unit 4, lesson 4, part 2, B Cells and Antibody

• Humoral adaptive immunity involves B cells; cellular adaptive immunity involves T cells.

CASE SCENARIO

• Tom, aged 35, has non-Hodgkin's lymphoma and is prescribed Rituximab, an anti-lymphocyte monoclonal antibody causing B lymphocyte lysis.

B Cells

• B cells respond specifically to pathogenic antigens through direct antigen binding.
• Antibodies, or immunoglobulins, mediate humoral immunity.
• B lymphocytes express clonally diverse cell surface immunoglobulin (Ig) receptors, known as B cell receptors (BCR), recognizing specific antigenic epitopes.

Activation of Antibody Synthesis

• Antigen and clonal expansion activates antibody synthesis.
• 10^10 antibody specificities exist.
• Naive B-lymphocytes undergo selective activation.
• These cells then proliferate clonally with the help of T helper cells (Th).
• This process results in the formation of plasma cells, which secrete antibodies, and memory cells.

B Cell Maturation: "Gene Rearrangement"

• B cell maturation occurs within bone marrow.
• A single antibody gene comprises variable, diversity, joining, and constant regions, each located on a different chromosome.
• Clonal diversity is due to multiple genes in each region, combinatorial freedom, and coordinated DNA cleavage and splicing.
• Approximately 10^10 antibody specificities are adequate to cover the range of pathogens likely to be encountered in life.

Antigens and Epitopes

• Antigens and epitopes refer to the amino acid epitopes of proteins.
• Conformational epitopes exist.
• Antibody responses to folded proteins target residues on the protein's surface.
• Linear epitopes exist.

B Cell Receptor

• B cell receptors bind B cell antigens.
• The B cell antigen, a substance or molecule, specifically interacts with an antibody (BCR), leading to antibody production and an immunologic response.
• Antigens are typically foreign proteins or polypeptides from sources like cells and bacteria; epitopes are specific chemical features.
• The antigen receptor on B lymphocytes, known as the B-cell receptor (BCR), is a cell-surface immunoglobulin.

B Cell Co-Receptors

• The specific B-cell receptor, an immunoglobulin molecule attached to the B cell membrane, recognizes the antigen in native form, unlike the TCR, which recognizes antigenic peptides.
• Co-stimulation of B cells is achieved by the molecule CD40.
• CD40 ligand (CD40L) is expressed on T cells.
• B cells present peptides to T cells, enabling a dialogue that leads to B-cell proliferation, optimal activation, and differentiation into plasma cells that produce antibodies.

B Cell Activation & T Helper Cells

• B-cell activation requires both antigen binding to the B-cell surface immunoglobulin, and interaction of the B cell with antigen-specific helper T cells.
• T helper cells recognize peptide fragments from the antigen internalized by the B cell, displayed as peptide:MHC class II complexes.
• T cell CD40L interaction with B cell CD40 ligand induces release of cytokines like TNFα.
• B cell activation and T helper cells cause cell proliferation.
• Some stimulated B-cells become plasma cells and secrete antibodies.
• Some cells become memory B-cells.

How B Cells Become Antibody-Secreting Plasma Cells

• B cells express immunoglobulin heavy chain transcripts that include transmembrane and cytoplasmic domains.
• Plasma cells express immunoglobulin heavy chain transcripts that stop after the C_H domains, thereby encoding the same antibody but in a secreted form.

Primary and Secondary Antibody Response

• Serum antibody concentration and class change after first and second antigen X exposures.
• The secondary response is faster, stronger, and primarily IgG, with higher antigen affinity due to the memory of B cells.

Basic Antibody Structure

• The variable region, or the antigen-binding fragment (Fab), recognizes the antigen.
• The Fc fragment is recognized by specific Fc receptors on effector cells.
• A monomeric IgG antibody has two antigen recognition sites.

Classes of Antibodies

• IgM is a pentamer released during the primary immune response.
• IgG, a monomer, is the most abundant and diverse antibody in primary and secondary responses, crossing the placenta to confer passive immunity.
• IgD is a monomer on the surface of B cells, important in B cell activation.
• IgA, a dimer, helps prevent pathogen attachment to epithelial surfaces.
• IgE, a monomer, binds to mast cells and basophils, releasing histamine when activated.

Effector Functions of Antibodies

• B-lymphocyte surface receptor (IgM) for antigen.
• Neutralization of antigen by IgG, IgM, IgA.
• Opsonisation (IgG, IgM).
• Steric hindrance (IgG, IgM, IgA) prevents injurious material from interacting with host cell membranes.
• Complement activated by IgG and IgM.
• Mucosal immunity mediated by IgA.
• Mast cells activated by surface-bound IgE interacting with antigen.
• Neonatal immunity mediated by maternal IgG.
• ADCC - Antibody-dependent cell-mediated cytotoxicity ADCC.

Antibody-Mediated Immunity

• Antibodies neutralize bacterial toxins by binding to them, preventing them from interacting with host cells.
• Antibodies also neutralize virus particles and bacterial cells by binding to and inactivating them.
• The antigen:antibody complex is eventually bound by and degraded by macrophages.
• Antibodies coating antigens enhance recognition by phagocytes for ingestion and destruction, called opsonization.
• Complement system activation by antibodies coating a bacterial cell can directly kill the bacterium.

Antibody-Dependent Cell Mediated Cytotoxicity (ADCC)

• Natural killer (NK) cells, neutrophils, and eosinophils act as effector cells capable of lysis.
• Antibody binding to the target cell's surface enables NK cells to kill it and depends on a prior antibody response by IgG.
• The Effector cell (NK cell) expressing Fc receptors on its the surface will bind to the Fc domains of antibody bound to a target cell.
• NK cells kill the target by apoptosis.

CASE SCENARIO - Non-Hodgkin's Lymphoma & Rituximab

• Non-Hodgkin's lymphoma originates in the lymphatic system, where disease-fighting networks spread throughout the body.
• In non-Hodgkin's lymphoma, tumors develop from lymphocytes.
• This lymphoma begins in B or T cells; most cases arise from B cells.
• B cell subtypes include diffuse large B-cell lymphoma, follicular lymphoma, mantle cell lymphoma, and Burkitt lymphoma.
• Its treatment includes chemotherapy, radiotherapy, or bone marrow transplant.
• Rituximab, a monoclonal antibody, attaches to B cells, marking them for destruction by the immune system, lowering B cell count.
• It targets the CD20 transmembrane protein, present from the stage B cells commit to development until it is downregulated.

Mechanism of Action of Rituximab

• Rituximab is a monoclonal antibody that targets the CD20 antigen on B cells.
• It mediates B cell lysis through Fc receptor recognition by NK cells, via antibody-dependent cell-mediated cytotoxicity (ADCC).
• The Fc portion induces the activation of the classical pathway of complement.
• Rituximab may also cause direct apoptosis of CD20 cells.