Jude Uzonna

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Immunology
Principles of the Adaptive Immunity
Jude Uzonna RM 425 APOTEX Ph 977-5659 Email
uzonna_at_cc.umanitoba.ca
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Why was it necessary for the immune system to
evolve?
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The perfect world
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The real world
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What are the hallmarks of the innate immune
system?
Review of Dr Kungs 2nd lecture
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• Do you think the innate immune system needed
  fixing ?
• - Why fix it if it isnt broken?

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Lecture Objectives
By the end of the Lecture, you will be able to

1. Know why adaptive immune system evolved
2. Know the key players of adaptive immunity
3. Differentiate b/w cell-mediated and humoral
   immunity
4. Understand the principles of Self/non-self
   discrimination and its significance
5. Understand the concepts of immunological memory
6. Differentiate b/w adaptive versus innate immunity
7. Integrate the functions of innate and adaptive
   immunity

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Adaptive Immunity

• Specific host defenses that are mediated by B and
  T lymphocytes following exposure to antigens, and
  exhibit diversity and memory.

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Why adaptive immunity evolved

• Shortcomings of innate immunity
• Non-specific
• Similar pattern of response for all pathogens
• Poor regulation
• Control mechanisms are poor or lacking
• Poor amplification
• Response magnitude same for all insults
• Lack of self discrimination
• Harm to self results for lack of specificity
• Short duration
• No memory

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The enemies are different
BACTERIA -Clostridium difficile (causes
antibiotic-associated colitis diarrhea)
FUNGUS -Epidermophyton floccosum (causes
athletes foot)
PARASITE - Tapeworm
VIRUS- Polio
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.therefore responses must be tailored for
specific enemies.

• Successful immune response is a huge
  investment!! Hence you need to remake it
• ? Faster
• ? Larger
• ? More specific
• ? Less damaging to self

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Characteristics/Hallmarks of Adaptive Immunity
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Types of Adaptive Immunity

• Humoral Immunity
• Antibodies produced by B cells
• Cell-mediated Immunity
• T cells directly (cytotoxicity) or indirectly via
  cytokines

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Major Cells of Adaptive Immunity

• Lymphocytes
• B cells
• T cells
• T helper cells (Th)
• Cytotoxic T cells (Tc)
• Antigen presenting cells (APCs)
• Dendritic cells
• Macrophages
• B lymphocytes

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T Lymphocytes

• Mature in thymus
• T cell receptors
• MHC restriction
• Class I
• Nucleated cells
• Necessary for CD8 T cell activation
• Class II
• APCs
• Necessary for CD4 T cell activation

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T Lymphocytes (contd)

• Progeny cells
• T helper cells
• Bear CD4 molecule
• Are Class II restricted
• Stimulates B and T cells (helper function)
• T cytotoxic cells
• Bear CD8 molecule
• Are Class I restricted
• Further differentiates
• CTLs (killing function)
• Memory T cells

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B Lymphocytes

• Originate and mature in bone marrow
• B cell receptor is membrane bound antibody
• Ag binding triggers division and differentiation
• Progeny
• Plasma cells
• Memory B cells

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Antigen Presenting Cells (APC)

• Cells with potential to capture, process and
  present antigens to T cells
• APCs also supply second signal to T cells
  leading to their proper activation
  (Proliferation, Differentiation and Effector
  activities)
• Key to their function
• Expression of MHC I and II on surface
• Ag internalization and degradation
• Co-stimulatory activities

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Key Antigen Presenting Cells
Professional Antigen Presenting Cells

• Dendritic cells
• Macrophages
• B lymphocytes

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Antigen Processing (Dr Babiuk)

• Chopping up of complex proteins into peptides
  that are recognized by T cells
• Exogenous antigens Antigens that enter the body
  of the organism from the outside, e.g. through
  inhalation, ingestion, or injection
• Phagocytosis
• Degradation
• Ag peptide/MHC II recognized by helper (CD4) T
  cells
• Endogenous antigens Antigens that are produced
  from within the cell as part of normal cell
  metabolism or when the cell is infected by
  bacteria or viruses
• Viral or tumor induced
• Complexes with Class I
• Recognized by Cytotoxic (CD8) T cells

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Functions of APCs

1. T cell selection in the thymus (only DCs)
2. Trap and capture antigen in the periphery
3. Process antigens into peptides
4. Storage of antigens (antigen depot)
5. Transport antigens to peripheral lymphoid tissues
6. Present antigenic peptides to T cells
7. Co-stimulate T cells

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Antigen Recognition (Dr Marshall)

