Biology of the B Lymphocyte

1
Biology of the B Lymphocyte

• Review
• B cells can develop a vast repertoire of
  antigenic specificities
• Diversity the ability to respond to many
  different antigenic determinants (epitopes) even
  if they have not been previously encountered
• Development of lymphocytes and how they are
  responsible for
• Specificity
• Memory
• Discrimination b/w self and nonself

2
Sites of Early B-Cell Differentiation

• Synthesis of Ab was shown to require the presence
  of an organ called the bursa of Fabricius
  (chickens)
• Cells that developed into mature Ab forming cells
  were called bursa-derived or B cells
• B cell differentiation (humans)
• Liver in early fetus
• During fetal development and throughout the rest
  of life switches to bone marrow
• Bone marrow is the primary lymphoid organ for
  B-cell differentiation

3
Ontogeny of the B Lymphocyte

• Differentiation pathway of B lymphocytes (Figure
  7.1 Handout)
• Pro-B Cell
• Earliest distinguishable cell in the B cell
  lineage
• DH-DH rearrangement
• No Ig product
• Pre-B Cell
• VHDHJH rearrangement
• Synthesizes m chain
• Surrogate light chains from two non-rearranging
  genes l5 and VpreB

4
B Cell Receptor (BCR)

• Pre BCR
• Iga (CD79a) and Igb (CD 79b)
• Associated with Ig molecules on all cells of the
  B cell lineage
• Do not bind Ag
• Signal transduction transmit signal into cell
  after binding of Ag to the V regions of Ig H and
  L chains
• Surrogate light chains m chain
• B-Cell Receptor
• H chain of the BCR may be m, d, g, a, or e

5
B Cell Ontogeny

• Cells that do not express pre-BCR die by
  apoptosis
• Cells expressing pre-BCR undergo positive
  selection
• Signals via the pre-BCR induce cells to
  proliferate
• Surrogate light chain synthesis is shut down
• Light chain rearrangement starts
• Further H chain rearrangement is stopped
• Immature B Cells
• Light chains pair with m chains (membrane-bound
  monomeric form)
• Immature B cells can recognize and respond to
  foreign Ag, but this interaction results in
  long-lasting inactivation rather than expansion
  and differentiation

6
Immature B Cells

• Interaction of self molecules and immature B
  cells is important in development of
  self-tolerance in the bone marrow
• B cells with potential reactivity to self are
  prevented from responding ? negative selection
• Deletion (apoptosis)
• Anergy (inactivation)
• Self reactive B cells may also undergo receptor
  editing to generate a new (foreign) specificity
• rescued from inactivation

7
Mature B Cells

• Development of IgMIgD mature B cells
• Predominantly in bone marrow
• Can also occur in secondary lymphoid organs
• Activation
• Response to foreign Ag
• Occurs primarily in secondary lymphoid organs
  (lymph node and spleen) in the germinal centers
• Enlarge to become B cell blasts
• Proliferate and differentiate
• Plasma cells ? class switching
• Memory B cells ? class switch but
  non-proliferating, long-lived

8
Memory B Cells

• Generation is associated with class switch and
  somatic hypermutation in the germinal centers of
  spleen and lymph node
• Germinal centers provide an environment where B
  cells with mutations for high affinity for Ag are
  clonally selected and expanded
• Serve as memory cells for subsequent responses
• Affinity maturation increases the production of
  high affinity Ab in the secondary response

9
B-1 or CD5 B Cells

• Most B cells are B-2 type
• B-1 cells
• Minor population in spleen and lymph nodes
• Predominate in the peritoneal and pleural
  cavities
• Express CD5
• Synthesize predominantly low affinity IgM in
  response to bacterial polysaccharide Ags

10
B Cell Membrane Proteins

• Ab production is a multi-step process that
  generally requires the mutual interaction b/w B
  cells and T cells
• Important molecules on the B cell can be
  categorized as
• Ag-binding molecules membrane Ig
• Distinguished B cells from other lymphocytes and
  mononuclear cells
• Signal transduction molecules associated with mIg
  transduce signals into the B cell following Ag
  binding to Ig
• Iga (CD79a) and Igb (CD79b)
• Immunoreceptor tyrosine-based activation motif
• other molecules increase the activatory
  signal
• CD19, CD21, CD81

