Cell-mediated immunity

Question 1

Describe the T lymphocyte response broadly

Answer 1

• ANTIGEN-INDEPENDENT (thymus)
  
  • T cell receptor genes undergo rearrangement. After thymic selection, “naïve” T cells expressing T cell receptor and CD4/CD8 are generated.
  • only ~5% leave the thymus!
• ANTIGEN-DEPENDENT (2ndry lymphoid tissue)
  
  • T cells activated by antigen presenting cells displaying MHCI/MHCII + peptide differentiate into “effector” T helper or T cytotoxic cells respectively. Generate cytokines or directly kill infected host cells.
• “Effector” T cells → cell-mediated immunity
  
  • T helper + T cytotoxic

Question 2

Describe how a Naive T cell becomes an effector T cell

Answer 2

• Naive T cell will go into lymphoid tissue, if it meets antigen displayed on appropriate MHC molecule + signal from the co-receptor CD4 or CD8
• NB! T cells get chosen for cytotoxic or helper cell in the thymus during thymic selection.

• Naive T cell:
  
  • Recognition of MHC + peptide + co-receptor (CD4/8) → SIGNAL 1
    
    • this allows the T cell to recognise antigen and become activated, however this is not the whole story
    • to activate a naive T cell in the first place, more is required, signal 2
  • Recognition of CO-STIMULATORY molecule(s) → SIGNAL 2
    
    • T cells exposed to Signal 1 in the absence Signal 2 become unresponsive or “tolerised”
    • co-stimulatory molecules are additional proteins found on the T cell and antigen-presenting cell that also have to engage in order for the T cell to become activated.
    • many of these, example is CD28 (on T cell) interacting with B7 (on antigen presenting cell)
    • → this is known as signal 2. without signal 2, the T cell becomes unresponsive or tolerised
  • Cytokines convert activated T cells into different subsets → (SIGNAL 3)

Question 3

Describe Co-stimulatory molecules and their function etc.

Answer 3

• • B7 is a molecule found on antigen-presenting cells e.g. dendritic, macrophages, B cells
  • B7 must interact with CD28 found on surface of T cell
  • B7 + CD28 interaction results in T cell inducing expression of IL-2 and IL-2 receptor
    
    • IL-2 acts in an autocrine fashion, made by T cell and binds to T cell IL-2 recpetor stimulating it further.
  • Required for activation of CD4+ve and CD8+ve cells.
  • Depending on what cytokines the antigen presenting cell is making when these interactions are occuring, these will stimulate the naive T cell to differentiate into a different subset of T helper effector cells
    
    • i.e. Other cytokines direct T cell differentiation into different subsets of CD4+ve T effector cells (Signal 3).
  • One of the best characterised is B7 expressed by dendritic cells, macrophages, B cells
  • Interacts with CD28 inducing expression of IL-2 and IL-2 receptor
  • IL-2 acts in autocrine fashion on CD4+ve T helper cells; also required for CD8+ve cytotoxic T cell activation
  • Other cytokines direct T cell differentiation into different subsets of CD4+ve T effector cells (Signal 3).

Question 4

Describe the general activity of effector T cells

Answer 4

• Naive T helper cell in 2ndry lymphoid signal gets signal 3 cytokines (vary depending onthe type of infection and the receptors that are activated on the antigen presenting cells) → causing the naive T cells to differentiate into 1 of the 5 main subsets of T helper cells: TH1, TH2, TH17, TFH, or TREG
• these subsets of T helper cells differ in which type of cytokines they produce, and their role in the immune response. THis helps ensure pathogen appropriate immunity

• CD4 +ve T cell subsets - differ in cytokines produced
  
  • Different cytokines induce activated “naïve” T helper cells (a.k.a. TH0 cells) to differentiate into various T cell effector subsets.
• Signal 3
  
  • Cytokines made by antigen presenting; vary depending on the type of infection.
• The T cell subsets in turn differ in the types of cytokines they make and their roles in immune responses. Helps ensure pathogen appropriate immunity.

Question 5

Describe TH1 cells

Answer 5

TH1 cells - IL-12 gamma interferon (Signal 3)
- typically most abundant in blood.

