7/31/2019 Immuno - Lec 14

1/12

Cell-Cell Interaction in Generating

Effector Lymphocytes

Tamara ShoterZiad Al-Nasser

Thursday, 21/7/2011

20

14

7/31/2019 Immuno - Lec 14

2/12

Immunology Lecture #14

Thursday, 21/7/2011

Done by: Tamara Shoter

The doctor started the lecture talking about the exam; its place,

time...Etc.

So, yesterday we were talking about the process of T-cell development

and we said that it occurs within the thymus gland. The earliest

thymocytes (Double negative cells, which express self-surface

molecules such as CD25 and CD44) enter the subcapsular zone, where

they start to mature. After that, they move further into the cortexwhere we have MHC antigens. Now, in the cortex, double negative cells

mature into double positive cells (CD4+ CD8+). Thymocytes' fate

depends on the ability of their receptors to recognize class I or class II

MHC molecules. Double positive (DP) cells that recognize calss I MHC

molecules will become CD4 + cells, while DP that recognize class II MHC

molecules will become CD8+ cells (this process is called Positive

Selection and it occurs in the medulla).

Notes:

1. T-cells that have high affinity for self-antigens are going to be

eliminated.

2. There's a mechanism called central tolerance related to the endocrine

gland, where all the tissues in our bodies supposed to be presented in

the thymus gland but it doesn't happen this way, so we have (?). The

auto regulatory immune gene can control this process with self-

restriction and if we have a mutation in this gene, then self tolerance or

self restriction is going to be affected and we call this

autoimmune polyendocrine syndrome I. ( Multiple endocrine organs

that have an autoimmune disease).

3. Some self-reactive cells may escape into the periphery and this will

cause the development of something called 'Peripheral Tolerance'.

7/31/2019 Immuno - Lec 14

3/12

Also, if you don't have accessory molecules like CD28 which develops at

the periphery, then peripheral tolerance will develop (Not fully sure).

4. We talked about T reg (known as suppressor T cells) which produce

suppressive cytokines like; IL-10 and the transforming growth factorbeta (TGF-). As I said the gene that codes for that is called Foxp3.

Mutations in it will result in a multisystem autoimmune disease.

5. 95% DP cells never mature because they lack TCRs that can

appropriately recognize self-MHC.

T helper cells are divided to Th1 and Th2 cells based on the nature of the

infectious process or the mechanism of the immune destruction that we

need. For example, if we need to destroy virally infected cells or tumor

cells, we require Interferon-gamma (IFN-) which activates

macrophages that express class I MHC that is need for cytotoxic T cells.

Remember, tumour necrosis factor and interleukin 6 cause elevated

body temperature and isotype switching to (IgG) which acts as anopsonin. Macrophages produce interleukin 12 which stimulates the

7/31/2019 Immuno - Lec 14

4/12

differentiation of Th0 to Th1 cells. The same thing here, we need other

types of antibodies we call them IgH (IgH means any antibody that's

not IgM, be it IgG, IgE ..Etc). Now, Interleukin 4 is produced by mast

cells which induce differentiationof Th0 cells to Th2 cells, which will

also produce interleukin 4 and interleukin 5, and interleukin 5 attracts

eosinophils.

We also talked about the gamma delta T-cell subset which represents a

small percentage (5%) of all T cells. The gamma delta T-cell receptor

can recognize certain peptide and non-peptide antigens without

processing and in the absence of MHC class I or class II molecules. These

cells help other T cells to activate macrophages, also they are involved

in the lysis of virally infected cells and they are a part of the first line of

defence. (Skin, gut mucosa..)

So here, in DiGeorge syndrome, the 3rd and 4th pharyngeal pouches

aren't developed. Therefore, the thymus and parathyroid glands will be

missing, of course patients with this syndrome will have a very low

number of T cells. However, the B cell count is normal .Since these

patients have T lymphocyte deficiency, they will be more susceptible to

viruses and tumours. There's a virus called Epstein-Barr (EB)virus that

binds to CD21 receptors present on the surface of B cells and this will

cause differentiation of B cells into antibody-producing plasma cells,

which in turn activate the CD8+ cells that are going to destroy infected

cells. Also, TNF and Il-6 cause elevated body temperature. See the

figure below:

http://en.wikipedia.org/wiki/Gammahttp://en.wikipedia.org/wiki/Delta_(letter)http://en.wikipedia.org/wiki/Gammahttp://en.wikipedia.org/wiki/Delta_(letter)http://en.wikipedia.org/wiki/Delta_(letter)http://en.wikipedia.org/wiki/Gammahttp://en.wikipedia.org/wiki/Delta_(letter)http://en.wikipedia.org/wiki/Gamma

