1Chapter 6: The Development of B Lymphocytes- Even though the human immune system can make B cells against almost all nuances, it doesn’t always have an abundance of every kind of B cell at any given timeo If it did, most of the body’s resources would be devoted to immune system - Cells in the bone marrow generate more than 60 billion new B cells every day- B cell development is divided into six distinct phases:o Phase 1: B cell precursors (in bone marrow) acquire functional antigen receptors through gene rearrangementso Phase 2: Negative selection  prevents the maturation of B cells that bear receptors that bind to normal human cellular components Negative selection starts in bone barrow and continues when the immature B cells travel to secondary lymphoid tissueo Phase 3: Positive selection  immature B cells compete for limited number of sites in follicles of secondary lymphoid tissues (where they complete maturation)o Phase 4: Mature B cells travel in lymph and blood looking for infection and pathogen-derived peptideso Phase 5: Activation of B cells by antigen Leads to proliferation and clonal expansion of antigen-specific B cells o Phase 6: Differentiation and diversification of B cells in each clone makes plasma cells that provide antibodies to attack ongoing infection and memory B cells for elimination of future interactions- Between every phase, a lot of cells don’t go on to the next phase because o Overall efficiency is very lowThe development of B cells in the bone marrow- Sole purpose of B cell is to make an immunoglobulin- Quality of gene rearrangements and proteins that are made are assessed at two checkpoints6-1 B-cell development in the bone marrow proceeds through several stages - In bone marrow: pluripotent hematopoietic stem cells  common lymphoid progenitor cells  T cells and B cellso Some of the cells develop into precursor cells that are committed to becoming B cellso All undifferentiated precursor cells are distinguished by cell-surface markers CD34: protein present on all human hematopoietic stem cell; used to distinguish hematopoietic stem cells from other bone marrow cells for transplantation- Earliest identifiable B-cell lineage: pro-B cellso Limited capacity for self-renewal  divide to produce more pro-B cells and also cells that will go on to develop furthero Main event in pro-B cell stage: rearrangement of the heavy-chain genes (always comes before rearrangement of light-chain genes)o Early pro-B-cell stage: joining of DH and JH o Late pro-B-cell stage: joining of VH to rearranged DJHo Once the gene is rearranged, it is transcribed through to the μ C-region gene, which is the nearest C geneo RNA transcript is spliced to produce mRNA for μ heavy chain  first immunoglobulin chain made by developing B cell- Once the μ chain is expressed, it becomes a pre-B cell (2 stages)2o First stage: large pre-B cells less mature at this stage, the cell has successfully rearranged a heavy-chain gene and made μ heavy chain, but hasn’t yet started the rearrangement of the light-chain geneso Second stage: small pre-B cells  At this stage, rearrangement of light-chain genes occurs Kappa light chain genes rearrange first; if kappa cannot be made successfully, λ light-chain genes are rearranged  When V and J segments combine (whether kappa or λ), light chain is synthesized and assembled in theER with the μ chain  forms membrane-bound IgM- IgM associated with IgAlpha and IgBeta to make a B-cell receptor complex that moves to the cell surfaceo Rearrangement of light-chain genes stops and small pre-B cell is finally an immature B cell6-2 B-cell development is stimulated by bone marrow stromal cells- B-cells in the bone marrow need stromal cells to provide a microenvironment at various stageso One function of stromal cells is to make specific contact with developing B cells through interaction of adhesion molecules and ligandso Another function is to produce growth factors that act on attached B cells membrane-bound stem-cell factor (SCF) that is a growth factor recognized by Kit (receptor on maturing B cells) Interleukin-7 (IL-7) is a growth factor that is a cytokine secreted by stromal cells (acts on late pro-B and pre-B cells)- The most immature stem cells are in the subendosteum, which is next to the interior surface of the boneo As it grows and matures, it moves closer and closer to the central axis of the marrow cavity (it still is in contact with stromal cells)o Eventually, the B cell becomes mature enough to leave the bone marrow and doesn’t need the help of the stromal cells anymore  goes to secondary lymphoid organs (like lymph nodes, spleen, or Peyer’s patches)where it will become mature B cell6-3 Pro-B-cell rearrangement of the heavy-chain locus is an inefficient process- The process of gene rearrangement is imprecise and inefficient3o Usually this happens with the addition of P and N nucleotides between the V, D, and J segments  reading frame of DNA is changed  no longer encodes immunoglobulin heavy chain If gene rearrangements don’t translate into useful proteins, they are nonproductive rearrangements  Those that do keep their reading frame and produce complete, functional immunoglobulin chains are productive rearrangements  At every rearrangement event, there is only a 1/3 chance that the reading frame will be maintained - Two copies of IG heavy-chain locus are inherited (one from the mother and one from the father) o The two copies are on homologous chromosomeso Increases likelihood that pro-B cell will make productive heavy-chain gene rearrangement because if the firstone makes a non-productive chain, the second one has a chance to redeem it o If both copies of the heavy-chain gene make nonproductive rearrangements, the cells lost their potential to make IGs  don’t develop further  die in bone marrow - In order for an early pro-B cell to rearrange IG heavy-chain genes, it must express recombination-activating genes RAG-1 and RAG-2 and other enzymes to cut, paste, and add DNAo The transcription factors E2A and EBF change this gene expression and also cause expression of Pax-5, a transcription factor that switches on genes for some proteins that are only in B cells (IGalpha and CD19, which is a cell-surface protein that helps cell respond to antigen)- First rearrangement of heavy chain: joining of DH to JH segment (this happens simultaneously on the two copies of