Post on 22-Dec-2015
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Antibodies and T Cell Receptor GeneticsLearning Objectives
• To understand mechanisms for creating diversity– Be able to identify changes at the
DNA level required to produce a functional immunoglobulin gene
• To understand isotype switching at the molecular level
• To recognize basic differences between antigen receptors on B and T cells
Generation of Antigen Receptor Diversity
• Survival requires B and T cell receptor diversity to respond to the diversity of pathogens
• The immune system must “Be Prepared” to respond to antigens it has never encountered
• One to 100 million different antigen receptors (Ig on B cells, TCR on T cells) can be produced
Generation of Diversity
• Diversity operates at the level of the lymphocyte
Cellular Solutions
• Each lymphocyte has a unique receptor for antigen
• Produce one million different lymphocytes per day
• Antigen selects cells by binding to a complementary receptor and stimulating cell division and differentiation (antibody-secreting plasma cells or effector T cells)
Generation of Diversity• Survival requires diversity to
respond to the diversity of pathogens
• One to 100 million different antibodies can be produced
• Nine isotypes• Similar numbers of T cell
receptors for antigenProblem - Not enough DNA to support observed diversity
10 x 106 genes X 103 base pairs DNA/gene = 10 x 109 bp> 3 X 109 bp DNA available
Generation of Diversity
• Functional genes for antigen receptors do not exist until they are generated during the development of lymphocytes
Genetic Solutions
• Variable region exons are formed by splicing together segments of genes inherited through the germline• The process is called Ig or TCR gene rearrangement, and generates tremendous diversity without monopolizing the genome
Variable region genes are constructed from gene segments
Germline DNAStem cell
B Cell DNA
mRNA
Protein
Somatic recombination at the Ig heavy chain locus
Germline DNA
D JH rearrangement
V DJH rearrangement
* *
*
Primary RNA transcript
Splicing and polyA mRNA
Nascent polypeptide
Mature μ heavy chain protein
Benefits of Antigen Receptor Gene Rearrangement
40 Vκ 30 Vλ 65 VH
x 5 Jκ x 3 Jλ 27 DH
x 6 JH
200 κ V regions 90 λ V regions 10,530 H V regions
How many antibodies can be made?(10,530 HC) x (200 κ LC) = 2.1 x 106 IgM κ antibodies(10,530 HC) x (90 λ LC) = 0.9 x 106 IgM λ antibodies
3 million totalNot bad from 176 gene segments!
One B cell or plasma cell only makes one antibody
Switch Recombination
Advantage No requirement for separate VDJH recombination for each isotype
Only cells that switch will be those responding to antigen
Features Irreversible
Individual plasma cell produces one isotypeone specificity
Primary and Secondary Antibody Responses
Primary Secondary
Higher titerHigher affinity
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The T Cell Receptor
• Heterodimer that only exists as a transmembrane antigen receptor
• It is not secreted since T cells function by direct cell contact
• The variable regions of the TCR are generated by somatic gene recombination as the T cells develop in the Thymus
• The process is identical to Ig gene rearrangement, but different genes are used
B cells recognize intact protein antigens
Lysozyme
Heavy Chain
Light ChainAntigenic DeterminantEpitope
Dangers in Diversity• Mechanism is essentially a
random draw• By chance, some Ig and TCR
will react with self antigens• Autoreactive B and T cells
must be eliminated or silenced to prevent autoimmune diseases
• Chromosomal translocations arising during VDJ recombination or isotype switching may lead to lymphoid malignancies