• TH1 cells are generated in response to IL-12 and gamma-interferon
  
  • i.e. thats the TH1 signal 3
• TH1 cells
  
  • Produce IL-2, gamma-interferon and TNF which →
    
    • Activate macrophages → inflammation (Classic CELL-MEDIATED immunity)
      
      • particularly gamma-inteferon is good at activating macrophages and inducing inflammation
      • activated macrophages become much better at killing organisms they ingested, e.g. they may be able to kill tubercle bacilli where before they weren’t able to
    • Important in intracellular infections
    • Induce B cells to make IgG1 and IgG3 (opsonizing) antibodies
      
      • produces a pro-inflammatory response
    • Important for the development of cytotoxic T cells
• overall TH1 cells are important in intracellular infections, but also important for extracellular infections (through stimulating IgG1 and IgG3 production)

Question 6

Describe TH2 cells

Answer 6

• TH2 cells are generated in response to IL-4
• TH2 cells produce IL-4, IL-5, and IL-13
  
  • activate eosinophils and mast cells
    
    • allergy- and parasitic-associated cells
  • IL-4 and IL-13 induce B cells to make IgE (promotes mast cell degranulation)
• important in helminth infections (large EC parasites) and allergy

Question 7

Describe TH17 cells

Answer 7

• tend to be found in lymphoid tissue near mucosal surfaces
• TH17 cells generate in response to TGF-beta and IL-6
• TH17 T cells produce IL-17 and IL-22.
  
  • Activates epithelial cells (which line mucosal surfaces) and fibroblasts.
    
    • Proinflammatory, especially at mucosal surfaces.
  • Recruit neutrophils to sites of infection
    
    • good at getting neutrophils to move out of the blood and onto the mucus surface where the infection is
• Important in fungal and extracellular bacterial infections, role in AUTOIMMUNE disease?

Question 8

Describe T-FH cells (Follicular helper T cells)

Answer 8

• Found specifically in lymphoid follicles
  
  • Lymph node anatomy:
    
    • Lymph node is divided into areas predominantly T cells (blue), and follicles around the outside, where B cells differentiate into plasma cells
      
      • when B cells undergoes clonal selection and clonal expansion, it forms these lymphoid follicles
      • TFH reside in these follicles, normally T and B cells separate in lymph nodes, but here they mingle
• TFH cells generate in response to IL-6
• Help naïve B cells differentiate into plasma cells and memory cells.
  
  • thymus-dependent antigens
• Promote somatic hypermutation + class switching
  
  • through producing cytokines that switch on the expression of AID (activation induce cytesinedeaminase) which promotes somatic hypermutation and class switching
• Contact dependent (requires interaction between co-stimulatory molecules CD154 and CD40), but also produce IL-21.
  
  • for this to occur, B and TFH cells have to interact with each other (in the lymph follicles), B cell presents antigen to T cell, co-stimulatory molecules CD154 (T cell) and CD40 (B cell) → activation of somatic hypermutation + class switching
• also produce IL-21

Question 9

Summary of T cell subset activation and effector functions

Answer 9

Question 10

Describe T Regulatory cells

Answer 10

• down regulates the immune response
• generated in response to TGF-beta (Signal 3)
• 2 types:
  
  • Natural Treg
    
    • Develop in the Thymus, recognise MHC + self-peptide
      
      • against what you would expect, recognising MHC + self-peptide, they should normally be selected against, but these don’t
    • Produce cytokines IL-10, TGF-beta which downregulate T cell responses
    • dependent on T cells interacting with cells directly → contact-dependent
  • Induced Treg
    
    • Developed in secondary lymphoid tissues, particularly mucosal lymphoid tissue
    • Recognise MHC + non-self peptide
    • produce cytokines that downregulate immune responses
    • act on T cells e.g. TH1 TH2 etc.
    • Downregulate inflammatory responses etc.

