B-Cell Development Stages

B-Cell Development Overview

• B cells produce immunoglobulins to target almost any antigen.
• While B cells aren't stored in large quantities, they can rapidly expand when needed.
• Approximately 60 billion new B cells are produced by bone marrow daily.
• Both negative and positive selection processes are crucial in B cell development.

B-Cell Development Phases

• Phase 1: Repertoire assembly involves generating diverse B cell receptors in the bone marrow.
• Phase 2: Negative selection eliminates or inactivates B cells that react to self-antigens.
• Phase 3: Positive selection promotes immature B cells to mature in secondary lymphoid tissues.
• Phase 4: Mature B cells circulate between the lymph, blood, and secondary lymphoid tissues, searching for infection.
• Phase 5: Antigen-derived antigens activate and clonally expand B cells in secondary lymphoid tissues.
• Phase 6: B cells differentiate into antibody-secreting plasma cells and memory cells in secondary lymphoid tissue to attack infection.
• Development occurs in the bone marrow (first 3 phases), and then migrates to secondary lymphoid tissues like the lymph nodes and spleen.

B-Cell Development Stages in Bone Marrow

• B cell development in the bone marrow proceeds through several stages.
• The main function of a B cell is to produce immunoglobulin.
• Pluripotent hematopoietic stem cells produce both B and T cells from common lymphoid progenitor cells.
• Pro-B cells have restricted self-renewal capacity.
• Early pro-B cells undergo DH and JH gene segment joining.
• Late pro-B cells undergo VH segment joining to the rearranged DJH.
• The pre-B cell expresses a µ heavy chain.
• Large pre-B cells are less mature.
• Small pre-B cells are more mature and rearrange light-chain genes.
• An immature B cell is also formed.

B-Cell Development Terminology

• Early pro-B cell and late pro-B cell are stages defined by heavy chain gene rearrangement.
• Large pre-B cell and small pre-B cell are stages defined by the maturity of light chain rearrangement.
• An immature B cell expresses IgM on its surface.

The Role of Stromal Cells in B-Cell Development

• Stromal cells are microenvironments that support B cell maturation in the bone marrow.
• These are non-lymphoid supportive cells that interact with developing B cells using adhesion molecules and their ligands.
• Stromal cells produce growth factors like SCF and IL-7, which stimulate attached B cells.
• The subendosteum houses immature stem cells.

Immunoglobulin Heavy-Chain Gene Rearrangement in Pro-B Cells

• Nonproductive rearrangements don't translate to useful proteins due to random nucleotide additions.
• Productive rearrangements allow for the production of a functional immunoglobulin.
• Two copies of the heavy-chain locus exist in homologous chromosomes.
• Gene rearrangements happen in both chromosomes during B cell development to ensure a functional heavy chain.
• If both copies are nonproductive, the B cell dies in the bone marrow.
• RAG1 and RAG2 are controlled by E2A, EBF, and Pax-5.
• Survival signals indicate that B cells should survive instead of undergoing apoptosis.

Quality Control of Immunoglobulin Heavy Chains by Pre-B-Cell Receptor

• Pro-B cells need to produce a µ heavy chain that can combine with an immunoglobulin light chain to survive.
• VpreB and λ5 bind to the µ heavy chain, imitating a light chain.
• Surrogate light chains can lead to B-cell immunodeficiency if defected.
• Pre-B-cell receptors lack an antigen-binding site and are not well represented at the cell surface.

Light-Chain Loci Rearrangement in Pre-B Cells

• Large pre-B cells divide to form a clone of around 100 small resting pre-B cells that have identical heavy chains and no surrogate light chains.
• RAG proteins help with rearranging light chains (κ first, then λ).
• Approximately 85% of pre-B cells successfully rearrange a light-chain gene.
• Less than half of the B-lineage cells in the bone marrow produce functional immunoglobulin heavy and light chains.
• 2/3 have k chains, which is aided by a second isotype, which increases the success rate by 50%.
• The upper respiratory tract is mainly a chain.

Post Light-Chain Loci Rearrangement in Pre-B Cells

• IgM forms when light chain formation is successful and combines with the heavy chain.
• B-cell receptors halt the rearrangement process.
• Regulation involves allelic and isotype exclusion.
• Light chain rearrangement is independent of other small pre-B cells.
• Numerous immature B cell clones with different antigenic specificities appear.

Checkpoints B Cells Encounter During Development

• Checkpoints:
  • (1st) cells need to make a heavy chain.
  • (2nd) cells need to make a light chain that can bind with the heavy chain.
• First checkpoint:
  • Late pro-B-cell stage (formation of a functional B-cell receptor).
• Second checkpoint:
  • Small pre-B-cell stage (after light-chain gene rearrangements).
• B cells must pass both checkpoints to successfully make immunoglobulin.

Protein Expression during B-Cell Development

• Expression changes in proteins involved in rearranging the immunoglobulin genes, testing the quality of the immunoglobulin chains, and driving cellular proliferation.
• RAG-1 and RAG-2 are essential.
• TdT adds N-nucleotides.
• Pax-5 is a transcription factor that is B-cell specific.
• Btk is essential for B-cell development beyond the pre-B-cell stage.
• X-linked agammaglobulinemia is recessive.
  • Patients lacking a functional Btk gene can't create circulating antibodies, which can lead to recurrent extracellular bacterial infections.

Chromosomal Translocations and B-Cell Tumors

• Mutations that convert B cells into tumor cells can arise from cutting, splicing, and mutating immunoglobulin genes.
• Translocations involve the fusion of a part of one chromosome with another.
• Proto-oncogenes are genes that cause cancer when their function or expression is perturbed.
• Oncogenes are viral genes that can cause transformation.

B-1 vs B-2 Cells

• B-1 cells produce low-affinity antibodies that bind to various antigens (polyspecificity).
• They typically have little or no surface IgD.
• CD5 is present.
• B-1 cells make up 5% of B cells in humans.
• They also lack N nucleotides.