Innate Immunity (part II) and Antigen Recognition by Adaptive Immunity

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Innate Immunity (part II) and Antigen
Recognition by Adaptive Immunity
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Innate Immunity against viruses

• Anti-viral immunity has 2 roles
• Blocking infection (antibodies, complement, etc.)
• Blocking viral replication (interferon, killing
  infected cells, anti-nucleic acid mechanisms)
• Many viruses evolve extremely rapidly, great
  challenge for innate immunity
• This problem is solved in part by a) targeting
  molecules that viruses have a hard time changing
  (dsRNA especially), and b) having multiple
  mechanisms, making it harder for a virus to evade
  all of them
• Viruses are amazingly good at evasion of immune
  defenses, but often the most lethal viruses are
  those that are not well adapted to their host
  (their goal is to grow and be transmitted to
  others, not to kill)

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INTERFERON cytokine critical for defense
against virus infections

• Virus-infected cell produces interferon to act on
  neighboring cells
• Uninfected cells respond to become refractory to
  viral growth (anti-viral state)

infected cell
Interferon-a??
type 1 interferon interferon a/b type 2
interferon interferon ?
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Induction of interferon by dsRNA in cytoplasm and
RLRs
RLRs
Mda5
IRF interferon regulatory-factor (family of
transcription regulators)
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Anti-viral effects of interferon a/b
Many other responses, less well understood
Note PKR and OligoA synthetase induced
in anti-viral state, but only have enzymatic
activity following virus infection
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IntracellularTLRs recognize pathogen nucleic
acids (TLR3, 7, 8, 9)Induce type 1 interferon
production, especially by specialized immune cell
(plasmacytoid dendritic cell)Location inside
cell aids discrimination from host nucleic acids
(Discrimination may fail in SLE)
Second mechanism of interferon production upon
virus infection Toll-like receptors recognize
viral nucleic acid in endosomes
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Natural Killer (NK) Cells innate lymphocytes
that protect against intracellular infections
Abbas and Lichtman Fig. 2-10
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NK cells are regulated by the balance between
activating and inhibitory receptors
NK cell has activating receptors and inhibitory
receptors killing believed to occur if
activating receptors dominate (relative numbers
of ligands on target cell)
stress-induced proteins (red)
stressed cell
healthy cell
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Summary of Innate Immunity to Viruses

• Interferon (type 1) is absolutely critical for
  anti-virus immune defense it is produced in two
  ways
• by infected cells via RLRs or similar sensors of
  DNA for DNA viruses
• by immune cells via nucleic acid-recognizing TLRs
• Interferon induces anti-viral state, which is
  fully engaged by recognition of dsRNA in
  cytoplasm and inhibits virus replication (also
  promotes adaptive immunity)
• Killing of infected cells is also performed by
  natural killer cells recognizing stress-induced
  molecules or loss of MHC class I molecule
  expression and by cytotoxic T cells which
  recognize virus antigens expressed by infected
  cells (MHC I)

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Pathogens and Innate Immunity

• Particular pathogens often have evolved ways to
  evade at least some elements of innate immunity
• Highly successful pathogens may also have
  mechanisms for evading adaptive immunity
• On the pathogen side, molecules the pathogen uses
  to evade immunity are among the virulence
  factors of that pathogen

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Part II antigen recognition

• Antigen recognition molecules structure and
  function
• B cell antigen receptors and T cell antigen
  receptors Role in clonal selection
• Classes of antibodies and their structures
• Monoclonal antibodies and their uses in medicine
  (lab tests, therapies)

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2 Types of Antigen Recognition

• 1) Antibodies
• Defense against extracellular microbes and
  viruses by soluble binding molecules linked to
  effector functions
• B cells also make a membrane-bound form of
  immunoglobulin that serves as their antigen
  receptor
• 2) T cell antigen receptor
• Peptide recognition mechanism for detecting
  antigens inside cells that are displayed on cell
  surface as peptides bound to MHC molecules

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The B cell antigen receptor is membrane Ig
signaling chains

• Antigen binding to more than one membrane Ig
  molecule triggers signaling reactions tells B
  cell that antigen is present, induces activation
  (clonal selection)
• Iga/Igb cytoplasmic domains have signaling
  function (ITAM sequence)
• Differential RNA splicing controls secreted vs.
  transmembrane forms of Ig heavy chain

Iga/Igb
Signal transduction
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The TCR also has ligand-binding chains
signaling chains
Figure 4-1 Abbas Lichtman
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Antibodies

• immunoglobulins
• Secreted by B lymphocytes (plasma cells)
• Great diversity and specificity gt109 different
  antibodies can distinguish between very similar
  molecules
• Block infectivity of viruses, action of toxins
  (neutralization)
• Tag particles for clearance/destruction (activate
  complement, opsonizefor phagocytosis)
• Protect against re-infection (vaccines)

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Antibody Structure
2 identical heavy chains and 2 identical light
chains Both made of repeating structural unit
The Ig domain
Light chain
k or l
Ig domain 110 amino acids globular domain used
in many proteins
Heavy chains
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Antibody Structure
Domains at ends of heavy chain and light chain
are highly variable and responsible for binding
antigen
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Variability in antibodies is clustered in the
loops in the variable domains of the heavy and
light chains (green) these regions are
responsible for binding to antigen (hypervariable
regions complementarity-determining regions)
Fab, variable domains in color
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Similarities between Ig and TCR