Question 11

Described Cytotoxic T cells

Answer 11

• Recognise foreign protein associated with MHCI protein
• Their job is to kill infected target cells

• Once activated, cytotoxic T cells bind specifically to infected target cells and induce the target cell to undergo apoptosis.
• 2 ways of doing this:
  1. proteases (granzymes) from cytotoxic T cell enter target cell via perforin channel
  
  1. Perforin perforates target cell membrane, granzymes (proteases (enzymes)) through channel → e.g. TNF-alpha bind to TNF receptor which has death domain (which dissociates upon TNF binding), this triggers caspase cascade to be activated leading to apoptosis
     
     2. T cells have a ligand which binds fas receptors, called fas ligand → causes clustering of fas-receptors → induces activation of Caspase cascade → apoptosis.
        - Fas pathway may be important in down-regulating immune responses

Question 12

Describe killing by CD8 Cytotoxic T cells

Answer 12

• Killing by cytotoxic T cells is
  
  • Specific
    
    • only infected cells bearing antigen are killed
    • this is what makes it different to NK cells
  • Efficient
    
    • granzymes are pre-formed; a single cytotoxic T cell can kill 100s of infected targets
  • “Clean”
    
    • enzymes formed during apoptosis degrade viral DNA and destroy non-viral pathogens. Apoptotic cells are taken up by phagocytes.
  • Other functions of Cytotoxic T cells:
    
    • Can also produce cytokines e.g. IFN-$\gamma$ and TNF-$\alpha$
    • help promote inflammation

• SPECIFIC - only infected cells bearing antigen are killed
• EFFICIENT - granzymes are pre-formed; a single cytotoxic T cell can kill 100s of infected targets
• “CLEAN” – enzymes formed during apoptosis degrade viral DNA and destroy non-viral pathogens. Apoptotic cells are taken up by phagocytes.
• Other functions of Cytotoxic T cells
  
  • Can also produce some CYTOKINES e.g. IFN g and TNF-a

Question 13

Describe the roles of effector T cells in the immune response as a summary

Answer 13

Question 14

Describe the effects of inappropriate activation of T cell subsets

Answer 14

• Inappropriate activation of T cell subsets is implicated in allergy and autoimmune disease
  
  • ALLERGY (Hypersensitivity) definition:
    
    • disease following an immune response to innocuous antigen (ALLERGEN). Mostly IgE-mediated a.k.a. Type I Hypersensitivity.
    • allergy is a immune response to an innoccus antigen (aka an allergen) mediated by IgE against the allergen: known as Type 1 hypersensitivity/allergy
  • Why is allergy increasing in developed countries?

Question 15

Describe the Hygiene hypothesis

Answer 15

• Children brought up on farms, or from large families, are less prone to develop allergy. Early, repeated childhood infections may be protective
• Insufficient exposure to certain types of infection (“dirt”) skews TH1/TH2 balance towards TH2?
• BUT there is a negative correlation between helminth infections and allergic disease.
• i.e. theory not really proved correct

Question 16

Describe the counter regulation or old friends hypothesis

Answer 16

• Childhood infection protects against allergy by promoting IL-10 production
  (Treg ↑ → TH1 ↓ and TH2↓)
  
  • i.e. you weren’t getting sufficient Treg cells in childhood if you weren’t infected a lot as a child
• Infection with microbes or larger parasites plays a critical role in driving immunoregulation e.g. promotes formation of Treg, IL-10
• Human immune system and “Old friends” co-evolved
  
  • old friends = the pathogens
• May also explain rise in autoimmune disease (TH1/TH17-driven)
• so: you need Treg cells that will dampen down other TH cell subsets

Question 17

Describe gamma-delta T cells

Answer 17

• gamma delta T cells (1-5% of T cells)
• remember: 95% of our T cells express $\alpha/\beta$ receptors on their surface and CD4 or CD8 on their cell surface
• However, a subset of T cells that have $\gamma\delta$ chains instead of the $\alpha\beta$ chains
  
  • don’t express CD4 or CD8 either
• Role of these cells?
  
  • Generated earlier in development than αβ T cells
    
    • so the first T cells we make, as the immune system is formed
  • Often found at mucosalepithelium
  • Less diverse, recognise a broader range of antigens (including lipids, phosphorylated antigens, DAMPs e.g. heat shock proteins)
    
    • compared to conventional T cells
  • Do not appear to require processing or presentation by MHCI/II.
    
    • so they are independent of antigen-presentation
  • Can make cytokines e.g. IL-17, IFN-γ, TNF but also cytotoxic. Can act as antigen presenting cells to αβ T cells.
    
    • can produce pro-inflammatory cytokines, can directly kill infected cells (be cytotoxic), and can present antigen to alpha-beta T cells
  • Role in various bacterial, viral and parasitic infections (mycobacteria, flu, HIV, malaria) and also in cancer.
    
    • normally found in mucosal surfaces, but during infection they can be found in the blood
  • May “bridge” innate and adaptive immunity.

