B-cell development

Definitions & Key takeaways

B-cell development consists of a series of cellular transitions, from hematopoietic stem cells into immunocompetent B cells. Depending on the step, these processes take place in different organs namely the bone marrow, lymph nodes, and spleen.

Like any other type of blood cell, B cells originate from hematopoietic stem cells (HSCs). HSCs give rise to common lymphoid progenitor cells, which in their turn become either B-cells or T-cells. B cell development takes place in a series of six main stages. First, they start as common lymphoid progenitor cells, which become early pro-B cells, then late pro-B cells, next large pre-B cells, then small pre-B cells, and finally, immature B cells. Immature B cells then migrate from the bone marrow into the lymph nodes and spleen to complete the process of maturation.

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Your immune system is like the military - with two main branches, the innate immune response and the adaptive immune response.
The innate immune response is immediate and non-specific, meaning that although it can distinguish an invader from a human cell, it doesn’t distinguish one invader from another invader.
In contrast, the adaptive immune response is highly specific for each invader, and that’s because the cells of the adaptive immune response have receptors that differentiate friendly bacteria and potentially deadly ones from their unique parts - called antigens.
This adaptive immune response takes days to weeks to become activated, but is also responsible for immunologic memory. Now, the key cells of the adaptive immune response are the lymphocytes- the B and T cells -which are generated during lymphopoiesis.
Lymphopoiesis has three goals - first, to generate a diverse set of lymphocytes - each with a unique antigen receptor, second, to get rid of lymphocytes that have receptors that are self-reactive meaning that they’ll bind to healthy tissue, and third, to allow lymphocytes that aren’t self-reactive to continue maturing in secondary lymphoid tissue.
Normally, hematopoietic stem cells, within the bone marrow mature into a common lymphoid progenitor cell, which then becomes either a B-cell or a T-cell.
To become a B cell, it has to develop into an immature B-cell in the bone marrow and then complete its maturation into an antibody secreting B cell, called a plasma cell, in the lymph nodes and spleen.
To become a T cell, it has to migrate to the thymus and become a thymocyte, where it completes its development into a mature T cell.
So, “B” for bone marrow and “T” for thymus. Throughout B cell development, the developing cells are interacting closely with the stromal cells of the bone marrow, which are largely composed of mesenchymal stem cells.
Mesenchymal cells are multipotent and can differentiate into various cells including macrophages and endothelial cells. Mesenchymal cells provide B cell with adhesion molecules they can use to attach and important growth factors like interleukin 7, they can use to grow and proliferate.
As they develop, B cells go through 6 stages: They start as common lymphoid progenitor cells, then become early pro-B cells, then late pro-B cells, then large pre-b cells, then small pre-B cells, and finally immature B cells.
As the cell develops it makes permanent changes in its DNA so that by the time it’s an immature B cell it has DNA that uniquely codes for a B cell receptor that can bind to foreign antigens but isn’t self-reactive.
The B cell receptor has two chains, a heavy chain and a light chain. The heavy chain contains regions that determine the type of antibody it will become as well as whether the B cell receptor will be surface bound or secreted, like an antibody.
The region where the heavy chain and light chain come together form a unique protein structure capable of binding proteins, carbohydrates, or lipids that the B cell might eventually encounter - and this is called the antigen binding site.
The antigen binding site of the B cell receptor is made up of three protein segments that are called V for variable, D for diversity, and J for joining.
The heavy chain is made up of 1 V segment, 1 D segment, and 1 J segment; while the light chain only contains a V and J segment - this is easy to remember - just think of it as the extra segment making the heavy chain heavier.
Every person inherits multiple genes that encode the V, D, and J protein segments, and these segments can be mixed and matched to make a unique structure.
A bit like how you might have several pairs of shoes, pants, and shirts and can mix and match them to create lots of different outfit combinations.
For the heavy chain, each person has 44 V gene segments, 27 D segments, and 6 J segments! And there are even more V and J segments for the light chain.
So one B cell might have a B cell receptor with a heavy chain that has a VH1-DH3-JH5 combination and a light chain that’s VL7-JL2, and another B cell might have a B cell receptor with a heavy chain that has a VH44-DH10-JH1 combination and a light chain that’s VL2-JL3.
And that would mean that these two B-cells would have completely different B cell receptors and therefore different antigen specificities.
Now, to have a fully functioning B cell receptor, a B cell has to get through a series of successful gene rearrangements, first for the heavy chain and then the light chain.
And if the B cell fails at any stage, it dies! It all starts with a common lymphoid progenitor cell which has various V, D, and J gene segments all lined up in its germline DNA.
Two enzymes, Rag-1 and Rag-2, start getting expressed, and that signifies that the cell is now an early pro-B cell. Rag-1 and Rag-2 help to splice together D and J segments on both chromosomes, and the chromosome that successfully rearranges first will then suppress the other chromosome from rearranging - a process called allelic exclusion.
