IMMUNOLOGY B cell and humoral immunity Wei Chen, Associate Professor The Institute of Immunology

IMMUNOLOGY

B cell and humoral immunity

Wei Chen, Associate ProfessorThe Institute of ImmunologyEmail: [email protected]

http://mypage.zju.edu.cn/566 Password: 8888

Study objectiveTo understand the activation and function of B cellsTo be aware of the differentiation and maturation of B cellsTo know the development of B cellsTo distinguish the subsets and surface markers of B cells.

B Lymphocytes (B cells)

B cells are an essential component of the adaptive immune system that play a large role in the humoral immune response. The principal functions of B cells are to make antibodies against antigens, perform the role of antigen-presenting cells (APCs) and eventually develop into memory B cells after activation by antigen interaction.

The abbreviation "B", in B cell, comes from the bursa of Fabricius in birds, where they mature. In mammals, immature B cells are formed in the bone marrow and resides in LN and spleen.

• B cells distribute in blood, lymphoid organs (lymph node, spleen, tonsil etc.) and mucosa. About 5-15% of the circulating lymphoid pool are B cells

• Naive mature B cells exit the bone marrow and migrate into the periphery. If these mature B cells encounter specific antigen, they become activated or tolerized.

• Following activation, Ag-specific B cells differentiate into antibody-forming cells (AFC) or memory B cells in the germinal centre.

B Lymphocytes (B cells)

ContentDevelopment of B cellsB cell surface markersB cell subsetsFunctions of B cellshumoral immunity

ContentDevelopment of B cellsB cell surface markersB cell subsetsFunctions of B cellshumoral immunity

Development and migration of B cells (An overview)

The phase of B cell development

B lineage commitment

HSC (hematopoietic stem cell) MPP (lymphoid/myeloid progenitor)ELP (earliest lymphocyte progenitor) ( 淋巴系髓系多能前体细胞 )CLP (common lymphoid progenitor) ETP (early T-lineage progenitor)

B cell development related events

1. B cell development dependent on BM stromal cells

2. Transcription factors important for B lineage development

3. B cell development stages characterized by stage‐specific surface markers

4. B cell development is coupled with rearrangement of heavy chain and light chain

5. The role of Ig Heavy Chain and pre‐BCR6. Immature B cells are tested for autoreactivity

before they leave the bone marrow

1. Regulators of Growth of Early B Lineage Cells

FLT3-L ： induce IL7RIL-7: survival factor for T and B cellsCXCL12(SDF-1): Retain precursors in BMOther cell-adhesion molecules VCM-1:binds to integrin VLA-4SCF: interact with Kit

2. B cell development is coupled with stage‐specific surface markers

3. B cell development is coupled with gene rearrangement

Pre-BCR 与 BCR 结构示意图前 B 细胞表面表达重链和替代轻链（ λ5 和 Vpre-B ），未成熟 B 细胞表达完整的重链和轻链

45 个 VH 23 个 DH 6 个 JH

The enzymes for gene rearrangement

The products of the two genes Rag-1 and Rag-2 (recombination-activating genes) comprise the lymphoid-specific components of the recombinase. Tdt (terminal deoxynucleotidyl transferase) modify the ends of the broken DNA. TdT adds N-nucleotides to the V,D, and J exons, enabling the phenomenon of junctional diversity.Other enzymes: DNA exonuclease, DNA synthetas, DNA ligase.

B lineage commitment

The results of gene rearrangement

Allelic exclusionIsotype exclusion

Gene rearrangement of BCRJunctional diversitySomatic Hypermutation

Affinity maturation in antibody responses

The diversity of BCR

Gene rearrangement of TCR and BCR

Junctional diversity

Somatic Hypermutation

4. Immature B cells are tested for autoreactivity before they leave the bone

marrow----Negative Selection

4. Immature B cells are tested for autoreactivity before they leave the bone marrow:

Receptor Editing

ContentDevelopment of B cellsB cell surface markersB cell subsetsFunctions of B cellshumoral immunity

BCR complex

co-receptor

CDs related to B cell activation

1. BCR complex• BCR (mIg): VH, VL----Ag

binding site mature B cells: mIgM and

mIgD. Function: specifically

recognizes antigen. • Ig/Ig (CD79a/CD79b):

heterodimer cytoplasmic domains contain ITAM.

