Today's lecture is on the Development and
differentiation of B cells.
We are surrounded all the time with millions
of bacteria, viruses, pathogens, and if it
was not for our very developed, well developed
immune system, we would be succumbing to various
infections from time to time.
You have already been, by now, introduced
to the several cells of the immune system
as well as the several interconnections they
make with each other, to provide now, this
very well balanced immune system which can
keep us safe from the myriads of antigens.
I would like to recapitulate your memory and
go back to the basics of immunology, immune
system, which can be studied under two separate
headings: one is the innate and the other
is the acquired.
Now, though as I have already said that there
is a network between the innate and the acquired
immune system, in fact, all the cells of the
immune system innate is different from the
acquired in that the cells, for example, neutrophils,
eosinophils, basophils, macrophages - all
of them act to protect us, but they do so
by recognizing patterns which are common amongst
viruses, bacteria, etcetera.
So, there are set of patterns that these cells
recognize.
On the other hand, the acquired immune system
which comprises mainly of T and B cells, and
of course, several accessory cells, which
we will come to, later.
The T and B cells are specialized cells that
recognize not only immune assay sequences
of a particular protein, but also the confirmations
associated with this, these sequences.
These cells can also recognize carbohydrate
structures.
So, like I said T and B CELLS are a part of
the acquired immune system, but they do need
help from cells such as monocytes or macrophages.
These cells are known to participate in the
humoral immunity.
And let us look at what are B cells.
right You probably have been already introduced
very briefly, I would like to talk a little
bit more with respect to the markers on the
cells as well as what the cells actually do
with respect to their function.
B cells are the only cells of the immune system
that synthesize immunoglobulins.
What are immunoglobulins?
You perhaps know these molecules as antibodies.
Immunoglobulins are either present as cells
of its antigen receptors, which would mean
that they anchor to the cell membrane of B
cells.
Alternatively, immunoglobulins are antibodies
which are secreted from the cells.
I have here a picture that depicts an antigen
activated B cell which has these cells of
its receptors - the immunoglobulins, recognizing
this antigen in response to this recognition
and binding, the B cell gets activated, and
through a large number of processes which
in fact happen in between, which is clonal
proliferation and differentiation, the ultimate
cells are plasma cells which secrete immunoglobulins.
Now, when they are secreted, these would be
called antibodies.
I would like to go into several of these processes
when I deal with the development of B cells.
So, when I talk about development, it also
describes the weight, as cells are synthesized
in the bone marrow and their cells of receptors
to the stage where they have started to now
become plasma cells to secrete antibodies.
I already told you that B cells have surface
immunoglobulin and there are a very large
number of these surface immunoglobulins on
every B cell, and the number appears quite
large, close to 0.1 million.
I just like to mention here, though we will
go into this in detail.
Every B cell has these 0.1surface immunoglobulin
as antigen receptors which are absolutely
identical.
I will be coming to this over and several
times, but everyone B cell recognizes only
one type of antigen, and therefore, all the
immunoglobulins or the antigen receptors are
identical.
Apart from surface immunoglobulin, there are
several other markers of B cells.
I will just list them out.
B220 which is a phosphatase; it is an enzyme
which is required for initiating signaling.
I will be dealing with signaling in my next
class.
So, I would like you to remember, apart from
B220 - the phosphatase, there is another molecule
which is very specific, only to be precise,
which is CD 19, and it is suffice you have
to remember that CD 19 is a co receptor.
MHC class II molecules - MHC (Major histocompatibility
complex), you have been introduced to this
molecule as well and you probably recall that
MHC class II, I mean MHC class II molecule
are present on antigen presenting cells.
Just to remind you that MHC class I, on the
other hand, are molecules that are present
on all nucleated cells; MHC class II molecules
are very necessary for the activation of B
cells.
Apart from monocytes as well as dendritic
cells, MHC class II molecules are present
also on B cells.
So, B cells are also antigen presenting cells.
Apart from these, there are also molecules
called complement receptors.
