1

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

2

Bone Marrow Stromal Cells SupportEarly B Lymphopoiesis

Ordered Rearrangement of Ig GenesDuring B-Cell Development in the Bone Marrow

3

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

4

Rearrangement of Ig alleles is orderedand regulated to achieve allelic exclusion

Checkpoints whichconfer allelicexclusion

pBCR

BCR

5

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

7

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

8

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.