Function: transduce the signals that lead to B cell activation.

CD19/CD21/CD81complex CD21=CR2, C3dR, EB virus receptor CD19/CD21/CD81 interactions with complement

associated with antigen play a role in antigen-induced B-cell activation.

2. Co-receptors

The role of the coreceptor in B cell activation

(1)CD40 interacts with CD40L (Th cell) (2)CD80(B7.1), CD86(B7.2) Expressed on activated B cells and other

APCs

(3)ICAM-1 （ CD54 ）、 LFA-1 （ CD11α/CD18) ： mediate cell-cell interaction and co-stimulation

3. Co-stimulatory molecules

Other receptorsCD20: function is unclear. It is suspected that it acts as a calcium channel in the cell membraneCD22 ： Inhibitory receptor with ITIM motifCD32 (FcγRII) ： Inhibitory receptorCytokine receptorsComplement ReceptorsToll-like receptorsMHC

ContentDevelopment of B cellsB cell surface markersB cell subsetsFunctions of B cellshumoral immunity

Subtype of B cells

1.Conventional B cells (B-2 cells)

2.B-1 cells (expression of CD5)

B-2 cells : conventional B cells Recirculating follicular B cells : circulate

between LN follicles and bloodmIg: IgM, IgD

Produce IgG after antigenic stimulation in the presence of T helper cells

• B1 cells (CD5+): Many of the first B cells that appear during ontogeny express CD5, a marker originally found on T cells. (express mIgM, no mIgD). They respond well to TI-Ag and may also be involved in the Ag processing and presentation to T cells.

Functions 1. produce anti-bacterial IgM the first line of

defence against microorganisms; 2. produce polyreactive Ab clearance of

denatured self components; 3. produce auto-Ab, thereby participating in the

pathogenesis of some autoimmune diseases.

ContentDevelopment of B cellsB cell surface markersB cell subsetsFunctions of B cellshumoral immunity

Functions of B cells1. Production of antibody Abs prevent microorganism from entry into

cells and eliminate microorganisms by opsonization causing phagocytosis, complement activation and toxin neutralization.

2. Ag presentation to T cells3. Immune regulation Secretion of cytokines (TNF, IFN, IL-12) →M,

DC, NK, B cell. Co-stimulation of T cells→T cell proliferation.

Phases of humoral immune responses

Functions of antibodies

ADCC

Neutralization By Antiviral Antibodies

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Antibody-mediated opsonization and phagocytosis of microbes

ADCC

Complement activation

Antigen presentation by B cells to T cells.

Mechanisms of Ig heavy chain class switching

ContentDevelopment of B cellsB cell surface markersB cell subsetsFunctions of B cellshumoral immunity

B cell‐mediated humoralimmune response

Humoral immunity is mediated by antibodies and is the arm of the adaptive immune response that functions to neutralize and eliminate extracellular microbes and microbial toxins.It is more important than cellular immunity in defending against microbes with capsules rich in polysaccharides and lipids.• TD‐Ag ： T cell‐dependent• TI‐Ag ： T cell‐independent

TD-Ag Most Ags in the naturemainly IgGCell-mediated immune responseMemory T and memory B cells

TI-AgPolysaccharides with repeated epitopesIgMno cell-mediated immunity, no memory

requirement for Th cell help.

The difference between TD-Ag and TI-Ag

B cell-mediated humoral immune response

1) PhasesAntigen recognition phaseproliferation and differentiation phaseEffector phase

2) Types• B2→TD-Ag• B1→TI-Ag

B cell-mediated immune response

Phases of humoral immune responses

Effector phase

“First signal”a. BCR recognizes B cell epitopes b. Ig/Ig transfer signalc. coreceptors (CD21/CD19/CD81)

Key point

1. B cells activated by Ag

Immune response of B cells to TD-Ag

B cell-mediated immune response

Antigen receptor-mediated signal transduction in B cells

Functional consequences of Ag-mediated B cell activation

2. Th cell-mediated activation and differentiation of B cells

Further activation Th cells → CD40L cytokines (IL-2, IFN-γ, IL-4, IL-5, IL-6, IL-13, etc.).

