B-Cell Generation, Activation, and Differentiation

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B-Cell Generation, Activation, and Differentiation

• B-Cell Maturation
• B-Cell Activation and Proliferation
• The Humoral Response
• In Vivo Sites for Induction of Humoral Responses
• Germinal Centers and Antigen-Induced B-Cell
  Differentiation
• Regulation of the Immune Effector Response

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Naïve B cells are short-lived
Class switch Affinity maturation
Ab
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Chap. 11
Chap. 11
Chap. 5
Chap. 11
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Committed Lymphocyte Progenitor
Adhesion molecules
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Progenitor B cells
Proliferative signal
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Precursor B cells
Proliferation and development
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Rearrangement of Immunoglobulin Genes during B
cell Development
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The pre-B cell receptor complex Membrane heavy
chain ? associates with surrogate light chain
(Vpre-B and a C-like sequence, ?5). This
associates with the Ig-?/Ig-? heterodimer.
Signaling complex
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Fig. 5-17
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B-1 B cells

• Innate-like subset of B cells.
• Appear during fetal life and express IgM but
  little IgD and display CD5. Are also found in
  peritoneum.
• Originates from stem cell in bone marrow, but
  also from proliferation of B-1 cells outside the
  BM.
• Responds poorly to protein antigen, but strongly
  to carbohydrate antigens.
• Antibodies produced are of low affinity.

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• Self-reactive B cells are eliminated in bone
  marrow (BM).
• BM produces 5 x 107 B cells/day, but only 5 x 106
  B cells/day or 10 actually enter the
  circulation.
• Some of this loss is due to negative selection
  and elimination or clonal deletion of immature B
  cells expressing autoantibodies to self-antigens.

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Experiment by D.A. Nemazee and K. Burki (1989)
demonstrated negative selection of B cells
DNA encoding IgM specific for H-2Kk, a class I
MHC molecule
Mice were H-2d or H-2d/k
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• Endogenous H-2k and H-2d class I MHC molecules
  are present on bone marrow stromal cells of
  transgenic mice.
• Transgenic B cells will express anti-MHC Kk IgM
  antibodies.

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anti-Kk IgM
Total IgM

61.6 of IgM antibody
Membrane IgM
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There is negative selection against any immature
B cell expressing auto-antibodies on their
membranes because these antibodies react with
self-antigens present on stromal cells, leading
to crosslinking and death of the B immature B
cells.
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• Tiegs and Nemazee (1993) demonstrated that
  self-reactive B cells can be rescued by
  expressing an edited light-chain gene.
• Found that a few mature B cells in the H-2d/k
  transgenic mouse expressed the transgene encoded
  ? chain but a different light chain.

B cell maturation is arrested
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B cell activation

• Depending on antigen, B-cell activation proceeds
  by two different routes.
• Thymus-dependent (TD) antigens require direct
  contact with TH cells to activate B cells.
• Thymus-independent (TI) antigens can activate
  B-cells in the absence of TH cells.

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• Thymus independent (TI) antigens are divided into
  two types, 1 and 2, and activate B cells by
  different mechanisms
• Most TI-1 antigens are polyclonal B-cell
  activators, or mitogens, for example, LPS.
• TI-2 antigens work by crosslinking the mIg
  receptor.
• TI-2 are different from TI-1 in three ways.
• Are not B-cell mitogens.
• Activate only mature B cells and inactivate
  immature B cells.
• Cytokines from TH cells are required for
  proliferation and class switching.

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Progression
G0
Competence Signals
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Transition of lymphocytes from G0 into S (cell
cycle), requires two steps - competence and
progression. Competence requires signal 1 and
signal 2. Progression requires interaction with
cytokines and other ligands with receptors on the
B cell membrane
Competence Signals
Signal 1
Signal 2
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Signal 1 gt Antigen binding the B cell receptor
(BCR) associated with Ig?/Ig? heterodimer. Signal
2 gt Interaction between CD40 on the B cell and
CD40L on the activated helper T cell.
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Lyn
BCR
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Lyn
BCR
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Similar to T cell receptor signaling
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2
1
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• Compartmentalization.
• Activation by receptor-mediated PTKs.
• Assembly of signaling complex.
• Recruitment of other signaling pathways.
• Changes in gene expression.

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CD22 (resting B cells) delivers a negative signal.
Amplification
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Effects of co-receptors on BCR signals.

• Without coreceptors, more molecules of IgM need
  to be engaged for B-cell activation.
• Mice immunized with a C3d-antigen fusion protein
  developed a stronger immune response.
• CD19 knockout mice had greatly diminished
  responses to most antigens.