• Both are composed of Ig domains
• Both have two variable subunits (H L for Ig
  TCR a and TCR b there are also gd T cells)
• Both have great diversity and exquisite
  specificity
• Both recognize antigen via hypervariable loops at
  the ends of the variable domains
• Both couple antigen recognition to lymphocyte
  activation (clonal selection) via signaling
  chains with very similar signaling mechanisms
  (ITAM sequences)
• (Only Ig is secreted and only Ig has class
  switching to change the constant domains)

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5 Different Antibody Classes
Heavy chains are named by the greek letter
corresponding to the type of antibody (m, g, d,
a, e) All classes can have k or l light chains
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Antibody Classes II Distinctive features

• IgM
• First antibody produced in immune response
  multimeric structure adds to avidity for
  antigen
• Antigen receptor of naïve B cells
• IgD
• Used primarily as antigen receptor, little
  secreted
• IgG
• Major antibody of serum and extravascular tissues
• Crosses placenta to provide protection in utero
• IgA
• Major antibody of secretions (mucus, saliva,
  milk, etc.)
• IgE
• Important for immune defense against helminths
  allergy and asthma

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Affinity and Avidity

• Affinity the strength of binding between a
  single binding site and a single ligand
• Avidity the strength of binding between a
  molecule and a complex ligand. If there are
  multiple binding sites then the avidity may be
  increased by increasing the number of binding
  sites or by increasing the affinity of those
  binding sites

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Affinity and Avidity II

• IgM is produced early in an immune response when
  the affinity for antigen often is low as an
  immune response continues, antibody affinity is
  improved, this is combined by class switching
  to the use of smaller molecules (IgG, IgE and
  IgA). The increased affinity compensates for the
  decrease in number of binding sites in
  maintaining the overall avidity for antigen

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Antibody Classes II Distinctive features

• IgM
• First antibody produced in immune response
  multimeric structure adds to avidity for
  antigen
• Antigen receptor of naïve B cells
• IgD
• Used primarily as antigen receptor, little
  secreted
• IgG
• Major antibody of serum and lymph
• Crosses placenta to provide protection in utero
• IgA
• Major antibody of secretions (mucus, saliva,
  milk, etc.)
• IgE
• Important for immune defense against helminths
  allergy and asthma

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Fc receptors

• FcR have two functions directing antibodies to
  correct locations in body and promoting effector
  function
• Polymeric Ig receptor is a transport receptor for
  polymeric IgA and transports it across epithelial
  cells
• FcRn is a transport receptor for IgG
• -Transports IgG across placenta
• -Responsible for long half-life of IgG in the
  blood

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Antibodies and medicine

• Vaccination works by inducing production of
  protective antibodies
• Antibodies from immunized or pooled donors
  (IVIG) can provide passive immunity (used for
  tetanus,snake bites, etc. to treat
  immunodeficiencies)
• Antibodies are often used to diagnose infectious
  diseases (e.g., presence of antibodies in
  patients blood), determine bloodtype, diagnose
  type of cancer, etc.
• Increasingly, antibodies are used to treat
  diseases (cancer, autoimmune disease, etc.)
  advantage of monoclonal antibodies

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Monoclonal Antibodies

• Normal antibodies are polyclonal, as they are
  mixtures of antibodies made by several different
  clones of B cells
• Monoclonal antibodies Single antibody (all same
  H and L chains) more reliable, consistent can
  be produced in unlimited quantities
• Made by fusion of B cells to a transformed cell
  line of the plasma cell type and selection for
  hybridomas that produce antibody with the
  desired properties

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Use of antibodies in lab tests
immunoprecipitate-based tests
(Polyclonal antibodies)
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Use of antibodies in lab tests RIA and
hemagglutination
Hemagglutination assay test blood type, etc.
Radioimmunoassay (RIA) measure hormone levels by
displacement of binding of a radioactive hormone
standard with hormone in a blood sample (etc.)
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Use of antibodies in lab tests ELISA
Enyzyme-linked immunosorbant assay (ELISA)
measure levels of antibody or antigen, depending
on assay design (sandwich ELISA is shown)
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Use of antibodies in lab tests flow cytometry
Flow cytometry measure the amount of a protein
on the surface (or inside) individual cells
measure the numbers of particular types of cells
in blood (etc.)
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Monoclonal antibodies used in medicine
Standardized, unlimited reagents for diagnosis or
therapy (human antibodies or humanized
antibodies can be made)
The list of monoclonal antibodies FDA-approved
for therapy is increasing by several per
year Names fully human -mumab humanized
-zumab chimeric -ximab murine
-omab
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Anti-TNF therapeutics
TNFR2
Adalimumab (Humira) etc.
Infliximab, etc. anti-TNF monoclonal antibodies
(more coming) Etanercept fusion between TNF
receptor extracellular domain and Fc part of
IgG1, which greatly extends half-life in
serum These agents are used to treat Rheumatoid
arthritis, Crohns disease, etc.