Boster Pathways-> Immunology

B-Cell Development Pathway

B cell development is a tightly controlled process in which functional peripheral subsets are generated from hematopoietic stem cells in the fetal liver prior to birth and in the bone marrow following birth.

Overview of B-Cell Development Pathway

The immune system is remarkable in its capacity to respond to a wide variety of antigens, including recently synthesized compounds. Antibody diversity exhibits unusual properties such as the presence of variable and constant regions on the same polypeptide chain and the use of identical V regions with distinct C regions. Somatic recombination is unique among mammalian genes for generating antibody and TCR diversity. Successful synthesis of both H and L chains and their expression on the membrane are required for B cell development and serve as a marker for its stages.

B cell development begins in the fetal liver and continues throughout our lives in the bone marrow. The following table summarizes the stages of B cell development. When a B cell expresses both the m and L chains on its membrane, it is considered to be a B cell. However, it is still immature and is easily killed by self antigen contact until it also expresses membrane IgD.

When a mature B cell migrates to the periphery, it can be activated by antigen and transformed into an antibody-secreting plasma cell or a memory B cell that responds more rapidly to a second antigen exposure. Apoptosis occurs when B cells do not successfully complete their development (programmed cell death).

Lymphoid progenitor cells initiate B cell development in response to signals from bone marrow stromal cells. Cytokines promote the production of TdT and recombinase (RAG-1 and RAG-2) in CD34+ lymphoid progenitors. The cells undergo D-J joining on the H chain chromosome, transforming them into early pro-B cells that express CD45 (B220) and Class II MHC. The late pro-B cell stage is completed by the joining of a V segment to the D-JH.

When pro-B cells express membrane m chains with surrogate light chains in the pre-B receptor, they differentiate into pre-B cells. Surrogate L chains are similar to natural L chains but are identical on all pre-B cells. IgaIgb signaling molecules are also included in the pre-B receptor complex. Ig heavy chains' cytoplasmic tails are too short to enter the cytoplasm and transmit an antigen-binding signal; Ig a and Ig b signal transduction molecules contain ITAMs (Immunoreceptor Tyrosine Activation Motifs) that become phosphorylated in response to antigen-BCR binding. Phosphorylation initiates a cascade of cytoplasmic signaling events. The cell inhibits H chain recombination and proliferates to form a clone of B cells that all produce the same m chain. Due to the fact that dividing cells are larger than resting cells, this stage is referred to as the large pre-B cell stage.

Following proliferation, non-dividing small pre-B cells undergo V-J joining on one L chain chromosome. Once the L chain is successfully synthesized, it is expressed on the cell membrane alongside the m chain, and the cell is referred to as an immature B cell. Because immature B cells are extremely sensitive to antigen binding, they die if they come into contact with self antigen in the bone marrow. B cells that do not recognize self antigen express d chain and membrane IgD alongside their IgM around the time they leave the bone marrow and mature into mature naive (resting) B cells.

Through cell-cell contacts and secreted signals, progenitor cells receive signals from bone marrow stromal cells. This microenvironment in the bone marrow is necessary for B cell development. SCF (stem cell factor) on the stromal cell membrane and kit (CD117) on the lymphocyte membrane are two CAMs involved in both B and T cell development. IL-7 is a secreted cytokine that is required for both B and T cell development. It is produced by the stromal cell and is bound to the developing lymphocyte's IL-7R. These binding events initiate cytoplasmic cascades that result in altered expression of developmentally required proteins. As B cells develop in the bone marrow, they migrate from the marrow's periphery to the core.

Somatic recombination can be productive (resulting in the synthesis of a functional H or L chain) or nonproductive (resulting in the introduction of a stop codon due to frame shift mutations) in developing B cells. Failure to make productive rearrangements and express Ig at critical developmental stages results in the death of the developing cell. B cells have two opportunities to rearrange their H chains productively (maternal and paternal chromosomes) and four opportunities to rearrange their L chains productively (paternal and maternal k and l loci). Normally, human B cells rearrange the DH and JH segments on both chromosomes concurrently. Additionally, because DH can be read in any reading frame, any D-J rearrangement is productive. Only about half of developing B cells, it is estimated, undergo productive H chain rearrangements. These successful pre-B cells divide to form B cell clones (the large pre-B cell stage), which can then undergo L chain recombination.