7/31/2019 Immuno - Lec 14

5/12

Chapter 16 - Cell-Cell Interaction in Generating

Effector Lymphocytes

So now the B, T cells left the thymus gland and the bone marrow as

nave/ virgin lymphocytes and we need those to be primed in order to

stay in the secondary lymphoid organs as long as we want an efficient

immune system.

If you are following the development of the B cells & T cells and the

markers that will develop on their surface when they go to the

secondary lymphoid organs, we see that after they go to the secondary

lymphoid organs and interact with an antigen we see other markers that

are so important for the activation of those B or T cells.

We can see that the binding of the antigen -when it's carried by the

MHC antigens- to the receptor is providing me with one signal but you

need other signals, and those come from the accessory molecules and

those are so important; sometimes for cytokine function, for isotype

switching.

So the immune system will not be completed unless those accessory

molecules that I am going to talk about are there and perform their

function, if they are missing then we are going to be

immunocompromised .This is what I am going to talk about in Cell-Cell

Interaction In Generating Effector Lymphocytes.

We are talking about an antigen taken by an APC, processed, presented

to the surface and then binding to the B & T cells; this is one factor, wehave other factors/ signals that are going to be involved in the

secondary lymphoid organs.

Refer to the Over-view figure at the beginning of the chapter:

We can see here in the Fig. the antigen taken by an APC (through the T

helper cells), T helper cells will be activated & produce cytokines which

act on the B cell, B cell will be differentiated into a plasma cell that will

give AB. This process is called effector B-cell generation. We are talking

7/31/2019 Immuno - Lec 14

6/12

about thymus dependent; this is the role of thymus (the process of

effector B cell generation in the involvement of T cell).

This is the activation of B cells. So it's not enough to bind the Ag to a B

cell, you require also the help of the thymus gland through CD4 by theproduction of cytokines to the B and to the differentiation to a plasma

cell.

The same thing here applies for T cytotoxic and the killing, it's not just

enough that the virus or the tumor Ag is presented to the T cytotoxic

cell through class-1 MHC Ag; this is one signal, the 2nd signal must come

also from a T helper cell.

We will see the cytokines that are going to be produced; sometimes if

the Ag presenting cell is already infected, certain viruses, extracellular

vaccines could be presented by class-1 MHC Ag to T helper. So both (?)

can provide cytokines through this process that will activate the CD8 to

change into an effector cell and then produce the cytolysin & kill the

target cell. (You better skip this paragraph).

So as you can see here (in the fig), in both* the T helper are required for

AB production or for killing process.

* Effector B cell generation & effector T cell generation.

T cell recognizes antigen that is only presented by an MHC Ag on

the APC (thymus dependent Ag). While B cell doesn't have to.

Co-stimulatory signals provided by the interaction with accessory

cells, so we require other signals beside the binding itself.

The outcome is plasma cells -as effector- that give me antibodyOR effector T cell either helper or cytotoxic. The helper will be

activated and act on the T cytotoxic activating it when this

cytotoxic is bound to the Ag of the surface of the cell that I need

to kill.

7/31/2019 Immuno - Lec 14

7/12

Refer to Fig.16.1/Pg.121:

So this is what's happening; here a B cell, it has a B cell receptor and the

Ag is bound here (to the BCR) will be taken inside, bind to class-2 MHC

Ag and then be presented (so B cell acts as an APC) to the T-helper.Then the T-helper is supposed to be activated. Now -as I said- when this

is taking place we require other mechanisms for the production of

cytokines.

Now this stage of T cell activation, makes the T cell (the prime T we call

it) form other accessory molecules that are needed for the production of

cytokines and for the activation of the T helper cell that's going to work

on the B cell, that's going to differentiate into plasma cell, that giveantibodies.

Fig. 16.2/Pg.121:

Look for example:

This is Lymphocyte Functional Antigen on T cell that's going to

bind to the ICAM-1 on APC.

CD28 & CD80(B7); this interaction is so important to tell you thatthis stage is a thymus dependent.

