Lymphocyte Development & Generation of Lymphocyte

Antigen Receptors

• References: 1. Abbas, A, K. et.al, Cellular and Molecular Immunology (6th ed., 2007), Chapter 8

QuestionsQuestions

What is the Bare Lymphocyte Syndrome?

What is the advantage of MHC Polymorphism? Is that good if MHC is as diverse as Ig or TCR?

An immunodeficiency disease Lacking Class-II MHC expression Mutations in transcriptional factors regulating Class II expression, ex. CIITA

MHC polymorphism may provide a broader protective immunity in a population during pathogen infection .

OutlineOutline

• Overview of Lymphocyte Overview of Lymphocyte

DevelopmentDevelopment

• Generation of Diversity in Lymphocyte Ag receptors

• B & T Lymphocyte Development

• Summary & Question

Key Concepts in lymphocyte development

1. Lymphocyte development-A process of differentiation

of lymphoid progenitor cells into mature lymphocytes (T

& B).

2. Rearrangement and expression of Ag receptor genes

are associated with lymphocyte development.

3. Selection events are involved in preserving cells w/ correct Ag receptors and eliminating dangerous

cells w/ self-recognition Ag receptor

4. Proliferation in the early lymphocyte development is

stimulated by IL-7.

Stages of lymphocyte Stages of lymphocyte development-Idevelopment-I

Checkpoints in lymphocyte Checkpoints in lymphocyte developmentdevelopment

Selections in Selections in lymphocyte lymphocyte developmentdevelopment

Pluripotent Pluripotent HSCs HSCs => lymphocyte => lymphocyte lineageslineages

OutlineOutline

• Overview of Lymphocyte

Development

•Generation of Diversity in Lymphocyte Ag receptors

• B & T Lymphocyte Development

• Summary & Question

Key Concepts in Diversity of lymphocyte Ag

receptors1. The germline organizations of Ig & TCR are similar. a. Multiple (V)ariable, (D)iversity, & (J)oining gene segments. b. These gene segments are spatially separated on the chromosomes

2. During lymphocyte development, gene rearrangement of Ag receptor genes occurs: a. Combinatorial diversity - Non-homologous DNA recombination (VDJ recombination) - Lymphoid-specific enzymes- RAG1 & RAG2 Other factors-DNA-PK, TdT, Ku, DNA ligase IV… b. Junctional diversity - Addition or removal of nucleotides among V (D)J joints -The largest contribution to diversity of Ag receptors

In 1970s, Scientists thought

105 genes in human body => Antibodies over 1010 ? => Limited gene segments Hybrid-DNA nature (Recombination) Diverse Ab products

Germline organization of Human Ig Germline organization of Human Ig lociloci

D segment in H chain

Germline organization of Human Germline organization of Human TCR lociTCR loci

D segment in chain

Domains of Ig & TCR Domains of Ig & TCR proteinsproteins

HV3 or CDR3

Generation of Diversity in Generation of Diversity in Ag receptor genesAg receptor genes

1. Recombination Signal

Sequence (RSS): Heptamer &

Nonamer => separated by 12- or 23- spacers => Recognized by

Recombinase

2. Deletion-VJ exons have the same orientation

3. Inversion – VJ have the different orientation

V(D)J V(D)J RecombinatioRecombinatio

n-In-I

V(D)J Recombination-V(D)J Recombination-IIII

V(D)J Recombination III V(D)J Recombination III ––Junctional DiversityJunctional Diversity

Junctional diversity=> the greatest variability at HV3 (CDR3)

Generation of Diversity in Ig or Generation of Diversity in Ig or TCRTCR

Generation of Diversity in Ig or Generation of Diversity in Ig or TCRTCR

Overview of Ig gene Overview of Ig gene rearrangementrearrangement

OutlineOutline

• Overview of Lymphocyte

Development

• Generation of Diversity in Lymphocyte Ag receptors

• B & T Lymphocyte DevelopmentB & T Lymphocyte Development

• Summary & Question

Pluripotent Pluripotent HSCs HSCs => lymphocyte => lymphocyte lineages IIlineages II

1. Regulated by transcriptional

factors

2. Notch-1 & GATA3 => T

EBF, E2A & Pax-5 => B

Features of B lymphocyte development

1. Maturation of B Lymphocytes development - Rearrangement & expression of Ig gene in a precise order - Selection & proliferation of pre-B cells via pre-Ag receptor

2. Selection of the mature B cell repertoire - Self Ag => Affect the strength of the BCR signal - Immature B cells => self Ag/high avidity => Receptor editing

=> Additional L chain recombination => Not Self-reactive => Fail to receptor editing => Apoptosis

3. During this maturation, B cell lineage cells go through distinct stages => A specific Ig gene expression => Distinct surface markers

4. At Pre-B cell stage, H chain recombination occurs first and associates w/ Surrogate light chains ( & VpreB). - 5 & VpreB are similar to & light chains but invariant - form pre-B cell receptor => Development

Stages of B lymphocyte development-I

Stages of B lymphocyte development-II

Pre-B cell & Pre-T cell receptors

Stages of B lymphocyte development-III

Receptor Editing–2nd Chance

Features of T lymphocyte development

1. Maturation of T Lymphocytes development - Sequential Rearrangement & expression of TCR genes - Selection & proliferation of T cell repertoire

2. Selection of the mature T cell repertoire occurs in Thymus

- Positive selection => Self MHC-restricted - Negative Selection => Self Ag-MHC/high avidity => Apoptosis => Central Tolerance

3. CD4 & CD8 are surface markers for differentiation of Thymocytes (immature T cells).

Stages of T lymphocyte development-I

Maturation of T lymphocytes in Thymus

CD4 & CD8 expression on Thymocytes

Selection of T lymphocytes in Thymus

TCR TCR transgenic transgenic mouse mouse model model => T cell => T cell selectionselection

OutlineOutline

• Overview of Lymphocyte

Development

• Generation of Diversity in Lymphocyte Ag receptors

• B & T Lymphocyte Development

• Summary & QuestionSummary & Question

SUMMARY1. Rearrangement and expression of Ag receptor

genes regulate B & T lymphocyte development.

2. Selection events are involved in preserving cells w/ correct Ag receptors and eliminating dangerous

cells w/ self-recognition Ag receptor

3. Gene rearrangement of Ag receptor genes occurs during lymphocyte development. - The basic mechanism is common to both B & T cells - Generate the Combinatorial diversity by randomly

combining different V(D)J gene segments - Generate the Junctional diversity by filling the gap

betweenjointed gene segments

QuestionQuestion

What is happened to the immunesystem if RAG1 & RAG2 are

mutated?