00:01
So, B cells,
as their name suggests B
developed in the bone marrow.
00:09
So the bone marrow
is where B cells develop.
00:11
And in fact, virtually all the cells
of the immune system start off
from multipotent
hematopoietic stem cells
in the bone marrow.
00:22
So let's have a look
at how B cell development
in the bone marrow occurs
and the stages in B cell maturation
in the bone marrow.
00:32
I've already mentioned
they start off
from hematopoietic stem cells.
00:37
These develop into pro-B cells,
those hematopoietic stem cells
that instead of becoming
a mast cell line,
instead of becoming
a dendritic cell,
those that are going
to become B cells
initially become Pro-B cells.
00:53
The next stage in their development
is to become a Pre-B cell.
01:00
Then they develop into
immature B cells,
and of course, eventually
into mature B cells.
01:07
There are two stages in this
development process
where these B cells undergo
very, very
extensive cell division
in other words proliferation.
01:16
That's at a very early stage
in their maturation
at the stem cell, pro-B cell stage.
01:22
And then later on at the pre-B,
Immature B cell stage,
they undergo further
massive rounds of proliferation.
01:30
They dividing all the time.
01:31
But these two stages
in their development
are characterized by particularly
extensive cell proliferation.
01:41
There are two genes that are
called RAG1 and RAG2.
01:45
Stands for Recombination
Activating Genes.
01:49
Already mentioned that the
immunoglobulin genes in the B cell
undergo recombination
to create diversity.
01:58
These recombination
activating genes,
obviously play a role
and that as the name suggests.
02:03
And these genes are also expressed
two different points
in the development and maturation
of B cells
that the Pro-B, Pre-B stage,
and then they can become
reexpressed later on
in immature B cells
to fine tune the antigen receptor
on the B cell.
02:24
There's another molecule
which we'll hear about shortly
called TdT,
Terminal deoxynucleotidyl
transferase
Bit of a mouthful that
isn't it?
We normally just refer
to this TdT
because terminal
deoxynucleotidyl transferase,
it's quite long word to say,
so TdT.
02:44
And that's expressed again,
at the stage between
pro-B cells and pre-B cells.
02:51
If we look at the
immunoglobulin gene DNA
in the B cell,
and the RNA
that's producing that DNA
in the stem cell stage,
and the pro B cell stage,
these immunoglobulin genes
are what we call in the
germline configuration.
03:12
That means how these genes
are inherited
through the sperm and the egg,
the germline,
and every single cell
in the body
will have this DNA just like
you have genes for
all the different genes
you have in all the different cells.
03:26
And in all the cells in the body,
they will be in the germline,
orientation or configuration.
03:32
And it's only once cells developed
to become B cells
that these immunoglobulin genes
recombine.
03:42
And this recombination initially
starts off at the Pre-B cell stage,
and some segments
within the heavy chain genes
for the immunoglobulins.
03:53
Because immunoglobulins consists
of a heavy chain, and a light chain.
03:57
The first event that says,
"Hmm, that cell is
going to become a B cell."
is when the B cell recombines
its heavy chain genes.
04:08
And it recombines
three gene segments
as we'll see shortly,
they're called V, D, and J
Variable, Diversity, and Joining.
04:18
Once that the combination processes
occurred in the pre-B cell,
messenger RNA is produced
for the heavy chain
and that includes a segment that is
from the constant region called mu.
04:34
So you get mu mRNA
in the pre-B cell,
As that cell then develops
to the final stage,
before full maturity,
in other words,
the immature B cell,
the recombined
heavy chain gene,
associates the
product of that gene
associates with the
product of a recombined
light changing
Then the light chains, the V gene
segments and J gene segments
that recombine,
produce a light chain protein.
05:11
So now we have Ig for the DNA
that's recombined in the heavy chain
and the light chain,
we have a mu heavy chain protein,
and either a kappa or lambda
light chain protein
produced from the capital
lambda messenger RNA.
05:30
And then finally, we end up
via alternative splicing
of the V, D, and J
with a constant region
of the IgM antibody
which is encoded by a Cmu gene
or the IgD antibody
encoded by a C Delta gene.
05:52
So the stage of maturation
early on
the immunoglobulin genes
are notly combined,
so there's no expression in order to
express these genes,
they need to be recombined,
they only recombine in B cells,
and they only recombine
at the pre-B cell stage.
06:10
So you start off
at the pre-B cell stage
with the first indication that this
cell is going to become a B cell.
06:17
It has cytoplasmic mu heavy chain.
06:20
And on the surface,
it has what is called
the Pre-B cell receptor,
which is a surrogate light chain.
06:27
It's not the proper light chain,
but it's a sort of intermediate
light chain that's produced
to put together with
the mu heavy chain.
