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CUT-AND-PASTE GENES. Latin immunis , means, "exempt" in English. The immune response begins when a white blood cell called a macrophage encounters a virus and consumes it. Meanwhile, other viruses look for nearby cells to infect.

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CUT-AND-PASTE GENES

Immunogenetics


Latin immunis, means, "exempt" in English.

Immunogenetics


The immune response begins when a white blood cell called a macrophage encounters a virus and consumes it. Meanwhile, other viruses look for nearby cells to infect.

Immunogenetics


Next, the macrophage digests the virus and displays pieces of the virus called antigens on its surface. Nearby cells have now become infected by the attacking viruses.

Immunogenetics


Unique among the many different helper T cells (another class of white blood cells) in the body, one particular helper T cell now recognizes the antigen displayed and binds to the macrophage.

Immunogenetics


This union stimulates the production of chemical signals-- such as interleukin-1 (IL-1) and tumor necrosis factor (TNF) by the macrophage, and interleukin-2 (IL-2) and gamma interferon (IFN-y) by the T cell -- that allow intercellular communication.

Immunogenetics


As part of the continuing process, IL-2 instructs other helper T's and a different class of T cells, the killer T's, to multiply. The proliferating helper T's in turn release substances that cause B cells to multiply and produce antibodies.

Immunogenetics


The killer T cells now begin shooting holes in host cells that have been infected by viruses.

Immunogenetics


The antibodies released by the B cells bind to antigens on the surfaces of free-floating viruses. Besides making it easier for macrophages to destroy viruses, this binding signals blood components called complement to puncture holes in the viruses.

Immunogenetics


Finally, as the infection is brought under control, the activated T and B cells are turned off by suppressor T cells. However, a few "memory cells" remain behind to respond quickly if the same virus attacks again.

Immunogenetics


Somatic recombination of antibody genes and the generation of antibody diversity

There are a million to a billion different B and T cells, each of which corresponds to a different antibody or T cell receptor protein.

The human genome is thought to contain less than 105 genes.

How can a human make more antibodies than there are genes in its genome?

Immunogenetics


Review of antibody diversity

  • Immunoglobulins [antibodies]

    Antibody molecules have 4 polypeptide chains [2 heavy & 2 light chains].

    Each polypeptide chain has constant and variable regions.

    An antibody molecule has two sites to bind to antigen.

    Secreted antibodies are grouped into 5 major classes:

    [IgG, IgA, IgM, IgD, IgE]

Immunogenetics


Light and heavy chains have variable regions and constant regions (relative to amino acid sequence).

Immunogenetics


Immunogenetics


Constant and variable regions
Constant and Variable Regions amino-terminal ends and form the antigen binding site.

Immunogenetics


Antibody domains
Antibody Domains amino-terminal ends and form the antigen binding site.

  • Both chains are composed of 110 amino acid repeating segments.

  • The repeating segments fold independently to form functional domains.

  • Each domains is held together by one intrachain disulfide bond.

  • The domains evolved by gene duplications.

Immunogenetics


Three dimensional structure
Three-Dimensional Structure amino-terminal ends and form the antigen binding site.

  • Each domain folds into a very similar three-dimensional structure.

  • In the variable domain are three hypervariable loops that form the antigen binding site.

  • Changing the length and amino acid sequence of the hypervariable loops does not disturb the three-dimensional structure.

Immunogenetics


Immunogenetics amino-terminal ends and form the antigen binding site.


Immunogenetics amino-terminal ends and form the antigen binding site.


Immunogenetics amino-terminal ends and form the antigen binding site.


Antibody diversity
Antibody Diversity amino-terminal ends and form the antigen binding site.

  • During B cell development, separate gene segments are assembled in a genetic recombination event before the gene is transcribed.

  • Experiments done in 1976 showed that the C-coding and V-coding regions on different parts of the chromosome were spliced together.

Immunogenetics


V-D-J-C amino-terminal ends and form the antigen binding site.

Immunogenetics


Light chain rearrangements only involve VJ and C rearrangements.

Immunogenetics


Immunogenetics rearrangements.


In the progenitor B cell, antibody gene rearrangement is initiated by a DH/JH rearrangement of one of the two H chain gene alleles.

As soon as one functional H chain gene and one functional L gene are created, the rearrangement process terminates.

However, if temporal cascade of rearrangements ultimately fails to create either a functional H chain or L chain gene, the B cell is eliminated.

