Effector Mechanisms of CellMediated Immunity

1
Chapter 6

• Effector Mechanisms of Cell-Mediated Immunity
• Eradication of Intracellular Microbes

2
Effector Phase of CMI

• Carried out by T-cells
• NOT antibodies that eradicate intracellular
  microbes
• Phases include
• activation
• proliferate
• differentiate
• elimination

3
Types of CMI

• 2 types based on type of intracellular microbes
• CD4 T-cells activate phagocytes to destroy
  microbes in vesicles of phagocytes
• CD8 T-cells kill cells containing microbes or
  microbial proteins in the cytoplasm
• Occasionally overlap in function but
  fundamentally different mechanism

4
Cell-Mediated Immunity
5
Intracellular Microbes

• Many bacteria and some protozoans live in
  phagocytic vesicles
• Viruses that infect phagocytic and nonphagocytic
  cells and live in the cytoplasm
• Naïve T-cell recognizes Ag, differentiate and
  move to site of infection
• Phagocytes ingest microbes display peptides on
  appropriate MHC, Ag recognition activates to
  perform function

6
2 Stages of Ag Recognition

• As naïve in lymph node/spleen to become effector
  cell
• As an effector it will recognize same Ag anywhere
  in the body and eliminate microbe

7
Migration of Effector T-Cells

• Movement to the sites of infection
• Effector T-cells have many adhesion molecules
  that bind ligand on endothelium on exposure to
  microbe
• Chemoattractants cytokines are produced at the
  infection site
• Profile of adhesion molecules change in response
  to differentiation of effector cells
• increase in expression of adhesion molecules bind
  to microbe or cytokine stimulated endothelium in
  tissues

8
Movement of Naïve / Effector T-Cells

• Innate immunity responds to infection by
  responding to secretion of cytokines by
  macrophages
• TNF and IL-1 act on endothelium cells on small
  blood vessels adjacent to infection
• stimulates endothelium to make more E- and
  P-selectin and integrins ICAM-1 (intracellular
  adhesion molecule) and VCAM-1 (vascular cell
  adhesion molecule) that bind LFA-1 (leukocyte
  function-associated Ag) and VLA-4 (very-late
  activation molecules) respectively
• Lymphocyte binds weakly and roll along
  endothelium
• comes across ligand and binds tightly and prepare
  to leave the blood vessel similar to monocytes
  ad neutrophils (Chapter 2)
• Also lose expression of L-selectin to keep active
  T-cells out of lymph nodes (L-selectin mediates
  naïve T-cells to move into lymph node)
• Activated T-cells on endothelium macrophages
  and endothelial cells produce another set of
  cytokines called chemokines

9
(No Transcript)
10
Glycoproteins on Cell Surface

• Ligands for E- and P-selectins and high-affinity
  integrins LFA-1 and VLA-4

11
Chemokines

• Attract and stimulate motility of leukocytes
• High local concentration bound to cell surface
  proteoglycans
• Concentration gradient created at extravascular
  site in response to infectious agents
• concentration gradient high at site but lower at
  blood vessel
• draws T-cells through vessel wall to site of
  infection follows low to higher concentration

12
Homing Effector T-Cells to Site of Infection

• Independent of Ag recognition
• dependent on chemokines and adhesions molecules
• All T-cells that can recognize microbial Ag will
  leave the blood vessel to these responses
• maximizes ability for specific recognition and
  elimination
• probably will undergo another activation phase to
  stay in contact and do its job
• increase in expression and binding affinity of
  VLA integrins on T-cells, binds to extracellular
  matrix and stays close to do job
• No recognition no 2nd activation go through
  lymph to lymph node and back to peripheral
  circulation to home to another site
• differentiated T-cells do their job away from the
  peripheral lymph tissue not as dependent on
  co-stimulation as naïve T-cell

13
Migration
14
Effector Function CD4 T-Cells

• Cell-mediated immunity
• Intracellular bacterial infections found to be
  able to transfer with T-cells but not serum Ab
• specificity due to T-cell but activated by
  macrophage

