Lecture 13 B cell activation and humoral immunity

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Some Chapter 10 study questions:
• What are the major steps required to generate robust B
cell activation? What cell types are involved?
• How do B cell homing properties change over the course
of B cell activation? What mediates this process?
• What critical events occur during formation of a B cell
immunological synapse with a helper T cell? What is the
function of the synapse?
• What is the basis of the hapten-carrier effect?
• What is the role of the germinal center? What cell types
are present there?
• How do antibodies act by neutralizing immunity, by fixing
complement, and by antibody-dependent cellular
cytotoxicity (ADCC)?
• Which antibodies are typically multimeric?
• What are the special functions of IgE?
Chapter 10
Humoral Immunity
Dean Tantin, PhD
Department of Pathology
Division of Microbiology & Immunology
University of Utah
JMRB 5200K
7-3035
[email protected]
Lecture - Oct. 9, 2014
B cell (blue) interacting
with a helper T cell
Hepatitis C virus coated
with neutralizing
antibodies (green)
Outline for today
1. Activation of naïve (mature) B cells
2. Effector functions of antibodies
a)
b)
c)
d)
Neutralization
Opsonization (coating) to enhance phagocytosis
Activation of complement system
Specialized functions of IgE
Development of B lymphocytes
Figure 8.39
YOU
ARE
HERE
Where this happens:
Spleen (blood Ag)
Lymph nodes (skin, mucosal)
GALT (intestinal)
Follicular B cells: circulate between secondary lymphoid organs
(2-3X/day). Most never encounter antigen and die.
a fibroblast…
Atlas of Blood Cells
Some proteins on surface: BCR (antigen recognition), CXCR5
(homing) and BAFF receptor (survival).
BCR
Strong, high-affinity
responses, esp to
protein antigens
MHC-II
Cytokine
receptor
Broad, abundant lowaffinity antibodies
against things like
polysaccharides
Figure 10.2
BCR
MHC-II
Cytokine
receptor
Figure 10.2
Figure 7.23
Complement augments BCR signal transmission
CD19:
•B cell-specific
•Ig superfamily member
•Associates with CR2/CD21
•Augments BCR signaling
Complement augments BCR signal transmission
Complement
receptor-deficient
mice fail to mount an
efficient immune
response
Response of B cells with BCR plus costimulation
Activate
TH cells
Receive
TH signals
Figure 10.5
Receptormediated
endocytosis
of antigen
MHC-II
Figure 10.5
Antigen
processing
and
presentation
T cell
recognition
of antigen
MHC-II
Figure 10.5
MHC-II
Full-blown B cell
activation. GC
reaction.
Plasma/memory
differentiation
Figure 10.5
Concept:
B and T lymphocytes respond to Ag separately
and migrate into proximity through the action of
chemokines, then physically interact.
Can we visualize
this in a REAL
situation?
YES!
•DC can also display Ag
to B cells
•B cells express TLRs
•T cells: CD28, CD40L
•B cells: B7-1/2, CD40
Gunzer, et al. Blood 104: 2801
Normally only 1 in 104-105 B cells
recognize a specific antigen
Transgenic mouse expressing
Added immunoglobulin genes:
Because of allelic exclusion, the
endogenous Ig genes do no
rearrange. As a consequence,
the mouse produces only 1 type
of immunoglobulin.
PROVIDES WHAT A SCIENTIST
NEEDS--EXPERIMENTAL
CONTROL.
Gunzer, et al. Blood 104: 2801
A T cell–B cell pair in cultured cells. B cells labeled green. Time
is 15 seconds per frame and approximately 80 minutes total
observation.
Gunzer, et al. Blood 104: 2801
T–B cell pair migrating at the interface of the T-cell zone and a B-cell
follicle of the inguinal lymph node in a living mouse 4 hours after
injection of T cells. T cells stained red and B cells green. Time is 45
seconds per frame and approximately 50 minutes total observation.