During the small pre-B cell stage, light chain V-J junctions are usually formed first, followed by k chain V-J junctions. If the rearrangement is successful, the cell develops a k chain and matures into an immature B cell expressing membrane IgM(k) BCR. B cells are capable of repeating V-J joining several times if the first attempt fails; this is referred to as light chain rescue. If k genes on either chromosome are not successfully rearranged, l genes are rearranged. Success results in the generation of IgM(l) BCR. If neither k nor l is rearranged productively, the bone marrow cell undergoes apoptosis. Only a small percentage of human pre-B cells mature into mature B cells.

During B cell development, genes encoding proteins required for somatic recombination and receptor expression are turned on and off at specific times. RAG-1, RAG-2, and TdT are expressed exclusively during somatic recombination events: early and late pro-B cell stages, as well as small pre-B cell stages. TdT is frequently turned off earlier than recombinases, which means that N nucleotide additions to gene segment joins are not as frequent in L chain sequences as they are in H chain sequences. F

or the pre-B receptor to appear on the cell membrane, chains for the surrogate L chain and Ig a and Ig b proteins must be expressed. Signal transduction molecules must be expressed at critical times; failure to express btk results in a human B cell immunodeficiency called Bruton's X-linked agammaglobulinemia.

Gene expression is regulated by soluble transcription factors that bind to regulatory regions of the DNA. Promoters are DNA sequences that bind RNA polymerase and initiate the synthesis of messenger RNA. Enhancers are non-coding (intron) regions of DNA that aid promoters in their function. Splicing gene segments with intervening DNA looped out brings promoters and enhancers closer together, stimulating mRNA synthesis. Additionally, tissue-specific enhancers are required; for instance, RAG-1 and RAG-2 recombine only Ig gene segments in developing B cells and only TCR gene segments in developing T cells, respectively.

Through cell-cell contacts and secreted signals, progenitor cells receive signals from bone marrow stromal cells. This microenvironment in the bone marrow is necessary for B cell development. SCF (stem cell factor) on the stromal cell membrane and kit (CD117) on the lymphocyte membrane are two CAMs involved in both B and T cell development. IL-7 is a secreted cytokine that is required for both B and T cell development. It is produced by the stromal cell and is bound to the developing lymphocyte's IL-7R. These binding events initiate cytoplasmic cascades that result in altered expression of developmentally required proteins. As B cells develop in the bone marrow, they migrate from the marrow's periphery to the core.

Somatic recombination can be productive (resulting in the synthesis of a functional H or L chain) or nonproductive (resulting in the introduction of a stop codon due to frame shift mutations) in developing B cells. Failure to make productive rearrangements and express Ig at critical developmental stages results in the death of the developing cell. B cells have two opportunities to rearrange their H chains productively (maternal and paternal chromosomes) and four opportunities to rearrange their L chains productively (paternal and maternal k and l loci). Normally, human B cells rearrange the DH and JH segments on both chromosomes concurrently. Additionally, because DH can be read in any reading frame, any D-J rearrangement is productive. Only about half of developing B cells, it is estimated, undergo productive H chain rearrangements. These successful pre-B cells divide to form B cell clones (the large pre-B cell stage), which can then undergo L chain recombination.

During the small pre-B cell stage, light chain V-J junctions are usually formed first, followed by k chain V-J junctions. If the rearrangement is successful, the cell develops a k chain and matures into an immature B cell expressing membrane IgM(k) BCR. B cells are capable of repeating V-J joining several times if the first attempt fails; this is referred to as light chain rescue. If k genes on either chromosome are not successfully rearranged, l genes are rearranged. Success results in the generation of IgM(l) BCR. If neither k nor l is rearranged productively, the bone marrow cell undergoes apoptosis. Only a small percentage of human pre-B cells mature into mature B cells.