Maturation of B cells

B-cell development begins as lymphoid stem cells differentiate into progenitor B cells (pro-B cells) which express a transmembrane tyrosine phasphatase called CD45R. Proliferation and differentiation of these pro-B cells into precursor B cells (pre-B cells) requires bone-marrow stromal cells which secrete various cytokines like IL-7 that support the developmental process. A receptor on the pro-B cell called c-Kit interacts with a stromal cell surface molecule causing the pro-B cell to divide and differentiate into the pre-B cells and start expressing a receptor for IL-7. IL-7 will induce down-regulation of adhesion molecules on the pre-B cells so that they can detach from the stromal cells.

B lymphoctyes come to express a unique antigen-binding receptor on their membrane. This antibody molecule consists of 2 identical heavy polypeptide chains and 2 identical light polypeptide chains which are held together by disulfide bonds.  Membrane-bound immunoglobulin on B cells associates with another membrane protein, the Ig-alpha/Ig-beta heterodimer, to form the B-cell receptor (BCR).

In germ-line DNA, multiple gene segments encode portions of a single immunoglobulin heavy or light chain. These gene segments cannot be transcribed and translated into complete chains until they are first rearranged into functional genes. During B-cell maturation in the bone marrow, certain of these gene segments are randomly shuffled. The maturation of a progenitor B cell progresses through an ordered sequence of Ig-gene rearrangements, coupled with modifications to the gene that contribute to the final product. By the end of this process, the individual B cell is antigenically committed to a specific epitope. After antigenic stimulation in peripheral lymphoid organs, further rearrangement of constant region gene segments can generated changes in the isotype expressed, which produce changes in the biological effector functions of the immunoglobulin molecule without changing its specificity.

The antibody on the B cell can recognize different epitopes (immunologically active region) with great precision. Even protein antigens that differ by only one amino acid can often be discriminated from each other. As a B cell matures in the bone marrow, its specificity is generated by random rearrangements of a series of gene segments encoding the antibody molecule. There are about 1.5 x 10⁵ molecules of antibody on the membrane of a single B cell. Other molecules are also expressed such as CD45 which is a marker of the B cell lineage since it first appears during maturation on the precursor B cells and remains throughout the life span. Since the majority of B cells also express class II MHC molecules B cells are also classified as an antigen presenting cell (APC). Indeed B cells can be thought of as helping their helpers because B cell activation can also depend on T cells (discussed below with Thymus cell dependent activation). However, because a B cell recognizes and internalizes antigen specifically by way of its membrane bound Ig, it is able to present antigen to T_H cells much less efficiently than dendritic cells or macrophages.

Very few B cells actually mature in the bone marrow. Some of this loss is due to negative selection of immature B cells that express antibodies against self-antigens in the bone marrow. In addition, the ones which do mature and circulate as naive B cells have short life spans. Thus only a small fraction of antibody diversity is displayed at any time by membrane immunoglobulin on recirculating B cells.

After B cells leave the bone marrow, activation and differentation will require antigen and occurs in the periphery. When a naive B cell encounters the antigen which matches its membrane bound antibody, the cell divides rapidly and its progeny differentiate into memory B cells and plasma (which are effector) cells. Memory cells continue to express the antibody whereas plasma cells do not produce the membrane antibody but secrete the antibody. Secreted antibodies are the major effector molecule of humoral immunity.

Cytokines/factors required for B cell development:   

• IL-7: is essental for B cell differentiation in the adult. Il-7 also induces histone acetylation of Vh genes. IL7-/- mice do not rearrange distal VHJ558 genes.

• TSLP: uses the IL7Ralpah and y-like receptor. TSLP also supports pro- cell proliferation.

• FLt3: Ligand: Flt3 ligand deficieny is less severe than IL7R deficiency for B cell development.

• SCF: (Stem cell factor): is a cytokine on stromal cells; its receptor is c-kit (a receptor tyrosine kinase)

Transcriptional Control of B cell development:   

Multipotential lineage cells (HSC) express low levels of genes for all potential lineages. In the next stage of development there is upregulation of sets of genes encoding downstream transcription factors, cytokines and cytokine receptors, signaling molecules, etc. that are appropriate for the next stage of development. In later development, transcription factors turn off genes whcih are no longer needed for earlier stage differentiaiton.

Transcription factor also aid in making chromatin in Ig loci accessible to bind RAG1/2. For example, E2A can bind histone acetyl transferases to make RAGs accessible.

• E2A: is present early on in B cell development.

• EBF: Early B cell Factor

• Pax5: is a master regulator of B lineage commitment. Pax5 represses expression of myeloid specific genes and activates B cell specific genes. Pax 5 -/- cells can give rise to many different lineages (e.g., T cell, NK cell, osteoclast, DC, granulocyte) given the appropriate stimulus (e.g., cytokine). But once have Pax5 the progenitor gives rise to B cell lineage. Pax 5 also plays a role in VH to DJH rearrangement.