T cell mediated responses II

• What are Foxp3 Treg cells?

  FoxP3 Tregs are the best studied Treg cell and express both CD4 and CD25 on their cell surface. They are characterised by the expression of the transcription factor FoxP3.
• What are Treg cells?

  These are T cells that suppress self reactive T cell responses and Treg defects are often exhibited in a variety of autoimmune diseases.
• What is IPEX?

  Immunodysregulation polyendocrinopathy enteropathy X-linked syndrome:

  • Mutations in the transcription factor FoxP3
  • Results in non-functional Treg cells
  • Development of severe auto-immune reactions
  • Bone marrow transplant is necessary for survival
• Describe how Foxp3 Tregs develop

  FoxP3 Tregs develop in two ways:

  • Natural or thymic Tregs: made in the thymus as part of normal T cell development
  • Induced or peripheral Tregs: develop from naive CD4+ T cells in the periphery
• Describe the two-step model for natural/thymic Treg development

  1. Treg precursor, self-reactive CD4+ T cell, exhibits strong TCR engagement resulting in the upregulation of CD25.
  2. IL2 signalling then stimulates the expression of FoxP3
• Describe the TGF-beta dependent model for natural/thymic Treg development

  Treg precursor, self reactive CD4+ T cell, exhibits TCR stimulus alongside TGF-beta signalling, which induces FoxP3 expression.
• How do induced/peripheral Tregs develop?

  1. In the periphery naive CD4+ T cell exhibits TCR activation alongside exposure to other factors:
  2. TGF-beta
  3. Retinoic acid
  4. IL-12
  5. These factors induce FoxP3 expression

  Specific dendritic cells in the gut can promote Treg formation, therefore this development is most prominent in the gut.
• Describe cytokine-dependent action of FoxP3 Tregs


  Cytokines play a key role in Treg mediated immune suppression:

  • TGF-beta and IL-10 are anti-proliferative cytokines produced by Tregs
  • They act on self-reactive CD4+ T cells and reduce proliferation
• Describe how FoxP3 Tregs induce T cell death

  FoxP3 Tregs secrete perforin, which creates pores in the CD4+ T cell membrane, and granzymes, which induce cell apoptosis.
• Describe how Foxp3 Tregs disrupt effector T cell metabolism

  IL-2 is important in driving proliferation of CD4+ T cells, due to activation of CD25, which is a high affinity IL-2 receptor. FoxP3 Tregs express high amounts of CD25, which allows it to mop up IL-2 so that it cannot be used by effector T cells. FoxP3 Tregs also express CD73 and CD39, which convert ATP to adenosine and remove ATP from the environment, which inhibits CD4+ T cell proliferation (Highly energy dependent process)
• Describe how FoxP3 Tregs target dendritic cells

  CTLA-4 dependent mechanism: FoxP3 Tregs express CTLA-4, which binds to CD80/86 and removes them the dendritic cell surface Via transendocytosis. These are costimulatory molecules that are required for T cell activation. Therefore, reducing activation of CD4+ T cells.

  Blocking T cell access to dendritic cells: Tregs bind to MHC II and prevent CD4+ T cells from binding
• Describe Treg plasticity

  Hybrid Tregs can be produced:

  • Tregs expressing Foxp3 and Tbet: better able to suppress Th1 cells
  • Tregs expression FoxP3 and gata3: gata3 is expressed in activated Tregs
• How are cytotoxic CD8+ T cells activated?

  CD8+ T cells differentiate into cytotoxic T cells, but before this can occur they must be activated:

  1. Naive CD8+ T cell engages with APC via its TCR
  2. IL-2 then stimulates proliferation and differentiation of T cell to form a CTL
  3. Active CTLs then kill virally infected cells
• Describe how dendritic cells present antigen to CD8+ T cells

  DCs are specialised to present antigen to CD8+ T cells:

  • MHC I presents intracellular antigens
  • However, DCs can also take up and present extracellular antigen on MHC I, via cross presentation
  • DCs produce high levels of IL-2 to generate high levels of co-stimulation
• Describe the role of CD4+ T cells in CD8+ T cell activation

  APCs can stimulate effector CD4+ T cells to induce CD40L and IL-2 production. These co-stimulatory signals induce dendritic cells to express more co-stimulators molecules and enhance co-stimulation of CD8+ T cells. IL-2 produced by activated CD4+ T cells can help drive the proliferation of CD8+ T cells.
• Describe the role of TH1 cells in CD8+ T cell activation

  Th1 cells can secrete IFN-gamma, which aids in the proliferation and differentiation of CD8+ T cell to form cytotoxic T cells.
• Do CTLs kill cells that don’t provide co-stimulation?

  After initial activation, CTLs do not require co-stimulation to kill a cell, they only require antigen recognition. However, these mechanisms of death induction need to be extremely well targeted.
• Describe how CTLs perform targeted cell killing

  1. TCR-MHC I interaction recruits LCK, which mediates docking of centrosome at the TCR.
  2. Centrosome then reorganises the microtubule and actin cytoskeleton, in order to move cytotoxic granules towards the immunological synapse
  3. These granules contain enzymes and factors that can induce cell death and they are released directly at the synapse.
• Describe the components of cytotoxic granules

  The main components of cytotoxic granules are:

  • perforin: forms pores in the cell membrane
  • granzymes: granzymes enter through pores directly into the target cell
• Describe how granzymes activate caspase-mediated induction of apoptosis

  Granzymes are delivered into the cytosol of infected cells and target and cleave pro-caspase 3 to form active caspase 3. Cas3 is an important enzyme for induction of cell apoptosis. Granzymes also cleave ICAD, which is an inhibitor of caspase activated DNase (CAD), therefore allowing CAD to cleave DNA and induce apoptosis.
• Describe how granzymes activate BID-mediated induction of apoptosis

  Granzymes can cleave BID, which, when truncated, can disrupt the mitochondrial outer membrane. This results in the release of cytochrome C from the mitochondria, which causes activation of caspase 9. Caspase 9 then activates cell apoptosis.
• Describe production of cytokines by CTLs

  CTLs produce cytokines to regulate immune responses:

  • IFN-gamma: activates macrophages, increases MHC I expression in infected cells, and directly inhibits viral replication
  • TNF-alpha: helps IFN-gamma activate macrophages
  • LT-alpha: can help directly kill some infected cells