PHAR 504: Biology of the B Cell

LECTURE 4: BIOLOGY OF THE B CELL

1. Clonal Selection: Fundamental Concept in Immunology

  • Definition: A core principle explaining how the adaptive immune system generates specific and targeted responses to non-self antigens.

  • Clonal Expansion: The process by which B cells (or their clones) that frequently interact with their cognate epitope are stimulated to reproduce.

    • This leads to the dominance of that clonal lineage and its immunoglobulins (Igs) in the immune response.

    • Selection for Affinity: This process actively selects for B cells producing higher-affinity Igs and against those producing lower-affinity Igs.

  • Establishment of Self-tolerance: Alongside clonal expansion, clonal selection also contributes to the establishment of self-tolerance, preventing the immune system from attacking the body's own tissues.

2. B Cell Development and Maturation

B cell maturation occurs in two distinct phases:

A. Phases of Maturation

  • Bone Marrow Phase (Antigen-independent):

    • Occurs in the bone marrow.

    • Independent of antigen exposure.

    • Involves V(D)J recombination for Ig gene rearrangement.

  • Peripheral Phase (Antigen-dependent):

    • Occurs in secondary lymphoid organs (e.g., spleen, lymph nodes).

    • Requires antigen exposure for activation and further differentiation.

    • Also leads to the generation of memory cells.

B. Plasma Cells and Memory B Cells

  • Plasma Cells:

    • Definition: Activated B cells that have become enlarged and proliferated, also known as lymphoblasts, plasmablasts ("blasts"), or effector B cells.

    • Proliferation: Divide 2−42 - 4 times daily for 3−53 - 5 days, with one activated B cell producing up to 10001000 clones.

    • Function: Produce large quantities of antibodies.

    • Identity: Clonally derived from a progenitor cell and produce the same immunoglobulin.

    • Lifespan: Days to months.

  • Memory B Cells:

    • Definition: Quiescent (resting) cells that can persist for decades (1010s of years).

    • Expression: Express antibody on their cell surface but do not secrete immunoglobulins.

    • Function: Crucial for generating an accelerated and more robust antibody-mediated immune response upon re-infection (the secondary immune response).

C. The Secondary Immune Response

  • Characteristics: Involves rapid production of high-quality Igs.

  • Efficiency: This response is faster and stronger because it bypasses the initial B cell maturation process, relying on pre-existing memory B cells.

D. Cell Surface Markers

These markers indicate the stage of B cell development and cell identity, crucial for classification and diagnostic purposes.

  • Progenitor and Pre-B Cells: CD34

  • B Cell Identification: CD19, CD20

  • Immature or Naïve B Cells: IgM

  • Mature B Cells (pre-class switch): IgD (expressed alongside IgM on mature naïve B cells that have not undergone class switch recombination)

  • Memory B Cells: CD27

  • Plasma Cells: CD38, CD138

  • Processes indicated by markers: V(D)J joining (earlier stages), Class switch recombination (later stages).

E. Clinical Relevance: Classification of B-Cell Malignancies

Cell surface markers are vital for diagnosing and classifying lymphomas and leukemias:

  • Lymphomas and Leukemias: CD19, CD20, and CD22

    • Examples include acute lymphoblastic leukemia (ALL), diffuse large B cell lymphoma, and chronic lymphocytic leukemia (CLL).

  • Acute Myeloid Leukemia (AML) and Acute Lymphoblastic Leukemia (ALL): CD34

  • Multiple Myeloma, Chronic Lymphocytic Leukemia (CLL), Plasmacytoma: CD38

F. Targeting B Cells at Different Stages

Monoclonal antibodies (mAbs) and CAR T-cell therapy target specific B cell markers for therapeutic intervention:

  • Anti-CD20 mAbs: Example: Rituximab, an immune modulator that depletes CD20+ B cells (used in certain lymphomas and autoimmune diseases).

  • Anti-CD19 mAbs: Example: Tafasitamab (Monjuvi), used for diffuse large B-cell lymphoma.

  • Anti-CD38 mAbs: Example: Daratumumab, used to treat multiple myeloma by depleting CD38+ malignant B cells.

