B Cells and Antibodies

Lecture 3 - B cells and Antibodies

Overview

• Key Concept: The evolution of adaptive immunity in higher life forms.

  • Occurred 200 million years ago in fish.

  • Adaptive immunity can respond to a wide range of invaders.

  • B lymphocytes: Critical cells that detect antigens using B cell receptors (BCR).

    • BCRs detect the antigen and signal the B cell to activate, divide, and produce antibody with the same specificity as the BCR

Objective 1.1 - BCR Structure and Function

• BCR Composition:

  • Made of light chains (Lc) and heavy chains (Hc).

  • Diversity arises from rearranging gene segments on chromosome 14.

  • Two copies of chromosome 14 (one from each parent) compete for selection.

Objective 1.2 - BCR Structure and Function

• Gene Segments Selected to make a BCR

  • Four segments: V, D, J, and C.

  • Heavy chain (Hc) is produced first, followed by testing for functionality.

    • Cut and paste in order, test to see if the receptor works.

  • The successful chromosome shuts down the other.

  • Heavy chain and light chain have to fit together to work

    

Objective 1.3 - BCR Structure and Function

How the BCR signals - two interacting components

• Antigen Recognition:

  • B cell epitope: Small region on the antigen (50-79 amino acids).

  • Binding occurs through Hc and Lc "antennae."

• Signaling Mechanism:

  • Proteins Igβ and Igα transmit signals to activate antibody production.

    

Objective 1.4 - BCR Structure and Function

How the BCR signals - concept of cross-linking

• Cross-Linking:

  • Polymeric antigens (one with repeats of an epitope) can bind multiple BCRs, enhancing signaling.

    • Many copies of the same antigen on the invader can do this.

      

    • Complement receptors on the B cell recognize a fragment of iC3b, enhancing the clustering that occurs on BCRs

      • This amplifies B cell signaling over 100-fold.

        

Objective 2.1 - Second Signals for B Cell Activation

• Activation Requirements:

  • B cells need two signals for activation.

  • Types of activation:

    • T-independent response (without T helper cell help).

    • T-dependent response (with T helper cell help).

• Regardless of which way the cell is activated, the process requires 2 signals

  • First signal: cross-linking (clustering) of BCR

  • Second signal: co-stimulatory signal determines the quality of the B cell response

Objective 2.2 - Second Signals for B Cell Activation

• T Cell-Dependent Activation: Interaction with co-stimulatory molecule on Th cell (T-helper cell)

  • Involves interaction with CD40 on B cells and CD40L (ligand) on T helper cells.

  • The second signal is crucial for the quality of the B cell response.

Objective 2.3 - Second Signals for B Cell Activation

• T Cell-Independent Activation: Danger signal

  • Involves recognition of PAMPs by Toll-like receptors (PRRs).

• Qualities of the T-independent B cell response

  • 1. Fast - Don’t need to wait for T helper cells to be activated

  • 2. Responds to non-protein antigens

The second, co-stimulatory signal helps prevent B cells from being activated against polymers from our own bodies (like DNA)

Objective 2.4 - Second Signals for B Cell Activation'

A third, bad way to stimulate B cells

• Mitogen Activation:

  • Mitogens can bind to molecules on B cells that just happen to be associated with BCRs

  • Mitogens can cluster BCRs without antigen recognition.

    • Results in the activation of many B cells (polyclonal activation, as many clones of B cells activate this way)

  • Results in polyclonal activation, which can confuse immune responses. This is a bad way to stimulate B cells

Objective 3.1 - Class Switching

What happens after B cells are stimulated? Maturation of B cells in 3 steps

• Post-Stimulation Processes:

  • 1. Class switching: Changing antibody class (e.g., IgM to IgG).

  • 2. Somatic hypermutation: Increases BCR affinity for antigens.

    • Done via mutations that increase the affinity of BCR for its cognate antigen

  • 3. Decision between becoming a plasma cell or memory cell.

Objective 3.2 - Class Switching

• Mechanism:

  • Activated B cells initially produce IgM or IgD.

  • Cutting and pasting heavy chain (Hc) genes on chromosome 14 leads to changes in antibody class

  • Heavy chain gene rearrangement leads to class switching.

    

Objective 4.1 - Antibody Classes and Functions

• Antibody Structure:

  • Constant region (Fc) determines function of Ab (antibody)

  • Fab (antigen binding) region determines antigen specificity.

    

Objective 4.2 - Antibody Classes and Functions

• IgM Antibody: First antibody in evolution, gene sequence, and

  expression activated by B cells

Held together by a J chain

• Very good at activating (“fixing”) complement (by the classical pathway)

  • Allows two C1_qrs protein complexes to come together

    • via the Fc region when IgM binds to the cognate antigen on an invader

  • Causes C1s to lose thier inhibitor molecule which leads to the activation of a C3 convertase —> same pathway as seen in the innate immune system

• IGM is a good neutralizer

: Objective 4.3 - Antibody Classes and Functions

• IgG Antibodies: Gamma globulins

  • Multiple subclasses; IgG3 and IgG1 have specific functions.

    • IgG3 fixes complement, can be recognized bt NK cells

      • Recognized by antibody dependent cell-mediated cytotoxicity, ADCC)

    • IgG1 opsonizes - coat cells or pathogens so cells of your immune system (phagocytes) can find them, attach to them, swallow them and break them apart

  • Crosses the placenta, providing fetal protection.

  • Good neutralizer

  • Most abundant class of antibodies in the blood; has a half life of 3 weeks (c.f IgM = one day)

    

Objective 4.4 - Antibody Classes and Functions

• IgA Antibody:

  • Most abundant - rare in blood but common on mucosal surfaces and mucsosal secretions.

    • Found in breast milk

  • Structure - 2 IgGs held together by a clip protein

    • Clip is a passport for secretion into the gut

  • Good neutralizer

    

Objective 4.5 - Antibody Classes and Functions

• IgE Antibodies:

  • Associated with allergic reactions and anaphylaxis.

  • Involved in defense against parasites.

  • Caused by mast cells degenerating

    

Objective 4.5 - Summary of Antibody Classes

Class Switching - controlled by the cytokines produced by T-helper cells. Ex.

ll Maturation

• Somatic Hypermutation:

  • High mutation rate in V, D, and J regions post-class switching.

  • Affinity for cognate antigens:

    • unchanged

    • reduced

    • increased

• B cells will keep dividing when constantly re-stimulated by antigen.

  • B cells that producer higher affinity Ab (bind more tightly to antigen) are favored, so we continually select for better Ab through this fine tuning

Objective 5.2 - B Cell Maturation

• Career Choices for B Cells:

  • Plasma cells: Secrete antibodies after migrating to spleen or bone marrow.

  • Memory B cells: retains memory of first antigenic stimulation to give faster response to subsequent encounter with cognate antigen.

    • Principle behind immunization

    • Only occurs with T-dependent antigens and Th help