Chapter 14

B Lymphocytes

Contents B cell receptor and B cell complexB cell accessory molecules B cell subpopulationsFunctions of B cellsB cell maturationBCR diversity and TCR diversity

I. B cell antigen receptor and B cell receptor complexBCRBCR complexFunction: Recognize antigen

1. B cell receptor----BCRMembrane immunoglobulin (mIg) on B cells: mIgM, mIgDRecognize and bind antigen specifically

2. BCR complexBCR and Ig - Ig (heterodimer) Ig (CD79a) , Ig (CD79b) Participate in BCR formationITAM------bind to tyrosine kinaseTransmit activating signal

II. Accessory membrane molecules on B cellsCD19, CD21(CR2), CD81(TAPA-1) ----B cell co-receptor complexCD21(CR2): receptor of C3dg,C3d and iC3b ----Enhance the binding of BCR and antigen ----Pass activating signal to CD19 ----EB virus receptor

CD19: Transmit activating signal into B cell

CD40 on B cell binds to CD40L on activated T cellTransmit an important co-stimulatory signal to B cells Upregulate expression of B7 on B cells Participate in class switching of antibody2. CD40----co-stimulatory receptor

3. Co-stimulatory molecule: B7B7-1 (CD80) and B7-2 (CD86) Expressed on B cells or other APCB7-CD28: activation signalB7-CTLA-4: inhibitory signal

4. MHC moleculesClass , MHC molecules

5. Mitogen receptorSPA, LPSPWM

6. Cytokine receptorIL-4R, IL-5R, IL-6R

7.Fc receptor (CD32---FcRII-b) ------Related to immunological regulation

III. B cell subpopulationsAccording to expression of CD5 or notB1 cell (CD5+) B2 cell (CD5-)

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

IV. Functions of B cellsProduce the antibody----HIPresent antigen----APC Participate in immunological regulation: secrete various cytokines, FcRII-b

The comparison of main membrane molecules between T and B cells

T lymphocyteB lymphocytefunctionsAntigen receptorTCRBCRBind to Ag,First signalAg receptor complexCD3 and Igand IgTransmit first signalCo-receptorCD4,CD8CD19-CD21 complexHelp to bind AgTransmit first signalCo-stimulatory receptorCD28CD40Second signal,Transmit second signalCo-stimulatory moleculesCD40LB7(CD80/CD86)Offer second signalMHC I expressionexpressionAntigen processingMHC II Activated expressionexpressionAntigen processing

Section Development and differentiation of B cellsDifferentiation of B cells in Bone marrow

Differentiation of B cells in peripheral lymphoid tissue (B cell mediate immune response, HI)

Differentiation of B cells in Bone marrow----Ag independentHematopoietic stem cells Lymphoid progenitor Pro-B cells( chain rearrangement) Pre-B cell( chain + surrogate light chain ) Immature B(mIgM, chain +chain orchain) Mature B(mIgM, mIgD) Functional B repertoire

Negative selection of B cells inbone marrow

2. Differentiation of B cells in peripheral lymphoid tissue----Ag dependantVirgin B/nave B cell most diePlasma cell AbMemory B cell secondary immune response

3. Events in the differentiation of B cells:

Gene rearrangement of Ig Negative selection

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

apoptosis or anergy surviving to develop

mature B cells

Questions?Why can TCR or BCR recognize so many Ag in nature? Why does IgM produce earlier than others?How does Ig produce BCR and Ab?How can B produce different type of Igs?---------------?

Part BCR diversity and TCR diversityBCR diversity

TCR diversity

Gene structure of Ig Gene rearrangement of Ig Characteristics of Ig gene expression Mechanism of Ig diversity Section BCR diversity

1. Germ line gene structure of Ig (human)H chain:14 chromosome V region encoding genes: VH (variable gene segments) 65 DH (diversity gene segments) 27 JH (joining gene segments) 6 Leader sequencesignal peptide C region encoding genes: CH (constant gene segments): C, C, C et al. (11)

L chain(--2 chromosome, --22 chromosome)

V region encoding genes: --V, J 40, 5 -- V, J 30, 4 Leader sequencesignal peptide

C region encoding genes: C (1); C(4)

In heavy chains, the V, D and J segments encode the variable domain while the C segment encodes the constant domain.

