Students: Hoang Kim Cuc
Vu Thi Luyen
Trinh Quynh Mai
Tran Duc Phuc
Nguyen Ngoc Quang

Class : K8-honor Biology


Contents

Development of immune cells
Receptors
Lympho cell selection
Diversity of antibodies

Development of immune cells
Lympho T receptor
2 types:
+ (95%)
+δγ
CD3 (γ, δ and ε)
Zeta chain
CD4 or CD8
Lympho B receptor
Fab region
Fc region
Heavy chain with one variable (VH) domain followed by a constant domain (CH1), a hinge region, and two more constant (CH2 and CH3) domains.
Light chain with one variable (VL) and one constant (CL) domain
Antigen binding site (paratope)
Hinge regions
Immunoglobin

Heavy chain: 5 types
# α - IgA 1, 2
# δ - IgD
# γ - IgG 1, 2, 3, 4
# ε - IgE
# μ - IgM
Light chain: 2 types
# κ
# λ
Diversity of antibodies due to constant region differences
The steps in immunoglobulin gene rearrangement at which cells can be lost
B cell development
A productively rearranged immunoglobulin gene is expressed immediately as a protein by the developing B cell
T cell selection
The rearrangement of T-cell receptor γ and δ genes in the mouse proceeds in waves of cells expressing different Vγ and Vδ gene segments.
The stages of gene rearrangement in α:β T cells
Diversity of antibodies
Isotype
The "immunoglobulin isotype" refers to the type of chain
All of induvidals in the same species
Only occurs after a given B cell has encountered antigen
Differ in C regon genes
9 isotypes
Heavy chain: 9 types
# α - IgA 1, 2
# δ - IgD
# γ - IgG 1, 2, 3, 4
# ε - IgE
# μ - IgM

Isotype
The organization of the immunoglobulin heavy-chain C-region genes in mice and humans
Human: Heavy chain 9 types Mice: Heavy chain 8 types
# α - IgA 1, 2 IgA
# δ - IgD IgD
# γ - IgG 1, 2, 3, 4 IgG 1,2a,2b,3
# ε - IgE IgE
# μ - IgM IgM


Allotype variation
immunoglobulin allotype is the allele of the antibody chains found in the individual
Differ in C region
The most important types are Gm (heavy chain) and Km (light chain).
Allotype
Human
Idiotype variation
In structure of variable region - antigen binding specificity

Specific for one antibody - produced cell clone
Contents of report
Discuss molecular mechanisms of generating diversity in immunoglobulins and TCRs:
Somatic recombination
Combinatorial diversity
Junctional diversity
Somatic hypermutation
Isotype switching.
The problem of Ig and TCR diversity:
Human immunoglobulin repertoire: ~ 1011
Number of genes in humans: ~ 2.5 x 104
How can there be so many Ig and TCR variants???
Germline theory
Somatic diversification theory
Healthy control
Nonlymphoid cells
Patient with chronic lymphocytic leukemia
Single B cell clone
Does the Ig gene locus differ in nonlymphoid
cells vs B cells?
The Ig gene is nonfunctional in germline DNA
The genes encoding each Ig chain are actually a family of gene segments located in one region of DNA (locus)
V gene segments
J gene segments
D gene segments (between V and J segments)

Must be physically rearranged to become a functional gene
Somatic recombination
Lymphocytes die during development if Ig/TCR recombination does not occur
Multiple gene segments increase Ig diversity
Pseudogenes
Gene duplications and diversification
Total length of heavy chain locus > 2 Mb
Multiple gene segments increase Ig diversity
Combinatorial diversity:

Heavy chains
40 x 25 x 6 = 6000

Light chains
40 x 5 = 200κ
30 x 4 = 120 λ

Total possible:
320 x 6000 = 1.9x106
Recombination occurs at specific sites
Recombination signal sequences (RSS) occur adjacent to coding sequences in V, D, and J segments
Heptamer-spacer-nonamer
12/23 rule
V-region gene segments are joined by recombination
Lymphocyte-specific and ubiquitous enzymes are required
Ubiquitous DNA repair enzymes are also required
DNA ligases
DNA-dependent protein kinase
Artemis
Many others
Junctional diversity



Diversity in the TCR gene locus
The TCR gene is most variable in the CDR3 region
CDR3
Summary: Ig vs TCR
Further Ig diversity arises through affinity maturation
Affinity maturation is due to somatic hypermutation
Silent
Neutral
Deleterious
Positive
Somatic hypermutation
Mechanism is not well defined

Requires enhancers and an active promoter

Requires activation-induced cytidine deaminase (AID)
Diversity of antibodies
Abs with the same idiotype (Ag specificity) can be of different isotypes
Isotype switching
Irreversible
All of induviduals in the same species
Only occurs after a given B cell has encountered antigen
Mechanism not fully understood
Requires AID
Requires DNA repair enzymes
Requires external signals (helper T cells)
Alternative RNA processing generates transmembrane or secreted Ig
IgM and IgD are coexpressed in mature naive B cells
Isotype switching occurs in activated B cells
The μ switch region (Sμ) consists of about 150 repeats of the sequence [(GAGCT)n (GGGGGT)]

Sγ, Sα, and Sɛ: repeats of the GAGCT and GGGGGT sequences
Diversity of antibodies: isotypes
Summary: mechanisms that generate diversity in lymphocyte receptors