  • CAR T-cell Therapy: Targets CD19, CD22, or CD38 and will be discussed in the context of Immunity to Tumors.

3. Secondary Lymphoid Tissues

These tissues are critical for initiating and amplifying adaptive immune responses.

A. General Functions and Characteristics

  • Locations: Spleen and lymph nodes.

  • Fluid Sampling: Filter and sample bodily fluids to detect infections.

    • Lymph Nodes: Collect antigens from the lymphatic system.

    • Spleen: Traps antigens circulating in the blood.

  • Key Processes: These tissues serve as sites for:

    1. Antigen accumulation and presentation.

    2. Gathering of leukocytes (primarily antigen-presenting cells (APCs), T cells, and B cells).

    3. Activation of mature naïve T and B cells.

    4. Clonal selection and proliferation of lymphocytes.

    5. Differentiation of lymphocytes into effector and memory cells.

B. The Spleen: Immune Response to Antigens in Blood

  • Structure: Contains follicles rich in naïve B cells, with T cells and APCs primarily located in the periphery.

  • Mechanism: When antigens enter the spleen, they are taken up by APCs, processed, and presented to B and T cells to stimulate an immune response.

  • Germinal Centers: Antigenic stimulation of B cells in follicles leads to the development of germinal centers, which contain activated B cells that are dividing, producing Igs, and undergoing genomic alterations (affinity maturation and class switch recombination).

C. Lymph Nodes: Immune Response to Antigens in Lymph

  • Structure: Encapsulated tissues located at junctions of draining lymphatics. They also contain follicles rich in naïve B cells, surrounded by APCs and T cells.

  • Mechanism: Antigens reaching a lymph node are taken up by APCs, processed, and presented to B and T cells to stimulate an immune response.

  • Germinal Centers: Similar to the spleen, activated B cells may form germinal centers, where they divide, produce Igs, and undergo genomic alterations (affinity maturation and class switch recombination).

D. Germinal Centers (GCs) in Secondary Lymphoid Tissues

  • Nature: Transient, specialized structures forming in response to antigenic stimulation and typically lasting for approximately 2−32-3 weeks after the initial immune response.

  • Key Sites for: Class Switch Recombination (CSR) and Affinity Maturation.

    • Affinity Maturation: Involves somatic hypermutation (SHM), which can sometimes lead to the recognition of "self" antigens.

    • Selection: Cells undergoing SHM must rigorous selection to remove self-reactive lymphocytes.

  • Peripheral Tolerance in GCs: This selection process in secondary lymphoid tissues constitutes Peripheral Tolerance.

  • Differentiation: Following the establishment of peripheral tolerance, surviving cells may undergo class switch recombination and then differentiate into plasma or memory cells.

  • Composition: GCs are functionally compartmentalized:

    • Dark Zone: Densely packed, rapidly-dividing B cells.

    • Light Zone: Contains non-dividing B cells, follicular dendritic cells (FDCs), and T helper cells, where selection, CSR, and differentiation processes occur.

4. B Cell Activation

B cells can be activated via T cell-dependent or T cell-independent pathways to produce immunoglobulins.

A. T Cell-Dependent Activation of B Cells

This pathway requires help from T helper cells and is critical for robust, high-affinity, and long-lasting responses.

  • Initial Antigen Binding: The B cell receptor (BCR) binds to its specific antigen.

  • Antigen Processing and Presentation: The antigen-BCR complex is internalized via receptor-mediated endocytosis. The B cell then processes the antigen and presents peptide fragments on Major Histocompatibility Complex Class II (MHC II) molecules on its surface.

  • T Helper Cell Interaction: A specific T helper cell, recognizing the MHC II-peptide complex, provides activation signals to the B cell.

    • Co-stimulation: The T helper cell expresses CD40 Ligand (CD40L), which binds to CD40 on the B cell, providing a crucial co-stimulatory signal.

    • Cytokines: The T helper cell secretes cytokines that further stimulate the B cell.

  • B Cell Activation and Outcomes: The activated B cell undergoes:

    • Proliferation: Rapid cell division.

    • Somatic Hypermutation: Introduces point mutations into the Ig variable region genes, leading to affinity maturation.