• Epitopes Motifs (conformational/primary
  sequences) on antigens that are recognized by B
  and T cell
• B cells recognize epitopes (conformational) alone
• T cells require MHC association (peptides)
• Major molecules involved in Ag recognition
• Membrane bound antibody (B cell receptor, BCR)
• T cell receptor (TCR)
• MHC I
• MHC II

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The Clonal Selection of Lymphocytes
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The Two-Signal Requirement for Lymphocyte
Activation

• Requirements Two signals
• Signal 1 specific recognition of antigen
• TCR-Peptide-MHC
• BCR-Native antigen
• Signal 2 Non-specific
• Microbial-induced molecules on/from APC
• Microbial molecule (LPS, CpG etc)
• Signal 1 alone leads of unresponsiveness
• Anergy, Deletion, Apoptosis

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Phases of Adaptive Immune Response
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MHC Restriction

• T cell receptors recognize antigenic peptide/MHC
  complexes
• CD4 T cells restricted by class II
• CD8 T cells restricted by class I

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Self/non-Self Discrimination

• Property of the adaptive immune system to mount
  specific/targeted responses to foreign antigens
  without responding to self
• Achieved by early and continuous presence of
  self-antigens
• Important for self tolerance and control of
  autoimmunity

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Danger vs non-Danger model

• Immune system does not discriminate b/w self and
  non-self
• Only interested in responding to dangers elicited
  via recognition of danger signals
• What makes an antigen dangerous??

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Key Differences b/w Self/non-Self and Danger
model

• Only foreign antigens can elicit immune response
  (self/nonself) vs.Even self Ag can elicit
  response if they become dangerous
• Keep looking for foreigners vs. ignore
  everybody and only respond if threatened
• Police vs Fire fighters

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T cell selection/education
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Shaping T Cell Repertoire by Positive and
Negative Selection
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T cell development Overview of the big picture

• 1. Developing T cells generate wide diversity of
  novel receptors
• 2. Each interacts with surrounding cells that
  express self MHC
• 3. Receptors on maturing T cells may
• - not bind MHC (are not positively selected)
• - bind very strongly (are not negatively
  selected to protect against autoimmunity)
• Cells that are both positively and negatively
  selected are exported to the periphery
• 99 of all maturing stem cells in the thymus
  die.

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B lymphocytes - development
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B cell development Overview
Stem cell ? B cell in Bone marrow (primary
organ) Two key goals for the system
Generate multiple Ag specific receptors (1 per
cell) Enzymes that join Ab gene components
together to get a functional Ab gene are error
prone introduction of random variability
Delete self reactive B cells generated by accident
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B cells also undergo selection

• Positive and negative selection in Bone marrow
• Selection is not MHC molecule-dependent
• Binding affinity and avidity more important

Is it really necessary for B cells to undergo
positive and negative selection?
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Self Tolerance

• Ability to remain tolerant to self while
  retaining the capacity to mount response to
  non-self.
• Self/non-self discrimination with in-built
  fail-safe mechanisms are key

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Significance of Self/non-self discrimination
(Tolerance)

1. Prevention of autoimmunity
2. Scarce resources are all directed against
   potential enemies

• What is the price for self/non-self
  discrimination?
• Why do we develop autoimmune diseases anyway?

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Cell-mediated Immunity

• Conferred via T lymphocyte activities (i.e.
  immunity can be transferred by T cells)
• Cell dependent
• Modulates humoral immunity
• Cytotoxic T cells

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CD4 T cells
CD4 T helper cell differentiation
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Key Differences b/w Th1 and Th2 cells
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Effector functions of Th1 and Th2 helper T cells
Th1
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Th17 Cells

• IL-17-producing CD4 T helper cells
• Secrete IL-17, IL17F, IL-21 and IL-22
• IL-17 and Il-21 receptor is ubiquitously
  expressed
• Differentiation factors TGF-? plus IL-6 or IL-21
• IL-23 is stabilization factor for Th17 cells
• Utilizes transcription factors STAT3, ROR?t and
  ROR?)
• Master regulator of inflammatory responses

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Differentiation of Th17 cells
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Take home review Natural and Inducible
Regulatory T cells Factors that
influence Regulatory T cell differentiation
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CD8 T cells
Mechanism of Cytotoxic T cell effector functions
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Humoral Immunity

• Conferred via serum (cell-free)
• Antibody dependent
• Antibody functions
• Enhanced elimination
• Neutralization
• C fixation/lysis

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B cells

• B cells produce antibodies (also known as
  immunoglobulins)
• B cell receptor Antibody
• Receptor is membrane bound (usually IgM, IgD)
• Unlike for T cells, the B cell receptor
• Recognize native (intact) protein
• Can be secreted, sometimes at high
  concentration.
• There are 5 main families (isotypes/classes) of
  Ab
• IgM, IgG, IgA, IgD, IgE

