Lecture 8 - B Cell Immunity 1 - Microimm 2500

B Cell Immunity Overview

Learning Objectives

• Understand B cell development.

  • Structure and generation of BCRs and antibodies through gene rearrangement.

  • Mechanisms of self-reactive B cell elimination via negative selection.

• Explore the structure and function of antibody classes: IgM, IgD, IgG, IgE, IgA.

• Learn effector mechanisms associated with different antibody classes.

• Distinguish between natural and artificial passive immunity.

What Are Antibodies?

• Antibodies are soluble polypeptide molecules produced and secreted by B cells.

• immune system can make antibodies in response to infections

  • have a broad range of effector function

• b cells can recognize any type of substance

• Functions:

  • Neutralizing pathogens and cytotoxic actions (cell-mediated and complement-mediated).

  • Capable of recognizing a wide array of substances.

  • Significant medical and commercial importance.

Bestselling Drugs (2024)

• Humanized Monoclonal Antibodies:

  • Keytruda (Merck & Co.): $27.2 billion

  • Ozempic (Novo Nordisk): $16.1 billion

  • Dupixent (Sanofi): $13.5 billion

  • Eliquis (BMS/Pfizer): $13.3 billion

  • Biktarvy (Gilead): $12.6 billion

  • Darzalex (J&J): $12 billion

  • Opdivo (BMS/Ono): $11.3 billion

  • Comirnaty (Pfizer/BioNTech): $10.8 billion

  • Gardasil (Merck & Co./CSL): $10 billion

  • Skyrizi (AbbVie): $9.9 billion

• ones marked with asterisk are antibodies called monoclonal antibodies bcs they are engineered to bind to specific antigens, providing targeted immune responses and enhancing the body's ability to fight infections.

B Cell Receptors and Antibodies

• Consist of four polypeptide chains linked by disulfide bonds.

• made up of four polypeptide chains

  • 2 immunoglobulin chains and 2 light chain proteins

  • linked by disulfide bonds

  • light chain

    • kappa

    • lamda

    • can use either or

  • variable region of the immunoglobulin chains is responsible for antigen recognition and binding, allowing B cells to produce a diverse range of antibodies.

    • has two distinct antigen binding site… heavy chain and light chain working together

  • constant region of the immunoglobulin chains determines the antibody's class and mediates its effector functions, such as activating complement or binding to Fc receptors on other immune cells.

• Components:

  • Light Chains: kappa (κ) or lambda (λ).

  • Heavy Chains: µ, δ, γ, α, or ε.

  • Antibody types are classified based on heavy chains.

• antibodies can be broken up into regions

• hinge region is flexible giving antibody a lot of flexibility

• modified by various carbohydrate chains

• antibody types are determined by which heavy chain they use

  • ex. IGM antibody uses mu heavy chain

• antibodies and b cell receptors are the same molecules but with differences in their structure and function; while antibodies are secreted into the bloodstream to neutralize pathogens, B cell receptors are membrane-bound forms that allow B cells to recognize specific antigens.

• development of antibodies (soluble) development is done by colonel selection

• encoded by immunoglobulin genes

Antibody Structure

• B cell receptor (BCR) vs. Antibody:

  • (a) Membrane-bound form (BCR).

  • (b) Secreted form (antibody).

• Components:

  • Iga, IgB

  • Spacer, hydrophilic segment, hydrophobic segment, cytosolic segment.

• heavy chain has 1 variable region and 3 constant regions in this antibody

• VDJ (immunoglobulin heavy gene) for heavy chain

  • genetic recombination is happening to make the variations of B cell receptors (BCRs) that allow for the diverse recognition of antigens.

• VJ (immunoglobulin kappa or lambda) for light chain

• 1million base pair longs so very long gene that encodes for the unique heavy chain of the antibody, allowing for a diverse range of antigen recognition.

Gene Rearrangement in Antibodies

• Antibody heavy and light chain proteins are encoded by V, D, J, and C gene segments.

B Cell Development in the Bone Marrow

• Stages include Pro-B, Immature, Transitional B Cell stages.

• IgH VDJ rearrangement, followed by Igκ/λ VJ rearrangement.

• Results in diverse BCR specificities.

• all b cells are generated in the bone marrow

• progenitor b cells undergo several stages of maturation before they can express functional immunoglobulin on their surface.

• if rearrangement is successful will show pre_BCR on surface of cell, indicating that the B cell is ready to progress to the next maturation phase. This pre-BCR is crucial for signaling and further development into a mature B cell capable of producing antibodies.

• Once mature, B cells can migrate to secondary lymphoid organs, where they can encounter antigens and undergo further activation and differentiation into plasma cells or memory B cells.

  • made up of heavy chain and some light cell chains

• immunoglobulin kappa or lambda light chains are rearranged… if successful light chain protein can be made and paired with heavy chain

  • can recognize molecules but are very sensitive at this time and can cause cell death when meeting ligands with high affinity. This affinity maturation process ensures that only the B cells with the highest affinity for the antigen will survive and proliferate, ultimately leading to a robust immune response.

• B cells don’t require antigen presentation, they can recognize any type of substance

Activation of Naive B Cells

• never engaged with antigen before

• Activation occurs via:

  • T-Independent (TI) mechanisms.

    • engagement of recognition of antigen activates and activating signal… must receive a second signal by a pattern recognition receptor to fully activate the B cell, allowing it to proliferate and differentiate into antibody-secreting plasma cells.

  • T-Dependent (TD) mechanisms using cytokines and T helper cells.

    • require activation by t cells, which provide necessary signals for B cell differentiation and antibody production.

