g.2014-immuno~ (10a.humoral immunity'bcell'-jyh)

B lymphocytesYanhong Ji

Xian Jiaotong University

Email: jiyanhong@mail.xjtu.edu.cn

Surface markers on B lymphocytes and their function

Subsets of B lymphocytes

Function of B lymphocytes

Contents

Surface Markers on B Lymphocytes and Their Functions

1. B cell receptor--BCR

• Membrane immunoglobulin(mIg) on B cell

• Recognize and bind antigen specifically

Part I B cell receptor complex and its associated molecules

2. BCR complex• BCR and Ig-(heterodimer) Ig(CD79a) , Ig(CD79b) • Participate in BCR formation• ITAM------bind to tyrosine kinase• Transmit activating signal

3. B cell co-receptor complex ----CD19, CD21, CD81

CD21

CD19CD81

CD21

CD19CD81

Antigen

C3d

CD21 occurs in a complex with CD19, CD81 . CD21 binds to the C3d fragment of complement. Complement-coated antigens therefore can cross-link CD21 with the BCR.

Co-crosslinking of CD21 with the BCR introduces more CD19 into the BCR and increases the signal through the BCR.

CD21

CD21: receptor of C3d,C3d and iC3b ----Enhance the binding of BCR and antigen and pass activating signal to CD19

CD19

CD19 is a transmembrane protein with a very large cytoplasmic domain. It is present on follicular dendritic cells and on B cells from the early pre-B stage until plasma cell differentiation. It is found associated with the BCR, or with CD21, or on its own in the plasma membrane.

The cytoplasmic domain has multiple tyrosines which are phosphorylated following BCR ligation. CD19 acts as a adaptor molecule and amplifies BCR signals.

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sykCD79CD79

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CD79CD79

*****

CD19: Transmit activating signal into B cell

CD21

CD19CD81

Antigen

C3d

CD81 occurs in a complex with CD19, CD21 . CD81: a transmembrane protein, stable CD21and CD19

CD81

Src family kinases then bind the phosphorylated CD19

Antigen recognition

C3d opsonized bacterium

• mIg and CD21 are cross-linked by antigen that has activated complement

C3d binds to CD21,

P P

• CD21 is phosphorylated and receptor-associated kinases phosphorylate CD19

P P

• Phosphorylated CD19 activates more Src family kinases• CD 81 and CD 21 also play a role as a coreceptor for the BCR

Transmission of signals from the cellsurface to the nucleus

• B cell-specific parts of the signalling cascade are associated with receptors unique to B cells - mIg, CD19 etc.

• Subsequent signals that transmit signals to the nucleus are common to many different types of cell.

• The ultimate goal is to activate the transcription of genes, the products of which mediate host defence, proliferation, differentiation etc.

Once the B cell-specific parts of the cascade are complete, signalling tothe nucleus continues via three common signalling pathways via:

1.The mitogen-activated protein kinase (MAP kinase) pathway2. Increase in intracellular Ca2+ mediated by IP3

3.The activation of Protein Kinase C mediated by DAG

1. CD40----co-stimulatory receptor• CD40 on B cell binds to CD40L on

activated T cell• Transmit an important co-stimulatory signal to B cells• Upregulate expression of B7 on B cells • Participate in class switching of antibody

Part II B cell Accessory molecules

CD40

2. B7• B7-1 (CD80) and B7-2 (CD86) • Expressed on B cells or other APC• B7-CD28• B7-CTLA-4

4. CD80/CD86(B7-1,B7-2

B

3. MHC molecules• Class , MHC moleculesⅠ Ⅱ

4. Mitogen receptor• SPA, LPS• PWM

5. Cytokine receptor• IL-4R, IL-5R, IL-6R

Part III Development and differentiation of B cells

Differentiation of B cells in Bone marrow

Differentiation of B cells in peripheral lymphoid tissue

1. Differentiation of B cells in Bone marrow----Ag independent

• Hematopoietic stem cells • Lymphoid progenitor • Pro-B cells( chain rearrangement) • Pre-B cell( chain + surrogate light chain ) • Immature B(mIgM, chain +κchain orλchain)

• Mature B(mIgM, mIgD) • Functional B repertoire

Overview of B-cell development

Peripheral

Stages of B cell development

Stem Cell Early pro-B cell Late pro-B cell Large pre-B cell

Small pre-B cell Immature B cell Mature B cell

Each stage of development is defined by rearrangements of IgH chain genes, IgL chain genes, expression of surface Ig, expression of adhesion molecules and cytokine receptors