11
B Cell Membrane Proteins

• Molecules involved in Ag presentation
• To activate T cells Ag must be presented by APC
• B cells (like other APC) act as APC for T cells
• B cells share important characteristics with
  other APC
• B cells express class II MHC molecules
  constitutively (always expressed)
• Increase MHC class II expression by IL-4
• Present Ag to CD4 T cells (helper T cells)
• MHC class II is expressed on all cells in the B
  cell lineage apart from the pro-B cell

12
B Cell Membrane Proteins

• Costimulatory molecules ? Interact with T cell
  membrane molecules to enhance activation
• B7
• Resting mature B cells
• Low levels B7
• Poor APC
• Activated B cells
• High levels of B7
• Very efficient APC
• CD40
• Critical role in isotype switching
• Interacts with CD154 (CD40L or CD40 Ligand) on T
  cells
• Human X-linked hyper-IgM syndrome
• Boys with a mutation in CD40 ligand gene (either
  not expressed or nonfunctional) make only IgM Ab
  cannot switch to any other isotype

13
B Cell Membrane Proteins

• Fc receptor FcgRII (CD32)
• Virtually all B cell express a low affinity
  receptor for the Fc portion of IgG
• Involved in Ab feedback to inactivate B cells
  to inhibit Ab production
• FcgRI (CD64) restricted distribution

14
The Major Histocompatibility Complex in the
Immune Response

• T cells evolved to deal with Ags inside the cell
• Viruses, bacteria and parasites that invade cells
• T cells use an Ag recognition system (TCR) that
  interacts with a fragment of an Ag presented on
  the surface of a cell bound to MHC gene product
• Major histocompatibility complex (MHC)
• Role is to bind to peptide fragments derived from
  protein Ags and then present them to T cells
• Binding of MHC molecules to peptide is selective
  binds to only certain peptides

15
MHC Molecules

• MHC molecules may be viewed as a third set of
  recognition molecules for Ag in the immune
  response, in addition to the Ag-specific T-cell
  and B-cell receptors.
• Important in rejection of tissues (mice studies)
• Every vertebrate species has MHC genes and
  products
• Transplantation rejection responses are dominated
  by T cells
• MHC plays a central role in T cell interactions ?
  both T cell development in the thymus and
  response of T cells to Ag
• MHC restriction of T-cell responses

16
Variability of MHC Genes Products

• Two major sets of MHC genes and products
• MHC class I
• MHC class II
• Human MHC region (chromosome 6) known as HLA
  (human leukocyte Ag)
• Murine MHC region (chromosome 17) referred to as
  H-2
• MHC molecules are members of the Ig superfamily
  and contain Ig-like globular domains
• Most other species follow the human nomenclature
• BoLA ? bovine
• SLA ? swine

17
MHC Complex

• MHC is referred to as a complex because the
  genes are closely linked and inherited as a unit
• The set of genes inherited by an individual from
  one parent is known as a haplotype
• MHC Class I (humans)
• Three independent human class I genes ? HLA-A,
  HLA-B, and HLA-C
• Always expressed at the surface in association
  with a molecule known as b2-microglobulin (b2m)

18
MHC Complex

• MHC Class II
• Produces three cell surface molecules ? HLA-DP,
  HLA-DQ and HLA-DR
• Each comprise an a and b chain
• DPa chain always pairs with DPb (DQ and DR behave
  similarly)
• The a and b chain of each molecule are coded by
  an A and a B gene, respectively
• The genes coding for DP a and b are known as DPA1
  and DPB1, DQ a and DQ b as DQA1 and DQB1,
  respectively
• DR region has seven DRB genes and one A gene
  the product of the A gene (DRA1) combines with
  the product of one of the DRB genes to generate a
  DR ab molecule

19
Murine MHC Complex

• Murine MHC, H-2 located on chromosome 17
• Murine MHC class I
• High degree of homology b/w human and mouse
  indicating a common ancestral origin
• Three mouse genes and products ? H-2K, H-2D and
  H-2L
• Expressed on cell surface with b2m
• Murine MHC class II
• I-Aab and I-Eab
• Genes are referred to as H-2I-Aa and Ab and
  H-2I-Ea and Eb
• Mouse I-A genes and products are homologous to
  human MHC class II DP
• Mouse I-E genes and products are homologous to
  human MHC class II DR