“Immunological Synapse”
Figure 10.3
“Immunological Synapse”
• A term stolen from neurobiology
• Between T helper cell and APC
MHC/peptide
ICAM-1
Overlay
ICAM-1
B7
CD40
MHC II/peptide
TCR
CD4
CD28
CD40 ligand
More on the immunological synapse…
Figure 10.6
From the T cell perspective…
•Some helper T cells migrate towards the follicle (they now
express CXCR5)
These can be
different
epitopes. The
BCR doesn’t
even have to
recognize
protein
antigens.
Antigen
processing
and
presentation
T cell
recognition
of antigen
MHC-II
Figure 10.5
“The Hapten-Carrier Effect”
--Exemplar: 2,4-Dinitrophenol
Haptens ONLY generate responses
when coupled to large carriers
Generation of secondary/memory
responses: The animal must be
immunized with the same carrier
conjugate (the carrier effect).
Memory T cells: to the CARRIER
Memory B cells: the the HAPTEN
Concept:
B and T lymphocytes
respond to Ag
separately and
migrate into proximity
by the action of
secreted chemokines,
then physically
interact.
Most CSR and all SHM takes place in germinal centers
Figure 1.18
Most CSR and all SHM takes place in germinal centers
• Boundary of mantle zone
defines a lymphoid follicle that
contains a germinal center
• GC=dark zone + light zone
• Dark zone – site of
proliferation and AID-mediated
diversification
• Light zone – site of selection
• Red cells are TFH
• MF gobble up the apoptotic B
cells
Figure 10.12
• Dark zone:
proliferation/
generation of
variability
• Light zone:
FDCs are
presenting
antigen…
Figure 10.13
• Dark zone:
proliferation/g
eneration of
variability
• Light zone:
FDCs are
presenting
antigen…
…only the
cells best able
to take up Ag
and present
to follicular
helper T cells
survive!
Figure 10.13
Outline for today
1. Activation of naïve (mature) B cells
2. Effector functions of antibodies
a)
b)
c)
d)
d)
Neutralization
Activation of complement system
Opsonization (coating) to enhance phagocytosis
Antibody-dependent cellular cytotoxicity (ADCC)
Ch.14 brief preview: specialized functions of IgE
Overview of humoral immunity
Fc
receptor/compl
ement binding
Figure 4.1
1.
Humoral immunity is mediated by secreted antibodies
a. Therefore they can act at a distance from the site of production
b. Action in, e.g., blood, mucosal surfaces, gut, lung
2.
Antibody functions are triggered by Ag binding to Antibody V regions
3.
Most blood protective antibodies made by long-lived plasma cells in BM
Secreted antibodies have multiple effector functions
Figure 10.1
Outline for today
1. Activation of naïve (mature) B cells
2. Effector functions of antibodies
a)
b)
c)
d)
d)
Neutralization
Activation of complement system
Opsonization (coating) to enhance phagocytosis
Antibody-dependent cellular cytotoxicity (ADCC)
Ch.14 brief preview: specialized functions of IgE
Neutralization of microbes and their toxins
Prevent microorganisms
and their toxic products
from binding to host cells
Requires only antigenbinding (V) region of the
antibody (i.e., passive
mechanism)
Neutralizing activity can
be mediated by any
secreted antibody
isotype
Hepatitis C virus coated
with neutralizing
antibodies (green)
Figure 10.26, 10.27
Neutralization of microbes and their toxins
Prevent microorganisms
and their toxic products
from binding to host cells
Requires only antigenbinding (V) region of the
antibody (i.e., passive
mechanism)
Neutralizing activity can
be mediated by any
secreted antibody
isotype
Figure 10.28
Neutralization of microbes and their toxins in the gut by IgA
Figure 10.22
Many of our best vaccines stimulate production of neutralizing antibodies
Outline for today
1. Activation of naïve (mature) B cells
2. Effector functions of antibodies
a)
b)
c)
d)
d)
Neutralization
Activation of complement system
Opsonization (coating) to enhance phagocytosis
Antibody-dependent cellular cytotoxicity (ADCC)
Ch.14 brief preview: specialized functions of IgE
The classical pathway is initiated by C1
Figure 2.17
The classical pathway is initiated by bound IgM/IgG
Interaction with C1
through constant regions
Fc
receptor/compl
ement (C1)
binding
Blue: IgM (L), IgG (R)
Yellow: C1q
Green/purple: C1r/s
Figure 10.29
Outline for today
1. Activation of naïve (mature) B cells
2. Effector functions of antibodies
a)
b)
c)
d)
d)
Neutralization
Activation of complement system
Opsonization (coating) to enhance phagocytosis
Antibody-dependent cellular cytotoxicity (ADCC)
Ch.14 brief preview: specialized functions of IgE
Antibody-mediated opsonization and phagocytosis of microorganisms
•Phagocytes bind to microbes coated with specific antibodies. IgG is
particularly good at this. IgG binding enhances phagocytosis and killing.
