BLOCK 3: IMM: (3.5) B-CELL DEVELOPMENT PART 1

B Cell Development

• B cell development has six main phases:
  • Repertoire assembly (bone marrow)
  • Negative selection (bone marrow)
  • Positive selection (bone marrow)
  • Recirculation (lymphoid tissue)
  • Activation (lymphoid tissue)
  • Differentiation (lymphoid tissue)
• Repertoire assembly forms the B cell receptor.
• Negative selection occurs before positive selection.
• Reversible modifications:
  • Transcriptional elements: irreversible process, involves co-expression of surface IgM and IgD
  • IgM and IgD expression: reversible due to loss of IgD after cell circulation
  • Splicing: reversible, involves changing the splicing and can be associated with isotype switching
• Irreversible modifications:
  • Isotype switching: regulated process, occurs with subsequent encounters and cell division
  • Somatic hypermutation: point mutations in genomic DNA, not reversible, provides diversity at the antigen-binding site
• Phases 1-3 occur in bone marrow
• Phases 4-6 occur in secondary lymphoid tissue (lymph nodes, spleen)
• B1 cells
  • Arise from the fetal liver during embryonic development
  • Provide early immune protection for the fetus
  • Possess limited isotype switching
  • Self-renew
  • Almost never requires T-cell help.
  • Mostly for conserved sequences, usually carbohydrates on bacteria
  • Predominantly found in areas of pathogen encounter (pleural and peritoneal cavities, upper respiratory and GI tract)
  • Produce many so-called "natural antibodies"
• B2 cells
  • Make up the vast majority of B cells in the body.
• Natural Antibodies
  • Directed against other blood types, potentially protecting the fetus from maternal immune rejection.
• Winifred Ashby
  • First to establish that red blood cell lifespan in humans is longer than 2-3 weeks
  • Used agglutination to determine this.

B Cell Development:

• Occurs in 2 phases, 3 in the bone marrow and 3 in secondary lymphoid tissue
• The first 3 phases of B cell development take place in the bone marrow.

B Cell Development Markers:

• CD127: α chain of the receptor for interleukin-7
• CD19: a subunit of the B-cell co-receptor, cooperates with the antigen receptor to produce activating signals when antigen is bound.
• CD34: Vascular addressin expressed on endothelial venules in lymph nodes and involved in the extravasation of white blood cells.
• CD10: May have a role in inactivating regulatory peptides favoring differentiation events. Normally expressed on pro-B cells and mature germinal center B cells.

B Cell Developmental Stages

• Stem cells: Ig genes in the germline configuration
• Early pro-B cell: Joining of DH to JH, defined by the rearrangement of the heavy chain, early pro-B cells turn into late pro-B cells
• Late pro-B cell: Joining of VH to DJH
• Large pre-B cell: Expression of a functional μ chain
• Large pre-B cells proliferate, producing small pre-B cells: Rearrangement of the light chain gene occurs
• Immature B cell: Successful light-chain gene rearrangement & expression of IgM on the cell surface

Proteins Involved in B Cell Development

• FLT3: Protein kinase and cell-surface receptor on stem cells, plays a crucial role in differentiation to CLP and B cell progenitor.
• Kit: Acts as a receptor for stem cell factor (SCF)
• IL-7R: Receptor for IL-7
• RAG proteins: Essential for gene rearrangement, selectively expressed at the 2 stages where rearrangements are made.
• TdT: Inserts N nucleotides in the VDJ recombination process.
• VpreB & λ5: Polypeptides that form the surrogate light chain.
• Igα & Igβ: Have long cytoplasmic tails with ITAMs that interact with intracellular signaling proteins & bind to the immunoglobulin C region on extracellular portions, forming the functional B-cell receptor complex.
• CD19: Part of the BCR co-receptor and involved in intracellular signaling.
• BTK: Kinase crucial for B cell receptor signaling.
• Pax-5: Nuclear transcription factor & B cell-specific activator protein.

B-cell Development: Role of Stromal Cells

• Early stages of B-cell development are dependent on bone marrow stromal cells
• Stem cells & early pro-B cells use integrin VLA-4 (Very Late Antigen 4) to bind to VCAM-1 (vascular cell adhesion molecule 1) on stromal cells
• Interactions of other CAMs promote binding of Kit on the B cell to stem-cell factor (SCF) on the stromal cell
• Activation of Kit causes B cell proliferation
• Interleukin-7 (IL-7) plays a crucial role in the early development of B cells, promoting proliferation & survival of B cell progenitors during the transition from pro-B to pre-B cell stage.