Viral Immunology

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Viral Immunology

• Innate Immunity
• Type I interferons
• NK Cells
• Specific Immunity
• B cell- and T cell-mediated

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Days after viral infection
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Action of IFN-?/?

• Activated by viral double-stranded RNA
• Binds to TOLL receptors on cells (TLR-3)
• State of resistance to viral replication in all
  cells
• Induce synthesis and expression of MHC class I
  molecules
• NK cell cytotoxicity is activated whenever the
  IFN-?/? cytokines are induced
• Increase expression of IL-12 receptors
• Stimulates the development of TH1 cells

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NK Cells

• Phylogenetically like primitive CTLs that lack
  TCR for antigen recognition
• Develop in the bone marrow from the common
  lymphoid progenitor cell and circulate in the
  blood
• Antiviral Functions of NK cells
• Cytotoxicity activated by
• Arenaviruses (lymphocytic choriomeningitis
  virus),
• Herpesviruses (herpes simplex virus)
• Orthomyxoviruses (influenza virus)
• Picornaviruses (coxsackie virus)
• Protozoan parasite Leishmania
• Bacterium Listeria moncytogenes

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NK activity to viruses in Humans

• Striking correlation between poor or no NK cell
  function and susceptibility to viruses
• Absence of NK cells associated with papilloma
  viral infections in a female patient
• Important in early defense
• Shown by using mice lacking T cells- Low NK cell
  cytotoxic activity increased sensitivity
  to murine cytomegalovirus

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NK Cell Activity

• Develop in the bone marrow from the CLP
• Have cytoplasmic granules
• Can kill certain tumor cell lines in vitro
• Mechanism of killing same as that used by
  cytotoxic T cells

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Cytotoxicity

• NK cells are activated by recognition of three
  types of targets
• 1. Antibody-coated cells (antibody-dependent
  cellular cytotoxicity, or ADCC)
• 2. Cells infected by viruses
• 3. Cells lacking MHC class I molecules (altered
  self)

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Activating ReceptorsActivating receptors trigger
killing by NK cell.

• 1. Fc Receptor-Fc?RIII (CD16)
• 2. Several classes of receptor- members of the
  Ig-like and C-type lectin family of proteins.
• Integrate signals from two types of surface
  receptors

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Cell Surface Receptors that Inhibit NK killing
activity

• Fall into two families
• C-type lectins--gt NK Receptor complex
• Ig-like domains Killer cell Immunoglobulin-like
  receptors or KIRS
• Mice lack KIR cluster

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NK receptors, contd

• Activating and inhibitory receptors fall into two
  broad structural types
• 1. NK cell lectin-like receptor complex (NKG)
• Killer cell-immunoglobulin-like receptors (KIR
  or NKp)
• Most receptors are expressed by subpopulations of
  NK cells. Individual NK cells express different
  combinations of receptors providing a
  heterogeneous response to pathogens

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Figure 8-29
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NK receptors, contd

• Same families contain both activating and
  inhibitory receptors

Human chromosomes
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Activating Receptors

• 1. Fc Receptor-Fc?RIII (CD16)
• infected cells can express viral proteins on
  surface of infected cells-recognized by
  Antibodies (Ab)
• Recognizes IgG1 and IgG3 Abs

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Antibody-dependent cell mediated cytotoxicity
(ADCC)
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Activation of NK cells through Fc?RIII
ITAM- containing ????????????????
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Activation of NK cells through Fc?RIII
ITAM- containing ????????????????
PLC?
IP3
JNK
p38
ERK
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Cell Surface Receptors that Activate NK killing

• In mice
• Ly49H interacts with MCMV encoded MHC-like
  protein m157
• In humans
• NKp46 interacts with sialic acid-binding
  influenza and parainfluenza viral receptors
• 2B4, cell surface receptor of the CD2 family

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B-EBV
CD48
NKp46-L
Lysis
NKp46
2B4
P


Ligand for 2B4 is CD48, a GPI-linked protein
whose expression is upregulated on
EBV-transformed B cells.

Activation
NK cell
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Activating receptors of NK cells
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Inhibitory receptors of NK cells
Inhibitory receptor engaged
NK cell not activated
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Ligands for NK-cell receptors

• Cell surface proteins, expression is altered in
  response to infection
• Many MHC class I or MHC class I-like molecules
• NKG2D binds to MIC-A and MIC-B, class I-like
  molecules that do not bind ?2-microglobulin

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Ligands for NK-cell receptors

• MIC-A and MIC-B expressed at high levels when an
  epithelial cell becomes infected, damaged, or
  cancerous
• Lead to release of cytotoxic granules and
  cytokines

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Inhibitory receptors of NK cells
Inhibitory receptor engaged
NK cell not activated
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Function of Inhibitory Receptors
Inhibitory receptor (Kir/CD94/Ly49)
Activating receptor (NKp46)
Activated tyrosine Kinases
SHIP-1
Tyrosine phosphorylated substrates
Activation
Killing
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Figure 8-30
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KIR family of inhibitory and stimulatory NK-cell
receptors

• All allotypes of HLA-C are KIR ligands.
• Not all allotypes of HLA-A and HLA-B are KIR
  ligands.
• Suggests that HLA-C particularly evolved towards
  control of the NK-cell response

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Figure 8-32
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Figure 8-33 part 2 of 2
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Cytokines and NK cell function

• Activated NK cells produce IFN-? to a subset of
  viruses including MCMV (murine cytomegalovirus)
  and influenza virus
• Produce TNF-?
• Function regulated by other cytokines including
• IFN-???, IL-12, IL-1?, IL1?, IL-6, IL-10, TGF-?,
  IL-15 and IL-18

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IL-12

• Induces NK cell IFN-? production
• Macrophage activation--gtkilling of microbes
• Contributes to preferential development of T
  helper type I cells (IFN-?-producing cells)
• Enhances the cytolytic functions of activated T
  lymphocytes (CTLs).