As the name suggests, these receptors are
able to fix complement.
Now, again, all this will be discussed in
the subsequent lectures.
I would like you also to remember FC receptors.
So, from all these molecules, B220, CD19,
MHC class II molecules, FC receptors for IgG
- all these participate in the signaling that
is initiated by the surface immunoglobulin,
after the surface immunoglobulin or the antigen
receptor here binds to its cognate antigen.
Why do we call these markers on B cells?
That is because one can have antibodies or
reagents which recognize separately each one
of them, and one can simultaneously bind all
the reagents to these molecule on a particular
B cell.
And if you have ways by which you can detect
each one separately, on any B cell you will
be able to detect each one.
Where are these B cells synthesized?
First of all, let me talk a little bit with
respect to evolution.
Now, during the course of evolution from a
huge cellular organism to a multi cellular
organism, there have to be a parallel development
of our system which would be able to keep
the self from the non self; in other words,
keeping the self, protected from invasion
by a non self.
So, the immune system, in fact, is developed
likewise; it is obvious then, that as the
complexities in evolution grew, even the immune
system developed very slowly to meet with
all the challenges that multi cellular organisms
had, trying to keep the unicellular organisms
from invading time.
When do you see the appearance of B cells,
and therefore, the immunoglobulins in the
evolutionary scale, if you remember, you have
non-cordata and cordata or the phyla, you
might remember that in vertebrata, you have
the jawless versus jawed fishes.
So, it is in the jawed fishes that B cells
first make the appearance, and you see a few
isotypes of the immunoglobulin, and of course,
isotypes is the new word now, but I will be
discussing that later.
.
Site of B cells synthesis, therefore, can
change; however, in mammals, in mice and human,
both of which have been studied extensively.
So, what is true of mouse is essentially true
in humans; before birth, B cells make the
appearance in the yolk sac; that is the first
site of synthesis, this is taken over subsequently
by the fetal liver, and then on to fetal bone
marrow.
After birth, it is only the bone marrow that
is the site of B cell synthesis.
The B cell development itself is the complex
process.
These cells start of as stem cell; well, stem
cells are these starting cells for almost
all cells of our system, but the good part
of our B cell development as for all lymphocyte
cells that there is a common progenitor.
So, the common progenitor is the stem cells
of the bone marrow and B cell developments
can be looked in distinctly two pathways:
one that is called antigen independent, which
would mean that inspite of no antigen being
present, B cells develop to their fully formed
surface immunoglobulin expressed cells, which
are ready to recognize the following antigen.
Antigen independent phase occurs in the bone
marrow.
The second phase is the antigen dependent
phase which occurs in the peripheral circulation
after encountering the specific antigen.
Are B cells immortal?
No.
The life span of B cells just 2 to 8 weeks.
Of course, it depends on what you call the
life span, as I will be coming to in the next
slide.
.
Every stem cell or the progenitor cell, when
it is committed to becoming a B cell goes
through these several stages and I will be
describing each and every one, but there is
always a large number of cells that are generated
from a single stem cell or single B stem cell.
Now, this is in the scenario for bone marrow
where you have large number of stromal cells
and it is absolutely essential that stromal
cells are in the vicinity of the dividing
developing B cell.
People have tried to look at the development
of B cells outside of the bone marrow.
And if only the stem cells are taken out and
even if they are provided several of the cyto
kinds that are required for development and
growth factors, the cells in fact do not develop
into mature B cells.
Therefore, people started looking at what
goes on between the developing B cell as well
as these stromal cells, and have come to the
following information.
A stem cell starts to express certain cell
addition molecules on the cell surface.
What are these cell addition molecules?
On the B cell it is VLA 4 and on the bone
marrow stromal cell its VCAM; that is actually
cell addition molecule.
These two molecules interact with each other,
rather specifically, and brings the pro B
cell in close proximity to the bone marrow
stromal cells.
So, one can look at this molecule is something
like tethering.
