T cell differentiation and function Flashcards
what are the different effector T cell populations?
cytotoxic CD8 cells
helper CD4 cells:
- Th1, Th2, Th17, Tfh, Treg
what controls T cell subset differentiation?
Signal 1, 2, 3
Signal 3 = cytokines from APCs
- epigenetic modifications can change CD4 cell fate
- hallmark TF control changes depending on subset
how is a T cell army generated?
- Activated by APC presenting peptide-MHC
- clonal expansion of pathogen-specific clones
- differentiation of clones
contraction into either
- effector pool
- memory pool which are long-lived
- re-exposure results in faster response
why do T cells need to clonally expand?
TCRs are highly diverse – only small proportion will respond to specific pathogen
- once the TCR has bound its cognate antigen, it needs to expand to generate more T cells which recognise the same antigen
what is a naive T cell?
Naïve T cell has undergone thymic selection so have rearranged their alpha-beta TCRs
- received selection signals in thymus
– exist in periphery but haven’t been activated in secondary lymphoid tissues
what induces a naive T cell to expand?
Naïve T cells activated by antigen presentation in secondary lymphoid organs proliferate and differentiate before exiting
- secondary lymphoid organs are the major site of T cell activation
How do T cells home to secondary lymphoid tissues (SLTs)?
Naive T cells express chemokine receptors/lymph node homing receptors CCR7 and I-selectin – homing into lymph node
what happens to naive T cells when in SLT??
APCs (DCs) enter lymph node via lymphatics
– naive T cells screen their TCRs for peptide-MHC
- Some T cells will be activated by APC presented peptide = proliferation and differentiation of T cell
- Mature T cells leave lymph node and change their homing receptors to enter sites of inflammation
Different Th subsets have different homing receptors to enter different sites of inflammation
what are the important lymph node homing receptors? how are they expressed by different T cells?
CD62L/L-selectin
CCR7
- naive T cells have high levels of these
- mature effector memory T cells lose homing receptors to lymph nodes to enter other tissues
- central memory T cells retain homing receptors to circulate around lymph nodes
how do naive T cells enter the lymph node?
Naïve T cells can enter lymph node using CD62L/L-selectin to become activated and adhere before differentiating and exiting to the tissues
what signals are important for naive T cell activation?
Signal 1 – TCR binding MHC-peptide
Signal 2 – co-stimulation CD28 with CD80/86
- this is required for full activation
- delivers signals for metabolic reprogramming - changes metabolism from inactivate to active for proliferation and clonal expansion
Signal 3 – cytokines produced from APC – induces subset which naïve T cell differentiates to
- cytokine environment is crucial
what are CD8 T cells?
CD8 can take on hallmarks of CD4 subsets, but are mostly cytolytic killing cells which produce IFNy (similar to TH1)
- capacity to kill but can be similar to Th1 with regards to cytokines
- CD8 kill cells with peptide + MHCI
how do CD8 T cells kill cells?
Virally-infected cell contains viral peptides presented in MHCI
- CD8 specific for peptide forms synapse with infected cell and release perforin and granzymes into the cell – direct granules to immunosynapse and secrete - forms holes in membrane to directly lyse cell
- Or can express death receptors e.g. Fas ligand
- if target cell expresses Fas, can induce apoptosis of infected cell
Directly kills infected cell or induces apoptosis via death receptor signalling
what are the CD8 killing mechanisms?
Perforin and granzyme B = lytic mechanism
CCL5, IFNy, TNFa, IL-17 = non-lytic
Fas ligand = apoptotic/death mechanism
how do CD8 T cells become exhausted in cancer?
Chronic antigen stimulation leads to exhausted fate of CD8
- limited killing and function so can’t kill tumours
- Immunotherapy can override exhaustion and induce effector function
what dictates what subset a CD4 cell differentiates to?
Signal 3 is crucial for differentiation
- Cytokines and CRs (cytokine receptors)
induce TFs to cause differentiation
how do dendritic cells control the environment for T cell differentiation?
APC: TCR-peptide-MHCII, costimulation (CD28 – CD80/86) induce activation and proliferation
- Cytokines from APC induce differentiation – innate immune system instructs T cell about nature of infection
- Can make Th1, Th2, Th17 polarising signals – instructions for response needed to be mount
what is the main cytokine produced by Th1 cells? What is its role?