•The process of coating microbes with IgG to enhance phagocytosis and
bacterial killing is called opsonization.
•IgG is bound by FcgRI, expressed on different phagocytes.
Fc: crystallizable fragment, g: IgG, R1: receptor 1 (FcgRI=CD64).
Not shown (well) here or in 10.34: clustering of Fc
receptor is obligatory
Complement receptors (e.g., CR1) can further
enhance
Figure 10.33
Outline for today
1. Activation of naïve (mature) B cells
2. Effector functions of antibodies
a)
b)
c)
d)
d)
Neutralization
Activation of complement system
Opsonization (coating) to enhance phagocytosis
Antibody-dependent cellular cytotoxicity (ADCC)
Ch.14 brief preview: specialized functions of IgE
•
NK cells bind to antibody-coated target cells by FcgRIII (=CD16). FcgRIII is low
affinity (requiring multivalent interactions). NK-mediated target cell destruction.
•
Monomeric IgG does not activate NK cells.
•
Once again, clustering is obligatory.
Figure 10.36
Outline for today
1. Activation of naïve (mature) B cells
2. Effector functions of antibodies
a)
b)
c)
d)
d)
Neutralization
Activation of complement system
Opsonization (coating) to enhance phagocytosis
Antibody-dependent cellular cytotoxicity (ADCC)
Ch.14 brief preview: specialized functions of IgE
Ig Class Switch Recombination (CSR) is instructed
Unlike V(D)J, which is largely random
CSR is initiated by CD40L:CD40 + appropriate cytokine
Figure 10.15
1. IgE Structure a) Like IgM, has an extra CH4 domain in the
constant region.
b) Binds FceRI with very high affinity.
2. IgE Properties
a) IgE does not fix complement.
b) The least common serum Ig because it binds tightly to Fc
receptors on basophils and mast cells in tissue even before
interacting with antigen.
c) IgE plays a role in parasitic helminth diseases. Since serum
IgE levels rise in parasitic diseases, measuring IgE levels is
helpful in diagnosing parasitic infections. Binding of
eosinophils to IgE-coated helminths results in killing of the
parasite.
d) As a consequence of its binding to basophils and mast cells,
IgE is also involved in allergic reactions. Binding of the
allergen to the IgE on the cells results in the release of
mediators (such as histamine) that result in allergic symptoms.
Very rapid (seconds) mast cell degranulation following IgE crosslink
Because of the high
affinity, IgE-coated mast
cells sit there in your
tissues…like a loaded
gun.
Figure 10.37
Some Chapter 10 study questions:
• What are the major steps required to generate robust B
cell activation? What cell types are involved?
• How do B cell homing properties change over the course
of B cell activation? What mediates this process?
• What critical events occur during formation of a B cell
immunological synapse with a helper T cell? What is the
function of the synapse?
• What is the basis of the hapten-carrier effect?
• What is the role of the germinal center? What cell types
are present there?
• How do antibodies act by neutralizing immunity, by fixing
complement, and by antibody-dependent cellular
cytotoxicity (ADCC)?
• Which antibodies are typically multimeric?
• What are the special functions of IgE?
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