B cells I. Differentiation of B cells in Bone marrow II. BCR and B cell accessory molecules III. The subsets of B cells

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B cellsI. Differentiation of B cells in Bone
marrowII. BCR and B cell accessory
moleculesIII. The subsets of B cells
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I. Differentiation of B cells in Bone marrow
1. process of B cell maturation2. events in the
differentiation of B cells3. mechanisms of Ig
diversity
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I. Differentiation of B cells in Bone marrow
1.Process pro-B cell ? chain pre-B cell
surrogate light chain ? chain immature B
cell ? chain or ? chain ? chain (membrane
IgM,mIgM) mature B cell mIgM, mIgD
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2. Events in the differentiation of B cells1)
Negative selectionimmature B cells
mIgM--self antigen mIgM self antigen
apoptosis or anergy surviving to develop

mature B cells2) gene rearrangement
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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.
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2) gene rearrangement (1) germline gene
structure of Ig(2) rearrangement of Ig genes
(3) characteristics of Ig gene rearrangement
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(1)   Germ-line gene structure of BCRH 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 , Cd, C? et al. (9)
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L chain(?-- chromosome 2, ?-- chromosome 22)
V region encoding genes ? --V?, J?
?-- V?, J? C region encoding genes C?
(1) C?(4)
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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.
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(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 RNA mRNA
splicing V-J rearrangement of L chain pre-B
cells V-J VJ DNA
immature B cells VJC?
VJ-C? RNA mRNA
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BCR(membrane type) and secretory type Ig
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(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.
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Allelic exclusion
Kuby Figure 5-10
Read Kuby pages 115-117 Allelic Exclusion
Ensures a Single Antigenic Specificity
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3. mechanisms of diversity of Ig (BCRor Ab)1).
Mutiple germline gene segments2). Combinatorial
V(D)J joining3). Junctional flexibility 4).
combinatorial assocination of heavy and
light chain 5). somatic
hypermutation
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1). Mutiple germline gene segmentsThere Are
Numerous Germ-Line V, D, and J Gene Segments.
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2). combinatorial V(D)J joining The
multiple germ-line gene segments are combined
randomly during the rearrangement of BCR genes.
human Ig 51VH27DH 6JH 8262 possible
combinations
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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.
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4). Combinatorial association of heavy and light
chains 5).Somatic hypermutationSomatic
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
diversityin already-rearranged gene segments
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Somatic hypermutation Ag
activated B cell proliferate gene mutation
in V region encoding genes affinity
maturation
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II. BCR and accessory molecules of B
lymphocytes B cell receptor complex B cell
accessory molecules
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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 ?
heterodimerBCRmembrane immunoglobulin on B
cell, mIg IgM, IgDIg?-Ig ?, (CD79)ITAMtransdu
ce an activation signal
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ITAMAn immunoreceptor tyrosine-based activation
motif (ITAM) is a conserved sequence of amino
acids (YXX(L/V)X7-11YXX(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.ITIMAn 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.
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2. Co-receptors complex

• 1) CD19, CD21, CD81,
• CD19
• CD21(CR2) receptor of iC3b and C3d

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• 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

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• B7(CD80,CD86)
• ------co-stimulator to T cells
• ligand of CD28

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• 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

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6. MHC moleculesMHC-I and MHC-? 7. Mitogen
receptors Receptors of mitogen SPA,PWM
(pokeweed), LPS 8. Cytokine receptors
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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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