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Antibody Response

• Immunological Discrimination B cells
• Neutralizing antibody responses against viruses
  prevent re-infection against the most important
  acute cytopathic human viruses, polio, measles,
  mumps
• In several model infections, the absence of B
  cells or CD4 T helper cells (or both) results in
  impaired virus clearance and death
• Vesicular stomatitis virus (VSV) in mice

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Mucosal Immunity

• MALT
• Collection of lymphocytes and accessory cells
• Organized into discrete lymphoid follicles
• Host defense at mucosal membranes
• 2-3X more IgA-secreting cells in mucosa than in
  systemic lymphoid tissue

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Ways that IgA neutralizes viruses

• Inhibit attachment and uptake
• Bulky Ab can hinder adhesion
• Bind to flagella or to the toxin
• Agglutinate viruses

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IgA Antibodies

• Largely secreted as dimers by plasma cells that
  underlie the mucosal epithelium

J-chain
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Days after viral infection
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T cell-mediated response to viruses

• Central tolerance. Not all self-encoded antigens
  expressed in thymus
• Peripheral tolerance, anergy, vetoing,
  suppression and deletion, 2 signal model

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T cell-mediated response to viruses
Principal mediator is the CTL or the activated
CD8 T cell
TCR/CD3 CD8
CTL
IFN-? and TNF-?
MHC-peptide
Viral infected Epithelial cell
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Rules for the induction of mature T cells

• After burst, antigens are taken up by mobile APC
• Or APC are infected
• Full differentiation of CD8 CTLs requires
  cytokines produced by CD4 helper cells (Th1
  cells)
• CTL detected in infected tissue 5-7 days after
  exposure

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Mechanisms of CTL-mediated lysis

• CTLs target cells by programming them to undergo
  apoptosis

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Mechanisms of CTL-mediated lysis

• CTL killing is antigen-specific. MHC-specific
• Requires cell contact. Kill only those cells to
  which they attach and bystander cells are not
  injured
• CTLs are not injured during lysis of target cells
• Each CTL is capable of sequentially killing
  multiple target cells
• Lysis of infection cells
• Stimulation of intracellular enzymes that degrade
  viral genomes

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CTL-mediated lysis

• Step 1. Recognition of antigen and conjugate
  formation, TCR,CD8, CD2, and LFA-1 on T cells
• Molecules on target cells
• Class I MHC
• LFA-3 (ligand for CD2)
• ICAM-1 or ICAM-2 (ligands for LFA-1)

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Activation of the CTL

• Step 2. Cross-linking or clustering of TCRCD3
  complexes on CTLs
• Step 3. Delivery of a lethal hit by the
  activated CTL

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Killing of target cell

• Ca-dependent release of lytic granules
• Contain perforin, Granzymes, Granulysin
• Perforin makes holes through which granzymes can
  move
• Is a protease that cleaves and activates Caspase 3

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Lethal Hit
Perforin creates osmotic defects
Ca2 H2O
Granule exocytosis a perforin-mediated lysis
Lysis of targets
CD8 CTL
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Granzyme-mediated killing
Granule exocytosis entry of granzymes activation
of caspases apoptosis of target
Granzymes enter through perforin holes activation
of caspases
Apoptosis and oxmotic lysis of cell
CD8 CTL
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Fas-mediated killing
FasL-Fas-mediated target cell apoptosis
Apoptosis of target
Fas-L
Fas
CD8 CTL
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4. Release of the CTL

• Decrease in the affinity of accessory molecules
  for their ligands

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Non-cytolytic viruses

• CTLs may be responsible for tissue injury
• i.e. LCMV infection in mice
• During some generalized infections with
  non-cytopathic viruses (LCMV) in mice and
  Hepatitis B or C in humans, all antigen-specific
  T cells may be deleted

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Bystander and CTLs are not killed
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Role of TH cells in viral clearance
CD4 helper T cell
cytokines
CD40L
CD8 T cell
MHC Class II
CD40
Activation
MHC Class I
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Contribution of cytokines in viral clearance

• Secretion of cytokines IFN-? and TNF-?
• CTLs are not likely to eradicate infections that
  affect large numbers of cells
• IFN-? and TNF-? can purge the virus from
  thousands of additional infected cells
  noncytopathically

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Mechanism for degrading viral RNA
IFN-?/TNF-?
AAAAAAAAAAAAA
Pre-formed HBV RNA
endonuclease
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Immunological Memory

• Poorly understood
• Mediated by increased antibody levels
• Necessary, but also sufficient, to provide
  protection against cytopathic virus infections
• Transfer of memory antibodies from mother to
  offspring

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• CTL Memory
• Lack of transfer of T cell memory from mother to
  child, T cell receptor expression requires a
  learning process
• CTLs are induced and disappear rapidly
• CD8 T cell frequencies are low in a normal host

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Evasion of Immune Mechanisms

• Viruses capable of great antigenic variation
• Infect cells of the immune system, impairing
  their function
• Mimic cellular genes
• EBV protein BCRF1-an analog of cellular IL-10,
  similar to that of human IL-10