destroy cells. When the

complement is going to be activated on the top of a cell its going to be

killed, so bacterial cells, fungal cells can be destroyed.

When the complement is activated, the larger components act as

enzymes and stays on the cell, and the smaller components act as

chemotactic factors, and anaphylatoxins as well. They call upon cells to

come to the area.

The end

Thanks to Elham for helping me