    • more complicated

    • t cells and b cells need to recognize each other

    • b cell interacts w/ antigen through bcr and will phagocytose that antigen, leading to the processing and presentation of antigen fragments on MHC class II molecules to T helper cells, which subsequently release cytokines that further stimulate B cell activation and proliferation.

    • t cell recognizes peptide on b cell and will deliver costimulatory receptors, which are crucial for the full activation of the B cell, enhancing its ability to produce antibodies and develop into memory cells.

    • CD40 expressed by b cell and CD40Ligand expressed by t cell

    • only happens if b cell and t cell are in close contact which happens if t cell recognizes peptide on b cell

• both need signal from bcr

Maturation and Clonal Selection of B Cells

• Development leads to antibody-secreting plasma cells or memory cells.

• Differentiation involves:

  • Gene rearrangement.

  • Clonal deletion (removal of self-reactive B cells).

• can make more than one type of antibody

• memory cells are formed after an initial immune response and can rapidly produce antibodies upon re-exposure to the same antigen.

Effector Mechanisms of Antibodies

1. Neutralization: Prevents pathogen binding.

2. Agglutination: Clumping of pathogens.

   1. cross link particles and can do glutination making large complexes of a tigens

3. Opsonization: Enhances phagocytosis.

4. Complement Activation: Lysis of pathogens.

   1. antibody is trigger activation of complement cascade

5. Antibody-Dependent Cell-mediated Cytotoxicity (ADCC): Induces destruction of infected cells.

   1. infected cell / cancer cell is being recognized by antibody molecules cell natural killer cells, leading to the targeted destruction of these cells through the release of cytotoxic granules.

6. Degranulation: Release of cytotoxic substances from immune cells.

   1. granulocytes degranulate and release substances such as histamine and proteolytic enzymes, which play a crucial role in amplifying the immune response and combating infections.

Production of Antibody Classes

• Heavy-chain C-region genes determine the Ig class.

• switch constant region gene segment to a different one (doesn’t happen in bone marrow)

  • during immune response it can switch use of constant region segments to encode different antibody classes (VDJ segment stays the same, constant region switches only)

IgM Structure and Function

• First antibody produced (primary response).

• it can form y shaped version w 2 binding sites but can also can exist as a pentamer, which allows it to have ten binding sites, enhancing its ability to agglutinate pathogens and activate the complement system.

• Exists as a monomer (2 binding sites) or pentamer (10 binding sites).

• Efficient in agglutinating antigens and initiating classical complement pathway.

• can bind in planar conformation (stick straight out), or staple form (pointing down)

IgD Structure and Function

• Expressed along with IgM by immature B cells.

• don’t know much about it

• can express IgM and IgD simultaneously, allowing them to participate in the initial immune response while also contributing to the maturation of B cells into more specialized forms.

• used as marker for immature b cells

• Constitutes only 0.2% of circulating antibodies with unclear function.

IgG Structure and Function

• Most prevalent antibody type in serum.

• Long-lived with multiple subclasses (IgG1-4).and most abundant

• dominate immune response second time you are exposed to something

• 4 subclasses

  • each have slightly different functions

• Effective in activating phagocytosis and complement pathways.

IgA Structure and Function

• Found in secretions like saliva, mucus, and breast milk.

  • mainly pressent in secretions

  • how its passed to babies and increase their immune system

• Present predominantly as dimeric form (two subclasses: IgA1, IgA2).

  • IgA neutralizes pathogens but doesn’t do much else

• Neutralizes pathogens but does not opsonize or activate complement.

IgA Transport Across Epithelial Barriers

• Dimeric IgA is actively transported into the gut lumen, neutralizing pathogens and toxins.

• actively transported across epithelial cells

  • has receptor pIgR, which facilitates its transcytosis and secretion into mucosal surfaces, playing a crucial role in mucosal immunity.

IgE Structure and Function

• Associated with allergic reactions and asthma, produced in minimal quantities yet potent.

• Activates mast cells, basophils, and eosinophils.

• responsible for allergies

• made in small quantities but very potent

• interacts with specific receptors and releases histamine, which contributes to the inflammatory response and exacerbates allergic reactions.

Blood Group Antigens and Antibodies

• Example of self-nonself discrimination.

• b cells subject to clonal deletion

• self non-self discrimination is a crucial mechanism that ensures B cells only respond to foreign antigens, preventing autoimmune reactions and maintaining immune system balance.

• ABO blood types:

  • Type A: A antigen, anti-B antibodies

    • have carbohydrate antigen A on blood cells

  • Type B: B antigen, anti-A antibodies

  • Type AB: Both antigens, no antibodies

  • Type O: No antigens, both anti-A and anti-B antibodies.

Natural Passive Immunity

• Transfer of IgG antibodies from mother to fetus through FcRn receptors during pregnancy.

• Post-birth, secretory IgA antibodies transferred via breastmilk.

• when you get antibodies from someone else

  • ex. pregnancy… antibodies go from mother to fetus

    • same with through breast milk

    • whatever mother is resistant to will make baby also resistant to it

Artificial Passive Immunity

• Intravenous Immunoglobulin (IVIG) treatments for antibody deficiencies.

• Use of humanized monoclonal antibodies for disease treatment and viral neutralization.

• patient infused w antibodies given to patients with antibody deficiencies

Summary

• BCR specificities generated through immunoglobulin gene rearrangement during B cell development.

• Self-reactive B cells eliminated via negative selection.

• Antibodies recognize diverse antigens not restricted by MHC.

• Different antibody classes perform specialized functions crucial for immunity.