Antibodies and T Cell Receptor Genetics

2011

Peter Burrows4-6529

406 Shelbypeterb@uab.edu

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)

Antigen Selects Lymphocytes

IsotypeSwitch

IgG, IgA, IgE

IsotypeSwitch

IgG, IgA, IgE

Antigen-Antibody Binding

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

Anatomy of a typical gene

Inside the cell

Cell membrane

Outside the cell

Anatomy of Immunoglobulin Genes in

B Lymphocytes

DNA

mRNA

Protein

V RegionExon

C RegionExon

Intron

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

Germline Ig Genes

V-region Gene Segments are Joined by Somatic

Recombination

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

VL

VL

VH

VH

IgM B Cell

Isotype Switching Also Occurs by Somatic Recombination

Isotype Switching Also Occurs by Somatic Recombination

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

© 1998 Gold Standard Multimedia Inc.

The T Cell Receptor for Antigen

TCR

Comparison of the TCR and the BCR (Immunoglobulin)

V regionsencoded byrearranging genes

Structure of the T-cell Receptor

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

T-cell Receptor - and -chain V Regions are Generated by Gene Segment

Rearrangement

B cells recognize intact protein antigens

Lysozyme

Heavy Chain

Light ChainAntigenic DeterminantEpitope

T cells recognize processed (degraded) protein antigens

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

NJEM