CellMediated Immune Response

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Chapter 5

• Cell-Mediated Immune Response
• Part 2

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Clonal Expansion

• Takes 1-2 days for proliferation to start for Ag
  specific lymphocytes
• CD8 T-cells
• before infection 1 in 105 to 106 is specific for
  Ag
• after viral infections, 10-20 of all lymphocytes
  in lymphoid organs may be specific for that
  virus, an increase of 10,000 fold increase in
  Ag-specific for that virus
• CD4 T-cells
• see only a 100 to 1000 fold increase
• Difference may be due to difference in function
• CD8 are effector cells themselves kill
  infected cells
• CD4 effector cells secrete cytokines that
  activated other effector cells small amount of
  cytokine needed to cause action

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Clonal Expansion Surprises

• Expansion is in specific T-cells for Ag but not
  other T-cells that do not recognize Ag
• Even though microbe may be complex usually lt5
  immunodominant peptides used for expansion

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Differentiation of Naïve Cells

• Need to become effector cells to eradicate the
  infection
• Changes in gene expression cytokines in CD4
  and CD8 cells or cytolytic proteins in CD8 CTLs
• Appear in 3-4 days and leave lymphoid tissue to
  move to the site of infection
• CD4 and CD8 perform different functions so
  differentiation is also different

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CD4 Effector Cells

• Respond to Ag by making surface molecules and
  cytokines to mainly activate macrophages and
  B-cells
• CD40L (ligand) is the most important expressed
  after Ag recognition and costimulation
• binds to CD40 on macrophages, B-cells or
  dendritic cells
• binding activates macrophages and B-cells
• binding of dendritic cells cause expression of
  co-stimulators and T-cell activating cytokines
• interaction produces T-cells activating cytokine
  causing positive feedback on APC-induced positive
  cell activation

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CD4 Effector Function
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Helper T-Cell Subsets

• Different types of CD4 effector T-cells with
  distinct function
• Subsets of effector cells produce distinct sets
  of cytokines that perform different functions
• TH1 cells and TH2

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TH1 Cells

• Produce IFN?, called that because it interfered
  with viral infections
• potent activator of macrophages
• stimulate Ab isotypes that promote phagocytosis
  by binding the FC receptor
• activate compliment and cause phagocytosis by
  binding phagocyte complement receptors
• Phagocyte-mediated ingestion and killing of
  microbes
• Stimulates the expression of MHCII and B7
  costimulators on APC especially macrophages
  which display Ags then amplify T-cells responses

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TH2 Cells

• Produce IL-4 stimulates IgE Ab production
• Produce IL-5 which activates eosinophils
• Stimulates phagocyte-independent, eosinophil
  mediated immunity
• usually against helminthic infections (parasites)
• IL-4, IL-10 or IL-13 inhibit macrophage
  activation and suppress TH1 cell-mediated
  response (return to in Chapter 6)

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TH1 and TH2

• Cell-mediated immune response to microbes is
  balanced between activation of TH1 and TH2

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KNOW ITEMS!!!
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Differentiation of TH1 and TH2

• Not a random process but is regulated by stimuli
  that naïve CD4 T-cell receive when they
  encounter microbial Ag
• Macrophage and dendritic cells make IL-12 in
  response to many bacteria and viruses
• T-cell encounters Ag on APC also get exposed to
  IL-12
• promotes TH1 proliferation and release of IFN? to
  activate macrophages
• innate immune response of IL-12 by APC will drive
  the formation of TH1helper cells (adaptive immune
  response)
• If there is no IL-12 as in helminth infection,
  then the T-cell will secrete IL-4 and get a TH2
  response
• may also be influenced by type of dendritic cell
  that can dictate the TH response
• specialization of immune response
• Differentiation of one type will inhibit the
  development of the other one to make most
  effective response to microbe

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TH1 or TH2
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CD8 T-Cell Differentiation

• Activated by Ag and costimulators
• Differentiate into CTLs that are able to kill
  infected cells expressing the Ag
• secrete proteins that make pores in membranes of
  infected cells and induce DNA and apoptotic death
  (Chapter 6)

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Development of Memory

• Long-lived T-cells even after infection is
  eradicated and innate immune reaction to
  infection pathogen is over
• Subsets of memory not known how it happens
• in mucosal barrier is effector memory rapid
  effector functions
• in lymphoid tissue is central memory cells
  populate lymphoid tissue and responsible for
  rapid clonal expansion after re-exposure
• Memory T-cells do not produce cytokine or kill
  cells
• rapidly do so when encounter Ag again