So, the B pro B cell is tethered to the bone
marrow stromal cells, such that they can be
the second set of interaction.
The second set is between the receptor which
is called C kit on the product B cell and
the stromal cell, stem cell factor.
The specific interaction between these two
cells of this protein now signals the pro
B cells to start dividing.
Because of lack of space here, I have not
been able to show more than two cells, but
in fact, this can go to several thousands.
What happens?
Also in response to this C kit binding to
the stem cell factor, there is expression
of the receptors on the B cells which were
not there earlier.
These receptors are called interleukin 7 receptors.
Stromal cells constitutively express the ligand
for these receptors and these are called interleukin
7.
Now, you have binding of the interleukin 7
which is being synthesized by the stromal
cells that bind specifically on the interleukin
7 receptor, which now the B cells have acquired.
Once the B cells, pro B cells have acquired
the interleukin 7 receptors, these are now
called pre B cell.
After the IL 7 receptor binding to its cognate
like and that is for the development of the
pre B cells to immature B cells.
During this process, not only are the cells
proliferating to make more and more of their
kind, but they also undergo a recombination
process which happens in the genes of the
immunoglobulin.
The recombination, event that happens in these
cells to, now, tell the cell that you make
a surface immunoglobulin and of the type that
can recognize a specific antigen.
This is a very complex process and I think
I will be dealing with that, and discussing
with you 3, 4 lectures from now.
Just like you to remember, that at this stage,
the reorganization of the immunoglobulin gene
because of which, now, this transcription
of the immunoglobulin gene and cells surface
expression - first of the isotype IgM.
All the cells here that you see on the screen
are immature B cells which express only IgM.
This is only about one molecule or two molecules
per cell which are depicted here, but remember,
each cell would have about 10 to the power
5 or 0.1million immunoglobulin on the cell
surface.
We are still with the antigen independent
phase which is still in the bone marrow up
to here.
So, we go back now to a cell which is immature
B cell which has only one isotype, which is
capable of recognizing or interacting with
this cognate antigen.
However such a immature cell is not ready
for the immune response that is seen, generated
upon activation.
It requires another immunoglobulin molecule
of the isotype IgD, and now from the immature
B cell, when two molecules are two different
molecules expressed on the cell surface IgM
and IgD, then such cells are mature cells.
I would like to emphasize here, however though
the isotypes are different, the capacity for
each of the molecules here to bind to only
one epitope or one antigen still remains.
Therefore, now every B cell has the capacity
to recognize only one type of antigen.
Once the cell has acquired even the second
isotype, IgM plus IgD cell leave the bone
marrow and enter the circulation.
I have talked about several of these molecules
so far.
So, let us just go over the antigen independent
phase once more, to know that you have the
stem cell which becomes a pro B cell which
then becomes the pre B cell, immature B cell,
and mature B cell.
The characters have moved a little bit away
from the cell, but this is the final cell
which is a mature cell bearing cell surface
immunoglobulin IgM, Immunoglobulin IgD, it
also had that CD19, just to make you remember
that it is a co receptor for this, and also
that phosphatase.
Now, you have the same on the cell which is
just before, which is the immature B cell,
which has everything this cell has, except
does not have IgD.
When you go back to the previous, you have
cell surface expression of CD19, B220, but
these cells have also the expression in the
cytoplasm of two enzymes RAG 1 and RAG 2.
RAG - recombination activating gene, one and
two.
Before that, the cells have again RAG 1 and
RAG 2, and again the same cell surface markers
which keep which are there, you can see throughout
the cell development.
There is also expression in these two stages
of an enzyme called terminal deoxyribose transferase
or TDT.
Both RAG 1, 2 as well as TDT participate in
the immunoglobulin gene rearrangement.
You have the immunoglobulin rearrangement
taking place in the pro B cell as well as
the pre B cell.
Like I told you earlier, this complex process
of reorganization of the immunoglobulin genes
will be dealt with later.
Now, we come to the antigen dependent phase.