    • Class Switch Recombination: Changes the isotype of the antibody produced (e.g., from IgM to IgG, IgA, or IgE) without altering antigen specificity.

  • Co-stimulatory Molecules: These molecules (e.g., CD40L-CD40) deliver secondary signals essential for complete T cell activation and subsequent T cell-dependent B cell responses. Monoclonal antibodies can block these pathways for therapeutic purposes (e.g., in kidney transplantation).

B. T Cell-Independent B Cell Activation

This pathway allows B cells to produce antibodies without direct T cell help, typically for specific types of antigens.

  • Antigen Types: Non-protein antigens that cannot be processed and presented by MHC molecules.

    • TI-1 Antigens: Can directly activate B cells without cross-linking BCRs (e.g., lipopolysaccharides (LPS) from bacteria), acting as polyclonal activators.

    • TI-2 Antigens: Highly repetitive structures (e.g., bacterial capsular polysaccharides) that cross-link multiple B cell receptors, providing the first activation signal.

  • Activation Mechanism:

    • First Signal: Cross-linking of multiple B cell receptors by repetitive antigens (for TI-2).

    • Second Signal: Provided by pattern recognition receptors (PRRs) recognizing pathogen-associated molecular patterns (PAMPs) or through complement factors.

  • Response Characteristics (Limited):

    • Speed: Rapid, but short-lived antibody response.

    • Affinity: Igs are generally of lower affinity compared to T cell-dependent responses.

    • Memory: Typically does not generate memory B cells.

    • Isotype: Mainly produces IgM antibodies.

5. B Cell Tolerance: Central and Peripheral

Immune tolerance is the unresponsiveness to specific antigens, vital for preventing autoimmune diseases by training the adaptive immune system to ignore autoantigens.

A. Central Tolerance

  • Definition: Established during lymphocyte development in primary lymphoid organs.

    • Location: Bone marrow for B cells (and thymus for T cells).

  • Process: Eliminates developing B lymphocytes that react strongly to self-antigens.

  • Mechanisms:

    • Receptor Editing: A rescue mechanism where self-reactive B cells can undergo another round of light chain V(D)J recombination to change their CDRs and antigen specificity. The new antibody must then pass central tolerance testing again.

    • Apoptosis: Programmed cell death that eliminates self-reactive B cells, preventing the establishment of auto-reactive clonal lineages.

    • Anergy: Self-reactive B cells are inactivated so they do not respond to antigen.

      • BCR signaling is blocked.

      • Anergic B cells remain in circulation but in low numbers.

      • A large dose of antigen (e.g., during infection or autoimmune conditions) can potentially overcome anergy, leading to clonal expansion and potentially autoimmune disease.

  • Conditions Impacted by Failures in Central B Cell Tolerance:

    • Systemic Lupus Erythematosus (SLE): Defects in central (and peripheral) B cell tolerance contribute to SLE development, leading to an increased proportion of autoreactive B cells.

    • Rheumatoid Arthritis (RA): Patients with RA show impaired central B cell tolerance, resulting in autoreactive B cells that contribute to disease pathogenesis.

    • Type 1 Diabetes (T1D): Defects allow autoreactive B cells to escape elimination in the bone marrow, infiltrate the pancreas or pancreatic lymph nodes, and present islet antigens to autoreactive T cells, promoting the destruction of pancreatic $\beta$ cells.

B. Peripheral Tolerance

  • Definition: Established in peripheral tissues.

  • Location: Occurs in secondary lymphoid tissues, particularly within germinal centers, after immune stimulation.

  • Context: During affinity maturation, somatic hypermutation can inadvertently generate B cells that recognize "self" antigens.

  • Mechanism: These potentially self-reactive cells must undergo a rigorous selection process to remove them.

  • Mechanisms for Peripheral B Cell Tolerance:

    • Apoptosis: Self-reactive B cells are eliminated through programmed cell death.

    • Anergy: Self-reactive B cells become inactivated and unresponsive to further stimulation. (Evidence for receptor editing in peripheral tolerance is limited.)

  • Outcome: Ensures that only non-self-reactive B cells survive, preventing autoimmunity in the periphery after the initial development in the bone marrow.