B Lymphocyte Development

THE IMMUNE SYSTEM

Development of B Lymphocytes

Chapter Overview

  • Chapter focuses on the development of B lymphocytes (B cells) in the immune system.

  • Describes key processes, checkpoints, genetic rearrangements, and interactions within the bone marrow and secondary lymphoid tissues.

6-1 B-cell Development in the Bone Marrow

  • B-cell development proceeds through several stages:     - Originates from pluripotent hematopoietic stem cells.     - Development is influenced by bone marrow stromal cells.

    • negative selection happens at bone marrow

  • Bone marrow: essentially blood, mesenchymal cells that traveled into bone

  • Figures:     - 6.1: Phases of B-cell development, highlighting that negative selection begins in the bone marrow and continues in secondary lymphoid organs.     - 6.2: Illustrates the migration of B cells from the bone marrow to secondary lymphoid tissues for maturation.

  • At every stage, many cells are prevented from entering the next phase

  • Body only expands and diminishes the clones according to circumstances as we make them every day as well as loose them

  • B-cells start development in bone marrow, circulates into blood stream and become mature in secondary lymphoid organ where they will be waiting to be activated.

  • Different stages can be marked by different surface proteins present

Stages of B cell development > immature B cell

  • Mostly rearranging their heavy and light chain arrangements in the DNA of their immunoglobulin genes, with some checkpoints

6-2 Influence of Bone Marrow Stromal Cells

  • Key Points:     - Bone Marrow Stromal Cells are essential for the development of B cells.     - These cells provide necessary signals and physical contacts.     - The dependence on stromal cell contact diminishes in later stages of B-cell development.

    • Bone marrow stromal cell stimulation relases IL-7, which will stimulate pro B cells to further develop

    • Stromal cell stimulation also contributes to prouction of necessary recombination enzymes such as repair enzymes etc. (heavy and light chain rearrangements in B cell)

  • Figures:     - 6.5: Early stages of B-cell development show dependence on stromal cells.

6-3 Rearrangement of Immunoglobulin Heavy-Chain Genes in Pro-B Cells

  • In pro-B cells, the rearrangement of heavy-chain genes is critical:     - Results in both productive and nonproductive rearrangements.     - Default pathway is apoptosis unless survival signals are received.

  • Key Factors:     - Important transcription factors (E2A, EBF, Pax-5) regulate somatic recombination involved in heavy-chain rearrangement.

  • Figures:     - 6.6: Representation of gene rearrangements in pro-B cells.

6-4 Pre-B-cell Receptor Function

  • The pre-B-cell receptor (pre-BCR) serves crucial functions:     - Monitors the quality of immunoglobulin heavy chains.     - Signals the cell to cease heavy-chain rearrangement and to switch to light-chain rearrangement.     - Induces allelic exclusion ensuring production of homogeneous B-cell receptors.     - A lack of this exclusion results in heterogeneous receptors with reduced avidity.

  • Figures:     - 6.7: Diagram showing the pre-BCR and its role in directing B-cell development.

6-5 Rearrangement of Light-Chain Loci in Pre-B Cells

  • Light-chain rearrangement occurs in pre-B cells and follows successful heavy-chain expression.     - Increases chances of producing functional immunoglobulin.     - The presence of functional surface immunoglobulin IgM inhibits further light-chain rearrangements.

  • Figures:     - 6.8: Organization of light-chain loci showing rearrangement processes.

6-6 Checkpoints in B-cell Development

  • B cells face critical checkpoints during development:     - First checkpoint: assesses productive rearrangement of heavy-chain genes ( pre- beta cell receptor.)     - Second checkpoint: ensures light-chain genes rearrangements have resulted in functional receptors ( B-cell receptor).     - Cells that fail at these checkpoints undergo apoptosis.

  • Figures:     - 6.10: Flowchart of the rearrangement and selection process in B-cell development.

6-7 Protein Expression in B-cell Development

  • Timing and expression of proteins are crucial to B-cell development stages.     - Varying proteins influence immunoglobulin gene rearrangements and B-cell maturation.