Though I have said this a couple of times
earlier, I would like to reemphasize that
everyone B cell has receptors; does not matter
if it is both IgM and IgD, both of them have
the capacity or the antigen binding pocket
which are identical.
Therefore, they both would be able to recognize
only one type of antigen or one set of amino
acid sequences.
This is the mature B cell now.
It is in the circulation in the periphery
out of the born marrow; it comes across its
cognate antigen.
This is a very specific binding and the strength
of the signal that is sent to the cell as
response to this antigen receptor binding,
determines how much of an immune response
in terms of antibody secreted happens.
Let us look at that little bit closely.
You have the antigen binding, now, to the
cognate receptor on the B cell, in which it
the cells now acquires an activated phenotype.
I have not shown here, but I like you to remember
that the activated phenotype would mean that
there would be larger number of MHC class
II molecules which are expressed.
There are also other molecules which are cells
of its receptors for two key cyto kinds called
IL4 and IL5, interleukin 4 interleukin 5 which
cell synthesizes.
These are synthesized by activated T cells.
Therefore, one can imagine that the same antigen,
now it is activating a B, a T cell.
The T cell, now, in its activated state starts
to make interleukin 4 and 5 which is recognized
by the cell and the cell starts proliferating.
So, the interleukin 4 and interleukin 5 are
also known as B cell growth factors.
They allow the cell to start proliferating.
All the cells of the acquired immune system,
T as well as B cells have this one property
that before they becomes effective cells,
in case of B cells the effector cells are
plasma cells, they need to undergo proliferation
at first and subsequently differentiation.
Now, the proliferating cells are differentiating
cells are very different from each other in
the way they go on with their lives.
Plasma cells are the antibody producing cells
or the n state cell.
So, once a B cell becomes a plasma cell is
destined to die; these cells do not live for
more than 3 to 4 days, and even with in experiments
where there are support systems that are given,
these cells would be able to live maximum
for 15 days.
What I have shown here is one cell which is
becoming 2, then becoming 4, after binding
to interleukin 4 and 5, but in fact they are
one cell could become as many as 100 1000
cells.
So, there would be a clone of cells which
is quite identical to the first cell.
There is one more aspect to B cell differentiation
that not all cells differentiate to antibody
producing plasma cells.
Now, what strikes you here I think would be,
what I have shown here as rounded cells during
proliferation, now become oval; then I call
them plasma cells.
In fact, that is precisely what happens.
You have each of these cells which is proliferating
till here we start binding to or they develop
receptors to interleukin 6 first.
So, then they are ready for the differentiation
and the proliferating cells develop receptors
for interleukin 6, and interleukin 6 binds
to these cognate receptors.
They differentiate now to antibody producing
cells.
They cannot proliferate any longer.
Now, most of the cells have become antibody
producing cells, and as you can see that,
the nucleus has become polar as compared to
the nucleus which is occupying the center
of the proliferating cell, and it is also
covering almost covering the entire cytoplasm.
You can see that the cytoplasm is increased
and there is a very well developed network
of endoplasmic reticulum which is absolutely
required for the protein synthesis here.
These cells need to synthesize and secrete
immunoglobulins.
Another important aspect here is that not
all cells become antibody producing cells,
plasma cells.
A few of them which could be because of lack
of adequate I l 6 become what are known as
memory cells.
I will talk about the memory cells a little
later, but memory cells are very important
part of the immune system, the acquired immune
system.
You do not have memory cells in the innate
immune system.
These memory cells, they are long lived; they
stay on quietly until the body is exposed
to the same antigen again; the good part of
the memory cells then is that they already
have receptors which can allow them to get
activated and differentiate very fast.
Therefore, memory cells are the ones that
actually generate a very fast and a robust
immune response.
Just like to mention here that in during the
development of vaccine, it is important to
have adequate amount or numbers of memory
cells generated; otherwise, the vaccine, well,
is of not much importance.