Formation of germinal centerAffinity maturationHeavy chain class switching

Key point

“Second signal”

Site: GC (germinal center)

B cell-mediated immune response

Ch. 11

p. 292

The interactions of Th cells and B cells in lymphoid tissues

The anatomy of humoral immune responses

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Mechanisms of Th cell-mediated activation of B cells

B cell-mediated immune response

The edge of lymphoid follicle

Fully activation Th cells → CD40L cytokines

Formation of germinal centerAffinity maturation

point mutations in the V regionsHeavy chain class switching

different Ab heavy chain classes

Key point

“Second signal”

Site: GC (germinal center)

2. Th cell-mediated activation and differentiation of B cells

Germinal centerFunction: to generate B cells that produce antibodies with affinity maturation

Germinal Center Reaction:Activated B cells give rise to Centroblasts （中心母细胞）

- localize in follicle, undergo rapid cell division and turn on machinery that causes somatic mutation in V-regionsCentroblasts give rise to Centrocytes （生发中心细胞）

- migrate to the FDC-rich region of the Germinal Center - survival is dependent on interaction with FDC-bound Ag and

presentation of Ag to Tfh cells - centrocytes that successfully compete to bind antigen (e.g.

by having higher affinity BCR) and to receive Tfh cell help are selected and may differentiate into long-lived plasma cells or memory B cells

Germinal center

Ch. 11

p. 294

Fully activation Th cells → CD40L cytokines

Formation of germinal centerAffinity maturation

point mutations in the V regionsHeavy chain class switching

different Ab heavy chain classes

Key point

“Second signal”

Site: GC (germinal center)

2. Th cell-mediated activation and differentiation of B cells

The anatomy of humoral immune responses

• Affinity maturation is the process by which the affinity of Abs produced in response to a protein Ag increases with prolonged and repeated exposure to that Ag.

• The increase in affinity is due to point mutations in the V regions, and particularly in the Ag-binding HVR, of the Abs produced.

• Affinity maturation occurs in the germinal centers of lymphoid follicles.

Affinity maturation in Ab responses

Affinity maturation in antibody responses

Ch. 11

p. 294

Fully activation Th cells → CD40L cytokines

Formation of germinal centerAffinity maturationHeavy chain class switching

is initiated by CD40L-mediated signals, and switching to different classes is stimulated by different cytokines

Key point

“Second signal”

Site: GC (germinal center)

2. Th cell-mediated activation and differentiation of B cells

B cell-mediated immune response

Isotype Switching Involves Recombination Between Specific Switch Signals

Ch. 11

p. 296

Effector phases of humoral immune responses

Effector phase

3. General features of Ab responses in vivo

Key point

Primary immune response- longer latent phase;- smaller peak response (lower Ab titer);- remaining in the serum at detectable levels for much shorter periods;- lower average affinity;- usually IgM;

B cell-mediated immune response

Secondary immune response(The immune response followed by

secondary antigenic response challenge)‐ shorter latent phase;‐ bigger peak response (higher Ab titer);‐ remaining in the serum at detectable

levels for much longer periods;‐ higher average affinity;‐ usually IgG.

Features of primary and secondary antibody responses

Immune response of B cells to TI-Ag

B cell-mediated immune response

Memory?

* TI-1 (B cell mitogen) activate B cells

* TI-2 activate mature B cells directly the repeated epitopes combine with BCR→ BCR cross-linking→produce IgM

No Th help, no memory, early effect, IgM

The mechanism of TI-Ag activating B1 cells

(A) TI-1 Ag (B) TI-2 Ag

B cell-mediated immune response

General conceptsB cell activation:

TD-Ag recoginition---First signal Th cell-mediated activation---Second signal

Affinity maturation Heavy chain class switching General features of Ab responses in vivo

B cell-mediated immune response

Movies for immune responseThe Immune Response:

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter22/animation__the_immune_response.html

Thank you!