TC & TH Cells

Question 1

Cytotoxic T Lymphocytes

(CTLs)

Characteristics

Answer 1

• Recognize Ag in class I MHC-restricted manner
• Kills infected cells or those expressing “altered-self” Ag
• Can make IL-2, IFN-α, and TNF-α
• Major role in defense against viral infections and malignant cells
• Causes damage during autoimmune diseases or transplant rejection

Question 2

Precursor CTLs

Answer 2

• Recently activated naïve CD8⁺ T-cells cannot kill due to insufficient granules
• Needs time to mature before effector functions active

Question 3

Methods of CD8⁺ T-cell Activation

Answer 3

1. T_H - cell independent
   
   • APC (Dendritic cells) ⇒ licensed/active APC
   • MHC-I/Peptide + costimulation signal (B7-1 or B7-2)
   • APC signal must be strong enough to stimulate CD8⁺ T-cells to produce IL-2.
2. T_H - cell independent (most effective)
   
   • T_H cell with same TCR specificity helps
   • Upregulates costimulatory molecules on APC’s (B7-1)
   • Produces IL-2 which helps activate CTL

Question 4

CTL

Killing Mechanism

Answer 4

Selective serial killers.

Requires cell-cell contact.

Cells die by apoptotic death to prevent lysis and release of the pathogen.

1. Fas/FasL pathway
   
   • Ca²⁺ dependent
   • “extrinsic”
   • recruitment of “death domain” containing molecules
   • caspase 8 ⇒ caspase 3
2. Performin/granzyme pathway
   
   • Pore can form on plasma membrane or endosomes
   • Ca²⁺ independent
   • “intrinsic”
   • Apaf-1 and procaspase -9 ⇒ caspase 9
   • Activate effector caspase (3) by cleavage

Question 5

Apoptosis

Answer 5

CTLs kill via apoptosis.

Neutrophils, macrophages, and complement induce a necrotic death.

Question 6

Natural Killer Cells

Answer 6

• Large lymphocytes that participate in the innate immune response
  
  • Keeps you alive during the primary response
• Defense against viruses and malignant cells
• Activated by:
  
  • IgG leading to ADCC
  • Lack of class I MHC on target cell
• Kills via:
  
  • Perforin and granzyme
  • Fas:FasL
• Makes IFN-γ and TNF-α
• Stimulated by:
  
  • IFN-α / IFN-β
    
    • From virus-infected cells
    • Favors development of cytotoxic effector function
  • IL-12
    
    • Made by macrophages
    • Favors IFN-γ production by NK cells (and Th1 cells)
      
      • Acts as a positive feedback loop further activating macrophages

Question 7

Viral Infection

Timeline

Answer 7

• Early during a viral infection:
  
  • IFNα, IFN-β, and IL-12 activate NK cells
    
    • Produce most of the IFN-γ
    • Provides most of the cytotoxicity against infected cells
• Later on in the infection:
  
  • Cytotoxic CD8+ T-cells specific for the virus generated
    
    • Become the main IFN-γ producers
    • Become main anti-viral cytotoxic cell

Question 8

Missing Self Model

Answer 8

• Normal cells present a ligand for the activating and inhibitory receptors (MHC I) to NK cells
• When viruses infect cells, some may inhibit MHC class I expression to evade CTLs
• Makes them prime target for elimination by NK cells
• NK cells recognize and kill infected and tumor cells by absence of MHC class I

Question 9

T_H Cell Cytokine Function

Answer 9

Helps to select the immune effector mechanisms engaged:

• CD8+ cell proliferation and activation
• macrophage activation
• NK cell activation
• B cell proliferation, activation, and isotype switching

Question 10

T_H Cell

Class Determination

Answer 10

Initial local cytokine environment at the time of T-cell activation, proliferation, and differentiation critical in determining subset created.

• IL-12 and IFN-γ favors TH1 cells.
• IL-4 favors TH2 cells.
• IL-10 inhibits TH1 pathway thus indirectly promotes TH2 production.
• Most immune reponses include both subclasses.

Self-Promoting Regulation:

• IFN-γ ⇒ STAT1 ⇒ T-Bet ⇒ Increased IFN-γ
• IL-4 ⇒ STAT6 ⇒ GATA-3 ⇒ Increased IL-4 & IL-5

Cross-Regulation:

• T_H2 cells produce:
  
  • IL-10 ⇒ down-regulates TH1 cells
  • IL-4 ⇒ down-regulates T-bet
• T_H1 cells produce:
  
  • IFN-γ ⇒ down-regulates TH1 cells by decreasing GATA-3

Question 11

Model of T_H Cell

Cytokine Secretion

Answer 11

Question 12

Comparison of T_H Cell Profiles

Answer 12

Question 13

T_H1 Functions

Answer 13

Recognizes bacterial peptides:MHC II ⇒ activates macrophages.

Question 14

T_H2 Functions

Answer 14

Recognizes antigenic peptide:MHC II ⇒ activates B-cells.

