b cell activation and antibody production lecture 15
TRANSCRIPT
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B Cell Activation and Antibody ProductionB Cell Activation and Antibody Production
Lecture 15Lecture 15
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Overview of B CellDevelopment,
Activation,Antibody Production
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B Cell AntigensB Cell Antigens
Proteins
Dependent Upon Helper T Cellsto Induce Antibody Production
Thymus Dependent Antigens
Polysaccharides, Lipids
Does Not Need Helper T Cellsto Induce Antibody Production
Thymus Independent Antigens
Antigens
Proteins
Dependent Upon Helper T Cellsto Induce Antibody Production
Thymus Dependent Antigens
Polysaccharides, Lipids
Does Not Need Helper T Cellsto Induce Antibody Production
Thymus Independent Antigens
Antigens
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B Cell Responses to Thymus-Dependent Antigens (T Cell-
Dependent Antibody Responses)
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Primary and Secondary Antibody Responses
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Phases of the Humoral Immune Response
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T Cell Epitope B Cell Epitope
Antigen
A T-Dependent Antigen Must Contain Both B and T Cell Epitopes
LINKED RECOGNITION
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(B Cells)
(T Cells)
Follicles
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Activation of B Cells by Antigen and Complement
1. Biochemical Signals2. Endocytosis of Antigen
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Antigen Recognition Phase of T-Dependent Antibody Response
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Interactions of B Cells with Helper T Cells
Initial Contact T-B Conjugate
TEM Picture
Note the broad area of membrane contact between B and T Cells.
B Cell B Cell
T Cell
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Helper T Cell-Dependent Activation Of B Lymphocytes
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B-Cell Activation by Thymus-Dependent Antigens
Cytokines
Linked Recognition
C’R
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Activated B Cells(Following Interaction with TH Cells
Extra-follicular Site Follicle
Antibody SecretingCells
Germinal Center
Antibodies
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Late Events in T Cell-Dependent Antibody Responses-Germinal Center Reaction
Late Events in T Cell-Dependent Antibody Responses-Germinal Center Reaction
• Affinity Maturation– Somatic Hypermutation
• Generation of Memory B Cells
• Affinity Maturation– Somatic Hypermutation
• Generation of Memory B Cells
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Somatic Hypermutation and Affinity Maturation of Antibodies
Affinity maturation is the process that leads to increased affinity of antibodiesfor a particular antigen as a result ofsomatic mutation in the Ig genes followed by selective survival of B cells producingthe antibodies with the highest affinity
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Affinity Maturation in Antibody Responses
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Selection of High AffinityB Cells in Germinal Center
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Phases of the Humoral Immune Response toT-Dependent Antigen
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Anatomy of Humoral Immune Responses
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Antibody Isotype Switching
Antibody Isotype Switching
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Isotype Switching Under the Influence of HelperT Cell-Derived Cytokines
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Mechanism of Ig Isotype Switching
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CD4 T Cell-Dependent Effects in Antibody Responses
• Memory B Cell Development
• Isotype Switching
• Affinity Maturation
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Thymus-Independent
Antigens
Thymus-Independent
Antigens
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B-Cell Activation by Thymus-Independent and Dependent Antigens
Most TI antigens are polyvalent and induce maximal Crosslinking of membrane Ig on B cells, without a Need for T cell help.