Recombination - as I have already told you,
recombination of the immunoglobulin genes
is going to be discussed in a later lecture,
but just to give you a flavor of what happens
in the pre B cells before they become immature
and mature B cells, here, I am depicting only
the heavy chain, as you all probably already
know that the immunoglobulin is made up of
two heavy and two light chains.
Both the heavy chains are identical in the
amino acid sequence and both the light chains
are also identical.
It is the combination of the light chain with
the heavy chain that forms the antigen binding
pocket.
Each of the heavy chain is made up of three
different gene segments at the antigen binding
site.
You have the V, D and J.
There are three 100 to 1000 different V gene
segment; there are fewer of D. In human, they
could be as many as 6 to 11, and J which are
even few may be 5 to 6.
Nature has it that during the recombination
process, any one of the J now binds to any
one of the D. So, you have a DJ segment which
is initiated by the two enzymes are talked
about a little while ago; the recombination
activating gene one and recombination activating
in gene two.
After DJ combination, there is recruitment
any one of the V. Therefore, now you have
VDJ recombination.
The entire event from pro B cell to pre B
cell and the immature B cell is under the
directive of this recombination which is through
these specific enzymes.
Now, VDJ that is heavy chain, after it is
ready, this transcription and you have generation
of only the heavy chain.
At this stage, the cell recruits a Psudo light
chain before the assembly of the light chain
recombination event that takes place.
Now, the Psudo light chain along with the
heavy chain is expressed on the cell surface.
Until now the D cell cannot recognize antigens.
It is only after recombination of the light
chain light chain which is again needed through
RAG 1 and 2 that you have the entire heavy
and light chain coming together as a cell
surface molecule.
Let us look at the basic structure of the
immunoglobulin.
You have the two heavy and the two light chains.
The heavy chains are as in black and the light
chains are shown here in red.
The light chains are bound to the heavy chain
through disulfide bounds. also the two heavy
chains are held together tightly by disulfide
bounds.
The light the heavy chain also has two distinct
carbohydrates, there is a slight move.
Here, it is CHO denoting carbohydrate.
This part of the immunoglobulin, it is forms
the antigen pocket, antigen binding pocket,
and therefore, it is called the binding domain
and two-thirds of the molecule is the effector
domain.
We will be dealing with the functions of immunoglobulins
in subsequent lectures.
Important here is the facts, that the C terminals
of the immunoglobulin can either have hydrophilic
or hydrophobic regions.
And if the replacements of a short stretch
of hydrophobic to hydrophilic determines whether
this molecule should stay as cells of its
receptor or should be secreted from the cell
as antibody.
Again, to keep happing on the same thing,
one B cell can make only one type of antibody,
which would mean that once the recombination
process of the immunoglobulin gene has happened,
then the variable domain remains the same,
and therefore, now every B cell and its progeny
will be able to recognize only one type of
antigen.
In fact, let us be a little bit more specific,
will be able to recognize only one set of
sequences.
Like you to see this, the antibody diversity
that we have, mammals have, we have the capacity
to recognize a 100 million different epitopes
or antigen determinants.
Antigen, the third point I have on the slide
says antigen induces clonal expansion.
What does this mean?
The B cell, when it is already when it is
produced in the bone marrow and comes into
circulation, in fact, already is predetermined
with respect to the kind of antigen it recognizes
because the variable domain has already been
established.
There could be some amount of receptor editing,
but that is minimal.
What I like, what I try to show here, there
are thee B cells here; each one of them seems
to have the same type of IgM and IgD on their
cell surfaces because these are mature cells,
but the small region are in the amino terminus
which actually binds, makes the antigen binding
pocket is very different, so that now cell
number 1 can recognize this type of antigens,
cell number 2 recognizes these yellow antigen,
and cell number 3 red.
And it is when, such as number 1 manages to
recognize its antigen; it comes across it
is cognate antigen; then there is clonal proliferation.
I would like to just show you this.
So, in the form of, let us say, an experiment
where a rabbit has been injected with an antigen.