15
T-Cell Response
A previously exposed T-cells decrease number of
bacteria in the spleen, but non-immune do not B
Antibodies to bacteria could not reduce number of
bacteria in spleen C in a test tube, activated
macrophages (T-cells turned on) could kill many
bacteria but T-cells and resting macrophages
could not
16
CD4 T-Cell TH1

• Activation of macrophages with phagocytosed
  microbes to increase the microbiocidal activity
  to kill the microbe
• can actually get by injecting the protein into
  the person with immunity
• Delayed-Type Hypersensitivity (DTH) happens 24-48
  hours after the challenge
• delayed because effector T-cell must home to the
  site of the protein

17
DTH

• T-cells and monocytes infiltrate the site
• edema, fibrin deposition due to the increased
  vascular permeability in response to the T-cell
  cytokines and tissue damage by macrophage
  activation by T-cells
• Used diagnostically to determine if previously
  exposed to antigen
• PPD to test for TB

18
Elimination of Phagocytosed Microbes

• TH1 subset recognizes macrophage-associated Ag
  activate macrophage by CD40LCD40 interaction
• Secrete the macrophage-activating cytokine - IFN?

19
Macrophage Activation

• Phagosome with the microbes and the lysosome fuse
  to make the phagolysosome that causes the
  processing of proteins to be placed in the MHC II
  
• MHC II interacts with CD4 T-cell causing release
  of IFN?, expresses CD40L which binds to CD40
  receptor on macrophage that initiates the
  biochemical pathways to signal transduction to
  make transcription factors
• interferon can also interact with the macrophage
  IFN receptor
• Gene expression turned on for lysosomal proteases
  and enzymes leads to microbicidal reactive
  oxygen intermediates and NO

20
Macrophage Activation

• CD40CD40L interaction is best because of contact
  with T-cell macrophage is also acting as Ag
  presenting cell
• IFN? enhances and amplifies response
  bidirectional between innate and adaptive immune
  response

21
macrophage microbe (innate)
activates phagocytes and kills microbe
IL-12
increase IFN
INF? when encounters Ag on macrophage
stimulate naïve T-cell(CD4) to TH1
cell (adaptive)
22
CD4 T-Cell Additional Functions

• Ag stimulated CD4 T-cell secretes TNF act on
  vascular endothelium to increase adhesion
  molecules and production of chemokines
• recruits more T-cells, neutrophils, monocytes to
  site of infection enhance phagocytes to help
  clear infection
• T-cell stimulated cellular infiltration
  inflammation
• Helps CD8 T-cells differentiate to CTLs and
  B-cells differentiate into Ab-producing
• Inflammation part of DTH sensitivity and innate
  immune response to enhance T-cell response

23
CD8 T-Cell

• MHC I (cytosolic proteins, may have passed out of
  the vesicle) can activate macrophage to kill
  intracellular microbes
• Similar to CD4 T-cells
• CD40L and IFN? activation
• not useful for viruses which replicate in
  cytoplasm only mostly because of vesicles is
  how many macrophages are activated

24
Elimination of Microbes

• Done by activated macrophages (Fig 6-7)
• Induces expression of enzymes that catalyze
  production of microbiocidal substances in
  phagosomes and phagolysosomes
• All three activated in the innate immune response
  with macrophages to microbes
• reactive O2 intermediates (ROI)
• NO
• proteolytic enzyme
• TH1 are also potent activators of macrophages
  mechanism in cell-mediated immunity

25
CMI Importance

• Need CMI when
• macrophage not activated by microbes (faulty
  innate response)
• microbes evolve a mechanism to evade the innate
  immune response
• CMI helps to overcome both of these and change
  balance back to macrophage/host

26
Macrophage Damage

• Macrophage molecules that attack intracellular
  microbes can also harm normal tissue if released
• why we see tissue damage in DTH
• Can happen when there is a long term CMI response
  and macrophage activation such as during
  infections such as with mycobacterium
• see granuloma formation