In light chains, the V and J segments encode the variable domain whilethe C segment encodes the constant domain.

VJC JC JC JC(a) Chain (22 chromosome))(2 chromosome)

2. Gene rearrangement of Ig

V-D-J rearrangement of H chain pro-B cells: D-J V-DJ VDJ DNA

pre-B cells: VDJC VDJ- C RNA mRNA

V-J rearrangement of L chain pre-B cells: V -J V J DNA immature B cells: V J C V J -C RNA mRNAtranscriptionsplicing

C C C3 C 1 C1 C2 C4 C C2C C C3 C1 C1 C2 C4 C C2C C C3 C1 C1 C2 C4 C C2CCCC

The expression of BCR Intranuclear:DNA rearrangement: ------- V region encoding gene (VDJ or VJ) Transcription and splicing -------leader sequence + V region encoding gene + C region encoding gene (L gene-V gene C gene)Extranuclear:Translation -------- nascent peptide L-V-CEndoplasmic reticulum:assembly--------H chain and L chain (IgM or IgD)

transportation------BCR (membrane Ig, mIg)

3. Characteristics of Ig gene expression recombination enzyme: RAG (recombination activating gene) TdT (terminal deoxynucleotidyl transferase) other DNA enzymes

Allelic exclusion and isotype exclusion Allelic exclusion: only one of the two alleles in homologous chromosomes can be expressed. Isotype exclusion: only one of the two types of light chain genes can be expressed(:=65:35).

Kuby Figure 5-10Read Kuby pages 115-117: Allelic Exclusion Ensures a Single Antigenic Specificity

Isotype switching ( class switching )

Ag

activated B cells proliferate

VDJ is switched to recombine with another C region encoding gene

IgM IgD, IgG, IgA, IgE

Switching region

Membrane type (BCR) and Secretory type Ig (Ab)

4. Mechanism of Ig diversity Combinatorial diversity human Ig: 65VH27DH 6JH=10530V 40V 5J =200V 30V 4J =120V

C C C3 C 1 C1 C2 C4 C C2C C C3 C1 C1 C2 C4 C C2C C C3 C1 C1 C2 C4 C C2CCCC

Junctional diversity

CDR3 lies in V-DJ or D-J junctionsLose or insert of several nucleotides will increase the diversity of CDR3.N-nucleotides insert by TdT without templateThere is no N-nucleotides insert in L chain

Somatic hypermutation Ag

activated B cells proliferate

gene mutation in V region encoding genes

affinity maturation

mature B cells which finished V gene rearrangement

Section Gene structure and rearrangement of TCRGene structure of TCR chain (14 chromosome): V, J, C chain (7 chromosome): V, D, J, C

(14 chromosome)(7 chromosome)

2. Gene rearrangement of TCR TCR chain rearrange first Inactivate gene within gene

3. Gene structure of TCR chain (7 chromosome): V, J, C chain (14 chromosome): V, D, J, C

(14 chromosome)(7 chromosome)4. Gene rearrangement of TCR No junctional diversity in TCR

5. Characteristics of TCR gene expressionWithout somatic hypermutationMore N- nucleotides insert than BCRMore valid rearrangement in V region of TCR BCR: 1014 TCR: 1016

Comparison of BCR and TCR functional genes

NO. of chain Chromosome V D J C----------------------------------------------------------------------- BCR H 14 65 27 6 11 2 40 5 1 22 30 4 4 TCR 14 70-80 61 1 7 52 2 13 2 7 12 5 2 14 4 3 3 1-------------------------------------------------------------------------

What you should know of this lectureDefinition of BCR/BCR complexMajor surface membrane molecules on B cells (comparison) Comparison of B1 cell and B2 cellFunctions of B cellsB cell maturation

*Clonal deletion: Functionally immature cells of a clone encountering antigen undergo a programmed cell death. For example, auto-reactive T-cell are eliminated in the thymus following interaction with self antigen during their differentiation (negative selection). Clonal deletion has been shown to occur also in the periphery. B cells expressing only IgM (no IgD) on their surface when exposed to antigen are eliminated.