06:34
And then once the light chain genes
have been recombined,
you get the full mature
antibody molecule
on the surface of
this immature B cell.
06:44
So it's a mature antibody,
but it's on the surface
of an immature B cell,
and this is membrane IgM.
06:52
And then, as well as IgM,
another class of antibody,
the IgD class is put on the surface
of the mature B cell.
07:03
As well as having antibodies on
their cell surface
B cells have
thousands of other molecules
on their cell surface
just like every cell does.
07:10
And we can use certain molecules
on the surface of B cells
to identify which stage
in maturation they are.
07:18
So for example,
the molecule CD34
is present on the surface
of the stem cells.
07:25
It's also present on the surface
of the pro-B cell.
07:29
But additional molecules
become expressed
such as CD19, and CD10.
07:35
And then, as those pro-B cells
develop into pre-B cells,
there is low expression of a
molecule called B220.
07:45
But as that pre-B cell develops
into an immature B cell,
the expression of CD34 is lost.
07:52
So if a cell has a molecule
on its surface,
we describe it as a CD,
whatever positive cell,
if it doesn't have expression,
we describe it as a CD,
whatever negative cell.
08:04
So in other words,
the stem cells, the Pro-B cells,
and the pre-B cells
are CD34 positive,
whereas the immature B cell
is CD34 negative.
08:16
At that stage,
of the immature B cell
there is a low level expression
of IgM antibodies
on the surface of the B cell.
08:24
And then as the B cell
further matures
to become
a fully mature B cell,
the level of expression
of IgM increases.
08:33
So, where are these
different types of B cell
these different stages
of maturation found?
Well, the stem cells,
the pro-B cells,
the pre-B cells are all within
the bone marrow.
08:44
And then as these cells develop
into immature B cells,
they can leave
the bone marrow
and enter what immunologists
refer to as the periphery
rather strange term that
the periphery
simply means anywhere outside
of the primary lymphoid tissues.
09:01
So anywhere in the body that isn't
the bone marrow or the thymus,
immunologists refer to
as the periphery.
09:07
And the mature B cells
are found in the periphery
or move to the periphery.
09:14
The response to antigen
clearly, if the B cell doesn't have
an antigen receptor on the surface,
it can't respond it,
it has no way of responding
to antigen.
09:22
So because there is no
surface antibody on stem cells,
pro-B cells, or pre-B cells,
there's no response to antigen.
09:31
At the immature B cell stage
when there is
low level expression of IgM.
09:36
the B cells can undergo
what is called
negative selection or deletion.
09:41
And this refers to the fact
that if those B cells
encounter molecules
from our own body,
what we call self antigen,
then they become deleted
because we don't want B cells
that can attack
our own body components.
09:55
We want B cells
that can fight infection
coming from outside the body.
09:59
So they'll be deletion of B cells
that recognize self antigen,
and there can be receptor editing.
10:05
And this is why
RAG1 and RAG2
get reexpressed
at this stage
so that the receptor
can be changed.
10:11
If it reacts itself, it can be
changed to a different receptor
that reacts with a foreign pathogen
rather than reacting itself.
10:19
And then,
once they're mature,
of course the B cells
are fully able to respond to antigen
I mean, that's how
it should be obviously,
you need the B cells
to be able to respond
And they can become activated,
they can proliferate,
divide up in number,
and they can differentiate
and become specialized,
particularly become specialized
to produce a progeny of a B cell
that is called a plasma cell.
10:42
And what plasma cells do
is they secrete
very large amounts of antibody
of the same specificity
that acts as the B cell receptor
on the surface of the B cell.
10:53
So in the bone marrow,
we have these pre-B cells,
this early stage of development,
and they have
on their cell surface,
not a mature form
of the antibody molecule,
but a preliminary version,
if you like the antibody molecule,
where you have the normal version
of the IgM heavy chain,
which is called mu heavy chain.
11:17
But it's not a mature version
of the light chain.
11:20
They have what we refer to
as a surrogate light chain,
and this is made up of
two components.
11:24
One is called V pre-B,
and the other is called lambda 5.
11:30
And then as the pre-B cell
matures into an immature B cell,
it swaps the surrogate light chain
for a proper, a real light chain,
and that can
either be the kappa light chain,
or the lambda light chain.
11:46
So the mature B cell
now has on its cell surface,
a proper mature version
of the antibody molecule.
11:55
And initially,
these mature B cells
that have not yet
encountered antigen
we call them naive B cells.
12:02
They're not yet met antigen at all.
12:04
They express on their cell surface
two different classes of antibody
IgM, surface IgM,
and surface IgD.
12:14
And these cells
have left the bone marrow
and travel to the
secondary lymphoid tissue.
12:20
And they sit in wait
in the secondary lymphoid tissue
in case their particular antigen
the antigen they recognize
comes along.