CD154 (CD40L) is going to bind to CD40 on the APC. This binding

makes the cytokines that are needed for isotype switching; so if

this binding is not formed only the B cell will produce the IgM (as

if it's thymus independent).

* We have a syndrome regarding this called X-linked hyper IgM

syndrome; the patient keeps producing IgM and doesn't produce IgGand you know how important the IgG is for opsonization, neutralization

and the high affinity of Igs and so on -while the IgM has a low binding

affinity.

So this syndrome results from the missing of CD154, and how can we

treat those? By Gene Therapy; we have to introduce the gene that's

missing so the isotype switching is going to take place.

7/31/2019 Immuno - Lec 14

8/12

*All these interactions occur when you have an APC in the paracortical

area of the lymph nodes.

*If any of the up-mentioned receptors is missing then we become

immune compromised.

Fig. 16.7/ Pg.125:

These are the differences between TH1 and TH2, we talked about that:

TH1: interferon- and the tumor necrosis factor-

responsible for fever, & IL-6 as well as IL-10. IL-4 & IL-5 not

produced here.

TH2: IL-4 & IL-5 and no INF- & TNF- .

Fig. 16.4/ Pg.122:

Look here into the complete picture that I was telling you about; class-2

presenting the Ag to the T helper providing the 1st signal. CD3 is so

important in the T helper for the passage of the signal. And then the

CD28 & the CD80 (B7) the same thing; give me an indication that this is

thymus dependent. And we talked about the CD40L and the CD40 that

will give a signal here for the isotype switching as well.

For B cell activation, it's not enough just to bind the Ag to BCR, this

gives you just one signal & you require other signal as we said many

times. This all doesn't happen in the bone marrow it happens when the

cell is primed; when the cells go to the secondary lymphoid organs &

meet the APCs in the interaction. This is naturally what should happen.

The explanation of stimulated or primed B & T cells in the secondary

lymphoid organs is that the APCs are present in the secondary lymphoid

organs so now what comes from the bone marrow or the thymus gland

(Nave or virgin lymphocytes) are going to be stimulated there.

The DC could produce cytokines, for example IL-12 for the

differentiation of TH1 &TH2, those they make the HEV to produce the

7/31/2019 Immuno - Lec 14

9/12

selectins & the addressins & those are important for the stoppage of

these lymphocytes & the passage through the assigned areas in the

secondary lymphoid organs; the paracortical region or the follicular

region.

The adhesion molecules communicate through the cytokines, so the

cytokines are also important for the adhesion molecules to develop.

* The function of T helper cells is cytokines production; interleukins &

other cytokines.

* T helper 2 activation usually leads to antibody production of the IgE

type and others (IgH: IgA, IgD, IgE), while IgG comes from T helper 1

and functions in opsonization.

* T helper 1: cell mediated immunity, interferon gamma, isotype

switching to IgG, then IL-6 and tumor necrosis factor.

B cell can present Ag to TH cell by class-2 MHC Ag. Those you see

between the APC at one side & the B or T cell on the other side we call

them the immunological synapse.

So when we talk about the immunological synapse, we are talking

about the Ag presented with class-2, ICAM-1, CD40, CD80 (B7) & on the

other side we have the receptor & the CD4, LFA-1, CD28, CD40LThis relationship in the synapse is VERY important in isotype switching.

So, what about Cd40 and Cd28? We said before that presentation with

class II MHC antigen to the receptor will provide just one signal, the

other signal should come from the accessory molecules in the synapse

as we said. These 2 signals will stimulate T helper cells to produceinterleukin 2 which induces the differentiation of B cells. Also,

interleukin 2 acts on a receptor called interleukin 2 receptor (Will be

explained in details later on). Remember that interleukin 2 is the major

cytokine needed for growth and differentiation, if it's not developed, we

will be severely immunocompromised. And in order for interleukin 2 to

be produced we need all the accessory molecules, so one signal here

isn't enough. The co-stimulatory receptor-ligand pairs CD40-Cd154

7/31/2019 Immuno - Lec 14

10/12

(CD40L) and CD80 (B7)-CD28 act synergistically to enable T-H cell

differentiation.

Hyper-IgM syndrome results from mutations in the CD154 gene, here

there will be no isotype switching to igG.

Here, the doctor is talking about figure 16.6 on p.124, just follow the

arrows. He added that cytokines are important for the development of

memory cells.