Let us say it is.
If one injects the rabbit on day one and one
starts to take blood samples from the rabbit
every other day, and then does an immunoassay
to find out what is the level of antibodies
specific to this antigen which has been injected
on day one.
Now, this is the pattern one would see most
of the time.
You see almost no antibody in the circulation
for something like 5 days, this would, of
course, depend on the sensitivity of the immunoassay.
Let us say one is using a Lisa to detect.
So, there is nothing, let us say, till about
day 5, and then slowly the amplitude increases,
which would be maximum at around day 9, 10
and then rapidly falls down.
If the antigen is now withdrawn subsequently,
if the anomaly is given with same antigen,
then one can see there is almost no lack phase
from day 1 to day 5, and if one measures the
immunoglobulin here again, sorry antibodies
here to the specific antigen, this could not
only stay in circulation for a longer time,
but also the amplitude of the response also
increases.
Now, why is there no lack phase here?
There is no lack phase here because of the
generation of memory cells during the course
of the primary stimulus.
This is the primary stimulus and this is the
secondary stimulus, and you have B cells which
are already now mature B cells which recognize
the antigen.
Only those cells which have cognate receptors
for this antigen would recognize, start to
proliferate, differentiate to plasma cells
and one clone of cell were to make now antibody
to that particular antigen.
There are memory cells generated here, which
sit quietly in the secondary lymphatic organs
which are, as you have already learned, the
spleen and the lymph nodes.
Once the animal sees the antigen, the same
antigen for the second time, then the memory
cells get into action.
They have receptors already for the interleukins.
So, they proliferate extensively very fast,
and of course, you have not only number of
cells increasing, but you also have the memory
cells which allow the memory to remain for
a longer period, and therefore, you have this
antibody which is large amount of antibody
over a larger period of time.
What would happen if you were to inject the
animal for the third time?
The amplitude of the response would again
perhaps increase.
This is determined by the antigen, but this
cannot go on.
There will be a decrease in the amplitude
with subsequent immunizations and we will
discuss that when we come to this signaling
events that B cells have.
Now, let us go back to the antigen induces
clonal expansion.
The clonal expansion, in fact, it is what
now gets translated to amount of antibodies
secretion.
So, if you have more antibody circulation,
you can imagine that there are more clones
that have been expanded and you have a very
large number of plasma cells.
Always the primary immune response and the
secondary immune response would have a difference
in the isotopes.
Again, we will be dealing with that detail
later, but suffice you have to say that the
primary immune response usually has IgM type
of antibody that is synthesized, and in the
secondary response you can have the other
isotype; you can have IgG you can have IgA
and you can also have IgG.
Are there any different types of B cells?
I have been talking about only one type, but
in fact, there are another type of B cells
- marginal zone B cells.
The type of B cells apart from B cells and
memory cells you also have marginal zone B
cells.
Now, these I should have probably talked about
earlier, but these have been discovered about
something like seven to eight years ago.
Marginal zone B cells are also called primitive
B cells.
They are totally T cell dependent.
Now, I have not talked about dependence of
T cells so far, but will be talking about
that in my next slide.
Now, marginal zone B cells are usually found
in the outside of the germinal center of the
spleen.
Germinal center - you have been introduced
to germinal centers; you know these primitive
B cells or marginal zone B cells are present
at the out skirts of this germinal center
and they generate and very early very fast
exuberant antibody response.
These cells are called primitive because they
do not take the time for clonal proliferation
the way I have explained in my last slide,
but they already have large number of interleukin
6 receptors which allow them to immediately
become plasma cells and start making antibodies.
A little bit about memory cells.
Are they different from the B cells? because
I said earlier, I did say that.
You know what I have been talking about so
far are these B cells and memory cells, but
I did not say much about the memory cells,
memory cells which are generated.
You can see that these are very long lived
because I told you that they participate again
in a very fast immune response.