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Decline of the Immune Response

• Clonal expansion and differentiation occurs in
  peripheral lymphoid organs
• As infection is cleared and stimuli for lymph
  activation disappears the response will decline
• Cells are deprived of survival factors die by
  apoptosis 1-2 wks after infection cleared
• only thing that remains is the memory T-cells

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Obstacles to Overcome

• Naïve cells must find Ag APC capture and take
  to area where naïve T-cells recirculate
• Correct type of T-cell must interact with Ag from
  extra- or intracellular compartments have MHC
  molecules for each type
• T-cells must interact with APC for long enough
  adhesion molecules stabilize the interaction
• T-cells should respond to microbial pathogens and
  not harmless proteins because T-cells must have
  costimulator molecules from APC
• Ag recognition by a small number of T-cell and
  converted to a response large enough to be
  effective amplification mechanism

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Biochemical Pathways

• T-cells express proteins involved in
  proliferation, differentiation and effector
  function
• naïve T-cells have low levels of expression of
  these proteins
• Ag recognition triggers new transcription and
  translation
• Pathways link Ag recognition with T-cell
  responses consist of activation enzymes, recruit
  adaptor proteins and production of transcription
  factors

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Signal Transduction

• Cross-linking of TCR at or near TCR complex
• orderly redistribution of other proteins on
  APC/T-cell
• bring in adhesion and signaling molecules
  important for optimal induction of activating
  signals in T-cells
• Cellcell link is called the immunological synapse

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Signal Transduction
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Start of Signal

• Cluster of CD4 or CD8 co-receptors activate Tyr
  protein kinase called Lck that is attached to the
  C end of co-receptor
• T-cell signaling proteins (CD3 and ? chain) have
  immunoreceptor Tyr-based activation motifs
  (ITAMs) critical for signaling
• Activated Lck phosphorylates ITAMs and becomes a
  docking site for a Tyr kinase called ZAP-70 (zeta
  chain associated protein of 70kD) also
  phosphorylated by Lck, which then phosphorylates
  various adapter molecules
• Ca-NFATpathways Ras/Rac-MAP kinase pathways

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Calcium-NFAT Pathway

• Nuclear factor of activated T-cells (NFAT)
  transcription factor
• dependent on Ca2 ions
• ZAP-70 phosphorylates phospholipase C (PLC) which
  cleaves plasma membrane phospholipid
  inositol-phospholipids making the by-product
  inositol 1,4,5-triphosphate (IP3) which
  stimulates Ca2 release from intracellular stores
• As this occurs, TCR complex signals cause
  extracellular Ca2 influx
• Cytoplasmic Ca2 binds calmodulin and this
  complex activates a phosphatase calcineurin
  removes a phosphate from inactive NFAT to make it
  active, moves to nucleus and then binds to the
  genes for IL-2 and components of IL-2 receptor
• Cyclosporin binds and inhibits calcineurin to
  inhibit T-cell activation against the organ
  transplant (Chapter 10)

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Ras/Rac MAP Kinase Pathway

• Includes GTP binding proteins Ras and Rac that
  are biologically active when bound to GTP
• requires several adapter proteins and a cascade
  of enzymes that eventually activates one of a
  family of mitogen-activated protein (MAP) kinase
• Initiated by ZAP-70 dependent phosphorylation and
  accumulation of adapter proteins at membrane
• recruit Ras or Rac and activate by exchange of
  GDP for GTP
• Ras-GTP and Rac-GTP initiate different pathways
  leading to distinct MAP kinases
• extracellular signal regulated kinase (ERK) and
  c-Jun-amino(N) terminal kinase (JNK) promote cFos
  expression and phosphorylation of c-Jun that
  creates the transcription factor AP-1 (activating
  protein-1) that enhances several T-cells genes

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Other Biochemical Events

• Actibation of Ser-Thr kinase activates protein
  kinase C (PKC) and activation of NF?B
  (transcription factor)
• PKC is activated by diacylglycerol (2nd
  by-product of PLC activity)
• PKC? (isoform of PKC) linked to activation of
  NF?B
• NF?B is usually inactive in naive T-cells because
  it is linked to I?B (inhibitor) which is released
  on activation of TCR NF?B moves to nucleus and
  activates transcription of several genes
• Transcription factors lead to production of
  cytokines, cytokine receptors, cell cycle
  inducers and effector molecules (such as CD40L)

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B7 and CD28

• B7 binding to CD28 is necessary for full T-cell
  response but signals from CD28 are not well
  understood
• may amplify TCR signals
• may initiate own set of signals