Produce IFNy – activation of infected macrophages – important for intracellular pathogens which can persist in macrophages e.g. TB, Leishmania
IFNy signalling in macrophage induces ROS and respiratory burst – inflammatory reaction is tightly controlled – macrophage increases its killing capacity via Th1 release of IFNy
what induces differentiation of naive T cell to Th1?
IL-12 is the Th1 polarising cytokine:
- IL-12 signals via IL-12R to induce STAT4 signalling
– this upregulates TBET (master TF for TH1)
– TBET binds and promotes IFNy expression
- TBET also binds IL-12Rbeta2 gene -
– increases expression of IL-12R and IFNy
- IFNy can further upregulates IL-12Rbeta2 signalling – positive feedback
- TBET antagonises GATA3 TF
what induces differentiation of naive T cell to Th2?
IL-4 is the polarising cytokine for TH2
- IL-4 activates STAT6 to induce master GATA3 TF, - induces signature cytokine IL-4 to increase IL-4R expression
what is the main dogma for Th1/Th2 differentiation?
cytokine binds cytokine receptor
- this activates a transducing molecule, usually a STAT
- this induces a master TF
- the master TF induces signature cytokine production
- together, the TF and cytokine can control cytokine receptor expression in positive or negative feedback
what are the key functions of Th1 cells?
IFN-g stimulates infected macrophages to help control infection, by increasing:
- MHC expression
- Co-stimulatory molecule expression
- Nitric Oxide (NO) production
- Phagolysosome maturation
- TNF-a production
how do Th1 cells control macrophages?
Important for full activation of macrophage
- Th1 delivers IFNy and CD40L signals to bind to CD40 on macrophage
- causes engulfment and fusion of bacteria into phagolysosome
- induces upregulation of inducible nitric oxide synthase, mature phagolysosome, upregulate ROS to kill pathogen – augments intracellular pathogen killing
how are Th1 cells involved in a response to TB?
M. tuberculosis is “controlled” by Th1 cells activating infected macrophages:
- TB infects macrophage and resists ROS
- Th1 forms granulomas, where a collection of infected multi-nucleated giant cells (fused macrophages) are surrounded by Th1s
- Th1 can then provide IFNy for macrophage activation and prevent TB spread
what do defects in Th1 result in for TB?
dissemination of TB:
- Deficiencies in Th1 cells results in reactivation of latent infections or inadequate control of de novo infection
– TB can escape from granuloma
how are Th1 cells implicated in disease?
Th1 cells are also found at affected sites in autoimmune and inflammatory disease and can cause type 1 IFN response against self-antigens:
- Multiple Sclerosis
- Autoimmune thyroiditis
- Rheumatoid Arthritis
- Type I Diabetes
- Psoriasis
- Crohn’s Disease
- Allograft rejection
what are Th2 cells important for?
Extracellular infections e.g. bacteria, helminths
Produces IL-4: induces B cell isotype switch to IgE for mast cell activation against parasites
what is the signature differentiation of Th2 cells?
IL-4 is the main cytokine that induces TH2:
- IL-4 upregulates GATA3
- GATA3 inhibits TBET due to IL-4 – forcing lineage choice via dose of TF to TH2 state
- GATA3 shuts down IFNy expression and activates IL-4 expression (vice versa for TBET inhibiting IL-4 expression) – antagonises TH1 programme
how do TBET and GATA3 antagonise one another?
T-bet and Gata3 reciprocally antagonise Th1/Th2 differentiation
T-bet will activate Ifng, repressing Il4 locus;
- suppress Th2 programme
Gata3 activates Il4, repressing Ifng
- suppress Th1 programme
what are the synergistic/switch processes in Th1/Th2 differentiation?
