1. Antibodies and T Cell Receptor Genetics 2011
Peter Burrows
4-6529
406 Shelby

2. Antibodies and T Cell Receptor Genetics Learning 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

3. 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

4. Generation of Diversity
Cellular Solutions
Diversity operates at the level of the lymphocyte
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)

5. Antigen Selects Lymphocytes
Isotype
Switch
IgG, IgA, IgE

6. Isotype
Switch
IgG, IgA, IgE

7. Antigen-Antibody Binding
Red 3 ball.

8. 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 antigen
Problem - 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

9. Anatomy of a typical gene
Inside the cell
Cell membrane
Outside the cell

10. Anatomy of Immunoglobulin Genes in B Lymphocytes
Intron
V Region
Exon
C Region
Exon
DNA
mRNA
Protein

11. Generation of Diversity
Genetic Solutions
Functional genes for antigen receptors do not exist until they are generated during the development of lymphocytes
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

12. Variable region genes are constructed from gene segments
Germline
DNA
Stem cell
B Cell
DNA
mRNA
Protein

13. Germline Ig Genes

14. V-region Gene Segments are Joined by Somatic Recombination

15. Germline
DNA
Stem cell
B Cell
DNA
mRNA
Protein

16. 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

17. Benefits of Antigen Receptor Gene Rearrangement
40 Vκ 30 Vλ 65 VH
x 5 Jκ x 3 Jλ DH
x 6 JH
200 κ V regions 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 total
Not bad from 176 gene segments!
One B cell or plasma cell only makes one antibody

18. Isotype Switching Also Occurs by Somatic RecombinationVL VH
IgM B Cell VH VL

19. Isotype Switching Also Occurs by Somatic Recombination

20. 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 isotype
one specificity

21. Primary and Secondary Antibody Responses
Higher titer
Higher affinity
© 1998 Gold Standard Multimedia Inc.

22. The T Cell Receptor for Antigen
TCR

23. Comparison of the TCR and the BCR (Immunoglobulin)
V regions
encoded by
rearranging
genes

24. Structure of the T-cell Receptor

25. 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

26. T-cell Receptor - and -chain V Regions are Generated by Gene Segment Rearrangement

27. B cells recognize intact protein antigens
Heavy Chain
Lysozyme
Antigenic Determinant
Epitope
Light Chain

28. T cells recognize processed (degraded) protein antigens

29. 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

30. NJEM