Generation of B-cells

Last post covering the stuff on the first midterm!

Describe the process of generation of B-cells
Describe the stages involved in the generation of B-cells
Describe the pre-B cell receptor

In an earlier post, I gave an outline of the overall process of haematopoiesis, or the making of blood cells. Now we're going to focus on B-cells in particular!

From multipotent progenitor to common lymphocyte progenitor (CLP)

All blood cells, including B-cells, start off as haematopoietic stem cells. These then differentiate into multipotent progenitor cells. Multipotent progenitor cells express the FLT3 receptor, which can bind to FLT3 ligand located on bone marrow stromal cells, leading to differentiation of the multipotent progenitor cell into a common lymphoid progenitor, or CLP.

It is imperative that the growing B-cell stays close to bone marrow stromal cells, as the latter provides a lot of signals to help the B-cell grow and differentiate, most notably the cytokine IL-7. The close proximity of the growing B-cell to the bone marrow stromal cell is maintained by the chemokine CXCL12 as well as cell adhesion molecules (CAMs). VCAM-1 (vascular cell adhesion molecule-1) on the bone marrow stromal cell can also bind to VLA-4 (very late antigen-4) on the CLP surface, and this binding serves as yet another "anchor" between the B-cell and the bone marrow stromal cell.

From CLP to pro-B cell

CLP eventually begins to express Kit, marking the start of the pro-B cell stage. Kit can bind to SCF (stem-cell factor), activating Kit which activates the early pro-B cell. During the early pro-B stage, the D to J rearrangement in the heavy chain takes place (see my previous post about gene rearrangements). Once this is done, the late pro-B cell stage, in which the V to DJ rearrangement is done, takes place. IL-7 continues to stimulate the cell throughout all of this.

From pro-B to pre-B cell

Pre-B cells cease to express Kit, but start to express other receptors. In the first pre-B cell stage, also known as the "large" pre-B cell stage, the heavy chain is made. It binds to a "surrogate light chain" which consists of two sections: VpreB (surrogate variable region) and λ5 (surrogate constant region). These two sections are conserved and as such are not made by a gene rearrangement step. The heavy chain paired with the surrogate light chain then appears on the cell surface as a "pre-B cell receptor." The pre-B cell receptor also associates with the Igα-Igβ heterodimer, which has cytoplasmic tails which help to conduct the signal. This heterodimer actually stays there for the lifetime of the B-cell and continues to aid in signalling when antigens bind to mature antibodies.

From pre-B cell to immature B-cell

When the pre-B receptor is activated, the cell enters the "small" pre-B cell stage, in which it stops rearranging its heavy-chain genes and starts rearranging its light-chain genes instead. Once that's done, IgM appears on the cell surface and the B-cell is considered to be an immature B-cell.

From immature B-cell to mature "naïve" B-cell

Last step! Immature B-cells can leave the bone marrow and enter the circulation. From there they are carried to secondary lymphoid organs, such as the spleen. In these organs, B-cells begin to produce IgD. Once it starts doing that, it's now considered to be a mature naïve B-cell (naïve because it hasn't seen its antigen yet). Yay!

Describe allelic exclusion, clonal deletion and receptor editing

B-cell development doesn't just end with the steps described above. There are many steps along the way to make sure that B-cells produce antibodies with a single antigen specificity, and that no B-cell will target any of the healthy cells of the body. Let's take a look at some of these processes...

Allelic Exclusion

Allelic exclusion is a simple way of making sure that only one kind of antibody is produced despite having multiple copies of each gene. Remember, we inherit one copy of each gene from both of our parents, so we actually have two heavy chain genes, two κ genes and two λ genes.

Essentially the way this works is that the cell tends to only rearrange one gene at a time, and if that fails then it works its way down the list until it finds one that works. Once it finds something that works, it won't rearrange any of the other genes.

Let's have a closer look at how this works! First off, we start with an exception: the DJ rearrangement in the early pro-B cell actually occurs on both chromosomes. Then, in the late pro-B cell stage, the VDJ rearrangement happens on one chromosome only. If this works, then the cell continues on its merry way, and if it doesn't, then the VDJ rearrangement happens on the other chromosome. If this also fails, then the cell feels worthless and useless and commits suicide. Poor thing.

If either rearrangement works, however, then the cell continues on to the pre-B cell stage. The first gene to be rearranged is the κ gene, again on one chromosome only. Then if that fails, the other κ gene is tried, and then the lambda genes. This is partially why humans have more κ light-chains than λ ones, as I mentioned in an earlier post. Again, if all of these rearrangements fail, the cell gives up on life.

Clonal Deletion

Clonal deletion, which occurs during the immature B-cell stage, is one way in which self-reactive B-cells (i.e. B-cells that react to the body's own cells) are removed from the system. If the B-cell binds to cells displaying self-molecules while still within the bone marrow, it receives signals to either undergo apoptosis or become non-reactive. The next process describes how a cell might become non-reactive...

Receptor Editing

Receptor editing also occurs during the immature B-cell stage, and is another way in which self-reactive B-cells can be avoided. As I mentioned, there are four genes that encode light chains: two κ genes and two λ genes. If any of these are still left over, and if there are still active RAG enzymes present, some immature B-cells might be able to take advantage of that by producing a new light chain. This creates a new antibody that might not be autoreactive.

And that's the end of all of the information that you need to know for the midterm! Happy studying!