B Cell Development Notes

Chapter 9: B Cell Development

Overview of B Cell Development

• B cell development initiates in the bone marrow and concludes in the periphery.

• Common lymphoid progenitors (CLPs) that remain in the bone marrow differentiate into immature B cells.

• The final maturation of these cells occurs in the spleen.

• Mature B cells will eventually develop into plasma cells that secrete antibodies, necessitating antigen recognition through the B cell receptor, which arises from gene rearrangement.

Key Components and Cells in B Cell Development

• Markers: CXCL12+, B220+, flt-3+, B220+c-kit+, IL-7+.

Elimination of Self-Reactive B Cells

• Like T cells, self-reactive B cells must be eliminated during development.

• Differences from T cell development include:

  • B cells can recognize intact antigens.

  • There is no MHC presentation involved and no reaction against self-MHC.

  • B cell development is predominantly completed in the bone marrow.

B Cell Development Stages in the Bone Marrow

• B cell precursors interact with bone marrow stromal cells.

  • Pre-pro-B cells require signaling from CXCL12 to progress to the pro-B cell stage.

  • Pro-B cells require IL-7 signaling to move to the pre-B cell stage.

Determination of B Cell Development Stages

• Evaluation of B cell stages includes:

  • Surface markers such as adhesion molecules and cytokine/chemokine receptors.

  • Specific transcription factors.

  • Rearrangement status of immunoglobulin genes.

  • Epigenetic changes (depicted in colors):

  • Black: Transcription factors

  • Green: Histone modifiers, DNA methylases, chromatin remodelers

  • Red: Micro-RNAs

Generation of the Common Lymphoid Progenitor

• Key transcription factors involved:

  • Ikaros: recruits chromatin remodelers.

  • PU.1 (Purine box factor 1):

  • High levels promote myeloid differentiation.

  • Low levels favor lymphoid differentiation, which is facilitated by down-regulation from Gfi1.

  • E2A is activated by Ikaros and PU.1 and is essential for B cell development.

Stages of B Cell Development

• Status: Various stages identified, such as Pre-pro-B, Pro-B, and Pre-B cells, each defined by the following:

  • Surface markers and Ig gene rearrangement status, leading to different checkpoints for development.

• Fractions (Fr.) A to E refer to the Hardy nomenclature for identifying stages of B cell development.

Pre-Pro-B Cell Development

• Characteristics:

  • Expresses B220 (CD45R), specific for B cells.

  • EBF1 (Early B cell Factor 1):

  • Binds to the heavy chain locus to prepare for rearrangement of D-JH segments.

  • EBF1 recruits the chromatin remodeling complex SWI/SNF to make necessary B lineage genes accessible.

  • Pre-pro-B cells interact with cells that secrete CXCL12.

Pro-B Cell Development

• At this stage, B cells migrate to regions producing IL-7.

• Notable processes include:

  • Completion of D/JH recombination in early pro-B cells.

  • Expression of Pax5 during late pro-B stage, crucial for controlling V/DJH recombination.

  • Pax5 remains expressed throughout B cell development, and it represses non-B cell genes.

Late Stage Pro-B Cell Development

• Expression of CD19 and CD79 is regulated in part by Pax5.

• Lack of Pax5 allows D/J recombination but blocks V/DJ recombination.

Pre-B Cell Development

• VpreB and λ5 are expressed, forming the surrogate light chain of the pre-BCR (pre-B cell receptor).

• The expression of the pre-BCR decreases during late stage, initiating light chain rearrangement.

Pre-BCR Signaling and Checkpoints

• Signaling through pre-BCR with IL-7 promotes proliferation prior to light chain rearrangement.

• Absence of pre-BCR signaling leads to apoptosis.

• VpreB and λ5 facilitate the aggregation of pre-BCR, necessary for initiating signaling.

• The proliferation rounds before light chain rearrangement result in a wider variety of receptors, akin to T cell development.

Immature B Cell Development

• The second checkpoint involves expressing a functional IgM on the cell surface.

• Failure to signal during this stage results in apoptosis; however, many immature B cells can survive due to having four potential alleles for light chain production (two κ and two λ).

B Cells and Self-Tolerance

• Following BCR expression, cells undergo tests for autoreactivity, leading to:

  • Clonal deletion (via apoptosis).

  • B cells exhibit low expression of anti-apoptotic proteins Bcl-2 and Bcl-XL.

  • BCR may undergo editing through re-expressing RAG, modifying BCR gene sequences.

  • Central tolerance involves removing autoreactive B and T cells from the repertoire.

Autoreactivity Results

• Cross-linking IgM prompts apoptosis in immature B cells while activating mature ones.

• Transgene experiments show specificity for MHC I types, confirming autoreactivity principles.

Splenic Development of B Cells

• Immature B cells mature in the spleen after leaving the bone marrow and are drawn towards the periphery due to sphingosine-1 phosphate (S1PR).

• They transition through two populations, T1 and T2, characterized by different surface markers.

• T1 B cells primarily differentiate into T2 B cells in the spleen.

• Only T2 cells can enter B cell follicles within the spleen and lymph nodes.

Surface Markers and Cytokine Receptors

• T1 B cells:

  • Express CD21 (BCR co-receptor for complement receptor C3d).

  • React to BAFF R (B cell Activating Factor R), crucial for survival.

• T2 B cells have different receptors compared to T1 B cells.

B Cell Trafficking in the Spleen

• B cells enter the spleen through central arterioles, progress through marginal sinuses, and migrate to T cell zones.

• T1 B cells convert into T2 B cells, which can enter follicles to mature into fully functional B-2 cells.

• B-2 cells encounter antigens and respond with T cell assistance.

Transitional and Additional B Cell Populations

• A newer type of transitional B cell, T3 B cells, recognizes antigens but does not activate, resulting in anergy.

• B-2 cells circulate in blood and lymphoid organs, particularly within the spleen and lymph node follicles.

• B-1 cells:

  • Secrete natural antibodies (mainly IgM) without antigen stimulation, providing an innate line of defense.

  • Recognize lipid and carbohydrate antigens of pathogens and help clear old red blood cells.

Differences Between B Cell Populations

• Summarized attributes and properties of B-1, Follicular (B-2), and Marginal Zone B cells:

Functionality of Marginal Zone B Cells

• Located in the marginal zone of the spleen, marginal zone B cells express CD1 (a non-classical MHC), which enables the presentation of lipid antigens to T cells.

• Their location allows interaction with blood-borne antigens, leading to the production of natural antibodies without needing T cell engagement.

Comparison of T and B Cell Development

• The developmental pathways differ concerning fetal development, signaling requirements, receptor editing, autoreactive elimination processes, and survival selection, with both B and T cells having unique characteristics and maturation checkpoints.