C410M B Cell Activation and Function II The Development of B Lymphocytes

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C4-10MB Cell Activation and Function IIThe
Development of B Lymphocytes

• Parham
• Chapt. 4 pgs. 85-105
• Michael Wolcott
• November 19, 2002

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B Cells

• The primary function of B cells is to produce
  antibodies.
• B-cells are derived from bone marrow precursor
  cells that arise continuously throughout life.
• B-cell development can be separated into two
  phases
• antigen-independent phase which occurs in
  primary lymphoid organs (fetal liver and adult
  bone marrow)
• antigen-dependent phase which occurs in secondary
  lymphoid organs (spleen, lymph nodes, gut Peyers
  patches.

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CORE MATERIAL Four Phases of B Cell Development

• Generation of B cell receptors in the bone
  marrow.
• Tolerance induction by self antigens in BM and
  periphery.
• B cell activation by foreign antigens (secondary
  lymphoid organs).
• Differentiation to antibody secreting (plasma
  cells) and memory cells in secondary lymphoid
  organs.

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CONCEPT B Cell Differentiation Proceeds Through
Several Definable Developmental Stages.
Bone Marrow
Periphery
Periphery
Y
Bone Marrow
Y
Y
Y
Y
Antigen
IgM
IgD
Y
Y
IgM
IgM
Y
Y
Y
Plasma Cell
Lymphoid
Immature
Mature
Pro-
Pre-
Activated
Stem Cell
B Cell
B Cell
B Cell
B Cell
B Cell
Memory
B Cell
Antigen Dependent
Antigen Independent
Primary Lymphoid Organ
Secondary Lymphoid Organs
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CORE MATERIAL The antigen specificity of each
individual lymphocyte is determined through the
assembly of V, (D), and J gene segments to
generate rearranged V genes encoding the
antigen-receptor varialbe (V) region. CORE
MATERIAL The rearrangement of antigen-receptor
gene segments controls lymphocyte development.
CORE MATERIAL Errors in immunoglobulin-gene
rearrangement can give rise to chromosomal
translocations that predispose B cells carrying
them to malignant transformation.
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CONCEPT B cells develop in the bone marrow and
then migrate to the secondary lymphoid tissues in
the periphery.
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As B cells develop they migrate through the bone
marrow and eventually egress to the venous blood
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CONCEPT The development of B cells proceeds
through stages defined by the rearrangement and
expression of the immunoglobulin genes.
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Pro-B-Cells
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Progenitor B-Cells

• Pro-Bells are the stage where D to J
  rearrangements are carried out. Generally the
  cell will rearrange the D and J segments on both
  alleles.
• Once DJ rearrangements are complete the cells
  progress through several intermediate stages in
  which an ordered rearrangement of the heavy chain
  V region is carried out.
• Pro B cells express lymphoid specific enzymes,
  RAG1/RAG2, and TdT, as well as characteristic
  surface proteins.

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Pre-B-Cells
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Pre-B Cells

• Pro-B to Pre-B-Cell transition entails V to DJ
  joining of the H chain on one allele.
• The productively rearranged immunoglobulin gene
  is expressed immediately as a protein by the
  developing B cell both in the cytoplasm and as a
  component of the pre-B cell receptor.
• Successful rearrangement of the heavy-chain
  immunoglobulin gene segments leads to the
  formation of a pre-B cell receptor that halts
  further VH to DJH rearrangement and triggers the
  cells to begin to divide.
• Once the developing B cells complete a productive
  rearrangement of the H chain locus, they begin an
  ordered rearrangement of the light chain loci.
• TdT is no longer expressed by cells at the pre-B
  cell stage.
• Pre-B cells do express RAG1/RAG2 in the nucleus.

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CONCEPT Success of the Developmental Journey, at
all Steps, Depends on Signaling Through Receptors
on the Surface of the Developing B Cells
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Sequential Expression of Membrane Ig and
Surrogate Light Chain at Different Stages of
B-Cell Development
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Pre-B-Cell Receptor

• Pre-B-cell receptor (pre-BCR) is composed of a
  µ-chain and a surrogate light chain and the
  signaling molecules Iga and Igß.
• Surrogate L-chain complex of two proteins a V
  like sequence called Vpre-B and a C-like sequence
  called ?5, which associate non-covalently to form
  a light-chain like structure.
• Gene-targeted ?5 knockout mice are blocked at the
  pre-B cell stage, suggesting that a signal
  generated through the receptor is necessary for
  pre-B cells to proceed to the immature B-cell
  stage.

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Immature B-Cells
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Immature B Cells

• Once a light-chain gene has been rearranged
  successfully, light chains are synthesized and
  combine with the heavy chain to form IgM (of a
  single specificity). IgM appears of the cell
  surface along with Iga and Igß to form the
  functional B cell receptor (BCR).
• Expression of the BCR signals the cells to down
  regulate RAG1/RAG/2 and to cease further
  rearrangements.
• This is the stage of development where negative
  selection of cells that recognize self antigens
  occurs.
• The immature B cells egress from the bone marrow
  and enter the circulation and ultimately arrive
  in the spleen. Any encounter with antigen on
  this journey results in initiation of the death
  program in the cells.