Immunogenetics


Rearrangement of antigen receptors is an irreversible event and absolutely required for lymphoid development.

Immunogenetics


Development of B-cells event and absolutely required for lymphoid development.

1) Gene rearrangements to produce highly variable antibody genes

2) Expression of surface immunoglobulin

3) Deletion or silencing of autoreactive B-cells

Immunogenetics


Stem Cell Factor (SCF), event and absolutely required for lymphoid development.

Interleukin-7

Immunogenetics


allelic exclusion event and absolutely required for lymphoid development.

Allelic exclusion is the process by which lymphocytes express antigen receptors from only one of two possible alleles.

Immunogenetics


Membrane bound antigens can crosslink the surface IgM on the surface of immature B cells.

Examples of such antigens are MHC class I and class II molecules.

Immature B cells that recognize such self antigens undergo apoptosis and are therefore removed from the repertoire.

Immunogenetics


Stem cell surface of immature B cells.

Germline DNA

Early pro-B cell

Late pro-B cell

Pre-B cell

IgM expressed on cell surface

Immature B cell

Mature B cell

Secreted Ig

Plasma cell

Immunogenetics


Immunogenetics surface of immature B cells.


Four sources of antibody diversity: surface of immature B cells.

1) Combinatorial diversity

2) Diversity due to heavy and light chain joining

3) Junctional diversity

4) Diversity due to somatic mutations

Immunogenetics


Combinatorial diversification
Combinatorial Diversification surface of immature B cells.

  • In the mouse variable light chain, about 300 different V-regions can be joined to 4 different J segments which are then joined to one C-region.

  • In the mouse variable heavy chain, 500 different V-regions can be joined to 4 J segments and 12 D segments.

  • What is the total number of possible combinations?

Immunogenetics


Combinatorial diversification1
Combinatorial Diversification surface of immature B cells.

  • Light Variable Chain:

  • 300 x 4 = 1200

  • Heavy Variable Chain:

  • 500 x 4 x 12 = 24,000

  • Total possible combinations:

  • 1200 x 24,000 = 28,800,000

Immunogenetics


Junctional diversification
Junctional Diversification surface of immature B cells.

  • Joining of the V segment to a J or D segment is not always precise in that random nucleotides can be inserted or lost at the junction.

  • It is estimated that junctional diversification increases the number of possible combinations by at least 108.

Immunogenetics


How does this occur? surface of immature B cells.

Immunogenetics


VDJ recombination involves several steps, beginning with a cut to the DNA.

These site-specific double- stranded breaks are initiated by two genes-called RAG-1 and RAG-2.

Immunogenetics


Immunogenetics


A protein, called Ku80, appears to do double duty in the processes of DNA repair and a kind of DNA recombination known as "VDJ joining."

Immunogenetics


VDJ recombination also takes place in the T cells. processes of DNA repair and a kind of DNA recombination known as "VDJ joining."

Immunogenetics


Class Switch processes of DNA repair and a kind of DNA recombination known as "VDJ joining."

In immature (and naïve) B cells, the m (and d) chains are located adjacent to the VDJ region. At this point, IgM

(and IgD) are expressed.

Constant region gene segments located further downstream can be brought into proximity to the VDJ region by looping out and deleting intervening sequences.

This process is irreversible and requires T cell signals.

Immunogenetics


Alternative polyadenylation sites processes of DNA repair and a kind of DNA recombination known as "VDJ joining."

Immunogenetics


Alternative mRNA splice sites processes of DNA repair and a kind of DNA recombination known as "VDJ joining."

Immunogenetics


B cell derived tumors processes of DNA repair and a kind of DNA recombination known as "VDJ joining."

Chronic lymphocytic leukemia

Blood

Malignant Plasma Cells

Multiple myeloma

Plasma cell (secreting

Ig of various isotypes,

IgG, IgA, IgE...)

Immunogenetics


Immunogenetics processes of DNA repair and a kind of DNA recombination known as "VDJ joining."


Monoclonal Antibodies processes of DNA repair and a kind of DNA recombination known as "VDJ joining."

A hybridoma is a fusion of a cancer cell with a functional, normal cell - a hybrid of a lymphoma) which results in the generation of an immortal cell-line.

Immunogenetics


Immunogenetics processes of DNA repair and a kind of DNA recombination known as "VDJ joining."


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