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B Cell Development
Assignment: Chapter 4 (Skip Figs. 4.6, 4.12)
Learning Objectives for Lecture 11
1. Discover how lymphoid stem cells become B cells destined
to make antibodies
2. Understand the importance of Ig gene rearrangement
3. Appreciate mechanisms leading to B-cell leukemias
Notes
1. Tens of billions of B cells generated each day in the bone marrow; only ½
survive
2. Bone marrow: primary lymphoid tissue
3. Development means the cell surface expression of a unique B Cell Receptor
(BCR), which is an Ig molecule (monomeric IgM and IgD)
Stage 1: Immunoglobulin (Ig) Gene Rearrangement
(Absence of Antigen)
RAG proteins responsible
for gene recombination
Notes
1. Pre-B-cell Receptor: rearranged HC, surrogate LC, Iga, Igb; initiates cell division resulting in 3070 small pre-B cell clones (all have same heavy chain, but with potential to have different light
chains)
2. Signal from pre-B-receptors halts HC gene re-arrangement & sLC synthesis; cell proliferation to
yield lots of small pre-B-cells; cell division stops and light chain gene rearrangement begins
3. Immature B cells selected for tolerance (prevents autoimmunity)
4. Tolerant immature and mature B cells enter periphery (immature cells mature in the spleen)
5. Naïve B cells (never seen antigen) circulate looking for foreign microbes
Iga and Igb are Signaling Subunits of the B
Cell Receptor (BCR; surface Ig molecule)
Notes
1. The Ig molecule (either pre-BCR or BCR) can not travel to the surface
of the B cell without Iga and Igb
2. The pre-BCR and BCR consist of an Ig molecule plus Iga and Igb
3. Iga and IgB genes turned on at the pro-B-cells stage and remain on
until cell becomes and antibody secreting plasma cell
4. Iga and Igb send signals when receptors are engaged (bound antigen)
Bone Marrow Stromal Cells Direct B Cell Development
Notes
1. Adhesions molecules: CAMS, VLAs, VCAMs
2. Signaling molecules: Kit (receptor); SCF (membrane bound growth factor), IL-7
Productive Gene Rearrangement is Key to
Developing B Cell Survival
Notes
1. Two copies of heavy chain and light chain loci (one on each chromosome;
remember there are 2 light chain loci k and l)
2. Most DJ rearrangements are successful (D can translate three reading frames)
3. VDJ rearrangement is consecutive; 50% success rate (remember: m chain)
4. Unproductive gene rearrangement results in apoptosis (ordered cell death)
Light Chain has Several Chances to Rearrange
Notes
1. Large pre-B-cells undergo cell
division before becoming
resting small B cells; LC
rearrangement
2. Starts with k and goes until all
possibilities have been tried
3. LC rearrangement, 85%
successful
4. Overall success of Ig gene
rearrangement is less than
50%
Ending Gene Rearrangement
Notes
1. Once productive gene rearrangement
has occurred, need to prevent further
gene rearrangement. Why?
2. The ligation (binding) of a rearranged
immunoglobulin gene at the cell surface
sends a signal to shut down gene
rearrangement. How is preventing gene
rearrangement accomplished?
3. B cell diversity: generation of different
heavy chain; each paired with a different
light chain
Combinatorial Diversity
LC: k: 40 V x 5J=200; l: 30V x4J=120 320 Total
HC: 40V x25D x6J = 6000
LC + HC 320 x 6000 = 1.9 x 106 possible combinations
Junctional Diversity
Random insertion of N and P nucleotides at VJ,
DV, VDJ junctions
Total Possible Different Ig Receptor Combinations
Combinatorial plus junctional = 1011
estimated possibilities
Regulating B Cell Development
1. Genes essential for gene recombination are turned on at selective
stages of B cell development
2. Genes encoding RAG;
-turned on in Early pro-B cell and late pro-B cell (HC rearrangement)
-turned off in Large pre-B cell (to allow proliferation)
-turned back on in Small-pre-B cell (LC rearrangement)
3. Terminal deoxynucleotidyl transferase (TdT)
-responsible for functional diversity (N nucleotides)
-turned on in pro-B cells, silent in small pre-B cells
4. Genes encoding Iga and Igb
-turned on in pro-B cells and remain on
5. Burton’s tyrosine kinase (Btk)
-signaling molecule whose deficiency prevent B cell development
X-linked Agammaglobulinemia
1. Iga and Igb signal to a signaling molecule: BTK
2. Btk needed to signal B cell to develop
3. Btk is located on X-chromosome
4. Patients lacking Btk (mostly boys) have B cell development
blocked at the pre-B-cell stage and therefore have no
circulating antibodies
5. Suffer from X-linked Agammaglobulinemia
6. Recurring infections: Haemophilus influenzae;
Streptococcus pneumoniae, Streptococcus pyrogenes;
Staphylococcus aureus
7. Treatment: antibiotics and infusion of antibodies
Formation of B Cell Tumors (Leukemias &
Lymphomas)
Notes
1. High transcriptional and splicing activity during B cell gene rearrangement
2. Mistakes made that can result in deregulated cell growth leading to leukemia
3. Ig gene segment is mistakenly joined to a gene regulating cell growth
-translocation: gene on one chromosome joined to a gene on a different
chromosome
-B cell tumors: Burkitt’s lymphoma; Ig gene segment mistakenly fused to a gene
called MYC that regulates the cell cycle; along with additional mutation leads
to Burkitt’s lymphoma
CD5+ B Cells (B-1 Cells)
1. Arise early in embryonic development
2. Express CD5 on surface
3. No surface IgD; restricted BCRs; Abs to bacterial polysaccharides
4. Predominate in pleural and peritoneal cavities
5. Capacity for self-renewal
6. Most B cell tumors causing chronic lymphocytic leukemia
(CLL) are transformed B-1 cells (express CD5 on surface)
7. Treatment: bone marrow transplant
Summary
1. B cell originate from lymphoid progenitor stem cells and
development in the bone marrow thoughout life
2. Consecutive gene rearrangements of Ig genes results in the
expression of a unique BCR (Ig molecule with H and L chains)
3. Several loci (2 HC; 4LC) to counter unproductive
rearrangements
4. mHC rearranges first and this must be productive to continue
-forms pre-BCR (rearranged mHC and surrogate LC); ligation
on cell surface halts HC gene rearrangement
5. LC rearrangement following proliferation of large pre-B cells
-4 loci; several attempts at each loci (85% success rate)
-productive light chain rearrangement halts further
rearrangement
6. B cell repertoire is diverse (1011)
7. Mistakes cause B-cell leukemias and lymphomas