When in the experimental animal the antigen
is injected for the second time, memory cells
have a very good number of these interleukin
6 receptors as well as interleukin 4 receptors,
both of which are absolutely essential; interleukin
4, first for proliferation.
That is how they are quite different from
the naive B cell; B cell which is not encountered
is antigen.
People have tried to look for specific markers
on memory cells, and as of now, there are
not too many which designate or differentiate
the memory cell from the naive B cell.
Another important aspect of the B lymphocyte
development is negative selection.
We always talk of the immune system with respect
to self versus non- self that the immune system
is able to recognize non-self from self, and
is able to mount an immune response only to
non self.
Now, how should this happen?
It is very it is very difficult to envisage
that B cells will be able to recognize each
and every pattern on each and every molecule,
and say and go through some kind of a computer
driven program, that says, yes, this these
are foreign and not self.
Therefore, we come down to looking at how
B cells can do this, and this process is known
as negative selection, in fact, quite a simple
process.
In the bone marrow, self-reacting B cells
are eliminated.
How does this happen?
Now, in the previous slides where I talked
about the differentiation sorry the development
of B cells, there are these I g yeah there
are these IgM bearing cells which are called
immature B cell and there are these mature
B cells which have both IgM as well as IgD.
Now, if such a cell which has only IgM on
its cell surface, if such a cell encounters
its cognate antigen, this cell is destined
to undergo apoptosis; apoptosis or program
cell.
Perhaps you will be dealing, we will be dealing
with apoptosis at a later stage, epitopes
itself is, probably it is a lot to learn in
this process, but the program cell that would
mean that the cell which undergoes apoptosis,
it kills itself signals to kill itself, such
that the cell is very nicely the death cell
is taken away very efficiently by microphages,
and death cells do not stay back which can
cause inflammatory responses.
So, if such an immature cell which has only
IgM innate cell surface is now exposed to
its cognate antigen, there is a signal for
such a cell not to undergo proliferation,
but to undergo apoptosis.
Now, soon after that, this stage, when it
is double positive which would mean as IgM
and IgD on the cell surface, and such a cell
now recognizes the antigen; that is the positive
signal.
Now, what does IgD do?
How is it that IgD can rescue IgM bearing
cells from apoptosis?
People were trying to look at several molecules
inside the cytoplasm, tried to see whether
the IgD receptor is anyway involved in anti-apoptotic
signal.
The only thing that one was able to decipher
that the study is that during the stage of
single immunoglobulin bearing cells to double
immunoglobulin bearing cell, there are large
regulation of large number of molecules which
are inside the cell and which are nothing
to do with the receptor, but they give the
anti-apoptotic signal or one can say pro survival
signal.
So, when such a cell meets with this cognate
antigen, the cell can undergo proliferation.
So, it is only when the cells leave the bone
marrow or rather when they have these two
receptors, can they leave the bone marrow.
In the bone marrow, one should have to think
in terms of total compartmentalization bone
marrow versus the circulation and the secondary
lymphatic organ.
Though bone marrow is the primary lymphatic
organ, it is well connected with the entire
circulatory system and you do have antigens
that travel from rest of the body to the bone
marrow.
There would always be cells which have only
IgM on the cell surface, and If this particular
cell for example, is destined to recognize
self-antigen, then the self-antigen would
have entered the bone marrow and now induce
apoptosis in the immature cell.
So, this is the basis for negative selection,
all well I cannot say all, because we do have
atomin disorders.
Very many, most of the self-reacting B cells
are eliminated in the bone marrow at the stage
when the cells have only IgM on the cell surface.
In mice, this has been studied greatly in
mice.
In mice, the bone marrow produces 50 million
B cells per day; that is very large number
for a small number mouse that is about 15
grams, but of these, only 10 percent get into
circulation.
So, only 5 million get into circulation and
it has been shown 90 percent of the cells
undergo apoptosis.
Now, the apoptosis here is not because of
only negative selection.
There are other reasons and we will be dealing
with that when we come to recombination of
the immunoglobulin genes.