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Ab isotypes (Classes)

• IgM
• First produced in primary responses
• 2nd most common serum Ab
• Opsonization, activates complement,
  neutralizing Ab
• IgG
• Dominates memory (20) responses in serum
• Highest concentration in serum
• Opsonization, activates complement,
  neutralizing Ab
• Transplacental transfer hence important for
  fetal immunity and immunopathologies

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Ab isotypes (continued)

• IgA
• Major Ab at mucosal surfaces
• In colostrum, tears, GI and respiratory
  secretions
• Opsonization, activates complement,
  neutralizing Ab
• IgD
• Who knows?
• IgE
• Parasite defense mediate immediate type
  hypersensitivity reactions
• 10,000x lower levels than IgG, even in
  allergic individuals

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Effector mechanisms of humoral immunity

• Neutralization binding to toxins or pathogens
  block their interaction with target cell
  receptors
• Antibody-dependent cytolysis binding of Ab
  couples pathogen to a cell with capacity to
  destroy that pathogen
• Opsonization Ab-coated particles are easier and
  more palatable for phagocytes to ingest
• Complement activation Leads to release of
  inflammatory mediators, deposition of opsonins
  and direct lysis of microbes

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Neutralization of microbes and toxins by
antibodies
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Antibody-dependent cellular cytotoxicity (ADCC).
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Antibody-mediated opsonization and phagocytosis
of microbes.
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Antibody-mediated complement activities
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Immunologic Memory

1. Ability of the immune system to respond more
   rapidly and effectively to pathogens that have
   been encountered previously either by previous
   infection or by vaccination
2. This reflects the pre-existence of clonally
   expanded lymphocytes with specificity for that
   antigen.
3. Hallmark of adaptive immunity

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Schematic Representation of Memory Response (B
cell)
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Effectiveness of Memory

• More responder cells available
• Frequency higher than naïve cells
• More efficient antigen recognition/activation
• May not require costimulatory signals for
  activation
• Rapid and more effective migration to tissues and
  lymph nodes
• Express different homing/chemokine receptors
• More effective function
• Produce qualitatively and quantitatively more
  cytokines (T cells) or antibodies (B cells) than
  naïve cells
• Longer lasting

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Innate versus Adaptive immunity
Innate Adaptive
Receptors Primitive and broad Highly specific (T and B cell receptors)
Kinetics Fast (hours-days) Slow (days-wks)
Regulation /-
Amplification No (insignificant) Yes
Self/nonself discrimination /-
Duration Short (days) Long (months/yrs)
Memory -
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Integration of Innate And Adaptive Immunity
Courtesy Abbas and Litchman Basic Immunology
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Innate Immunity Shapes Adaptive Immunity

• Innate immune cells participate at both priming
  and effector phases of the adaptive immunity
• Macrophages and DCs present antigens to T cells
• IFN-? produced by NK cells can activate
  macrophages
• NK cells can directly lyse infected cells
• Innate immune response generates molecules that
  act as costimulatory (second) signals for T and B
  cell activation
• APCs express costimulatory molecules
• Production of cytokines (e.g. IL-1, IL-2, IL-4,
  IL-10, IL-12, TNF-?, IFN-?)

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Confused Then Ask Questions!!
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Summary

• Evolutionary need for adaptive immunity
• Self/non-self discrimination, specificity,
  amplification, regulation and memory
• T and B cells are mediators of adaptive immunity
• T cells cell-mediated immunity
• B cells humoral immunity
• Cells of innate immunity also participate (DCs,
  MØ, NK cells)
• Effector mechanisms of adaptive immunity
• Cell-mediated immunity direct cytotoxicity and
  production of cytokines control intracellular
  pathogens and tumors
• Humoral immnunity by antibodies via
  neutralization, ADCC, opsonization, complement
  activation
• Activation steps for T and B cells are different
• T cells specific recognition of peptide/MHC
  complex (signal 1) and costimulatory signals by
  APC (Signal 2)
• B cells recognize native proteins (signal 1).
  May/may not require signal 2 from CD4 Th cells
  (TD and TI antigens)
• Immunologic memory an important hallmark
• Faster and rapid response on a second antigen
  encounter
• Innate immune response shapes the adaptive
  immunity

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Recall Objectives

• Why adaptive immune system evolved
• Principles of Self/non-self discrimination and
  its significance
• MHC restriction
• Central and peripheral tolerance
• Key players of adaptive immunity
• Differences b/w cell-mediated and humoral
  immunity
• Principles of immunological memory
• Adaptive versus innate immunity

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Now you are an expert on adaptive immunity and
will answer my question in the exam