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 B-Cell Receptor
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 IgM+IgD+ mature B cells Activation
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