TBET (gene is tbx21):
- Synergistc effect: IL-12 signals via STAT4 to induce IFNy (early activation),
- STAT4 induces TBET
- TBET induces own expression and upregulates IFNy (later activation) – multiple pathways and TFs coverging on same fate
Switch effect: TBET inhibits GATA3 expression and function
- Once IFNy is in motion – TBET-IFNy reinforcement loop to stabilise TBET to commit to TH1 rather than TH2 (TH1 is a highly stable phenotype so will not revert to Th2)
what are the functions of Th2 cells?
secrete IL-4, IL-5, IL-9 and IL-13:
- Target basophils, mast cells, eosinophils
- IL-4 induce IgE class switch
- Confers resistance to helminth – coats in IgE to crosslink mast cell - leads to degranulation and gut expulsion
- Stimulate intestinal mucus secretion and peristalsis
how are Th2 cells implicated in disease?
- Implicated in hayfever – IgE response to grass pollen – mast cells crosslinked to allergen via IgE – histamine release
- Asthma – release of I-L4,IL-13 - smooth muscle spasms, eosinophilic inflammation and hypersecretion of mucus
Th2 drives allergic cell disease
- unwanted effects of Th2-derived cytokines on innate immune effectors in airway
how does a Th17 fate differ from Th1/Th2?
Th1 and th2 can commit easily
Th17 are highly plastic
- Phenotype depends on environment, can become TH1 or TH2 if cytokine environment changes
what induces the differentiation of naive T cells into Th17?
polarising cytokines IL6 and TGFb, IL-1b and IL-21 induce TFs RORgt and RORa – these activate signature cytokines:
- IL17A/F, IL-22, CCL20
Th17 is maintained by IL-23
what key cytokine does Th17 produce?
IL-17 protects against fungal and mucosal infection – increase neutrophil recruitment and production in BM via cytokine release, tighten epithelial barriers
what diseases have Th17 cells been implicated in?
Also implicated in autoimmunity, for example Multiple Sclerosis, Crohn’s Disease, Rheumatoid Arthritis
what are the key functions of Th17 cells?
- Produce IL17, IL22 – act on epithelial cells to produce antimicrobial peptides and prevent pathogen invasion – inhibit bacterial growth
- IL-22 increase epithelial cell turnover to shed bacteria
- produce G-CSF which acts on BM to increase neutrophil production
- Recruit neutrophils via chemokines
- Produce CCL20, which is recruitment chemokine for Th17 cells which express CCR6 – recruitment of more TH17 to amplify immunity
how are Th17 cells in an unstable lineage?
Unstable phenotype – highly plastic:
- TH17 cells are between a naïve cell and TH1/TH2
- IL6 and TGFb induce TH17
- Cytokines instruct which CRs will be expressed by the T cell that makes them sensitive to certain fates
how many chains does the IL-12 receptor have?
2:
IL-12beta2 - provides sensing of IL-12
IL-12beta1
what is the function of IL-12R?
it phosphorylates STAT4 to induce TBET and IFNy
- IFNy signals back via IFNyR to increase expression of IL-12Rbeta2
positive feedback - Th1 increases expression of IL-12R to increase sensitivity to IL-12
what IL-12 subunit do Th17 cells use? how does this affect differentiation?
TH17 use common chain IL-12RB1 paired with IL-23R:
- When IFNy is low – IL- 23 signals by STAT3 to reinforce TH17 fate
- When IFNy is high – IFNy upregulates IL-12RB2 so more susceptible to IL-12 – TH17 becomes TH1
what regulates Th17 to Th1 lineage plasticity?
Chromatin structures and modifications:
Epigenetics – modifications that can be heritable but don’t directly change DNA sequence
what are the two key chromatin modifications in Th17 plasticity?
H3K27me3 modification: compaction of chromatin
- DNA is tightly wound up, so not accessible for TFs – turns off gene expression
H3K4me3: relaxation of chromatin
- unwinding of DNA – TFs can access – active gene transcription
– gene turned on
how is TBET modified to enable Th17 plasticity?
Naïve T cell has both H3K27me3 and H3K4me3 modifications at TBET – bivalent chromatin
- gene is poised to choose different differentiation fate (same in stem cells)
- Gene could be expressed or turned off depending on signal
what modification occurs in TBET during Th1 differentiation?
- For Th1: TBET locus loses K27 modification
– DNA unwinds and opens IFNy locus whilst shutting of RORc (RORyt gene) and IL-17 - reinforces TH1 fate