Clinical Applications:

There's a disease called Leprosy ( ), it's caused by

Mycobacterium leprae. This disease affects the terminal (?), so you

are going to have nodules and paraesthesia. They have noticed that if

you have (?) infection, then lepromatous leprosy results, and if you

have a localized infection we call that tuberculoid leprosy. In

lepromatous leprosy, Th2 cells are going to be activated (Note:

Mycobacterium tuberculosis activates Th1 cells which will

activatemacrophages..), antibodies will be produced (IgE type)

which have no effect, so patients usually die. Now, in tuberculoid

leprosy, small amounts of Th1 cells will be activated and interferon

gamma, interleukin 6, and TNF will be produced, these cytokines will

activate macrophagesEtc. (Note: Patients with tuberculoid leprosy

have a positive skin test reaction) Also, patients can fully recover

from this disease and they become immune to this bacterium.

Refer to page p.126, Acid-fast bacilli are present in all parts of thebody in lepromatous leprosy.

Refer to p.127, here is a patient with hyper IgM syndrome, he's

immunocompromised because IgM antibodies have low affinity and

no memory. So patients with this syndrome require gene therapy.

Remember, the molecule that is missing here is CD154 or CD40L (L

stands for ligand).

http://en.wikipedia.org/wiki/Mycobacterium_lepraehttp://en.wikipedia.org/wiki/Mycobacterium_lepraehttp://en.wikipedia.org/wiki/Mycobacterium_leprae

7/31/2019 Immuno - Lec 14

11/12

You can see how the cooperation is going to take place for the killing

process as well. Now, activation of CD8 T cells in order to develop

effector functions requires: (1) First signal: An antigen presented by

class IIMCH molecules. (2) Second signal: Interleukin 2 which we can

get from the same virus that has been trafficked through the

extracellular pathway or from T helper cells. (Note: We always require T

helper cells in the thymus-dependent mechanism, to provide us with

interleukin 2).

Notes:

1. In multiple scelrosis, sheaths around the axons will be destroyed byTh1 cells. We notice that during pregnancy Th2 are going to be

activated, so more antibodies will go to the baby. (Pregnancy reduces

the activity of this disease Beneficial effect). Th1 and Th2 can't be

activated at the same time; if we activate Th1 cells, we suppress Th2

cells and vice versa, we can use that in the treatment of autoimmune

diseases.

2. We talked about conjugate vaccines against Streptococcus

pneumoniae and Haemophilus influenzae B. The polysaccharides of

these encapsulated bacteria stimulate thymus-independent antigens,

resulting in the production of IgM antibodies only , so no memory cells

will develop and there will be no isotype switching. So, how are going to

deceive our immune system? We get the polysaccharide antigen and

mix it with a protein. The protein component of the conjugate can then

be processed and presented on the surface of B cell associated with

MHC class II molecules; this complex is going to be recognized by T

helper cells which in turn will produce cytokines needed for the

production of memory cells.

7/31/2019 Immuno - Lec 14

12/12

Chapter 17 Immunological Memory

After Initial exposure to a certain antigen, memory cells will be formed

and they will stay in our bodies for a long period of time. On a

subsequent exposure (Secondry exposure) to the same antigen, there

will be a more rapid and higher affinity immunological response and

these memory cells will be released. Refer to figures (17.1, 17.2) They

are very important. You have to remember that vaccines are used to

create memory cells for pathogens before we encounter them. Also,

there are qualitative and quantitative differences between naive cells

and memory cells (Will be explained later on). Keep in mind that a single

vaccine results in production of small amounts of memory cells, that'swhy it's important to give booster shots from time to time in order to

achieve full immunity. (Since, the length of survival differs from one

memory cell to another). One significant characteristic of memory cells

is that they have anti-death genes antibodies which will neutralize the

effect of death genes, so they can survive for a long period of time.

Note:

The expression of L-selectin (CD62L), the receptor that facilitates

homing to peripheral lymph nodes in T memory cells is variable,

because they are already activated, so they do need to go lymphoid

tissues. Effector T cells low; because they aren't needed anymore,

they are ready for action. Navie T cells High; because we need them

to bind and go to the assigned areas.

Done!

I'd like to give a shout-out to my dearest friend; Mai Mazen. Thanks a

bunch for helping me with this lecture :]

Tamara Shoter