So B-cells, as their name suggests, ‘B’, develop in the bone marrow.
So the bone marrow is where B-cells develop.
And in fact, virtually all the cells of the immune system start of
from multipotent hematopoietic stem cells in the bone marrow.
And B-cells can become fully mature within the bone marrow.
They don’t need to go anywhere else to become mature B-cells, they develop within the bone marrow.
Once they are mature, they can leave the bone marrow and travel to the secondary
lymphoid tissues; the mucosa-associated lymphoid tissue, the lymph nodes and the spleen.
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.
I’ve already mentioned, they start off from hematopoietic stem cells.
These develop into pro-B cells.
Those hematopoietic stem cells, that instead of becoming a mast cell, or instead of becoming
a dendritic cell, those that are going to become B-cells initially become pro-B cells.
The next stage in their development is to become a pre-B cell.
Then they develop into immature B-cells and of course eventually into mature B-cells.
There are two stages in this development process where these B-cells
undergo very, very extensive cell division, in other words proliferation.
That’s at a very early stage in their maturation, at the stem cell, pro-B cell stage.
And then later on, at the pre-B, immature B-cell stage, they undergo further massive rounds of proliferation.
And they’re dividing all the time, but these two stages in their development
are characterized by particularly extensive cell proliferation.
There are two genes that are called RAG1 and RAG2, stands for Recombination Activating Genes.
We already mentioned that the immunoglobulin genes in the B-cell undergo recombination to create diversity.
These Recombination Activating Genes obviously play a role in it as their name suggests.
And these genes are also expressed at two different points in the
development and maturation of B-cells, at the pro-B pre-B stage.
And then they can become re-expressed later on in immature B-cells, to fine tune the antigen receptor on the B-cell.
There’s another molecule which we’ll hear about shortly called TdT - Terminal Deoxynucleotidal Transferase.
Bit of a mouthful that isn’t it?
We normally just refer to it as TdT because Terminal Deoxynucleotidal Transferase is
quite a long word to say, so TdT. And that’s expressed again at the stage
between pro-B cells and pre-B cells. If we look at the immunoglobulin gene DNA in the B-cell
and the RNA that’s produced from that DNA. In the stem cell stage and the pro-B cell stage, these immunoglobulin
genes are what we call, in the germ line configuration. 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. And in all the cells in the body, they will be
in the germline orientational configuration. And it’s only once cells develop to become B-cells
that these immunoglobulin genes recombine. And this recombination initially starts off at the
pre-B cell stage, and some segments within the heavy
chain genes for the immunoglobulins… because immunoglobulins
consist of a heavy chain and a light chain. The first event that says, “Mmm, that cell, that
cell’s going to become a B-cell”, is when the B-cell recombines its heavy chain genes, and it
recombines three gene segments as we’ll see shortly. They’re called V, D and J -
Variable, Diversity and Joining.
Once that recombination process has 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 (µ).
So you get µ 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 chain gene.
Because in the light chains, the V gene segments and J gene segments that recombine produce a light chain protein.
So now we have Ig for the DNA that’s recombined in the heavy chain and the light chain.
We have a µ heavy chain protein and either a kappa (k) or lambda (λ)
light chain protein produced from the k or λ messenger RNA.
And then finally we end up via alternative splicing of the V, D and J with the constant region
of the IgM antibody which is encoded by a Cµ gene or the IgD antibody encoded by a Cδ gene.
So the stage of maturation; early on the immunoglobulin genes are not recombined, 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.
So you start off at the pre-B cell stage with the first indication that this cell is going to become a B-cell.
It has cytoplasmic µ heavy chain and on the surface it has what is
called the pre-B cell receptor, which is a surrogate light chain.
It’s not the proper light chain but it’s a sort of intermediate
light chain that’s produced to put together with the µ heavy chain.
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.
So it’s a mature antibody but it’s on the surface of an immature B-cell, and this is membrane IgM.
And then as well as IgM, another class of antibody, the IgD class is put on the surface of the mature B-cell.
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.
And we can use certain molecules on the surface of B-cells to identify which stage in maturation they are.
So for example, the molecule CD43 is present on the surface of the stem cells.
It’s also present on the surface of the pro-B cell.
But additional molecules become expressed, such as CD19 and CD10.
And then as those pre-B cells develop into pre-B cells, there is low expression of a molecule called B220.
CD43 is still there.
But as that pre-B cell develops into an immature B-cell, the expression of CD43 is lost.
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.
So in other words, the stem cells, the pro-B cells and the pre-B cells are CD43 positive,
whereas the immature B-cell is CD43 negative because it has lost expression of CD43.
At that stage of the immature B-cell, there is a low level expression of IgM antibodies on the surface of the B-cell.
And then as the B-cell further matures to become a fully mature B-cell, the level of expression of IgM increases.
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.
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.
So anywhere in the body that isn’t the bone marrow or the thymus, immunologists refer to as the periphery.
And the mature B-cells are found in the periphery, or move to the periphery.
The response to antigen; clearly if the B-cell doesn’t have an antigen receptor
on its surface, it can’t respond, it has no way of responding to antigen.
So because there is no surface antibody on stem cells, pro-B cells or pre-B cells, there’s no response to antigen.
At the immature B-cell stage, when there is low level expression of IgM,
the B-cells can undergo what is called negative selection or deletion.
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.
We want B-cells that can fight infection coming from outside the body.
So there’ll be deletion of B-cells that recognize self-antigen and there can be receptor editing.
And this is why RAG1 and RAG2 get re-expressed at this stage, so that the receptor can be changed.
If it reacts with self, it can be changed to a different receptor
that reacts with a foreign pathogen rather than reacting with self.
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.
And what plasma cells do is they secrete very large amounts of antibody of the
same specificity that acts at the B-cell receptor on the surface of the B-cell.
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
of the antibody molecule where you have the normal version of the IgM heavy chain which is called µ heavy chain.
But it’s not a mature version of the light chain.
They have what we refer to as a surrogate light chain, and this is made up of two components.
One is called V pre-B and the other is called λ5.
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 k light chain or the λ light chain.
So that the mature B-cell now has on its cell surface a proper mature version of the antibody molecule.
And initially these mature B-cells that have not yet encountered antigen, we call them naïve B-cells.
They’ve not yet met antigen at all.
They express on their cell surface two different classes of antibody - IgM, surface IgM and surface IgD.
And these cells have left the bone marrow and travelled to the secondary lymphoid tissue.
And they sit in wait in the secondary lymphoid tissue in case
their particular antigen, the antigen they recognize comes along.