If a cell successfully joins a D segment to a J segment, then it’s considered a late pro-B cell. Next, the late pro-B cell has to attach its D-J gene segment to a V gene segment, with the help of an additional enzyme called V(D)J recombinase.
Once the VDJ segment are combined, the full heavy chain’s antigen binding site is complete and needs to be recombined with the mu gene, which codes for the constant region of the antibody.
The mu gene codes for the protein that makes the IgM constant region and it’s the first of the different types of antibody constant regions that are expressed on B cells.
Once a cell has successfully completed V-D-J rearrangement on the heavy chain and bound it to the mu constant gene it’s considered a large pre-B cell.
At this point, it’s time to test out if the heavy chain is functional, by seeing if it can bind to a surrogate light chain and get expressed on the cell surface.
The surrogate light chain is made of two proteins - VpreB and lambda 5, and it’s basically something for the heavy chain to practice with until the real light chain is eventually made.
The surrogate light chain and heavy chain are joined together placed in a vesicle, and then that vesicle is sent up to the cell surface.
At the cell surface, the surrogate light chain and heavy chain are placed near two additional chains, immunoglobulin alpha and immunoglobulin beta, which are called side chains.
If the surrogate light chain and heavy chain successfully make a functional B cell receptor on the surface of the large pre-B cell, these side chains send down a signal to the nucleus.
The cell starts to rapidly proliferate, and each daughter cell is called a small pre-B cell. The small pre-B cells now begin rearranging the light chain, which means that the pre-B cells will all have the same heavy chain, but each will have its own distinct light chain.
And there are actually two types of light chains, a kappa light chain and a lambda light chain. The kappa light chain starts rearranging first, and it’s an iterative process.
In other words, initially a pre-B cell might create a kappa light chain that doesn’t bind well to the heavy chain, or it might result in a B cell receptor that reacts with human antigens.
If that happens, the cell will either get a chance to iterate and try a different arrangement or in some cases the cell might simply get killed off.
Not allowing B cell receptors to be self-reactive is very important. Because if these faulty or self-reactive B cells are released, then they might simply not work or they might attack self-tissue and cause disease.
To identify the self-reactive B-cells, there’s an autoimmune regulator gene, called AIRE for short. AIRE is expressed in primary lymphoid organs and allows them to express antigens which are normally found all over the body - the brain, ovaries, pancreas, etcetera.
It allows these primary lymphoid organs to serve as a microcosm of the entire body, a bit like how the Epcot Center theme park in Florida is a microcosm of the world.
So that allows the body to test out these newly formed B cell receptors in a safe environment. If a small pre-B cell binds strongly to a self-antigen then it dies swiftly and silently by apoptosis, whereas if a small pre-B cell binds with only intermediate strength to a self-antigen, then the small pre-B cell will attempt to rearrange its light chain in the hopes that it will reduce the strength of its binding to a self-antigen.
Only small pre-B cells that don’t recognize self-antigens at all, develop into immature B cells. Now, if the B cell does have a faulty B cell receptor or a self-reactive B cell receptor, then the pre-B cell tries again by splicing out more DNA segments from its light chain to generate a new V and J rearrangement.
In fact, the cell will keep trying to do this over and over doing more and more rearrangements, until it has gone through all of the V and J segments on the first chromosome, and then it starts to splice out DNA segments on the second chromosome.
Once all of the v or j segment rearrangements on the kappa chain of both chromosomes have been tried, the cell will start trying to rearrange the lambda chain.
And it will keep iterating with the lambda chain as well, until it runs out of genetic material. Ultimately, if there’s a rearrangement where the resulting light chain binds to the heavy chain, and the resulting B cell receptor isn’t self-reactive, then, and only then, does the cell become an immature B cell.
Now, to get into the blood, an immature B cell has to signal to the bone marrow that it has completed its development. Now, remember that the B cell already has a B cell receptor with a mu heavy chain constant region which made it an IgM antibody.
But the B cell now undergoes a process of alternative splicing, which is where some of the B cell receptors continue to use the IgM constant region whereas some start using the IgD constant region.
But all of the antibodies - both IgM and IgD still have the same antigen binding specificity. The presence of IgD constant regions on some of the B cell receptors, is a bit like a driver’s license - it signals to the bone marrow that the B cell is a fully rearranged immature B cell that should be allowed out on the open road.
As a quick recap, B cells development includes rearrangement of the heavy chain, rearrangement of the light chain, and making sure that the B cell receptor isn’t faulty or self reactive.
Heavy chain rearrangement means combining the D-J segments and then the V-DJ segments, and then the V-D-J segment is joined to an IgM heavy chain and then it’s paired with a surrogate light chain before the cell proliferates.
Each daughter cell rearranges its light chain combining the V-J segments, first with the kappa chain genes, then the lambda chain genes.
If the B cell receptor isn’t faulty or self-reactive, then it will express the IgD constant region which will allow it to be released into the periphery as an immature B cell.