27
Other Roles of Activated Macrophages

• Secrete cytokines TNF and IL-1 and chemokines
• stimulate recruitment of neutrophils, monocytes
  and effector T-cells to site of infection
• Secrete platelet derived growth factor (PDGF)
  stimulate fibroblasts and endothelial cells
  repairs the tissues after infection
• Also increases expression MHC II and
  costimulators which enhance Ag-presentation and
  T-cell amplification and CMI

28
Role of TH2 Cells in CMI

• Function to enhance eosinophil rich inflammation
  and to limit the tissue damage caused by
  macrophages activation
• TH2 cells produce IL-4 (IgE Ab) and IL-5
  (activates eosinophils) as well as IL-10 which is
  also produced by other cells
• usually helminthic infections
• IgE coats helminth, eosinophil recognizes IgE and
  causes the release of granules that kill the
  helminth
• TH2 cells also secrete IL-4, IL-10 and IL-13
  which inhibit macrophage activation terminated
  TH1-mediated DTH limiting the tissue injury

29

• TH2 and TH1 relative activation may determine
  outcome of infection
• Activated macrophages are against microbes in
  vesicles
• CTLs are against cytoplasmic and proteins that
  escape from phagosomes

30
Effector Functions of CTLs

• CD8 CTL recognize MHC class 1 and kill these
  cells
• protein Ag synthesized in cytoplasm and those
  that escape phagocytic vesicles
• recognize MHC I peptides on infected cell (target
  cell) by the TCR on CD8 T-cell and CD8
  co-receptors
• cells hold together by integrins on the CTL by
  binding ligands on the target cell
• Ag receptors and co-receptors cluster at site of
  contact on the target cell
• dont require co-stimulation or T cell help
• can kill any infected cell with appropriate Ag in
  surface

31
Signal Transduction

• Ag recognition by CTL causes signal transduction
  to make protiens
• causes exocytosis of CTL granule contents which
  will cause pores to form in the cell
• introduce things that can induce DNA
  fragmentation and apoptosis
• Pore forming protein is perforin inserts into
  cell membrane and polymerizes by high
  concentration of Ca2 ions in extracellular
  environment to make pore
• CTLs secrete granzyme that gets into the cell
  thru the pores or by receptor mediated
  endocytosis
• granzyme activates caspases that induce apoptosis
• caspases are cysteine protease that cleaves at
  aspartic acid residues

32
Cell Death by 2nd Pathway

• Activated CTLs have membrane protein called Fas
  ligand which binds to death inducing CD95 (Fas)
  on target cells (Chapter 9)
• activates caspases and induces target cell death
• DOES NOT require granzyme
• minor pathway

33
Net Outcome

• Infected cells are killed
• Apoptotic cells phagocytosed and eliminated
• Also kills microbes growing inside of cell
• CTLs can detach and go kill another cells
• Also secrete cytokine IFN? - activates
  macrophages to destroy phagocytosed microbes and
  recruit other leukocytes
• CD8 CTLs also help eliminate microbes by
  phagocytosis

34
CD4 and CD8 Cells

• Function together to eradicate intracellular
  microbes
• only in the phagosome CD4 T-cell and IFN?
  macrophage
• escape from phagosome CD8 CTL

35
Resistance of Microbes to CMI

• Different microbes evade the diverse mechanisms
  to resist T-cell mediated host defense
• inhibit the fusion of phagosomes with lysosomes
  and create pores in phagosomal membranes and
  escape
• many viruses inhibit MHC I expression, block
  transport of Ag by TAP1 and TAP2 and remove new
  MHC I molecules from the ER
• all lead to reduced peptide loading and viral Ag
  presentation
• some viruses produce inhibitory cytokines or
  decoy receptors to remove IFN? to prevent the
  CMI response
• kill cells of the immune system like HIV killing
  CD4 cells

36
Must Know These!!!
37
Rescue Mechanisms

• NK cells are activated by MHC I deficient cells
  help overcome viral immune evasion
• Protection from microbes that evade the immune
  response is through vaccination of host