  • Figures:     - 6.11: Development timeline of immunoglobulin-related proteins.

  • Different proteins expressed at different points orchestrtate development of functional B cells.

6-8 Tumors and Chromosomal Translocations

  • Many B-cell tumors exhibit chromosomal translocations leading to Ig genes joining with genes that regulate cell growth, leading to malignancies:     - E.g., Burkitt’s lymphoma associated with translocations that alter cell growth regulation.     - Distinction between proto-oncogenes (normal growth-regulating genes) and oncogenes (cancer-promoting genes).

  • Figures:     - 6.13: Illustrates chromosomal rearrangements in B-cell tumors.

6-9 Distinction of CD5+ and CD5- B cells

  • CD5+ B cells (often B-1 cells) differ from typical B-2 cells:     - Produced mainly in the fetal stage, have limited receptor diversity, and a unique anatomical distribution (peritoneal and pleural cavities).     - No T-cell help required for activation, low somatic hypermutation rate, etc.

  • Figures:     - 6.14: Comparison chart between B-1 and B-2 cells.

Selection and Further Development of B-cell Repertoire

6-10 Purging of Self-Reactive B-cell Receptors

  • Immature B cells that recognize self-reactive receptors are eliminated during development in the bone marrow:  »   - Critical for establishing tolerance and preventing autoimmunity.

  • Figures:     - 6.16: Process showing how multivalent self antigens activate self-reactive B cells.

6-11 Receptor Editing of Autoreactive B Cells

  • Receptor editing allows immature B cells to modify their receptors if they initially interact with multivalent self-antigens:     - Light-chain rearrangement allows self-reactive receptors to adapt.     - Failure to produce a non-self-reactive receptor results in apoptosis.   

  • Chance to modify their receptors so they can not recognize self.

  • Figures:     - 6.17: Illustration of receptor editing mechanics in self-reactive B cells.

6-12 Non-responsiveness to Monovalent Self Antigens

  • Immature B cells that encounter monovalent self antigens typically become nonresponsive (anergic):     - Characterized by reduced surface IgM and shorter half-life, aiming to limit potential autoimmunity.

  • Figures:     - 6.18: Graphical representation of B cells undergoing anergy.

6-13 Maturation in Lymphoid Follicles

  • After immature B cells exit the bone marrow, they mature in lymphoid follicles:     - This phase is essential for generating diverse and clonally expressed receptors.

  • **Phases of Maturation: **     - Phase 1: Generation of diverse B-cell receptors in the bone marrow.     - Phase 2: Negative selection of self-reactive receptors.     - Phase 3: Positive selection to promote maturation.     - Phase 4: Recirculation for infection searching.     - Phase 5: Activation by specific antigens and clonal expansion.     - Phase 6: Differentiation into plasma cells and memory B cells.

  • Figures:     - 6.19: Routes of B-cell circulation in tissues.

  • go to lymphoid follical to get stimulated and matured

6-14 Activation and Differentiation Upon Antigen Encounter

  • Antigen exposure leads to activated B cells differentiating into plasma cells and memory B cells:     - Plasma cells secrete IgM and stop proliferating, while memory B cells participate in later immune responses.

  • Figures:     - 6.21: Workflow of B-cell activation and differentiation.

6-15 Diversity in B-cell Tumors

  • The variety in B-cell tumors correlates with differentiation stages of B cells, reflecting the complexity of their development:     - Understanding this diversity helps in tailoring therapeutic strategies.  

  • Figures:     - 6.22: Summary of the relationship between tumor diversity and B-cell development.

Summary of Key Concepts

  • The development of B lymphocytes incorporates stages of cellular development, genetic rearrangements, checkpoints, and interactions with stromal cells.

  • Various pathways and decisions made during development play crucial roles in B-cell functionality, tolerance, and malignancy potential in certain contexts.

  • B cells undergo rigorous selection processes to ensure autoreactive cells are eliminated or rendered inactive, culminating in a highly specialized immune response upon activation.