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 IgM+IgD+ 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