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”

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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