Question 18

Describe how the immune response is terminated and why its important

Answer 18

• Important becausse its wasteful and can cause damage if immune responses continue to be produced
• Once an infection is cleared, 99% of activated and effector cells die, so hopefully we are just left with memory B cells
• Mechanisms of down-regulation: Tregs and Cytotoxic T cells
• Immune checkpoints are related to molecules expressed on lymphocytes such that when these molecules bind a ligand, it causes inactivation: examples
  
  • CTLA-4 (induced on activated T cells) binds strongly (with high avidity) to B7 (one of the co-receptor molecules)
  • normally on a naive T cell, B7 interacts with CD28 causing activation of naive T cell
  • once T cell has been activateed during later stages of immune response, it starts expressing CTLA-4 on its surface instead of CD28.
  • CTLA-4 binds with B7 with high avidity → resulting in switching off the T cell (inhibits T cell activation)
    
    • CTLA-4 has higher avidity to B7 than CTLA-4
  • this is one example of an immune checkpoint

• Once an infection is cleared, 99% of activated and effector cells die.
• Mechanisms of down-regulation: Tregs, cytotoxic T cells
• Inhibitory “immune checkpoints” expressed on lymphocytes
  
  • e.g. CTLA-4 (induced on activated T cells) binds with high avidity to B7.
• NB! Engagement of CD28 on naïve T cells with B7 provides the co-stimulatory signal for activation
• CTLA-4 engagement inhibits T cell activation

Question 19

Giveexamples of other immune checkpoints

Answer 19

• e.g. lymphocyte receptors with immunoreceptor tyrosine inhibitory motifs (ITIM)
  
  • associated with their cytoplasmic regions
  • ITAM motifs cause activation, where ITIM cause inhibit cell activation when phosphorylated.
  • 2 examples of this is an Fc receptor on B cells, and PD-1 on B and T cells
  • FcγRIIb – on B lymphocytes
    
    • When the FC regions binds IgG, instead of activating B cell, it switches the B cell off.
  • PD-1 on activated B and T lymphocytes – interacts with PD-ligand, which is widely expressed.
    
    • (PD = programmed death)
    • PD-1 found on activated B cells and T cells, interacts with PD ligand found on many cell types → causes downregulation and ultimately apoptosis of B/T cell
• Cancer cells often subvert “immune checkpoints”.
• Agents that target immune checkpoints may be used in cancer immunotherapy.

Question 20

Summary1: when do naive CD4/CD8 T cells differentiate into “effector” helper or cytotoxic T cells

Answer 20

On recognising antigen + MHCII/I, naive CD4/CD8 +ve T cells differentiate into “effector” helper or Cytotoxic T cells

Question 21

Summary2: What do naive T cells also require for survival

Answer 21

Naive T cells also require co-stimulatory signals for survival, provided by e.g. B7 and CD28 (signal 2)

Question 22

Summary3: Signal 3 =

Answer 22

Cytokines made by antigen presenting cells influence the subset of effector CD4+ ve T cells generated (signal 3)

Question 23

How do CD4+ve T helper cells differ

Answer 23

CD4+ve effetor T cell subsets differ in the cytokines they make and therefore in their role in the immune responses

Question 24

TH1 cells are:

Answer 24

important for intracellular infections (activate macrophage and help development of Cytotoxic T cells)

Question 25

TH2 cells are

Answer 25

important in parasitic infections and in allergy, by promoting the production of IgE

Question 26

TH17 CD4+ve T cells are

Answer 26

pro-inflammatory, help recruit neutrophils and are important in extracellular bacterial and fungal infections

Question 27

TFH CD4+ve T cells :

Answer 27

help “naive” B cells in lymphoid tissues differentiate and undergo class switching

Question 28

Treg CD4+ve T cells help

Answer 28

suppress immune responses

Question 29

Cytotoxic T cells

Answer 29

kill infected host cells primarily by secreting “granzymes” through perforin channels to induce apoptosis

Question 30

Killing by cytotoxic T cells is:

Answer 30

specific, efficient and “clean”

Question 31

Imbalances in T helper subsets may contribute to:

Answer 31

the incidence of allergy and autoimmune disease

Question 32

“Immune checkpoints” are important for:

Answer 32

dpwnregulating immune responses once an infection is cleared