Transcript B cells

IMMUNOLOGY
B cell and humoral
immunity
Wei Chen, Associate Professor
The Institute of Immunology
Email: [email protected]
http://mypage.zju.edu.cn/566 Password: 8888
Study objective
To understand the activation and function of B
cells
To be aware of the differentiation and maturation
of B cells
To know the development of B cells
To 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 antigenpresenting 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 Lymphocytes (B cells)
• 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.
Content
Development of B cells
B cell surface markers
B cell subsets
Functions of B cells
humoral immunity
Content
Development of B cells
B cell surface markers
B cell subsets
Functions of B cells
humoral 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‐BCR
6. 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 IL7R
IL-7: survival factor for T and B cells
CXCL12(SDF-1): Retain precursors in BM
Other cell-adhesion molecules VCM-1:binds to integrin VLA-4
SCF: 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 exclusion
Isotype exclusion
The diversity of BCR
Gene rearrangement of BCR
Junctional diversity
Somatic Hypermutation
Affinity maturation in antibody responses
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
Content
Development of B cells
B cell surface markers
B cell subsets
Functions of B cells
humoral 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.
2. Co-receptors
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.
The role of the coreceptor in B cell activation
3. Co-stimulatory molecules
(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 cellcell interaction and co-stimulation
Other receptors
CD20: function is unclear. It is suspected that it acts as a
calcium channel in the cell membrane
CD22：Inhibitory receptor with ITIM motif
CD32 (FcγRII)： Inhibitory receptor
Cytokine receptors
Complement Receptors
Toll-like receptors
MHC
Content
Development of B cells
B cell surface markers
B cell subsets
Functions of B cells
humoral 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 blood
mIg: 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
against microorganisms;
2. produce polyreactive Ab
components;
the first line of defence
clearance of denatured self
3. produce auto-Ab, thereby participating in the
pathogenesis of some autoimmune diseases.
Content
Development of B cells
B cell surface markers
B cell subsets
Functions of B cells
humoral immunity
Functions of B cells
1. 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 cells
3. 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
Antibody-mediated opsonization and phagocytosis of microbes
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© 2005 Elsevier
ADCC
Complement
activation
Antigen
presentation by
B cells to T cells.
Mechanisms of Ig heavy chain class switching
Content
Development of B cells
B cell surface markers
B cell subsets
Functions of B cells
humoral immunity
B cell‐mediated humoral
immune 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
The difference between TD-Ag and TI-Ag
TD-Ag
Most Ags in the nature
mainly IgG
Cell-mediated immune response
Memory T and memory B cells
TI-Ag
Polysaccharides with repeated epitopes
IgM
no cell-mediated immunity, no memory
requirement for
Th cell help.
B cell-mediated immune
response
B cell-mediated humoral immune
response
1) Phases
Antigen recognition phase
proliferation and differentiation phase
Effector phase
2) Types
• B2→TD-Ag
• B1→TI-Ag
Phases of humoral immune responses
Effector phase
B cell-mediated immune
Key
response
point
Immune response of B cells to TD-Ag
1. B cells activated by Ag
“First signal”
a. BCR recognizes B cell epitopes
b. Ig/Ig transfer signal
c. coreceptors (CD21/CD19/CD81)
Antigen receptor-mediated signal transduction in B cells
Functional
consequences of
Ag-mediated B
cell activation
Key
point
B cell-mediated immune
response
2. Th cell-mediated
activation and
differentiation of B cells
Site: GC (germinal center)
Further activation
Th cells → CD40L
“Second signal”
cytokines (IL-2, IFN-γ, IL-4, IL-5, IL-6, IL-13, etc.).
Formation of germinal center
Affinity maturation
Heavy chain class switching
p. 292
Ch. 11
The interactions of Th cells and B cells in lymphoid tissues
The anatomy of humoral immune responses
B cell-mediated immune
response
Mechanisms of Th cell-mediated activation of B cells
The edge of lymphoid follicle
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Key
point
2. Th cell-mediated activation and
differentiation of B cells
Site: GC (germinal center)
Fully activation
Th cells → CD40L
“Second signal”
cytokines
Formation of germinal center
Affinity maturation
point mutations in the V regions
Heavy chain class switching
different Ab heavy chain classes
Germinal center
Function: 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-regions
Centroblasts 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
p. 294
Ch. 11
Key
point
2. Th cell-mediated activation and
differentiation of B cells
Site: GC (germinal center)
Fully activation
Th cells → CD40L
“Second signal”
cytokines
Formation of germinal center
Affinity maturation
point mutations in the V regions
Heavy chain class switching
different Ab heavy chain classes
The anatomy of humoral immune responses
Affinity maturation in Ab 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 antibody responses
p. 294
Ch. 11
Key
point
B cell-mediated immune
response
2. Th cell-mediated activation and
differentiation of B cells
Site: GC (germinal center)
Fully activation
Th cells → CD40L
cytokines
“Second signal”
Formation of germinal center
Affinity maturation
Heavy chain class switching
is initiated by CD40L-mediated signals, and switching to
different classes is stimulated by different cytokines
Isotype Switching Involves Recombination Between
Specific Switch Signals
p. 296
Ch. 11
Effector phases of humoral immune responses
Effector phase
Key
point
B cell-mediated immune
response
3. General features of Ab responses in vivo
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;
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
B cell-mediated immune
response
Immune response of B cells to TI-Ag
No Th help, Memory?
no memory, early effect, IgM
* 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
B cell-mediated immune
response
(A) TI-1 Ag
(B) TI-2 Ag
The mechanism of TI-Ag activating B1 cells
B cell-mediated immune
response
General concepts
B 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
Movies for immune response
The Immune Response:
http://highered.mcgrawhill.com/sites/0072507470/student_view0/chapter22/animation__the_i
mmune_response.html
Thank you!