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Membrane-bound immunoglobulins associate with two
other proteins, Iga and Igß, to form the B-cell
Receptor (BCR)
Antigen Recognition
Signal Transduction
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CONCEPT Membrane bound and secreted forms of
Igs are created by alternative RNA splicing
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CONCEPT B cells undergo a strictly programmed
series of gene rearrangements in the bone marrow.

• The recombination process is imprecise due to the
  random addition and deletion of nucleotides at
  the joins between gene segments. Thus, by a
  matter of chance stop codons can be introduced
  which prevent synthesis of a complete chain
  (termed non-productive rearrangements).
• B-development has evolved to preserve and
  multiply those B cells that have made productive
  joins and eliminate that have not.
• Since there are two alleles for each
  immunoglobulin locus in the diploid genome, each
  of which can rearrange, the cell must prevent
  both alleles from making productive joins, lest
  the cell express two or more receptors of
  different specificities. This is accomplished by
  checking for a productive join as soon as the
  allele has rearranged.
• When a productive join is made it signals the
  cell to cease the current phase of rearrangements
  and progress to the next stage.
• Assembly of genes for a complete BCR requires
  three separate recombination events, which occur
  at different stages of development. These are,
  in order that they occur DH to JH, VH to DJH to
  produce the functional heavy chain gene, and the
  joining of VL to JL to produce the functional
  light chain gene. The kappa chain locus is
  generally rearranged before the lambda chain
  locus.

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CONCEPT Even though a B cell is diploid, it
expresses the rearranged heavy-chain genes from
only one chromosome and the rearranged
light-chain genes from only one chromosome.
Allelic exclusion is the term that signifies the
expression of only one of two alleles of a given
gene in a diploid cell.
Each B Cell Produces Immunoglobulin of a Single
Antigen Specificity
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Model of Allelic Exclusion
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Summary of B-cell Development
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Saga of the Immature B Cell(life on the streets)

• B cell development differs significantly from T
  cell development in that negative selection of
  autoreactive B cells can occur in the same
  microenvironment in which productive immune
  responses begin, i.e. B cells grow up on the
  streets.

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Mechanisms of Self-Tolerance

• Unresponsiveness to self antigens, or
  self-tolerance, is maintained by mechanisms that
  actively prevent the maturation or stimulation of
  potentially self-reactive lymphocytes.
• Several fundamental concepts are relevant to our
  understanding of self-tolerance.

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Fundamental Concepts

• Tolerance to self antigens is an actively
  acquired process (rather than an inherited
  property), in which self-reactive lymphocytes
  either are prevented from becoming responsive to
  self antigens or are inactivated after
  encountering these antigens.
• Self-tolerance may be induced at various stages
  of lymphocyte development and activation.

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Fundamental Concepts

• Central Tolerance is the induction of tolerance
  in generative (primary) lymphoid organs as a
  consequence of immature self-reactive lymphocytes
  recognizing self antigens.
• The bone marrow compartment has two mechanisms
  for dealing with autoreactive B cells.
• Clonal Deletion by apoptosis
• Receptor editing

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Fundamental Concepts

• Peripheral Tolerance is the induction of
  unresponsiveness in peripheral sites as a result
  of self-reactive lymphocytes encountering
  self-antigens under particular conditions. In B
  cells this results in apoptosis, or anergy which
  leads to eventual elimination from the B cell
  pool.

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Mature B Cells
Bone Marrow
Periphery
Periphery
Y
Y
Y
Bone Marrow
Y
Y
Antigen
IgM
IgD
Y
Y
Y
Y
IgM
Y
Plasma Cell
Immature
Mature
Pro-
Lymphoid
Pre-
Activated
B Cell
Stem Cell
B Cell
B Cell
B Cell
B Cell
Memory
B Cell
Antigen Dependent
Antigen Independent
Primary Lymphoid Organ
Secondary Lymphoid Organs
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CONCEPT Co-expression of IgM and IgD results
from alternative RNA splicing.
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CONCEPT The B-1 subset of B cells has a
distinct developmental history and expresses a
distinctive repertoire or receptors.

• A minority subset of B cells arises during fetal
  development, is self renewing, and has a
  restricted reperetoire.
• B-1 B cells are the origin of the common B cell
  tumor chronic lymphocytic leukemia.
• The B-1 population responds poorly to protein
  antigens but much better to bacterial
  polysaccharides (thymus independent antigens).
• The B-1 population secretes mostly IgM antibodies
  of low affinity, does not undergo affinity
  maturation or class switching.
• The B-1 population does not form memory cells.

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Comparison of the properties of B-1 and B-2 cells.
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CONCEPT Errors in immunoglobulin-gene
rearrangement give rise to chromosomal
translocation carrying them to malignant
transformation.
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Chromosomal Rearrangements in Burkitts Lymphoma
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CONCEPT The different B-cell tumors reflect the
heterogeneity of developmental and
differentiation states of the normal B-cell
population.