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Antigen-Independent B-Cell Development
1. DNA rearrangements establish theprimary repertoire, creating diversity
2. Allelic exclusion ensures that each cloneexpresses a single antibody on the surface,establishing specificity
3. Deletion of self-reactive clones establishestolerance
Bone Marrow
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Bone Marrow Stromal Cells SupportEarly B Lymphopoiesis
Ordered Rearrangement of Ig GenesDuring B-Cell Development in the Bone Marrow
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Heavy chain rearrangement occurs first:DJ on both allelesV-DJ on one allele
Productive rearrangement(1/9)
Mu and preBCR (surrogate L.C.)
1. STOP H.C.rearrangement2. Proliferation3. Begin L.C. rearrangement
Non-productive rearr.(8/9)
V-DJ on second allele
Mu and pBCR Non-prod.
DEATH
PRD-J
NPRPR
NPRNPR
(H.C.Alleles)
Productive rearrangement produces IgM and theB CELL RECEPTOR on the surface
Light Chain Rearrangement: 4 possible alleles, eachwith 1/3 chance of a productive rearrangement
Kappa usually precedes lambda
STOP further L.C. rearrangement
κλ
PR,GL
GL,GL
NPR,PR
GL,GL
NPR,NPR
PR,GL
NPR,NPR NPR,NPR
NPR,NPRNPR,PR
DEATHCONTINUE DEVELOPMENT
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Rearrangement of Ig alleles is orderedand regulated to achieve allelic exclusion
Checkpoints whichconfer allelicexclusion
pBCR
BCR
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THE B CELL RECEPTOR
1. Bound antigen is inter-nalized and presentedto T cells.
2. Bound antigen givessignals to the B cellto proliferate anddifferentiate.
Signalling from the BCR
Lack of Btkcauses Bruton’sXLA
(blocked at preBstage)
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No self recognition1. Proliferation2. Maturation3. Exit to periphery
IgM on B Cell Surface
L.C. editing
Recognition of self
DEATH
Recognition of self
Light Chain Receptor Editing to Change theSpecificity of Self-reactive Clones
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Ig GeneStatus atDifferent StagesOf
Antigen-Independent B-Cell Development
1. DNA rearrangements establish theprimary repertoire, creating diversity
2. Allelic exclusion ensures that each cloneexpresses a single antibody on the surface,establishing specificity
3. Deletion of self-reactive clones establishestolerance
Bone Marrow
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Antigen-Dependent B Cell DevelopmentIn Periphery (spleen and LN)
Antigen and TH cells give B cells two signals:1) proliferate 2) differentiate
T-cell dependent responses are refined two ways: 1) higher affinity antibodies
2) IgG/A/E (“switched”) isotypes
Two products of B cell development:1) plasma cells secrete Ig (final effector)2) memory cells respond to IIo antigen
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Marginal Zone
Germinal Center
Naïve ImmigrantFrom Marrow
Low affinity IgM
High affinity IgG
Memory Cell
antigenIgM
MHCII
IgG
2. FDC-
1. Ag-sp T cell1.Affinity Maturation
2. IsotypeSwitching
Plasma Cell
B CellActivationBy T-CellDependent And T-CellIndependentAntigens
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The Germinal Center
T Cell-B Cell Communication
1. Cell-cell signals from CD40L/CD40 andother surface molecules.
2. Secreted cytokines
(B cells signal T cells by presenting Ag inassociation with MHC II)
T cells provide 2 kinds of help to B cells:
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T Cell:B CellSynapse
The Germinal Center
1. Affinity maturationa. Somatic hypermutationb. Selection for high affinity clones
2. Isotype switch recombination
4. Final maturation to memory or plasma cell.3. Peripheral tolerance
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Proliferation
Ag(FDC) + T cell help
(Iterative cycles)
SURVIVAL
butT help and no Ag(eliminates low affinity clones)
orAg and no T help(eliminates self-reactive clones, giving tolerance)
DEATH
+ Somatic Hypermutation
AFFINITY MATURATION IN THE GC
Darkzone
Lightzone
Pattern of V Gene Mutations Provides EvidenceOf Cyclical Mutation and Selection Events
Random mutation combined with selection.
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B Cells MakingIg with HighAffinity for Antigen AreSelectivelyProtected fromApoptosis inthe GerminalCenter.
SELECTIVE SURVIVAL IN GC
1. Selects clones producing high affinityantibody--i.e.affinity maturation
2. Eliminates self-reactive clones--peripheral tolerance.
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Hyper IgM Syndrome
1. Mutations in CD40L
2. Mutations in CD40
3. Mutations in AID (or repair enzymesdownstream of AID)
4. One or more other genes defined by human disease!
Marginal Zone
Germinal Center
Naïve ImmigrantFrom Marrow
Low affinity IgM
High affinity IgG
Memory Cell
antigenIgM
MHCII
IgG
antigen
FDC
Ag-spec. T cell1. Affinity Maturation
2. IsotypeSwitching
Plasma Cell
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1. Memory B cellsSurface Ig, usually IgGHigh affinity for antigenLong-lived, even in the absence of antigenRespond rapidly to secondary stimulation
2. Plasma CellsSecrete copious amounts of Ig, no surface IgNon-dividingSome are short-lived, some becomelong-lived in the bone marrow
Different Ig Isotypes
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Ig Isotypes Have Different Functionsand Distributions
Secreted AntibodiesFunction in Various WaysTo EliminateForeign Invaders
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AntibodiesCan NeutralizePathogens
AntibodiesActivate NKCell Killing by Engaging FcReceptors
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AntibodiesActivateComplement-Mediated Lysis
Opsonization ofPathogens by Antibodies
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IgE, residentOn Mast Cells,Causes De-Granulation WhenAntigen Is Encountered
SUMMARY1. Antigen-indpendent B-cell development occurs in the bone marrow:
DNA rearrangements create a diverse primary repertoirepBCR and BCR provide developmental checkpointsSelf-reactive clones are edited or deleted, providing centraltolerance
2. Antigen-dependent B-cell development occurs in the spleen and lymphnodes:
TI responses involve repeating epitopes and TLR activationTD responses involve cell-cell contact and soluble mediators
3. Peripheral B-cell tolerance occurs by editing, anergy or clonal deletionin the spleen.
4. Affinity maturation and CSR occur in germinal center B cells and requireT cells, follicular dendritic cells and antigen. Memory cells and plasma cells emerge from the germinal center reaction.
5. Immune deficiencies result from gene defects in Btk, CD40, CD40L & AID.