Question 15

T_H17 Cells

Answer 15

• Proinflammatory cells
• Generated early in immune response when mature microbe-activated dendritic cells make high levels of IL-23, IL-6, and TGF-β which stimulate naive CD4+ T-cells
• Down-regulated to IFN-γ or IL-4 (Th1 or Th2 response)
• Produces IL-17 and IL-6:
  
  • Induce neutrophil recruitment
  • Induce fibroblast/epithelial cytokin production
• Role in bacterial and fungal defense

Question 16

T regulatory cell

Function

Answer 16

• Role in inducing peripheral tolerance
• Prevents immune responses against self-Ag or commensal microorgansims
• Generates inhibitory cytokines

Question 17

Treg Cell Induction

Answer 17

• In absence of infection, dendritic cells make TGF-β > IL-6.
  
  • Continue to present self & environmental Ag
• Naive CD4+ T cell stimulated by dendritic cell producing high TGF-β but no other cytokines involved in Th cell differentiation
  
  • Leads to transcription factor FoxP3
  • Results in Treg cell generation
  • Deficiency in FoxP3 causes absence of Treg = IPEX disease

Question 18

Treg Cell

Mechanism

Answer 18

Expresses both CD4 and CD25 (α-subunit of IL-2R).

Produces IL-10 and TGF-β.

Treg cells inactivate traditional T cells by:

1. Cytokine deprivation by binding to IL-2 in the microenvironment
2. Generation of inhibitor cytokines like TGF-β
3. Inhibiting APCs
4. Cytotoxicity

Question 19

Th Cell Class Summary

Answer 19

Question 20

Thymic Indepedent B-cell Activation

Answer 20

• Thymic (or T cell) indepedent Ag able to activate naive and/or mature B cells without activating T cells
• Tend to be polysaccharides, lipopolysaccharides, and proteoglycans with repetitive epitopes
• Without T-cell help, there is little isotype switching, somatic mutation, or development of memory
• Age dependent

Question 21

T-independent B cell Responses

Answer 21

• Primary, quick response
• Lower concentrations of Ab
• IgM >>>> Ig anything else
  
  • Agglutination & complement activation
• Typical secondary responses largely do not happen because memory T cells not involved
• Speed of innate w/ specificity of adaptive

Question 22

Thymic Dependent B-Cell Activation

Answer 22

1. B-cell binds its Ag
   
   • Ag cross-linking of surface Ig activates B cell
   • Induces B cell proliferation
   • IgM synthesis
2. Surface Ig also facilitates receptor-mediated endocytosis leading to presentation of peptides from Ag on B cells MHC II
3. T-cell with same Ag specificity binds peptide/MHC II (TCR:MHC/peptide and CD28:B7) on B-cell promoting T-cell activation
4. Allows Th2 cell to signal B cell through:
   
   • CD40:CD40L interaction
   • cytokines
5. Leads to B cell proliferation, Ab synthesis, and isotype switching
6. After ~ 1 week, germinal centers formed in secondary lymphoid organs
7. Activated B cells undergo somatic hypermutation and affinity maturation within GC
8. Development of memory B cells largely dependent on T cell help

Question 23

T-dependent B-cell Response

Answer 23

• Slower initiation
  
  • Both T & B need to be stimulated then find each other
  • Both T & B need to recognize similar Ag
• Longer lasting
  
  • Memory exists in both B and T
• Quicker response on subsequent exposure
  
  • Due to permanent changes secondary T and B
• Isotype switching occurs
• Majority of AG induce thymic dependent B cell activation
• Basis for vaccine modification using T-cell dependent Ag to stimulate both

Question 24

Ig Class Switching

Answer 24

• In primary response, CD4+ T-cells are already activated by dendritic cells or macrophages.
• Interaction between T and B cells is dependent on:
  
  • TCR recognition of peptide + MHC Class II on B cell
  • B cell has internalized Ag through recognition of Ag by surface Ig receptor-endocytic route
    
    • Better than macrophages at targeted immune response
  • Processed Ag and re-expressed peptides in context of MHC II
  • Optimal response when both epitopes on same molecular complex allowing cell-cell contact
• Co-Stimulatory signals required
  
  • CD28 on T cells and B7 on B cells
  • CD40 ligand on T cells and CD40 on B cells
    
    • Defect = Hyper-IgM Syndrome

Question 25

Summary of T-B Collaboration

Answer 25

• B cells meet T cells in secondary lymphoid tissue
• If they recognize the same antigen and if that antigen is present, they interact.
• T cells make cytokines that the B cell needs to become activated, switch their isotypes,
• become memory B cells, and improve the affinity of their antibody
• The main effects of cytokines produced by activated T cells on B cells
• IL-2, IL-4, IL-6 lead to B cell activation, proliferation, differentiation
• Isotype switching is dependent on the cytokine produced by the T cell
• IL-4 - role in switching to IgE (allergy)
• IL-10 and TGFβ - role in switching human B cells to make IgA