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Features of Antibody Responses toT-Dependent and T-Independent Antigens
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Antibody Response to T-Dependent Antigens
Antibody Response to T-Dependent Antigens
• Role of Helper T Cells– Cytokines
– CD40/CD40L interactions
• Isotype Switching– Switch Recombination
– Cytokines and Isotypes
• Affinity Maturation– Somatic hypermutation
– Selection for B cells which produce High Affinity Antibodies
• Memory B Cells
• Role of Helper T Cells– Cytokines
– CD40/CD40L interactions
• Isotype Switching– Switch Recombination
– Cytokines and Isotypes
• Affinity Maturation– Somatic hypermutation
– Selection for B cells which produce High Affinity Antibodies
• Memory B Cells
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Antibody Effector Functions
Antibody Effector Functions
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Effector Functions of Antibodies
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Neutralization of Microbes by Antibodies
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Neutralization of Toxins by Antibodies
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Opsonization of Microbes by Antibodies
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Antibody-Dependent Cellular Cytotoxicity (ADCC)
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Functions of Complement
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Complement-Mediated Lysis of E. coli
Alive Killed
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Cellular Interactions in Immune Responses
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The Immune Response: A Summary
The Immune Response: A Summary
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WHY can immune system recognize somany different epitopes??
Antibody heavy and light chains arecomposed of gene segments
Variable regions are uniqueA limited variety of constant region sequences
are used
They must be rearranged into functional genesbefore they can be transcribed
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p. 106
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Organization of Ig genes
Germline DNA- gene segments surrounded bynoncoding regions
These are rearranged to form functional genes
Light chains- V, J and C segments
Heavy chain- V, D, J, C
V regions rearrange first
A single V can rearrange to more than one C
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Multigene families
or
In humans: 40 V, 5 J, 1 C
Similar number of genes in humans; this is rare in mice
Heavy-chain gene families are similar but morecomplex (D segment)
CH regions formed from exons
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One of many possible combinations
p. 111
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Heavy chain DNA
D-J and V-DJ rearrangements must occurseparately
On a mature B cell, both mIgM and mIgDare expressed on the cell surface
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How does rearrangement occur?
Each V, D and J is flanked by RSS(Recombination signal sequences)
Mechanism is controlled by RAG-1 and RAG-2proteins and an enzyme TdT
If any of these proteins is defective no mature B cells can form; nor T cells
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p. 112
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“Junctional flexibility” contributes to diversity
But not all rearrangements are “productive”
p. 115
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B cells are diploid and contain chromosomes from both parents
However, heavy chain genes are rearrangedfrom only one chromosome, as are light chaingenes.Therefore, any one B cell will contain
one VH and one VL (antigen specificity)
How? Allelic exclusion (Yancopoulos and Alt, 1986)
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Model for allelic exclusion
p. 116
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Generation of antibody diversity(why are there so many possible antigencombining sites?)
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Multiple germline gene segments
In human germline:
51 VH, 27 D, 6 JH
40 V, 5 J
30 V , 4 J
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Combinatorial V-J and V-D-J joining
57 V X 27 D X 6 J= 8262 possible combinationsfor VDJ joining
40 V X 5J = 200 possible V 120 possible V
8262 X (200+120) = 2.64 X 106 possible combinations
Without taking into account other sources of diversity
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Junctional flexibility in V-J or V-D-J junction
Additional nucleotides added at junctions(P or N addition), if a single-strandedregion is created during the joiningprocess
Somatic hypermutationmutations occur AFTER rearrangementtends to occur in CDR regionsaffects antigen affinity (tends to increase):“affinity maturation”occurs in B but not T cells
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Class switching
After antigen stimulation heavy-chain DNA canrearrange so VDJ joins to any isotype
Cytokines help determine the isotype
IgG2a or IgG3 (mice): IFN-IgM: IL-2, IL-4, IL-5IgE: IL-4
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p. 122
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Membrane-bound or secreted?
Alternative splicing, p. 124
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Mature B cells express both mIgM and mIgD
No switch site between C and C
The VDJCC contains 4 polyadenylation sites
mIgM or mIgD can be generated dependingon which polyadenylation site is used
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Regulatory elements of transcription
Promoters
Enhancers
Gene silencers
Gene rearrangement brings enhancers closeto the promoter they influence
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Why aren’t Igs produced in B cells?
In T cells a protein may bind to the -enhancer and prevent V-J joining
Arrangement of immunoglobulin genes (andformation from exons) and greatlyfacilitated formation of geneticallyengineered antibodies