People have shown this innumerable number
of times that if you take immature cells and
you cross link the receptors of the of these
cells by anti IgM, the cells undergo apoptosis.
Now, my next class is going to be signaling,
but before that I would just like to introduce
to you antigens which would be thymus independent
and thymus dependent.
What does this mean - Thymus dependents versus
independents?
B cells also bearing those type of receptors
for either the let us say polysaccharides
or a globular protein would determine whether
that that B cell would require T cell help
or not.
Thymus dependent, actually one can even think
it with their also commonly known as T cell
dependent T cell independent antigens; of
the two antigens, let us just look at the
independent one.
95 percent of the B cells require T cell help.
You remember where the T cell help comes?
It is in the generation or of the interleukin
4 and 5 receptors on the B cell upon activation,
which require interleukin 4 and 5 synthesize
and provided by the T cell.
In fact, not only the T and B cells interact
with each other, and that is what we will
see in the next class.
Thymus dependent antigens, like I said, are
T cell dependent.
Antigens constitute over 95 percent of the
total antigens that we can we have.
People have studied thymus independent antigens
are very few.
They also can be looked at two different types:
one is type one which polyclonaly activate;
they are called polyclonal B cell activators
are just like lipopolysaccharide.
These are recognized by toll like receptors.
Toll like receptors, you may have already
studied, are receptors which are present on
the innate the cells of the innate term of
the immune system.
Type two, on the other hand, is polymeric
proteins or polysaccharides.
These are able to cross link receptors and
activate them.
Therefore, they do not need all this.
This will become much more clearer when I
start with my signaling class.
I would like to, however, point out that of
the two, the thymus dependent versus thymus
independent antigens of B cells, the immune
response to T dependent antigens is much stronger,
which would mean that the amplitude of the
antibody response, that I talked about little
while ago, increases much is much higher for
T cell dependent antigens.
Importantly, it is only the T cell dependent
antigens which induce B cells to generate
memory cells.
Another important aspect is that of isotype
switching.
I have not talked too much about that except
I just mention that a B cell makes IgM type
of antibodies first to the same antigen, after
it becomes the memory cell and generates a
second response, the IgM isotype is switched
to IgG or any one of the three other isotypes.
It is only the thymus dependent antigens that
induce the B cells to switch the isotype and
other important part is it is only the T cell
dependent antigens that induce the cells B
cells to undergo affinity maturation.
All these I am going to able to discuss in
detail after I talk about signaling, and as
we deal with also the recombination events
that take place for the different regions
of the immunoglobulin gene to come together,
and make a complete heavy chain or a complete
light chain.
Another thing what I would like to mention
here before we go to the next class is, and
perhaps you might like to look at or at least
get to know about the different signaling
molecules that are present in the B cell.
Now, though B cells and T cells are part of
the acquired immune system, the way they recognize
antigens are totally different.
Both of them are able to recognize antigens
of different types, different sequences of
amino acid, but whereas, T cells require presentation
of the peptides or amino acid sequences in
the context of major histocompatibility complex
one or two, B cells recognize antigens in
the native confirmation.
So, let us look at the difference between
B and T cells now.
When there is a pathogen that enters the body,
B cells which are in circulation, and if they
have the cognate receptors for this antigen
bind to them get activated, start proliferating
and differentiating.
The same B cells can internalize this antigen
again after binding to the surface immunoglobulin
or the antigen receptors, internalize the
antigen, process it with respect to making
small fragments or peptides of the antigens.
This is loaded on the class II molecules and
presented to.
So, you can see that B cell receptors are
so different from T cell receptors in this
one or the first event that happens, that
is recognition of the antigen.
In my next class, I will be dealing mostly
with B cell signaling, but one can imagine
that B cells and T cells are so similar in
the way they divide and they differentiate
to their final differentiated or the n cell
stage.
The signaling molecule should be very similar.
However, in my next class, I will be able
to bring all this out and discuss this in
detail.
alright.
Thank you.