Organization of Ig gene segments in the mouse V-variable

D-diversity

J-joining

C-constant

L-leader

V gene

Heavy-chain gene rearrangement and RNA processing

RAG1/RAG2

RAG1/RAG2

Stages of differentiation in the bone marrow aredefined by Ig gene rearrangement

B CELL STAGE

IgH GENECONFIGURATION

Stem cell Early pro-B Late pro-B Large pre-B

Germline DH to JH VH to DHJH VHDHJH

Pre-B cellreceptor

expressed

Ig light chain gene has not yet rearranged

Kappa light-chain gene

rearrangement and RNA processing

RAG1/RAG2

V D J C

D JV CD J

V CD JV

Germline

DH-JH joining

VH-DHJH joining

V J C

V CJV

Germline

VL-JL joining

light chain rearrangement

Largepre-B

Smallpre-B

Y ImmatureB cell

Peripheral

Checkpoint in B cell development

Stem Cell Early pro-B cell Late pro-B cell Large pre-B cell

Small pre-B cell Immature B cell Mature B cell

Y

B cell receptor

Expressed when VHDHJH CH is productively rearranged

VpreB/5 - the surrogate light chain, is required for surface expression

Ig & Ig signaltransductionmolecules

CH

Heavy chainVHDHJH

Light chainVLJLCL

VpreB

5

Pre-

LargePre-B

LargePre-B

LargePre-B

LargePre-B Large

Pre-BLargePre-B Large

Pre-BLargePre-B Large

Pre-BLargePre-B

Proliferation

Y ImmatureB cell

Light chain expressedIgM displayed on surface

IgM

Ligation of the pre-B cell receptor triggers entry into the cell cycle

Largepre-B

Many large pre-B cells with identical pre-B receptors

Large pre-B

Intracellular VDJCH chainVL-JL rearranges

Proliferation stops

Small pre-B

B cell receptor

Ig & Ig signaltransductionmolecules

CH

Heavy chainVHDHJH

Light chainVLJLCL

Diversity of BCR Combinatorial diversity (2.5x108)

Junctional diversity Somatic hypermutation

Total: 1014

2. Events in the differentiation of B cells:

Gene rearrangement of Ig

Clonal deletion

Immature B cells : mIgM--self antigen mIgM X self antigen

apoptosis or anergy surviving to develop

mature B cells

Y

Y

Y Y

B

Immature B cellCell surface Ig expressed

Able to sense Ag environment

Can now be checked for self-reactivity

Acquisition of antigen specificity creates a need

to check for recognition of self antigens

1. Physical removal from the repertoire DELETION2. Paralysis of function ANERGY3. Alteration of specificity RECEPTOR EDITING

B cell self tolerance: clonal deletion

Immature B cell recognizesMULTIVALENT

self Ag

B

Clonal deletion byapoptosis

YYBImmatureB

BSmallpre-B

Small pre-B cellassembles Ig

Y

B cell self tolerance: Anergy

B

YY

YB

Anergic B cell

IgD normal IgM low

Immature B cell recognizessoluble self Ag

No cross-linking

YYB

ImmatureB

BSmallpre-B

Small pre-B cellassembles Ig

IgM

IgD

IgD

IgD

Receptor editingA rearrangement encoding a self specific receptor can be replaced

V CD JVV V

Y

BB!!Receptorrecognizes

self antigen!!

B Apoptosisor anergy

YBB

Edited receptor now recognizesa different antigen and can be

rechecked for specificity

CD JVV VV

Arrest developmentAnd reactivate

RAG-1 and RAG-2

YYY

YYY

Mature B cellexported to the

periphery

Y

Y

B cell self tolerance: export of self tolerant B cells

IgD and IgM normal

IgMIgD

IgD

IgD

IgD

IgMIgM

IgM

Immature B cell doesn’t recognize any

self Ag

YYB

ImmatureB

BSmallpre-B

Small pre-B cellassembles Ig

B

2. Differentiation of B cells in peripheral lymphoid tissue----Ag

dependant

• Plasma cell Ab• Memory B cell secondary

immune response

B cell recognizesnon-self antigen

in periphery

Ig-secreting plasma cell

Differentiation in the periphery

YY Y YY YYY Y

BY Y

YYYYY

BY Y

YY

YYYB

Mature peripheralB cell

Control of Affinity & Affinity Maturation

Five B cell antigenreceptors - all specificfor , but withdifferent affinitiesdue to somatichypermutationof Ig genes in the germinal center B B B B B

Only this cell, that has a high affinity for antigenOnly this cell is rescued from apoptosis

The cells with lower affinity receptors die of apoptosis by neglect

Isotype Switching Under the Influence of HelperT Cell-Derived Cytokines

Mechanism of Ig Isotype Switching

Plasma cells

Surface Surface High rate Growth Somatic Isotype Ig MHC II Ig secretion hypermutation switch

B

BMature B cell

Plasma cell

High Yes No Yes Yes Yes

Low No Yes No No No

B cell subpopulations

According to expression of CD5 or notB1 cell (CD5+) B2 cell (CD5-)

CD5

Two B cell lineages

B

B cell precursor

B

Mature B cell

B2 B cells

Plasma cell

YY Y YY YYY Y

PC

IgG

BYYYYYYYYYY

YYYYY YYYYY

IgM - no other isotypes

B

Distinct B cellprecursor

B1 B cells‘Primitive’ B cells found in pleura and peritoneum

Comparison of B1 and B2 cells B1 B2

Development early late BCR mIgM mIgM and mIgD CD5 + - Reproduction self-renewing from pre-B cell in BM Recognized Ag TI-Ag and auto-Ag TD-Ag Ab type IgM >IgG IgG >IgM Ab avidity low high Second IR - + Function innate immunity adaptive immunity

Function of B cell

Produce the antibody----HI Present antigen----APC Participate in immunological regulation