Download Part I T lymphocyte - Shandong University

B cells
I. Differentiation of B cells in Bone
marrow
II. BCR and B cell accessory molecules
III. The subsets of B cells
I. Differentiation of B cells in Bone marrow
1. process of B cell maturation
2. events in the differentiation of B cells
3. mechanisms of Ig diversity
I. Differentiation of B cells in Bone marrow
1.Process
pro-B cell
 chain
pre-B cell
surrogate light chain +  chain
immature B cell
κ chain or λ chain +  chain (membrane IgM,mIgM)
mature B cell
mIgM, mIgD
2. Events in the differentiation of B cells
1) Negative selection
immature B cells :
mIgM--self antigen
apoptosis or anergy
mIgM
self antigen
surviving to develop
mature B cells
2) gene rearrangement
It is estimated that in the mouse the bone marrow
produces about 5x107 B cells/day but only 5x106
(or about 10%) are actually recruited into the
recirculating B-cell pool. This means that 90% of
the B cells produced each day die without ever
leaving the bone marrow.
negative selection
Immature B cells that express auto-antibodies
against self-antigens are eliminated in the bone
marrow.
2) gene rearrangement
(1) germline gene structure of Ig
(2) rearrangement of Ig genes
(3) characteristics of Ig gene rearrangement
(1) Germ-line gene structure of BCR
H chain: chromosome 14
V region encoding genes:
VH (variable gene segments),
DH (diversity gene segments),
JH (joining gene segments)
C region encoding genes:
CH (constant gene segments): Cμ , Cδ, Cγ et
al. (9)
L chain(-- chromosome 2, -- chromosome
22)
V region encoding genes:
 --V, J
-- V, J
C region encoding genes:
C (1); C(4)
In heavy chains, the V, D and J
segments encode the variable
domain while the C segment encodes
the constant domain.
In light chains, the V and J segments
encode the variable domain while
the C segment encodes the constant
domain.
(2) Gene rearrangement of BCR
VDJ rearrangement of H chain
pro-B cells: D-J
V-DJ
VDJ
DNA
transcription
pre-B cells: VDJCμ
VDJ- Cμ
mRNA splicing
V-J rearrangement of L chain
pre-B cells:
V-J
VJ
immature B cells: VJC
mRNA
RNA
DNA
VJ-C
RNA
BCR(membrane type) and secretory type Ig
(3) Characteristics of BCR gene
rearrangement
Allelic exclusion:
only one of the two alleles in homologous
chromosomes can be expressed.
isotypic exclusion:
only one of the two types of light chain
genes can be expressed.
Allelic exclusion
Kuby Figure 5-10
Read Kuby pages 115-117: Allelic Exclusion Ensures a Single Antigenic Specificity
3. mechanisms of diversity of Ig (BCR
or Ab)
1). Mutiple germline gene segments
2). Combinatorial V(D)J joining
3). Junctional flexibility
4). combinatorial assocination of heavy and
light chain
5). somatic hypermutation
1). Mutiple germline gene segments
There Are Numerous Germ-Line V, D, and J
Gene Segments.
2). combinatorial V(D)J joining
The multiple germ-line gene segments are
combined randomly during the rearrangement
of BCR genes.
human Ig:
51VH×27DH ×6JH= 8262 possible combinations
3). Junctional flexibility
imprecise joining
In the junction of V-J, V-DJ or D-J, several
nucleotides are lost to increase the diversity of
the V region encoding gene of L chain or H chain.
N-nucleotides addition
During the D-J and V to D-J joining process,
several nucleotides are inserted to increase the
diversity of V region encoding gene of H chain.
N-nucleotides insert by TdT(terminal
deoxynucleotidyl transferase) without template
There is no N-nucleotides insert in L chain.
4). Combinatorial association of heavy and
light chains
5).Somatic hypermutation
Somatic hypermutation occurs at a frequency
approaching 10-3 per base pair per generation.
This rate is at least a hundred thousand-fold
higher (hence the name hypermutation) than
the spontaneous mutation rate, about
10-8 /bp/generation, in other genes.
Somatic hypermutation adds diversity
in already-rearranged gene segments
Somatic hypermutation
Ag
activated B cell proliferate
gene mutation in V region encoding genes
affinity maturation
II. BCR and accessory
molecules of B lymphocytes
B cell receptor complex
B cell accessory molecules
1.BCR complex
a group of membrane molecules on B cells
that can specifically bind to the antigen and
pass an activation signal into B cells,
consisting of BCR and Ig-Ig  heterodimer
BCR
membrane immunoglobulin on B cell, mIg: IgM,
IgD
Ig-Ig , (CD79)
ITAM
transduce an activation signal
ITAM
An immunoreceptor tyrosine-based activation motif (ITAM)
is a conserved sequence of amino acids (YXX(L/V)X711YXX(L/V)) in the cytoplasmic tails of certain cell surface
proteins of the immune system. The tyrosine residues
within these motifs become phosphorylated following
interaction of the receptor molecules with their ligands
and transduce an activation signal.
ITIM
An immunoreceptor tyrosine-based inhibition motif (ITIM),
is a conserved sequence of amino acids (S/I/V/LxYxxI/V/L)
in the cytoplasmic tails of many inhibitory receptors of the
immune system. After ITIM-possessing inhibitory
receptors interact with their ligand, their ITIM motif
becomes phosphorylated and tranduce an inhibitory
signal.
2. Co-receptors complex
1) CD19, CD21,
CD81,
 CD19:
 CD21(CR2):
receptor of iC3b
and C3d
3.CD40
---co-stimulatory receptor
CD40 on B cell binds to CD40L on activated T
cell---pass a costimulatory signal into B cells
4. B7
5. CD45
6. MHC molecules
7. Mitogen receptors
8. Cytokine receptors
B7(CD80,CD86)
------co-stimulator to T cells
ligand of CD28
3.CD40
---co-stimulatory receptor
CD40 on B cell binds to CD40L on activated T cell---pass a costimulatory signal into B cells
4. B7
5. CD45
The cytoplasmic domain of CD45 has an
intrinsic phosphatase activity that removes
an inhibitory phosphate group on a tyrosine
kinase called Lck (in T cells) or Lyn/Fyn/Lck
(in B cells) and activates it.
6. MHC molecules
7. Mitogen receptors
8. Cytokine receptors
6. MHC molecules
MHC-I and MHC-Ⅱ
7. Mitogen receptors
Receptors of mitogen:
SPA,PWM (pokeweed), LPS
8. Cytokine receptors
III. Subset of B cell
--------------------------------------------------------------------Comparison of B1 and B2 cells
-----------------------------------------------------------------------------------B1
B2
CD5
location
+
thorax, abdominal cavity
lymph organs
lamina propria of intestine
Recognized Ag
TI Ag and auto-Ag
TD Ag
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